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The piezoelectric and fatigue behaviour of lead-free BCTZmaterial
Author:Zhang, Yichi
Publication Date:2015
DOI:https://doi.org/10.26190/unsworks/18419
License:https://creativecommons.org/licenses/by-nc-nd/3.0/au/Link to license to see what you are allowed to do with this resource.
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The Piezoelectric and Fatigue Behaviour of Lead-free BCTZ Material
Yichi Zhang
A thesis in fulfilment of the requirements for the degree
Doctor of Philosophy
School of Materials Science & Engineering
Faculty of Engineering
2015
Acknowledgements
i
ACKNOWLEDGEMENTS
I would firstly like to thank my supervisor, Professor Mark Hoffman, for the great
opportunities and continuous support during my time at University of New South Wales.
I appreciate his ideas, time and patience on my project. In addition, I also thank him for
supporting me to attend trainings, workshops and conferences and to have overseas
research collaborations with Purdue University. I would also like to thank my co-
supervisor, Doctor Julia Glaum, for her German-style teaching. It has always been a
great fun to learn from her. Thank her for all the teachings, discussions, encouragements,
comments, coffees and smiles.
I also want to acknowledge researchers who helped me during my study time: Joseph
Arsecularatne, Tania Vodenitcharova, Manuel Hinterstein, John Daniels, Yunqi Wu,
Tian Hang, Neil Lazo, Sayedeh Emami, Lu Chen, Neamul Khansur, Hugh Simons and
many others in UNSW; Professor John Blendell, Matthias Ehmke, Tony Chung and
Xiao Ma in Purdue University; Professor Keith Bowman in Illinois Institute of
Technology; Michael Hoffmann in Karlsruhe Institute of Technology; Professor Jürgen
Rödel, Yo-Han Seo, Kyle Webber and Claudia Groh in the Technische Universität
Darmstadt; Professor Jacob Jones in North Carolina State University; Orapim Namsar
in Chiang Mai University. I am thankful to George Yang, Rahmat Kartono, Twin Htoo,
Anthony Zhang, Jane Gao and Danny Kim for their technical support. I would specially
thank Bill Joe for his great support in the piezoelectric lab. I am also thankful to Yu
Wang who made the temperature dependent X-ray diffraction measurement available
for me and Andrew Studer who assisted the neutron diffraction experiment in ANSTO.
Acknowledgements
ii
I am also grateful to UNSW workshop and Zhenyu Liu in Faculty of Engineering for
their contributions to my self-made fatigue sample stage.
The research work is financially supported by Australian Research Council and
University of New South Wales. Parts of the living cost in the US during research
collaboration was supported by Purdue University.
Last, I owe special gratitude to my wife, Yingyu Huang, who makes my life not boring
anymore. I am also greatly indebted to my parents. They have been great supporters and
comforters to me during my life. I thank all my friends, Church brothers and sisters for
their companies during these years. Thank God for His blessing, predestination,
redemption and salvation.
Abstract
iii
ABSTRACT
Piezoelectric ceramics can generate electric charges upon the application of mechanical
pressure and can also deform accordingly upon the application of electric field. They
have been widely used for sensors and actuators. The most commonly used
piezoelectric materials are modified lead zirconate titanate (PZT) due to their high
piezoelectric response, broad working temperature range and ease of modification for
specific applications. However, the PZT contains toxic lead. During the production and
disposal, lead could be released to the environment and cause potential environmental
hazards. Therefore, significant research has been conducted to develop lead-free
alternatives. Unfortunately, none of the lead-free piezo-ceramics developed to date can
replace PZT completely. Some do not show comparable piezoelectric response to PZT;
some have high piezoelectric response but cannot operate at high temperature; some are
difficult to synthesise.
Due to a high piezoelectric coefficient, (Ba100-xCax) (Ti100-yZry) O3 (BCTZ) piezoelectric
ceramics have been considered as a promising lead-free alternate piezoelectric material
at room temperature range. However, the BCTZ system has not been fully understood
and only limited compositions in the system have been synthesised and investigated in
the literature.
In this thesis seven specifically selected compositions in the BCTZ system are
investigated including six new compositions that have not been previously reported.
The compositions were selected, based on a prediction determined from analysis of the
literature, such that they have phase transition and would exhibit high piezoelectric
coefficient at room temperature. Grain size, density, pores morphology, phase structure,
piezoelectric response and field-dependent permittivity were characterized for all new
Abstract
iv
compositions. The results confirmed all compositions exhibit well developed hysteresis
loops and a large piezoelectric coefficient at room temperature. This is due to the
coexistence of several phases where the major phase is likely to be orthorhombic and
the second phase is proposed to be tetragonal. The phase transition was found to occur
over a broad temperature range instead of at a specific temperature only. A relationship
between the tetragonal-orthorhombic phase transition temperature and Ca2+ and Zr4+
content was proposed. This enables clear determination of other BCTZ compositions
with high piezoelectric coefficient at a desired operation temperature.
Ageing behaviour usually occurs in the acceptor-doped piezoelectric material (e.g. hard
PZT), and exhibits as the development of a pinched or shifted hysteresis loop over time.
Although no pinched hysteresis loop was observed for lead-free BCTZ material, it was
serendipitously noticed during the piezoelectric property measurements that the
hysteresis loop of poled BCTZ changed over time. Samples were then prepared for a
systematic investigation of ageing behaviour. The samples were left at poled state up to
36 days and showed a shift of the hysteresis loop along the field direction and
development of asymmetry in strain and permittivity hysteresis loop over time. The
origin of this ageing behaviour is proposed as the local defect dipoles and the migration
of the charged defects to the grain boundaries. The reorientation of the defect dipole
contributes to a fast but unstable ageing mechanism in this material while the migration
of the charged defects contributes to a slow but more stable mechanism.
Consistent piezoelectric performance over long periods of operation is an important
factor in terms of application and commercialization, in addition to high piezoelectric
response. Many piezoelectric materials experience property degradation and mechanical
damage during continuous electromechanical cycling. Therefore, the stability of the
Abstract
v
piezoelectric performance during both unipolar and bipolar cycling is investigated. For
unipolar cycling, samples were cycled at 10Hz up to 5×106 cycles. It is found that the
unipolar fatigue behaviour is similar to soft PZT. Development of a bias field, offset
polarization, and asymmetry in strain and dielectric hysteresis loops are observed during
bipolar measurements. These changes are mainly attributed to the migration of charged
carriers driven by the unscreened depolarization field at grain boundaries. Re-
distribution of the accumulated charged carriers by electric bipolar cycling or thermal
annealing can significantly recover the unipolar fatigued state. The high unipolar strain
response experiences a slight decrease and stabilized after 1000 cycles, which is a good
characteristic for actuator applications.
For bipolar cycling, two compositions with different Ca and Zr doping were cycled at
10 Hz up to approximately 107 cycles. Both investigated compositions exhibited high
bipolar fatigue resistance compared to other ceramics reported in the literatures. The
high fatigue resistance is originated from the lack of mechanical damage and a weak
domain wall pinning effect due to their location in the phase transition region. It was
also found that the pore morphology affected the bipolar fatigue behaviour.
List of Publications, Conferences and Workshops Arising from the Research Presented in this Thesis
vi
LIST OF PUBLICATIONS, CONFERENCES AND WORKSHOPS
ARISING FROM THE RESEARCH PRESENTED IN THIS THESIS 1. ZHANG, Y., GLAUM, J., GROH, C., EHMKE, M. C., BLENDELL, J. E.,
BOWMAN, K. J. & HOFFMAN, M. J. 2014. Correlation Between Piezoelectric Properties and Phase Coexistence in (Ba,Ca)(Ti,Zr)O3 Ceramics. Journal of the American Ceramic Society, 97, 2885-2891.
2. FRANZBACH, D. J., SEO, Y.-H., STUDER, A. J., ZHANG, Y., GLAUM, J., DANIELS, J. E., KORUZA, J., BENČAN, A., MALIČ, B. & WEBBER, K. G. 2014. Electric-field-induced Phase Transitions in co-doped Pb(Zr1−xTix)O3 at the Morphotropic Phase Boundary. Science and Technology of Advanced Materials, 15, 015010.
3. ZHANG, Y., GLAUM, J., Ehmke, M. C., BOWMAN, K. J., BLENDELL, J. E. & HOFFMAN, M. J. 2015. The Ageing Behaviour of Poled BCTZ Lead-free Piezoelectric Ceramics. Journal of the Applied Physic (submitted)
4. ZHANG, Y., GLAUM, J., Ehmke, M. C., BOWMAN, K. J., BLENDELL, J. E. & HOFFMAN, M. J. 2015. Unipolar Fatigue Behaviour of BCTZ Lead-free Piezoelectric Ceramics. Journal of the American Ceramic Society (submitted)
5. ZHANG, Y., GLAUM, J., Ehmke, M. C., BOWMAN, K. J., BLENDELL, J. E. & HOFFMAN, M. J. 2015. High Bipolar Fatigue resistance of BCTZ Lead-free Piezoelectric Ceramics. Journal of the American Ceramic Society (submitted)
6. ZHANG, Y., GLAUM, J., GROH, C., EHMKE, M. C., BLENDELL, J. E., BOWMAN, K. J. & HOFFMAN, M. J. “Correlation Between Piezoelectric Properties and Phase Coexistence in (Ba,Ca)(Ti,Zr)O3 Ceramics”, oral presentation, EMA (Electronic Materials and Applications) 2014 conference, United states.
7. ZHANG, Y., GLAUM, J. & HOFFMAN, M. J. “Fatigue Behaviour of Lead-free BCTZ Piezoelectric Material”, oral presentation, CAMS (Combined Australian Materials Societies) 2014 conference, Australia
8. ZHANG, Y., GLAUM, J. & HOFFMAN, M. J. “Fatigue Behaviour of Lead-free Piezoelectric Ceramics”, poster presentation, ANSTO-AINSE Neutron School on Order and Disorder, 2011, ANSTO, Australia
a. Workshop: ANSTO-AINSE Neutron School on Order and Disorder 2011, Australia
b. Workshop: Quantitative X-ray Diffraction Analysis, UNSW, Australia
List of Symbols and Abbreviations
vii
LIST OF SYMBOLS AND ABBREVIATIONS
piezoelectric coefficient (small signal)
∗ piezoelectric coefficient (large signal)
bias field
coercive field
mean coercive field
positive coercive field
negative coercive field
maximum polarization
remanent polarization
2 switchable polarization
maximum strain (unipolar loading)
maximum strain in positive half cycle (bipolar loading)
maximum strain in negative half cycle (bipolar loading)
minimum strain in positive half cycle (bipolar loading)
minimum strain in negative half cycle (bipolar loading)
∆ strain amplitude in positive half cycle (bipolar loading)
∆ strain amplitude in negative half cycle (bipolar loading)
List of Symbols and Abbreviations
viii
T temperature
Curie point
phase transition temperature between orthorhombic and tetragonal phases
ε permittivity
strain asymmetry (gama)
ANSTO Australia Nuclear Science and Technology Organization
BCT barium calcium titanate
BCTZ barium calcium titanate zirconate
BT barium titanate
BNT bismuth sodium titanate
BZT barium zirconate titanate
EDS energy-dispersive X-ray spectroscopy
FIB focused ion beam
KNN potassium sodium niobate
MPB morphotropic phase boundary
PPT polymorphic phase transition
PZT lead zirconate titanate
SEM scanning electron microscopy
XRD X-ray diffraction
Table of Contents
ix
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ........................................................................................................ i
ABSTRACT ....................................................................................................................... iii
LIST OF PUBLICATIONS, CONFERENCES AND WORKSHOPS ARISING FROM THE RESEARCH
PRESENTED IN THIS THESIS ............................................................................................... vi
LIST OF SYMBOLS AND ABBREVIATIONS .......................................................................... vii
TABLE OF CONTENTS ........................................................................................................ ix
LIST OF FIGURES ............................................................................................................ xiii
LIST OF TABLES .............................................................................................................. xxi
Chapter 1 Introduction ...................................................................................................... 1
1.1 Background ............................................................................................................. 1
1.2 Preview ................................................................................................................... 2
Chapter 2 Literature Review ............................................................................................. 4
2.1 Basic Principle of Piezoelectric Materials .............................................................. 4
2.1.1 Dielectric materials .......................................................................................... 4
2.1.2 Piezoelectricity ................................................................................................. 5
2.1.3 Pyroelectricity .................................................................................................. 8
2.1.4 Ferroelectricity ................................................................................................. 8
2.1.5 Domain poling and switching .......................................................................... 9
2.2 Properties of Piezoelectric Materials .................................................................... 14
2.2.1 Pb (Zr, Ti) O3 ................................................................................................. 14
Page
Table of Contents
x
2.2.2 Background of Lead-free piezoelectric materials .......................................... 17
2.2.3 Bismuth-based materials ................................................................................ 18
2.2.4 Sodium-potassium-niobate-based materials .................................................. 20
2.2.5 Ba1-xCaxTi1-yZryO3 ......................................................................................... 22
2.3 Transient Degradation mechanisms ...................................................................... 37
2.3.1 Ageing behaviour ........................................................................................... 37
2.3.2 Fatigue studies on PZT-based materials ........................................................ 40
2.3.3 Fatigue studies on lead-free piezoelectrics .................................................... 45
Chapter 3 Thesis Objective, Hypotheses and Approaches ............................................. 50
3.1 Aim of Thesis ........................................................................................................ 50
3.2 Hypotheses ............................................................................................................ 51
3.3 Research Approach ............................................................................................... 55
Chapter 4 Methodology and Materials ........................................................................... 58
4.1 Materials Preparation ............................................................................................ 58
4.1.1 Preparation of the BCTZ samples .................................................................. 58
4.1.2 Surface preparation ........................................................................................ 59
4.1.3 Electrode ........................................................................................................ 59
4.2 Characterization methods ...................................................................................... 61
4.2.1 Density measurement ..................................................................................... 61
4.2.2 Grain size determination ................................................................................ 62
4.2.3 Diffraction measurement ............................................................................... 62
Table of Contents
xi
4.2.4 Piezoelectric property measurement .............................................................. 68
4.2.5 Frequency and temperature dependent permittivity and dielectric loss ......... 70
4.3 Testing of Ageing Behaviour ................................................................................ 71
4.4 Fatigue Testing ...................................................................................................... 71
4.4.1 Fatigue setup .................................................................................................. 71
4.4.2 Fatigue test process ........................................................................................ 75
4.4.3 Improvement of electrode for fatigue test ...................................................... 78
Chapter 5 Characterization of BCTZ .............................................................................. 81
5.1 Introduction ........................................................................................................... 81
5.2 Experimental Results ............................................................................................ 81
5.2.1 Density, grain size and pore morphology ...................................................... 81
5.2.2 Piezoelectric and dielectric properties ........................................................... 83
5.2.3 Temperature dependent XRD, permittivity and d33 ....................................... 85
5.3 Discussion ............................................................................................................. 89
5.3.1 Phase transition .............................................................................................. 89
5.3.2 Piezoelectric performance .............................................................................. 92
5.3.3 Influence of Ca and Zr doping on orthorhombic-tetragonal phase transition 94
5.4 Summary ............................................................................................................... 96
Chapter 6 Ageing Behaviour of BCTZ ........................................................................... 97
6.1 Introduction ........................................................................................................... 97
6.2 Experimental Results ............................................................................................ 99
Table of Contents
xii
6.3 Discussion ........................................................................................................... 104
6.4 Summary ............................................................................................................. 111
Chapter 7 Unipolar Fatigue Behaviour of BCTZ ......................................................... 113
7.1 Introduction ......................................................................................................... 113
7.2 Experimental Results .......................................................................................... 114
7.3 Discussion ........................................................................................................... 119
7.4 Summary ............................................................................................................. 126
Chapter 8 Bipolar Fatigue Behaviour of BCTZ ............................................................ 127
8.1 Introduction ......................................................................................................... 127
8.2 Experimental results ............................................................................................ 128
8.3 Discussion ........................................................................................................... 133
8.3.1 Comparison to other materials ..................................................................... 133
8.3.2 Fatigue resistance ......................................................................................... 135
8.3.3 Ca and Zr contents ....................................................................................... 138
8.3.4 Micro-cracking ............................................................................................. 139
8.3.5 Anisotropy in BC8TZ5.5 ................................................................................ 141
8.4 Summary ............................................................................................................. 143
Chapter 9 Discussion .................................................................................................... 144
Chapter 10 Conclusions ................................................................................................ 153
REFERENCE .................................................................................................................... 156
APPENDIX A - FATIGUE CONTROLLING PROGRAMMING (LABVIEW GRAPHICAL CODE) 165
List of Figures
xiii
LIST OF FIGURES
Figure 2-1. (a) A cubic ABO3 perovskite structure; (b) Tetragonal phase with a dipole in
the temperature under Tc ................................................................................................... 9
Figure 2-2. Changes of lattice and dipole upon electric field (a) along and (b)
orthogonal to spontaneous polarization .......................................................................... 10
Figure 2-3. Domain switching under an external electric field....................................... 11
Figure 2-4. A polarization hysteresis loop of a ferroelectric material ............................ 12
Figure 2-5. A two dimensional schematic sketch of an ideal butterfly strain hysteresis
loop. ................................................................................................................................ 13
Figure 2-6. PbTiO3-PbZrO3 sub-solidus phase diagram (Jaffe et al., 1971) ................... 15
Figure 2-7. Polarization hysteresis of ‘hard’ and ‘soft’ PZT (Shrout and Zhang, 2007) 16
Figure 2-8. Phase diagram of BNT–BT. Fα , ferroelectric rhombohedral phase; Fβ,
ferroelectric tetragonal phase; AF, antiferroelectric phase; P, paraelectric
phase(Takenaka et al., 1991). ......................................................................................... 19
Figure 2-9. Temperature dependences of dielectric constant ɛs and loss tangent tan ϭ in
the temperature range from RT to 500 °C for poled BNT. (Hiruma et al., 2009) .......... 20
Figure 2-10. Phase diagram for the KNbO3-NaNbO3 system. Regions labeled with Q, K,
and L are monoclinic, although angular distortions are such that they are commonly
regarded as orthorhombic ferroelectric; M and G are orthorhombic ferroelectric; F, H,
and J are tetragonal ferroelectric. Region P is orthorhombic antiferroelectric. (Rödel et
al., 2009) ......................................................................................................................... 21
Figure 2-11. Phase diagram of BaZr0.2Ti0.8O3-xBa0.7Ca0.3TiO3 system (Liu and Ren,
2009). .............................................................................................................................. 23
Page
List of Figures
xiv
Figure 2-12. The dielectric constant of Ba0.85Ca0.15Zr0.1Ti0.9O3 as a function of
temperature (Xue et al., 2011) ........................................................................................ 24
Figure 2-13. a,b: Piezoelectric coefficient and large signal dS/dE as a function of
composition respectively; c,d: piezoelectric coefficient and k33 as a function of
temperature respectively (modified from (Liu and Ren, 2009, Xue et al., 2011)) ......... 24
Figure 2-14. The phase diagram of Ba0.85Zr0.15TiO3-xBa0.8Ca0.2TiO3 (Bao et al., 2010) 26
Figure 2-15. The dielectric permittivity (a1-a4) and piezoelectric coefficient (b1-b4) of
Ba0.85Zr0.15TiO3-xBa0.8Ca0.2TiO3 as a function of temperature (Bao et al., 2010) .......... 27
Figure 2-16. Solubility and phase relationships in the system (Ba, Ca) (Ti, Zr)O3. The
peripheral numbers indicate the value of the lattice constants in Å. T, tetragonal phase;
C, cubic phase 0, orthorhombic phase. (modified image as solid black lines are original
graph from (McQuarrie and Behnke, 1954) and dashed line is later correction in
(Hennings and Schreinemacher, 1977)) .......................................................................... 28
Figure 2-17. Variation of Curie point of BCZT, summarised from literatures. The
circles represent published data that show a tetragonal to orthorhombic phase transition
and the crosses represent reports of a orthorhombic to rhombohedral phase transition . 30
Figure 2-18. Variation of transition temperature for (a), Ba1-xCaxTi0.93Zr0.07O3 (Hennings
and Schreinemacher, 1977); (b), Ba0.9Ca0.1Ti1-0.9xZr0.9xO3 (Ravez et al., 1999a); (c),
Ba0.9Ca0.1Ti1-xZrxO3 (Favarim et al., 2010) ..................................................................... 31
Figure 2-19. (a) The phase diagram of (Ba1-xCax)(Zr0.1Ti0.9)O3 ceramics. (b) The phase
diagram of (Ba0.85Ca0.15)(ZryTi1-y)O3 ceramics. (Tian et al., 2013b) .............................. 32
Figure 2-20. XRD patterns of the milled powders at room temperature and annealed at
temperatures from 800 to 1200 C for 2 h (Frattini et al., 2012)...................................... 34
Figure 2-21. DTA and TGA curves of milled source powder mixture (Frattini et al.,
2012). .............................................................................................................................. 34
List of Figures
xv
Figure 2-22 Illustration of a defect dipole. (image taken from (Genenko et al., 2015)) . 39
Figure 2-23 Room temperature P-E loops of well-aged Ba(Ti1-xFex)O3 ceramics
measured at about 50 KV/cm and 100 Hz. (Huang et al., 2014) .................................... 40
Figure 2-24. Loading regimes: (a) unipolar, (b) sesquipolar, and (c) bipolar electric; (d)
compression-compression loading (depolarisation), (e) electrical and in phase
mechanical loading (blocking force scenario, anti-resonance of ultrasonic motors), and
(f) out of phase electrical and mechanical loading (mixed loading, resonance of
ultrasonic motors). (Lupascu and Rödel, 2005) .............................................................. 41
Figure 2-25. Polarization P (a), strain S (b), dielectric constant 33 (c), and piezoelectric
constant d33 (d) hystereses for a lead zirconate titanate sample doped with 1% La with
sputtered Pt electrodes and a silver paint protective layer after 1, 3 , and .
bipolar cycles. (Balke et al., 2007a) ....................................................................... 42
Figure 2-26. Macroscopic crack pattern observed by optical microscopy. The schematic
drawing shows the overall appearance of the cracks and the location of the above image.
The cracks initiated from the edge of the electrodes are formed due the difference of
mechanical stress between electroded and non-electroded areas. (Nuffer et al., 2002) . 44
Figure 2-27. Scanning electron microscope pictures of a fracture surface of unfatigued
(a) and fatigued lead zirconate titanate (1% La + 0.5% Fe) samples with sputtered Pt
electrodes and silver paint after fatigue cycles with two different
magnifications. (Balke et al., 2007a) .............................................................................. 44
Figure 2-28. P(E) (a), S(E) (b), 33 (c), and d33(E) (d) before fatigue, after 104 and 106
bipolar cycles. (Luo et al., 2011a) ................................................................................... 47
Figure 2-29. Schematic summary of the fatigue process in BNT-6BT. Poling of the
cubic/relaxor structure is observed in the first 10-100 cycles. After this point, domain
List of Figures
xvi
fragmentation occurs, causing a loss in polarization and a diffraction pattern reminiscent
of unpoled state (Simons et al., 2012). ............................................................................ 47
Figure 2-30. Fatigue results for a typical 5% BZT tested at 50 kV/cm and 10 Hz for (a)
polarization hysteresis and (b) strain hysteresis with invcreasing number of cycles
completed. (Patterson and Cann, 2012) .......................................................................... 49
Figure 3-1. The composition diagram of BCZT system. Dots represent studied
composition. The circled points represent the best compositions reported by each study
on piezoelectric properties. The dash line illustrates the solubility line which is shown in
Figure 2-16. ..................................................................................................................... 52
Figure 4-1 The in-situ neutron diffraction pattern of BC15TZ10 at (a) un-poled state and
(b) during poling at 5Ec. The wavelength was 2.95872 Å. The small peaks were
introduced by contamination of 1/3 wavelength and were fit with a model with 3 times
the lattice parameter of the normal BCTZ model. .......................................................... 63
Figure 4-2 The extracted (111)pc and (200)pc reflection in the temperature dependent
neutron diffraction pattern. The temperature ranged from 200 – 450 K and the
wavelength used was 1.4963 Å. Colours represents intensity of the reflections, where
red signified high intensity while blue signified low intensity. ...................................... 64
Figure 4-3 The lattice parameter calculated from the Rietveld refinement on the
temperature dependent neutron diffraction data. The circles highlight the sudden
changes of the lattice parameter. ..................................................................................... 65
Figure 4-4 A schematic of the temperature-dependent XRD stage configuration. ......... 68
Figure 4-5 An image of TF Analyzer 2000 system. ....................................................... 69
Figure 4-6 The picture of the self-built fatigue setup ..................................................... 72
Figure 4-7 (a) the picture of the sample holder; (b) the schematic of the sample slot in
the sample holder ............................................................................................................ 73
List of Figures
xvii
Figure 4-8 The User interface of the fatigue controller programmed using LabView ... 74
Figure 4-9 The loss of sputtered silver coating after bipolar cycling. ............................ 79
Figure 4-10 The loss of polarization and permittivity during bipolar cycling at 2Hz due
to the damage to electrode. Electrode of the samples was sputtered silver coating plus
the silver paste. Most of the lost properties were recovered by re-applying the electrode.
......................................................................................................................................... 80
Figure 5-1 SEM images of thermochemical etched of (a) BC8TZ5.5, (b) BC10TZ6.7, (c)
BC12TZ8, (d) BC14TZ9, (e) BC15TZ9.5 and (f) BC15.5TZ10............................................... 82
Figure 5-2 (a) Polarization and (b) strain hysteresis loops of all compositions measured
at 0.1 Hz at 25 ˚C. ........................................................................................................... 84
Figure 5-3 Piezoelectric properties of compositions of BCxTZy with x = 8–15.5 and y =
5.5–10. (a) Average of positive and negative coercive field, (b) maximum polarization,
(c) remanent polarization, (d) maximum strain at approximately five times the coercive
field measured at 0.1 Hz; (e) d33 in the remanent state and (f) d33* at approximately five
times coercive field. ........................................................................................................ 84
Figure 5-4 The whole XRD pattern of all compositions at 25 ˚C. .................................. 86
Figure 5-5 Detailed scan of (a) (200)pc and (b) (222)pc peaks at different temperatures.87
Figure 5-6 Permittivity and dielectric loss of BC15TZ9.5 on the function of temperature.
The pink dash lines indicate how the transition temperatures were obtained by using the
tangent method. ............................................................................................................... 88
Figure 5-7 Temperature-dependent d33 curve on BC15TZ9.5. .......................................... 89
Figure 5-8 Phase transition temperature for different compositions with increasing
doping amount. The black points are the phase transition temperature obtained from the
permittivity data. The orange stars are the temperatures that have been investigated with
List of Figures
xviii
XRD measurement. The blue and green area are the phase mixture region according to
the results of XRD measurement. ................................................................................... 91
Figure 5-9 Distribution of the tetragonal-orthorhombic phase transition over
compositions in BCTZ system as a function of temperature and Ca2+ and Zr4+ content.
Stars are the compositions from this study and data of black dots are from literature.
(Ravez et al., 1999b, Mitsui and Westphal, 1961, Li et al., 2010a, Li et al., 2010b, Li et
al., 2010c, Pisitpipathsin et al., 2013, Tian et al., 2013b) ............................................... 95
Figure 6-1 the change of (a) polarization and (b) bias field on the function of time ...... 99
Figure 6-2 (a) the strain hysteresis loop before and after ageing; (b) the decrease of the
minimum strain at the positive side and (c) corresponding change of strain asymmetry
on the function of time .................................................................................................. 101
Figure 6-3 The change of permittivity hysteresis loop after ageing for 36 days. ......... 102
Figure 6-4 The decrease of the bias field during the bipolar cycling. .......................... 102
Figure 6-5 (a) the recovery of the minimum strain at the positive side and (b)
corresponding change of strain asymmetry during the bipolar cycling. ....................... 103
Figure 6-6 The influence of the bipolar measurement on the bias field. ...................... 107
Figure 7-1 The changes of (a) unipolar polarization and (c) strain loops during the
unipolar fatigue cycling at 10Hz, and their extracted maximum (b) polarization and (d)
strain values on the function of number of cycles ........................................................ 114
Figure 7-2 (a) polarization, (b) strain, (c) d33 and permittivity hysteresis loops before
and after 5 million cycles of unipolar cycling at 10 Hz with Emax = 4 Ec ..................... 116
Figure 7-3 the changes of (a) polarization, (b) maximum strain at the negative side and
strain asymmetry, (c) d33 and (d) bias field on the function of number of cycles during
unipolar fatigue cycling at 10Hz and Emax = 4 Ec ......................................................... 116
List of Figures
xix
Figure 7-4 the recovery of (a) polarization, (b) strain, (c) d33 and (d) permittivity
hysteresis loops by 10000 bipolar cycles on the 5 million cycles unipolar fatigued
sample ........................................................................................................................... 117
Figure 7-5 The changes of (a) unipolar polarization and (b) strain, bipolar (c)
polarization, (d) strain, (e) d33 and (f) permittivity hysteresis loops of unipolar fatigued
sample after annealing at 400 °C .................................................................................. 118
Figure 7-6 Changes of normalized remanent polarization and maximum strain in bipolar
and unipolar measurements, respectively, for different materials. ............................... 121
Figure 8-1 The changes of (a) polarization, (b) strain, (c) current, (d) dielectric loss, (e)
d33 and (f) permittivity hysteresis loops of one selected BC8TZ5.5 sample during bipolar
fatigue cycling at 10 Hz with Emax = 3Ec ...................................................................... 129
Figure 8-2 The changes of (a) polarization, (b) strain, (c) current, (d) dielectric loss, (e)
d33 and (f) permittivity hysteresis loops of one selected BC15TZ10 sample during bipolar
fatigue cycling at 10 Hz with Emax = 3Ec ...................................................................... 130
Figure 8-3 The changes of mean values with standard deviations of (a) remanent
polarization, (b) coercive field, (c) d33 and (d) strain asymmetry factor of both
compositions during on the function of number of bipolar fatigue cycles ................... 130
Figure 8-4 (a) Photo of an unfatigued sample and a bipolar fatigued sample; (b) the
damaged material on the bipolar fatigued BC8TZ5.5 surface where discolouration
appears; (b) the crack observed on the subsurface leading to a minimization of the
domain size ................................................................................................................... 131
Figure 8-5 The changes of (a) polarization, (b) strain, (c) current, (d) dielectric loss, (e)
d33 and (f) permittivity hysteresis loops of one bipolar fatigued BC8TZ5.5 sample after
annealing at 400 °C for 10 mins ................................................................................... 132
List of Figures
xx
Figure 8-6 The changes of (a) polarization, (b) strain, (c) current, (d) dielectric loss, (e)
d33 and (f) permittivity hysteresis loops of one bipolar fatigued BC15TZ10 sample after
annealing at 400 °C for 10 mins. .................................................................................. 133
Figure 8-7 The loss of normalized remanent polarization on the function of number of
bipolar cycles for different piezoelectric materials ....................................................... 135
Figure 8-8 the surface image of (a) BC8TZ5.5 and (b) BC15TZ10 under micro-scope
before fatigue ................................................................................................................ 140
List of Tables
xxi
LIST OF TABLES
Table 2-1. Crystallographic symmetries of piezoelectrics and pyroelectrics (Yang, 2002)
........................................................................................................................................... 5
Table 2-2. The synthesis parameters from literature....................................................... 33
Table 5-1. Microstructural properties of each composition ............................................ 82
Table 6-1 Characters of BC15TZ10 .................................................................................. 98
Table 7-1 Testing parameters of unipolar fatigue cycling in different studies ............. 120
Table 8-1. The testing parameters used in the bipolar fatigue studies .......................... 135
Table 9-1 The effects of defect dipoles and oxygen vacancies in different scenarios for
BCTZ ............................................................................................................................ 151
Page
Introduction
1
Chapter 1 Introduction
1.1 Background
Piezoelectric materials can transfer mechanical energy into electrical energy, and vice
versa. Due to these unique properties, these materials can be used in many applications
such as sensors, ultrasonic transducers, piezoelectric motors and sparkers in lighters.
The advantages of these materials in general are small size and light weight, quick
response and high precision. Their use has led to the replacement of many old designs,
the creation of innovative components and even new devices and applications. Although
there are many piezoelectric materials in nature, such as Rochelle salt, cane sugar,
quartz, etc., artificial piezoelectric ceramics are more applicable as they can be modified
to suit specific requirements. Most of these piezoelectric ceramics have a perovskite
structure and exhibit ferroelectricity. Among all the piezoelectric ceramics, Pb(Ti,Zr)O3
(PZT) based-materials are most popular due their high piezoelectric response, high
Curie temperature and easy to be modified piezoelectric response through the addition
of different doping materials (Jaffe et al., 1971). The commercialization of this class of
materials has been developed over many years and it has been the subject of a
significant body of scientific research.
Despite the outstanding performance of PZT materials, there is a rising concern of
toxicity of lead in these compositions. In last ten years, there has been a dramatic
increase in research on lead-free piezoelectric compositions. Many potential materials
have been investigated, such as BaTiO3 (BT), (Bi,Na)TiO3 (BNT), (K,Na)NbO3 (KNN),
(K,Bi)TiO3 (KBT) as well as modifications of these base structures. Although PZT-
based materials may be difficult to replace by a single alternative material, some lead-
free piezoelectric materials have demonstrated great potential for certain applications
Introduction
2
(Rödel et al., 2015). For example, KNN based materials may be able to replace PZT in
high temperature applications while Ca and Zr doped BT materials (BCTZ) may be
suitable for low temperature ranges.
The high piezoelectric response of BCTZ is comparable to soft PZT at room
temperature as firstly reported in 2009 (Liu and Ren, 2009). The small signal
piezoelectric coefficient (d33) of this material is approximately 550 pm/V and the large
signal piezoelectric coefficient (d33*) can reach as high as 1200 pm/V. The high d33 is a
promising characteristic for sensor applications as the high d33* is for actuator
applications. Although the properties of BCZT are strongly temperature dependent,
there is still a chance it may be used in ultrasonic cleaner, ultrasonic machining tool,
nano-motor and other applications around room temperature (Rödel et al., 2015).
However, the high piezoelectric performance is not the only factor that needs to be
considered in terms of application. Reliability is another important factor (Rödel et al.,
2015). So far, the piezoelectric long term performance of BCTZ in applications has not
been reported in the literature and is still unknown.
1.2 Preview
This thesis will present an investigation of the BCTZ system including the piezoelectric
performance and fatigue behaviour.
Chapter 2 is a comprehensive literature review introducing the basic knowledge of
piezoelectric materials, progress on lead-free compositions including BCTZ and
previous studies of the fatigue behaviour of piezoelectric materials.
Chapter 3 introduces the aim of this thesis, posits a number of hypotheses and outlines
the approaches taken to address them.
Introduction
3
Chapter 4 introduces detailed procedures of material fabrication and testing methods.
Chapter 5 shows the characterization of the fabricated materials, including the
correlation between phase coexistence and doping concentration.
Chapter 6 shows the ageing behaviour of BCTZ.
Chapter 7 presents the fatigue behaviour of BCTZ subjected to continuous unipolar
loading.
Chapter 8 presents the fatigue behaviour of BCTZ subjected to continuous bipolar
loading.
Chapter 9 summarizes the findings of the research in regard to piezoelectric, ageing and
fatigue behavior and discusses these in relation to the hypotheses posited in Chapter 3.
Chapter 10 presents the conclusions of the study.
Literature Review
4
Chapter 2 Literature Review
2.1 Basic Principle of Piezoelectric Materials
2.1.1 Dielectric materials
Dielectric materials (also called electrically insulating materials) exhibit an electric
dipole displacement, a separation of positively and negatively charged entities on a
molecular or atomic level, upon application of electric field. The magnitude of the
polarity is called dipole moment which is expressed by vector , where q is the
charge and is the distance between positive charge and negative charge. The
polarization as a sum over all the dipole moment vectors of a volume ( ) is expressed
by ∑ / .
Dielectric materials are utilized as insulators and/or in capacitors. Relative permittivity
r, also called dielectric coefficient, is the property exploited in such applications.. It is
calculated by
/
where is the permittivity (the property to store charges on an slab with electrodes) of
material brought to a given voltage and 0 is called the permittivity of vacuum which is
a constant value of 8.85 × 10-12 F/m.
The value of r is 2~5 for most solid polymers and less than 20 for most inorganic
solids. However, ferroelectric ceramics, an advanced ceramic type, generally have a
much higher dielectric coefficient, typically several hundred to several thousand
(Callister, 2007, Jaffe et al., 1971).
Literature Review
5
2.1.2 Piezoelectricity
Piezoelectricity (from the Greek “piezin”, meaning to press) is a ability to generate
electric charges upon application of mechanical pressure, and vice versa (Shields, 1966,
Jaffe et al., 1971). Only an originally non-isotropic material exhibits piezoelectricity
due to the existence of dipoles. This applies to the 21 non-centrosymmetric crystal
classes out of 32 crystal classes, except cubic class 432 (details in Table 2-1).
Table 2-1. Crystallographic symmetries of piezoelectrics and pyroelectrics (Yang, 2002)
For solid materials, a stress merely leads to a proportional strain γ, related to their
elastic compliance M1: . For piezoelectric materials, there is an additional
creation of electric charges upon applied stress, which is called direct piezoelectric
effect. Because of the non-centrosymmetric structure being able to develop a dipole in
the crystal, the pressure applied on the material leads to a change of the dipole moment
so that opposite electrical charges appear at the opposite side of the crystal. If the circuit
is completed through a high impedance, these charges flow, resulting in a measurable
1 Elastic compliance M is the reciprocal from of elastic modulus E:
Literature Review
6
electric current (Shields, 1966). The dielectric displacement D is proportional to the
stress , which can be written as (Jaffe et al., 1971, Randall et al., 2005):
(d expressed in C/N)
where the dielectric displacement D is a vector component, σ is the stress tensor, and d
coefficient is the third rank tensor called piezoelectric coefficient.
Similarly, an electric field will not only cause electrostriction1 but also a geometric
strain (deformation) proportional to an applied voltage, which is called converse
piezoelectric effect. When an electric field is applied to a piezoelectric material, due to
the non-centric crystalline structure, the dipole is enlarged as the function of electric
field. The dipole displacement in the crystal results in a physical strain on the macro
scale. The strain γ is proportional to applied electric field E, namely
γ dE (d expressed in m/V)
where γ is a strain tensor, vector E is the applied electric field, and piezoelectric
coefficient d is the third rank tensor.
It has been proved that:
It denotes the ratio of short-circuit electric charge developed per unit area of electrode
to the mechanical stress applied to the piezoelectric element. High d coefficient is
1 In electrostriction, deformation of materials is proportional to applied electric field but independent with the orientation of the applied field. This phenomenon is present very weakly in all materials.
Literature Review
7
desirable for materials intended to develop motion or vibration, such as sonar or
ultrasonic cleaner transducers.
For poled polycrystalline ceramics, applying an electric field from different directions
with respect to the poling direction results in different d coefficients. Thus, the d
coefficient needs to be expressed as diλ, of which the value of i and λ are determined by
the crystalline symmetry. Taking the simplest tetragonal 4mm symmetry as an example,
the applied field can have three directions with respect to the poling direction. The
poling direction is by convention the 3-direction (i = 3) and 1 and 2 refer to arbitrarily
chosen orthogonal axes in the plane normal to 3. Six numbers are used to express the
changes in ferroelectric ceramics upon the application of an electric field. λ = 1, 2 and
3 indicates the strain along the three orthogonal directions where 3 denotes the poling
direction. λ = 4, 5, 6 indicate the shear strains on the planes perpendicular to the axis 1,
2 and 3, respectively. Therefore, the matrix of d is written as:
.
It was reported that most these d is 0 (Jaffe et al., 1971). Thus, the matrix becomes
0 00 0
0 00
0
00 00 0
.
Due to symmetry, d32 = d31 and d24 = d15. Hence, the only three independent coefficients
are d33, d31, and d15, among which d33 is the most commonly quoted coefficient for
ferroelectric materials as it represents the changes of strain along the direction of
applied field.
2.1.3 Pyr
Among 20
polarizatio
moment is
surface ch
and was
(2006). Its
surfaces o
2.1.4 Fer
Some pyr
the absenc
orientation
ferroelectr
Glass, 200
Ferroelect
point, dipo
where εr i
Curie tem
Pe2.1.4.1
Perovskite
is signifie
point (see
roelectricit
0 non-centr
on direction
s proportion
harges upon
first publis
s existence
on change of
rroelectric
roelectric cr
ce of an ele
ns upon a
ricity, and t
01).
tricity is tri
oles are abs
is the relati
mperature.
erovskite str
e structure i
ed by chem
e Figure 2-1
ty
rosymmetric
n, a so cal
nal to temp
n change of
hed by Joh
can be det
f temperatu
ity
rystals have
ectric field
application
the relevant
iggered by
sent and the
ive dielectri
ructure
is one the m
mical compo
1a). It is a
c crystallin
lled polariz
perature, the
temperatur
han George
tected by ob
ure. (Yang, 2
e two or m
and the ele
of an el
t materials
the Curie
dielectric c
1
ic coefficie
most commo
osition ABO
cubic struc
ne classes, 1
zed point-gr
ese 10 cryst
re. This phe
e Schmidt i
bserving the
2002).
ore spontan
ectric dipole
ectric field
are called f
point. For
coefficient o
ent, C is the
on structures
O3 with spac
cture with l
10 of them
roup (Table
talline class
nomenon is
in 1707 as
e flow of c
neous polar
es can be sw
d. This ph
ferroelectric
temperatur
obeys Curie
e Curie con
s for ferroel
ce group
large cation
Literatu
can develo
e 2-1). As
ses are able
s called pyr
mentioned
charge to an
rization orie
witched bet
henomenon
c materials
res above t
e-Weiss Law
nstant, and
lectric mate
3 above
ns (A) on t
ure Review
8
op a unique
the dipole
to develop
roelectricity
d by Katzir
nd from the
entations in
tween these
n is called
(Lines and
this critical
w, which is
is called
erials which
e the Curie
the corners,
w
8
e
e
p
y
r
e
n
e
d
d
l
d
h
e
,
smaller ca
The oxyge
holes. Usu
cation filli
Figure 2-1the temper
At a tem
symmetry
leads to a
depends o
phase, tw
rhombohe
2.1.5 Dom
When fer
polarizatio
where a
polarizatio
a global p
ations (B) in
en atoms fo
ually, if a st
ing in the do
1. (a) A cubirature unde
mperature be
y. The small
a dipole (Fi
on the what
elve <110>
edral phase (
main polin
rroelectric m
on is gener
region of
on is called
polarization
n the body c
orm an octah
tructure has
odecahedro
ic ABO3 perer Tc
elow the C
l cation mov
igure 2-1(b)
phase the
> directions
(Yang, 2002
ng and swi
materials a
rated and t
neighbour
a ‘domain’
n due to th
centre, and o
hedron and
a small cat
on hole is al
rovskite str
Curie point
ves away fr
)). The dire
material is,
s for orthog
2).
itching
are cooled
the orientat
ring cells
’ (Yang, 20
he countera
oxygen atom
the smaller
tion filling i
so reckoned
ucture; (b)
t the perov
rom the cen
ections that
, such as six
gonal phase
down from
tion of the
with ident
002). The m
action of ne
ms (O) in th
r cation is fi
in the octah
d as a perov
Tetragonal
vskite struc
ntre of the o
t the small
x <100> di
e and eight
m the Curi
dipoles is
ical orienta
material norm
eighbouring
Literatu
he centres o
filled in the
hedron hole
vskite struct
phase with
cture loses
oxygen octa
cation tend
irections for
t <111> dir
ie point, s
randomly
ation of s
mally does
g domains
ure Review
9
f each face.
octahedron
and a large
ure.
h a dipole in
its centro-
ahedron and
ds to move
r tetragonal
rections for
pontaneous
distributed
pontaneous
not present
in order to
w
9
.
n
e
n
-
d
e
l
r
s
d
s
t
o
maintain l
are called
on the cry
However,
domains c
cell is aff
2-2(b) sho
spontaneo
achievable
2-2(b) can
cell is com
directions
Figure 2-2
If an exte
domains t
parallel to
of the gra
lowest free
‘domain w
ystal structur
upon appl
can be enlar
fected by th
ow the chan
ous polariza
e under me
n also be ac
mpressed a
to move bu
2. Changes o
ernal electri
tend to alig
o the extern
ains. Figure
energy. Th
wall’. There
res.
lication of
rged or rota
he external
nges of lattic
ation, respe
echanical p
chieved by
along (001)
ut four direc
of lattice ands
ic field is
gn with th
al field due
2-3 demon
he boundarie
are differen
an externa
ated. In a vi
electric fie
ce and dipol
ectively. S
pressure. Fo
pulling the
direction,
ctions either
d dipole upospontaneous
applied to
e direction
e to the limi
nstrates how
es between
nt kinds of
al electric f
iew of cell
eld. For ex
le upon elec
imilarly, th
or instance,
e cell along
the small
r along (100
on electric fis polarizatio
a ferroelec
n of the fie
itations imp
w domain w
domains of
domain wa
field or me
scale, the d
ample, Figu
ctric field al
he switchin
, the dipole
(010) direc
cation wou
0) or (010).
ield (a) alongon
ctric polycry
eld though
posed by the
walls move u
Literatu
f different o
alls allowed
echanical st
dipole mom
gure 2-2(a)
long and or
ng of dipo
e switching
ction. How
uld not only
g and (b) or
rystalline m
some cann
e crystallog
upon applic
ure Review
10
orientations
d depending
tress, these
ment of each
and Figure
rthogonal to
les is also
g in Figure
wever, if the
y have two
rthogonal to
material, the
not reorient
graphic axis
cation of an
w
0
s
g
e
h
e
o
o
e
e
o
e
t
s
n
electric fi
‘poling’. I
the maxim
electric fie
a few dom
When the
direction p
and dipol
aligned, th
dominated
the stretch
P εE
material.
contribute
switching
signified b
ield. This p
It is shown
mum point
eld. The pol
mains (main
applied fie
parallel to
le reorienta
he growth o
d by the intr
hing of the
P where
The polari
ed by the re
of unstable
by .
Figure
process, to
in Figure 2
a (commo
ling can be
ly 180 degr
ld keeps gro
the applied
ation are ca
of the polar
rinsic effect
e unit cells
Ps is the
ization dec
estoration o
e dipoles. T
2-3. Domain
turn all the
2-4 that the
only terme
roughly div
ree domains
owing, mos
d field. The
alled extrin
rization bec
t which is r
s. Thus, the
spontaneou
creases upo
of the stret
The residual
n switching
e domains
polarization
d as
vided into th
s) start to ro
st of the dom
ese processe
nsic effect.
omes slow
related to th
e maximum
us polarizati
on removal
ched dipole
l polarizatio
under an ex
close to on
n increases
) wit
hree stages.
otate with in
mains begin
es involving
After mos
but still inc
he growing
m polarizati
ion; ɛ is th
l of the e
e moment a
on is called
xternal elect
Literatu
ne direction
from the ze
th increasin
In the first
ncreasing ap
n to align to
g domain w
st domains
crease linea
dipole mom
ion can be
he permitti
external fie
adding to s
d remnant p
tric field
ure Review
11
n, is called
ero point to
ng external
stage, only
pplied field.
o the closest
wall motion
have been
arly. This is
ment due to
express as
vity of the
eld, mainly
some back-
polarization,
w
d
o
l
y
.
t
n
n
s
o
s
e
y
-
,
The overa
opposite p
coercive f
domains r
such rever
term ‘ferr
similarity
However,
polarizatio
applied el
different d
all polarizat
polarity. Th
field, signif
reorientate t
rsal of the a
roelectric’
with the h
contrary to
on direction
lectric field
directions m
Figure 2-4
tion become
he electric
fied by .
to align wit
applied field
is used in
hysteresis lo
o ferromagn
ns, thus they
. In other w
may result in
4. A polariza
es zero aga
field wher
If the rev
th the rever
d a polariza
n analogy w
oop of mag
netic materi
y only deve
words, elect
n different v
ation hystere
ain upon app
re the pola
versed elect
rsed field. A
tion hystere
with the te
gnetic dipol
ials, ferroele
elop a close
tric field ap
values of po
esis loop of a
plication of
arization be
tric field ke
As shown i
esis loop is
erm ‘ferrom
es in a rev
ectric mater
est direction
pplied to fer
larization.
a ferroelectr
Literatu
f an electric
ecomes zer
eeps increa
in Figure 2
developed.
magnetic’ d
versing mag
rials only h
n of polariza
rroelectric m
ric material
ure Review
12
c field with
o is called
asing, more
-4, through
In fact, the
due to this
gnetic field.
have limited
ation to the
materials in
l
w
2
h
d
e
h
e
s
d
e
n
Besides th
(Figure 2-
the polariz
the switch
close to th
the strain
dominated
restore to
dipoles ar
dipoles sta
is applied
field incre
with the fi
Figure 2-5
he polarizat
-5), illustrat
zation hyste
hing of 180
he coercive
rapidly. Wh
d the stretch
their origin
re not able t
art to switch
and the stra
eases furthe
ield directio
5. A two dim
tion hystere
ting the defo
eresis loop,
degree dom
field, 90 de
hen the ext
hing of dipo
nal state, re
to switch b
h back (perp
ain reaches
er, the mate
on again.
mensional sch
esis loop, th
formation (s
there are a
mains does n
egree domai
trinsic proce
oles. With t
esulting in
back thus th
pendicular t
a minimum
erial develop
hematic ske
here is also
strain) chan
also three st
not lead to a
in switching
esses are co
the remova
the loss of
his materials
to the field
m around the
ps a strain
etch of an ide
a butterfly
ges upon ap
tages of pol
a strain. Wh
g starts to ta
ompleted, th
l of the app
f strain. Ho
s exhibits a
direction) w
e coercive fi
as the dipo
eal butterfly
Literatu
y strain hyst
pplied field
ling. At the
hen the app
ake place an
he increase
plied field,
owever, mo
a remnant st
when the rev
field. When
oles are driv
y strain hyst
ure Review
13
teresis loop
d. Similar to
beginning,
lied field is
nd increases
of strain is
the dipoles
st switched
train. Some
versed field
the electric
ven to align
teresis loop.
w
p
o
,
s
s
s
s
d
e
d
c
n
2.2 Prop
2.2.1 Pb
The ferro
discovered
started to
areas due
for specifi
Ph2.2.1.1
PZT exhib
position. F
PbZrO3 (m
The subst
ultimately
symmetry
(MPB) ex
the MPB
rhombohe
from one
developed
MPB betw
2002, Kr
nanodoma
(Schönau
acceptable
perties of
(Zr, Ti) O
oelectric na
d by Japan
become one
to its large
ic applicatio
hase diagram
bits a perov
Figure 2-6
moderate pi
titution of Z
y causes the
y. The mater
xhibit the hig
occurs for
edral and tet
phase to an
d a debate. S
ween the rh
eisel et al
ains lies at
et al., 200
e, there is s
Piezoelec
O3
ature of l
nese researc
e of the mo
e piezoelectr
ons.
m and prop
vskite struct
shows the
iezoelectrici
Zr4+ for Ti4
e appearanc
rials at the
ghest coupl
r a ratio b
tragonal pha
nother. How
Some resea
hombohedra
., 2009, A
the MPB,
07, Schmitt
still a need
ctric Mate
ead zircon
chers in ear
ost common
ric response
perties of PZ
ture with Pb
phase diag
ity) and Pb
4+ in PbTiO
ce of anothe
tetragonal-r
ling factor a
between Ti
ases equally
wever, the
archers supp
al and tetrag
Ahart et al
which lea
t et al., 20
of further i
erials
nate titanat
rly 1950s (
ly used ferr
e, high Cur
ZT
b at the A p
gram betwe
bTiO3 (non-p
O3 reduces t
er ferroelec
rhombohed
and dielectr
and Zr of
y coexist, w
further stud
ported that t
gonal phase
l., 2008); o
ds to the e
008). Curren
investigatio
e
Jaffe et al.
roelectric m
rie point and
position, Zr
een the two
piezoelectri
tetragonal d
ctric phase o
ral morphot
ic constant.
f 0.48: 0.5
which leads t
dies on the
there is a m
es (Noheda
others argu
ease of pola
ntly, both o
n. It is easi
Literatu
, (
, 1971). Si
materials in a
d ease of m
r and Ti sh
o componen
ic) (Jaffe et
distortion (P
of rhomboh
otropic phas
. At room te
52. At this
to an ease o
principle o
monoclinic p
a et al., 199
ued that a
arization re
of these co
ier for dipo
ure Review
14
(PZT) was
ince then it
all different
modification
aring the B
nts of PZT:
t al., 1971).
P4mm) and
hedral R3m
e boundary
emperature,
MPB, the
of transition
of the MPB
phase at the
99, Noheda,
region of
eorientation
oncepts are
les to align
w
4
s
t
t
n
B
.
d
m
y
,
e
n
B
e
,
f
n
e
n
and switch
PZT at th
temperatu
temperatu
from MPB
many appl
Fi
‘H2.2.1.2
PZT can
particular
behaviour
and ‘soft’
h with the e
he MPB. An
ure (morpho
ure increase
B at high tem
lications an
igure 2-6. P
Hard’ and ‘s
be further
properties
r, doped PZ
PZT.
electric field
nother adva
otropic) whi
occurs duri
mperature. D
nd investigat
bTiO3-PbZr
soft’ PZT
r modified
according
ZT composi
d, leading t
antage of PZ
ich means th
ing applicat
Due to the u
tions are do
rO3 sub-soli
through d
to specific
itions are ge
o the impro
ZT is that i
hat the phas
tion. Thus,
unique prop
one using th
idus phase d
doping with
requiremen
enerally div
oved piezoe
its MPB is
se is not aff
the properti
perties of th
ese compos
diagram (Jaf
h different
nts. Based
vided into t
Literatu
electric perf
nearly inde
fected signif
ies of PZT
he material a
sitions.
ffe et al., 19
elements t
on their pi
two types:
ure Review
15
formance of
ependent of
ficantly if a
still benefit
at this MPB
71)
to improve
iezoelectric
‘hard’ PZT
w
5
f
f
a
t
B,
e
c
T
Shrout and
dopants (s
dopants in
with accep
cells, whil
free oxyge
of the dom
that, elect
and reduc
be trapped
seen in Fi
stress so th
Figure 2
‘Soft’ PZT
1971, Shr
results in
reduction
d Zhang (20
substitutive
nduce oxyge
ptor dopant
le the other
en vacancie
main config
tric losses a
ed piezoele
d at the grai
gure 2-7a. ‘
hat it is suit
2-7. Polariza
T is typica
rout and Zh
a rise in pe
in coercive
007) summ
cations wit
en vacancie
ts creating
unbounded
es can be tra
guration and
are reduced
ctric activit
in boundari
‘Hard’ PZT
table for hig
ation hyster
lly associat
hang, 2007)
ermittivity,
e field (Figu
marise that ‘h
th lower va
es in the ma
defect dipo
d oxygen va
apped at th
d thus redu
d but at the
ty (Figure 2
ies thus crea
T has a low
gh power an
resis of ‘hard
ted with A
). It has the
elastic com
ure 2-7b). T
hard’ PZT i
alence than
aterials. Som
oles which
acancies can
e domain b
uce the dom
expense of
2-7a). The ch
ating a loca
piezoelectri
nd high volt
d’ and ‘soft’
-site vacanc
e opposite
mpliances a
This is prob
is commonl
the original
me oxygen
can influen
n diffuse wit
boundaries l
main wall m
f an increas
harged oxyg
al internal b
ic coefficien
tage applica
’ PZT (Shro
cy or dono
effect of ac
nd piezoele
ably due to
Literatu
ly doped wi
al one). The
vacancies a
nce the neig
ithin the ma
leading to s
mobility. As
sed coerciv
gen vacanci
bias field wh
nt but can s
ations.
out and Zha
or doping (J
cceptor dop
ectric coeffi
o the stress
ure Review
16
ith acceptor
se acceptor
are coupled
ghbour unit
aterials. The
stabilization
a result of
e field (Ec)
ies can also
hich can be
sustain high
ng, 2007)
Jaffe et al.,
ping, which
ficient but a
released by
w
6
r
r
d
t
e
n
f
)
o
e
h
,
h
a
y
Literature Review
17
the donor dopants leading to a high mobility of non-180 degree domain walls. ‘Soft’
PZT, offering high resistivity and piezoelectric coupling, is suitable for medical
ultrasound transducers, pressure sensors, and actuators.
2.2.2 Background of Lead-free piezoelectric materials
Despite the outstanding performance of the PZT piezoelectric family, lead is of extreme
toxicity and there is a growing concern with use of lead due to the potential hazards to
the environment and human beings in production and disposal of lead-contained devices.
On 1st July 2006, Waste Electrical and Electronic Equipment (WEEE) and Restriction
of the use of certain Hazardous Substances in electrical and electronic equipment
(RoHS) have been proposed to restrict the use of some toxic materials in a specified list
of electrical and electronic equipment. The maximum concentration of lead is set to 0.1
wt% (Rödel et al., 2009). Other countries all over the world have also started to follow
the European Union to establish similar regulations to limit the use of lead-based
materials. However, PZT, though containing a high concentration of lead (63 wt%), has
got an exemption from the limitation due to technically or scientifically lack of lead-
free piezoelectric alternatives with comparable piezoelectric properties and costs. There
is an urgent demand of lead-free piezoelectric materials to prevent the potential hazard
from the high lead contained materials. A review of the exemption for lead-based
piezoelectrics given by the legislative authorities will be held every four years. The
lead-based materials will be prohibited as soon as there is a practicable substitution
available.
In the history, a great number of natural lead-free piezoelectric materials have been
discovered, such as barium-titanate, quartz, tourmaline, topaz, cane sugar, and Rochelle
salt (sodium potassium tartrate tetrahydrate). However, only quartz and Rochelle salt
exhibit high piezoelectric properties and they all have drawbacks that prevent them
Literature Review
18
from wide application (Shields, 1966). PZT dominates the application range for
piezoelectric materials since its discovery and the research of lead-free piezoelectrics
seized for approximately half a century. In recent decades, with the increasing concern
of the lead toxicity, lead-free piezoelectric materials as alternative to PZT became one
of the most popular topics in this field. Currently, the most promising lead-free
piezoelectric materials are in the bismuth-based and the sodium-potassium-niobate-
based families.
2.2.3 Bismuth-based materials
Bismuth-based perovskite materials such as Bi0.5Na0.5TiO3 (BNT) and K0.5Bi0.5TiO3
(KBT) receive attention due to the similarity in atom size and lone-pair electrons
between Bi and Pb.
Pure BNT exhibits a rhombohedral phase (R3c) at room temperature with strong
ferroelectricity, reflected in the remnant polarization Pr= 38µC/cm2 (Panda, 2009).
Although BNT has a relatively high Curie point at 320 °C (Rödel et al., 2009), it starts
to depolarize at a low temperature of 187 °C (Figure 2-9) (Hiruma et al., 2009). The d33
of pure BNT lies only in the range of 57-64 pC/N and additional drawbacks of these
materials are its high conductivity and high coercive field which lead to problems in the
poling process. However, these problems can be solved by adding BaTiO3 into the BNT
system, leading to the solid solution BNT-xBT. The BNT-BT system has an MPB
between a rhombohedral and a tetragonal phase at x = 6 % (as shown in its phase
diagram Figure 2-8). At the MPB, comparing with pure BNT, the d33 is increased to
117-125 pC/N, and d15 up to 194 pC/N (Rödel et al., 2009, Panda, 2009, Damjanovic et
al., 2010).
Literature Review
19
KBT is similar to BNT, except that it has a tetragonal phase at room temperature. KBT
exhibits a slightly higher d33 (82 pC/N reported by Rödel et al. (2009)) with its Curie
point at 370 °C. Compared to BNT, it has a higher depolarization temperature at 270 °C
(Aksel and Jones, 2010). However, it also has a higher coercive field as BNT which
makes it more difficult to pole.
There are some further investigations on KBT-BT, BNT-KBT, and even BNT-KBT-BT,
but none of them exhibits promising properties (Aksel and Jones, 2010, Rödel et al.,
2009, Panda, 2009).
Figure 2-8. Phase diagram of BNT–BT. Fα , ferroelectric rhombohedral phase; Fβ, ferroelectric tetragonal phase; AF, antiferroelectric phase; P, paraelectric phase(Takenaka et al., 1991).
Literature Review
20
Figure 2-9. Temperature dependences of dielectric constant ɛs and loss tangent tan ϭ in the temperature range from RT to 500 °C for poled BNT. (Hiruma et al., 2009)
2.2.4 Sodium-potassium-niobate-based materials
Besides the BNT and KBT families, the sodium potassium niobate (K0.5Na0.5NbO3 or
KNN) family is also considered as one of the most promising compositions for the lead-
free piezoelectric materials. Attractive properties of KNN such as high Curie point
(above 400°C) and good ferroelectric performance (Pr = 33 µC/cm2 and d33 = 160 pC/N)
make it popular during last decade. Unique from BNT and KBT, KNN is a specific
composition (50/50) on a complete solid solution of NaNbO3 and KNbO3, which lies on
a MPB between two orthorhombic phases (ferroelectric for KNbO3 and antiferroelectric
for NaNbO3) at room temperature. The phase diagram of KNbO3-NaNbO3 is shown in
Figure 2-10. Although there are numerous MPBs in the phase diagram, only the
composition at 50/50 exhibits attractive piezoelectric response (Jaffe et al., 1971).
According to its high loss and moderate Ec (approximately 5~9 kV/cm) (Shrout and
Zhang, 2007), it is classified as a ‘soft’ piezoelectric. The piezoelectric coefficient is
further increased by doping with suitable elements (Rödel et al., 2009). However,
Literature Review
21
undoped KNN also has two main drawbacks related to its processing: one is that high
densification cannot be achieved by using conventional proceedures; the other is that
the volatility of potassium oxide makes it difficult to maintain stoichiometry (Panda,
2009).
Figure 2-10. Phase diagram for the KNbO3-NaNbO3 system. Regions labeled with Q, K, and L are monoclinic, although angular distortions are such that they are commonly regarded as orthorhombic ferroelectric; M and G are orthorhombic ferroelectric; F, H, and J are tetragonal ferroelectric. Region P is orthorhombic antiferroelectric. (Rödel et al., 2009)
Many modifications of KNN-based materials have been investigated including BNT-
KNN, KNN-BT, and BNT-BT-KNN (Rödel et al., 2009, Panda, 2009, Aksel and Jones,
2010, Shrout and Zhang, 2007). One of the most successful approaches is reported by
Saito et al. (2004) in Japan. Inspired by the MPB of PZT, they doped KNN with Li, Ta,
and Sb to obtain a MPB between tetragonal and orthorhombic phases near room
temperature. One of the compositions at the MPB, (K0.44Na0.52Li0.04)(Nb0.84Ta0.10-
Sb0.06)O3 (they named it LF4), exhibits a piezoelectric coefficient as high as 416 pC/N
near room temperature. The Curie point is also as high as 253°C. These properties grant
these mate
and Sb is
2.2.5 Ba1
Ba2.2.5.1
Barium t
piezoelect
coefficien
115°C~13
This mate
piezoelect
Th2.2.5.2
Similar to
(rhomboh
having a M
shown in
the entire
be expres
rhombohe
29.5 °C on
et al., 201
erials a high
also toxic (R
1-xCaxTi1-yZ
arium titana
itanate (Ba
trics found a
nt of approx
30°C) (Jaffe
erial is mor
tric properti
he BZT-xBC
o the disc
edral) and
MPB near r
Figure 2-11
compositio
sed as Ba0.
edral and te
n heating as
1, Liu and R
h potential
Rödel et al.
ZryO3
ate
aTiO3, sho
and studied
imately 190
e et al., 197
re famous f
ies.
CT system
covering of
(Ba0.7Ca0.3
room tempe
1. Although
onal range, i
.85Ca0.15Zr0.
tragonal ph
s shown in F
Ren, 2009).
to substitut
, 2009).
ortened as
d (Jaffe et al
0 pC/N at ro
71, Shrout
for its high
f LF4, Liu
3)TiO3 (tetr
erature. The
h it has a cu
it shows on
1Ti0.9O3 in
hase around
Figure 2-12
.
te PZT. Ho
BT) was
l., 1971). It
oom temper
and Zhang
h dielectric
u and Ren
ragonal) se
e phase diag
urved MPB
e interesting
this article
d room temp
2), and it ha
wever, Ta i
one of th
has a relativ
rature but a
g, 2007, Ak
coefficient
n (2009) m
eking if th
gram of the
and a low C
g compositi
), which is
perature (20
s a low Cur
Literatu
is relatively
he earliest
vely high pi
a low Curie
ksel and Jo
t (1700-210
mixed Ba(Z
here is a c
BZT-xBCT
Curie point
ion, BZT-5
s at the MP
0.5 °C on c
rie point at
ure Review
22
y expensive
perovskite
iezoelectric
point (only
nes, 2010).
00) than its
Zr0.2Ti0.8)O3
composition
T system is
throughout
0BCT (will
PB between
cooling and
93 °C (Xue
w
2
e
e
c
y
.
s
3
n
s
t
l
n
d
e
Literature Review
23
Figure 2-11. Phase diagram of BaZr0.2Ti0.8O3-xBa0.7Ca0.3TiO3 system (Liu and Ren, 2009).
At this MPB, its d33 is stable in the range of 546~600 pC/N around room temperature
(the large signal dS/dE reaches 1150 pC/N). The remnant polarization at the MPB is
14.8 µC/cm2, and the coercive field is 170 V/mm, indicating that Ba0.85Ca0.15Zr0.1Ti0.9O3
is extremely ‘soft’. It is interesting to note that the elastic stiffness constants and elastic
compliance constants are all different from pure BaTiO3 but similar to soft PZT
(PZT5A) (Xue et al., 2011). For this Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 system, the
enhancement of the piezoelectric effect at the MPB is significant. However, the MPB of
this BZT-xBCT system strongly depends on temperature. No matter if composition
(horizontal in the phase diagram) or temperature (vertical in the phase diagram) shifts
away from the MPB, the piezoelectric coefficient decreases dramatically (Figure 2-13).
Additionally, the Curie point of this material, approximately 93 °C, is lower than for
most piezoelectric materials. All these characteristics indicate that this material is better
operating under isothermal conditions. Additionally, Ehmke et al. (2012a) found that
the BCTZ can be easily de-poled by mechanical pressure. This characteristic may
hinder this material from applications that are associated with clamping pressure. It is
also found that the hysteresis asymmetry of strain and permittivity loops can easily
Literature Review
24
develop during poling process (Li et al., 2013, Ehmke et al., 2013a). It may indicate that
unipolar cycling may induce a significant change to this composition.
Figure 2-12. The dielectric constant of Ba0.85Ca0.15Zr0.1Ti0.9O3 as a function of temperature (Xue et al., 2011)
Figure 2-13. a,b: Piezoelectric coefficient and large signal dS/dE as a function of composition respectively; c,d: piezoelectric coefficient and k33 as a function of temperature respectively (modified from (Liu and Ren, 2009, Xue et al., 2011))
Initially, it was thought that the high piezoelectric performance is attributed to the fact
that the composition is close to a C-R-T triple point (Liu and Ren, 2009) (also called
convergence region in some literature (Acosta et al., 2014)). As observed for the PZT
Literature Review
25
system, compositions at the C-R-T triple point usually have very high piezoelectric
coefficient and dielectric properties. Damjanovic et al. (2010) summarized that the main
difference between PZT and the two most popular lead-free piezoelectrics (BNT-BT
and KNN) is that the MPB of PZT ends at a tricritical point where rhombohedral and
tetragonal and cubic phase meet, while for modified KNN and BNT-BT, the MPBs do
not end at the cubic phase, and the crystallographic distortion between the two phases
which form the MPB is much less pronounced compared to PZT. At the triple point, the
free energies of the cubic, tetragonal and rhombohedral phases become equal. In other
words, there is no energy barrier for polarization reorientation from one phase to
another. Thus, the free energy flattening along the tetragonal-cubic and the
rhombohedral-cubic path in addition to the tetragonal-rhombohedral path near the triple
point is isotropic. Damjanovic et al. pointed out that the two former phase transitions
lead to enhanced propensity for polarization extension, and the latter transition lead to
strong effects of polarization rotation. The ease of polarization extension can result in a
very large increase of the piezoelectric properties. Therefore, the triple point was
thought to be an essential factor for the high piezoelectric performance of
Ba0.85Ca0.15Zr0.1Ti0.9O3 and the compositions near the triple point are expected to have
even higher piezoelectric coefficient and dielectric properties. However, Liu and Ren
(2009) did not really produced a sample at the triple point. Studies by Acosta et al.
(2014) found that the compositions near the triple point do not have high piezoelectric
coefficient due to the low spontaneous polarization.
With a concern of the low Curie point of Ba0.85Ca0.15Zr0.1Ti0.9O3, Bao et al. (2010)
modified the system to Ba(Zr0.15Ti0.85)O3-x(Ba0.8Ca0.2)TiO3. As seen from its phase
diagram, shown in Figure 2-14, the Curie point increases with the addition of BCT.
When x> 50%, the Curie point is stable at a level close to 120 °C. However, the
Literature Review
26
piezoelectric properties decrease dramatically with increasing BCT (shown in Figure
2-15). Composition BZT-53BCT (will be expressed as Ba0.894Ca0.106Zr0.07Ti0.93O3 in this
article) was focused on in this system as it has the MPB at room temperature, though
does not have the best dielectric and piezoelectric properties in this system (seen from
Figure 2-15). For Ba0.894Ca0.106Zr0.07Ti0.93O3, the piezoelectric coefficient d33 is reported
to be 450 pC/N at room temperature and Curie point is 114 °C, slightly higher than for
Ba0.85Ca0.15Zr0.1Ti0.9O3.
Figure 2-14. The phase diagram of Ba0.85Zr0.15TiO3-xBa0.8Ca0.2TiO3 (Bao et al., 2010)
Figure 2-1Ba0.85Zr0.15
Ot2.2.5.3
Actually,
BCZT) w
BCZT wa
McQuarri
system an
found tha
Lichteneck
exerted by
accurate.
work in th
Figure 2-1
findings th
15. The die5TiO3-xBa0.8
ther compos
lead-free m
were not fir
as not reckon
e and Behn
nd even det
at the solub
ker rule to
y the secon
Hennings
he low Zr
16, with the
hat compos
electric perm8Ca0.2TiO3 as
sitions in B
materials wit
st found by
ned as a pie
ncke (1954)
termined th
ility of Ca
determine
ndary phas
and Schrein
and Ca reg
e dashed lin
sitions in th
mittivity (a1s a function
BCZT system
th all Ba, Ca
y Ren’s gr
ezoelectric m
) investigat
heir lattice
and Zr in
secondary
se into con
nemacher (
gion, and co
ne. This fin
he region en
1-a4) and pn of tempera
m
a, Ti, and Z
roup but kn
material but
ted the solu
dimensions
BT was v
phases wh
sideration,
(1977) repe
orrected the
nding coinc
nclosed by
piezoelectricture (Bao et
r (Ba1-xCaxZ
nown since
t a dielectric
ubility of th
s, shown in
ery limited
hich did no
thus this d
eated McQ
e insolubilit
ides much
the dashed
Literatu
c coefficientt al., 2010)
Zr1-yTiyO3 s
e the 1950s
ic material a
he (Ba, Ca
n (Figure 2
d. However,
ot take dilut
diagram wa
Quarrie and
ty region a
better with
d line do no
ure Review
27
t (b1-b4) of
signified by
s. Although
at that time,
)(Ti, Zr)O3
2-16). They
, they used
tion effects
as not very
Behncke’s
as shown in
h other later
ot exhibit a
w
7
f
y
h
,
3
y
d
s
y
s
n
r
a
Literature Review
28
secondary phase. They also found that the appearance of this secondary boundary
depends on the quenching temperature or cooling rate. The higher the quenching
temperature or the cooling rate, the harder the secondary phase would appear.
Figure 2-16. Solubility and phase relationships in the system (Ba, Ca) (Ti, Zr)O3. The peripheral numbers indicate the value of the lattice constants in Å. T, tetragonal phase; C, cubic phase 0, orthorhombic phase. (modified image as solid black lines are original graph from (McQuarrie and Behnke, 1954) and dashed line is later correction in (Hennings and Schreinemacher, 1977))
Despite these early investigations by these pioneers, not much attention was paid on
BCTZ and its high piezoelectric performance was not discovered. In 1977, Hennings
and Schreinemacher already investigated compositions very close to
Ba0.894Ca0.106Zr0.07Ti0.93O3. Unfortunately, piezoelectric properties were not investigated
in that study. In the first 50 years, BCTZ was mainly studied as a ferroelectric relaxor
for compositions with Zr amount over 20 mol%. However, after 2009, in which the high
piezoelectric performance was reported, many compositions in BCTZ system have been
25 %
40 %
Literature Review
29
produced and investigated in a few years. There were mainly three groups that
investigated series of new compositions: Ren’s group (Liu and Ren, 2009, Bao et al.,
2010, Gao et al., 2011, Xue et al., 2011), Zhang et al. (Zhang et al., 2010, Zhang et al.,
2009), Li et al. (Li et al., 2010a, Li et al., 2010b, Li et al., 2010c, Li et al., 2011b)
between 2009 and 2011.
Figure 2-17 summarizes the reported Curie point of each composition reported in the
literature. The Curie point decreases with the addition of Zr4+, and Ca2+. This coincides
with the theory that isovalent substitution (i.e. Ca2+ for Ba2+, Zr4+ for Ti4+) lowers the
Curie point (Shrout and Zhang, 2007). Thus, in the BCZT system, the maximum Curie
point cannot be much higher than the Curie point of BT which is approximately 115 °C
to 120 °C. As reported in the literature, the highest Curie point, i.e. 137 °C, located at
Ca≈0.12, Zr=0. The addition of Zr4+ content appears to have a larger influence on the
Curie point than the addition of Ca2+. This was also reported by Lin et al (2013).
Furthermore, there is a dramatic reduction in the Curie point when the Zr amount is
higher than approximately 0.20 approaching the region of relaxor ferroelectrics. It
worth to mention that Li et al. (2010c) (the compositions are circled in Figure 2-17 with
a dashed line) and Lin (2013) investigated the BCZT compositions around
Ba0.85Ca0.15Zr0.10Ti0.9O3. The values of the Curie point of those compositions were at a
level of 60 °C close to Ba0.85Ca0.15Zr0.10Ti0.9O3, which are much lower than the Curie
point reported by Liu and Ren (2009). The piezoelectric properties they reported were
also significantly lower than what other publications report. It might indicate that the
properties of BCZT can be significantly influenced by the processing conditions which
will be reviewed in the next section.
Figure 2-1represent crosses rep
For BCTZ
the tetrag
Schreinem
by other r
(Ravez et
orthorhom
approxima
the orthor
literature.
orthorhom
investigate
indicates t
with incre
investigati
17. Variationpublished dpresent repo
Z, there com
gonal and
macher (197
researches w
t al., 1999
mbic phase
ately 0<Ca<
rhombic ph
It is inte
mbic phase
ed the Ba(Z
that there is
easing x fro
ions of the
n of Curie pdata that shoorts of a orth
mes a debate
d rhomboh
77) (in Figu
who were s
9a, Favarim
in the BCZ
<0.22 and 0
hase adding
eresting tha
while most
Zr0.2Ti0.8)O3
s a tetragona
om 0 % to
phase tran
point of BCow a tetragohorhombic t
e about the
hedral phas
ure 2-18a).
studying the
m et al., 2
ZT system
0<Zr<0.10. F
g to the Cu
at most stu
t studies on
3-x(Ba0.7Ca0
al to orthorh
1 %. How
nsition of th
ZT, summaonal to orthoto rhomboh
existence o
se as e.g
This orthor
e relaxor pr
2010). As
is about 4
Figure 2-17
urie point
udies on t
n BZT-BCT
0.3)TiO3 syst
hombic pha
wever, Gao
he MPB ma
arised from orhombic p
hedral phase
of an orthorh
g. reported
rhombic pha
roperties (in
seen from
0K~80K in
7 also summ
of composi
the BCTZ
T did not. B
tem with X
ase transitio
et al. (201
aterial BZT-
Literatu
literatures. hase transit
e transition
hombic pha
d by Hen
ase was als
n Figure 2-
m these dia
n width and
marizes the e
itions repo
system re
Bhardwaj et
XRD. The X
on at room t
1) also rep
-50BCT. T
ure Review
30
The circlestion and the
ase between
nnings and
so observed
-18b and c)
agrams, the
d ranges in
existence of
rted in the
eported the
t al. (2010)
XRD pattern
temperature
orted TEM
he samples
w
0
s e
n
d
d
)
e
n
f
e
e
)
n
e
M
s
exhibit na
room temp
results pre
the step ra
been miss
range of
happened
(Tian et
Ba0.85Ca0.
with varyi
also confi
high resol
Figure 2-1and SchrBa0.9Ca0.1T
ano-scale do
perature. B
esent either
anges are w
sed. For ex
the orthorh
to the varia
t al., 20
15Zr0.1Ti0.9O
ing tempera
irmed the ex
ution tempe
18. Variatioeinemacher
Ti1-xZrxO3 (F
omains of c
y varying c
an entirely
wide in this
xample, only
hombic pha
ation range
13a) iden
O3 and its r
ature, as sho
xistence of
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coexisting t
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ombic pha
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Literatu
rhombohedr
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were studied
K in width.
nd 0.6. A r
ase in c
ric loss me
y (Keeble e
ZT-BCT sy
Ti0.93Zr0.07Oz et al., 1
ure Review
31
ral phase at
nt, the TEM
evertheless,
could have
d, while the
. This also
ecent study
composition
easurements
et al., 2013)
ystem using
O3 (Hennings1999a); (c),
w
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Figure 2-1diagram o
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As the pa
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calcination
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1300 °C a
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19. (a) The of (Ba0.85Ca0.
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uggest that
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ttini et al.
peratures ab
.1Ti0.9)O3 cel., 2013b)
ocess play i
the synthesi
and sinterin
nificantly. A
fferent stud
the desired
aterials are
can increase
on. Henning
ve 1200 °C
rious tempe
purities gen
still some
fter the sinte
(2012) als
bove 1200
Literatu
eramics. (b)
important r
is process u
ng, as bein
As seen from
dies. The
compositio
able to mel
e the reacti
gs and Schr
C. Wang et
eratures fro
nerally disap
impurities
ering proce
so reported
°C, as seen
ure Review
32
The phase
roles to the
used in the
ng the most
m Table 2-2
purpose of
on. There is
lt and react.
on rate and
einemacher
al. (2011)
om 1000 to
ppear when
even after
ess possibly
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w
2
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t
2,
f
s
.
d
r
)
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n
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y
t
e
Literature Review
33
2-20. From the refinement of peak 110, they also found that at 800 °C the calcined
powders are CaCO3, BaZrO3, BZT and CaZrO3; at 900 °C the calcined powders are
CaZrO3, BaZrO3 and BZT; at 1000 °C the calcined powders are BaZrO3 and BZT.
Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) were
also conducted in this research. From the results of these analysis (Figure 2-21) it seems
that most mass loss happens within the temperature range of 600 – 900 °C. The
emission of CO2 was found to be completed at 800 °C, followed by the crystallization at
900 °C. They suggest that 800 °C is sufficient for calcination as the left over impurities
would disappear during the sintering process.
Table 2-2. The synthesis parameters from literature
Calcination Temp (°C)
Calcination time (h)
Sintering Temp (°C)
Sintering Time (h)
Hennings et al. 1200 6 (1995) 1430 (1977) 1400 (1995)
15 2
Liu and Ren (2009) 1350 2 1450-1500 3
Bao et al. (2010) 1300 2 1450 3
Li et al.(2011b, 2011d, 2011c)
1200 4 1450 4
Zhang et al. 1200 6 1500 (2007) 1200-1400 (2009)
2
Bhardwaj et al. (2010) 1100 12 1250 8
Wu et al. (2011) 1200 3 1300-1500 2
Wang et al. (2011) 1000-1300 2 1500-1550 2
Pisitpipathsin et al. (2013) 1200 4 1300-1450 4
Ye et al. (2012) 1150 6 1500 6
Frattini et al. (2012) 600-1200 2 1250-1400 1-3
Literature Review
34
Figure 2-20. XRD patterns of the milled powders at room temperature and annealed at temperatures from 800 to 1200 C for 2 h (Frattini et al., 2012).
Figure 2-21. DTA and TGA curves of milled source powder mixture (Frattini et al., 2012).
Literature Review
35
The purpose of sintering is to bind calcined BCTZ powder together by diffusion at high
temperature to a solid mass of ceramic at a given shape. The temperature required
during sintering is higher than during calcination. Zhang et al. (2009) reported that a
secondary phase of Ba3Ca2Ti2O9 appears after sintering at 1200 °C, which gives the
minimum sintering temperature of BCZT. They also found that the sintering
temperature would affect the sample phase, grain size and density. Samples have a
tetragonal phase when sintered below 1320 °C, and the phase transforms to pseudo-
cubic phase (a phase with coexistence of orthogonal and tetragonal phases) above
1320 °C. Grain size and density increase with increasing sintering temperature. It
appeared that the properties d33, ɛr and k33 of the composition peaked when sintering at
1350 °C. Similar peaking phenomenon (especially ɛr) was also observed by Li et al.
(2011b). Bhardwaj et al. (2010) sintered their samples at 1250 °C for 8 hours and no
impurities were reported. Frattini et al. (2012) selected sintering temperatures from
1250 to 1400 °C and found that the dielectric constant was substantially improved at
1400 °C compared to 1300 °C. Wang et al. (2011) conducted an investigation with
sintering temperatures from 1500 °C to 1550 °C for 2 hours, which suggest the 1540 °C
is the optimum sintering temperature for BZT-50BCT. Wu et al. (2012a) investigated
the optimum sintering temperature with a calcination temperature at 1200 °C and found
that the Ba0.85Ca0.10Ti0.9Zr0.10O3 sintered at 1440 °C exhibited highest properties.
Additionally, they also suggested that 5 hours is the optimum sintering dwelling time
for that composition at 1500 °C sintering temperature (Wu et al., 2011).
The highest piezoelectric coefficient reported in the literature varied significantly from
440 pC/N to 650 pC/N. It is difficult to identify the origin of this variation as the
properties can be easily affected by composition, processing condition and
measurement techniques. From these studies, the calcination temperature of BCTZ is
Literature Review
36
suggested to be 1300 °C. Lower than this temperature, impurities still exist after
calcination; higher than this temperature, there is a large chance of sintering, as 1250 °C
is a critical temperature for sintering. In order to achieve a high density and high
piezoelectric properties, the ideal sintering temperature should be 1400 to 1550 °C.
In the industry, the temperature of sintering has been a major concern for the sake of the
cost. Raising and maintaining a high temperature is substantially expensive. With this
concern, there were some attempts to reduce the sintering temperature but maintain the
same piezoelectric properties. With an inspiration that CuO decreases the sintering
temperature of (Ba,Sr)(Ti,Zr)O3, Chen et al. (2012) investigate the effect of CuO content
from 0 mol% to 3.0 mol % on the sintering process of BCZT. It is found that a
secondary phase appears when the content of CuO exceed 0.5 mol % at a sintering
temperature of 1400 °C for 2 hours. Though the grain size does increase substantially
but d33 decreases with the increase of CuO doping. With 0.5 mol % CuO doping, d33
dramatically increases when the sintering temperature increases from 1350 °C to
1400 °C. Although the d33 continues to rise slightly with the temperature increased to
1500 °C, there is an obvious decline of the Curie point with increasing temperature.
Thus, 1400 °C with 0.5 mol % CuO for sintering Ba0.85Ca0.15Zr0.1Ti0.9O3 is advised in
this research. However, even after a long dwelling time of 6 hours for sintering, the
piezoelectric properties of the obtained samples are not attractive, with a d33 of
approximately 400 pC/N. Cui et al. (2012) also conducted a similar study but with less
amount of CuO doping compared to the previous study. They investigated the doping
range from 0 to 0.1 wt % (0.1 wt % quotes to approximately 0.003 mol % for CuO in
Ba0.85Ca0.15Zr0.1Ti0.9O3). They were able to reduce the sintering temperature from
1540 °C to 1350 °C and the piezoelectric properties remain the same level. The
optimum content of CuO was suggested to be 0.04 wt %with which the d33 can achieve
Literature Review
37
510 pC/N after sintering at 1350 °C. It was also found that the phase changes from
tetragonal to rhombohedral phase when CuO content exceeds 0.06 wt%. That might
explain the low piezoelectric properties of samples doped with 0.5 mol % CuO in the
previous study. This may also explain the poor piezoelectric properties and low Curie
point of samples in a study of Lin et al. (2013) who added 1 mol % CuO to all the
samples.
2.3 Transient Degradation mechanisms
For applications, it is ideal if the piezoelectric performance is consistent and persistent.
Unfortunately, it has been found that piezoelectric materials experience property
degradation over time or during electric or mechanical cycling. This section will
introduce the existing knowledge regarding to these changes.
2.3.1 Ageing behaviour
Ageing behaviour of piezoelectric materials is usually defined as the change in
properties on the function of time. It is usually pronounced in acceptor-doped
piezoelectric materials, including the lead zirconate titanate (PZT) family (Glaum et al.,
2012, Kamel and de With, 2008), barium titanate (BT) family (Dechakupt et al., 2010,
Huang et al., 2014, Sareein et al., 2011, Zhao et al., 2015, Liu et al., 2006, Mitoseriu et
al., 2001), and some other lead-free compositions (Feng and Ren, 2008). Degradation of
the dielectric and piezoelectric properties with time was observed starting from the
moment of cooling from the Curie point. The fully aged materials commonly express a
pinched double hysteresis loop (Yun et al., 2011, Liu et al., 2006, Huang et al., 2014,
Glaum et al., 2012, Feng and Ren, 2008). When the materials are de-aged thermally or
electrically, the pinched hysteresis loop opens gradually. Many theories were developed
to explain the ageing phenomenon including the domain splitting (Ikegami and Ueda,
Literature Review
38
1967), domain wall trapping by electronic charges (Warren et al., 1995), stress
clamping (Sareein et al., 2011), ionic drift (Morozov and Damjanovic, 2008, Lambeck
and Jonker, 1986, Hagemann, 1978), space-charge formation (Genenko, 2008, Genenko
et al., 2009, Lupascu et al., 2006) and relaxation of defect dipoles (Carl and Härdtl,
1977). So far, it is well-accepted that the substitution of lower valence elements
(acceptor-dopants) effectively introduce the ageing behaviour as the ageing rate is
usually proportional to the amount of acceptor-dopants (Lupascu et al., 2006). An
oxygen vacancy usually exists in the unit cell which contains the acceptor dopants in
order to neutralize the valence. These unit cells also generate dipoles when the materials
are cooled from the Curie point and are called defect dipoles, shown in Figure 2-22. The
reorientation of these defect dipoles involves the hopping of oxygen vacancies and is
more difficult than for the normal dipoles. When the materials are cooled below the
Curie point, normal dipoles as well as the defect dipoles start to appear with random
orientations. As time elapses, these defect dipoles tend to align with the neighbouring
dipoles during the relaxation process. When the external field is applied, these defect
dipoles cannot be easily switched with the external field and they switch the
neighbouring dipoles back to their original orientation when the external field is
removed. Thus the polarization hysteresis exhibits a greatly reduced global remanent
polarization, seen as the double hysteresis loop (Figure 2-23).
Literature Review
39
Figure 2-22 Illustration of a defect dipole. (image taken from (Genenko et al., 2015))
This process has been widely accepted and is called volume effect (Lambeck and
Jonker, 1986). The aged material can be de-aged by poling, annealing or electrical
cycling (Granzow et al., 2006, Kamel and de With, 2008). During these processes, the
bonded dipoles are gradually released from the defect dipoles and the hysteresis loop is
thus opened up. Besides the volume effect, the oxygen vacancy can also migrate to the
grain boundaries driven by the residual depolarization field appearing at mismatching
neighbouring grains (Masao, 1970). The accumulation of the oxygen vacancies at the
grain boundaries generates an internal bias field that screens the depolarization field in
the grain and influences the local domains. The internal bias field can be seen as the
shift of the hysteresis loop along the field axis.
Literature Review
40
Figure 2-23 Room temperature P-E loops of well-aged Ba(Ti1-xFex)O3 ceramics measured at about 50 KV/cm and 100 Hz. (Huang et al., 2014)
2.3.2 Fatigue studies on PZT-based materials
Most materials would exhibit property degradation or even fracture during a long period
of application. The phenomenon is normally called fatigue behaviour. In a machine, the
failure of one part may cause the loss of function of the whole machine. Piezoelectric
materials are usually applied with cyclic electric field or mechanic loading or even both.
Fatigue has become the major obstacle keeping piezoelectric materials from being used
in applications.
The fatigue behaviour of PZT-based materials has been studied for a long time as they
have been widely used. Either thin films or bulk materials were studied for PZT.
Because thin films and bulk materials have different conditions and different nature, the
fatigue behaviours are also different. In general, thin films are much more sensitive. For
Literature Review
41
example, thermal, optical, and electrical variations can all suppress the polarization
switchability (Warren et al., 1994). Also, the interface between thin film and substrate
has a significant influence on the fatigue behaviour (Colla et al., 1998). Bulk materials
are more stable, of which the domain switchability is usually affected by applied
electric field or mechanic stress. However, bulk materials have more defects, cracks,
and domain boundaries than thin film materials. Thus, the investigations on the thin
films can be a complement to bulk materials studies as a defect-free condition.
Figure 2-24. Loading regimes: (a) unipolar, (b) sesquipolar, and (c) bipolar electric; (d) compression-compression loading (depolarisation), (e) electrical and in phase mechanical loading (blocking force scenario, anti-resonance of ultrasonic motors), and (f) out of phase electrical and mechanical loading (mixed loading, resonance of ultrasonic motors). (Lupascu and Rödel, 2005)
For the phenomenon of fatigue behaviour of PZT, it is widely accepted that the fatigue
behaviour can be influenced by surface contamination (Jiang et al., 1994), porosity
(Jiang and Cross, 1993), existing cracks (Cao and Evans, 1994, Shieh et al., 2006, Zhu
and Yang,
magnitude
Shieh et a
Wang et a
cycling pr
this article
Figure 2-2constant dsputtered bipolar cy
Bip2.3.2.1
During bi
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, 1998), freq
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igue degrad
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33 (c), and poped with 1
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42
al., 2011a),
et al., 2008,
ödel, 2005,
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piezoelectric1% La with.
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r
Literature Review
43
cycling suffer from polarization, dielectric coefficient, and piezoelectric coefficient
degradation, associated with bias field and strain hysteresis loop asymmetries (seen
Figure 2-25). Different fatigue mechanisms were found for PZT materials with different
dopants. For Pb0.99[Zr0.45 Ti0.47 (Ni0.33 Sb0.67)0.08]O3, macro/micro cracks were found
under the electrodes after fatigue testing, as seen in Figure 2-26 (Nuffer et al., 2002,
Luo et al., 2012, Luo et al., 2011a). These cracks propagate perpendicular to the applied
field reducing the electric field in the bulk piezoelectric material. These cracks are
normally found approximately 100 μm from the electrodes and no obvious cracks are
found in the bulk. For PZT with La doping, Balke et al. (2007a) found that a 50 µm
thick layer of damaged grains, as seen in Figure 2-27, leading to surface discolouration
and a reduction of the switchable polarization. When the damaged layer was removed,
materials were capable to be recovered up to 90% of the original values. This partial
recovery indicates that other mechanisms may also contribute to the fatigue degradation.
Apart from the mechanical damage, domain pinning was suggested to play an important
role in bipolar fatigue (Glaum et al., 2010). It was found that point defects such as free
oxygen vacancies or electrons can diffuse within the material driven by electrical cyclic
loading and often are trapped at domain boundaries (Nuffer et al., 2000). The
agglomeration of these point defect at domain boundaries restricts domain switchability
and domain wall mobility, leading to an increase of coercive field and sometimes a bias
field (Glaum et al., 2010, Glaum et al., 2011). Normally, the effect of domain wall
pinning can be recovered at high temperature by redistributing the trapped point defects.
Figure 2-2drawing shThe crackmechanica
Figure 2-2and fatiguelectrodes (Balke et a
Un2.3.2.2
It has bee
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roscopy. Theion of the ab
d due the duffer et al., 2
urface of unples with spifferent mag
es much hig
005). Comp
witching. Th
ributor to th
unipolar c
by the ext
ure Review
44
e schematicbove image.ifference of2002)
fatigued (a)puttered Ptgnifications.
gher fatigue
pared to the
hus, charge
he unipolar
cycling, the
ternal field.
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2007b). It
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2.3.3 Fat
With the
essential f
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Ba2.3.3.1
Although
fatigue stu
microstruc
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found that
the cracks
fraction of
due to the
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aused by an
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umulate at
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nd it turned
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nate (BT) ba
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grains sign
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2007c). The
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d-free piezo
d-free piezo
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out that the
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ant roles in
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ong the inter
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fatigue beh
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less fatigu
ation under
n.
materials, th
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be determin
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limited. It
haviour. Sin
rowth (Chen
associated w
o abnormal
oth dielectr
Literatu
domains can
depolarizat
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umulation o
obal anisotr
strain by a
field. Comp
ue effect (B
sesquipolar
heir reliabil
lead-free s
ned by differ
ed in applic
was report
ntering bari
n et al., 20
with micro-
l grains. An
ric and fatig
ure Review
45
nnot have a
ion field at
d can drive
f the point
ropy of the
applying an
pared to the
Balke et al.,
r loading is
lity is also
systems has
rent fatigue
cations, the
ed that the
um titanate
009). It was
-cracks and
n increasing
gue strength
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increases
microstruc
leakage cu
and allow
area is me
called ava
BN2.3.3.2
Similar to
electrical
were also
fatigue in
cracks but
terial (Lu et
tests using
the form
ctural defe
urrent to flo
s higher con
elted by the
alanche arcin
NT-xBT
o lead-based
bipolar cyc
observed u
n BNT-6BT
t restores th
t al., 2010).
DC loadin
mation of
cts can be
ow. The flow
nductivity e
e high temp
ng associate
d piezoelect
cling, as sh
under the e
T seems to
he piezoelec
Similar inf
ng (Chen et
microstruc
e interconn
w of the lea
encouraging
perature and
ed with ther
trics BNT-6
hown in Fig
electrodes.
be weak ,
ctric propert
fluence of ab
al., 2010).
ctural defe
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akage curre
g more leak
d eventually
rmal runawa
6BT experie
gure 2-28 (
However, t
for the 40
ties (Ehmke
bnormal gra
The presen
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form a co
nt increases
kage current
y breaks dow
ay scenario
ences proper
Luo et al.,
the contribu
0 °C annea
et al., 2011
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ains was als
nce of abnor
g DC loa
onductive c
s the local t
t. As a resu
wn. The me
.
erty degrada
2011a). M
ution of cr
aling doesn
1).
ure Review
46
so observed
rmal grains
ading. The
channel for
temperature
ult, the local
echanism is
ation during
Micro-cracks
acks to the
n’t heal the
w
6
d
s
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r
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l
s
g
s
e
e
Literature Review
47
Figure 2-28. P(E) (a), S(E) (b), 33 (c), and d33(E) (d) before fatigue, after 104 and 106 bipolar cycles. (Luo et al., 2011a)
Figure 2-29. Schematic summary of the fatigue process in BNT-6BT. Poling of the cubic/relaxor structure is observed in the first 10-100 cycles. After this point, domain fragmentation occurs, causing a loss in polarization and a diffraction pattern reminiscent of unpoled state (Simons et al., 2012).
The fatigue behavior of BNT-6BT can be divided into two regimes, seen as Figure 2-29
(Simons et al., 2012). In the first regime, 0-102 cycles, Ec increases; Prem remains
relatively constant but Pmax decreases rapidly. In the second regime, starting from 102
cycles, Pmax, Prem, Ec and permittivity all decrease steadily. It was found that the non-
ergodic relaxor nature of BNT-6BT plays an important role in the first stage (Simons et
al., 2012, Ehmke et al., 2011). BNT-6BT irreversibly transforms from a highly
disordered pseudo-cubic phase to a ferroelectric state within the first 10 electrical cycles
at room temperature. It also transforms the material from rhombohedral-favored
symmetry
switching
102 cycles
pinning le
and pinne
the electri
BN2.3.3.3
Luo et al.
with KNN
the charge
increased
and space
doping am
the degrad
defect at
materials
ferroelectr
state upon
a reversibl
BZ2.3.3.4
The fatigu
by Patters
fatigue to
BZT even
shown in
y to tetrago
ability, gre
s, the migrat
eads to the
d domains.
cal cycling,
NT-BT-KNN
(2011b) in
N as a subst
e disorder
at high-cyc
e charge ac
mount up to
dation rate.
domain wa
due to the a
ric order up
n removal o
le field-indu
ZT-BKT-BN
ue behaviou
son and Ca
the invest
n presents
n Figure 2-
onal-favore
eatly induci
ting charge
formation o
Therefore,
, gradually l
N
nvestigated
titution to B
and leads t
cle numbers
cumulation
3% leads to
Domain w
alls. The ch
absence of d
pon applicat
f the field.
uced phase
NT
ur of Bi(Zn
ann (2012).
igated num
an increasi
-30. The e
ed one. Th
ing fatigue t
carries agg
of new dom
the domain
leading to d
the bipolar
BNT. It is fo
to more do
s due to me
n in case of
o an elimina
wall pinning
harged defe
domain wal
tion of an e
Therefore,
change.
n0.5Ti0.5)O3-
Interesting
mber of cyc
ing polariz
xplanation
his transfor
to samples
glomerate an
mains in ord
ns become s
degradation
and unipol
ound that th
main wall
echanical d
f unipolar l
ation of the
is associat
ects cannot
lls. The BN
lectric field
the pinning
-(Bi0.5K0.5)T
gly, this ma
cles. Surpris
zation and
to the fat
rmation su
in the first
nd pin doma
der to respon
smaller, mo
of global p
lar fatigue b
he addition
pinning. Th
amage in c
loading. Fu
ferroelectri
ted with acc
t accumulat
NT-BT-3KN
d but returns
g effect cann
TiO3-(Bi0.5N
aterial does
singly, the
strain after
igue resista
Literatu
uppresses th
hundred cy
ain walls. T
nd to the ap
obile sub-do
polarization.
behaviour o
of 1% KNN
he degrada
case of bipo
urther increa
ic order, an
cumulation
te in non-f
NN exhibits
s to a non-f
not be stabi
Na0.5)TiO3 w
sn’t exhibit
compositio
r electrical
ance of th
ure Review
48
he domain
ycles. After
This domain
pplied field
omains with
.
of BNT-BT
N increases
ation rate is
olar loading
asing KNN
d decreases
of charged
ferroelectric
long-range
ferroelectric
ilized under
was studied
significant
on with 5%
cycling as
his material
w
8
n
r
n
d
h
T
s
s
g
N
s
d
c
e
c
r
d
t
%
s
l
proposed b
sintering
after elect
Figure 2-3polarizatiocompleted
by the auth
temperature
trical cyclin
30. Fatigue on hysteres
d. (Patterson
or is due to
e. However
g is still not
results for asis and (b) n and Cann,
o the low lev
r, the reaso
t clear.
a typical 5%strain hys
2012)
vel of intrin
on why the
% BZT teststeresis wit
nsic defects
polarizatio
ed at 50 kVth invcreasi
Literatu
resulting fr
on and stra
V/cm and 10ing number
ure Review
49
rom the low
ain increase
0 Hz for (a)r of cycles
w
9
w
e
) s
Thesis Objective, Hypotheses and Approaches
50
Chapter 3 Thesis Objective, Hypotheses and Approaches
3.1 Aim of Thesis
Lead-free BCTZ ceramics have been identified as a potential alternative to lead-based
soft PZT due to their high piezoelectric response at room temperature range. However,
BCTZ is still a very young material compared to PZT, which has been systematically
studied for a number of decades. It is seen in the previous chapter that most publications
are limited to a binary BZT-BCT system and usually only one of the compositions in a
study exhibits high piezoelectric performance at room temperature in this system. This
approach only provides limited information about the BCTZ system. Other potential
compositions suitable for applications in the system could be missed. Studying a more
comprehensive set of compositions not only increases the number of potential
compositions for applications, but also provides a broader view upon the BCTZ system
and may help to understand the mechanism behind the extraordinary piezoelectric
behaviour at morphotropic phase boundary (MPB). The influence of Ca and Zr doping
on the MPB is also of interest warranting further investigation.
Furthermore, high piezoelectric response is not the only factor of interest in the
application of piezoelectric materials. Reliability is another important factor that needs
to be taken into consideration. In application, piezoelectric materials are usually
required to convert mechanical pressure to an electrical signal or deform upon the
application of an electric field repeatedly. Whether the material is able to maintain the
same performance with increasing number of cycles determines the durability of the
material. Whether performance is sensitive to the environment, such as temperature,
moisture and contact pressure, may also limit the material to certain applications.
Thesis Objective, Hypotheses and Approaches
51
Furthermore changes in properties of materials during standby for a period of time can
also affect the commercialisation. No matter what reason, the failure of the piezoelectric
materials during application is of major significance. The replacement of the failed
components may also be complicated and expensive. Therefore, the ageing and fatigue
behaviour of BCTZ needs to be studied.
The aim of this study is to determine the potential of a number of BCTZ compositions
for usage in piezoelectric applications. To achieve this, compositions with high
piezoelectric coefficient shall be identified. Although Ba0.85Ca0.15Ti0.9Zr0.1O3 has been
reported to have high piezoelectric coefficient, it may not be the best composition in the
BCTZ system. Understand the influence of Ca and Zr doping on the change of MPB
may help to identify other composition with good piezoelectric performance. After
identifying the promising compositions, fatigue behaviour of these compositions shall
be investigated to determine the reliability during potential applications.
3.2 Hypotheses
Based on the current knowledge outlined in Chapter 2, the above aim and chosen
compositions, the following hypotheses are posited as a basis for the research in this
thesis:
I. The piezoelectric properties of BCTZ are similar to a soft PZT but are more
sensitive to temperature and mechanical pressure.
It has been reported that BCTZ exhibits soft piezoelectric behaviour, which
is more similar to a soft PZT than to barium titanate (BT) (Xue et al., 2011).
However, unlike soft PZT, the high piezoelectric response of BCTZ strongly
depends on temperature. The material expresses the optimum piezoelectric
coefficient at a temperature near where the MPB is located. Moving away
Thesis Objective, Hypotheses and Approaches
52
from this temperature results in a significant loss of piezoelectric properties
(Xue et al., 2011). It has also been reported that mechanical pressure can
easily de-pole the sample and lead to a strong reduction in piezoelectric
coefficient (Ehmke et al., 2012a).
II. The phase transition temperature can be adjusted by changing the doping
amounts of Ca and Zr. The compositions with phase transition at room
temperature can be predicted.
Figure 3-1. The composition diagram of BCZT system. Dots represent studied composition. The circled points represent the best compositions reported by each study on piezoelectric properties. The dash line illustrates the solubility line which is shown in Figure 2-16.
It has been reported that the increase of Ca dopant slightly reduces the MPB
temperature, while the increase of Zr dopant significantly raises the MPB
Thesis Objective, Hypotheses and Approaches
53
temperature (Tian et al., 2013b). The proposal is whether these two effects
can compensate each other so that the MPB remains at a certain temperature
with changing of both dopants according to a certain ratio. Based on a
survey of all existing studies on the BCTZ system, a composition map is
summarized in Figure 3-1. The dots in the map represent compositions
investigated in literature. It is interesting to find that all the compositions
reported high piezoelectric coefficient at room temperature lie along a linear
line as shown in Figure 3-1; thus the optimal Ca to Zr ratio is very likely to
be a constant value. Therefore, it is proposed that other compositions along
this line could also have a MPB at room temperature and exhibit high
piezoelectric coefficient.
III. The BCTZ ceramics should not exhibit ageing behaviour, similar to the soft PZT,
as they are not intentionally acceptor doped.
Ageing behaviour is defined as change of properties with time. This effect
may significantly influence the design of experiments and measurements.
Ageing behaviour often occurs in hard piezoelectric ceramics and is
considered to be associated with acceptor doping (Kamel and de With, 2008).
Heavily acceptor-doped piezoelectric ceramics usually show pinched
hysteresis loops during initial application of the electric field. The hysteresis
loops develop a more open shape after repeated poling or electric cycling.
For donor doped soft PZT, this phenomenon has not been observed. Since
BCTZ is similar to soft PZT and exhibits well developed hysteresis loops
upon the first application of an electric field, it is not expected that the
properties of BCTZ change with time.
Thesis Objective, Hypotheses and Approaches
54
IV. The BCTZ ceramics may experience severe unipolar fatigue.
Poling studies on BCTZ (Ehmke et al., 2013a, Li et al., 2013) find that
BCTZ materials are sensitive to unipolar electric loading. Instead of
improving the development of hysteresis, the poling process induces large
anisotropy in the strain hysteresis loop and field dependent permittivity.
Strong bias field and offset polarization are observed after the poling process.
Repeated poling can even lead to a decrease of the polarization. Since
unipolar cycling is similar to a repeated poling process, it is expected that a
similar effect would occur during the unipolar cycling. With the increase in
number of cycles, the material may exhibit severe fatigue of the piezoelectric
properties.
V. Property degradation is expected to be more severe under bipolar cycling than
unipolar cycling. The BCTZ ceramics are expected to have similar bipolar
fatigue behaviour to soft PZT. The mechanical damage and domain wall pinning
are the major reasons for the loss of piezoelectric properties.
The studies of bipolar and unipolar fatigue (Luo et al., 2011a, Balke et al.,
2007a, Balke et al., 2007c, Lupascu, 2004, Lupascu and Rödel, 2005)
suggested that the bipolar cycling, which involves two domain switches per
cycle, causes more property degradation than unipolar cycling. Mechanical
damage and domain wall pinning, which are not reported in unipolar fatigue
studies, are often found in bipolar fatigue studies. Therefore, it is expected
that BCTZ would show more property degradation during bipolar cycling
than unipolar cycling. As BCTZ has similar piezoelectric properties to soft
PZT, one would expect a similar magnitude of bipolar fatigue behaviour.
Thesis Objective, Hypotheses and Approaches
55
Both mechanical damage and domain wall pinning may occur during the
bipolar cycling and would thus be responsible for the loss of piezoelectric
properties.
VI. The doping of Ca and Zr increases the lattice distortion and may increase the
property degradation during bipolar cycling.
In Ca and Zr doped BT, Ca replaces the position of Ba while Zr replaces the
position of Ti. Since Ca and Ba, and Zr and Ti, respectively, are of different
atom size, the increasing amount of Ca and Zr doping will introduce more
point lattice distortion in the BT matrix. This would provide more available
defects in the materials which may serve as initial pinning centres for free
defects to accumulate, leading to the pinning of neighbouring domains.
3.3 Research Approach
The approach for addressing the above hypotheses is outlined as follows.
A series of BCTZ compositions with increasing doping amount, outlined in Figure 3-1
will be fabricated. All the compositions are predicted to have a phase transition at room
temperature. The density, grain size, surface morphology, purities, phase structures and
piezoelectric properties of each composition will be characterised and compared to
elucidate the effect of the doping amount on this system.
The density will be determined using Archimedes’ method. The grain size and surface
morphology will be determined by optic microscopy and SEM on etched samples. XRD
will be conducted on calcined and sintered samples to determine the purity level and
phase structure at room temperature. These characterization will confirm the qualities of
the new compositions fabricated in this study. Piezoelectric and dielectric properties
Thesis Objective, Hypotheses and Approaches
56
will be measured using TF Analyzer 2000 system. For the piezoelectric measurements,
the temperature shall be maintained constant at 25 ˚C, and the contact pressure on
samples shall be carefully adjusted to avoid mechanical de-poling. The small signal (d33)
and large signal (d33*) piezoelectric responses are the two key properties that determine
the potential piezoelectric performance during application. However, the phase
transitions which are keys to understand the origins of high piezoelectric performance
are also important to be investigated. Temperature-dependent permittivity and
temperature dependent XRD or neutron diffraction measurements can help to reveal the
phase transitions with temperature change. In situ field-dependent neutron diffraction or
synchrotron measurements can help to resolve if there is field-induced phase transitions.
In order to identify the reliability of BCTZ during potential applications. Bipolar and
unipolar cyclic loading will be conducted on BCTZ with promising piezoelectric
performance at room temperature. As fatigue tests are time consuming, a special
custom-built fatigue setup will be developed. The fatigue setup should be able to hold
multiple samples for the consideration of time saving and statistical analysis. The
clamping force on the samples during the fatigue test should be minimized. The samples
need to be immersed in silicone oil during fatigue cycling to avoid electrical arcing. In
case of dielectric breakdown during cycling, the fatigue setup should be able to detect
current change in the circuit and terminate the test. Simultaneously, the time of break
down shall be recorded in order to calculate the cycle number. In order to observe
significant fatigue in an achievable practical time, the maximum field used needs to be
sufficient to induce saturated polarization switching. However, the maximum field
cannot be too large as it may lead to dielectric breakdown of the sample or cause arcing.
Therefore, 4 times the coercive field is chosen for the maximum field during unipolar
fatigue testing and 3 times the coercive field is chosen for the maximum field during
Thesis Objective, Hypotheses and Approaches
57
bipolar fatigue testing. The difference in maximum field is due to the consideration that
bipolar cycling may introduce more damage to the samples and result in earlier failure.
The changes of switchable polarization, piezoelectric coefficient, dielectric coefficient
and strain hysteresis loop shall be monitored during the fatigue process.
As mechanical damage and domain wall pinning are the two major contribution for the
bipolar fatigue, these two mechanism should be investigated first. After fatigue testing,
the effect of domain wall pinning can be determined by annealing sample at high
temperature (400 ˚C) and remeasure the piezoelectric and dielectric properties. This
temperature is high enough to redistribute the point defects trapped at domain walls but
not sufficient to change the mechanical damage. The degree of recovered can indicate
the proportions of these two contributions to the fatigue behaviour. The mechanical
damage can be further confirmed by optical microscopy or SEM observation on the
surface, subsurface and cross-section. If mechanical damage is observed, removal of
damaged layer could recover the piezoelectric and dielectric properties of residual
sample.
For unipolar fatigue, charged defect agglomeration at grain boundaries is the most
polarization explanation used in literature. Both bipolar cycling and thermal annealing
should redistribute the charged defects and will be conducted after unipolar cycling.
Ageing behaviour, though not expected to occur on BCTZ, can significantly affect the
fatigue testing and thus worthwhile to be investigated. As poling of BCTZ has been
studied in the literature and un-poled sample is not related to fatigue testing, the
investigation will be more focused on the poled BCTZ.
Methodology and Materials
58
Chapter 4 Methodology and Materials
4.1 Materials Preparation
4.1.1 Preparation of the BCTZ samples
Ba100-xCaxTi100-yZryO3 (shortened as BCxTZy) ceramic powders were prepared using a
conventional solid oxide method. BaZrO3 (99.5%, Sigma Aldrich, St. Louis, MO, USA),
CaCO3 (99.0%, Sigma-Aldrich, St. Louis, MO, USA), BaCO3 (99.8%, Alfa Aesar,
Ward Hill, MA, USA), and TiO2 (99.8%, Sigma-Aldrich, St. Louis, MO, USA) were
used as starting chemicals. Seven compositions, BC8TZ5.5, BC10TZ6.7, BC12TZ8,
BC14TZ9, BC15TZ9.5, BC15TZ10, BC15.5TZ10, along the predicted phase transition line
mentioned in the previous chapter were produced for this study. Each starting chemical
was weighted using an electronic balance according to the calculation for each
composition. All the weighted chemicals were mixed with ethanol and put into a ball-
milling bottle for ball-milling in order to obtain a homogeneous mixture. After ball-
milling for 24 hours, the suspensions with mixed chemicals were transferred into a
beaker and dried in an oven at 90 ̊C for 24 hours. The dried powders were crushed in a
mortar and transferred into a zirconate crucible for calcination. The chemicals were
calcined in a furnace to react to the desired BCTZ composition at 1300 ̊C for 2 hours
with a heating rate of 350 ̊C per hour and a cooling rate of 400 ̊C per hour. The
calcined powders were again ball-milled in ethanol for 24 hours to break the powder
agglomeration and then dried for 24 hours. The dried powders were crushed in the
mortar and passed through a 70-mesh sieve. The prepared BCTZ powders were stored
in plastic bottles.
Methodology and Materials
59
The prepared BCZT powders were weighted as 0.5-0.6 g per sample and then were
uniaxially pressed into cylindrical discs using a set of Ø12mm dies. The pressure was
~80 MPa. The pressed discs were placed on a zirconate crucible and sintered in the
furnace at 1450 ̊C for 3 hours in air with a heating rate of 350 ̊C per hour and a cooling
rate of 400 ̊C per hour. Zirconia crucibles were used for calcination and sintering
because alumina crucibles would react with this material.
4.1.2 Surface preparation
Samples were ground using 320 grade wet/dry silicon carbide grinding paper followed
by 600, 1200 grade successively. Between each grade, the samples were washed with
tap water and dried with compressed air. The sample surfaces were examined using a
microscope to ensure that the scratches from the previous grade have been completely
removed by the current grade before moving on to the next grade. After the last step of
grinding, the samples were cleaned in an ultrasound bath for 2 minutes using soap water
and then rinsed with tap water. Afterwards, the samples were polished with 3 µm
diamond suspension followed by 1 µm as finish. After each grade of polishing, the
samples were cleaned by soap water using the ultrasonic bath and then examined under
the microscope ensuring that all the diamond particles have been removed.
Approximately 0.1-0.2 mm of the surface was removed on each side during the whole
grinding and polishing process. Special preparation was applied for particular
measurements which will be described in the corresponding sections.
4.1.3 Electrode
Three types of electrodes were used in this study: brushed silver paste, burnt-in silver
paste and sputtered silver coating.
Methodology and Materials
60
Brushed silver paste (RS components Ltd., Northants, UK) was a layer of adhesive
silver paint that could easily be applied by a brush or a stick and was also easy to be
removed by acetone or isopropyl alcohol. After application of the silver paint, it
required half an hour drying in air. However, it is difficult to ensure that the silver paint
was homogeneously covering all the surface area. It was also noticed that the contact
between sample and electrode was not good leading to an increase of the observed
coercive field compared to samples with sputtering coated electrodes. Heat treatment of
the samples at 121-148 ˚C for 5 to 10 minutes on a hot plate could improve the
adherence and address this issue effectively.
Burnt-in silver paste (Gwent Group, Pontypool, UK) was a different type of silver
paste that requires a heat treatment to above 450 ˚C. The silver paste would diffuse into
the material at high temperature and thus is the most stable type of electrode compared
to the other two electrodes. However, it has been found that samples with this type of
the silver paste had significantly lower piezoelectric coefficient than the samples with
the other two types of electrodes. It might be the diffusion of the silver paste reduced
the effective insulating thickness of the material and also influenced the domain
switching. Therefore, this type of electrode was only used in the temperature-dependent
permittivity test.
Sputter coated silver was a 50 nm silver particle layer deposited on the sample surface
using a sputter coating machine (Leica EM SCD050, Leica Mikrosystem GmbH, Wien,
Austria). It takes 150 s for the deposition with an applied voltage of 60 mV on the silver
target. The working distance is 50mm and the vacuum level is kept to 5 × 10-2 bar. The
sputtered silver coating had a homogeneous covering on the sample surface which also
provided the best contact with the sample without influence on the piezoelectric
properties. It can be removed by polishing a few cycles with 1 µm diamond suspension.
Methodology and Materials
61
Due to these advantages, the sputter coated electrodes were used for most tests in this
study. Nevertheless, it was noted that the sputtered electrode could disappear at
temperatures higher than 100 ˚C when heating up using a hot plate.
4.2 Characterization methods
4.2.1 Density measurement
Density of the samples was measured according to Archimedes’ principle. Five as-
sintered samples per composition were used for a statistical analysis. To measure the
density of an individual sample, the sample was firstly dried in a furnace at 110 °C for 4
hours and the mass of the dried sample, m0, was measured using an electronic balance
with an accuracy of 0.0001 g. The second step was to measure the mass of the sample
immersed in the water, m1. The sample was immersed in distilled water for 4 hours
ensuring that all the open pores were filled with water. Mass m1 was measured in water
with a basket hooked to the electronic balance. The last step was to measure the mass of
the wet sample in the air, m2. The sample was retrieved from the water and wiped with
a wet tissue to remove the attached water but keep the water in the pore, and then the
weight was taken. The second and third steps were repeated five times to obtain average
values of m1, m2, respectively.
The density of the sample was calculated using
where ρwater is the density of water at the tested temperature.
4.2.2 G
Image ana
surfaces o
µm diamo
samples w
water plus
heated to
cooled at
a scannin
Corporatio
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number o
calculated
The avera
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4.2.3 D
N4.2.3.1
The neutr
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and silver
Smithfield
experimen
Grain size d
alysis on a p
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ond suspen
were firstly i
s 2% of HF
1250 ̊C in
300 ̊C per h
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. Photoshop
f grains the
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Japan).For
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Lucas Heig
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was filled w
nd Materials
62
in size. The
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rocess, the
with distilled
mples were
10 min and
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echnologies
rawn across
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attern of BC95872 Å. Th
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un-poled st
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63
Step poling
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445K using
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3 wavelengtmal BCTZ m
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ks towards
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cation of di
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n temperatu
ent in gene
sults and di
on in the teed from 20ensity of the.
hodology an
ith 3 times
ion of 1.49
mprehensiv
the limited
4-2. The on
ermal expa
eresting to
ures which
eral did no
iscussion.
emperature00 – 450 Ke reflections
nd Materials
64
the lattice
Å intended
ve Rietveld
d resolution
nly changes
ansion with
observe the
h might be
ot reach its
dependentK and the, where red
s
4
e
d
d
n
s
h
e
e
s
t e d
Figure 4-temperatuof the latti
The synch
JEEP of
Hamburg,
none of th
statistical
X4.2.3.2
Two diffe
measurem
measurem
During th
strong sur
on how m
significant
used for X
3 The latture dependenice paramete
hrotron mea
the Diamo
, Germany.
hese experi
distribution
X-ray diffra
ferent kinds
ment at roo
ment with a i
he XRD exp
rface effects
much surfa
t when mor
XRD measu
tice parament neutron der.
asurements
ond Light
However,
iments succ
n.
action meas
s of XRD
om temper
in-house ma
periment, it
s from the a
ace was gr
re than 0.2 m
urements we
eter calculadiffraction d
for step po
Source, U
due to the
cessfully pr
surement
experimen
rature, whi
ade sample
t was found
as-sintered
round off.
mm surface
ere ground
ated from data. The ci
oling study
UK and be
large grain
roduced dif
nts were co
le the oth
stage.
d that the p
surface. Th
However,
e layer was
down by at
Meth
the Rietvelrcles highlig
were attem
eamline B2
n size of the
ffraction pa
onducted. O
her is a te
peak pattern
he peak shap
the surfac
removed. T
least 0.2mm
hodology an
ld refinemeght the sudd
mpted at bea
2, HASYL
he as sintere
atterns with
One is no
emperature
ns were aff
pe changed
ce effect be
Therefore, t
m and the s
nd Materials
65
ent on theden changes
amline I12-
AB/DESY,
ed samples,
h satisfying
rmal XRD
dependent
flicted with
d depending
ecame less
the samples
surface was
s
5
e s
-
,
,
g
D
t
h
g
s
s
s
Methodology and Materials
66
then polished to 1µm. The samples were annealed at 400 ̊C using a furnace before the
XRD measurements to release the strain induced by the grounding and polishing
process.
The normal XRD measurements at room temperature were carried out in an X’pert
Multipurpose X-ray diffraction system (MPD) (PANanlytical B.V., Almelo,
Netherlands) using Cu Kα radiation. 0.02 radian soller slit was used for a high
resolution scan. In addition to the full XRD pattern (from 20 to 85 2ϴ°), the (200) and
(222) peaks were scanned in detail. The scan rate was set to roughly half an hour per
scan in order to obtain an adequate resolution and measurements were undertaken at
room temperature (~25 ̊C). In order to clearly demonstrate the peak splitting, the
reflection of the Kα2 wavelength is stripped from all the XRD patterns.
Temperature-dependent X-ray diffraction measurement was conducted using Empyrean
X-Ray diffraction with self-built temperature controller (PANanlytical B.V., Almelo,
Netherlands). The sample stage as shown in Figure 4-4 consists of a copper plate, a
Peltier temperature-controlling system, a heat sink and two thermocouples (one on the
surface of the copper plate and the other one inside of the copper plate). The Peltier
element can be cooled to -15 °C or heated up to 100 °C with an accuracy of 0.1 °C
controlled by a voltage source. During the experiment, the sample was placed on the top
of the copper plate with a thin layer of silicone grease in between to improve the
thermal conductivity and was centralised using a camera fixed on the celling of the
camber. When the target temperature is set in the computer, a signal will be sent to the
voltage supplier so that the voltage will be applied to the Peltier element. The
temperature change of the copper plate was monitored by the thermocouples and the
magnitude of the voltage was changed accordingly. When the surface temperature
reached the target temperature, the Peltier element will try to stabilise the temperature
Methodology and Materials
67
fluctuation within 0.1 °C. The XRD patterns were taken from -10 °C to 45 °C. This
measurement system had physical constraints from its design that could not be
addressed in the timeframe of the thesis project. On the one hand, the sample was
desired to be as thin as possible in order to reduce the temperature gradient from the
sample bottom surface, where the temperature is measured, to the sample top surface,
where the diffraction patterns were taken. However, the background, the surface of the
copper plate in this case, was too close to the sample surface, and induced high intensity
of diffraction peaks to the measured pattern. Therefore, there was a competition
between measurement duration and beam size for an adequate intensity. On the other
hand, a layer of ice was formed if the temperature was equal or below 0 °C. The layer of
ice not only induced many peaks in the diffraction pattern but also greatly reduced the
intensity of the BCTZ pattern. Additional to that, water drops sometimes form on the
sample surface at low temperature which induces amorphous reflections and reduces
intensity. Although this issue could be solved by covering samples with a transparent
vacuum chamber, it was not completed for use in the research in this thesis. Therefore,
full XRD patterns with high quality could not be obtained, which hinders quantitative
Rietveld refinement on the structure. Fortunately, 200pc and 222pc diffraction peaks of
the investigated compositions were not affected by the noise from the background and
the ice layer. The qualitative analysis is still available. Thus, only these two peaks were
taken for analysis. The parameter settings are the same as for the measurement
described in the previous paragraph except the soller slit is 0.04 radian. Temperature-
dependent XRD full pattern measurements and Rietveld refinement analysis may be
carried out in the future after completion of the system.
Figur
4.2.4 Pi
A TF Ana
measure t
five comp
1. The
in
tem
sam
2. The
fiel
and
d33
re 4-4 A sch
iezoelectri
alyzer2000
the piezoele
ponents:
e sample ho
silicone oil
mperature se
mple with a
e laser devi
ld direction
d detecting
of the samp
hematic of th
ic property
system (aix
ectric prope
older is capa
l to avoid t
ensor and a
spring.
ice can dete
n by sending
the return s
ple.
he temperat
y measure
xACCT syst
erties. As s
able of load
the arcing
metal plate
ermine the d
g a laser be
signal. It en
ure-depend
ement
tems GmbH
hown in Fi
ding with on
at the edge
e (that funct
displacemen
eam to the
nables the T
Meth
ent XRD sta
H, Aachen, G
igure 4-5, t
ne sample w
e. The sam
tions as a m
nt of the sam
metal plate
TF Analyzer
hodology an
age configur
Germany) w
the system
which can be
mple holder
mirror) that c
mple along
e in the sam
r to read the
nd Materials
68
ration.
was used to
consists of
e immersed
also has a
contacts the
the applied
mple holder
e strain and
s
8
o
f
d
a
e
d
r
d
Methodology and Materials
69
Figure 4-5 An image of TF Analyzer 2000 system.
3. The PID temperature controller (Eurotherm Model 2416, Worthing, UK) can
read the current temperature in the sample holder and heat the sample holder if
high temperature is required for the measurement.
4. The high voltage amplifier (Trek 20/20C, Trek Inc., New York, USA) can
multiply input signals by 2000 times and output to the sample holder with a
resolution of ~20V. An engineering interlock is built in the machine which cuts
of the powder supply when the current in the circuit exceeds up to 20 mA in
"trip" mode or limited the current up to 20 mA in "limit" mode. The amplifier
can be remotely controlled or activated locally. It is in "remote" mode when used
in TF Analyzer 2000 system.
5. TF Analyzer controller can output signals as a waveform generator to high
voltage amplifier, communicate with the laser device and temperature controller,
and read the change of voltage drop of the sample. The polarization is measured
Methodology and Materials
70
using a method similar to the Sawyer-Tower circuit. The high voltage is applied
to the stacks of the sample and a reference capacitor. The charge of the sample is
the same with the charge calculated from the reference capacitor. Therefore, the
relative polarization of the sample can be measured.
Before the measurement, both sides of the polished samples were sputter coated with
silver. The samples were poled at five times the coercive field (Ec) for 10 minutes at
25 ̊C. The bipolar polarization and strain hysteresis loops were measured at 0.1 Hz.
From the strain hysteresis loops the value Smax was extracted, which is the difference
between maximum and remnant strain under the positive electric field. This value was
used to calculate the large signal piezoelectric coefficient Smax/Emax (noted as d33*). The
piezoelectric coefficient d33 was measured at different electric fields using the TF
Analyzer’s small signal capacitance vs. voltage (CV) function. The small signal
frequency was 1000 Hz, and the vibration amplitude 30 V (approximately 24.5 ~ 31
V/mm depending on sample thickness). The error range of the d33 measurements was
approximately 6 pm/V in the remanent state. The CV function also measured field-
dependent permittivity (ε) and dielectric loss (tanθ) at the same time. The measurements
were conducted at various temperatures from room temperature up to 100 °C; however,
for the measurements at room temperature, the temperature is normalised to 25 °C.
4.2.5 Frequency and temperature dependent permittivity and dielectric
loss
Temperature-dependent permittivity and dielectric loss measurements were conducted
on the Quatro Cryo system (Novo control Technologies GmbH and Co. kG,
Hundshagen, Germany) at the Technical University Darmstadt, Germany. All eight
composition were measured with the temperatures ranging from -40 ̊C to 140 ̊C at an
Methodology and Materials
71
increment of 0.25 ̊C per step. The electrode used in this study is the burned-in silver
paste. Five frequencies were used, i.e. 102 Hz, 103 Hz, 104 Hz, 105 Hz and 106 Hz.
4.3 Testing of Ageing Behaviour
Four BC15TZ10 samples were poled by applying a 10 Hz bipolar triangular loading of
750 V/mm, which is approximately five times of the coercive field. The initial state of
the polarization and strain was immediately measured by applying another bipolar
loading and the piezoelectric coefficient (d33) and permittivity were measured by a
bipolar field-dependent small signal measurement (vibration of 30V at 1000Hz). The
ageing time was considered starting from the end of the small signal measurement. The
polarization and strain hysteresis loop measurement was carried out on day 2, 5, 11, 22
and 37. Each measurement consisted of two bipolar cycles. The samples were short
circuited by a tweezer after each measurement. The piezoelectric coefficient was only
measured at the end of the ageing study to minimize the influence of this measurement
on the ageing state. On the last day of the ageing measurement, 10000 cycles of a 10 Hz
bipolar loading were applied to the samples in order to test the stability of the aged state.
During the bipolar cycling, a polarization and strain measurement was taken for every
decade. After bipolar cycling, the samples were annealed in a furnace at 400 ˚C for 10
min with increasing and decreasing ramp of 5 ˚C/min.
4.4 Fatigue Testing
4.4.1 Fatigue setup
The electrical fatigue test was conducted using both the TF Analyzer 2000 system and a
self-built fatigue setup. The fatigue test usually takes a long period of time, therefore,
the TF Analyzer 2000 system is not ideal as it only holds one sample at a time. The self-
built fatigue setup is made to cycle up to five samples at the same time.
Methodology and Materials
72
The self-built fatigue setup consists of five components (as shown in Figure 4-6):
Figure 4-6 The picture of the self-built fatigue setup
Methodology and Materials
73
Figure 4-7 (a) the picture of the sample holder; (b) the schematic of the sample slot in the sample holder
1. The sample holder, as shown in Figure 4-7, can hold up to five samples at the
same time. The samples are contacted with copper pins which are held by an
ABS plastic frame (modified Lego® bricks). The upper pin has an adjustable
nut that is lifted by a spring. This design on the one hand can hold up the upper
pin to minimize the pressure of the wire and pins to the samples as BCTZ is
very sensitive to the pressure; on the other hand it avoids short circuit by lifting
the whole upper pin up when the slot is not loaded with sample. The frame is
Methodology and Materials
74
fixed in an ABS plastic container which holds the silicone oil to avoid edge
arcing. Two terminals are mounted on the container for wiring.
2. The function generator (Agilent 33220A) can output electrical signals.
3. The high voltage amplifier (Trek 20/20C, Trek Inc., New York, USA) can
amplify the signal from function generator by 2000 times and output to the
sample holder. The amplifier in this case is set to "local" mode with "trip"
mode for tripping.
4. The oscilloscope (Agilent 6220) can monitor the voltage change in the circuit
by reading the voltage output signal from the high voltage amplifier.
Figure 4-8 The User interface of the fatigue controller programmed using LabView
5. The computer with a software programmed using LabView shown in Figure
4-8) can control the function generator and record the feedback data from the
o
to
sa
ap
For safety
enclosed i
door is op
down and
samples a
high volta
activated b
after the d
fatigue tes
"burst" m
because th
of voltage
field, whic
4.4.2 Fa
P4.4.2.1
Samples u
the same e
of the sam
with 1 µm
4.1.2).
oscilloscope
o be applied
ample break
ppendix A.
y reasons,
in a rack, a
pened durin
thus no inp
are loaded i
age amplifie
before clos
door is clos
st can start
mode as the
he first appl
e usually dra
ch in return
atigue test
Prior to fati
used for the
electric fiel
mples in the
m polishing
. The softw
d or high v
k down, sho
the sample
nd an engin
ng the test, t
put to the hi
in the samp
er is witche
ing the doo
sed. The osc
t by running
normal mo
lied voltage
aws a high c
trips the am
t process
igue test
fatigue test
d to the sam
same batch
before the
ware can term
voltage is de
ort circuit o
e holder, c
neering inte
the power s
igh voltage
ple holder f
ed on. The
or of the rac
cilloscope c
g the softw
ode usually
e may not s
current if th
mplifier.
t were grou
mples withi
h is controlle
electrode w
minate the e
etected in th
or arcing. T
computer an
erlock is bu
supply to th
amplifier is
first. Then,
output of t
ck. The fun
can be turne
ware. The e
y induced tr
start at 0 or
he applied v
und to appro
n the same
ed within 0
was applied
Meth
experiment
he circuit w
The detailed
nd higher
uilt on the d
he function
s given. To
the power
the amplifie
nction gener
ed on at an
lectrical cy
ripping in t
180 ° phas
voltage is hi
oximately 1
batch, the d
.05 mm. Th
(detailed m
hodology an
if the volta
which migh
d code can b
voltage am
door of the
generator w
start a fatig
for the com
er has to b
rator can be
ny time. Aft
ycling is us
the first cyc
se and sudd
igher than th
mm. In ord
difference i
he surface w
method is m
nd Materials
75
age is failed
t indicate a
be found in
mplifier are
rack. If the
will be shut
gue test, the
mputer and
e manually
e turned on
ter that, the
ing infinite
cle. This is
den increase
he coercive
der to apply
in thickness
was finished
mentioned at
s
5
d
a
n
e
e
t
e
d
y
n
e
e
s
e
e
y
s
d
t
The surfac
technolog
checked w
put into a
Resin and
container
min and r
bubbles in
were take
until the e
polished w
and 1 µm
Hitachi Hi
B4.4.2.2
For the bi
were elect
at 2 Hz an
setup is n
The PE lo
Analyzer.
using the
higher num
Every cer
transferred
cycled at 1
ce of an un
ies Corpora
with a proce
container w
d hardener w
to cover th
remained in
n the resin.
en out from
exposed cro
with 320, 6
polishing in
igh-technol
Bipolar fati
ipolar fatigu
trically cycl
nd 10 Hz, re
not capable
oop, d33 an
The first 1
TF Analyz
mber of cy
rtain period
d back to TF
10 Hz using
fatigued sam
ation, Toky
edure as fol
with the sam
were mixed
he whole sa
n the vacuum
. After 24 h
m the contai
ss section w
00, 1200 gr
n sequence.
ogies Corpo
igue test
ue test, two
led at three
espectively.
to cycle at
nd ε33 of un
0000 cycle
zer. The re
ycles, the sa
d of time, t
F Analyzer
g the same p
mple was c
yo, Japan).
llowed. The
mple surface
d by a ratio
ample. The
m environm
hrs solidifi
iner and gro
was close to
rades silico
. The polish
oration, Tok
sets of five
e times of th
Due to the
a frequenc
nfatigued sa
s at 10 Hz
elated prope
amples wer
the fatigue
to be meas
procedure m
checked usin
The cross-
e selected s
e perpendicu
of 4:1 and
container w
ment for an
cation of th
ound using
o the middle
on carbide g
hed surface w
kyo, Japan)
e samples e
he coercive
limit of the
cy higher th
amples wer
(1000 at 2
erties were
re cycled to
test was in
sured. Five s
mentioned ab
Meth
ng SEM (TM
-section of
sample held
ular to the b
the mixtur
was then va
additional
he resin, th
120 grade
e point. The
grinding pap
was viewed
.
ach of the B
field (appr
e tripping cu
han 10 Hz w
re firstly m
Hz) were ca
measured
ogether usin
nterrupted a
samples of B
bove.
hodology an
TM3000, Hi
f the surfac
d by a plast
bottom of th
re was pour
acuum-pum
15 min to
he moulded
e silicon car
e samples w
per followe
d using SEM
BC15TZ10 c
roximately 4
urrent (i.e. 2
with multip
measured us
arried out i
for each d
ng the self-
and the sam
BC8TZ5.5 w
nd Materials
76
tachi High-
e was also
tic clip was
he container
red into the
mped for 15
remove the
specimens
rbide paper
were further
ed by 3 µm
M (TM3000
composition
450 V/mm)
20 mA), the
le samples.
sing the TF
ndividually
decade. For
built setup.
mples were
were bipolar
s
6
-
o
s
r.
e
5
e
s
r
r
m
0,
n
)
e
.
F
y
r
.
e
r
U4.4.2.3
For the un
of the u
(approxim
bipolar pi
samples in
unipolar c
P4.4.2.4
After the c
fatigued s
in acetone
diamond s
optical m
Microscop
samples w
FIB-SEM
beam and
holes were
An etchin
purpose o
rest was c
domains.
Epiphot 2
Unipolar fa
nipolar fatig
unipolar loa
mately 600 V
ezoelectric
n sequence
cycling.
Post-fatigue
completion
amples wer
e solution a
suspension
microscope
pe (SEM) (
was investig
, Germany)
sub-surface
e melted for
g process w
f compariso
chemically e
The prepa
00, Japan) a
atigue test
gue test, fiv
ading was
V/mm) and
and bipolar
. Same mea
e test
of the fatig
re examined
and the resid
for several
(Nikon E
Hitachi TM
gated using
). The FIB
es (20 µm w
r each samp
was added to
on, half area
etched using
ared cross-
and the SEM
ve samples o
selected
d the freque
r CV measu
asurements
gue test, the
d. The silve
dual sputter
passes. Th
Eclipse ME
M3000, Tok
dual focuse
milled rect
wide and 4
ple.
o the observ
a of expose
g 1:1 HCl w
sections w
M.
of BC15TZ1
to be fou
ency was 10
urements w
in the sam
e surface, su
r paste was
r coated sil
he exposed
E600L, Jap
kyo, Japan).
ed ion beam
tangular hol
µm deep) w
vation proce
d cross-sect
with 2 % HF
were observ
Meth
10 were used
ur times o
0 Hz. The u
were conduct
me sequence
ubsurface an
washed off
ver was po
surfaces we
pan) and a
The sub-su
m (FIB) (Ze
les in the s
were expose
edure of the
tions was co
F in order to
ved using a
hodology an
d. The max
of the coe
unipolar pi
cted on the u
e were cond
nd cross-sec
ff in the ultr
olished off u
ere examine
a Scanning
urface of tw
eiss Auriga
surface usin
ed to electr
e cross-secti
overed by t
o reveal the
a microsco
nd Materials
77
ximum field
rcive field
ezoelectric,
un-fatigued
ducted after
ction of the
rasonic bath
using 1 µm
ed using an
g Electron
wo fatigued
Crossbeam
ng a Ga ion
ron beam. 3
ion. For the
ape and the
e grains and
ope (Nikon
s
7
d
d
,
d
r
e
h
m
n
n
d
m
n
e
e
d
n
Methodology and Materials
78
4.4.3 Improvement of electrode for fatigue test
Three different methods of applying electrodes were used on the samples for fatigue
tests: (1) sputtered sliver coating along, (2) sputtered sliver coat plus painted silver paste
and (3) sputtered silver coating plus painted silver paste heated at 140 °C using hot plate
for 5 minutes.
During the first fatigue experiments, only sputtered sliver coating was used, consistent
with the other piezoelectric measurements. It was found that the sputtered layer was
easily damaged during the continuous cycling, especially bipolar cycling. As shown in
the SEM image (Figure 4-9), the surface of a fatigued sample exhibited different
colours, light and dark. An energy-dispersive X-ray spectroscopy (EDS) measurement
confirmed that the light area had high concentration of silver which was absent in the
dark area. It can be seen from Figure 4-9 that the dark area was always surrounding the
open pores. It is very likely that the mismatching of displacement between ceramic and
pores during bipolar cycling initiated the loss of silver coating at the edge of the pores.
The damaged area then enlarged due to the strain mismatching between coated area and
un-coated area.
Methodology and Materials
79
Figure 4-9 The loss of sputtered silver coating after bipolar cycling.
In the later experiment, a layer of silver conductive paste (RS components Ltd.,
Northants, UK) was painted on the top of the sputtered layer. Although the painted
silver paste could serve as a protection to the covered sputtered layer to a certain extent,
the silver paint itself also experienced partial peeling off during bipolar cycling. After
the painted layer peeled off, the exposed sputtered layer was found to be damaged again.
As seen from Figure 4-10, the polarization and permittivity decreased drastically when
the area covered by the electrode was reduced. When the electrode was re-applied, the
lost properties were restored close to the original value. However, this does not indicate
that there was no other degradation within the bulk material during the bipolar cycling.
Due to the continuous loss of covered area with increasing cycles, the electric field
across the ceramics was also reduced and became inhomogeneous. Therefore, the
external field applied became less effective as expected and the data became
untrustworthy. It was found that the damage condition of the silver paste was sample
dependent, which can be seen form the increase of error bar with number of cycles.
Methodology and Materials
80
Therefore, the major reason was possibly due to the lack of adherence and
inhomogeneous painting, resulting in the peeling off from the edge of the sample.
Figure 4-10 The loss of polarization and permittivity during bipolar cycling at 2Hz due to the damage to electrode. Electrode of the samples was sputtered silver coating plus the silver paste. Most of the lost properties were recovered by re-applying the electrode.
With this consideration, the samples were further heated up to 140 ˚C on a hot plate for
5 minutes after application of the silver paste to improve the adherence. It was found
that this process successfully prevented silver paste from peeling off during bipolar
cycling. The results will be shown in Chapter 8. This method was also used for unipolar
fatigue experiment in Chapter 7.
Characterization of BCTZ
81
Chapter 5 Characterization of BCTZ
5.1 Introduction
Parts of content of this chapter has been published in the Journal of the American
Ceramic Society in “Correlation between piezoelectric properties and phase coexistence
in (Ba, Ca) (Ti, Zr)O3 ceramics” (Zhang et al., 2014b).
Among the seven compositions fabricated in this study, six of them, lying along the
predicted phase transition line at room temperature in Figure 3-1, have not been
reported in other literature. Therefore, their basic piezoelectric behaviour is still unclear.
This chapter shows the characterization of the six compositions, including grain size,
density, pore morphology, phase structure, piezoelectric properties and dielectric
properties. The methods used for these characterizations have been introduced in the
previous chapter.
5.2 Experimental Results
5.2.1 Density, grain size and pore morphology
The density and grain size of all compositions are listed in Table 5-1. All compositions
had densities of over 90% theoretical density (calculated from the XRD data). Images of
the etched surfaces reveal the microstructure (Figure 5-1). The grain boundaries are
clearly observed on the surface. Light coloured grain boundary regions were observed
for example in Figure 5-1(a). The origin of these regions is unclear but might be related
to small amounts of a secondary phase. The irregular holes are considered to be intrinsic
pores among grains while the large black holes are considered to result from grains
pulled out during polishing. No clear relationship between density, grain size and
doping content was observed.
Characterization of BCTZ
82
Table 5-1. Microstructural properties of each composition
Composition BC8TZ5.5 BC10TZ6.7 BC12TZ8 BC14TZ9 BC15TZ9.5 BC15.5TZ10
Density (g/cm3) 5.6 5.5 5.4 5.3 5.4 5.5
Theoretical density (g/cm3)
5.88 5.83 5.80 5.76 5.74 5.74
Mean grain size (µm)
17 25 16 32 24 19
Standard deviation for grain size
2.2 4.2 6.1 3.6 7.6 3.1
Figure 5-1 SEM images of thermochemical etched of (a) BC8TZ5.5, (b) BC10TZ6.7, (c) BC12TZ8, (d) BC14TZ9, (e) BC15TZ9.5 and (f) BC15.5TZ10
Characterization of BCTZ
83
5.2.2 Piezoelectric and dielectric properties
As all compositions have a different coercive field, the piezoelectric property
measurement results are plotted as a function of electric field normalised to the
respective coercive field in Figure 5-2. It can be seen that all the compositions exhibited
well developed hysteresis loops at a maximum electric field of 5 Ec at 25 ̊C. The
average coercive field ( ) calculated from the positive and negative coercive fields,
remanent and maximum polarization (Pr and Pmax, respectively), Smax, d33 and d33* are
plotted in Figure 5-3. The effect of the doping fraction on the piezoelectric properties
can be seen in this figure. Generally, all the compositions have a similar coercive field
between 130 ~ 145 V/mm (Figure 5-3(a)). The values of both maximum and remanent
polarization decreased with the increasing doping amount (Figure 5-3(b) and (c)).
Figure 5-3(d) and (e) show that the maximum strain and d33 increased with doping
content up to x=15, y=9.5 and then slightly fell for the highest doping content. d33*
(Figure 5-3(f)) increased dramatically from BC8TZ5.5 to BC10TZ6.7, and then only rose
slightly with further doping. Composition BC15TZ9.5 had the largest piezoelectric
coefficient of all the compositions investigated in this study (d33=458 pm/V and
d33*=740 pm/V). However, composition BC12TZ8 seemed to be an exception compared
to the other compositions. This composition had the highest coercive field and Smax at 5
Ec, but lowest polarization and d33. The d33 trends with grain size, however both d33 and
d33* show no correlation with density. The measured piezoelectric properties are
generally consistent with the closest compositions reported in the literature (Li et al.,
2010b, Bao et al., 2010, Wu et al., 2012b, Tian et al., 2013b) with some variations.
Variations also exist between values in the literature for these compositions. These
differences are likely to be caused by a number of factors: Intrinsic microstructural
properties such as grain size, density and chemical homogeneities, etc.; measurement
Characterization of BCTZ
84
parameters such as frequency, maximum electric field and temperature etc.; and the
measurement method as it is noticed that the Berlincourt d33 meter usually reports a
higher d33 comparing to TF Analyzer.
Figure 5-2 (a) Polarization and (b) strain hysteresis loops of all compositions measured at 0.1 Hz at 25 ˚C.
Figure 5-3 Piezoelectric properties of compositions of BCxTZy with x = 8–15.5 and y = 5.5–10. (a) Average of positive and negative coercive field, (b) maximum polarization, (c) remanent polarization, (d) maximum strain at approximately five times the coercive field
Characterization of BCTZ
85
measured at 0.1 Hz; (e) d33 in the remanent state and (f) d33* at approximately five times
coercive field.
5.2.3 Temperature dependent XRD, permittivity and d33
In Figure 5-4, the XRD patterns of all the compositions (un-etched) shows a perovskite
structure hence the indexing of these peaks was done according to a cubic perovskite
structure. Also, no secondary phase was found in the pattern. The XRD measurements
on the thermo-chemically etched samples (figures not presented) shows additional
peaks in the diffraction pattern indicating that the secondary phase observed in the SEM
micrographs might have formed during the thermal etching process. This secondary
phase was not observed in samples after chemical etching only (shown in Figure 8-4)
could be an indirect evidence that secondary phase was introduced during thermal
etching process. Figure 5-5 shows detailed scans of the (200)pc and (222)pc reflections
for all compositions at different temperatures. It can be seems that both (200)pc and
(222)pc reflections experienced significant changes within the temperature range
investigated. To be specific, for the (200)pc reflection (Figure 5-5(a)), the peak intensity
at 45.21 decreased and the peak intensity at 45.4 increased with increasing temperature.
Around 25 ˚C intensity peak at 45.1 started to appear and its intensity kept increasing
with increasing temperature. At high temperature, i.e. about 35 ˚C, all compositions
exhibit two peaks (45.1 and 45.4) with 1:2 ratio. The (222)pc reflection (Figure 5-5(b))
exhibited multiple peaks in the low temperature range, i.e. below 25 ˚C, which
gradually merge to one single peak when the temperature increased to 35 ˚C.
At room temperature, i.e. around 25 ˚C, one broad shoulder and one sharp peak were
observed for all compositions for the (200)pc reflection; the asymmetric peak shape of
1 This is an approximate value as the exact value varies according to the composition. Same applies to the other descriptions.
Characterization of BCTZ
86
the (222)pc reflection indicated that at least two peaks contributed to it. It is also noticed
that the distance between splitted peaks at (200)pc reflections decreased with increasing
doping amount, though BC10TZ6.7 seems to be an exception to this trend.
Figure 5-4 The whole XRD pattern of all compositions at 25 ˚C.
Characterization of BCTZ
87
Figure 5-5 Detailed scan of (a) (200)pc and (b) (222)pc peaks at different temperatures.
The result of temperature-dependent permittivity measurements on BC15TZ9.5 is shown
in Figure 5-6 as a typical example. Three significant peaks can be seen in the dielectric
loss curve at -6 ̊C, 25 ̊C and 82 ̊C and the corresponding inflection points can be clearly
located at 3.5 ̊C, 31 ̊C and 86 ̊C in the permittivity curve by using the tangent method
(indicated by the pink dash lines). These inflection points imply three phase transitions
within the investigated temperature range, which were found for all compositions.
Three corresponding phase transition points were located using the systematic approach
as introduced by Zhang et al. (2008). Similar results were found for the other
compositions.
Characterization of BCTZ
88
Figure 5-6 Permittivity and dielectric loss of BC15TZ9.5 on the function of temperature. The pink dash lines indicate how the transition temperatures were obtained by using the tangent method.
Measurement of the temperature dependent piezoelectric coefficient showed that d33
reached the highest values around the second transition point extracted from the
permittivity curve at approximately 30 ̊C (the result of BC15TZ9.5 is shown in Figure
5-7). Further increase in temperature caused d33 to decrease abruptly over a few degrees
and then to decrease further but at a slower rate.
Characterization of BCTZ
89
Figure 5-7 Temperature-dependent d33 curve on BC15TZ9.5.
5.3 Discussion
5.3.1 Phase transition
The phase transition points, depending on composition, are extracted from the
permittivity measurements and summarized in Figure 5-8. Three lines separate the
graph into four phases. According to the BaTiO3 phase diagram, from high temperature
to low temperature, these phases are currently accepted as cubic (P 3 mm), tetragonal
(P4mm), orthorhombic (Amm2) and rhombohedral (R3m) (Kwei et al., 1993). Keeble
et al. (2013) have conducted a high resolution temperature-dependent synchrotron study
and confirmed the existence of these four phases in the BCTZ system. As seen in Figure
5-8, the phase transition between tetragonal and orthorhombic phases lies within a small
range between 30 - 33 ̊C for all the compositions, which is consistent with the premise
Characterization of BCTZ
90
that all compositions would have a similar phase transition temperature near room
temperature.
The determination of the phase transition temperatures in the range from -10 ˚C to 50
˚C is confirmed by the results of XRD measurements. However, the results further show
that the phase transition is not completed at a single temperature but occurs over a broad
temperature region, which agrees with the findings in other studies (Ehmke et al., 2012b,
Bjørnetun Haugen et al., 2013, Keeble et al., 2013).
For temperatures above 35 ˚C, all the compositions exhibit tetragonal phase (space
group P4mm) as confirmed by the appearance of two peaks for the (200)pc reflection
(intensity ratio 1:2) and only one peak at the (222)pc position. However, when the
temperature decreases below 35 ˚C, the minimum between the two peaks of tetragonal
phase starts to increase, indicating the appearance of a second phase. It can be seen that
around 10 ˚C the XRD patterns show features of an orthorhombic phase (Amm2 space
group), which is known to give two peaks for the (200)pc (intensity ratio 2:1) and two
peaks for the (222)pc reflection (intensity ratio 1:1) (Kwei et al., 1993). Therefore, the
orthorhombic phase is identified as the second phase that starts to appear when cooling
down from 35 ˚C. The proportion of orthorhombic phase increases while the proportion
of tetragonal phase decreases with decreasing temperature. This gradual transition
region has been indicated in blue in Figure 5-8. When the temperature is further
decreased from 10 ˚C, a rhombohedral phase begins to appear adding to the
orthorhombic phase. This is reflected by the increase of intensity of the reflections at
45.25 and 83.6. Similar to the phase transition between tetragonal and orthorhombic
phases, the structure also gradually transfer from orthorhombic to rhombohedral phase,
as shown in green in Figure 5-8. However, the rhombohedral-orthorhombic phase
transition line is curved compared to the phase transition line between orthorhombic-
Characterization of BCTZ
91
tetragonal. For example, BC8TZ5.5 at -10 ˚C is still in the orthorhombic phase while
BC15.5TZ10 exhibits almost1 pure rhombohedral phase (R3m) showing only one peak for
the (200)pc reflection and two peaks for the (222)pc reflection (intensity 1:3). From
Figure 5-8, it is also notice that the phase transition line obtained by the tangent method
from the temperature dependent permittivity curve (Figure 5-6) tends to reflect the high-
temperature-end boundaries of the phase coexisting region.
Figure 5-8 Phase transition temperature for different compositions with increasing doping amount. The black points are the phase transition temperature obtained from the permittivity data. The orange stars are the temperatures that have been investigated with XRD measurement. The blue and green area are the phase mixture region according to the results of XRD measurement.
1 The slight asymmetry of (200)pc of BC15.5TZ10 implies that the phase transition has not finished completely. A small portion of residual orthorhombic phase reflection is still hidden under the shoulder of the peak.
Characterization of BCTZ
92
5.3.2 Piezoelectric performance
It is known that coexisting phases at the phase transition point ease dipole switching and
domain wall motion under electric field drive condition, thus increasing the
piezoelectric performance. This study shows that the piezoelectric performance is not
enhanced at one particular temperature (MPB), but over the whole phase coexisting
region. The Figure 5-7 shows that the d33 of the composition was able to maintain above
370 pm/V from 23 ˚C to 40 ˚C with a maximum of 475 pm/V at 30 ˚C. The coexistence
of two ferroelectric phases is a well-known phenomenon at morphotropic phase
boundaries (MPB) and for polymorphic phase transitions (PPT) enabling not only
switching of the polarization vector but also a rotation which leads to enhanced
piezoelectric properties in the two phase regime. The similarity of polarization rotation
for the composition-dependent MPB and the temperature-dependent PPT phase
transformations was analyzed by Damjanovic (2005) using a thermodynamic approach.
The reason for the phase coexistence near MPBs and PPTs is often related to
microstructural inhomogeneities due to local differences in chemistry. In the case of
PZT with a MPB it is shown that the coexistence region between the tetragonal and
rhombohedral phases becomes narrower for chemically more homogeneous materials
(Kakegawa et al., 1977). A strong influence of the local homogeneity on the sharpness
of the PPT has been demonstrated in modified KNN ceramics (Morozov et al., 2011).
This provides the possibility that the applied electric field can induce a transition from
one phase to the other leading to increased ferroelastic contributions. However, the
electric fields used to measure the piezoelectric coefficient are quite small and an
electric field induced phase change might not occur at these field levels. Another source
of the high piezoelectric contribution can be found in the prevalence of the
orthorhombic structure at room temperature. The existence of 9 ferroelastic directions
Characterization of BCTZ
93
(6 from the orthorhombic phase and 3 from the tetragonal phase) leads a higher number
of grains being oriented favourable to the electric field such that a close alignment of
the polarization vectors to the applied electric field can occur and thus, the piezoelectric
performance is improved.
As seen in Figure 5-8, the Curie point decreases with doping content while the phase
transition temperature between orthorhombic and rhombohedral phases increases. This
means that the temperature region where the orthorhombic phase is present appears
narrower for higher Ca2+ and Zr4+ content. According to the study of Tian et al. (2013b),
the orthorhombic phase region will become even narrower, if the doping content is
further increased and, theoretically, at a certain point, the two phase transitions (T-O
and O-R) would join such that three phases coexist. Also, the Curie point will decrease
with increasing doping level, and there seems to be a tendency to form a critical region
of four coexisting phases, similar to what was discussed for BC9.6TZ13.6 (Keeble et al.,
2013) and in the system BaTiO3-xBaSnO3 (Yao et al., 2012). However, in practice, this
point cannot be reached because of the limitation of solubility (McQuarrie and Behnke,
1954, Hennings and Schreinemacher, 1977) (as demonstrated in Figure 3-1) and Gibb’s
phase rule (Keeble et al., 2013). The region where two phase transitions are close was
termed a diffuse phase transition region by Tian et al. (2013b). As the two phase
transitions come closer, the threshold energy for transferring the orthorhombic phase
into either tetragonal or rhombohedral phase may be comparable, facilitating dipole
switching into more directions. As seen in Figure 5-5, the reflections of different phases
are over lapping within a small range of 2theta (0.2-0.32), indicating that the lattice
distortions (c/a) of these different phases are similar. This could explain the high
piezoelectric performance of BC15TZ9.5 close to the diffused phase transition region.
This may also explain why the orthorhombic phase is usually difficult to observe and
Characterization of BCTZ
94
refine in the XRD results at the higher substitution regions, while it was easily found in
the lower doping area (Hennings and Schreinemacher, 1977, Ravez et al., 1999a, Li et
al., 2010a, Li et al., 2010b, Zhang et al., 2010).
From Figure 5-3, it can be seen that BC12TZ8 is off the trend compared to other
compositions. The reason can be seen from the Figure 5-8, where BC12TZ8 has almost
finished phase transition to tetragonal phase at room temperature while other
compositions show significant phase coexistence. The material loses the benefit from
phase coexistence, resulting in the increase of coercive field and decrease of
polarization and d33.
5.3.3 Influence of Ca and Zr doping on orthorhombic-tetragonal phase
transition
Keeble et al. (2013) proposed a three dimensional phase diagram depicting the
distribution of the four phases depending on Ca-content, Zr-content and temperature. It
demonstrates that the transition temperature between tetragonal and rhombohedral
phases decreases with increasing Ca2+ content and decreasing Zr4+ content. The results
of our study related to the TT-O phase transition agree with this statement. Since high d33
values of the BCTZ compositions originate from the mixed phase region, the structural
information can be used to easily identify the region of high d33 compositions. To do so,
the dependence of the tetragonal-orthorhombic phase transition temperature upon Ca
and Zr content, including results from this study and previous studies (Tian et al., 2013b,
Mitsui and Westphal, 1961, Li et al., 2010b, Li et al., 2010a, Li et al., 2010c, Ravez et
al., 1999a, Pisitpipathsin et al., 2013), is summarized in a contour map (Figure 5-9).
This map assists identification of BCTZ compositions with optimum piezoelectric
properties at a desired operating temperature. It can be seen that Ca2+ doping reduces
Characterization of BCTZ
95
TT-O slightly while Zr4+ doping increases TT-O dramatically. The balancing ratio is
approximately 0.6 (Zr4+ : Ca2+). High d33 at the same temperature range can be obtained if the doping
amounts of Zr4+ and Ca2+ accord to this ratio.
Figure 5-9 Distribution of the tetragonal-orthorhombic phase transition over compositions in BCTZ system as a function of temperature and Ca2+ and Zr4+ content. Stars are the compositions from this study and data of black dots are from literature. (Ravez et al., 1999b, Mitsui and Westphal, 1961, Li et al., 2010a, Li et al., 2010b, Li et al., 2010c, Pisitpipathsin et al., 2013, Tian et al., 2013b)
It should be noted that the piezoelectric values reported in the literature vary
significantly from study to study, and there is an ongoing discussion regarding the
existence of the orthorhombic phase between the tetragonal and rhombohedral phase
(Kwei et al., 1993, Keeble et al., 2013, Bjørnetun Haugen et al., 2013). Although the
orthorhombic phase is observed in these studies, it is difficult to resolve the
orthorhombic phase quantitatively using Rietveld refinement due to the coexistence of
several phases and the small peak splitting. For other studies, the small region of
orthorhombic phase could often be missed due to limited temperature steps and
insufficient resolution of the measurement. Furthermore, the high sensitivity of BCTZ
Characterization of BCTZ
96
materials to processing and post-processing variations, as well as the different
methodologies used by different groups might have also influenced results. In the
current study, it was also noticed that significant structural differences were observed
depending on the distance to the as-sintered surface. This may be due to surface
diffusion and oxidation during the sintering process and it is certainly an important
factor to consider when attempting structural analyses on this material
5.4 Summary
The approach of producing compositions which all lie in the MPB region was
successfully implemented and all the compositions exhibit strong piezoelectric
behaviour at room temperature which was found to originate from co-existing phases.
The phase transition between orthorhombic and tetragonal phases seems to occur over a
broad temperature range with a median temperature at approximately 31 ̊C in all
investigated compositions. The co-existing phases improve the piezoelectric
performance at room temperature. Furthermore, it was found that the temperature range
in which the orthorhombic phase appears becomes narrower with increasing Ca2+ and
Zr4+ doping. Two phase transitions (T-O and O-R) become closer and may even overlap
at higher doping levels. Therefore, the piezoelectric coefficient tends to increase with
increasing doping amount.
Ageing Behaviour of BCTZ
97
Chapter 6 Ageing Behaviour of BCTZ
6.1 Introduction
Parts of this chapter has been submitted to the Journal of Applied Physics for
publication with the title “The ageing behaviour of poled BCTZ lead-free piezoelectric
ceramics”.
As mentioned in the literature review (2.3.1), ageing behaviour of piezoelectric
materials is usually defined as the change in properties over time. So far, most ageing
behaviour is explained by migration of the oxygen vacancies in the long- or short- range
as the ageing rate is usually proportional to the amount of acceptor-dopants (Lupascu et
al., 2006). Contrary to hard PZT, soft PZT does not exhibit significant ageing behaviour
and the polarization is well-developed as long as the maximum field is greater than the
coercive field (Kamel et al., 2007).
Since, the piezoelectric properties of BCTZ ceramics are similar to soft PZT (Xue et al.,
2011), it is not expected to exhibit ageing behaviour such as pinched or shifted
hysteresis loop for this material. However, during our measurements on BCTZ, a
significant development of a bias field over time was noticed for electrically poled
samples indicating that the ageing behaviour might still occur to this material. If it is
true, this behaviour must be taken into consideration as the ageing behaviour could
greatly influence the measured values. With this concern, the ageing behaviour of poled
BCTZ was systematically studied in this study prior to fatigue experiments. Four
samples were left in a negative remanent polarization state after a bipolar measurement
for 36 days. Detailed methodology can be found in 4.3. The result showed that the bias
field developed continually over this period of time and consists of a stable portion and
Ageing Behaviour of BCTZ
98
an unstable portion, respectively. Two mechanisms, migration of the charged defects
and reorientation of the defect dipoles, may explain these two contributions.
The composition used for the chapter and following two chapters is
Ba0.85Ca0.15Ti0.9Zr0.1O3 (BC15TZ10), which is close to the compositions BC15TZ9.5 and
BC15.5TZ10 characterized in the last chapter. A simple summary of characters of
BC15TZ10 is given in Table 6-1.
Table 6-1 Characters of BC15TZ10
Characters Values
Theoretical density 5.74 g/cm3
Measured density 5.45 g/cm3
Mean grain size 26.34 µm
Coercive field 146 V/mm
Spontaneous polarization 11.6 µC/cm2
d33 422 pm/V
d33* 1055 pm/V
6.2 Expe
Figur
Figure 6-1
The maxi
µC/cm2 an
decrease r
erimental
re 6-1 the ch
1(a) shows
imum and
nd 1 µC/cm
resulted fro
l Results
hange of (a) p
that the pol
positive rem
m2, respectiv
om the shift
polarization
larization de
manent pol
vely. It is w
t of the hys
n and (b) bia
ecreased ex
larization d
worthy to p
steresis loop
Agein
as field on th
xponentially
decreased b
point out tha
p along the
ng Behaviou
he function
y with the a
by approxim
hat the majo
e field axis
ur of BCTZ
99
of time
ageing time.
mately 0.25
ority of this
(bias field,
Z
9
.
5
s
,
Ageing Behaviour of BCTZ
100
). If this contribution is taken into account, the residual decrease on the
maximum polarization and the positive remanent polarization were found to be 0.1
µC/cm2 and 0.4 µC/cm2, respectively. The development of a negative bias field was the
most significant change induced during ageing. As shown in Figure 6-1(b), the bias
field was around 0 V/mm on the first day and quickly increased to 9 V/mm within the
first 24 hours. The development slowed down after that and continued to grow to 28
V/mm in the following 36 days. From Figure 6-1(b) a rapid increase in development
rate of the bias field is observed after day 22.
Figure 6-2(a) showed the change of the strain hysteresis loop between the positive and
negative field side after ageing. There was a small development of an asymmetry which
can mainly be contributed to the loss of the minimum strain on the positive field side
(S+min). As seen from Figure 6-2(b), the decrease of S+
min approximately followed the
exponential function within the error range. The reduction of S+min seemed to be quicker
in the earlier stage which is reflected in the increase of the strain asymmetry factor,
gamma (∆ ∆
∆ ∆), shown in Figure 6-2(c). The corresponding change was also
observed in the permittivity hysteresis loop. As shown in Figure 6-3, there was a loss of
permittivity especially at points A and B and the permittivity hysteresis loop becomes
slightly asymmetric after ageing. The permittivity values at high field (i.e. above 300
V/mm) appear to be independent of the ageing process.
Figure 6-2minimum the functio
After the
recovered
2 (a) the strastrain at the
on of time
e applicatio
. As seen in
ain hysteresie positive sid
on of 1000
n Figure 6-4
is loop beforde and (c) c
00 bipolar
4, the bias
re and after orrespondin
cycles, th
field decrea
Agein
ageing; (b) ng change of
he propertie
ased from 2
ng Behaviou
the decreasf strain asym
es were s
28 V/mm to
ur of BCTZ
101
se of the mmetry on
ignificantly
o 11 V/mm.
Z
y
Ageing Behaviour of BCTZ
102
Data fitting indicated that the recovery followed an exponential function except during
the first few cycles. The recovery in the early stage appears to be faster as the first
bipolar cycle could already reduce the bias field by 4 V/mm. The recovery slowed down
after approximately 100 cycles.
Figure 6-3 The change of permittivity hysteresis loop after ageing for 36 days.
Figure 6-4 The decrease of the bias field during the bipolar cycling.
The asym
bipolar cy
hysteresis
the positiv
that the st
at around
Figure 6-5correspond
mmetry of t
ycling (Fig
loop was r
ve side was
train asymm
±0.01 for h
5 (a) the recoding change
the strain h
gure 6-5).
recovered c
s restored to
metry was m
igh cycle nu
overy of the e of strain as
hysteresis l
As seen fr
completely a
o its origina
mostly recov
umber.
minimum ssymmetry d
loop was a
from Figure
after 10000
al value. Fr
vered withi
strain at theduring the bi
Agein
also recover
e 6-5(a), th
0 cycles as t
rom Figure
n the first 1
positive sidipolar cyclin
ng Behaviou
red signific
he asymme
the minimu
6-5(b), it c
10 cycles. γ
de and (b) ng.
ur of BCTZ
103
cantly with
etry of the
um strain at
can be seen
γs stabilized
Z
h
e
t
n
d
Ageing Behaviour of BCTZ
104
6.3 Discussion
Before discussing the ageing behaviour, it is worthwhile reviewing the basic
characteristics of Ba0.85Ca0.15Ti0.9Zr0.1O3 at room temperature. Three aspects related to
the ageing effect should be considered: (1) amount of defects, (2) phase coexistence and
(3) Curie point:
(1). The material has no purposely induced acceptor or donor dopants. Both Ca2+
and Zr4+ have the same valence to their alternative ions, Ba2+ and Ti4+, respectively. In
principle, these dopants do not introduce oxygen vacancies into the system. However,
defects can still exist. As e.g., free oxygen vacancies can be introduced by non-
equilibrium conditions during the sintering and cooling (Upadhyay et al., 2012) and
defect ions can also be introduced from impurities in the source powders. These defect
ions (i.e. acceptor ions in A- or B- sites) can couple with oxygen vacancies to form
various point defects, e.g. defect dipoles. The other un-bonded oxygen vacancies could
diffuse within the grain and compensate the unscreened local charges. It has been
reported that un-poled pure BT can also exhibit a pinched hysteresis loop after ageing
(Upadhyay et al., 2012, Dechakupt et al., 2010), although there exist contrary results
from other studies (Huang et al., 2014, Mitoseriu et al., 2001). For the samples
produced in our study, the total amount of these impurities calculated from the starting
chemicals should be less than 0.4%. Although this value (0.4%) includes iso-valence
ions, it was reported in literature (Morikawa and Ishizuka, 1987, Wu and Schulze, 1992)
that acceptor ions such as Na1+, Al3+ and Fe3+ are very likely to exist in the source
powder. Despite that fact that the possible amount of the acceptor ions is small,
Genenko (2008) reported that for acceptor-doped BaTiO3 materials, a concentration as
low as 0.01% can already introduce a significant ageing behaviour. The development of
the internal bias field in our study was not saturated after 3×106 seconds which agrees
Ageing Behaviour of BCTZ
105
with the simulation of Genenko for a low doping concentration doped material (up to
0.06%)(Genenko, 2008).
(2). The composition under investigation has a tetragonal and orthorhombic
phase coexisting at room temperature (Keeble et al., 2013, Zhang et al., 2014b). The co-
existence of two phases provides more possible dipole orientations. Additionally, the
small lattice distortion (c/a ratio) between the two phases indicates that it is possible that
one phase transforms into the other upon electric field application (Zhang et al., 2014b,
Keeble et al., 2013, Zhang et al., 2014a). These two contributions allow excellent dipole
alignment to the external field thus this material exhibits a large remanent polarization
and a high d33 despite the low spontaneous polarization.
(3). The Curie point of this material is only 90 ˚C (Li et al., 2011a) which is
close to room temperature. (Zhang et al., 2014b)
The characteristics discussed above will help to explain why no pinched polarization
hysteresis loop is observed for un-poled BCTZ ceramics even through a significant Ebias
develops in the poled state. First, there is no intentional acceptor-doping for this
material. Although defects might be introduced from the impurities in the source
material, the overall concentration should be low and the influence to the global
polarization can be too low to be noticed. Therefore, the ageing behaviour is less
pronounced than for acceptor-doped (e.g. Mn and Fe) BT (Huang et al., 2014, Zhao et
al., 2015). Second, the small c/a ratio at room temperature resulting from the phase
coexistence might influence the ageing behaviour. As it was observed that small lattice
distortion (c/a ratio) can significantly suppress the ageing behaviour (Huang et al.,
2014). The small c/a ratio implies a shallow energy well for dipole switching. Therefore,
the pinned dipoles can easily escape the captivity of the defect dipoles and switch with
Ageing Behaviour of BCTZ
106
the external field. This factor distinguishes BCTZ from un-doped BT (Upadhyay et al.,
2012, Dechakupt et al., 2010) and some iso-valence doped materials (e.g. BaSrTiZr
(Portelles et al., 2005) and Hf, Zr-doped BT (Mitoseriu et al., 2001)) as these materials
all have a single phase at room temperature. It also explains why the hysteresis loop can
be easily developed with just one bipolar cycle and is independent of the increasing
poling field (Su et al., 2011, Ehmke et al., 2013a, Li et al., 2013) or temperature (Li et
al., 2011a, Su et al., 2011) compared to the PZT (Granzow et al., 2006, Ehmke et al.,
2013b, Ehmke et al., 2013a, Li et al., 2011a). Third, it was reported for PZT that the
aged material can be easily de-aged with the application of just one bipolar electric
cycle at a temperature of one third of the Curie point temperature (Kamel and de With,
2008). This also supports the finding that the BCTZ material can be easily de-aged at
room temperature as the Curie point of this material is closer to room temperature
compared to the PZT family. Forth, the free oxygen vacancies is randomly distributed
in the ceramic at the un-poled state and does not affect the polarization during the
poling process.
For the poled state, the change of the bias field is the most pronounced change.
Although 28 V/mm over 36 days is not large in value, it is a mean value over the bulk
material. The local bias field can be much larger than this value and it affects the local
dipoles greatly as this material only has a coercive field as low as 150 V/mm. A fast
change in the early stage was found in both the aging and the de-aging curves (Figure
6-1(b) and Figure 6-4, respectively). The increasing development rate after 21 days in
Figure 6-1(b) is possibly due to the increase of the time interval between the
measurements as it was noticed that the first bipolar measurement can significantly
recover a large proportion of the bias field (Figure 6-6). Thus, the frequent
measurements recover parts of the bias field. At the same time, it is also evident in
Ageing Behaviour of BCTZ
107
Figure 6-6 that the recovery rate was significantly higher during the first bipolar cycle.
Additional evidence for the two recovery rates can be found in the change of S+min and
γs during the ageing and de-ageing process, though not as obvious as the change in bias
field. The two recovery rates indicate that two ageing contributions exist. One that is
unstable and can be easily restored by the external field, while the other is stable and
only slowly activated by the external field. These two contributions are proposed as the
volume effect and the grain boundary effect.
Figure 6-6 The influence of the bipolar measurement on the bias field.
Volume effect: When the external field is applied, the spontaneous dipoles tend
to align with the field direction. Even after the removal of the external field, most
dipoles still maintain the poled orientation. However, the reorientation of the defect
dipoles requires hopping of oxygen vacancy which has a slower response to the external
field than spontaneous dipoles. Therefore, when the poling effect is weak, e.g. one 10
Hz bipolar cycle in this study, the orientation of the defect dipoles may remain random
as in the annealed state. The randomly orientated defect dipoles can interact with the
Ageing Behaviour of BCTZ
108
general dipoles in two possible ways. The defect dipoles can switch the surrounding
dipoles back to the un-poled orientation or the defect dipoles tend to reorient to align
with the orientation of the remanent polarization. If the first case occurs, the global
remanent polarization would be significantly decreased and the hysteresis loop would
express as the pinched loop. However, such a pinched loop was not observed for BCTZ
materials. Also, the reversal switching of dipoles would increase the local stress and
increase the local energy which is less likely to happen during the energy relaxation
process. Therefore, the first case is not likely to occur. The second case would be more
reasonable as the local energy is minimized when defect dipoles couple with the
neighbouring dipoles aligning at the same direction. When a positive external field is
applied to the negative remanent state, the dipoles switch 180 degree to the positive
direction. However, the defect dipoles are not easy to reorient and those dipoles coupled
with the defect dipoles are also harder to be switched to the positive direction. When the
external field is reduced or reversed, the coupled dipoles tend to switch back to the
orientation of the defect dipole, i.e. in the negative direction. Therefore, globally, the
material requires a higher field to switch to the positive side but lower field to switch to
the negative side. The hysteresis loop shifts along the field axis towards the positive
field direction.
Grain boundary effect: Although the dipoles in the material tend to align
parallel to the negative field in an experimental study, there still exists a residual
depolarization field at the grain boundaries due to the mismatched orientation of the
grains. The unscreened depolarization field drives the free oxygen vacancies introduced
during sintering process to the grain boundaries. The accumulation of the oxygen
vacancies at the grain boundaries provide an internal bias field that stabilizes the
domain structure. When a positive external field is applied, these stabilized domains
Ageing Behaviour of BCTZ
109
become more difficult to switch to the positive direction. The bipolar cycling can
redistribute the oxygen vacancies and thus reduce the internal bias field.
It seems that the volume effect and the grain boundaries model lead to the same
phenomenon and it is challenging to distinguish their individual contributions. However,
the origin of these two effects requires distinct conditions. The volume effect only
requires the existence of defect dipoles within the material and applies to both un-poled
and poled state. The reorientation of these defect dipoles originate from hopping of
oxygen vacancies within the local unit cell which is a short-range effect and requires
low activation energy (Morozov and Damjanovic, 2008). The low lattice distortion
(Zhang et al., 2014b) and flat energy wells (Lohkämper et al., 1990), as present in the
phase transition temperature range of BCTZ, can further ease the reorientation of the
defect dipoles. However, it also weakens the force on the general dipoles so that trapped
dipoles can be easily released during a few bipolar cycles. Therefore, the volume effect
may contribute to the fast development rate in the ageing and de-ageing curve. The
grain boundary effect is the migration of the oxygen vacancies to the grain boundaries
which is a long-range effect, and only applies to poled state. It requires higher activation
energy and significantly depends on the grain size and the driving force (Lohkämper et
al., 1990) but is less influenced by phase coexistence. The development of accumulation
of the charge carriers at the grain boundaries is expected to be slower than the hopping
of oxygen vacancies within a unit cell. The accumulated charge carriers are also harder
to be redistributed by bipolar cycling with a moderate field (i.e. only 750 V/mm in this
study). Therefore, the grain boundary effect may contribute to the slow development
rate in the ageing and de-ageing curve.
The ratio of these two effects depends on the grain size and the concentration of these
two types of defects. In term of grain size, the grain size of the tested material is about
Ageing Behaviour of BCTZ
110
26 µm which is considered to be large (Kamel and de With, 2008). In this case, the
volume effect prevails and the grain boundary effect is limited as the unscreened
depolarization field influences only the subsurface area of the grains (Genenko, 2008).
The concentration of oxygen vacancies is likely to be much larger than that of defect
dipoles. However, the real concentration of oxygen vacancies still needs to be
determined experimentally (e.g. post annealing method) in future work Therefore, it is
difficult to determine which effect is dominating and both of them have significant
contribution in the ageing behaviour in this study.
In the permittivity hysteresis loop, the value of permittivity around 0 field (between
±0.5 Ec) is related to the 180-degree domain wall density and mobility while peaks at
±Ec are related to the non-180 degree domain wall density and mobility(Bar‐Chaim et
al., 1974). Therefore, the decrease of the permittivity at point A (in Figure 6-3) indicates
that 180 degree domain switching is suppressed when the external field is removed. The
asymmetry of Smin (Figure 6-2) and the permittivity at ±Ec (Figure 6-3) implies that the
non-180 degree domain switching is partially restricted when a positive field is applied.
Ehmke et al. investigated the poling effect on the BCTZ material and they proposed the
accumulation of the charge carriers at the grain boundaries driven by the unscreened
depolarization field (Ehmke et al., 2013a, Ehmke et al., 2013b) which is a similar model
to the grain boundary model of ageing behaviour. It is noticed that the asymmetry of the
non-180 degree domain switching we have observed in this study, is also observed in
the poling study. For the poling process, the non-180 degree domain switching in phase
with the poling field increases while the out-of-phase side does not change; for the
ageing process at the poled state, the non-180 degree domain switching in phase with
the remanent polarization remains while the out-of-phase side decreases. The poling
study (Ehmke et al., 2013b) suggests that the charge carriers trapped at grain and
Ageing Behaviour of BCTZ
111
domain boundaries suppress the out-of-phase non-180 degree domain switching and the
magnitude of the suppression strongly depends on the scale of migration which is
proportional to the scale of the poling field and poling time. A similar process could
occur during the ageing behaviour but with much smaller magnitude as the migration of
the charge carriers happens at the remanent state, where the driving force for charge
migration is much weaker than upon electric field application. Compared to the ageing
process, the poling process induces much larger change to the strain maxima and
introduced significant asymmetry to the 180-degree domain switching. The differences
may come from the distinct nature of the poling and ageing process where the poling
induces the energy to the material, generating an unstable domain structure while
ageing is the relaxation and stabilization of domain structure to minimize the energy in
the system. The charge migration under external field could be much faster and also
occur on a larger scale than the ageing process and lead to a strong suppression of the
side switching in poling direction. In contrast to that for the ageing process, the
relaxation and stabilization result in less 180 degree domain walls and less domain wall
mobility exhibiting as the decrease of permittivity at the low offset field region.
Furthermore, although this study is based on an assumption that the ceramic is
stoichiometric, local non-stoichiometry may exist due to chemical inhomogeneity. This
local non-stoichiometry may induce local defects, i.e. A-site or B-site vacancies.
However, the effect of A-site or B-site vacancies on ageing effect has not been
systematically reported, which may be interesting for the future work.
6.4 Summary
Although no pinched hysteresis loop is observed for un-poled BCTZ material, this study
reveals that changes in the piezoelectric properties especially the internal bias field
Ageing Behaviour of BCTZ
112
occur over time when the material is left at a remanent polarization state. The origin of
this development may be attributed to the reorientation of local defect dipoles and the
migration of the oxygen vacancies to the grain boundaries. Although both mechanisms
can introduce the same ageing phenomenon, the former model gives a fast contribution
to the ageing and de-ageing process while the later model explains a slow contribution.
The ageing behaviour of BCTZ could also occur in other homovalent doped
piezoelectric materials. It not only affects the scientific measurements but also induce
reliability and performance issues during the application.
Unipolar Fatigue Behaviour of BCTZ
113
Chapter 7 Unipolar Fatigue Behaviour of BCTZ
7.1 Introduction
Parts of the chapter have been submitted to the Journal of the American Ceramic
Society for publication with the title “Unipolar fatigue behaviour of BCTZ lead-free
piezoelectric ceramics”.
The small signal piezoelectric coefficient (d33) and large signal piezoelectric coefficient
(d33*) are two key properties for sensor or actuator applications. The d33 is the
piezoelectric response to a small electric or mechanical signal, while the d33* is usually
obtained from the unipolar loading up to several 100 or 1000 V where d33*=Smax/Emax. A
large d33* is desired for the actuator applications such as fuel-injector, nano-motor and
haptic technology (Rödel et al., 2015). Since BCTZ was reported to exhibit large d33*
(Liu and Ren, 2009), it has high potential to be used as a lead-free actuator replacing the
currently used PZT. However, it has been reported unipolar loading, i.e. poling, can
introduce a properties degradation and anisotropic hysteresis loops (Ehmke et al.,
2013a). With this concern, investigations of the unipolar fatigue behaviour of BCTZ
become important.
This chapter shows the effects of unipolar cycling on (Ba0.85Ca0.1)(Ti0.9Zr0.1)O3. The
changes in unipolar and bipolar piezoelectric response including the d33 and permittivity
are measured after subsequent unipolar cycling steps. Detailed experimental methods
can be found in 4.4.2.3. The results are compared with the unipolar fatigue studies on
PZT and poling studies on BCTZ.
Unipolar Fatigue Behaviour of BCTZ
114
7.2 Experimental Results
Figure 7-1 shows the results of unipolar measurements during the cycling. As seen in
the Figure 7-1(a) and (b), the maximum polarization decreased significantly to 70%
within the first 1000 cycles and did not change significantly afterwards. The unipolar
hysteresis also decreased with increasing number of cycles (Figure 7-1(a)). Figure 7-1(c)
and (d) show that the unipolar maximum strain decreased slightly to the vicinity of 91%
of the virgin cycle after the 1000 cycles and became stable at that level.
Figure 7-1 The changes of (a) unipolar polarization and (c) strain loops during the unipolar fatigue cycling at 10Hz, and their extracted maximum (b) polarization and (d)
strain values on the function of number of cycles
Figure 7-2 shows the change of piezoelectric properties of one selected sample before
and after fatigue and Figure 7-3 shows the change of the mean values obtained from
five samples with increasing cycle number. It can be seen from the hysteresis loops that
all loops shifted along the x-axis towards the negative field showing a development of a
Unipolar Fatigue Behaviour of BCTZ
115
positive bias field ( ) during the unipolar cycling. As shown in Figure
7-3(d), the bias field increased starting from 100 cycles and grew to approximately 26
V/mm after 5*106 cycles. Figure 7-2(c) also reveals a shift of d33 hysteresis loop
towards the positive y-axis direction at low field, i.e. -300 V/mm < E < +300 V/mm. It
can be seen in Figure 7-3(c) that d33 decreased slightly in the first 100 cycles then
started to increase with additional cycles. The unipolar fatigued samples also exhibited
a decrease in maximum and remanent polarization. It can be seen that the positive
remanent polarization did not change significantly (remains at 98%) after the unipolar
fatigue while the negative remanent polarization experienced a drastic decrease to 87%.
The measured strain hysteresis loops were normalized by shifting the crossing point of
the curve with the y-axis to the zero point of the axis and are presented in the Figure
7-2(b). It can be seen that the positive strain amplitude (ΔS+) increased slightly by the
unipolar cycling, while the negative strain amplitude (ΔS-) was greatly reduced. The
decrease of the normalised S-max with the number of cycles followed an exponential
function and approximately 45% of S-max was lost after 5×106 cycles. It was also noticed
that there were increases of minimum strain at both the positive and negative side with a
similar magnitude, Figure 7-2(b). Due to the large change of S-max, the strain hysteresis
loop became asymmetric and the change of asymmetric factor γ (∆ ∆
∆ ∆)
corresponded to the change of the S-max (Figure 7-3(b)). The field-dependent
permittivity loop also developed strong asymmetry. As seen in the Figure 7-2(d), peak
A and B are suppressed while the peak C and D are encouraged. The permittivity under
high offset field, i.e. E>300 V/mm or E<-400 V/mm, were not affected by the unipolar
cycling.
Unipolar Fatigue Behaviour of BCTZ
116
Figure 7-2 (a) polarization, (b) strain, (c) d33 and permittivity hysteresis loops before and after 5 million cycles of unipolar cycling at 10 Hz with Emax = 4 Ec
Figure 7-3 the changes of (a) polarization, (b) maximum strain at the negative side and strain asymmetry, (c) d33 and (d) bias field on the function of number of cycles during unipolar fatigue cycling at 10Hz and Emax = 4 Ec
Unipolar Fatigue Behaviour of BCTZ
117
Figure 7-4 shows the influence of bipolar cycles on one unipolar fatigued sample. As
seen from Figure 7-4(a), the negative remanent polarization was significantly increased
after 10000 bipolar cycles. The hysteresis loops were also slightly shifted towards the
un-fatigued position. Figure 7-4(b) shows that the fatigue-induced asymmetry in the
strain hysteresis loop was almost completely restored. Not only the decrease of the S-max
was recovered, but also the increase of minimum strain at both positive and negative
fields were restored to the un-fatigued values. As seen in Figure 7-4(c), the d33
hysteresis was also slightly shifted back towards the un-fatigued state but not fully
restored. Significant recovery of the permittivity asymmetry is shown in Figure 7-4(d)
where the permittivity on the negative side was significantly increased after the bipolar
cycling.
Figure 7-4 the recovery of (a) polarization, (b) strain, (c) d33 and (d) permittivity hysteresis loops by 10000 bipolar cycles on the 5 million cycles unipolar fatigued sample
Unipolar Fatigue Behaviour of BCTZ
118
Figure 7-5 The changes of (a) unipolar polarization and (b) strain, bipolar (c) polarization, (d) strain, (e) d33 and (f) permittivity hysteresis loops of unipolar fatigued sample after annealing at 400 °C
Unipolar Fatigue Behaviour of BCTZ
119
Figure 7-5 shows the recovery during the thermal annealing process on the unipolar and
bipolar measurement. All the measurements suggested that the unipolar induced fatigue
had been fully restored by the thermal annealing process. All the piezoelectric and
dielectric properties after thermal annealing were identical to the un-fatigued state, and
the unipolar maximum polarization and strain were even higher than the original state
(Figure 7-5(a) and (b)).
7.3 Discussion
As shown in the results, the unipolar cycling introduces significant changes to both
unipolar and bipolar measured parameters.
For unipolar cycling, a decrease for the maximum polarization and maximum strain is
observed in the first 1000 cycles, however, the values subsequently stabilize. In the
early stage of cycling both irreversible and reversible processes contribute to the
unipolar properties. With repeated unipolar loading, the polarization reaches better
alignment. The proportion of the irreversible contribution is reduced which can be seen
by the closing of the hysteresis in Figure 7-1. After 1000 cycles, the changes of
polarization and strain are mainly contributed by the reversible contribution and become
stable. It also reveals that unipolar cycling has a small effect on the reversible
contribution, which includes an intrinsic effect and a reversible extrinsic effect. This
high and stable d33* after a pre-cycling process is very attractive for the actuator
applications.
The major changes visible in the bipolar measured parameters can be summarized as the
development of bias field, decrease in polarization and increasing asymmetry in strain
and permittivity hysteresis loop (Figure 7-2). These changes are also commonly
reported from unipolar fatigue studies on other piezoelectric materials (Luo et al., 2011a,
Unipolar Fatigue Behaviour of BCTZ
120
Balke et al., 2007c, Verdier et al., 2002, Luo et al., 2011b). Figure 7-6 shows the
comparison of properties changes during unipolar cycling between this study and other
materials found in literature (Luo et al., 2011a, Luo et al., 2011b, Balke et al., 2007c,
Yao et al., 2013). The correlated testing parameters are shown in Figure 7-1. Although
the frequency used in this study is much lower than other studies, the unipolar fatigue
behaviour is less likely to be influenced if frequency can cause complete switching
(Balke et al., 2007c). As show in Figure 7-6, the magnitude of changes on BCTZ is
much smaller than for other lead-free compositions such as BNT-BT (Luo et al., 2011a)
and BNT-BT-KNN (Luo et al., 2011b), and is more similar to soft PZT (Balke et al.,
2007c). Both BCTZ and soft PZT exhibit a small reduction of the polarization during
unipolar cycling, suggesting that domain wall pinning is not effective for both of them
(Verdier et al., 2002). Therefore, a comparison with the unipolar fatigue behaviour of
PZT is made in the discussion.
Table 7-1 Testing parameters of unipolar fatigue cycling in different studies
Material Frequency (Hz) Normalized Emax Source
BNT-BT 50 2 Ec (Luo et al., 2011a)
PIC 151 50 2 Ec (Balke et al., 2007c)
BNT-BT-KNN 50 3 Ec (Luo et al., 2011b)
Modified KNN 50 2 Ec (Yao et al., 2013)
BCTZ 10 4 Ec
Unipolar Fatigue Behaviour of BCTZ
121
Figure 7-6 Changes of normalized remanent polarization and maximum strain in bipolar and unipolar measurements, respectively, for different materials.
Based on the studies of soft PZT (Balke et al., 2007c, Verdier et al., 2002), the
mechanism for unipolar fatigue is the diffusion and agglomeration of the charged
defects at the grain boundaries but not at domain walls. During unipolar loading, the
dipoles are driven to align antiparallel to the external field so that the depolarization
field of these dipoles can compensate the external field. However, due to the nature of
the piezoelectric material, the dipoles have limited switchable orientations that are
restricted by the prevalent crystallographic phase and the crystallographic axis of the
specific grain. For BCTZ materials at the phase transition near room temperature
dipoles have more available orientations leading to potentially better alignment with the
external field. But, perfect parallel alignment is still impossible for these polycrystalline
Unipolar Fatigue Behaviour of BCTZ
122
ceramics. The different crystallographic axis of neighbouring grains will result in an
unscreened depolarization field. This local unscreened depolarization field can be even
stronger than the external field (Lupascu, 2004) and affects the charged defects at the
vicinity of the grain boundaries. The affected charged carriers are driven to the grain
boundaries with time or increasing number of unipolar cycles. The grain boundaries act
as the sink for these charged defects to accumulate (Lupascu, 2004). The accumulated
charge carriers generate an internal bias field that clamps the domain structure more
than the external field direction. As a result, in the bipolar measurement, the hysteresis
loop shifts to the negative direction along the field axis as the reverse domain switching
is suppressed.
The approximately exponential development of the piezoelectric properties in Figure
7-3 which is also observed in other studies (Luo et al., 2011a, Balke et al., 2007c) can
also be explained by this mechanism. On the one hand, the unscreened depolarization
field should be the strongest leading to the fastest migration rate after the first few
cycles as it has not yet been compensated by the migrated charged defects. With an
increasing number of accumulated charge carriers it starts to decrease leading to a
coincident decrease in the driving force for charge migration. Therefore, this is likely
the reason for the exponential decline. The existence of this mechanism can also be
witnessed by the ageing behaviour on poled BCTZ material. It was observed that the
bias field evolved over time for poled BCTZ ceramics in Chapter 6 and that
accumulation of charged defects at the grain boundaries contributed to the ageing
mechanism. Therefore, the migration and accumulation of the charged defects is very
likely to be a decisive mechanism during unipolar fatigue as well.
The development of asymmetry in the bipolar strain hysteresis loop in Figure 7-2 is
usually associated with the development of an offset polarization π (Verdier et al.,
Unipolar Fatigue Behaviour of BCTZ
123
2003). According to modified Landau-Devonshire theory for polycrystalline
piezoelectric material, strain is produced by both switchable and offset polarizations as
(Lupascu, 2004). The offset polarization refers to the domains
that are entirely clamped and can only contribute to the intrinsic effect but not the
extrinsic effect. Although the offset polarization cannot be revealed in the polarization
hysteresis which is a relative measurement, it can be viewed through the d33 hysteresis.
As seen from Figure 7-2(c), the shift of the d33 hysteresis along the d33 axis is evident to
a development of offset polarization. The increase of the d33+ in Figure 7-3(d) after 100
cycles is mainly contributed by the increasing offset polarization.
The clamping of domains is also revealed in the permittivity loop in Figure 7-2(d). In
the permittivity loop the peaks B and C are contributed by the first non-180 degree
domain switching, while the peak A and D are contributed by the second non-180
degree domain switching and 180 degree domain switching (Bar‐Chaim et al., 1974).
The difference between 180 degree domain switching and non-180 degree domain
switching originated from different types of domain walls, i.e. 180 degree domain wall
and non-180 degree domain wall. For domains separated by non-180 degree to reverse
their direction, they have to go through two non-180 degree rotations, and thus
contribute two peaks to the permittivity loop when the external field is reversed. In
Figure 7-2(d), it can be seen that the second non-180 degree domain rotation on the
negative side is strongly supressed (peak A). Due to this suppression, less non-180
degree domains switch to align parallel to the negative external field. Therefore, less
back-switching also occurs when the external field reverses leading to the increase to
the peak C. The clamped domains may be responsible for the development of
asymmetry. A similar phenomenon is also observed in the poling study on BCTZ
(Ehmke et al., 2013a, Ehmke et al., 2013b).
Unipolar Fatigue Behaviour of BCTZ
124
Generally, the poling and unipolar fatigue loading result in similar effects as both
loading methods apply the electric field in only one direction (Balke et al., 2007c).
Comparing the results from the poling study conducted by Ehmke et al. with the current
unipolar fatigue study, one can notice that the poling study induces much stronger bias
field, offset-polarization and asymmetry than the unipolar fatigue. Although the
calculated effective field during unipolar fatigue in this current study is actually 32.5
times of the one used in the poling study, the maximum field used in the unipolar
fatigue study is much less than the one used in the poling study (5 Ec in the fatigue
study compared to 18.1 Ec in the poling study). This suggests that the magnitude of the
maximum field may play a more important role than the effective field. This is likely
due to the large maximum field inducing large dipole displacement which may increase
the depolarization field and also larger local stress. The increased depolarization field
increase the migration range of the oxygen vacancies leading to more accumulation at
the grain boundaries. The local stress may clamp the domains and hinder the domain
wall motions when the reversed field is applied.
Another possible contribution to the unipolar fatigue could be the local defect dipoles
which are the coupled dipole between oxygen vacancies and acceptor impurities (Carl
and Härdtl, 1977). The reorientation of these defect dipole requires the hopping of the
oxygen vacancies within the unit cell, thus is of slower response compared to the
normal spontaneous dipoles. Once the orientation of these defect dipoles is stabilized by
the unipolar cycling, the defect dipoles cannot be easily reversed during bipolar
measurements. Furthermore, the defect dipoles affect the surrounding unit cells. It is
harder for the neighbouring dipoles coupled to the defect dipoles to be activated by the
external electric field compared to the free dipoles. The existence of defect dipoles
would definitely contribute to the development of the bias field and asymmetry of
Unipolar Fatigue Behaviour of BCTZ
125
hysteresis loops but the contribution is considered to be much smaller than the charge
carrier accumulation at grain boundaries as this material is not intentionally doped with
acceptor dopants.
Piezoelectric properties degraded from unipolar fatigue cycling often can be restored by
the applications of a few hundred bipolar cycles (Balke et al., 2007c, Verdier et al.,
2003). During the bipolar cycling process, the non-switchable dipoles affected by the
defect dipoles should be released quickly. The charge carriers at the grain boundaries
can be also re-distributed, leading to a decrease of the local bias field (Balke et al.,
2007c). As BCTZ is a “soft” piezoelectric ceramic which is sensitive to electric field, it
is expected the unipolar cycling induced bias field is not stable and can be quickly
decreased during bipolar measurements. Although the unipolar fatigue induced
asymmetries, bias field and offset polarization exhibit reduction during bipolar cycling
in this study, the recovery rate is much slower than expected when compared with PZT
(Balke et al., 2007c). Figure 7-4 shows that changes introduced by unipolar cycling are
much more stable than expected and only approximately half the loss of the properties
is restored after 10000 bipolar cycles. Compared to the unipolar fatigued PZT (Balke et
al., 2007c), the recovery rate of the bipolar cycling on BCTZ is much slower. It seems
that bipolar cycling cannot redistribute the charges accumulated at the grain boundaries
effectively; however, the exact reason is still unclear. As shown in the Figure 7-5, the
thermal annealing process at 400 ˚C restored all the properties back to the original state.
The thermal process provides the charged carriers more energy and also reduces the
migration barriers by transforming all unit cells into cubic phase. The re-distribution of
the charged carriers can occur on a large scale and thus the properties after thermal
annealing appears as the un-fatigued state.
Unipolar Fatigue Behaviour of BCTZ
126
7.4 Summary
The unipolar fatigue study on BCTZ material reveals that this material can have a stable
unipolar performance after a pre-cycling stage, which is appealing for actuator
applications. Significant development of bias field, offset polarization and asymmetry in
strain and permittivity loops were observed during bipolar measurements on unipolar
fatigued samples. However, the loss of the piezoelectric properties is much smaller than
some other investigated lead-free piezoelectric materials and the unipolar fatigue
behaviour of BCTZ was found to be similar to the unipolar fatigue behaviour of soft
PZT material. These developments mainly originate from migration of the charged
carriers accumulating at grain boundaries. The unipolar fatigued material can be
completed restored to the original state by thermal annealing which re-distributes the
accumulated charged carriers.
Bipolar Fatigue Behaviour of BCTZ
127
Chapter 8 Bipolar Fatigue Behaviour of BCTZ
8.1 Introduction
Parts of this chapter have been submitted to the Journal of the American Ceramic
Society for publication with the title “High Bipolar Fatigue resistance of BCTZ Lead-
free piezoelectric ceramics”.
The bipolar fatigue can greatly hinder materials from being transferred into application
and commercialization. Although, the bipolar fatigue behaviour of piezoelectric
ceramics in general has been intensively studied, the bipolar fatigue behaviour of BCTZ
has not been reported in the literature. It was shown in that last chapter that BCTZ can
be greatly influenced by unipolar cycling. It is often reported that the bipolar cycling
can introduce more severe properties degradation and mechanical damage than the
unipolar cycling (Luo et al., 2011b), which leads to a doubt to the reliability of BCTZ
for bipolar application Therefore, it is important to investigate the bipolar fatigue
behaviour of BCTZ.
This chapter shows the bipolar fatigue behaviour of BCTZ and the role of Ca and Zr
dopants in the fatigue behaviour. Two BCTZ compositions were tested upon 10 Hz
bipolar cycling: Ba0.92Ca0.08Ti0.945Zr0.055O3 (BC8TZ5.5) and Ba0.85Ca0.15Ti0.9Zr0.1O3
(BC15TZ10). Detailed methodology can be found in 4.4.2.2. This study shows that the
BCTZ materials degrade with increasing number of cycles but have higher fatigue
resistance than soft PZT and the addition of the Ca and Zr elements improves the
bipolar fatigue resistance.
Bipolar Fatigue Behaviour of BCTZ
128
8.2 Experimental results
Figure 8-1 shows the changes of the piezoelectric hysteresis loops of one selected
BC8TZ5.5 sample before bipolar fatigue and after 106 and 8.5*106 bipolar cycles. After
8.5*106 cycles, there were slight decreases for the maximum and remanent polarizations
(Figure 8-1(a)) and also developments of the asymmetry in the strain (Figure 8-1(b))
and permittivity (Figure 8-1(f)) hysteresis loops. Figure 8-1(c) shows that the peaks in
the current hysteresis loop were broadened and moved towards higher electric field. As
shown in Figure 8-1(e), the d33 value was also decrease from approximate 500 pm/V to
400 pm/V. In addition, there was a slight increase in the dielectric loss (Figure 8-1(d))
and a decrease in permittivity (Figure 8-1(f)). Figure 8-2 shows the hysteresis loops of
piezoelectric properties for one selected BC15TZ10 sample before and after 106 and 107
bipolar cycles. The major changes were similar to the BC8TZ5.5, such as decrease of
polarization, broadening of current peak, asymmetry of strain hysteresis and decrease of
d33. However, the BC15TZ10 did not exhibit asymmetry in the permittivity hysteresis
loop after bipolar cycling. Instead, there were a large increase of dielectric loss and a
large decrease permittivity, which were significantly different from BC8TZ5.5.
Figure 8-3 shows the statistical changes of extracted values from the hysteresis loops
over all five tested samples for both compositions. Despite the increasing standard
deviation (i.e. error bar) highlighting a significant sample dependency of the fatigue
process, the general trends (i.e. mean values) are consistent with the hysteresis loops
shown in Figure 8-1 and Figure 8-2. The differences in fatigue behaviour between
BC8TZ5.5 and BC15TZ10 can be seen more clearly in Figure 8-3. BC8TZ5.5 suffered 17%
more remanent polarization loss after 105 cycles but was able maintain 15% less loss of
d33 and 7% less increase of coercive field ( ) compared to BC15TZ10.
Bipolar Fatigue Behaviour of BCTZ
129
Development of strain asymmetry factor γ (where ∆ ∆
∆ ∆) was much larger in
BC8TZ5.5 (0.24) than BC15TZ10 (0.09).
Figure 8-1 The changes of (a) polarization, (b) strain, (c) current, (d) dielectric loss, (e) d33 and (f) permittivity hysteresis loops of one selected BC8TZ5.5 sample during bipolar fatigue cycling at 10 Hz with Emax = 3Ec
After removal of the electrode on fatigued samples, both compositions exhibited
discolouration on the surface, i.e. uniform dark marks and dark areas as shown in Figure
8-4(a). BC8TZ5.5 samples had much more severe discolouration than BC15TZ10. Under
SEM, damaged surface was usually observed in the centre of the dark area in BC8TZ5.5,
as illustrated in Figure 8-4(b). For the cross-section and subsurface observations,
damaged layer near electrode which was often reported for PZT was not found for both
compositions. However, some micro-cracks were found on the cross-section of the
BC8TZ5.5 sample. As seen from the Figure 8-4(c), the domain size near these micro-
cracks were much smaller than the domains in the unaffected grains.
Bipolar Fatigue Behaviour of BCTZ
130
Figure 8-2 The changes of (a) polarization, (b) strain, (c) current, (d) dielectric loss, (e) d33 and (f) permittivity hysteresis loops of one selected BC15TZ10 sample during bipolar fatigue cycling at 10 Hz with Emax = 3Ec
Figure 8-3 The changes of mean values with standard deviations of (a) remanent polarization, (b) coercive field, (c) d33 and (d) strain asymmetry factor of both compositions during on the function of number of bipolar fatigue cycles
Bipolar Fatigue Behaviour of BCTZ
131
Figure 8-4 (a) Photo of an unfatigued sample and a bipolar fatigued sample; (b) the damaged material on the bipolar fatigued BC8TZ5.5 surface where discolouration appears; (b) the crack observed on the subsurface leading to a minimization of the domain size
Bipolar Fatigue Behaviour of BCTZ
132
The Figure 8-5 and Figure 8-6 show the hysteresis loops after thermal annealing
compared to the unfatigued and fatigued hysteresis loops for one selected BC8TZ5.5 and
BC15TZ10 samples, respectively. For both compositions, the most of the loss in
piezoelectric properties were restored to the original state. However, the loss in d33 for
the BC8TZ5.5 was not recoverable for all annealed samples. After thermal treatment, the
discolouration of the BC15TZ10 disappeared while the darkened areas were still
observed on the surface of BC8TZ5.5.
The conductivity of BC8TZ5.5, BC15TZ10 and soft PZT are 3.66, 1.93 and 1.19 *10-11
S/m respectively.
Figure 8-5 The changes of (a) polarization, (b) strain, (c) current, (d) dielectric loss, (e) d33 and (f) permittivity hysteresis loops of one bipolar fatigued BC8TZ5.5 sample after annealing at 400 °C for 10 mins
Bipolar Fatigue Behaviour of BCTZ
133
Figure 8-6 The changes of (a) polarization, (b) strain, (c) current, (d) dielectric loss, (e) d33 and (f) permittivity hysteresis loops of one bipolar fatigued BC15TZ10 sample after annealing at 400 °C for 10 mins.
8.3 Discussion
8.3.1 Comparison to other materials
In Figure 8-7 a comparison of the loss of 2Pr on the function of bipolar cycles is made
between the BCTZ ceramics and other piezoelectric materials reported in the literature
(Balke et al., 2007a, Nuffer et al., 2000, Pojprapai and Glaum, 2012, Luo et al., 2011b,
Simons et al., 2012). It can be seen that the lead-based materials, even including the
commercial PIC 151, degrade at fewer electric field cycles compared to BCTZ. The
other two lead-free piezoelectric materials shown also exhibit inferior fatigue resistance
compared to the BCTZ materials of the present study. It is worthy to point out that the
bipolar fatigue behaviour strongly depends on the testing parameters, i.e. maximum
field and cycling frequency. The magnitude of the maximum field is the most effective
Bipolar Fatigue Behaviour of BCTZ
134
factor in the bipolar fatigue process as the domain wall motions are directly driven by
the external field. It has been found that the material experiences most severe fatigue
degradation when dipoles are fully switching by the external field during every field
reversal, where Emax >> Ec (Nuffer et al., 2002, Nuffer et al., 2000, Zhao et al., 2008,
Shieh et al., 2006). Frequency is another factor that influences the bipolar fatigue
behaviour as lower frequency allows more time for the dipoles to switch with the
external field and usually leads to an earlier degradation (i.e. lower cycle number) than
electric cycling with higher frequency (Simons et al., 2012, Zhang et al., 2001, Luo et
al., 2011a). The testing parameters of the studies in Figure 8-7 are given in Table 8-1. It
can be seen that not only the maximum field (normalized by the coercive field) used in
the present study is the highest, but also the frequency is the lowest. This indicates that
BCTZ experiences less degradation on the switchable polarization during bipolar
electric cycling and may, based on these findings, outperform even commercial PZT.
Hence, BCTZ can be considered to have high fatigue resistance to bipolar electric
cycling.
Bipolar Fatigue Behaviour of BCTZ
135
Figure 8-7 The loss of normalized remanent polarization on the function of number of bipolar cycles for different piezoelectric materials
Table 8-1. The testing parameters used in the bipolar fatigue studies
Materials Normalized Emax Frequency (Hz) Source PZT + 1% La 2 Ec 50 (Balke et al., 2007a)
PIC 151 2 Ec 50 (Nuffer et al., 2000)
PZT 1.2 Ec 50 (Pojprapai and Glaum, 2012)
93BNT-6BT-1KNN 3 Ec 50 (Luo et al., 2011b)
94BNT-6BT 1.5 Ec 10 (Simons et al., 2012)
BC8TZ5.5 3 Ec 10
BC15TZ10 3 Ec 10
8.3.2 Fatigue resistance
The high fatigue resistance may be due to several reasons:
(1). The competition between domain wall pinning and unpinning effects. Domain
wall pinning is a major reason for the loss of piezoelectric properties during bipolar
Bipolar Fatigue Behaviour of BCTZ
136
cycling (Glaum and Hoffman, 2014, Lupascu, 2004). Defects in materials, such as ionic
defects, free oxygen vacancies and defect dipoles (oxygen vacancies bonded by the
ionic defects), can re-orient driven by the unscreened depolarization field and the
external field. Domain boundaries act as sinks for free defects to accumulate. The
accumulated defects reduce the domain wall motion resulting in the decrease of
switchable polarization. Domain wall pinning is also very likely the main reason for the
fatigue behaviour of the BCTZ materials observed in this study as expressed by the loss
of polarization, increase of coercive field, loss of permittivity and loss of d33. The
thermal annealing process redistributes the trapped defects and restores the degraded
properties back to the original state. However, the degree of degradation of BCTZ
during bipolar fatigue is much smaller compared to other piezoelectric materials (Figure
8-7) indicating that the domain wall pinning effect is weak. The weak domain wall
pinning could originate from two aspects: a low defect concentration and an unpinning
process. As the BCTZ material does not have intentional acceptor dopants, the only
source for the ionic defects are the impurities in the starting chemicals. The total
concentration of the impurities in this study is approximately 0.4% and the possible
proportion for acceptor elements should be even less than this. Free oxygen vacancies
induced during sintering at high temperature are possible major defects that pin the
domain walls (Patterson and Cann, 2012). The trap depth for the defects at domain
walls depends on the type of charges and the magnitude of spontaneous polarization
(Al-Shareef et al., 1996). The increase of the spontaneous polarization raises the
activation energy needed for de-trapping the defects (Dontzova et al., 1989). The
spontaneous polarization of BCTZ is considerably lower than in PZT and BNT-BT
materials, indicating shallow trapping wells at the domain boundaries. As such, the
trapped oxygen vacancies are likely to be unpinned by the external field (Al-Shareef et
Bipolar Fatigue Behaviour of BCTZ
137
al., 1996, Dimos et al., 1996) and domain wall pinning and unpinning can occur at the
same time during bipolar cycling leading to the slow degradation rate observed.
Furthermore, it is noted that the BCTZ has a higher conductivity than both the soft PZT
which implies that pinning charge carrier is harder in the BCTZ than the PZT.
(2). The small local stress. Mechanical damage is often one of the main reasons for the
degradation of piezoelectric properties for bulk ceramics in bipolar cyclic loading. If the
external field is high enough to induce polarization switching it can induce crack
growth under the electrodes (Nuffer et al., 2002, Lupascu et al., 2000). Macro-cracks
are usually initiated by the rise of local stresses due to strain mismatch between
switchable and un-switchable or partially switchable grains, especially close to the
electrodes (Shvartsman et al., 2005b, Nuffer et al., 2002, Luo et al., 2011a). These
cracks propagate during the continuous field reversal due to the stress concentration at
the crack tip (Zhu and Yang, 1998). The un-switchable or partially switchable grains
can be caused by inhomogeneous electrode covering, pinning effects (Nuffer et al.,
2002) or orientation to the applied field (Jones et al., 2005). The development of macro-
cracks greatly reduces the remanent polarization (Lupascu et al., 2000, Nuffer et al.,
2002). Also, a development of a damaged layer under the electrode is reported for PZT-
based ceramics (Balke et al., 2007a, Luo et al., 2012). The damaged layer is found to
have different dielectric properties from the bulk material and screens the external field
leading to a reduction of the piezoelectric response. The origin of this damaged layer is
also thought to be the development of micro-cracks along the grain boundaries due to
the rise of mechanical stress during repeated domain switching. However, for our BCTZ
compositions a third factor has to be taken into account when discussing the origin of
the fatigue behaviour. Our previous study has shown that both compositions under
investigation experience a gradual phase transition around room temperature (Zhang et
Bipolar Fatigue Behaviour of BCTZ
138
al., 2014b). In the transition temperature region, the material exhibits excellent
piezoelectric response indicating that domain walls are highly mobile. The material has
a higher number of possible dipole orientations originating from the two co-existing
phases and small lattice distortion ratio (c/a) (Zhang et al., 2014b). This results in good
dipole alignment and response to the external field implying small local stresses during
the domain switching process. Additionally, the weak pinning effect limits the build-up
of internal stresses between grains and reduces as such the chance of developing macro-
cracks. Therefore, no mechanical damaged layer and macro-crack growth were
observed in the subsurface and cross-section examination.
8.3.3 Ca and Zr contents
The results also suggest that the fatigue endurance can be further improved if the Ca
and Zr contents are optimized. Although BC15TZ10 seemingly experiences less
polarization loss during bipolar cycling than BC8TZ5.5, it may suffer from more severe
domain wall pinning.
The first evidence for this hypothesis can be found in the increase of the coercive field,
which indicates that domain wall switching gets suppressed and requires in average
more activation energy from the external field to take place. The peaks in the current
hysteresis loop are induced during the switching of domains. The broadening of these
peaks implies that domains are pinned at different degrees, requiring different activation
energies for them to switch. From the results, it can be seen that BC15TZ10 exhibits a
larger increase of the coercive field and the dipoles are harder to be switched after
bipolar fatigue compared to BC8TZ5.5.
The second evidence can be found in the small signal measurement which measures the
piezoelectric response to small voltage vibrations at different offset fields in this study.
Bipolar Fatigue Behaviour of BCTZ
139
The strong degradation of d33 and permittivity, adding to the significant increase of
dielectric loss for BC15TZ10 shows that a large amount of dipoles are not able to be
activated by the small signal after bipolar fatigue. In contrast, the change of small signal
response is much smaller for BC8TZ5.5. Therefore, the domain wall pinning effect is
strengthened if more Ca and Zr dopants are induced. The doping of Ca and Zr induces
local point lattice distortions to the barium titanate matrix, which can trap the space
charges and act as pinning centres. It means that BC15TZ10 may have more pinning
centres than BC8TZ5.5 due to the higher level of doping.
8.3.4 Micro-cracking
In addition to the pinning effect, the development of micro-cracks observed on the
cross-section (Figure 8-4(c)) may cause further loss of polarization. As no such micro-
cracking is found for BC15TZ10, it may explain why BC8TZ5.5 appears to have more
polarization loss compared to BC15TZ10. The distinct differences in the development of
micro-cracks for these two compositions is possibly originated from the different pore
morphology. For BC8TZ5.5, the size of the pores is found to have large deviation. As
shown in Figure 8-8(a), large pores (~100 µm), medium pores (~10 µm) and small
pores (~1 µm) can all be found on the surface. More importantly, the large pores and
some medium pores are often of irregular shape. Stress concentration during the dipole
reorientation is likely to occur at the sharp edges of the irregular pores and may initiate
micro-cracks. However, for BC15TZ10, it is observed that the pores on the sample
surface are round and of similar size, approximately 10 µm in diameter, as shown in
Figure 8-8(b).
Bipolar Fatigue Behaviour of BCTZ
140
Figure 8-8 the surface image of (a) BC8TZ5.5 and (b) BC15TZ10 under micro-scope before fatigue
The presence of the micro-cracks reduces the size of domains in their vicinity as shown
in Figure 8-4(c). This can be associated with the presence of large local stresses around
crack tips that force the macroscopic domains to split into fine ferroelastic twins for
stress relieve (Shvartsman et al., 2005b, Shvartsman et al., 2005a, Zhu and Yang, 1998).
The formation of these ferroelastic domains reduces the number of dipoles aligned with
the poling direction and as such reduces the polarization. The progression of the micro-
cracking process can even lead to delamination of material, which for the materials in
our study is often observed in the centre of the discoloured areas (Figure 8-4(b)). As
these micro-cracks cannot be recovered during the thermal annealing at 400 ˚C (Verdier
Bipolar Fatigue Behaviour of BCTZ
141
et al., 2004), it explains the residual proportion of the piezoelectric properties in
BC8TZ5.5, especially the d33. The small vibration signal is not high enough to override
the effect of the local stresses, therefore, the loss of d33 associated with micro-cracking
cannot be restored by the annealing process.
Although both compositions exhibit surface discolouration after bipolar fatigue, their
origins may be different. For BC15TZ10, no micro-cracks are found on the investigated
surface, subsurface and cross-section. The discolouration is likely due to change of local
conductivity as charge carriers that were originally bound to deep trapping centres
become mobile during the bipolar cycling and migrate into less deep traps (Lupascu,
2004). The discolouration disappeared as the thermal treatment redistributed these
trapped charge carriers again. However, for BC8TZ5.5 micro-cracking appears to be the
major contribution to the discolouration even though migration of charge carriers
certainly occurs as well (Lupascu, 2004, Patterson and Cann, 2012, Zhang et al., 2005).
As the micro-cracks are not healed during the thermal treatment, the discolouration
remains after annealing.
8.3.5 Anisotropy in BC8TZ5.5
Development of the anisotropy in strain hysteresis and field dependent dielectric
measurement during bipolar fatigue is often explained by offset polarization (Nuffer et
al., 2000). The offset polarization should be observed in the d33 hysteresis loop as a shift
of the hysteresis along the y-axis. However, no significant shift was found in the d33
hysteresis for BC8TZ5.5. Therefore, offset polarization may not be an effective factor for
bipolar fatigue in this case. Instead, the formation of micro-cracking may be responsible
for the development of the bias field and asymmetric hysteresis loops (strain and
permittivity) for BC8TZ5.5. This might occur, when the formation of cracks prevails on
one side of the sample (Lupascu, 2004), which is the case for the BC8TZ5.5 samples as
Bipolar Fatigue Behaviour of BCTZ
142
they exhibit the very large and connect pores shown in Figure 8-8(a) only on one side of
the samples. However, the asymmetries have developed in the same way for all samples.
If the anisotropic distribution of pores is the only reason for the asymmetry
development, it would require that the sides with the large pores were connected to the
same electric pole for all samples. However, as this factor was not intentionally
controlled during the experiment it is not likely that all samples were connected the
same way regarding to their pore distribution.
The consistent development of hysteresis asymmetries indicates that it is related to the
polarity of the electric field upon first application. When the material is in the un-poled
state, all the domains are randomly orientated as a result of thermal relaxation. When
the first bipolar cycle is applied, the first half cycle breaks the relaxed state and
restructures the domain morphology. During the first time of polarization switching,
stresses around the pores develop, especially at those sharp corners of irregular pores
where cracks are most likely to be initiated. These stresses will initiate the formation of
twin domains which aligns with the direction of the first applied external field. When
the field is reversed to opposite, these twin domains may be clamped by the stress and
cannot reorientation completely. This could be the reason for the anisotropic
development in the measured permittivity data (Figure 8-1(f)). The permittivity peaks at
2 Ec are associated with non-180 degree domain wall density and mobility (Bar‐
Chaim et al., 1974). After the first bipolar cycle, the peak at the positive side is already
slightly higher than the negative side. With the increase of the number of bipolar cycles,
the asymmetry between these peaks is further increased.
Bipolar Fatigue Behaviour of BCTZ
143
8.4 Summary
The bipolar fatigue behaviour of two BCTZ compositions has been investigated in this
study. Both compositions exhibited higher bipolar fatigue resistance at room
temperature compared to PZT-based and BNT-BT-based materials. The high fatigue
resistance was benefitted from the small local stress and weak pinning effect at the
phase transition region. BC8TZ5.5 is found to experience less domain wall pinning than
BC15TZ10. However, the pore morphology can affect the fatigue behaviour. Pores of
irregular shape are likely to initiate micro-cracks. The propagation of the micro-cracks
darkens the surface colour, reduces switchable polarization and even induces surface
delamination. The parameters used in the material fabrication process in this study are
optimised for BC15TZ10 but can still be improved for BC8TZ5.5. Therefore, the irregular
pores may be reduced by optimisation of fabrication parameters and the fatigue
resistance of this material may be further improved.
The high fatigue resistance grants lead-free BCTZ material another credit towards
replacement of lead-based piezoelectric materials in the room temperature range.
However, specific fatigue behaviour during a real application is still associated with the
working conditions and requirements, such as contact pressure, cycling frequency and
cycle numbers. It is worth to point out that the piezoelectric properties started to
decrease significantly after 106 cycles and had no sign of saturation at the maximum
number of cycles investigated in the current study. The bipolar fatigue behaviour at the
high number of cycles is beyond the scope of the current study but might also be
interesting for application and commercialization.
Discussion
144
Chapter 9 Discussion
The piezoelectric properties of (Ba,Ca)(Zr,Ti)O3 ceramics with compositions that show
phase coexistence at room temperature have been presented. Eight compositions with
increasing doping amount of Ca and Zr were fabricated and characterized. The ageing
behaviour, unipolar and bipolar fatigue behaviour were investigated for a representative
composition, BC15TZ10. The influence of Ca and Zr on the bipolar fatigue was also
investigated by applying the same bipolar cycling conditions to the lower doped
composition, BC8TZ5.5.
Detailed experimental results and discussion has been presented in each of the previous
four chapters. This chapter will overview the main findings based on the hypotheses
posited in chapter 3:
I. The piezoelectric properties of the BCTZ are similar to a soft PZT but are
more sensitive to temperature and mechanical pressure.
This hypothesis has been confirmed. All compositions investigated in this thesis
have high piezoelectric coefficients comparable to the soft PZT. The high
piezoelectric coefficient originates from multiple phase coexistence around room
temperature. The coexistence of phases provides the dipoles more switchable
orientations, leading to an easier polarization alignment to the external field.
However, contrary to the soft PZT, the piezoelectric coefficient of BCTZ strongly
depends on temperature. However, it is found in this study that the morphotropic
phase boundary is not located at a single temperature but exists over a broad phase
transition region where the phase gradually transforms from one to the other.
Although the proportions of these two phases vary in this region, the piezoelectric
Discussion
145
coefficient benefits from the existence of the second phase. The material can
maintain a high piezoelectric property within a small change of temperature. This
feature slightly reduces the temperature limitation of BCTZ for applications, though
a stable temperature is still preferred for best performance.
The BCTZ materials are also sensitive to the mechanical pressure. It is found that
the piezoelectric coefficient keeps decreasing if measured with a d33 meter which
applies mechanical vibrations to measure the generated electric voltage. In the
piezoelectric measurements conducted in the present study, the contact force is
carefully adjusted to avoid large pressure on the samples as otherwise the
piezoelectric coefficient may be reduced. Thus, BCTZ ceramics are preferred to be
used in pressure-free conditions - an issue that should be taken into consideration
when choosing a material for a specific application.
II. The phase transition temperature can be adjusted by changing the doping
amounts of Ca and Zr. The compositions with phase transition at room
temperature can be predicted based on previous studies in the literature.
All the compositions fabricated in this study have an excellent agreement with the
prediction. All of them show phase coexistence between orthorhombic and
tetragonal phases at room temperature. It is found that the Ca:Zr doping ratio of
approximately 1.5:1 does not change the temperature of phase transition between
tetragonal and orthorhombic phases. Increased doping amount of Ca and Zr
according to this ratio decreases the Curie temperature and increases the phase
transition temperature region between orthorhombic and rhombohedral phases. Due
to the phase coexistence, all the compositions exhibit similar piezoelectric
properties. Additionally, the polarization and strain hysteresis loops are well
Discussion
146
developed for all compositions. The results provide confidence to predict the phase
transition temperatures for other un-investigated compositions in the BCTZ system.
The selection of potential suitable compositions for applications at different working
temperatures is broadened and the time wasted on fabricating unwanted
compositions can be avoided. However, the pore morphologies of different
compositions indicate the optimum condition for fabrication changes with the
doping amount. The fabrication parameter for new compositions may still require
further investigation in order to achieve the best performance.
III. BCTZ ceramics should not exhibit ageing behaviour, similar to the soft
PZT, as it is not intentionally acceptor doped.
BCTZ has no intentional acceptor doping and un-poled samples can be easily poled
with a bipolar or unipolar cycle. Therefore, it is usually considered that this material
does not age, similar to the soft PZT. However, during the measurements, it is
observed that poled BCTZ develops a bias field over time. Through a systematic
study, it is found that the BCTZ material does exhibit ageing effects and that the
defects within the material are responsible. Two types of defects are taken into
consideration, i.e. point ionic defect dipoles and mobile oxygen vacancies. The
former is induced by the impurities within the starting powders and the latter is
generated during the sintering process at high temperature.
The ageing behaviour at the un-poled state is dominated by the point defect dipoles.
In the un-poled state, the dipoles are randomly oriented and the defect dipoles align
with neighbouring dipoles as the result of thermal relaxation over time. The
reorientation of the defect dipoles is associated with hopping of oxygen vacancies
and is slower than the reorientation of the dipoles. During the application of the
Discussion
147
external field, the neighbouring dipoles bound by the defect dipoles may not be able
to switch and exhibit ageing behaviour. However, the concentration of these point
defects is low as the total percentage of impurities is less than 0.4% and the portion
of the accepter elements shall be even lower than this value. Although these defect
dipoles may restrict the neighbouring dipoles and prevent them from switching with
an external field during the first a few cycles, the influence to the total polarization
is be too low to be noticed.
However, the characteristics for poled BCTZ are different. In the poled BCTZ, the
dipoles are not randomly oriented but aligned to the direction of the last applied
external field. However, the orientation of dipoles is limited by the crystallographic
axes of grains. Dipoles usually cannot perfectly align with the field direction. The
depolarization field of different grains generated by the dipoles cannot be mutually
compensated. The unscreened depolarization field acts as a driving force for the
diffusion of free oxygen vacancies to the grain boundaries. The accumulation of the
oxygen vacancies at the grain boundaries over time develops a local field across the
neighbouring grains. During the bipolar measurement, the external field is
compensated by the local field and a bias field is exhibited in the hysteresis loops.
The ageing behaviour not only influences the laboratory measurements, but also
may influence the piezoelectric performance in applications. If poled soft PZT also
shows similar ageing behaviour as BCTZ, the magnitude of the bias field is two
orders of magnitude less than its large coercive field and is usually not noticed.
However, as BCTZ has a small coercive field, the change of the bias field
introduces a reduction in polarization. Although the decrease of polarization and
strain observed in the study are small and may not cause a significant issue for
application, the bias field might significantly increase on a longer time scale as its
Discussion
148
development was not saturated for the time length investigated. Therefore, the
ageing behaviour is worthwhile taken into consideration during applications.
IV. BCTZ ceramics may experience severe unipolar fatigue.
The unipolar fatigue behaviour of BCTZ is found to be comparable to the poling
behaviour of BCTZ, which supports the hypothesis. The migration of free oxygen
vacancies, which are introduced in the sintering process, also plays an important
role for the unipolar polar fatigue behaviour. With the application of external field,
the polarization is increased and the unscreened depolarization field at grain
boundaries can be much stronger than the remanent state. Therefore, more free
oxygen vacancies are drawn to the grain boundaries compared to the ageing
behaviour of the poled state, leading to more pronounced development of bias field
and asymmetry.
The unipolar fatigue behaviour of BCTZ is slightly more severe in percentage
compared to the soft PZT. This is mainly due to its small coercive field. The local
bias field developed at the grain boundaries is more likely to be higher than the
coercive field and have a strong pinning effect on the domains within the
neighbouring grains. Nevertheless, the strain unipolar response does not fatigue and
is able to be maintained at a high level after a few cycles. Thus, the BCTZ exhibits a
high potential to replace soft PZT for some actuator applications.
V. Piezoelectric property degradation is expected to be more severe in bipolar
cycling than unipolar cycling. BCTZ ceramics are expected to have similar
bipolar fatigue behaviour to soft PZT. Mechanical damage and domain wall
pinning are the major reasons for the loss of piezoelectric properties.
Discussion
149
As expected, bipolar cycling does introduce greater piezoelectric property
degradation than unipolar cycling. However, it is found that BCTZ ceramics have
better bipolar fatigue resistance than other piezoelectric materials including
commercial soft PZT. Mechanical damage under the electrode, such as macro-
cracks and a damaged layer, which are usually observed for bipolar fatigued soft
PZT, are not observed in BCTZ. Lack of mechanical damage appears to be a reason
for higher fatigue resistance.
Domain wall pinning should be responsible for the loss of piezoelectric properties
during fatigue. Similar to the reasons for ageing and unipolar behaviour, the free
oxygen vacancies introduced during the sintering process dominate the bipolar
fatigue mechanism. Instead of being dragged to grain boundaries by the one
directional force in the scenario of unipolar cycling, the oxygen vacancies are more
likely to be trapped by charged domain wall boundaries appearing during the
repeated field reversal in the bipolar cycling. The accumulation of defects at domain
wall boundaries has a strong influence on the neighbouring domains. These domains
become more difficult to switch with an external field, leading to an increase of the
coercive field. For small signal measurement, these affected domains may not
respond to small signal vibrations leading to a decrease in piezoelectric coefficient
and permittivity.
However, due to the low lattice distortion and low spontaneous polarization, the
pinning effect appears to be weak in this material. Domain wall pinning and
unpinning may happen at the same time during the bipolar cycling. This could be
another reason adding to the lack of near-electrode mechanical damage leading to
less bipolar fatigue degradation compared to soft PZT.
Discussion
150
In terms of application, the high bipolar fatigue resistance certainly demonstrates
BCTZ higher potential than other lead-free piezoelectric materials. However, it still
depends on the specific application requirements
VI. Doping of Ca and Zr increases the lattice distortion and may increase
piezoelectric property degradation during bipolar cycling.
It is observed in this study that the small signal piezoelectric response changes more
significantly due to bipolar fatigue in BC15TZ10 than in BC8TZ5.5. This can be
explained by the higher lattice distortion which makes it more likely to trap defects
at the domain walls. Although it is also observed that BC15TZ10 experiences less
reduction of polarization during bipolar cycling than BC8TZ5.5, the loss of
polarization for BC8TZ5.5 may come from the development of micro-cracks. The
micro-cracks originated from the large and irregular pores in the BC8TZ5.5 where
local stress can concentrate at the sharp corner of irregularly shaped pores.. The
origin of the different pore morphologies in different compositions is not clear. It is
possible that the pore morphology of BC8TZ5.5 can be improved by optimizing the
sintering parameters leading to even better bipolar fatigue performance. In this sense,
the lower doped composition might be more attractive for bipolar application as it
has comparable piezoelectric properties to the higher doped composition but with
higher Curie temperature and fatigue resistance.
Based upon the above, it can be seen that the phase coexistence at room temperature
and the free oxygen vacancies introduced during production play important roles in the
piezoelectric behaviour of BCTZ. Any changes to these two conditions may result in a
significant change in the piezoelectric and fatigue behaviour.
Discussion
151
If the operating temperature rises or decreases and the material becomes single phase,
not only the piezoelectric properties will decrease significantly, the fatigue resistance
will also drop. The domain wall pinning will be more severe and the local stress will
also increase, leading to macro-cracking. Pure BT is a good example of a single phase
material at room temperature. Low piezoelectric response and strong fatigue
degradation has been reported in the literature.
Oxygen vacancies are considered to be the major source of defects, which develop into
two types: (1) oxygen vacancies coupled with acceptor ions becoming defect dipoles
and (2) un-bonded oxygen vacancies that can diffuse within the grain. All four
behaviours (ageing in the un-poled or poled state, fatigue with uni- or bi-polar cycles)
are associated with these two types of dipoles. Table 9-1 summaries that the roles of
two types of defects at these four scenarios. If the free oxygen vacancies can be reduced
by using improved sintering procedures, the fatigue resistance could be further
improved.
Table 9-1 The effects of defect dipoles and oxygen vacancies in different scenarios for BCTZ
Scenario Defect dipole Free Oxygen vacancies Effect
Ageing
Un-poled
● Align: local dipoles ● Weak effect
● Compensate depolarization field ● At local domain boundaries ● No global effect
None
Poled
● Align: Pr
● Fast ● Unstable
● Compensate unscreened depolarization field ● At grain boundaries ● Slow ● Stable
Moderate
Fatigue
Unipolar
● Align: Eexternal
● Unstable ● Driven by unscreened depolarization field ● At grain boundaries ● Stable
Strong
Bipolar
● No alignment ● Pinning domain walls ● At domain boundaries ● Easily unpinned by electric field
Weak
Discussion
152
After all, the studies of BCTZ materials show that it not only has high piezoelectric
response, but also high reliability. As such, a high potential for BCTZ to replace soft
PZT on certain applications is proposed.
Conclusions
153
Chapter 10 Conclusions
In this thesis, the piezoelectric, dielectric, ageing and fatigue behaviours of BCTZ
compositions demonstration phase coexistence at room temperature were investigated.
The studies show that phase coexistence at room temperature plays an important role in
all these materials’ piezoelectric behaviour. A deep understanding of how Ca and Ba
doping affects the phase transition temperature and how BCTZ behaves when subjected
to unipolar and bipolar electrical loading is also provided.
From these studies, it can be concluded that:
1. All the compositions fabricated in these studies have phase coexistence at room
temperature.
2. The temperature of this phase coexistence region can be adjusted by modifying
the doping amounts of Ca and Zr. The study provides a method to confidently
predict the phase transition temperature for different compositions in the BCTZ
system and may save time in identifying high performance compositions for
applications at certain working temperature.
3. The co-existing phases improve the piezoelectric performance. The phase
transitions of orthorhombic-tetragonal and orthorhombic-rhombohedral occur
over broad temperature ranges.
4. The temperature range in which the orthorhombic phase appears becomes
narrower with increasing Ca2+ and Zr4+ doping. Two phase transitions (T-O and
O-R) become closer and may even overlap at higher doping levels. Therefore,
the piezoelectric coefficient tends to increase with doping amount.
5. Changes in the piezoelectric properties, especially the internal bias field, occur
over time when the material is left at a remanent polarization state. The origin of
Conclusions
154
this development may be attributed to the reorientation of local defect dipoles
and the migration of the oxygen vacancies to the grain boundaries. Although
both mechanisms can introduce the same ageing phenomenon, the former model
gives a fast contribution to the ageing and de-ageing process while the later
model explains a slow contribution.
6. BCTZ can have a stable unipolar performance after a pre-cycling stage, which is
appealing for actuator applications.
7. Significant development of bias field, offset polarization and asymmetry in
strain and permittivity loops occur during bipolar property measurement of the
unipolar fatigued samples. However, the loss of the piezoelectric properties is
much smaller than for some other lead-free piezoelectric materials and the
unipolar fatigue behaviour of BCTZ was found to be similar to the unipolar
fatigue behaviour of soft PZT material. These developments primarily originate
from the migration of charged carriers accumulating at grain boundaries.
8. Both BC8TZ5.5 and BC15TZ10 compositions exhibit higher bipolar fatigue
resistance at room temperature compared to PZT-based and BNT-BT-based
materials. The high fatigue resistance originates from the small local stress and
weak pinning effect at the phase transition region.
9. As BC8TZ5.5 and BC15TZ10 are the compositions with the lowest and one of the
highest doping levels, respectively, in the current study, it can be predicted that
the compositions with medium doping amount also have high fatigue resistance
to bipolar loading. BC8TZ5.5 is found to have less domain wall pinning than
BC15TZ10, indicating that the fatigue resistance may decrease with increasing
doping amount which induces more lattice defects.
Conclusions
155
10. The pore morphology can affect the fatigue behaviour. Pores of irregular shape
are likely to initiate micro-cracks. The propagation of the micro-cracks darkens
the surface colour, reduces switchable polarization and even induces surface
delamination. The fatigue resistance of this material may be further improved by
optimisation of fabrication parameters.
Reference
156
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Appendix A - Fatigue Controlling Programming (LabView Graphical Code)
165
APPENDIX A - FATIGUE CONTROLLING PROGRAMMING
(LABVIEW GRAPHICAL CODE)
Form the implement of self-built multipole sample fatigue setup, a program needed to
control the waveform applied on samples and the fatigue time length. This program
should also be able to send instructions to waveform generate and terminate experiment
if target time is achieved or an abnormal signal in the circuit is detected, e.g. sample
break down. If the experiment failed in any circumstance, the program should also
suggest the reason of ceasing and record the timestamp which tell the number of fatigue
cycles that has been applied.
As LabView is often used to communicate with machines, it has been chosen to be used
in this setup. The LabView language is a graphical based programming language thus
only the figure of the program can be provided. The user interface is called front panel
which has been shown in Figure 4-8, and the programming code is located in the block
diagram which is shown in this section. The basic logic of the program for bipolar
fatigue test will be introduced in this section. Although there are a few differences
between programs for bipolar and unipolar cycling, the basic ideas are the same. The
major differences will be mentioned when introducing the bipolar fatigue controller.
Appendix A - Fatigue Controlling Programming (LabView Graphical Code)
166
In the initial stage, the program collect all the user input data, such as waveform shape,
frequency, amplitude, DC offset and target number of cycles. The program then
calculates the time length of the experiment in second as target number of cycles
divided by frequency and displays to users in hour/minute/second format. After
receiving confirmation from users, the program sends the information, such as
waveform shape, frequency, calculated amplitude (i.e. amplitude divided by the amplify
factor of high voltage source which is 2000 in the case of Trek 20/20c), DC offset (0 for
bipolar, half of calculated amplitude for unipolar), to waveform generator. The burst
model of waveform generator is then activated which waits and starts to output the
signal to high voltage source when the waveform passes 0 phase (i.e. voltage is 0 V). If
the burst model is not used, there is a great chance to have the first output signal higher
than the coercive field. As there is a sudden application of large field, the switching of
polarization in extremely short time would draw a strong current in the circuit and leads
to triggering of Trek 20/20c which has an interlock at 20mA.
Appendix A - Fatigue Controlling Programming (LabView Graphical Code)
167
After enabling the output of the waveform generator, the program starts to record the
time elapsed and displays in the front panel. The experiment is considered to be finished
when elapsed equates or greater than the target time. At the same time, the program also
retrieve the data from the oscilloscope and record it in a dat file. It then quickly analyses
the data by comparing the maximum voltage in the data to the desired amplitude to see
if there is abnormal signals, i.e. voltage is unreasonably high or low. A high voltage
may imply occurrence of arcing while a low voltage may imply short circuit. Both the
threshold values can be adjusted in the beginning of the test. If any abnormal signal is
detected, the program will terminate the experiment and displays error light adding to a
message stating the reason of stopping. Users can also trace the waveform and obtain
the last timestamp from the recorded data file. The code for cutting off for unipolar
loading is very different in this part as there is no negative voltage.
The program can still be improved as there are still a few defects in it.