RSC Advances

PAPER

Publ

ishe

d on

24

Nov

embe

r 20

14. D

ownl

oade

d by

Ric

e U

nive

rsity

on

10/1

2/20

14 1

7:37

:19.

View Article OnlineView Journal | View Issue

Crystallization an

aDepartment of Physics, Indian Institute of

Varanasi-221005, India. E-mail: yadav.av11bDepartment of Physics, University of Luckn

Cite this: RSC Adv., 2015, 5, 2819

Received 26th September 2014Accepted 21st November 2014

DOI: 10.1039/c4ra11301b

www.rsc.org/advances

This journal is © The Royal Society of C

d dielectric properties of Fe2O3

doped barium strontium titanate borosilicate glass

Avadhesh Kumar Yadav,*a C. R. Gautamb and Prabhakar Singha

An attempt has been made to prepare barium strontium titanate borosilicate glasses in the system,

64[(Ba1�xSrx)$TiO3]–30[2SiO2$B2O3]–5[K2O]–1[Fe2O3] (0.4 # x # 1.0), using the conventional

melt-quench method. The prepared glasses were characterized by differential thermal analysis, X-ray

diffraction, scanning electron microscopy and impedance spectroscopy techniques. On the basis of the

DTA results, the glasses were crystallized via regulated heat treatment process. The activation energy for

crystallization was 818 � 1.6 kJ mol�1. A high dielectric constant of about 107 070 was obtained for the

6 hour heat treated glass ceramic sample. Fe2O3 plays an important role in enhancing crystallization,

dielectric constant and retardation of dielectric loss in the samples.

1. Introduction

Numerous studies of borosilicate glasses are important invarious elds such as optoelectronics, solar energy technology,astronomical reecting telescope and in micro-electromechanical systems.1 The high density and high refrac-tive index of these glasses make them suitable for shieldingmaterials and laboratory glassware.2 The glasses, generally,contain both network formers and modiers. The alkali andalkaline earth oxides are mainly used to modify the networks ofglasses. The silicate network has higher chemical and thermalstability than the borate (B2O3) network.3 Alkaline earth oxideslike strontium oxide (SrO) and barium oxide (BaO) improve theglass forming ability and provide good optical properties. Theaverage molar refraction, molar polarizability, electronicpolarizability of oxide ions and optical basicity increase with thereplacement of SrO in the place of BaO.4 The doping of Fe2O3 inborosilicate glasses plays an important role in the networkmodication of a triborate unit into a tetraborate unit.5

The crystallization process is widely used in many materialsto crystallize glassy or amorphous states. In the crystallizationprocess, nucleating agents play important roles in initiating andcontrolling the process of crystallization.6 The controlled crys-tallization of glasses converts the glass into glass ceramics,which offers advantages over conventional ceramics such asner crystallites and lower levels of porosity.7 The crystallizationkinetics and nucleation mechanism of BaxSr1�xTiO3–Al2O3–

SiO2,8 BaO–SrO–Nb2O5–B2O3–SiO2–CeO2,9 21BaO–21SrO–18Nb2O5–30SiO2–10B2O3,10 (1 � y)(Ba0.70Sr0.30)TiO3:y(BaO:Al2-O3:2SiO2),11 (1 � y)(Ba0.70Sr0.30)TiO3–y(B2O3–xSiO2),12 and

Technology (Banaras Hindu University),

@gmail.com

ow, Lucknow-226007, India

hemistry 2015

(Ba0.6Sr0.4)TiO3]–[2SiO2–B2O3]–[K2O]–La2O3 (ref. 13) have beenreported. CeO2 in BaO–SrO–Nb2O5–B2O3–SiO2 reduces theactivation energy of crystallization, whereas La2O3 increases theactivation energy.

The recent demand to increase the energy storage capabilityand reliability of capacitors is necessary for future applicationprospects. In recent years, signicant efforts have been made todevelop high constant dielectric materials, which are capable ofa high energy storage density.14,15 Among all glass ceramicmaterials investigated, barium strontium titanate (BST) glassceramics has been found to be a promising candidate for highenergy storage.16 BST glass ceramics are currently fabricatedmainly in presence of borosilicate and aluminosilicate networkforming systems with certain dopants such as CeO2, La2O3 andNb2O5.17 To the best of our knowledge, no one has attempted tostudy crystallization kinetics and dielectric behaviour of Fe2O3

doped BST borosilicate glass. In the present article, we reportthe crystallization and dielectric properties of BST borosilicateglass in the glass system, 64[(Ba1�xSrx)TiO3]–30[2SiO2–B2O3]–5[K2O]–1[Fe2O3].

2. Experimental procedure

Barium strontium titanate borosilicate glasses were prepared infour basic compositions by varying the Ba/Sr ratio using BaCO3

(99%), SrCO3 (99%), TiO2 (99%), SiO2 (99.5%), H3BO3 (99.8%),K2CO3 (99.9%) and Fe2O3 (99.9%) Himedia, India. An appro-priate amount of ingredients were melted at 1200 �C into a high-grade alumina crucible in the open air using a programmableelectric furnace. Themelt was quenched at room temperature inan aluminium mould and immediately transferred to a pre-heated programmable muffle furnace for annealing at 450 �Cfor 3 hours to remove the internal residual stresses produceddue to the temperature gradient during rapid cooling.

RSC Adv., 2015, 5, 2819–2826 | 2819

Fig. 1 (a) X-ray diffraction pattern and (b) SEM image of the glasssample BST5K1F0.4.

Fig. 2 DTA patterns of the glass sample BST5K1F0.4 for heating rates5, 10, 15 and 20 �C min�1.

RSC Advances Paper

Publ

ishe

d on

24

Nov

embe

r 20

14. D

ownl

oade

d by

Ric

e U

nive

rsity

on

10/1

2/20

14 1

7:37

:19.

View Article Online

Differential thermal analysis (DTA) was carried out on thecoarse powdered glass samples at heating rates of 5, 10, 15 and20 �C min�1. On the basis of the DTA results, the glass sampleswere subjected to regulated heat treatment schedules for crys-tallization. X-ray diffraction (XRD) of the glass and glass ceramicsamples was carried out on a Rigaku Miniex-II X-ray diffrac-tometer using Cu-Ka radiation. SEM images of well-polishedglass and glass ceramic samples were taken by a scanningelectron microscope (model LEO 430 Camridge InstrumentsLtd. U. K.). The dielectric/impedance spectroscopic measure-ments were taken usingWayne Kerr 6500 P (high frequency LCRMeter, Frequency: 20–5 MHz) over the temperature range of 0 to150 �C of both side electrode glass ceramic samples over thefrequency range from 20 Hz to 100 kHz.

3. Results and discussion

The nomenclature of the prepared glass samples was as follows:BST5K1F0.4, where BST refers to (Ba, Sr)TiO3 borosilicate, 5 Kdesignates 5% of K2O, F indicates Fe2O3 and 0.4 represents thecontent of SrO. The nomenclature of the glass ceramic samplesfollowed the parent glass sample code, e.g. BST5K1F0.4T844.Here, the letter T refers to the soaking time of crystallization of 3hours and the last three number digits denote the crystalliza-tion temperature of the glass. The nomenclature of the BSTborosilicate glass and glass ceramic samples are listed inTable 1.

3.1. Structure and morphology of glass

The XRD pattern of a glass sample BST5K1F0.4 is shown inFig. 1(a). Only single diffusive broad peak was observed in thepattern. This shows the short-range ordering of the borate andsilicate network structures in the glass, which is characteristicof an amorphous phase. This result was conrmed by the SEMimage in Fig. 1(b) by absence any crystallites on the surface ofthe glass sample. No grain or grain-boundary was detected inthe SEM image.

3.2. Crystallization kinetics

The crystallization kinetics of BST borosilicate glasses werestudied by DTA. The DTA pattern of the glass sampleBST5K1F0.4 is shown in Fig. 2. The pattern indicates theevolution of the crystallization exotherm dependence on theheating rate. The crystallization temperature, Tcry, of glassincreased with increasing heating rate (4) of DTA. The intensityof the peak also increased with increasing heating rate. This

Table 1 Nomenclature of the glass and glass ceramic samples

Composition (x) Glass sample code Holding time (ho

0.4 BST5K1F0.4 36

0.6 BST5K1F0.6 60.8 BST5K1F0.8 61.0 ST5K1F1.0 6

2820 | RSC Adv., 2015, 5, 2819–2826

indicates the activation controlled crystallization of glassymatrix, and the heating rate and crystallization temperaturesare listed in Table 2.

The method reported by Kissinger18 was used to determinethe kinetic parameters associated with the crystallization of theBST phase. The Kissinger equation is given as follows:

urs) Holding temp. (�C) Glass ceramic sample code

844 BST5K1F0.4T844844 BST5K1F0.4S844878 BST5K1F0.6S878860 BST5K1F0.8S860872 ST5K1F1.0S872

This journal is © The Royal Society of Chemistry 2015

Table 2 Heating rate, crystallization temperatures (Tcry) and crystalgrowth dimension of glass sample BST5K1F0.4

4 Tcry (K) DTFWHM n

5 1117 23.92 1.3610 1121 19.71 1.6315 1125 20.40 1.5620 1131 16.18 1.96

Paper RSC Advances

Publ

ishe

d on

24

Nov

embe

r 20

14. D

ownl

oade

d by

Ric

e U

nive

rsity

on

10/1

2/20

14 1

7:37

:19.

View Article Online

ln4

Tcry2¼ � Ea

R

1

Tcry

þ lnA

Ea=R(1)

where 4 is the heating rate, Ea is the activation energy of crys-tallization, R is the universal gas constant, A is the pre-exponential frequency factor and Tcry is the crystallizationtemperature. The slope of this plot between ln(4/Tcry

2) and1/Tcry (Fig. 3) gives �Ea/R. The activation energy, Ea, for crys-tallization of BST was determined from the slope and was foundto be 818� 1.6 kJ mol�1. This value is higher than that reportedfor BST borosilicate glass (705 kJ mol�1)12 and BST borosilicateglass doped with 1 mol% of La2O3 (762 kJ mol�1).13 This resultindicates the sluggish crystallization of the glassy matrix. Theobserved activation energy was also higher than the dissocia-tion energies of the B–O bonds (515 kJ mol�1)19 and Si–O bonds(424 kJ mol�1).8 Therefore, the networks of borate and silicatewere modied by the addition of 1 mol% Fe2O3. Thus, Fe2O3

acts as a network modier and nucleating agent forcrystallization.

The possibility of the diffusion of ions, such as Ba2+ (175 kJmol�1), Sr2+ (165–255 kJ mol�1) in the glass, was also ruled outbecause the activation energies needed for the diffusivity aremuch lower than the obtained values. This conrms that graingrowth is controlled by interface kinetics, and not diffusion.The pre-exponential frequency factor, A, and reactionrate constant, k, were determined by the Arrhenius equation(k ¼ Ae�Ea/RT). The values at 844 �C were found to be 1.046 �1035 s�1 and 1.09 s�1, respectively. The reaction rate constant

Fig. 3 Plot of ln(4/Tcry2) versus 1/Tcry.

This journal is © The Royal Society of Chemistry 2015

indicates the normal rate of the crystallization in the glassmatrix due to 1 mol% doping of Fe2O3. A non-systematic vari-ation of DTFWHM corresponding to the scanning rate of the DTAmeasurement shows that the contribution of surface andvolume crystallization in BST glass ceramics is independent ofthe scanning rate. The non-systematic variation was alsoobserved by Liu et al.10 The dominant nucleationmechanism, asdetermined by peak prole analysis of crystallization exotherms(as represented by eqn (2)),20 shows the simultaneous surfaceand volume crystallization occurring in BST glass ceramicsbecause the crystal growth dimension ‘n’ was between 1.36–1.96.11,21 The ‘n’ can be determined by the following relation:11

n ¼ 2:5

�R

Ea

� Tcry

2

DTFWHM

!(2)

where DTFWHM is the full width at half maximum of the crys-tallization peak.

The DTA patterns of the remaining BST borosilicate glasssamples are shown in Fig. 4(a)–(c). The DTA results wereexplored over the temperature range of 100–1050 �C. Above thistemperature, melting of glasses began. For all the glasssamples, there was a shi in the baseline within the tempera-ture range of 641–672 �C, depending on the composition. Theshi in the baseline represents a change in the specic heatattributed to the glass transition temperature, Tg. The glasstransition temperature, Tg, of the different glass samples arelisted in Table 3. The glass transition temperature, Tg, of theglassy matrix increased with increasing concentration of SrO.The increase in Tg is related to the increase of the viscosity of themelt with increasing concentration of SrO in the glass melt.32,33

The glass transition temperatures were found to lie between 641and 672 �C. In all glass samples, there was only one exothermic

Fig. 4 DTA patterns of the glass samples (a) BST5K1F0.6, (b)BST5K1F0.8 and (c) ST5K1F1.0.

RSC Adv., 2015, 5, 2819–2826 | 2821

Table 3 Glass transition temperature, Tg and crystallization temper-ature Tcry of the BST borosilicate glass samples

Glass sample code

DTA Peaks (�C)

DT ¼ Tcry � Tg (�C)Tg Tcry

BST5K1F0.6 641 878 237BST5K1F0.8 657 860 203ST5K1F1.0 672 872 200

RSC Advances Paper

Publ

ishe

d on

24

Nov

embe

r 20

14. D

ownl

oade

d by

Ric

e U

nive

rsity

on

10/1

2/20

14 1

7:37

:19.

View Article Online

peak, which corresponds to the crystallization of glass into glassceramic and is known as the crystallization temperature (Tcry).In the glass samples, BST5K1F0.8 and ST5K1F01.0, theexothermic peak was sharp in comparison to BST5K1F0.4 andTcry increased with increasing concentration of SrO. The stabi-lization temperature, DT, decreased with increasing SrOcontent. This suggests that the substitution of SrO promotes theformation of the required crystalline phase.22

3.3. X-ray diffraction

The glass sample BST5K1F0.4 was crystallized for heat treat-ment schedules of 3 and 6 hours with a heating rate of 5 �Cmin�1 at 844 �C. The crystallized samples are named asBST5K1F0.4T844 and BST5K1F0.4S844. The XRD pattern ofthese samples are shown in Fig. 5. The pattern was indexed tostandard JCPDS data le nos 13-0522 (Ba1.91Sr0.09TiO4) and 84-0923 (Ba2TiSi2O8). Fig. 5(a) presents the XRD pattern of the glassceramic sample, BST5K1F0.4T844. The glass sample crystallizedin the major phase of Ba1.91Sr0.09TiO4 along with a secondaryphase, Ba2TiSi2O8 (BTS). The major crystalline phase had anorthorhombic structure. The XRD pattern of the 6 hour heattreated glass ceramic sample was similar to that of the 3 hour

Fig. 5 X-ray diffraction patterns of the glass ceramic samples (a)BST5K1F0.4T844 and (b) BST5K1F0.4S844, inset (a) and (b) theirWilliamson and Hall (WH) plots.

2822 | RSC Adv., 2015, 5, 2819–2826

heat treated sample, and only a slight difference in nucleationand growth was observed. The amount of secondary phaseformation in BST5K1F0.4S844 was less than that inBST5K1F0.4T844. This was conrmed in the microstructuralstudies. The crystallite size was estimated by the Williamsonand Hall (WH) plot (inset Fig. 5) as well as the Debye–Scherermethod. The crystallite sizes of these two samples were 41.7 nm,42.8 nm and 44.0 nm 46.7 nm, as determined by the Debye–Scherrer method and (WH) plot, respectively. The latticeparameters were determined to be a ¼ 6.19 A, b ¼ 10.54 A, c ¼7.64 A and a ¼ 6.17 A, b ¼ 10.50 A, c ¼ 7.66 A, respectively, forthe 3 hour and 6 hour heat treated samples. These values arewell matched with the already reported results.23

The XRD patterns of the glass ceramic samplesBST5K1F0.6S878, BST5K1F0.8S860 and ST5K1F1.0S872 areshown in Fig. 6(a)–(c). These XRD patterns of the glass ceramicsamples, crystallized over the 6 hour heat treatment schedulebecause the crystallization was found better in the 6 hour heattreated samples. The XRD pattern of BST5K1F0.6S878 was ofsimilar to that of BST5K1F0.4S844. The XRD patterns ofBST5K1F0.8S860 and ST5K1F1.0T872 show the major crystal-line phase of SrTiO3, ST (JCPDS le no. 73-0661) along withsecondary phase formation of Sr2B2O5, SB (JCPDS le no. 89-4488). The d-values corresponding to the XRD peaks were wellmatched with the standard cubic perovskite SrTiO3 crystallinephase. The crystallite sizes and strain broadening are listed inTable 4. A negative sign indicates compressive strain, whereas apositive sign represents tensile strain. The lattice parameters of

Fig. 6 X-ray diffraction patterns of glass ceramic samples (a)BST5K1F0.6S878, (b) BST5K1F0.8S860 and (c) ST5K1F1.0S872.

This journal is © The Royal Society of Chemistry 2015

Table 4 Major crystalline phase, crystal structure, lattice parameters and crystallite size in the different barium strontium titanate glass ceramicsamples

Glass ceramic sample code Major crystalline phase Crystal structure

Lattice parameters (A) Crystallite size (A)

Strain (�10�4)a b c tDS tWH

BST5K1F0.4T844 Ba1.91Sr0.09TiO4 Orthorhombic 6.19 10.54 7.64 417 440 30.5BST5K1F0.5S844 Ba1.91Sr0.09TiO4 Orthorhombic 6.17 10.50 7.66 428 467 27.0BST5K1F0.6S878 Ba1.91Sr0.09TiO4 Orthorhombic 6.11 10.51 7.64 363 481 28.5BST5K1F0.8S860 SrTiO3 Cubic 3.93 3.93 3.93 463 405 �15.0ST5K1F1.0S872 SrTiO3 Cubic 3.95 3.95 3.95 547 435 �8.0

Paper RSC Advances

Publ

ishe

d on

24

Nov

embe

r 20

14. D

ownl

oade

d by

Ric

e U

nive

rsity

on

10/1

2/20

14 1

7:37

:19.

View Article Online

the major crystalline phase were also similar to reported resultsand are listed in Table 4.24,25

3.4. Microstructural behavior

The microstructure of BST glass does not show any grains, asindicated in Fig. 1(b). When the glass sample was heat treated,nucleation was rst started in the form of crystal grains in theglassy matrix. Aer nucleation, grain growth took place andnearby grains touched each other. Fig. 7(a) and (b) depicts thescanning electron micrographs of BST5K1F0.4T844, andBST5K1F0.4S844. A dendritic microstructure with nonuniformgrain growth26 was observed in the 3 hour heat treated sample

Fig. 7 Scanning electron micrographs of the glass ceramic sample (a)BST5K1F0.4T844 and (b) BST5K1F0.4S844.

This journal is © The Royal Society of Chemistry 2015

BST5K1F0.4T844 (Fig. 7(a)), whereas 6 hours heat treatmentplayed an important role in reducing the secondary phase andmaking a uniform structure. The separation between the grainsarises due to a reduction of the secondary phase. The dendriticstructure of the crystalline phase is generally formed in diffu-sion limited cases as a way of preventing any material excludedfrom the crystal from reducing the growth rates.6 The systematicgrains growth is completed in four steps, as shown in Fig. 8. Theglass has a smooth amorphous microstructure and there are nocrystallites present. During the crystallization of glasses, rst, afew nuclei (crystal grains) are formed randomly in the glassymatrix, and then these grains are likely to grow in someparticular direction and form a dendritic microstructure.27

Finally, the grains of major crystalline phase and secondaryphase begin overlapping. Secondary grain growth was observedin the 6 hour heat treated glass ceramic sample. This may beoccur aer complete growth when the crystal surface area nolonger increases but decreases, as observed in Fig. 7(b). Thesurface crystal growth is closely related to surface nucleation.6

Fig. 9(a)–(c) depict the SEM images of the samplesBST5K1F0.6S878, BST5K1F0.8S860 and ST5K1F1.0S872. Themicrograph of BST5K1F0.6S878 shows well developed randomlydistributed grains in the glassy matrix. The microstructure ofthis sample was similar to that of the BST5K1F0.4T844. Thesurface morphology of BST5K1F0.8T860 shows a clear andrandomly distributed dendritic structure. The microstructure of

Fig. 8 Systematic sketch map of the crystallization process for theBST5K1F0.4T844 sample.

RSC Adv., 2015, 5, 2819–2826 | 2823

Fig. 9 Scanning electronmicrographs of the glass ceramic samples (a)BST5K1F0.6S878, (b) BST5K1F0.8S860 and (c) ST5K1F1.0S872.

RSC Advances Paper

Publ

ishe

d on

24

Nov

embe

r 20

14. D

ownl

oade

d by

Ric

e U

nive

rsity

on

10/1

2/20

14 1

7:37

:19.

View Article Online

the glass ceramic sample, ST5K1F1.0T872, shows the inter-connected grains of perovskite SrTiO3. A secondary phase,Sr2B2O5, was clearly observed in the form of whitish grains(Fig. 9(c)). In this micrograph, some pores were also present.This type of structure was found to be very useful in gas andhumidity sensing, because gases or moisture are absorbed bythe surface pores, which changes the resistivity of thematerial.28

Fig. 10 Variation of (a) dielectric constant, 3r, and (b) dissipation factor,tan d, with temperature at different frequencies for the glass ceramicsample BST5K1F0.4T844.

3.5. Dielectric behaviour

The variation of the dielectric constant, 3r, and dissipationfactor, tan d, with temperature at few selected frequencies (20,

2824 | RSC Adv., 2015, 5, 2819–2826

100 Hz, 1, 10 and 100 kHz) for glass ceramic samplesBST5K1F0.4T844 and BST5K1F0.4S844 are shown in Fig. 10 and11. The dielectric constant increases with increasing tempera-ture at all frequencies up to a particular temperature, known asthe Curie temperature, and decreases gradually thereaer withincreasing temperature. The dielectric constant was found to bea maximum at a frequency of 20 Hz. At higher frequencies, thedielectric constant decreased due to a reduction of the netpolarisation. This can be explained by the Maxwell–Wagnermodel of the dielectric constant.34 The dielectric loss forBST5K1F0.4T844 was almost independent of temperature(Fig. 10). The dielectric behaviour of the glass ceramics wasobserved to be independent of the heat treatment process ofglass but a difference in the dielectric constant and dielectricloss was found in these two samples. The peak value of dielec-tric constant for these samples was observed to be 24 250 and107 070, respectively. The higher dielectric constant in the 6hour heat treated sample, BST5K1F0.4S844, was also supportedby XRD and SEM. The addition of 1 mol% Fe2O3 to the BST glasspromotes crystallization, as discussed earlier in the crystalliza-tion kinetics. The Fe3+ ions present in the glassy network arediffused into the crystalline BST at higher temperatures, whichmake the glass ceramic samples semiconducting. SEM showedthat the grains of BST5K1F0.4S844 are well separated by grainboundary in the glassy matrix. Hence, a large conductivitydifference is introduced between the semiconducting grainsand insulating grain-boundary. The conductivity difference isresponsible for space charge polarization and hence the highdielectric constant.17,29 This peak value (3r ¼ 107 070) of thedielectric constant was higher than that reported for La2O3

doped BST glass ceramics.16

The variation of the dielectric constant, 3r, and dissipationfactor, tan d, with temperature at few selected frequencies (20,100 Hz, 1, 10 and 100 kHz) for the BST5K1F0.6S878,BST5K1F0.8S860 and ST5K1F1.0S872 samples, which werecrystallized using the 6 hour heat treatment schedules, areshown in Fig. 12, 13 and 14. Fig. 12(a) and (b) depicts thedielectric behaviour of the glass ceramic sample

This journal is © The Royal Society of Chemistry 2015

Fig. 11 Variation of (a) dielectric constant, 3r, and (b) dissipation factor,tan d, with temperature at different frequencies for the glass ceramicsample BST5K1F0.4S844.

Fig. 12 Variation of (a) dielectric constant, 3r, and (b) dissipation factor,tan d, with temperature at different frequencies for the glass ceramicsample BST5K1F0.6S878.

Fig. 13 Variation of (a) dielectric constant, 3r, and (b) dissipation factor,tan d, with temperature at different frequencies for the glass ceramicsample BST5K1F0.8S860.

Fig. 14 Variation of (a) dielectric constant, 3r, and (b) dissipation factor,tan d, with temperature at different frequencies for the glass ceramicsample ST5K1F1.0S872.

This journal is © The Royal Society of Chemistry 2015

Paper RSC Advances

Publ

ishe

d on

24

Nov

embe

r 20

14. D

ownl

oade

d by

Ric

e U

nive

rsity

on

10/1

2/20

14 1

7:37

:19.

View Article Online

BST5K1F0.6S878. The pattern shows that the dielectric constantincreases gradually at 20 and 100 Hz with increasing tempera-ture, and remains invariant at other frequencies. The dielectricloss also increased gradually with increasing temperature. Thepeak dielectric constant at 20 Hz was 9590. The dielectricconstant for the BST5K1F0.8S860 sample was invariant up to120 �C at lower frequencies and was independent of tempera-ture at 10 and 100 kHz (Fig. 13). Similar behavior in the glassceramic sample, ST5K1F1.0S872, was also observed. Thedielectric constant and loss were not inuenced signicantly inthe glass ceramic samples BST5K1F0.8S860 andST5K1F1.0S872. The dielectric constant was strongly dependenton the composition, and decreased with the addition of SrO inplace of BaO in the glassy matrix. The dielectric constant in thestrontium rich sample was 3220, which was higher than thevalues reported for the CoO and La2O3 doped strontium boro-silicate glass ceramics.17 This also supports the previousdiscussion in this section. The peak dielectric constants arelisted in Table 5. The dielectric loss was found to be very low inthis glass ceramic system. The low dielectric loss was attributedto the addition of Fe3+ ions in the glassy matrix, which havesmaller ionic radii than that of Ti4+ ions. Fe3+ ions acts asacceptor ions and are very useful in reducing Ti4+ ions. Fe3+ ionsmay even substitute for Ti4+ sites and prevent the reduction ofTi4+ to Ti3+ by neutralizing the donor action of the oxygenvacancies, causing a decrease in the dielectric loss.30,31

4. Conclusions

A series of barium strontium titanate borosilicate glasses withthe addition of Fe2O3 were prepared by the conventional melt-quench method. Kinetic studies showed that simultaneoussurface and internal volume nucleation occur in the glassmatrix. The activation energy for crystallization was observed tobe 818 � 1.6 kJ mol�1. A high dielectric constant of about107 070 was found in a 6 hour heat treated glass ceramicsample. The high dielectric constant was due to space chargepolarization, which was attributed to the conductivity differencebetween the semiconducting grains and the insulating grain-

RSC Adv., 2015, 5, 2819–2826 | 2825

Table 5 Dielectric constant of various glass ceramic samples in the system 64[(Ba1�xSrx)TiO3]–30[2SiO2$B2O3]–5[K2O]–1[Fe2O3]

Glass ceramic sample code

Maximum value of dielectric constant, 3r

20 Hz 100 Hz 1 kHz 10 kHz 100 kHz

BST5K1F0.4T844 24 250 18 654 16 334 16 327 8222BST5K1F0.4S844 107 070 78 129 37 061 16 294 10 021BST5K1F0.6S878 9590 5313 1742 1425 2059BST5K1F0.8S860 6664 5285 3228 1053 1307ST5K1F1.0S872 3220 2206 1897 1786 2068

RSC Advances Paper

Publ

ishe

d on

24

Nov

embe

r 20

14. D

ownl

oade

d by

Ric

e U

nive

rsity

on

10/1

2/20

14 1

7:37

:19.

View Article Online

boundary in the glass matrix. Fe2O3 plays an important role as anucleating agent, and it enhances the activation energy; more-over, it reduces the dielectric loss in the samples.

Acknowledgements

The authors gratefully acknowledge the University GrantCommission (UGC), New Delhi, (India) for nancial supportunder Major Research Project F. no. 37-439/2009 (SR).

References

1 T. G. V. M. Rao, A. R. Kumar, K. Neeraja, N. Veeraiah andM. Rami Reddy, J. Alloys Compd., 2013, 557, 209–217.

2 R. Martens and W. Muller-Warmuth, J. Non-Cryst. Solids,2000, 265, 167–175.

3 G. Kaur, O. P. Pandey and K. Singh, J. Non-Cryst. Solids, 2012,358, 2589–2596.

4 S. Lakshimi, S. Rao, G. Ramadevudu, M. Shareefuddin,A. Hameed, N. M. Chary and M. L. Rao, Int. Trans. J. Eng.,Manage., Appl. Sci. Technol., 2012, 4, 25–35.

5 A. K. Yadav and C. R. Gautam, Spectrosc. Lett., 2014 (in press).6 N. Karpukhina, R. G. Hill and R. V. Law, Chem. Soc. Rev.,2014, 43, 2174–2186.

7 A. K. Yadav and C. R. Gautam, Adv. Appl. Ceram., 2014, 113,193–204.

8 J. Chen, Y. Zhang, C. Deng and X. Dai, Mater. Chem. Phys.,2010, 121, 109–113.

9 T. Y. Liu, G. H. Chen, J. Song and C. L. Yuan, Ceram. Int.,2013, 39, 5553–5559.

10 T. Y. Liu, G. H. Chen, J. Song and C. L. Yuan, Trans.Nonferrous Met. Soc. China, 2014, 24, 729–735.

11 P. V. Divya, G. Vignesh and V. Kumar, J. Phys. D: Appl. Phys.,2014, 40, 7804–7810.

12 P. V. Divya and V. Kumar, J. Am. Ceram. Soc., 2007, 90, 472–476.

13 A. K. Yadav, C. R. Gautam and P. Singh, New J. Glass Ceram.,2012, 2, 126–131.

14 Q. Tan, P. Irwin and Y. Cao, IEEJ Transactions onFundamentals and Materials, 2006, 126, 1153–1159.

15 M. J. Pan and C. A. Randall, IEEE Electr. Insul. Mag., 2010, 26,44–50.

2826 | RSC Adv., 2015, 5, 2819–2826

16 Y. Zhang, T. Ma, X. Wang, Z. Yuan and Q. Zhang, J. Appl.Phys., 2011, 109, 084115.

17 A. K. Yadav and C. R. Gautam, J. Mater. Sci.: Mater. Electron.,2014, 25, 5165–5187.

18 H. E. Kissinger, J. Res. Natl. Bur. Stand., 1956, 57, 217–221.19 Y. Zeng, X. Qin, S. Jiang, G. Zhang and L. Zhang, J. Am.

Ceram. Soc., 2011, 94, 469–473.20 J. A. Augis and J. E. Bennet, J. Therm. Anal., 1978, 13, 283–

292.21 E. P. Gorzkowski, M. J. Pan, B. A. Bender and C. C. M. Wu,

J. Am. Ceram. Soc., 2008, 91, 1065–1069.22 X. Wang, Y. Zhang, J. Zhu, X. Song, R. Lvc, J. Liuc and

M. Zhang, Ceram. Int., 2014 (in press).23 W. Kwestroo and H. A. M. Paping, J. Am. Ceram. Soc., 1959,

42, 292–299.24 S. A. Markgraf, A. Halliyal, A. S. Bhalla, R. E. Newnham and

C. T. Prewitt, Ferroelectrics, 1985, 62, 17–26.25 G. M. Meyer, R. J. Nelmes and J. Hutton, Ferroelectrics, 1978,

21, 461–462.26 J. Chen, Y. Zhang, C. Deng, X. Dai and L. Li, J. Am. Ceram.

Soc., 2009, 92, 1350–1353.27 W. Zhang, S. Xue, S. Liu, J. Wang, B. Shen and J. Zhai,

J. Alloys Compd., 2014, 617, 740–745.28 O. Lupan, V. Cretu, M. Deng, D. Gedamu, I. Paulowicz,

S. Kaps, Y. K. Mishra, O. Polonskyi, C. Zamponi, L. Kienle,V. Trom, I. Tiginyanu and R. Adelung, J. Phys. Chem. C,2014, 118, 15068–15078.

29 D. Kumar, C. R. Gautam and O. Parkash, Appl. Phys. Lett.,2006, 89, 112911.

30 S. B. Herner, F. A. Selmi, V. V. Varadan and V. K. Varadan,Mater. Lett., 1993, 15, 317–324.

31 X. Liang, Z. Meng and W. Wu, J. Am. Ceram. Soc., 2004, 87,2218–2222.

32 C. R. Gautam, D. Kumar and O. Parkash, Adv. Mater. Sci.Eng., 2011, 2011, 1–11.

33 A. Bahrami, Z. Ali Nemati, P. Alizadeh and M. Bolandi,J. Mater. Process. Technol., 2008, 206, 126–131.

34 G. Catalan, D. O. Neill, R. M. Bowman and J. M. Gregg, Appl.Phys. Lett., 2000, 77, 3078–3080.

This journal is © The Royal Society of Chemistry 2015