Effect of sintering temperature on the ferroelectric property and electrocaloric effect of Pb0.8Ba0.2ZrO3 ceramics

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Effect of sintering temperature on the ferroelectric property and electrocaloric effect of Pb0.8Ba0.2ZrO3 ceramics Guoming Qiana, Kongjun Zhua,∗, Jun Lia, Kang Yana, Jing Wanga, Weiqing Huanga,b,∗∗, Junfeng Maoc a

State Key Laboratory of Mechanics and Control of Mechanical Structures, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, No. 29 Yudao Street, Nanjing, 210016, China b School of Mechanical and Electric Engineering, Guangzhou University, Guangzhou, 510006, China c Military Physical and Psychological Training Center, Naval Command College, No. 21 Banshan Garden, Xuanwu District, Nanjing, 210016, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Pb0.8Ba0.2ZrO3 ceramics Sintering temperature Ferroelectric property Electrocaloric effect

This work presents the effects of sintering temperature ranging from 1200 °C to 1300 °C at intervals of 20 °C on the crystal structure, ferroelectric properties, and electrocaloric effect (ECE) of Pb0.8Ba0.2ZrO3. Samples sintered at 1240 °C, 1260 °C, and 1280 °C have large remanent polarization and small coercive field. Meanwhile, samples sintered at 1260 °C, 1280 °C, and 1300 °C possess large breakdown field strength. Samples sintered at 1260 °C for 4 h exhibit the optimal ferroelectric properties. Antiferroelectricity-ferroelectricity (AFE-FE) phase transition occurs at room temperature T1 (279 K). Directly examining ECE at this temperature is meaningful, and the temperature change is 0.068 K at approximately 60 °C and 30 kV/cm. Results laid the foundation for studying the performance of ferroelectric and ECE within this phase-transition temperature range and provide a reference for new solid-state refrigeration technology.

1. Introduction Lead zirconate (PbZrO3, PZ) ceramics have been extensively used because of their excellent electrical performance during storage, energy conversion, large displacement brakes, and other properties [1–5]. At approximately room temperature, PZ ceramics display an antiferroelectric phase (AFE), the Pb ions show an antiparallel shift along the [110] direction, and their crystal structure is an orthorhombic system [6,7]. At a certain temperature (~235 °C), the dielectric constant sharply reaches the maximum and is converted into paraelectric phase (PE), the crystal structure is then transformed into a cubic system [8]. Some studies have reported the existence of ferroelectric phase (FE). AFE→FE (or AO-FR) phase transition first occurs followed by FE→PE (or FR-PC) phase transition with increasing temperature [9]. However, the two phase transitions differ only by few degrees due to the narrow temperature range of the ferroelectric phase [10]. The ferroelectric performance is poor at near room temperature, greatly limiting its application range [8]. Nevertheless, the ferroelectric temperature region can be enlarged by replacing Pb with Ba. The ferroelectric temperature region can be expanded to near room temperature as the concentration

of Ba2+ ions increases [11]. When an external electric field is applied, AFE→FE phase transition will occur, which is a first-order process, and will produce large strain and entropy changes [10]. This finding is meaningful in the application of the energy storage and actuator. Meanwhile, the large entropy change will change the temperature of the material itself. Thus, this condition is also very advantageous in electrocaloric (EC) refrigeration [12–15]. The phase transition, lattice structure, dielectric constant, hardness, and piezoelectric coefficient of (Pb1-xBax)ZrO3 (0 ≦ x ≦ 0.30) have been reported in detail [16–19]. However, no research has investigated the electrocaloric effect (ECE). ECE occurs when the polar material is applied/removed in the electric field, leading to alteration in the material's polarization state and induced entropy or electrothermal change [20–23]. This phenomenon then leads to temperature variation in the material. Within the phase transition temperature range, the change in polarization value will be intense, and high △T values will be generated under the same electric field [24]. In 2006, A. Mischenko and others [25] evaluated the AO-PC phase transition of PbZr0.95Ti0.05O3 near ~222 °C and obtained △T = 12 K. In 2017, E. Glazkova-Swedberg et al. [26] showed that PbZrO3 film undergoes AO-FR phase

∗

Corresponding author. Corresponding author. State Key Laboratory of Mechanics and Control of Mechanical Structures, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, No. 29 Yudao Street, Nanjing, 210016, China. E-mail addresses: [email protected] (K. Zhu), [email protected] (W. Huang). ∗∗

https://doi.org/10.1016/j.ceramint.2019.12.162 Received 2 December 2019; Received in revised form 15 December 2019; Accepted 19 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Guoming Qian, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.162

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transition near 230 °C and reported △T = 15 K. In 2013, Biaolin Peng et al. [27] prepared Pb0.8Ba0.2ZrO3 film on Pt(111)/TiOx/SiO2/Si substrate by sol–gel method to produce AO-FR phase transition near room temperature; they reported ΔT = 43.5 K and ΔS = 46.9 J/(kg⋅K) at 598 kV/cm. In 2006, A. S. Mischenko et al. [28] synthesized 0.9PbMg1/ 3Nb2/3O3-0.1PbTiO3 film; FR-PC phase transition occurred at 60 °C, and △T = 5 K was also obtained. In 2013, Xavier Moya et al. [29] reported the phase transition of BaTiO3 at 120 °C and obtained ΔT = 0.9 K. The phase transition at the quasi-homogeneous phase boundary also results in excellent ECE. In 2009, T. M. Correia et al. [30] proved that 0.93PbMg1/3Nb2/3O3-0.07PbTiO3 film undergoes ferroelectric phase transition at 35 °C and that the ΔT is 9 K. Therefore, when the applied electric field is constant, the vicinity of the phase transition temperature is the primary choice to obtain a large ECE. This strategy is one of the effective methods for regulating the phase-transition temperature by element doping and realizing EC refrigeration at room temperature. In this study, Pb0.8Ba0.2ZrO3 (PBZ) polycrystalline ceramics were synthesized by the traditional solid-state method. The effects of sintering temperature on crystal structure, density, and grain size were studied, and the corresponding ferroelectric properties were tested. The phase transition temperature at approximately room temperature was determined by dielectric temperature test. The ECE performance of ceramic within the vicinity of room temperature was determined by direct methods.

temperature was 20 °C) of the samples were measured by using a Premier II ferroelectric test system (Radiant Technologies, Inc.). Ceramic samples were polarized by CC2671 E type withstanding voltage tester (Nanjing Changchuang Technology Co., Ltd.) in a silicone oil bath at 120 °C for 40 min with the electric field of 20 kV/cm, then the heating device was turned off and the polarization was cooled for 15 min. After that, the d33 was tested using the ZJ-3AN type quasi-static d33 tester (The Institute of Acoustics of the Chinese Academy of Sciences). The variation in the dielectric constant and dielectric loss of the ceramics at different frequencies and at 173 K–373 K were carried out using TH2827C precision LCR meter (Heilongjiang Hengya Economic and Trade Co., Ltd.). TZDM-200-300 was used for temperature monitoring. Under the set conditions, the temperature variation was directly detected by a thermistor, which was adhered to the selected ceramic. A Cu conductive paste was used to connect the sample with high-voltage DC device. Signals were released upon the application of the electric field and represent the adiabatic temperature change of the electrocaloric effect. The output signals were recorded by a computer (The direct testing process of electrocaloric performance was completed at the School of Materials Science and Engineering, Tongji University.). 3. Results and discussion 3.1. XRD and SEM testing

2. Experimental procedure The XRD patterns of the Pb0.8Ba0.2ZrO3 solid solutions in the twotheta range 10°–60° at different sintering temperatures are presented in Fig. 1a. The XRD data reveal the formation of a polycrystalline perovskite structure obtained at all sintering temperatures. As the sintering temperature increases, the intensities of (130)/(112) and (101) peaks also increase. Increasing temperature probably causes the volatilization of lead during the sintering, leading to a change in the lattice structure. This change will gradually become apparent with increasing temperature [31,32]. Locally magnified (111) and (200) peaks are shown in Fig. 1b to clearly demonstrate the variety of the crystal structure of the PBZ ceramics. These ceramics are slightly split into the (111) and (200) peaks, indicating two crystalline phases, namely, rhombohedral (ferroelectric phase) and orthorhombic phases (antiferroelectric phase). The Ba2+ ions enter into A-site to substitute to the Pb2+ ions. This phenomenon expands the ferroelectric temperature region and allows the analysis of the morphotropic phase boundary at approximately the room temperature [33]. The SEM surface and section micrographs of the sintered pellets at different sintering temperatures are shown in Fig. 2a and b. The relatively dense surface structure and the clear boundary layer indicate that good-quality samples can be obtained by conventional immobilization at the set sintering temperature range. The sintering temperature greatly influenced the grain growth. As the sintering temperature increases, the grain grows continuously. However, if the sintering temperature is too high, PbO volatilization will be serious and grain growth will be affected. Using a suitable sintering temperature results in full crystal formation, uniform grain size, and compact structure. The theoretical density of the PbZrO3–BaZrO3 system was reported by Pokharel et al. [33] and had values of 8.055 and 6.229 g/cm3, respectively. Thus, the density (ρ) of the PBZ ceramic can be calculated using the empirical estimate [8]: ρ=(1-x)*8.055 + x*6.229, where x is 0.2 in this work and the theoretical density of PBZ ceramic is 7.6898 g/cm3. The variations in the average density and relative specific density are shown in Fig. 2c. The relative specific densities of all PBZ ceramics are more than 90% of the theoretical density. However, the average density of ceramics decreases with increasing sintering temperature. The main reason is that due to the volatilization of lead, the internal defects increase, the densification decreases, and the density also decreases [34]. The normal distribution of the grain size at different sintering temperatures is shown in Fig. 1Sa–f. The calculated average grain size

PBZ ceramics were fabricated via a traditional solid-phase method. Lead oxide (PbO), barium carbonate (BaCO3), and zirconia (ZrO2) were used as raw materials and purchased from Sinopharm Chemical Reagent Co. (all contents are greater than 99.0%). A certain amount of raw materials was weighed on the basis of the stoichiometric ratio by considering the volatilization of PbO during sintering. The excess of PbO was ~1 wt%. The weighed material was dissolved in ethanol solution and milled for 12 h with an agate ball at a rotation speed of 350 rpm. The weight ratio of the ball, material, and liquid was 1:3–5:0.5–1.0. The sample was dried at 65 °C for 24 h and filtered through a 100-mesh sieve. The powder was pressed at 6–8 MPa, placed in a crucible sealed with alumina, and calcined at 900 °C for 2 h. The block was dried, crushed in an agate mortar, sifted (200 mesh), milled again for 12 h, dried at 65 °C for 24 h, and sieved (100 mesh). The powder was pressed into circular disks with a diameter of 10 mm and a thickness of 1 mm. The sintering temperature was set from 1200 °C to 1300 °C for 4 h, with a total of six temperature gradients and a temperature interval of 20 °C, followed by cooling to room temperature in the furnace. After sintering, the ceramic samples sintered at corresponding temperatures selected were crushed into granules with an agate rod and used for X-ray diffraction (XRD) measurement on Bruker D8 Advance at room temperature. The density of the sintered samples was measured by Archimedes’ drainage method, and deionized water was used as the liquid medium. The selected samples were polished by a 2000 mesh sandpaper and thermally etched for 40 min at the preset temperature, which is 20 °C-80 °C lower than the sintering temperature. Hot-etched samples and partially fractured ceramics were used for surface and section morphological observation and characterized by field emission scanning electron microscopy (FE-SEM S4800, Hitachi). Grain size was measured by software Nano Measure. The maximum, minimum, and average grain sizes were calculated based on 150 grains, and the statistical distribution of the grain size was analyzed with Origin. The samples were polished to 0.4 mm and coated with conductive silver paste on the front and back surfaces to test their ferroelectric, dielectric, and electrocaloric performance. The samples were dried and kept at 500 °C for 20 min. The polarization-electric field (P-E, Frequency was 5 Hz) hysteresis loops and the leakage current characteristics (I-t, the applied electric field was 10 kV/cm, and the ambient 2

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Fig. 1. (a) The XRD patterns of the Pb0.8Ba0.2ZrO3 solid solutions at different sintering temperatures are presented in. (b) Locally magnified (111) and (200) peaks. Fig. 2. (a) and (b) The SEM surface and section micrographs of the sintered pellets at different sintering temperatures. (c) The variations in the average density and relative specific density.

of PBZ ceramics with sintered 1200 °C–1300 °C is 2.0–5.0 μm. At sintering temperatures of 1200 °C and 1220 °C, the average grain size is small (2.84 and 2.65 μm, respectively). The average grains grow and increase to size of 3.72 and 4.37 μm at 1240 °C and 1260 °C, respectively. At 1280 °C and 1300 °C, some of the grains melt and transformed into a glass phase because of high-sintering temperatures. In addition, the grain size is considerably non-uniform between 3.95 and 3.51 μm, respectively. The ceramic porosity further increases. Therefore, a ceramic with a good performance can be obtained at a sintering temperature of 1260 °C. Meanwhile, the maximum grain size, minimum grain size, and difference between the maximum minus the minimum can reflect the homogeneity and grain growth of the sample, as presented in Table 1. The difference value is 3.16, 3.54, 5.98, 5.89, 5.20, and 3.47 at sintering temperatures of 1200 °C–1300 °C. When the sintering temperature is from 1200 °C to 1260 °C, the grain gradually grows with increasing temperature. With increasing temperature from 1260 °C to 1300 °C, lead element volatilizes seriously, and grain growth is hindered. The grain size of the ferroelectric ceramics affects their dielectric

Table 1 The maximum grain size, minimum grain size, and difference between the maximum minus the minimum. The sintering temperature (oC)

Maximum

Minimum

Difference

1200 1220 1240 1260 1280 1300

4.40 4.16 6.69 7.53 7.30 5.42

1.24 0.62 0.71 1.64 2.10 1.95

3.16 3.54 5.98 5.89 5.20 3.47

and electrocaloric properties [35–37]. Small grain size means small domain size and abundant grain boundaries in the ceramic sample. When an electric field is applied, several grain boundaries participate in and are converted into new domains. The domains are turned toward 3

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Fig. 3. (a) The hysteresis loops at different sintering temperatures, and the difference is obvious. (b) The maximum residual polarization value (Pr+) and the breakdown field strength (E+) at the different sintering temperatures. (c) The leakage current characteristics of the sintering temperature-dependent ceramic samples. (d) Piezoelectric coefficient d33 of Pb0.8Ba0.2ZrO3 ceramics at various sintering temperature.

the electric field. At the same time, the domain size defines the entropy value of the material in the initial state. The small domain size indicates the large entropy value of the system. This condition is also beneficial for improving the performance of the electrocaloric effect. However, the performance of large ECE is unusually found in the system with the smallest grain size. The optimal size distribution and higher relative density make it easier to obtain high EC and dielectric properties, which means homogeneous composition distribution. The chemical homogeneity is closely related to the sintering. Thus, the sintering temperature has an important position. However, small samples also achieve excellent EC performance primarily because they can withstand the high breakdown field strengths [38].

3.2. Effect of sintering temperatures on ferroelectric properties Sintering temperature directly affects the structure and properties of ceramics [33]. For example, the sintering temperature range of piezoceramics is narrow; that is, if sintering temperature is lower than the lower limit of sintering temperature, the samples are underfired, and the porosity of the samples is high. Consequently, they cannot be made into porcelain. If the sintering temperature exceeds the upper limit, then overburning occurs, thereby causing a serious loss of lead, decreasing the density, and deteriorating the properties of samples. Therefore, an appropriate sintering temperature should be set to make a compact structure of samples and to optimize the performance. Fig. 2Sa–f show the hysteresis loop at different sintering temperatures. 4

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leakage current, and the optimal ferroelectric property are obtained. 3.3. Characterization of ECE at room temperature The variation in dielectric constant and dielectric loss was determined at different frequencies within 173–373 K as shown in Fig. 4. Moreover, the ceramic samples are sintered at 1260 °C for 4 h. The AFEFE phase transition occurs at room temperature (279 K) from the dielectric spectrum, and this temperature is recorded as T1. At this temperature, the maximum dielectric constant is reached to 3101, and the leakage current is 1.03. At 508 K [8], a large dielectric constant is obtained, and this temperature is the FE-PE phase transition temperature or Curie temperature (Tc). This study focuses on the corresponding electrocaloric performance when the AFE-FE phase transition occurs near T1. Fig. 5a and b are the temperature changes measured by the direct method. The sintering temperature is 1260 °C, the external temperature is selected at approximately 10 °C, and the field strength is 10 and 30 kV/cm. When the DC electric field is applied instantaneously at the two poles of the sample, the dipole flips toward the direction of the electric field. In addition, the degree of ordering is increased, the overall entropy value is decreased, and the temperature of the sample is instantaneously increased. However, excess heat is monitored and regulated by the temperature control system to maintain the temperature constant. Thus, the temperature of the system will stabilize again as time goes by. When the electric field is removed, the dipole orientation is weakened, the ordering degree is decreased, the overall entropy value is increased, and the temperature is instantaneously decreased. The PBZ sample presents a typical positive ECE [39]. As the electric field increases, the temperature change gradually increases when the electric field is cancelled (Fig. 5c). The temperature change of the all samples increase with increasing electric field at different sintering temperatures. Fig. 5d shows the temperature change of the sample at 10°C, 40 °C, and 60 °C. In addition, the applied DC electric field is 30 kV/cm. The illustration shows the temperature change at approximately 40 °C, and the diagram reveals that the external temperature has a certain influence on the temperature change in the sample [40–43]. Under the condition of constant external electric field, the temperature change of the sample tends to increase when the external temperature increases. External temperature affects the ordering degree of the dipole and the temperature change in the electric card of the sample.

Fig. 4. The variation in dielectric constant and dielectric loss was determined at different frequencies within 173–373 K.

The remanent polarization and coercive field of the pieces gradually increase to the saturation state and then decrease. The sintering temperature is different, and the ferroelectric property is distinct. The maximum residual polarization in the maximum electric field reflects the advantages and disadvantages of the ferroelectric properties of ceramics. The optimal sintering temperature is also chosen for the next step to examine the ECE. During sintering, the surface energy is the driving force. The powder with a large specific surface and high surface energy will shift to the low one. This phenomenon will cause matter migration, grain boundary movement, pores’ gradual elimination, and ceramics shrinkage. Finally, the ceramics become compact and have a certain strength after the sintering. Fig. 3a shows the hysteresis loops at different sintering temperatures, and the difference is obvious. The maximum residual polarization value and the highest breakdown field strength are different. The coercive field is basically the same, and the difference in ferroelectric properties is significant. The maximum residual polarization value (Pr+) and the breakdown field strength (E+) at the different sintering temperatures are presented in Fig. 3b. When the maximum remanent polarization value is prioritized for ferroelectricity properties, an excessively low sintering temperature is unfavorable for grain growth and structure densification. If the sintering temperature is too high, lead volatilization is severe, thereby affecting its properties. The optimum sintering temperatures to be selected are 1240 °C, 1260 °C, and 1280 °C, and the corresponding maximum remanent polarization values are 8.49, 9.62, and 8.89 μC/cm2. The degree of grain growth is unique similar to the densification when the sintering temperature is different. Thus, the highest field strength is used as the evaluation condition of the optimal sintering temperature. Furthermore, 1260 °C, 1280 °C, and 1300 °C should be selected, and the corresponding maximum breakdown field strengths are 55, 52, and 52 kV/cm. In practical applications, optimum ferroelectric properties, safety concerns, and electric field application should be considered. The leakage current characteristics can also reflect the performance of the ceramic samples in Fig. 3c. When the sintering temperature increases, the grains grown gradually, the leakage current reduces and reaches a minimum at a temperature of 1260 °C, which is on the order of 10−9 A. When the sintering temperature increases continuously, the pores and holes in the sample become more and more, which affect the leakage current value. The piezoelectric constant d33 at room temperature is shown in Fig. 3d. When the sintering temperature is 1260 °C, d33 is ~55.33 pC/N, which is the largest. The sample sintered at 1260 °C is selected as the research object of the ECE. At this sintering temperature, the maximum residual polarization value, the highest breakdown electric field, the minimal

4. Conclusion The crystal structure, microstructure, ferroelectric properties, dielectric properties, and electrocaloric effect of PBZ samples at different sintering temperatures were investigated. When the samples were sintered at 1260 °C, the relative density of the ceramics was 91.36%, and average grain size was 4.37 μm. The breakdown field strength was 55 kV/cm, and residual polarization was 9.62 μC/cm. The leakage current was on the order of 10−9 A, the piezoelectric constant d33 was ~55.33 pC/N. Hence, sample sintered at 1260 °C showed the best ferroelectric performance. The AFE→FE phase transition temperature of 279 K was measured near room temperature. The ECE temperature change tested by direct method near room temperature. △T, is 0.068 K at 60 °C and 30 kV/cm. This result lays a foundation for studying the ferroelectric and solid-state electrocaloric refrigeration near room temperature. Acknowledgement This work was supported by the National Nature Science Foundation of China (NSFC No. 51672130and 51572123), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and astronautics) (Grant 5

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Fig. 5. (a) The EC temperature changes of the ceramic, the sintering temperature is 1260 °C, the external temperature is approximately 10 °C, the field strength is 10 and (b) 30 kV/cm, (c) Temperature changes of all samples under different electric fields, (d) the temperature change of the sample at 10, 40 and 60 °C, the applied DC electric field is 30 kV/cm, the illustration shows the temperature change around 40 °C.

Appendix A. Supplementary data

No. MCMS-0518K01), the special fund of 333 high-level talents training project in Jiangsu province (BRA2017424),A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and The Key Research and Development Program of Jiangsu Province (Grant No. BE2018008-2).

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