Combined effects of diffuse phase transition and microstructure on the electrocaloriceffect in Ba1xSrxTiO3 ceramicsYang Bai, Xi Han, Kai Ding, and Li-Jie Qiao Citation: Applied Physics Letters 103, 162902 (2013); doi: 10.1063/1.4825266 View online: http://dx.doi.org/10.1063/1.4825266 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/16?ver=pdfcov Published by the AIP Publishing

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Combined effects of diffuse phase transition and microstructureon the electrocaloric effect in Ba12xSrxTiO3 ceramics

Yang Bai (白洋),a) Xi Han (韩茜), Kai Ding (丁凯), and Li-Jie Qiao (乔利杰)Key Laboratory of Environmental Fracture (Ministry of Education), University of Science and Technology Beijing,Beijing 100083, China

(Received 9 August 2013; accepted 28 September 2013; published online 16 October 2013)

This paper demonstrates the combined effects of diffuse phase transition and microstructure on the

electrocaloric effect (ECE) in Ba1�xSrxTiO3 (x¼ 0.2–0.4) ferroelectric ceramics. The ECE was

characterized by direct heat flow measurement and an indirect thermodynamic method, which both

provided consistent results. With increasing amount of Sr, the phase transition diffused and the

grains grew larger. Since larger grains favor a high DTmax while a diffuse phase transition does not,

the combination of these two effects induced nonmonotonic variation of the ECE. The x¼ 0.35

sample showed the best performance, with an ECE strength of 0.23K�m/MV near room

temperature.VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4825266]

Recently, the electrocaloric effect (ECE) of ferroelec-

trics has attracted a lot of interest, because of its potential

applications in solid-state refrigeration. The ECE is regarded

as the best solution for cooling microelectronic devices-it is

easy to miniaturize, highly efficient, and low cost. Since

the observation of a giant ECE, DTmax¼ 12K, in

PbZr0.95Ti0.05O3 thin films in 2006,1 giant ECEs have been

reported for various ferroelectric ceramic and polymer

films.2–11

The ECE is only significant in the near vicinity of a

phase transition, so it is thought to be closely related to the

features of the phase transition.12 Our previous work

revealed that the ECE near first order phase transitions

(FOPT) was dominated by the energy change of the lattice

structure, rather than the entropy change from polarization

ordering.13 Moya reported that the ECE strength (jDT/DEj)is determined by the sharpness of a transition.14 It has also

been shown, both theoretically and experimentally, that

when a transition is diffuse, the peak value of the ECE may

be reduced, and the peak may be widened.15,16 Besides the

intrinsic nature of the material, the ECE of ferroelectric

ceramics is sensitive to the microstructure, as are many other

ferroelectric properties. However, the combined effect of the

nature of the phase transition and the microstructure of sin-

tered ceramics has not previously been considered and is the

subject of this paper. We selected Ba1�xSrxTiO3 ceramics

for investigation, because the substitution of Sr for Ba can

gradually make the typical FOPT of BaTiO3 diffuse, and

modify the microstructure at the same time.

Ba1�xSrxTiO3 (x¼ 0.2� 0.4) ceramics were prepared

from analytical reagent grade BaCO3, SrCO3, and TiO2 by a

conventional ceramic method. After calcining at 1000 �C, apure perovskite phase was obtained. The resulting powders

were dry-pressed and sintered at 1350 �C.Scanning electron microscopy images of the microstruc-

tures of the sintered samples are shown in Fig. 1. All samples

exhibited dense microstructures with the measured density

over 95% of the theoretical density. With increasing amount

of Sr, the grains became larger. At x¼ 0.25, the grain size

was 1� 2 lm, and it became >20 lm at x¼ 0.4. When Ba2þ

is partly substituted by Sr2þ (which has a smaller radius), the

lattice deficiencies can promote grain growth during the sin-

tering process; the effects of this become more obvious

when the proportions of the two ions are similar.

The thermal characteristics of the tetragonal-cubic (T-C)

phase transition were measured with a differential scanning

calorimeter (DSC, TA Instruments Q2000) and are shown in

Fig. 2. With increasing x, the endothermic peak shifts to lower

temperature and becomes less sharp, indicating that the phase

transition has gradually become diffuse. The phase transition

temperature and the latent heat are plotted in the inset of

Fig. 2. Both show linear decreases: T1¼�339xþ 130 (�C)and H¼�1.67xþ 0.87 (J/g). The phase transition temperature

is taken here as T1 in the Landau-Devonshire theory, i.e., the

highest temperature where the low-temperature phase exists in

a metastable state under zero-field. The intercepts of 130 �Cand 0.87 J/g in the linear fits agree well with the transition tem-

perature and latent heat of pure BaTiO3. This implies that the

substitution of Sr for Ba diffuses the phase transition, and

reduces the latent heat.

FIG. 1. The microstructure of the samples sintered at 1350 �C.

a)Author to whom correspondence should be addressed. Electronic mail:

baiy@mater.ustb.edu.cn. Fax: þ86-10-62332345.

0003-6951/2013/103(16)/162902/4/$30.00 VC 2013 AIP Publishing LLC103, 162902-1

APPLIED PHYSICS LETTERS 103, 162902 (2013)

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The temperature dependence of the permittivity (er) wasmeasured with an HP4192 impedance analyzer (Fig. 3(a)).

The diffuse phase transition was characterized based on

Smolenski’ composition fluctuation theory and the Curie-

Weiss law, where the diffusion exponent a and the variance

r (indicating the width of the phase transition) were calcu-

lated by

1

erðTÞ �1

erm¼ ðT � TmÞa

2ermr2; (1)

where erm and Tm stands for the maximum of permittivity

and the corresponding temperature. With increasing x, aincreases gradually, indicating a gradually diffused transi-

tion. However, a is always close to 1 and r is small, even for

the x¼ 0.4 sample (a¼ 1.29 and r¼ 3.9), meaning that the

phase transition is not particularly diffuse. In addition, the

obtained values of Tc (the temperature at which two phases

have same energy potential) and T0 (the lowest temperature

where the high-temperature phase exists in a metastable

state) both decrease with x. The difference between Tc and

T0 reflects the thermal hysteresis of the FOPT. With increas-

ing x, the value of (Tc-T0) decreases but does not vanish,

again indicating the FOPT is only somewhat diffuse.

The indirect ECE characterization is based on the

Maxwell relation: (@P/@T)E¼ (@S/@E)T. The ECE DT of the

samples is calculated17 and plotted in Fig. 4. The samples

have remarkable ECE in the vicinity of the phase transition

and DTmax occurs above the transition temperature. The

ECE DT increases, and DTmax shifts to higher temperature,

with increasing electric field. The values and positions of

DTmax are listed in Table I. The composition dependence of

DTmax is not monotonic. At E¼ 20 kV/cm, DTmax decreases

slightly when x goes from 0.2 to 0.3, and then increases

again when x increases further to 0.35. Under E¼ 10 kV/cm,

DTmax decreases monotonically. Among all samples exam-

ined, the x¼ 0.35 sample has the best ECE performance

with an ECE strength of 0.23K �mm/kV.

The direct ECE characterization was based on meas-

uring the heat flowment in an isothermal process,17 and the

results are plotted in Fig. 5. The overall behavior is similar

FIG. 2. Heat flow of Ba1�xSrxTiO3 ceramics in a heating process. The insets

show the variation of T1 and H.

FIG. 3. Temperature dependence of permittivity. The insets show the

composition dependence of a, r, T0 and Tc.

FIG. 4. Indirect ECE characterization of Ba1�xSrxTiO3 ceramics.

TABLE I. Comparison of ECE performance obtained by indirect and direct

characterizations.

Indirect method Direct method

E¼ 2 kV E¼ 1 kV E¼ 1 kV

T (�C) DTmax (K) T (�C) DTmax (K) T (�C) DTmax (K)

x¼ 0.2 76 0.39 74 0.15 – –

x¼ 0.25 61 0.37 47 0.16 47 0.15

x¼ 0.3 30 0.36 26 0.18 33 0.16

x¼ 0.35 23 0.42 16 0.23 15 0.18

FIG. 5. Direct ECE characterization of Ba1�xSrxTiO3 ceramics.

162902-2 Bai et al. Appl. Phys. Lett. 103, 162902 (2013)

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to that found in the indirect measurement, i.e., a significant

peak occurs just above the phase transition temperature. The

good agreement between the results of the direct and indirect

measurements confirms their validity.

The ECE depends on the change of ordering entropy,

lattice energy, and electric polarization energy during the

application or removal of the field. Previous reports indicated

that the ECE near a FOPT is dominated by the energy

change associated with the change of the lattice structure,

which can be two orders of magnitude higher than the en-

tropy change of dipolar order.13 The energy change associ-

ated with the diffused transition will be spread over a range

of temperatures, and result in a lower DTmax.15,16 The typical

FOPT of pure BaTiO3 tends to be diffused by Sr substitution,

and this diffusion is expected to lower DTmax. However, the

ECE of ferroelectric ceramics is also affected by microstruc-

ture. It has been shown theoretically that larger grain sizes

should promote the ECE, because the grain boundaries have

weaker ferroelectricity than that of the grains.18,19 Taking an

extreme example, a BaTiO3 single crystal has a much larger

ECE than that of polycrystalline BaTiO3 ceramics, although

both materials have the same composition and even similar

density. In our previous work, a BaTiO3 single crystal

showed an ECE of DTmax¼ 4.8K under 10 kV/cm, while

that of the corresponding ceramic was only 0.5K. The ECE

of the Ba1�xSrxTiO3 ceramics has a complicated composi-

tion dependence, because of the combined effects of the

diffuse phase transition and the microstructure.

Among the Ba1�xSrxTiO3 samples, the x¼ 0.35 sample

has the largest ECE, with an ECE strength of 0.23K �mm/kV

at 16 �C near room temperature (RT). Previously reported

ECEs in ferroelectric ceramics were always at high tempera-

tures, >100 �C, but there is an urgent need for RT microre-

frigerator for microelectronic devices. The operating

temperature near RT will facilitate RT applications. Among

the ECE materials with operating temperatures near RT,20–26

the Ba0.65Sr0.35TiO3 in this work has a fairly high DTmax, as

shown in Table II; its ECE strength is higher than those of

most ceramics, and is only slightly lower than that of the

PMN-PT single crystal. Recently, Liu reported the ECE of

spark-plasma-sintered Ba0.65Sr0.35TiO3 ceramics, and found

an ECE strength similar to that found on our work.26

The ECE of Ba1�xSrxTiO3 ceramics was investigated by

direct heat flow measurement and an indirect thermodynamic

method. The ECE performance was determined by the fea-

tures of the phase transition and the microstructure of the

ceramics. In Ba1�xSrxTiO3 ceramics, a greater amount of Sr

led to a more diffuse phase transition (which lowers DTmax)

and induced larger grains (which increases DTmax), so the

variation of the ECE with Sr content for our samples was

complicated. Of the samples examined, Ba0.65Sr0.35TiO3 had

the best performance, with an ECE strength of 0.23K�mm/kV

near RT. This material may potentially be applied in RT re-

frigeration for microelectronic devices.

This work was supported by Grants from the National

Science Foundation of China (51172020, 51072021, and

51372018), the Program for New Century Excellent Talents

in University (NCET-12-0780), the Fundamental Research

Funds for the Central Universities (FRF-TP-09-028A), and

the Program for Changjiang Scholars and Innovative

Research Team in University (IRT1207).

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