Enhanced Electrocaloric Effect in Sr2+-Modified Lead-Free BaZrxTi1

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Functional Inorganic Materials and Devices

Enhanced electrocaloric effect in Sr2+ modified lead-free BaZrxTi1-xO3 ceramics Xiaodong Jian, Biao Lu, Dandan Li, Yingbang Yao, Tao Tao, Bo Liang, Xiongwei Lin, Jinhong Guo, Yijiang Zeng, and Sheng-Guo Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04036 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Applied Materials & Interfaces

Enhanced electrocaloric effect in Sr2+ modified lead-free BaZrxTi1-xO3 ceramics Xiao-Dong Jian,1,2 Biao Lu,1 Dan-Dan Li,1 Ying-Bang Yao,1,2 Tao Tao,1,2 Bo Liang,1,2 Xiong-Wei Lin,1,2 Jin-Hong Guo,1 Yi-Jiang Zeng1 and Sheng-Guo Lu1,2

(1Guangdong Provincial Research Center on Smart Materials and Energy Conversion Devices, 1Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, 1School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China; 2Dongguan South China Design Innovation Institute, Building D-1, University Innovation City Area, Songshan Lake, Dongguan, 523808, China) *Author to whom any correspondence should be addressed. Electronic mails: [email protected]. (S. G. Lu)

ABSTRACT The barium strontium zirconate titanate ceramics ((BaSr)(ZrTi)O3-BSZT) with Zr4+ ionic contents of 15 and 20 mol%, and Sr2+ ionic contents of 15 , 20 , 25 , and 30 mol% were prepared using a solid-state reaction approach. The XRD, SEM were used to characterize the lattice structure and the morphologies of the ceramics. Permittivity and polarization as a function of temperature were characterized using an impedance analyzer and a Tower-Sawyer circuit. The electrocaloric effect (ECE) was measured directly and calculated using the Maxwell relation (indirectly). Results indicated that



Electronic mail: [email protected]

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the BSZT ceramics change from a normal ferroelectric to a relaxor ferroelectric with the increasing Zr4+ ionic content, which can be further modified by the addition of Sr2+ ionic content. The optimized adiabatic temperature change T obtained is 2.43 K in (Ba0.85Sr0.15)(Zr0.15Ti0.75)O3 ceramics, and T>1.6 K over a wide temperature span of 120 C was obtained.

Keywords Barium strontium zirconate titanate, phase transition, permittivity, polarization, electrocaloric effect.

1 INTRODUCTION Electrocaloric effect (ECE) is the inverse of pyroelectric effect. ECE can be defined as the adiabatic temperature change or isothermal entropy change in polar materials induced by the application or removal of the external electric field, due to the change of electric polarization. 1-5 The ECE has been considered as a promising alternative for modern refrigeration industry, e.g., food storage, heat sink for electronic devices, and medicine treatment, to meet the growing demand on control of the global warming. 6-8 Traditional vapor-compression refrigeration has been extensively studied and commercialized widely for decades since the first vapor-compression refrigeration with diethyl ether as the refrigerant was invented by Jacob Perkins in 1834.

9

However, the low thermal conversion efficiency and the environment-harmful refrigerants, e.g., Freon, are the key drawbacks. Compared to the vapor-compression


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technology, electrocaloric solid-state cooling as a novel refrigeration technology has drawn increasing attentions for researchers because of its high energy conversion efficiency over 60% of the Carnot cycle

10

and environment-friendly refrigerant,

which has shown a great potential to replace the traditional refrigeration technology. 11-14

It is well known that finding a high ECE performance material is the crucial step

for developing a refrigeration device. ECE data were firstly reported in Rochelle salt by Wiseman and Kuebler in 1963 although the ∆T was only 0.0036 K, 15 which is too small to be applied in the cooling devices. After that, researchers made much effort on lots of other materials. Unfortunately, the adiabatic temperature changes ∆Ts were still below 2.5 K until the giant ECE was found in Pb(ZrTi)O3 (PZT) (95/05) thin films, in which ∆T=12 K was obtained at 226 ℃ under 48 MV·m-1 in 2006 reported by Mischenko et al.

16

Afterwards, more and more high ECE materials with different

forms, such as PLZT (8/65/35) thin film with ∆T=1.8 K at 6.8 MV·m-1

17

and

copolymer P(VDF-TrFE) 55/45 mol% with ∆T=12 K at 209 MV·m-113, 18, have been reported in recent years. But for the fabrication of solid-state cooling devices, a substrate which is required to support the thin film, is one of the disadvantages for refrigeration design. In addition, although polymer materials can easily withstand a high electric field to achieve a high adiabatic temperature change, as shown above, the extremely low thermal conductivity is not appropriate for applications. To find a promising ECE material with excellent ECE performance and suitable for practical applications is still a challenge for researchers. Barium titanate (BTO) is considered as a potential alternative and is in fact the



most widely used ferroelectrics due to its good dielectric properties, high ferroelectric performance and lead-free environment-friendly nature. 19, 20 The ECE characteristics in BTO in various forms had been extensively investigated in BTO single crystal, 21 polycrystalline bulk/thick/thin ceramics and multilayer ceramics (MLCC) by many groups. It was reported that, a temperature change of 7.1 K was obtained at 96 C under an external electric field of 80 MV·m-1 in BTO MLCC by Bai et al. 22 However, since BTO is a first-order phase transition ferroelectric with a discontinuous phase transition, and ECE is strongly related to the change of polarization, the ECE as a function of temperature shows a jump and the maximum ∆T is always obtained near the ferroelectric-paraelectric phase transition point. Therefore, for practical cooling devices, BTO ceramics cannot provide a wide operation temperature range. Moreover, the Curie temperature of BTO is as high as 120 C, which is far from the room temperature. Therefore, it is better to make the big ECE occur over a broad temperature range near the room temperature in lead-free BTO based ceramics. Fortunately, it has been found that some ions such as Zr4+, 23-25 Sr2+, 26 Mn4+, 27 Ca2+, 28-30 Hf4+, 31 Sn4+32,33 can shift and broaden the phase transition regime, e.g., Zr4+ ions can shift the Curie temperature towards low temperatures, and convert the BTO from a normal ferroelectric to a relaxor ferroelectric, which has a broad phase transition temperature range near the room temperature. However, the disadvantage of Zr4+ ionic doping will lead to the reduction of ECE performance in the diffused phase transition range. To address the problem, the Sr4+ ions are considered to be added into the Zr4+ doped BTO to improve the ECE performance. The dielectric and ferroelectric


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properties in barium strontium titanate ceramics have been investigated for a long time, and the shift of phase transition temperature towards low temperatures, and the normal ferroelectric to relaxor ferroelectric transition were found to exist with the increasing Sr2+ ionic content. This means that the addition of Sr2+ ions will shift the Curie temperature towards low temperatures, and keep the large ECE values in a wide temperature range. Therefore, the (BaxSr1-x)(ZryTi1-y)O3 (BSZT) ceramics show a great potential to meet the demand for practical solid-state cooling devices. In this work, Sr2+ and Zr4+ ions co-doped BTO ceramic bulk ceramics were prepared using a conventional solid-state reaction process. The structural and electric properties were characterized in terms of XRD, SEM, impedance analyses and polarization-electric field (P-E) hysteresis loop measurements. The ECE performance was evaluated by both the direct and indirect methods, and the results obtained were discussed based on the phenomenological theory.

2. EXPERIMENTAL SECTION 2.1 Preparation of BSZT ceramics BSZT ceramics with x= 0.15, 0.20, 0.25, 0.3 and y=0.15, 0.2 were prepared using a conventional solid-state reaction method. Raw materials used in this work were BaCO3 (purity ≥ 99.0 %, Aladdin Reagent), SrCO3 (purity ≥ 99.0 %, Aladdin Reagent), ZrO2 (purity ≥ 99.0 %,

Aladdin Reagent) and TiO2 (purity ≥

99.0 % Aladdin Reagent). Batch materials were weighed according to the stoichiometry, and ball milled at 256 rpm for 24 h using a planetary miller. The milled



slurry was dried at 65 C for 6 h. Then the mixed-powders were ground, sieved and calcined at 1350 ~1375 C for 4 h to form the perovskite crystallites. Then the BSZT crystallite powders were mixed with 7 wt% PVB solution as the binder, sieved and pressed into disks of 1.2 cm in diameter and 1 mm in thickness under an axial pressure of 3 MPa, and then an isostatic pressure of 250 MPa. Ceramic samples were then sintered in a temperature range of 1425 C to 1450 C for 8 h in order to obtain the dense bulk ceramics. Both sides of the as-sintered ceramics were polished and sputtered gold as electrodes for the following electrical measurements.

2.2 Characterization of BSZT ceramics X-ray diffraction (XRD) measurement at room temperature was used to confirm the purity and the formation of perovskite structure with a Rigaku Ultima Ⅳ diffractometer using Ni-filtered CuKα radiation (λ=1.5406 Å) with a scan step of 0.08° and a counting time of 1 s per step, from 20° to 80°. Hitachi SU3400 scanning electron microscope was employed to observe the morphology and the densification of ceramics. The relative densities of BSZT bulk ceramics were calculated in terms of the ratio of theoretical density which was estimated from the XRD data and the density measured using the Archimedes’ method. Dielectric constants as a function of frequency and temperature were obtained using the Agilent 4284A Impendence analyzer. The P-E hysteresis loops as a function of temperature and electric field, were measured using a RADIANT TRek MODEL-609B standard ferroelectric analysis system. The ECE in this study was directly obtained by a specially designed


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calorimeter with a thermometer, details can be referred to in Ref. (34-36). Results were compared with the ECE results calculated using the Maxwell relation (Eq. (1)) based on the polarizations extracted from the P-E hysteresis loops, P (E, T). The ECE adiabatic temperature change (ΔT) and isothermal entropy change (ΔS) were obtained by the following Eqs. (1) and (2). 𝑬

∆𝐒 = ∫𝑬𝟐 𝟏

( )𝒅𝑬 ∂𝑷

(1)

∂𝑻

𝑬 𝟏

( )𝒅𝑬

∆𝐓 = ― ∫𝑬𝟐𝝆𝑪𝑬 𝟏

∂𝑷

(2)

∂𝑻

3. RESULTS AND DISCUSSIONS 3.1 XRD analyses XRD patterns of Ba(Zr0.15Ti0.85)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics doped with different Sr2+ ionic contents at room temperature are presented in Fig. 1(a, b). The high diffraction intensities and no impurity peaks indicate that a pure perovskite structure has been formed. All diffraction peaks shift towards higher angles with the increasing Sr2+ ionic content due to the replacement of Ba2+ ions by Sr2+ ions which has a smaller ionic radius (1.18 Å, while radius of Ba2+ ions is 1.35 Å). The pseudo-cubic symmetry or coexistence of tetragonal and cubic phases with a P3m3 point group at room temperature for various Sr2+ and Zr4+ ionic contents are indicated by the single (200) diffraction peak at ~ 45°, which is shown in the right of Figs.1 (a) and (b). It is different from the peak-split pattern for a typical tetragonal BTO, and the c/a ratio (approach 1) is calculated from the lattice parameter, which implies the



pseudo-cubic structure. The main reason is the high doping concentration of Zr4+ (15% and 20%) ions which enter the lattice sites of Ti4+ ions, resulting in the random distribution of Zr4+ and Ti4+ ions, and the nanosized polar domains, leading to the relaxor ferroelectric behaviors. 37,38

3.2 SEM observation The SEM images of Sr2+ doped Ba(Zr0.15Ti0.85)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramic morphologies are shown in Figs. 2 and 3, which indicate that dense ceramic structures and distinct grain boundaries have been formed via a conventional solid-state reaction approach at temperature ranged from 1425 C to 1450 C for soaking 8 h. All ceramic samples with a relative density over 95% were confirmed using the Archimedes’ method, meaning that the sintering process was well carried out. Furthermore, relatively small and uniform polycrystalline grains with 5~10 μm in size are obtained under the sintering conditions shown above.

3.3 Dielectric characterization Permittivity as a function of temperature for Sr2+ doped Ba(Zr0.15Ti0.85)O3 (a) and Ba(Zr0.2Ti0.8)O3 (b) bulk ceramics was measured at 100 Hz, 1 kHz and 10 kHz, and presented in Fig. 4. Some dielectric characteristic parameters are summarized in Table 1. The BSZT bulk ceramic samples of all compositions show good dielectric characteristics and low loss tangents (< 10% at 1 kHz). Particularly, for 20 mol% Sr2+ ions doped Ba(Zr0.15Ti0.85)O3 ceramic sample, the maximum εr reaches 23,000 at


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Curie temperature (Tc=32.8C), with a loss tangent of 6.6% at 1 kHz. With the increasing Zr4+ ionic content, Tc shifts towards low temperatures, from 67.5C for 15 mol% Zr4+ to 38.5 ℃ for 20 mol% Zr4+, εr-max decreases from 15,987 to 10,752, and the peak of dielectric constant versus temperature becomes broader due to the transformation from a normal ferroelectric to a relaxor ferroelectric due to the addition of Zr4+ ions. Furthermore, the doping of Sr2+ ions reduces the Curie temperature of the ceramic samples which is analogous to the behaviors of Zr4+ ionic doping. With the increasing Sr2+ ionic content, the εr-max of bulk ceramics increases first

and

then

decreases.

The

εr-max

over

23,000

was

obtained

in

(Ba0.8Sr0.2)(Zr0.15Ti0.75)O3 ceramics at Tc.

3.4 P-E hysteresis loop The P-E hysteresis loops measured at -40 C, RT and 100 C at 10 Hz under an applied electric field of 50 kV·cm-1 for BSZT bulk ceramics are shown in Fig. 5. Moreover, the parameters of polarization are presented in Table. 2. The P-E hysteresis loops become slimmer with higher Zr4+ contents and reveal a typical hysteresis loop of relaxor ferroelectric due to the ferroelectric domain structure transforming from the microdomains into nanodomains. Furthermore, for both 15 mol% and 20 mol% Zr4+ samples, the Sr2+ ions will enter into the A-sites in ABO3 structure, and enhance the polarization properties, e.g., increasing the saturate polarization (Ps), adjusting the remanent polarization (Pr) and modifying the ceramics between normal ferroelectric (NFE) and relaxor ferroelectric (RFE) phases. With the increasing Sr2+ ionic content,



Ps increases first and then decreases, similar to the tendency of εr-T, the maximum value of Ps about 26.58 μC·cm-2 was obtained at -40 C at 50 kV·cm-1 in (Ba0.8Sr0.2)(Zr0.15Ti0.75)O3 ceramic, and Ps of 20.16 μC·cm-2 obtained at -40 C at 50 kV·cm-1 in (Ba0.8Sr0.15)(Zr0.20Ti0.75)O3 ceramic. In addition, Sr2+ ions modify the domain structure in ferroelectrics and weaken the relaxor ferroelectric characteristics, revealed by the shape of P-E hysteresis loops shown in Fig. 4 and Table 2. Compared to nanodomains which cause the relaxor ferroelectric behaviors and broaden the permittivity-temperature peaks, Sr2+ ions doped BZT samples show dielectric and ferroelectric characteristics close to the normal ferroelectrics. Therefore, by controlling the Sr2+ ionic content, and the ratio of NFE and RFE can be adjusted, the optimized properties can be obtained.

3.5 ECE properties ECE refers to the change of electric dipole directions inducing the change of entropy inside the ferroelectric materials under the application or removal of external electric field, which means the change of polarization entropy and lattice entropy, and the heat change in terms of the endothermic/exothermic behavior occurred in materials macroscopically. Therefore, under an adiabatic condition, the temperature change (∆T) can be procured. Adiabatic temperature change (∆T) is a function of temperature, and usually shows the maximum value at the vicinity of Curie temperature. Figure 6 shows the adiabatic temperature change (∆T) as a function of temperature in different BSZT bulk ceramics under an external electric field of 50


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kV·cm-1. Whit the increasing Zr4+ ionic content, ∆T reduces from 1.75 K for 15 mol% Zr4+ to 1.55 K for 20 mol% Zr4+, because the nanodomain structure induced by the doped Zr4+ ions leads to the relaxor ferroelectric behaviors, i.e., broadening the phase transition temperature range, in which the ECE values are reduced due to the reduction of polarizations in the broadened temperature range. Compared with the nanodomains, the microdomains induce the normal ferroelectric phase with larger polarizations, thus leading to larger ECE values. It was found that ∆T increases with the increasing Sr2+ ionic content, the maximum value of ∆T=2.43 K was obtained in (Ba0.85Sr0.15)(Zr0.15Ti0.75)O3 ceramics at 68 C at 50 kV·cm-1. Moreover, the ECE efficiency (│ΔT·ΔE-1│) increases, and a value of 0.48 K·MVm-1 was procured for (Ba0.85Sr0.15)(Zr0.15Ti0.75)O3 ceramic. As a result of codoping of Zr4+ and Sr2+ ions, the ceramic samples present a broad temperature range with ∆T ≥ 1.5 K. Especially, for (Ba0.8Sr0.2)(Zr0.15Ti0.75)O3 ceramics, a ∆T of 1.7 K was obtained in the temperature range from -20 C to 80 C. By modifying the ratio of nanodomains to microdomains, or called the ratio of RFE phase to NFE phase, one can optimize the ferroelectric material with both a high ECE value and a wide applicable temperature range. Besides, both the direct and indirect measurement results of ECE are offered in this work, which is shown in the Supporting Information. Indirect measurement is based on the polarizations extracted from the P-E hysteresis loops shown in Fig. 5, and calculated using the Maxwell relation. However, as a conventional method to estimate the ECE characteristics, indirectly calculated values based on the Maxwell relation are smaller, which are shown in Figs. S1 – S9 in the Supporting Information.



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The reason is due to the polarizations extracted from the P-E hysteresis loops, which are usually obtained via a parallel plate capacitor configuration, in which only the projected polarization along the electric field can be measured, thus the polarization used for ECE calculation is always less than the real polarization of the material. In addition, for the first-order phase transition, the Clausius-Clayperon equation should be employed due to the discontinuous order parameter (polarization) near the phase transition temperature, which should be taken into account besides the Maxwell relation. Although the relaxor ferroelectric shows a continuous change of polarization, which is the averaged result, because the local polar region may demonstrates a first-order phase transition, thus the Maxwell relation may not take all the polarization changes into account. Therefore, the calculated ECE values using Maxwell relation are less than those procured directly. Table 3 presents the directly measured results of ECEs in various electrocaloric materials, e.g., single crystals, disc ceramics, MLCCs, thick/thin films, and polymers. Although those large electrocaloric responses were obtained in those PbTiO3-based thin films and polymers, e.g., PLZT, 17 P(VDF-TrFE)

39

and P(VDF-TrFE-CFE)

40,

the outstanding performance of ∆T were only achieved under an extremely high electric field. Moreover, Pb as a toxic heavy metal pollutant cannot be widely applied owing to environmental concern. In addition, BTO ceramic with various forms shows a promising prospect to be commercialized, and a ceramic composition with a large ECE temperature change over a broad temperature range has also been procured

42.

However, high Curie temperature (over 120 C) and discontinuing ∆T curve related to


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the ambient temperature would be the critical factors. To modify the ECE performance, various dopants were wieldy used, for examples, ∆T =2.63 K was achieved at 25 C under 110 kV·cm-1 in BSTMY ceramic,

27

and ∆T =1.03 K was

achieved at 110 ℃ under 35 kV·cm-1 in Hf-BZT . 31 In this work, compared with those results shown above, the investigations in Sr2+ and Zr4+ co-doped BTO ceramics provide a potential material with high electrocaloric responses, lower Curie temperature and lower electric fields. The maximum ∆T value of 2.43 K was obtained in (Ba0.85Sr0.15)(Zr0.15Ti0.75)O3 ceramic at 68 C under 50 kV·cm-1 with high ECE efficiency (│ΔT·ΔE-1│) of 0.48 K·MVm-1 . Moreover, the variation of ∆T was continuous with the ambient temperature and the ∆T – T curve was broad as well, which is beneficial to the further refrigeration applications.

4 CONCLUSIONS In conclusion, barium strontium zirconate titanate ceramics with Zr4+ ionic contents of 0.15, 0.20, and Sr2+ ionic contents of 0.15, 0.2, 0.25, 0.30, were prepared using a solid-state-reaction approach. The permittivities as a function of temperature and frequency show the characteristics of transition from a normal ferroelectric to a relaxor ferroelectric with the increasing Zr4+ ionic content, and can be further modified by the Sr2+ ions. The highest adiabatic temperature change obtained is 2.43 K in (Ba0.85Sr0.15)(Zr0.15Ti0.75)O3 ceramics. Furthermore, the ECE properties are enhanced with the doping of Sr2+ ions, shows a broad temperature range near the room temperature, which shows potential for the practical solid-state cooling devices.



ASSOCIATED CONTENT Supporting Information The direct and indirect ECE measurement results for each BSZT bulk ceramic samples are summarized in the Supporting Information. ACKNOWLEDGEMENTS This work was supported by the Natural Science Foundation of China (Grant No. 51372042, 51872053), the Guangdong Provincial Natural Science Foundation (2015A030308004), and the NSFC-Guangdong Joint Fund (Grant No. U1501246). REFERENCES (1) Correlia, T; Zhang, Q. Electrocaloric Materials: New Generation of Coolers. 2014, 34. (2) Peräntie, J.; Correia, T.; Hagberg, J.; Uusimäki, A. Electrocaloric Effect in Relaxor Ferroelectric-based Materials. In Electrocaloric Materials. Springer, Berlin, Heidelberg. 2014, 47-89. (3) Trček, M.; Lavrič, M.; Cordoyiannis, G.; Zalar, B.; Rožič, B.; Kralj, S.; Kutnjak, Z. Electrocaloric and Elastocaloric Effects in Soft Materials. Phil. Trans. R. Soc. A. 2016, 374, 2074, 20150301. (4) Kutnjak, Z.; Rožič, B.; Pirc, R. Electrocaloric Effect: Theory, Measurements, and Applications. Wiley encyclopedia of electrical and electronics engineering. 2015. (5) Pirc, R.; Kutnjak, Z.; Blinc, R.; Zhang, Q. M. Electrocaloric Effect in Relaxor Ferroelectrics. J. Appl. Phys. 2011. 110, 7, 074113.


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(23) Qian, X. S.; Ye, H. J.; Zhang, Y. T.; Gu, H.; Li, X.; Randall, C. A.; Zhang, Q. M. Giant Electrocaloric Response over a Broad Temperature Range in Modified BaTiO3 Ceramics. Adv. Funct. Mater. 2014, 24, 1300-1305. (24) Ye, H. J.; Qian, X. S.; Jeong, D. Y.; Zhang, S. J.; Zhou, Y.; Shao, W. Z.; Zhen, L.; Zhang, Q. M. Giant Electrocaloric Effect in BaZr0. 2Ti0. 8O3 Thick Film. Appl. Phys. Lett. 2014, 105, 152908. (25) Ma, Y. B.; Molin, C.; Shvartsman, V. V.; Gebhardt, S.; Lupascu, D. C.; Albe, K.; Xu, B. X. State Transition and Electrocaloric Effect of BaZrxTi1−xO3: Simulation and Experiment. J. Appl. Phys. 2017, 121, 024103. (26) Djemel, I.; Kriaa, I.; Abdelmoula, N.; Khemakhem, H. The Effect of Low Sn Doping on the Dielectric and Electrocaloric Properties of Ferroelectric Ceramics Ba0.95Sr0.05Ti 0.95Zr0.05O3. J. Alloy. Compd. 2017, 720, 284-288. (27) Xu, Z.; Qiang, H. Enhanced Electrocaloric Effect in Mn + Y co-doped BST Ceramics near Room Temperature. Mater. Lett. 2017, 191, 57-60. (28) Kaddoussi, H.; Lahmar, A.; Gagou, Y.; Manoun, B.; Chotard, J. N.; Dellis, J. L.; El Marssi, M. Sequence of Structural Transitions and Electrocaloric Properties in (Ba1-xCax)(Zr0.1Ti0.9)O3 Ceramics. J. Alloy. Compd. 2017, 713, 164-179. (29) Kaddoussi, H.; Lahmar, A.; Gagou, Y.; Asbani, B.; Dellis, J. L.; Cordoyiannis, G.; El Marssi, M. Indirect and Direct Electrocaloric Measurements of (Ba1− xCax)(Zr0. 1Ti0. 9)

O3 Ceramics (x= 0.05, x= 0.20). J. Alloy. Compd. 2016, 667, 198-203.



(30) Zhou, Y.; Lin, Q.; Liu, W.; Wang, D. Compositional Dependence of Electrocaloric Effect in Lead-free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 Ceramics. RSC. Adv. 2016, 6, 17, 14084-14089. (31) Wang, X.; Wu, J.; Dkhil, B.; Zhao, C.; Li, T.; Li, W.; Lou, X. Large Electrocaloric Strength and Broad Electrocaloric Temperature Span in Lead-free Ba0.85Ca0.15Ti1−xHfxO3 Ceramics. RSC. Adv. 2017, 7, 10, 5813-5820. (32) Hou, Y.; Yang, L.; Qian, X.; Zhang, T.; Zhang, Q. M. Electrocaloric Response near Room Temperature in Zr-and Sn-doped BaTiO3 Systems. Phil. Trans. R. Soc. A. 2016, 374, 2074, 20160055. (33) Upadhyay, S. K.; Fatima, I.; Reddy, V. R. Study of Electro-caloric Effect in Ca and Sn Co-doped BaTiO3 Ceramics. Mater. Res. Express. 2017, 4, 4, 046303. (34) Jian, X. D.; Lu, B.; Li, D. D.; Yao, Y. B.; Tao, T.; Liang, B.; Guo, J. H.; Zeng, Y. J.; Chen, J. L.; Lu, S. G. Direct Measurement of Large Electrocaloric Effect in Ba(ZrxTi1-x)O3 Ceramics. ACS Appl. Mater. Interfaces 2018, 10, 4801-4807. (35) Lu, B.; Yao, Y. B.; Jian, X. D.; Tao, T.; Liang, B.; Zhang, Q. M.; Lu, S. G. Enhancement of the electrocaloric effect over a wide temperature range PLZT ceramics by doping with Gd3+ and Sn4+ ions. J. Euro. Ceram. Soc. 2019, 39, 1093-1102. (36) Lu, B.; Li, P. L.; Tang, Z. H.; Yao, Y. B.; Gao, X. S.; Kleemann, W.; Lu, S. G. Large Electrocaloric Effect in Relaxor Ferroelectric and Antiferroelectric Lanthanum Doped Lead Zirconate Titanate Ceramics. Sci. Rep. 2017, 7, 45335.


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(37) Buscaglia, M. T.; Buscaglia, V.; Viviani, M.; Nanni, P.; Hanuskova, M. Influence of Foreign Ions on the Crystal Structure of BaTiO3. J. Eur. Ceram. Soc. 2000, 20, 12, 1997-2007. (38) Ghosh, S. K; Rout, S. K; Deshpande, S. K. Structural and Scaling Behavior in Relaxor Ferroelectric BZT Ceramic Doped with Rare Earth Europium Ion. In: Applications of Ferroelectric, International Symposium on Integrated Functionalities and Piezoelectric Force Microscopy Workshop (ISAF/ISIF/PFM), 2015 Joint IEEE International Symposium on the. IEEE, 2015, 17-20. (39) Li, X.; Qian, X. S.; Gu, H.; Chen, X. Z.; Lu, S. G.; Lin, M. R.; Bateman, F.; Zhang,

Q.

M.

Giant

Electrocaloric

Effect

in

Ferroelectric

Poly

(Vinylidenefluoride-Trifluoroethylene) Copolymers Near a First-Order Ferroelectric Transition. Appl. Phys. Lett. 2012, 101, 132903. (40) Li, X.; Qian, X.; Lu, S. G.; Cheng, J.; Fang, Z.; Zhang, Q. M. Tunable Temperature Dependence of Electrocaloric Effect in Ferroelectric Relaxor Poly (Vinylidene

Fluoride-Trifluoroethylene-Chlorofluoroethylene

Terpolymer.

Appl.

Phys. Lett. 2011, 99, 052907. (41) Sanlialp, M.; Shvartsman, V. V.; Acosta, M.; Dkhil, B.; Lupascu, D. C. Strong Electrocaloric Effect in Lead-free 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 Ceramics Obtained by Direct Measurements. Appl. Phys. Lett. 2015, 106, 6, 062901. (42) Peng, B. L.; Zhang, Q.; Lyu, Y.; Liu, L. J.; Lou, X. J.; Shaw, C.; Huang, H. T.; Wang, Z. L. Thermal strain induced large electrocaloric of relaxor thin film on



LaNiO3/Pt composite electrode with the coexistence of antiferroelectric and ferroelectric phases in a broad temperature range. Nano Energy, 2018, 47, 285-293.


Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions Table of Contents graphic: Schematic diagram shows the electrocaloric effect. A real temperature

drop

is

shown

when

the

electric

field

is

removed

for

(Ba0.85Sr0.15)(Zr0.15Ti0.85)O3 ceramic. Fig. 1 X-ray diffraction patterns for BSZT bulk ceramics with different doping contents. (a) Sr2+ doped Ba(Zr0.15Ti0.85)O3; (b) Sr2+ doped Ba(Zr0.2Ti0.8)O3. Fig. 2 SEM images of Sr2+ doped Ba(Zr0.15Ti0.85)O3 bulk ceramics sintered at the temperature ranging from 1425 C to 1450 C for 480 min. (a) Sr2+-0%, (b) Sr2+-15%, (c) Sr2+-20%, (d) Sr2+-25% and (e) Sr2+-30%. Fig. 3 Microscope images of Sr2+ doped Ba(Zr0.2Ti0.8)O3 bulk ceramics sintered at the temperature ranging from 1425 C to 1450 C for 480 min. (a) Sr2+-0%, (b) Sr2+-15%, (c) Sr2+-20% and (d) Sr2+-25%. Fig. 4 Permittivity as a function of temperature for BSZT bulk ceramics measured at 100 Hz, 1 kHz and 10 kHz. (a) Sr2+ doped Ba(Zr0.15Ti0.85)O3; (b) Sr2+ doped Ba(Zr0.2Ti0.8)O3. Fig. 5 Polarization as a function of electric field in Sr2+ doped Ba(Zr0.15Ti0.85)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics measured at -40 C, room temperature and 100 C under an electric field of 50 kV·cm-1 . Fig. 6 Direct measurement of electrocaloric response as a function of temperature in Sr2+ doped Ba(Zr0.15Ti0.85)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics under an electric field of 50 kV·cm-1 .



Page 22 of 31

Table 1 Dielectric characteristic parameters of Sr2+ doped Ba(Zr0.15Ti0.85)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics. Sr2+ doped Ba(Zr0.15Ti0.85)O3 Tc (℃) Sr2+-0% Sr2+-15% Sr2+-20%

67.5 41.7 32.8

εr-max 15987 17550 23082

Sr2+ doped Ba(Zr0.2Ti0.8)O3 Tc (℃)

εr-max

𝐭𝐚𝐧 𝜹

9.2%

Sr2+-0%

38.5

10752

8.5%

7.4%

Sr2+-15%

22.2

10160

7.1%

6.6%

Sr2+-20%

12.8

11911

6.6%

Sr2+-25%

1.4

7125

6.5%

𝐭𝐚𝐧 𝜹

Sr2+-25%

25.1

17855

6.3%

Sr2+-30%

13.4

14480

6.1%


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Polarization parameters of Sr2+ doped Ba(Zr0.15Ti0.85)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics at room temperature under an applied electric field of 50 kV·cm-1 . Sr2+ doped Ba(Zr0.15Ti0.85)O3 Sr2+-0% Sr2+-15% Sr2+-20%

Sr2+ doped Ba(Zr0.2Ti0.8)O3

1P s

1P r

2E c

18.26

4.72

3.59

20.99 22.59

4.2.1 5.32

1P s

1P r

2E c

Sr2+-0%

12.27

1.76

2.12

2.81

Sr2+-15%

17.09

2.92

2.08

2.96

Sr2+-20%

16.11

2.63

2.65

Sr2+-25%

15.18

3.26

1.73

Sr2+-25%

18.72

2.83

1.94

Sr2+-30%

17.97

1.19

0.92

(μC·cm-2)

1 P s, P r 2 Ec (kV·cm-1)



Page 24 of 31

Table 3 Comparison of ECE temperature change T and ECE efficiency of various materials Material

Form

T(℃)

∆T(℃)

∆E(kV·cm-1)

∆T·∆E-1(k·m·MV-1)

Method

Reference

Ba(Zr0.15Ti0.85)O3

bulk

67.5

1.75

50

0.35

Direct

This work

(Ba0.85Sr0.15)(Zr0.15Ti0.85)O

bulk

41.7

2.43

50

0.48

Direct

This work

(Ba0.8Sr0.2)(Zr0.15Ti0.85)O3

bulk

32.8

2.20

50

0.44

Direct

This work

(Ba0.75Sr0.25)(Zr0.15Ti0.85)O

bulk

25

1.95

50

0.39

Direct

This work

(Ba0.7Sr0.3)(Zr0.15Ti0.85)O3

bulk

13.4

1.71

50

0.34

Direct

This work

Ba(Zr0.2Ti0.8)O3

bulk

38.5

1.55

50

0.31

Direct

This work

(Ba0.85Sr0.15)(Zr0.2Ti0.8)O3

bulk

22.2

1.84

50

0.37

Direct

This work

(Ba0.8Sr0.2)(Zr0.2Ti0.8)O3

bulk

12.8

1.96

50

0.39

Direct

This work

(Ba0.75Sr0.25)(Zr0.2Ti0.8)O3

bulk

1.4

1.75

50

0.35

Direct

This work

PLZT

thin film

45

40

120

0.33

Direct

(17)

BTO

single crystal

129

0.9

12

0.75

Direct

(21)

BTO

ceramic MLCC

96

7.1

800

0.09

DSC

(22)

Ba(Zr0.2Ti0.8)O3

ceramics-glass

39

4.5

145

0.31

Direct

(23)

0.65BZT–0.35BCT

ceramic

65

0.33

20

0.16

DSC

(38)

BSTMY

ceramic

25

2.63

110

0.23

Indirect

(27)

Hf-BCT

ceramic

133

1.03

35

0.29

Indirect

(31)

P(VDF-TrFE)

polymer

33

35

180

0.13

Direct

(36)

P(VDF-TrFE-CFE)

polymer

30

15.7

150

0.1

Direct

(37)

3

3


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 1 X-ray diffraction patterns for BSZT bulk ceramics with different doping contents. (a) Sr2+ doped Ba(Zr0.15Ti0.85)O3;

(b) Ba(Zr0.2Ti0.8)O3.

231x250mm (300 x 300 DPI)



Fig. 2 SEM images of Sr2+ doped Ba(Zr0.15Ti0.85)O3 bulk ceramics sintered at the temperature ranging from 1425 ℃ to 1450 ℃ for 480 min. (a) Sr2+-0%, (b) Sr2+-15%, (c) Sr2+-20%, (d) Sr2+-25% and (e) Sr2+30%. 595x297mm (300 x 300 DPI)


Page 26 of 31

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Fig. 3 SEM images of Sr2+ doped Ba(Zr0.2Ti0.8)O3 bulk ceramics sintered at the temperature ranging from 1425 ℃ to 1450 ℃ for 480 min. (a) Sr2+-0%, (b) Sr2+-15%, (c) Sr2+-20% and (d) Sr2+-25%. 254x190mm (300 x 300 DPI)



Fig.4 Permittivity as a function of temperature for BSZT bulk ceramics measured at 100 Hz, 1 kHz and 10 kHz. (a) Sr2+ doped Ba(Zr0.15Ti0.85)O3; (b) Sr2+ doped Ba(Zr0.2Ti0.8)O3. 189x287mm (300 x 300 DPI)


Page 28 of 31

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Fig. 5 Polarization as a function of electric field in Sr2+ doped Ba(Zr0.15Ti0.85)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics measured at -40℃, room temperature and 100℃ under an electric field of 50 kV•cm-1. 375x191mm (300 x 300 DPI)



Fig.6 Direct measurement of electrocaloric response as a function of temperature in Sr2+ doped Ba(Zr0.15Ti0.85)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics under an electric field of 50 kV•cm-1. 401x381mm (300 x 300 DPI)


Page 30 of 31

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Table of Contents Graphic 238x190mm (300 x 300 DPI)


Enhanced Electrocaloric Effect in Sr2+-Modified Lead-Free BaZrxTi1

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