1. Introduction {#sec1}
===============

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.^[@ref1]−[@ref3]^ 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-field-operated phase transition.^[@ref4]^ 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.^[@ref5]−[@ref12]^ The negative EC effect was discovered by researchers in Pb(Mg~1/3~Nb~2/3~)O~3~-PbTiO~3~ materials.^[@ref8]^ In particular, it is expected that the negative EC effect could be applied in cooling devices, which requires a large negative adiabatic temperature change (Δ*T*) and isothermal entropy change (Δ*S*). To obtain a larger Δ*T* 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.^[@ref3],[@ref13]^ Furthermore, the actual cooling capacity and the EC strength of  their bulk counterparts, 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 solid-state 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 {#sec2}
=========================

[Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"} 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,^[@ref14]^ indicating that the fabricated samples were confirmed to be dense.

![SEM patterns for the fractured surface microstructure of PLZT2/95/5 ceramic samples.](ao9b02149_0001){#fig1}

As shown in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a,b, the dielectric constant (ε~r~) and dielectric loss (tan δ) of PLZT2/95/5 ceramic samples varied with temperature at different frequencies from 1 to 20 k Hz. The ε~r~*--T* curves with varying frequencies from 1 to 20 k Hz have no obvious variation at temperatures ranging from 0 to 400 °C. Two anomalous peaks could be seen in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a, 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 AFE~O~ to AFE~T~ occurs in this region, and the second anomalous peak at 225 °C corresponds to another phase transition from AFE~T~ to PE.^[@ref15]−[@ref22]^

![(a) Dielectric constant and (b) dielectric loss tan δ changes as a function of temperature for PLZT2/95/5 at various frequencies.](ao9b02149_0002){#fig2}

[Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"} 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 [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}a,b, the polarizations measured at 40 and 150 °C are very small. The typical AFE double P--E loops are obtained in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}c--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 *E*~F~ 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 [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}f, indicating that the AFE phase in this region is quite weak, thus the electric-field-inducting 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 AFE~T~; similar results were reported by Hao.^[@ref47]^ On the other hand, look at the forward electric field in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}c--e when the electric field drops. It can be seen that when the electric field drops to a certain value (corresponding to *E*~A~ 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](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}, indicating a complete anti-ferroelectric phase. As shown in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}d, 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 [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}e is relatively large, indicating that this is the coexistence phase of the AFE and FE. Many PLZT ceramics have similar phase-transition reactions,^[@ref23],[@ref24]^ 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 *E*~A~, the phase translates from FE to AFE, and when the electric field reaches *E*~F~, the phase recovered from AFE to FE. The decrease of *E*~F~ 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).

![*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.](ao9b02149_0003){#fig3}

On the basis of *P--E* loops, the isothermal entropy change Δ*S* and the adiabatic change in temperature Δ*T* of the ceramics can be calculated by using Maxwell's relations as follows^[@ref25]−[@ref29]^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 *E*~1~ to *E*~2~, Δ*S* represents the isothermal entropy change, and Δ*T* 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.^[@ref30]^ The calculated results are presented in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}a, and the applied electric field varied from 40 to 65 kV/cm. As is shown in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}a, 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 [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}a, suggesting the two phase transitions.

![(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.](ao9b02149_0004){#fig4}

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 [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}a, we can get the thermoelectric coefficient. As is shown in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}b, 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 [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}a, which were extracted from the upper branches of the *P*--*E* loops in *E* \> 0, as seen from [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}, and was fitted with a fourth-order polynomial equation. [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}b 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 [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}b. 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^.^[@ref31]^ The two peaks induced by the applied electric field correspond to the FE--AFE and AFE--PE phase transitions,^[@ref32]^ respectively. The positions of the two peaks were *T*~o~ = 428 K and *T*~c~ = 498 K (155 and 225 °C), respectively, consistent with the dielectric measurements.

According to [eqs [1](#eq1){ref-type="disp-formula"}](#eq1){ref-type="disp-formula"} and [2](#eq2){ref-type="disp-formula"}, the value of Δ*S* and Δ*T* could be obtained from the *P--E* hysteresis loops. Temperature dependence of Δ*S* and Δ*T* for PLZT ceramics at several electric fields is given in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}c,d, respectively. As you can see from [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}, the ceramic shows a negative card effect. The emergence of two peaks for Δ*S and* Δ*T* at different electric fields in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}c,d can be explained by the two phase transitions. With an increase in the electric field, the two peak locations in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}c,d decrease gradually to a lower temperature, similarly corresponding to the results in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}a,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.^[@ref33]^ Samples with relatively large Δ*S* and Δ*T* values at the phase-transition temperature agree with previous literature.^[@ref34]−[@ref36]^ The maximum entropy change Δ*S*~max~ and temperature change Δ*T*~max~ occur in the second phase-transition zone at high temperature. The values of Δ*S*~max~ and Δ*T*~max~ are listed in [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"} according to [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}c,d. Comparing the maximum values obtained by different electric fields in [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}, it was found that upon increasing the applied electric field from 40 to 65 kV/cm, the maximum value of Δ*T*~max~ 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 anti-ferroelectric ceramics may be caused by the synergy of relaxation in anti-ferroelectric ceramics.

###### Comparison of the ECE Properties of Other Materials from the Literature

  material                                                    Δ*T* (K)   refs
  ----------------------------------------------------------- ---------- ------------
  PbZT-5                                                      0.15       ([@ref37])
  Pb~0.99~Nb~0.02~(Zr~0.75~Sn~0.20~Ti~0.05~)~0.98~O~3~        2.5        ([@ref38])
  (Pb~0.88~Sr~0.08~)(Nb~0.08~ (Zr~0.53~Ti~0.47~)~0.92~)O~3~   --0.38     ([@ref39])
  Pb(Zr~0.455~Sn~0.455~Ti~0.09~)O~3~                          1.6        ([@ref40])
  Pb(Mg~0.067~Nb~0.133~Zr~0.8~)O~3~                           0.91       ([@ref41])
  Bi~4~Ti~3~O~12~ nanoparticles                               --1.9      ([@ref42])
  PbZrO~3~                                                    --1.5      ([@ref43])
  Na~0.5~Bi~0.5~TiO~3~-BaTiO~3~                               --0.33     ([@ref44])
  0.91Pb(Zn~1/3~Nb~2/3~)O~3~-0.09PbTiO~3~                     0.9        ([@ref45])
  (Pb~0.97~La~0.02~)Zr~0.95~Ti~0.05~O~3~                      --7.47     this work

As we all know, Δ*T* 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 Δ*T* are shown in [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}.^[@ref37]−[@ref45]^ From [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}, it is noticed that the absolute value of Δ*T* (−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](#eq1){ref-type="disp-formula"}](#eq1){ref-type="disp-formula"} and [2](#eq2){ref-type="disp-formula"} are less than that from direct measurement.^[@ref46]^ 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.

3. Conclusions {#sec3}
==============

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.

4. Experimental Procedure {#sec4}
=========================

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, TiO~2~, PbO, ZrO~2~, and La~2~O~3~ 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(Zr~0.95~Ti~0.05~)O~3~ 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 field-emission 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 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.

The Supporting Information is available free of charge on the [ACS Publications website](http://pubs.acs.org) at DOI: [10.1021/acsomega.9b02149](http://pubs.acs.org/doi/abs/10.1021/acsomega.9b02149).*P*--*E* hysteresis loops of PLZT2/95/5 ceramics measured at different temperatures from 40 to 210 °C ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b02149/suppl_file/ao9b02149_si_001.pdf))

Supplementary Material
======================

###### 

ao9b02149_si_001.pdf

The authors declare no competing financial interest.

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).
