HAN Liu-Yang, GUO Shao-Bo, YAN Shi-Guang, RéMIENS Denis, WANG Gen-Shui, DONG Xian-Lin

Electrocaloric Effect in Pb0.3CaSr0.7-xTiO3Ceramics Near Room Temperature

HAN Liu-Yang1,2,3, GUO Shao-Bo1, YAN Shi-Guang1, RéMIENS Denis3, WANG Gen-Shui1, DONG Xian-Lin1

(1. Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. Université Polytechnique Hauts-de-France, Valenciennes 59313, France)

The electrocaloric (EC) effect is strongly related to interaction of polarization and temperature changes, showing great potential in high-efficient solid state refrigeration. This work focuses on the Pb0.3CaSr0.7–xTiO3(PCST(),= 0.00, 0.05, 0.10, 0.15) ceramics in which the influence of Ca content on dielectric and ferroelectric property under electric field was studied, and the EC temperature change was calculated through indirect method. Substitution of Ca largely modifies the diffused phase transition behaviors of PCST ceramics, which the diffusion exponent of PCST(0.05) increases with electric field up, indicating a promising wide temperature range of large electrocaloric effect. Thus, the largest adiabatic temperature change (1.71 K) is obtained near the room temperature in PCST(0.05) by indirect method. With an electric field of 8 kV/mm, PCST(0.05) ceramic shows good EC effect in a wide temperature range that the adiabatic temperature change is larger than 1 K from 5 ℃ to 70 ℃.

electrocaloric effect; ferroelectrics ceramics; diffused phase transition

When an electric field is applied or removed, there is a reversible temperature change in dielectric materials that can be exploited as promising solid-state refrigeration candidates to replace vapor-compression systems[1-3]. In 2006, the giant EC response with an adiabatic temperature change (Δ) of 12 K was demonstrated in Pb(Zr0.95Ti0.05)O3(PZT) antiferroelectric films near the Curie temperature (C) for a huge polarization change[4].From then on, a booming development of EC effect started, and many advancements have been achieved[3,5-7].

The pyroelectric and EC effects of ferroelectrics are strongly correlated with each other. The EC effect is the thermodynamically reverse process of pyroelectric effect due to Maxwell relationship. Thus many pyroelectrics can also be good EC materials for solid-state refrigeration, such as PZT, BaSr1–xTiO3(BST) and PbSc1/2Ta1/2O3(PScT)[5, 8-11]. Much attention has been especially paid on BST and PScT for its large pyroelectric effect near the room temperature[5, 8, 10-11].Recently, Pb0.3CaSr0.7–xTiO3[PCST(),= 0.00, 0.05, 0.10, 0.15] was reported to show high pyroelectric coefficient near room temperature[12], and the maximum of pyroelectric coefficient is obtained under a very low electric field of 200 V/mm. The diffused phase transitions occur in PCST() ceramics, which may lead to a wide EC temperature span. The enhanced pyroelectric properties and the low induced-electric-field of PCST() ceramics predict high EC effect in PCST() ceramics, indicating great potential in electrocaloric solid-state refrigeration devices.

This work focuses on the EC effect of Pb0.3CaSr0.7–xTiO3(PCST(),=0.00, 0.05, 0.10, 0.15) ceramics. The PCST() ceramics experience typical diffused phase transition, thus good EC effects were observed in a wide temperature span. The optimized EC effect was obtained in 0.05 Ca-doped ceramic, and the indirect EC method was carried out to verify Δvalues.

1 Experimental

2 Results and Discussion

2.1 Dielectric properties

The temperature dependence of dielectric permittivity for PCST() ceramics is given in Fig. 1(a). The ferroelectric- paraelectric phase transition of PCST() ceramics happens near the room temperature. The electric field is believed to stabilize the ferroelectric phase when the temperature is higher thanC. Thus the peak value of dielectric permittivity is suppressed with an electric field of 0.5 kV/mm. To reveal it clearly, the diffusion exponent of the phase transition can be characterized by[8]Eq(1):

whereand TCare the peak value of dielectric constant and the corresponding temperature, γ the diffusion exponent, and σ the variance. The diffusion exponent of samples with electric field were given in Fig. 1(b). As it was reported, the phase transition of PCST(x≤0.10) is second- order transition, while the phase transition order is first order in PCST(0.15)[12]. In general, γ increases with electric field when a second order phase transition occurred (x≤0.10). For x=0.15, where the first order phase transition happened, γ firstly decreases then increases with electric field up. The diffusion exponent of PCST(0.05) rises from 1.36 to 1.68 with an electric field changing from 0 to 0.5 kV/mm, indicating an enhanced diffused transition happened with electric field increasing. These diffusion behaviors under electric field give us expectation for a temperature-broadened EC effect in PCST(x) ceramics with application of large electric field[13-14].

2.2 Ferroelectric properties

Fig. 2 shows theloops of PCST() ceramics at 5 ℃, and inset shows the composition-dependentCin PCST() ceramics. The samples show the similar slim ferroelectric hysteresis loops with small coercive field. The maximums of the polarization (max) of samples are different and peak at=0.05.

2.3 Electrocaloric properties

Fig. 3(a) shows theloops of PCST(0.05) ceramic with an electric field of 8 kV/mm at different temperatures, and the inset illustrates the temperature dependence of the polarization under different electric fields. It is seen that the polarization decreases sharply just aboveCunder low electric fields but decreases slowly under high electric field. Based on the Maxwell relationship[15], the adiabatic temperature change (Δ) of EC effect can be calculated by,

Where ρ is the density and c is the specific heat (426 J/(kg·K)). The temperature dependence of the ΔT for PCST(0.05) under different electric fields is given in Fig. 3(b). The maximum ΔT is obtained at the temperature slightly higher than TC and increases gradually with the increase of the electric field.

Fig. 3 (a) P-E loops under different temperatures, and (b) calculated ΔT-T curves under different electric fields of PCST(0.05) sample

The indirect Δas a function of temperature in PCST() ceramics is shown in Fig. 4. The maximum of Δreaches 1.71 K under an electric field of 8 kV/mm in PCST(0.05) ceramic at 22 ℃, and the diffused phase transition contributes to a wide temperature range, where the Δof PCST (0.05) ceramic is higher than 1 K even at 70 ℃. The span from 5 to 70 ℃ is the main operating temperature range for many devices, as well for cooling applications.

Fig. 4 Calculated ΔT-T curves of PCST(x) ceramics

In Table 1, the EC properties of PCST() are listed, and other EC materials that show good EC effect are given for comparison. Since the practical cooling devices work at room temperature to a large extent, PCST(0.05) ceramic exhibits good performance at room temperature compared to other EC materials. Meanwhile, the Δof PCST(0.05) ceramic larger than 1 K from 5 ℃ to 70 ℃. All these superior performances demonstrate that PCST (0.05) is a good EC material with high cooling efficiency.

3 Conclusions

In summary, the dielectric diffusion behaviors of PCST()ceramics under electric field were systematically studied, all samples show the increasing diffusion exponent with high electric field applied. When Ca substitution is 0.05, the sample shows the largestmax. The enhanced EC effect near the room temperature with the broadened range is obtained by the indirect method based on the Maxwell relationship. The EC response of PCST(0.05) reaches 1.71 K at 20 ℃ , and it is larger than 1 K in a wide tem-perature range from 5 ℃ to 70 ℃. Therefore the EC effect near the room temperature with the wide range exhibits great potential for practical cooling applications.

Table 1 Comparison of EC properties of common reported materials

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Pb0.3CaSr0.7-xTiO3陶瓷的室溫電卡效應(yīng)

韓劉洋1,2,3, 郭少波1, 閆世光1, RéMIENS Denis3, 王根水1, 董顯林1

(1. 中國科學(xué)院 上海硅酸鹽研究所，無機功能與器件重點實驗室，上海 200050； 2. 中國科學(xué)院大學(xué)，北京 100049； 3. 上法蘭西理工大學(xué), 瓦朗謝納 59313, 法國)

電卡效應(yīng)是極性材料中極化強度和溫度的相互作用, 具有電卡效應(yīng)的鐵電陶瓷材料在高效固態(tài)制冷領(lǐng)域有很好的應(yīng)用前景。本研究以Pb0.3CaSr0.7–xTiO3[PCST(),= 0.00, 0.05, 0.10, 0.15]陶瓷為對象, 系統(tǒng)研究了在電場作用下Ca含量對材料介電性能和鐵電性能的影響, 并通過間接法計算了不同溫度下材料的電卡溫變。研究結(jié)果顯示: Ca含量可顯著調(diào)控PCST陶瓷的彌散相變特性, PCST(0.05)的相變彌散因子隨外加電場的增大而增大, 可利用彌散相變在較寬溫度區(qū)間內(nèi)獲得較大的電卡效應(yīng)。經(jīng)計算可得: PCST(0.05)在室溫下可產(chǎn)生1.71 K的溫變。當(dāng)電場為8 kV/mm時, PCST(0.05)陶瓷在5~70 ℃的溫度范圍內(nèi), 絕熱溫變均大于1 K, 表現(xiàn)出優(yōu)異的電卡效應(yīng)。

電卡效應(yīng); 鐵電陶瓷; 彌散相變

TQ174

A

2018-11-20;

2019-01-30

National Natural Science Foundation of China (11604354, 11774366); Chinese Academy of Sciences President’s International Fellowship initiative (2017VEA0002); Research Equipment Program of Chinese Academy of Sciences (YJKYYQ20170018)

HAN Liu-Yang (1991-), female, candidate of PhD. E-mail: hanliuyang@student.sic.ac.cn

WANG Gen-Shui, professor. E-mail: genshuiwang@mail.sic.ac.cn; GUO Shao-Bo, senior engineer. E-mail: guoshaobo@mail.sic.ac.cn

1000-324X(2019)09-1011-04

10.15541/jim20180551