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Giant Negative Electrocaloric Effect in Anti-Ferroelectric (Pb0.97La0.02)(Zr0.95Ti0.05)O3 Ceramics Ying-Cheng Zhao, Qiu-Xiang Liu,* Xin-Gui Tang, Yan-Ping Jiang, Bi Li, Wen-Hua Li, Li Luo, and Xiao-Bin Guo School of Physics & Optoelectric Engineering, Guangdong University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, P. R. China
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S Supporting Information *
ABSTRACT: A giant electrocaloric effect is reported in (Pb0.97La0.02)(Zr0.95Ti0.05)O3 antiferroelectric ceramics. These samples were fabricated by a solid-state mixed oxide technique. Dielectric analyses were employed to investigate the anti-ferroelectric (AFE) and ferroelectric (FE) phase transitions of the sample. During the heating process, the phase transition from the orthorhombic anti-ferroelectric phase (AFEO) to the tetragonal anti-ferroelectric phase (AFET) occurs at 155 °C, and the phase transition from AFET to PE occurs at 225 °C. Using the Maxwell relationship, the entropy change ΔS and adiabatic temperature change ΔT were obtained at different electric fields ranging from 40 to 65 kV/cm. The maximum adiabatic temperature change (ΔTmax = −7.47 K) was obtained at 50 kV/cm, which was attributed to the field-induced phase transformation between the anti-ferroelectric and ferroelectric phases. These results showed that PLZT2/95/5 ceramics possess a large negative electrocaloric effect value, which could be applied in achieving cooling power as refrigerants.
1. INTRODUCTION An electrocaloric (EC) effect, the temperature of polar materials changes if an extra electric field is applied or removed, is very likely to be applied in advanced cooling devices for integrated electronics and sensors. For instance, compared with the traditional cooling technology, the refrigeration system based on EC presents a remarkable improvement in cooling efficiency, and the cooling process has no environmental pollution.1−3 Among the methods proposed to obtain a large EC effect, based on the perovskite-structured oxides with significant temperature-dependent spontaneous polarization, many research studies focus on the concerns thermodynamic phenomena occurring in ferroelectric (FE) and anti-ferroelectric (AFE) materials due to the electric-fieldoperated phase transition.4 These materials show large potential as dielectrics in high-energy-density electrical capacitors and in improving the on-board cooling requirements of advanced electronics. In recent years, numerous studies have shown that positive or negative EC effects exist in various FE/AFE membrane systems.5−12 The negative EC effect was discovered by researchers in Pb(Mg1/3Nb2/3)O3-PbTiO3 materials.8 In particular, it is expected that the negative EC effect could be applied in cooling devices, which requires a large negative adiabatic temperature change (ΔT) and isothermal entropy change (ΔS). To obtain a larger ΔT value, most researchers paid attention to thin films that are suitable for a large applied electric field. Although thin films work better in miniature cooling equipment, bulk materials have more advantages in the middle- and large-scale equipment.3,13 Furthermore, the actual cooling capacity and the EC strength of their bulk counter© XXXX American Chemical Society
parts, such as single crystals and ceramics, are generally larger than those of thin films. Hence, the exploitation of the EC effect in bulk materials is significant and necessary. In this work, ceramic samples were synthesized by a solidstate mixed oxide technique. The phase transition of the samples was determined by dielectric measurements. Furthermore, a giant negative EC value of −7.47 K was calculated according to the Maxwell relation and the thermal variation of polarization−electric field (P−E) hysteresis loops, which was related to the electric-field-induced AFE phase. Based on the giant negative electrocaloric effect (ECE) value in the study, this sample could be applied in ceramic capacitors with ECE cooling devices.
2. RESULTS AND DISCUSSION Figure 1 exhibits the scanning electron microscopy (SEM) images of PLZT2/95/5 ceramic samples. We can see from the image that the grain boundary is obvious and the average size of the boundary is about 5−7 μm for fine-grained ceramics. Based on Archimedes’ principle, the measured sample density is equal to ∼95% of the theoretical density,14 indicating that the fabricated samples were confirmed to be dense. As shown in Figure 2a,b, the dielectric constant (εr) and dielectric loss (tan δ) of PLZT2/95/5 ceramic samples varied with temperature at different frequencies from 1 to 20 k Hz. The εr−T curves with varying frequencies from 1 to 20 k Hz have no obvious variation at temperatures ranging from 0 to Received: July 12, 2019 Accepted: August 13, 2019
A
DOI: 10.1021/acsomega.9b02149 ACS Omega XXXX, XXX, XXX−XXX
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Figure 3 shows the P−E loops of the samples measured at different temperatures with the maximum applied field of 60 kV/cm. As can be seen from Figure 3a,b, the polarizations measured at 40 and 150 °C are very small. The typical AFE double P−E loops are obtained in Figure 3c−e at 60 kV/cm. First, the forward direction of the electric field is analyzed. Under a low electric field, the hysteresis loop shows a straight line. When the electric field is added to a certain intensity (corresponding to the EF in the diagram), the sample undergoes phase transition from the AFE to FE metastable phase, which causes a sudden increase of polarization. The critical field values for the phase transitions at different temperatures are not the same. As the temperature increases, this critical field value decreases from 52 kV/cm at 155 °C to 45 kV/cm at 175 °C; when the temperature increases to 205 °C, it continues to drop to 30 kV/cm. When the temperature further increases to 210 °C, a typical ferroelectric hysteresis loop diagram is shown in Figure 3f, indicating that the AFE phase in this region is quite weak, thus the electric-fieldinducting phase transition is very small. With the temperature further increasing to 225 °C, a phase transition from the AFE to PE occur, and the AFE phase at high temperature is probably AFET; similar results were reported by Hao.47 On the other hand, look at the forward electric field in Figure 3c−e when the electric field drops. It can be seen that when the electric field drops to a certain value (corresponding to EA in the diagram), a transition from the FE phase to the AFE phase occurs, which causes a sudden decrease of polarization. When the applied voltage drops to 0, the remnant polarization of the sample at 155 °C is 0 as shown in Figure 3, indicating a complete anti-ferroelectric phase. As shown in Figure 3d, the phase transition from AFE to FE occurs at a lower electric field when the temperature increases to 175 °C, indicating that the AFE phase is more unstable at this temperature. The polarization at 205 °C shown in Figure 3e is relatively large,
Figure 1. SEM patterns for the fractured surface microstructure of PLZT2/95/5 ceramic samples.
Figure 2. (a) Dielectric constant and (b) dielectric loss tan δ changes as a function of temperature for PLZT2/95/5 at various frequencies.
400 °C. Two anomalous peaks could be seen in Figure 2a, and the corresponding dielectric constants for them are 742 and 4489, respectively. The first anomalous peak at 155 °C illustrates that a phase transition from AFEO to AFET occurs in this region, and the second anomalous peak at 225 °C corresponds to another phase transition from AFET to PE.15−22
Figure 3. P−E hysteresis loops of PLZT2/95/5 ceramics measured at different temperatures: (a) 40 °C, (b) 150 °C, (c) 155 °C, (d) 175 °C, (e) 205 °C, and (f) 210 °C. B
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indicating that this is the coexistence phase of the AFE and FE. Many PLZT ceramics have similar phase-transition reactions,23,24 so a small electric field can cause a sudden change of polarization. This result demonstrates that the FE phase becomes more stable as the temperature increases, so the phase transition process from AFE to FE becomes more easier. When the electric field reaches EA, the phase translates from FE to AFE, and when the electric field reaches EF, the phase recovered from AFE to FE. The decrease of EF with increasing temperature suggests that the AFE phase was more and more unstable while the FE phase became more and more stable. According to the dielectric analysis results, the sample will be transformed into the PE phase above 225 °C. However, owing to the fact that the ceramics were immersed in silicone oil in the measurements and would break down when the temperatures were beyond 220 °C, P−E data were obtained from 40 to 210 °C (which were not up to the transition temperature). On the basis of P−E loops, the isothermal entropy change ΔS and the adiabatic change in temperature ΔT of the ceramics can be calculated by using Maxwell’s relations as follows25−29 ΔS = −
ΔT = −
1 ρ
∫E
1 ρ
E2
1
∫E
E2
1
ij ∂P yz jj zzdE k ∂T {
T ji ∂P zy jj zzdE C k ∂T {
Figure 4. (a) Polarization, (b) pyroelectric coefficient, (c) isothermal entropy change, and (d) adiabatic temperature change as a function of temperature under different electric fields for the PLZT2/95/5 ceramics.
(1)
(2)
where ρ represents the density of the PLZT ceramic, C represents the specific heat capacity, ∂P/∂T represents the pyroelectric coefficient, the external electric field ranges from E1 to E2, ΔS represents the isothermal entropy change, and ΔT represents the adiabatic temperature change. The measured density of the sample is 5.53 g cm−3. Based on a fourth-order polynomial fitting to the cubic-spline interpolation of the raw P−E hysteresis loop data, the upper branch of the hysteresis loops was used to calculate the curves of polarization as a function of temperature under different applied electric fields.30 The calculated results are presented in Figure 4a, and the applied electric field varied from 40 to 65 kV/cm. As is shown in Figure 4a, the polarization at low temperatures (below 400 K) at all measured electric fields is negligible. With an increase in temperature, the polarization increases gradually. There are two polarization peaks observed in Figure 4a, suggesting the two phase transitions. The small peak corresponds to the FE−AFE transition and the big one corresponds to the AFE−PE transition, respectively. For different external electric fields, the corresponding temperature of the polarization peak is different. With an increase of the applied electric filed, the peak moves toward a lower temperature, indicating the changing trend of the phase-transition temperature. According to the slope of the curve in Figure 4a, we can get the thermoelectric coefficient. As is shown in Figure 4b, the pyroelectric coefficient under different bias fields is a function of temperature. The pyroelectric coefficient (∂P/∂T) can be obtained by numerical differentiation of the P−T data in Figure 4a, which were extracted from the upper branches of the P−E loops in E > 0, as seen from Figure 3, and was fitted with a fourth-order polynomial equation. Figure 4b shows that the pyroelectric coefficient (∂P/∂T) decreased with the increase of the external electric field. The temperature dependence of the specific heat capacity of PLZT2/95/5 ceramics on cooling runs is shown in
the inset of Figure 4b. As we all know, structural phase transitions can be reflected by changes in specific heat capacity. For the sample, the specific heat capacity (C) calculated from the inset was about 0.48 J g−1 K−1.31 The two peaks induced by the applied electric field correspond to the FE−AFE and AFE−PE phase transitions,32 respectively. The positions of the two peaks were To = 428 K and Tc = 498 K (155 and 225 °C), respectively, consistent with the dielectric measurements. According to eqs 1 and 2, the value of ΔS and ΔT could be obtained from the P−E hysteresis loops. Temperature dependence of ΔS and ΔT for PLZT ceramics at several electric fields is given in Figure 4c,d, respectively. As you can see from Figure 4, the ceramic shows a negative card effect. The emergence of two peaks for ΔS and ΔT at different electric fields in Figure 4c,d can be explained by the two phase transitions. With an increase in the electric field, the two peak locations in Figure 4c,d decrease gradually to a lower temperature, similarly corresponding to the results in Figure 4a,b. Changes in the position of the entropy change peak and the temperature change peak at different voltages may be related to the effect of temperature on the stability of the FE, AFE, and PE phases.33 Samples with relatively large ΔS and ΔT values at the phase-transition temperature agree with previous literature.34−36 The maximum entropy change ΔSmax and temperature change ΔTmax occur in the second phasetransition zone at high temperature. The values of ΔSmax and ΔTmax are listed in Table 1 according to Figure 4c,d. Comparing the maximum values obtained by different electric fields in Table 1, it was found that upon increasing the applied electric field from 40 to 65 kV/cm, the maximum value of ΔTmax increases first and then decreases. The maximum change is −7.47 K, which occurs when the electric field is 50 kV/cm. The change in the electrical card effect in antiferroelectric ceramics may be caused by the synergy of relaxation in anti-ferroelectric ceramics. C
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dielectrics, the polarization−electric field (P−E) loops, and the density of all of the ceramics were measured by Agilent E4980A, Radiant Technologies Precision Premier II (Albuquerque, NM) and a differential scanning calorimeter (DSC8000), respectively.
Table 1. Comparison of the ECE Properties of Other Materials from the Literature material
ΔT (K)
refs
PbZT-5 Pb0.99Nb0.02(Zr0.75Sn0.20Ti0.05)0.98O3 (Pb0.88Sr0.08)(Nb0.08 (Zr0.53Ti0.47)0.92)O3 Pb(Zr0.455Sn0.455Ti0.09)O3 Pb(Mg0.067Nb0.133Zr0.8)O3 Bi4Ti3O12 nanoparticles PbZrO3 Na0.5Bi0.5TiO3-BaTiO3 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 (Pb0.97La0.02)Zr0.95Ti0.05O3
0.15 2.5 −0.38 1.6 0.91 −1.9 −1.5 −0.33 0.9 −7.47
37 38 39 40 41 42 43 44 45 this work
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02149.
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As we all know, ΔT is an important parameter to determine the ECE, which depends on the external electric field and the magnitudes of the external electric fields, where the latter is effected by electrical breakdown strength of the materials. Many reports about calculated peak values of ΔT are shown in Table 1.37−45 From Table 1, it is noticed that the absolute value of ΔT (−7.47 K) for this sample is obviously larger than previously reported values, and it implies that the ceramic materials we prepared have excellent negative electrocaloric performance. Generally, the theoretical calculation results according to eqs 1 and 2 are less than that from direct measurement.46 Therefore, the actual ECE temperature change of PLZT2/95/5 ceramics may be larger than the calculated value in this work, which will be greatly beneficial for the application of EC-based solid-state cooling systems.
P−E hysteresis loops of PLZT2/95/5 ceramics measured at different temperatures from 40 to 210 °C (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qiu-Xiang Liu: 0000-0001-9821-6042 Xin-Gui Tang: 0000-0001-6429-2098 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant nos. 11574057, 51604087 and 11574058), the Guangdong Provincial Natural Science Foundation of China (Grant no. 2016A030313718), and the Science and Technology Program of Guangdong Province of China (Grant nos. 2016A010104018, and 2017A0104022).
3. CONCLUSIONS In conclusion, excellent ECE properties had been found in PLZT2/95/5 ceramics. The ECE results demonstrated that PLZT2/95/5 ceramics had a large negative ECE (−7.47 K), which was relevant to the applied electric-field-induced FE phase and AFE phase. These data demonstrated the existence of a large negative ECE in PLZT2/95/5 ceramics. Based on these results, it can be concluded that PLZT2/95/5 ceramics possess a large recoverable energy-storage density and electrocaloric effect value, which could be applied in high energy-storage density ceramic capacitors and in achieving cooling power as refrigerants.
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REFERENCES
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4. EXPERIMENTAL PROCEDURE In this paper, the samples were successfully prepared by means of the mixed oxide method. The detailed processes are as follows: First, on the basis of the formula, TiO2, PbO, ZrO2, and La2O3 were weighted in the standard stoichiometry. Considering the evaporation effect, PbO was 5% overdose. Then, we mixed precursor oxides in ethanol for 24 h and presintered it at 850 °C in an alumina crucible for 2 h. After several cycles of milling and drying, the calcined fine powder was cold pressed into cylindrical pellets with 10−12 mm diameter and 1−2 mm thickness by applying a hydraulic press. Finally, Pb(Zr0.95Ti0.05)O3 was used as a spacer powder to maintain the PbO atmosphere. The ceramics were sintered at 1320 °C for 5 h. The surface morphology was characterized by a fieldemission scanning electronic microscope (JSM-7001F, JEOL, Japan). To conduct the electric properties, all ceramic samples are polished to 0.8 mm and both sides were covered with silver paste to serve as electrodes. The temperature dependence of D
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