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 

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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

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K in width.

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ZT-BCT sy

Ti0.93Zr0.07Oz et al., 1

ure Review

31

ral phase at

nt, the TEM

evertheless,

could have

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composition

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et al., 2013)

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O3 (Hennings1999a); (c),

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impurities

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so reported

°C, as seen

ure Review

32

The phase

roles to the

used in the

ng the most

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purpose of

on. There is

lt and react.

on rate and

einemacher

al. (2011)

om 1000 to

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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

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al., 2011a),

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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

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ric and fatig

ure Review

45

nnot have a

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46

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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

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n0.5Ti0.5)O3-

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Na0.5)TiO3 w

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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

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was studied

significant

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2012)

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resulting fr

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V/cm and 10ing number

ure Review

49

rom the low

ain increase

0 Hz for (a)r of cycles

w

9

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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

the image

number o

calculated

The avera

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4.2.3 D

N4.2.3.1

The neutr

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Smithfield

experimen

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ond suspen

were firstly i

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1250 ̊C in

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62

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63

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s (i.e. phase

fferent phas

(with cubic

n Figure 4-

parameter

nsitions, the

included in

pc and (200he temperaColours repe signified lo

e fit with a

fraction test

heta degree

e transition)

ses, as seen

c phase) w

3. Although

r at certain

e experime

the later re

0)pc reflectioature rangeresents inte

ow intensity.

Meth

a model wi

, the selecti

e for a com

. However,

n in Figure 4

were the the

h it was inte

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

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Agein

as field on th

xponentially

decreased b

point out tha

p along the

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he function

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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.

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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.

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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

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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

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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

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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

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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).

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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.

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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.

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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

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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.

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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)

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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.