Influence of Sr/Ba ratio on the energy storage propertiesand dielectric relaxation behaviors of strontium barium titanateceramics

Ting Wu • Yongping Pu • Pan Gao •

Dan Liu

Received: 29 March 2013 / Accepted: 26 June 2013

� Springer Science+Business Media New York 2013

Abstract SrxBa1-xTiO3 (x = 0.50–0.70) ceramics were

prepared by conventional solid-state method. The effects of

Sr/Ba ratio on the microstructures, energy storage proper-

ties and dielectric relaxation behaviors of ceramics were

systematically investigated. Scanning electron microscopy

observations revealed that the grain size was inhibited with

increasing Sr molar fraction. The Sr0.6Ba0.4TiO3 ceramics

obtained the highest energy density of 0.3629 J/cm3

attributed to the increase of average breakdown strength

resulting from the decrease of grain size and the optimizing

of microstructure. In order to investigate the influence of

Sr/Ba ratio on the dielectric relaxation behaviors, the

activation energy has been calculated from the relaxation

of dielectric loss and the complex impedance spectra by the

Arrhenius relationship, respectively. The same results

indicated that the decrease of grain size resulting in more

grain boundaries, it was difficult for transferring charge and

making an orientation under external electric field. Mean-

while, more defects existed at grain boundary and accel-

erated the thermally activated motions of defects, leading

to the increase of activation energy.

1 Introduction

Barium strontium titanate (SrxBa1-xTiO3, BST) is a fer-

roelectric solid solution between barium titanate (BaTiO3,

BT) and strontium titanate (SrTiO3, ST) with perovskite

structure. For its favorable dielectric and ferroelectric

properties such as high dielectric constant, alterable Curie

temperature, low dielectric loss and high tunability of

dielectric behavior, it has been widely used in preparation

of dynamic random access memories, dielectric capacitors,

microwave phase shifters, transducers, positive tempera-

ture coefficient resistors and energy storage ceramics [1–4].

Many efforts have been put to improve the microstruc-

ture and electrical properties of BST ceramics synthesized

by other chemical processes [5–10]. The BST-based glass–

ceramics have also been studied in the recent years

[11–14]. However, very limited literature reported the

relationship between the microstructures and the energy

storage properties or the dielectric relaxation behaviors in

the BST ceramics, especially for a wide range of Sr/Ba ratio.

In this paper, BST series of ceramics (x = 0.50, 0.55,

0.60, 0.65, 0.70) have been prepared by traditional sinter-

ing method. In order to express conveniently, they were

abbreviated for BST50, BST55, BST60, BST65 and

BST70, respectively. The impact of composition on crystal

structures, breakdown strength and storage prosperities of

BST ceramics have been studied systematically. The rela-

tionship between dielectric relaxation behaviors and

microstructures has also been investigated.

2 Experimental procedure

The SrxBa1-xTiO3 ceramics were synthesized by the con-

ventional solid-state route. The compositions discussed in

this article were prepared according to the formulas of

SrxBa1-xTiO3 with x = 0.50, 0.55, 0.60, 0.65, 0.70 (mole

fraction). The starting raw materials of BaCO3 (C99.9 %),

SrCO3 (C99.9 %), and TiO2 (C99.9 %) powders were ball-

milled using ZrO2 balls and distilled water as milling

media, the slurry was dried and the mixture was calcined at

T. Wu (&) � Y. Pu � P. Gao � D. Liu

School of Materials Science and Engineering, Shaanxi

University of Science and Technology, Xi’an 710021,

People’s Republic of China

e-mail: 420501558@qq.com

123

J Mater Sci: Mater Electron

DOI 10.1007/s10854-013-1368-y

1,150 �C for 4 h. Then the synthesized powders were ball-

milled again and pressed into pellets. The pellets were

baked at 600 �C for 0.5 h for binder removal and then

sintered at temperatures ranging from 1,300–1,380 �C for

2 h in air.

For dielectric measurements, Ag paste was printed on

both sides of the pellets and heat treated at 600 �C for

15 min. The frequency dependence of dielectric properties

was measured using a precision LCR Meter (E4980A,

Agilent Tech., CA, US) over a frequency range from 10 Hz

to 1 MHz at room temperature, the temperature depen-

dence of dielectric properties were measured over a tem-

perature range from 20 to 600 �C, and the impedance data

were measured over frequencies from 20 Hz to 2 MHz in a

temperature range of 200–600 �C without bias voltage. The

DC BDS measurement was performed using a withstanding

voltage tester at room temperature. All samples were

immersed in silicone oil to prevent surface flashover. At

least 10 specimens were used for each composition during

BDS testing. The polarization–electric field (P–E) hystere-

sis loops were measured using a ferroelectric tester (TF

Analyzer 2000, aixACCT, Aachen, Germany) at room

temperature. The surfaces of the disks were polished and

thermally etched before observing the microstructure with

scanning electron microscopy (SEM).

3 Results and discussion

In order to determine the suitable sintering temperature

(SST) [15], the bulk density of all samples sintered at dif-

ferent temperatures was measured by the Archimedes

method. The measured density showed that nearly 97.0 % of

the theoretical density was obtained for BST ceramics when

the sintering temperature reached to 1,350 �C. In the rest of

this paper, all the samples were prepared at their SSTs.

Figure 1 shows the XRD patterns for BST ceramics with

varying Sr/Ba ratio. According to the XRD patterns, all

peaks could be indexed and correspond to a perovskite

phase, which indicated the formation of a single phase. A

small change in intensity of the reflection peaks may be due

to the variation of the concentration of Sr2? at the Ba-site.

A careful examination of the XRD patterns revealed that

the diffraction peaks shifted towards high-angle direction

with increasing Sr2? content, evident from the enlarged

view of the peak (110) shown as inset. It was because that

the radius of Sr2? (1.12 A) was smaller than that of the

Ba2? (1.35 A) [16, 17], which contributed to the decrease

of lattice parameter.

Figure 2 presents the frequency dependence of dielectric

constant and dielectric loss for BST ceramics with varying

Sr/Ba ratio. In the measured frequency range, the dielectric

constant of samples showed good frequency stability. The

ion radius has enormous influence on the dielectric constant

in the dielectrics with ion-displacement polarization. In the

SrxBa1-xTiO3 ceramics, the dielectric constant decreased

with increasing Sr mol fraction, which can be attributed to

the smaller radius of Sr2? than that of the Ba2?. The polar-

ization corresponding to the Sr2? was small, which has little

contribution to the dielectric constant and made the decrease

of dielectric constant. The SrxBa1-xTiO3 (x = 0.50–0.70)

ceramics processed parelectric phase at room temperature

[17, 18], which contributed to the gradually decrease of

dielectric loss.

The microstructures of BST ceramics with varying Sr/

Ba ratio are shown in Fig. 3. The grain size decreased

obviously and a uniform distribution were observed with

increasing Sr contents. The optimizing of microstructures

resulting from the decrease of lattice parameter and the

densification of microstructure, which can be proved by the

Fig. 1 and the decrease of dielectric loss. As shown in

Fig. 1 XRD patterns for BST ceramics: (a) BST50, (b) BST55,

(c) BST60, (d) BST65, (e) BST70

Fig. 2 Frequency dependence of dielectric constant and dielectric

loss for BST ceramics with varying Sr/Ba ratio

J Mater Sci: Mater Electron

123

Fig. 3a, the microstructure consisted essentially of large

grains, while the grain size decreased and distributed more

uniform with increasing Sr/Ba ratio (Fig. 3b–d). A micro-

structure consisting entirely of uniform small grains was

obtained in Fig. 3e.

Figure 4 shows the Weibull distribution of the dielectric

BDS for BST ceramics with varying Sr/Ba ratio. This

distribution was usually used for the BDS analysis. The

reasonable values of BDS could be described by

Xi ¼ lnðEiÞ ð1Þ

Yi ¼ ln ln 1= 1� Pið Þð Þð Þ ð2Þ

Pi ¼ i=ðnþ 1Þ ð3Þ

where Xi and Yi were the two parameters in Weibull dis-

tribution function, Ei was the specific breakdown voltage of

each specimen in the experiments, Pi was the probability

for dielectric breakdown, n was the sum of specimens of

each sample, and i was serial number of specimen.

According to the Weibull distribution equation [19],

there was a linear relationship between Xi and Yi, where the

slope was the Weibull modulus m relating to the range of

BDS, and the intercept reflected the magnitude of BDS.

The value of the Weibull modulus m was obtained by

linear fitting of the experimental data and presented in

Fig. 4. All the five samples fitted well with Weibull

Fig. 3 SEM micrographs of the polished surface of BST ceramics: (a) BST50, (b) BST55, (c) BST60, (d) BST65, (e) BST70

Fig. 4 Weibull distribution of BDS for BST ceramics with varying

Sr/Ba ratio

Fig. 5 P–E hysteresis loops for BST ceramics with varying Sr/Ba

ratio

J Mater Sci: Mater Electron

123

distribution, thus the samples had a concentrative distri-

bution of BDS.

The P–E hysteresis loops for BST ceramics with varying

Sr/Ba ratio are plotted in Fig. 5, which were measured at

1 Hz until the samples undergone a breakdown. It can be

seen that the breakdown strength of ceramics increased,

which can be attributed to the decrease of grain size and

densification microstructure in the BST ceramics. The

polarization density decreased with increasing Sr mole

fraction resulting from the decrease of dielectric constant.

The dielectric constant, average breakdown strength and

energy density of BST ceramics are given in Table 1. The

energy density was calculated with the formula followed

J ¼ 1

2e0erE

2b ð4Þ

where Eb was the average breakdown strength, e0 was the

permittivity of free space, er was the relative permittivity.

To obtain average breakdown strength, ten samples for

each composition were measured. According to the for-

mula, we can found that high average breakdown strength

and high dielectric constant were two key parameters to

obtain high energy density, and high average breakdown

strength made a more pronounced contribution toward the

energy density. The samples of BST60 showed the highest

energy storage density, which can be attributed to the

increase of average breakdown strength resulting from the

decrease of grain size and the optimizing of microstructure.

Figure 6 shows the variation of dielectric constant and

dielectric loss with temperature at different frequencies for

BST ceramics with varying Sr/Ba ratio. The dielectric

constant and dielectric loss remain constant up to a certain

temperature and thereafter increased rapidly with increas-

ing temperature. The temperature dependence of dielectric

loss plot showed a peak. The position of the peak shifted to

higher temperature with increasing frequency. Similar

results were observed in other samples (BST55 and

Table 1 Dielectric constant, average breakdown strength and energy

density of BST ceramics with varying Sr/Ba ratio

BST50 BST55 BST60 BST65 BST70

Dielectric constant 1800 1110 998 852 724

Average breakdown

strength (kV/mm)

6.41 7.93 9.06 9.75 10.44

Energy storage

density (J/cm3)

0.3273 0.3089 0.3629 0.3562 0.3492

Fig. 6 Variation of dielectric constant and dielectric loss with temperature measured at different frequencies for BST ceramics: (a) BST50,

(b) BST60, (c) BST70

J Mater Sci: Mater Electron

123

BST65). It is indicated that some relaxation polarization

mechanism were existed in BST ceramics [20, 21].

In order to investigate the influence of Sr/Ba ratio on the

dielectric relaxation behaviors, the activation energy has

been calculated from the relaxation of dielectric loss in

BST ceramics. Figure 7 plots the dependence of the

lnf versus 1,000/T for BST ceramics with varying Sr/Ba

ratio, the activation energy can be calculated from the slope

using the following equation

ln f ¼ ln f0 þ�Ea

kB

1

Tð5Þ

where f was the value of measurement frequency at the

extreme point of each dielectric loss peak at a certain

measurement temperature, Ea was the activation energy for

the relaxation process, kB was the Boltzmann constant and

T was the absolute temperature. The activation energy for

relaxation obtained from the slope of solid lines was 0.816,

1.002 and 1.024 eV for BST50, BST60 and BST70,

respectively. In our measurement condition, Ea corresponds

to the relaxation of space charge, which characterizes the

effective energy barriers that impeded the transfer of free

charge and made an orientation under external electric

field. The grain size decreased with increasing Sr/Ba ratio,

resulting in more grain boundary and space charge in the

BST ceramics. It was difficult for transferring free charge

and making an orientation under external electric field,

leading to the increase of activation energy.

The complex impedance spectrum has been proved to be

a powerful method for investigating the relaxation polari-

zation mechanism of grain and grain boundary in ceramics

[22]. The samples were measured at different temperatures

at every 10 �C interval in order to get series of Cole–Cole

images. The complex impedance spectra measured at dif-

ferent temperatures for BST ceramics with varying Sr/Ba

ratio are presented in Fig. 8. Similar results were observed

in other samples (BST55 and BST65). There was only one

semicircle can be observed in the diagrams, which repre-

sented the contributions from the grain boundary phases. It

is indicated that in the samples existed the same dielectric

relaxation processes. As can be seen in Fig. 8a–c, the

impedance semicircle of grain boundary phases increased

obviously with increasing Sr/Ba ratio. It is suggested that

the effect of grain boundary phases on the relaxation

mechanism of ceramics was enhanced. Meanwhile, the

impedance semicircle become smaller with increasing

measuring temperature, thus the observed dielectric relax-

ation behaviors corresponding to the grain boundary phases

were correlated with the thermally activated motions of

defects.

Fig. 7 The dependence of the lnf versus 1,000/T for BST ceramics: (a) BST50, (b) BST60, (c) BST70. The solid lines are linear fits through the data

J Mater Sci: Mater Electron

123

According to the Arrhenius relationship, the activation

energy for relaxation can be calculated from the slope

using the following equation. The relaxation time s was

calculated from the peak position of the -Z’’ versus fre-

quency plot (Fig. 8) using the relation [23] 2pfr = 1, where

fr was the relaxation frequency

ln s ¼ 1

kBTEa þ ln s0 ð6Þ

where s0 was a pre-exponential factor, Ea was the activa-

tion energy for the relaxation process, kB was the Boltz-

mann constant and T was the absolute temperature.

Figure 9 plots the relaxation time as a function of mea-

suring temperature for BST ceramics with varying Sr/Ba

ratio, the activation energy corresponding to the grain

boundary phases can be obtained from the slope of solid

lines which linear fits through the data. The activation

energy calculated from the complex impedance spectra was

increased from 1.172 to 1.231 eV, which was consistent

with the activation energy calculated from the relaxation of

dielectric loss. The decrease of grain size with increasing

Sr/Ba ratio resulting in more defects at grain boundary,

which accelerated the thermally activated motions of

defects and made the activation energy increased.

4 Conclusion

SrxBa1-xTiO3 (x = 0.50, 0.55, 0.60, 0.65, 0.70) ceramics

have been prepared by traditional sintering method. The Sr/

Ba ratio had enormous influence on the microstructures,

energy storage properties and dielectric relaxation behaviors

of BST ceramics. On one hand, the grain size was inhabited

by the increase of Sr/Ba ratio which proved by the SEM

micrographs. The samples of BST60 ceramics obtained the

highest energy density attributed to the increase of break-

down strength which resulting from the optimizing of

microstructure. On the other hand, the activation energy for

the relaxation mechanism calculated from the relaxation of

dielectric loss and the complex impedance spectra have

been increased by the decrease of grain size. This was

because that more grain boundary in the BST ceramics with

increasing Sr/Ba ratio, it was difficult for spreading of

charge and making an orientation under external electric

field. At the same time, the decrease of grain size leading to

more defects existed at grain boundary, which accelerated

the thermally activated motions of defects and made the

activation energy increased. These results provided useful

information for BaxSr1-xTiO3 ceramics in the application of

capacitor ceramics.

Fig. 8 Complex impedance spectra measured at different temperatures for BST ceramics: (a) BST50, (b) BST60, (c) BST70

J Mater Sci: Mater Electron

123

Acknowledgments This research was supported by the National

Natural Science Foundation of China (51072106, 51102159), the New

Century Excellent Talents Program of Chinese Education Ministry

(NCET-11-1042), Foundation of Shaanxi Educational Committee

(12JK0447), International Science and Technology Cooperation

Project Funding of Shaanxi Province (2012KW-06), the Academic

Leaders Cultivation Program and Graduate Innovation Fund of Sha-

anxi University of Science and Technology.

References

1. Q.M. Zhang, L. Wang, J. Luo, Q. Tang, J. Du, Ba0.4Sr0.6TiO3/

MgO composites with enhanced energy storage density and low

dielectric loss for solid-state pulse-forming line. Int. J. Appl.

Ceram. Technol. 7, E124–E128 (2010)

2. H.V. Alexandru, C. Berbecaru, A. Ioachim, M.I. Toacsen, Oxides

ferroelectric (Ba, Sr)TiO3 for microwave devices. Mater. Sci.

Eng. B 109, 152–159 (2004)

3. A. Kumar, S.G. Manavalan, Characterization of barium strontium

titanate thin films for tunable microwave and DRAM applica-

tions. Surf. Coat. Technol. 198, 406–413 (2005)

4. M.H. Badr, L.M. Sharaf El-Deen, A.H. Khafagy, D.U. Nassar, Struc-

tural and mechanical properties characterization of barium strontium

titanate (BST) ceramics. J. Electroceram. 27, 189–196 (2011)

5. Y. Wang, B.Y. Liu, F. Wei, Z.M. Yang, J. Du, Effect of (Ba ? Sr/

Ti) ratio on the dielectric properties for highly (1 1 1) oriented (Ba,

Sr)TiO3 thin films. J. Alloy. Compd. 475, 827–831 (2009)

6. C. Wang, B.L. Cheng, S.Y. Wang, H.B. Lu, Y.L. Zhou, Z.H.

Chen, G.Z. Yang, Improved dielectric properties and tunability of

multilayered thin films of (Ba0.80Sr0.20)(Ti1-xZrx)O3 with com-

positionally graded layer. Appl. Phys. Lett. 84, 5 (2004)

7. Z. Wang, S.L. Jiang, G.X. Li, M.P. Xi, T. Li, Synthesis and

characterization of Ba1-xSrxTiO3 nanopowders by citric acid gel

method. Ceram. Int. 33, 1105–1109 (2007)

8. J.L. Zhao, X.H. Wang, L.T. Li, X.X. Wang, Y.X. Li, Stoichi-

ometry control and structure evolution in hydrothermally derived

(Ba, Sr)TiO3 films. Ceram. Int. 34, 1223–1227 (2008)

9. Y.P. Ding, C.Y. Jin, Z.Y. Meng, Investigation on the amorphous-

crystalline transition and microstructure of sol-gel derived (Ba1-

xSrx)TiO3 thin films. Mater. Res. Bull. 35, 1187–1193 (2000)

10. K.A. Razak, A. Asadov, W. Gao, Properties of BST ceramics

prepared by high temperature hydrothermal process. Ceram. Int.

33, 1495–1502 (2007)

11. Y. Zhang, J.J. Huang, T. Ma, X.R. Wang, C.S. Deng, X.M. Dai,

Sintering temperature dependence of energy-storage properties in

(Ba, Sr)TiO3 glass-ceramics. J. Am. Ceram. Soc. 94, 1805–1810

(2011)

12. B. Wu, L.Y. Zhang, X. Yao, Low temperature sintering of

BaxSr1-xTiO3 glass-ceramic. Ceram. Int. 30, 1757–1761 (2004)

13. K. Kageyama, J. Takahashi, Tunable microwave properties of

barium titanate-based ferroelectric glass-ceramics. J. Am. Ceram.

Soc. 87, 1602–1605 (2004)

14. J.C. Chen, Y. Zhang, C.S. Deng, X.M. Dai, Improvement in the

microstructures and dielectric properties of barium strontium

titanate glass–ceramics by AlF3/MnO2 addition. J. Am. Ceram.

Soc. 92, 1863–1866 (2009)

Fig. 9 Relaxation time as a function of 1,000/T for BST ceramics: (a) BST50, (b) BST60, (c) BST70. The solid lines are linear fits through the

data

J Mater Sci: Mater Electron

123

15. Q.M. Zhang, L. Wang, J. Luo, Q. Tang, J. Du, Improved energy

storage density in barium strontium titanate by addition of BaO–

SiO2–B2O3 glass. J. Am. Ceram. Soc. 92, 1871–1873 (2009)

16. S.B. Herner, F.A. Selmi, V.V. Varadan, V.K. Varadan, The effect

of various dopants on the dielectric properties of barium stron-

tium titanate. Mater. Lett. 15, 317–324 (1993)

17. J.W. Liou, B.S. Chiou, Dielectric characteristics of doped Ba1-x

SrxTiO3 at the paraelectric state. Mater. Chem. Phys. 51, 59–63

(1997)

18. S.W. Kim, H.I. Choi, M.H. Lee, J.S. Park, D.J. Kim, D. Do, M.H.

Kim, T.K. Song, W.J. Kim, Electrical properties and phase of

BaTiO3–SrTiO3 solid solution. Ceram. Int. 39, S487–S490 (2013)

19. J.J. Huang, Y. Zhang, T. Ma, H.T. Li, L.W. Zhang, Correlation

between dielectric breakdown strength and interface polarization

in barium strontium titanate glass ceramics. Appl. Phys. Lett. 96,

042902 (2010)

20. D. Kumar, C.R. Gautam, O. Parkash, Preparation and dielectric

characterization of ferroelectric (PbxSr1-x)TiO3 glass ceramics

doped with La2O3. Appl. Phys. Lett. 89, 112908 (2006)

21. M. Filippi, B. Kundys, R. Ranjith, A.K. Kundu, W. Prellier,

Interfacial contribution to the dielectric response in semicon-

ducting LaBiMn4/3Co2/3O6. Appl. Phys. Lett. 92, 212905 (2008)

22. S.H. Yoon, C.A. Randall, K.H. Hur, Influence of grain size on

impedance spectra and resistance degradation behavior in

acceptor (Mg)-doped BaTiO3 ceramics. J. Am. Ceram. Soc. 92,

2944–2952 (2009)

23. S. Sen, R.N.P. Choudhary, Impedance studies of Sr modified

BaZr0.05Ti0.95O3 ceramics. Mater. Chem. Phys. 87, 256–263

(2004)

J Mater Sci: Mater Electron

123