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Tuning the electrocaloric effect in 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by relaxor phase blending
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Tuning the electrocaloric effect in 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by relaxor phase blending Feng Lia,d,∗, Jiahao Lib, Shandong Lib, Tianyu Lic, Renjun Sic, Chunchang Wangc,∗∗, Jiwei Zhaid,∗∗∗ a
Institutes of Physical Science and Information Technology, Anhui University, Hefei, 230601, PR China College of Physics, Qingdao University, 308 Ningxia Road, Qingdao, 266071, PR China c Laboratory of Dielectric Functional Materials, School of Physics and Material Science, Anhui University, Hefei, 230601, PR China d Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai, 201804, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrocaloric effect Dielectric property PFM images Composites
The pseudo-first-order phase transition in 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics leads to a sharp increase in temperature change (ΔT) in the vicinity of the ferroelectric-to-relaxor transition temperature TFR (~100 °C) [Appl. Phys. Lett. 110 (2017) 182904]. In this study, we add the 0.78Bi0.5Na0.5TiO3-0.06BaTiO30.16(Sr0.7Bi0.2)TiO3 relaxor phase to the 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ferroelectric matrix to tune its electrocaloric effect. The results show that the addition of the relaxor phase plays a vital role in phase and localstructure evolution. A transition occurs between the ferroelectric and ergodic relaxor phases when the mass fraction of the latter increases to 30% (x = 0.3), as verified by X-ray diffraction analysis, Raman spectroscopy, and polarization-electric field (P-E) hysteresis loops. Furthermore, addition of the relaxor phase reduces the TFR from 76 °C at x = 0.1–55 °C at x = 0.2; however, this transition disappears at x = 0.3 and 0.4 composite. In-situ piezo-force microscopy (PFM) images illustrate that domains can be written into x = 0.1 and 0.2 ceramics with a valley in the piezoresponse curves. Increasing the temperature agitates the domain arrangement and decreases the contrast for PFM images; this indicates a gradual phase transition in the composite. The temperature corresponding to maximum ΔT exhibits a downward shift (0.58 K at 80 °C for x = 0.1 and 0.5 K at 65 °C for x = 0.2), while the temperature-ΔT curves are flat when x = 0.3 and 0.4. Moreover, the maximum ΔT shows a decrease with an increase in the relaxor phase content; this is believed to be related to a decrease in the latent heat due to a pseudo-first-order to second-order transition. Thus, we suggest that the incorporation of a relaxor phase into ferroelectric matrices is an effective technique to tune their electrocaloric effect and improve the thermal stability of ceramic composites.
1. Introduction Zero-global-warming techniques are gradually gaining importance in the modern world as the earth continues to witness a steady increase in temperature, which in turn is responsible for extreme weather condition [1]. It is well known that hydrofluorocarbon gases, often used as refrigerants, are major contributors to the green-house effect. To overcome this drawback, several environmentally-friendly and novel techniques have been developed, such as the magnetocaloric effect and elastocaloric effect. However, these two caloric-effects can be only induced under large magnetic/stress fields and exhibit low cooling efficiency, thus restricting miniaturization and integration when
assembling cooling devices. The electrocaloric effect (ECE, including positive ECE and inverse ECE), defined as a change in entropy/temperature upon applying/removing an electric field in polar dielectrics under adiabatic conditions [2,3], has been extensively explored in the past decade for refrigeration applications ever since a giant ECE (ΔT ~ 12 K) was discovered in antiferroelectric inorganic thin films of PbZr0.95Ti0.05O3 near its antiferroelectric-paraelectric transition temperature [4]. Subsequently, a giant ECE was also detected in P(VDFTrFE-CFE) ferroelectric polymers at room temperature [5]. More recently, a high cooling-power device is designed by adopting flexible EC polymer films. The result show that when the cooling device is connected to a battery, its surface temperature decreases drastically by 8 °C
∗
Corresponding author. Institutes of Physical Science and Information Technology, Anhui University, Hefei, 230601, PR China. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (F. Li), [email protected] (C. Wang), [email protected] (J. Zhai). ∗∗
https://doi.org/10.1016/j.ceramint.2019.10.171 Received 5 October 2019; Received in revised form 16 October 2019; Accepted 18 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Feng Li, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.171
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x = 0.1, 0.2, 0.3 and 0.4). The mixed powders were milled again for another 8 h. The detailed experimental procedures can be found in Ref. [26]. The phase structure was detected by X–ray diffractometer (Rigaku Smart-lab Beijing Co, Beijing, China). Raman spectra were measured by a Horiba Lab-Ram iHR550 spectrometer with a Linkam THMSE 600 heating stage. The morphology of ceramics was observed using fieldemission SEM (Regulus 8230; Hitachi Co, Tokyo, Japan). The dielectric properties were obtained by using an LCR meter (Agilent E4980A, Santa Clara, CA) within temperature range of 25–400 °C. Polarizationelectric field (P-E) hysteresis loops were measured at room temperature with a frequency of 10 Hz by ferroelectric testing system (Precision LC, Radiant Technologies, Inc. Albuquerque, NM). The composites were poled under E = 4 kV/mm at room temperature and the d33 was measured by a quasi-static d33 m (ZJ-6A, Institute of Acoustics, Beijing, China). The piezoresponse force microscopy (PFM) images were obtained based on a commercial atomic force microscopy (AFM 5300E, Hitachi, Japan). The central area (5 × 5 μm2) was pre-poled at a dc voltage of 40 V. The images were then recorded within the area of 10 × 10 μm2 after removing the external electric field. The home–made adiabatic calorimeter was made to characterize the ECE behaviors for composites, and the measurement device is shown in Ref. [26].
within a very short period of time (~5s), thus demonstrating that ECE is a promising method for refrigeration applications [6]. The ECE has been investigated in lead-free relaxor ferroelectric (RF) ceramics and antiferroelectric ceramics, including those based on BaTiO3- [7–10], K0.5Na0.5NbO3- [11–13], Bi0.5Na0.5TiO3- (BNT-) [14–18], and NaNbO3 [19,20]; most of these ceramics exhibit excellent machinability and thermal stability. Among these ceramics, ECE in BNT-based compositions is of special interest. Due to the particular dielectric relaxor characteristics of BNT-based ceramics, a large ECE occurs over a wide temperature window. Furthermore, the existence of polar nanoregions (PNRs) contributes to ECE due to PNRs alignment in the presence of electric field [21]. For example, Weyland suggested that the existence of a diffuse critical point contributed to an increase in ECE and EC responsivity of 0.75BNT-0.25SrTiO3 ceramics [22]. Goupil illustrated that the maximal directly-measured ECE (~0.73 K) was located in the vicinity of the ferroelectric-to-relaxor transition temperature (TFR ~ 160 °C) in 0.82BNT-0.18(Bi0.5K0.5)TiO3 ceramics [23]. Later, Goupil also demonstrated that the shoulder temperature, Ts, in the dielectric permittivity curves of BNT-KNbO3 ceramics represented the upper temperature limit of the ECE peak, which facilitated the selection of novel lead-free ceramics for ECE cooling [24]. Recently, Li stated that ECE in 0.94BNT-0.06BaTiO3 (BNT-0.06BT) ceramics was maximum at the pseudo-first-order phase transition temperature [25]. Moreover, the addition of a relaxor ferroelectric (Sr, Bi)TiO3 member into BNT-0.06BT binary systems can lower their TFR and a large ECE may be observed at ambient temperature [26]. These studies indicate that modifying the ECE behavior of BNT-based ceramics makes them promising solid-state materials for several applications. With respect to the exploration of ECE in BNT-based ceramics, both direct and indirect methods have been adopted. Direct ECE methods refer to directly measuring entropy/temperature change, such as modified differential scanning calorimetry (DSC) and adiabatic calorimetry. In indirect methods, ΔT is calculated according to the Maxwell 1
E2
3. Results and discussion Fig. 1(a and c) shows the room-temperature X-ray diffraction (XRD) patterns of fresh and poled x = 0.1–0.4 composites. It can be seen that at all compositions, a pure perovskite-type structure is obtained without the inclusion of any other phase. To clearly detect differences in the phase structure of the fresh and poled composites, locally magnified (111) and (200) diffraction peaks are displayed in Fig. 1(b and d). Clear differences could be observed in phase structures of the two composites. In fresh composites, only a single diffraction peak could be detected, which indicates a pseudocubic phase. However, splitting occurs in the (111) and (200) peaks at poled x = 0.1 and 0.2, as shown by the arrows, demonstrating the coexistence of rhombohedral (R) and tetragonal (T) phases. With an increase in the RF content (x = 0.3 and 0.4), the splitting of diffraction peaks reduces; instead, a hump emerges at the lower angle of (200) peaks, as shown by diamond markers, owing to the presence of tetragonal PNRs. Notably, the macroscopic structure of poled x = 0.3 and 0.4 composites is still pseudocubic. This phenomenon was described in previous studies as well [32,33]. Therefore, it may be deduced that electric field application changes the phase structure of the matrix from pseudocubic to a co-existing R and T ferroelectric phase at x = 0.1 and 0.2. Meanwhile, the pseudocubic phase is maintained in the x = 0.3 and 0.4 composites, regardless of the application of an electric field. In fact, transition between non-ergodic relaxor and ferroelectric phases is irreversible, while the transition between ergodic relaxor and ferroelectric phase is reversible [34]. Therefore, compositions with x = 0.1 and 0.2 are classified as non-ergodic relaxors while those with x = 0.3 and 0.4 are ergodic relaxors. Fig. 2(a–d) show the SEM images of x = 0.1–0.4 composites, respectively. A dense microstructure without porosity and voids can be observed at all compositions, which indicates that the RF inclusions form a tight interface with the ferroelectric matrix. Notably, such a compact microstructure is advantageous in withstanding high-voltage pulses in ECE measurement. Meanwhile, grain size decreases slightly with an increase in the RF content. The grain size decreases from 1.73 μm for x = 0.1–1.48 μm for x = 0.4 composite, respectively, as clearly shown in their insets. Fig. 3(a–d) illustrate variations in dielectric constant (εr) and loss tangent (tanδ) of poled (x = 0.1–0.4) composites, respectively, in the temperature range of 25–400 °C and at frequencies of 1, 10, and 100 kHz. At all compositions, Tm (the temperature corresponding to dielectric constant maxima) at high temperatures could be clearly observed along with a dielectric shoulder, Ts, at low temperatures. These two dielectric anomalies are related to the mutual transitions between
∂P
equation using temperature-dependent P-E loops: Δ T= − Cρ ∫ T( ∂T )E dE E1
(C is the heat capacity and ρ is the density of ceramic pellets). Therefore, it is not difficult to analyze why both positive and inverse ECE are reported for BNT-based ceramics in previous reports [14,15,25,27]. For example, large discrepancies are observed in BNT0.18BKT ceramics using indirect and direct (DSC) methods, which can be ascribed to an increase in polarization and a decrease in the coercive field with an increase in temperature, leading to an inverse ECE [23]. Such large discrepancies are also observed in RF polymers. Lu et al. emphasize that the validity of the non-ergodic RF analyzed using Maxwell relation is a concern [28]. Therefore, the Maxwell equation is not suitable for non-equilibrium systems such as the nonergodic relaxor phase in BNT-based ceramics; in such condition, it is essential to calculate the actual ECE [27]. Hence, in this study, a home-made adiabatic calorimeter is designed to explore the evolution of ECE. Recently, ferroelectric/relaxor composites have been developed to tune the electrostrain properties of BNT-based ceramics. The results show that a large strain performance accompanied by a low hysteresis may be obtained at a relatively low electric field, which is thought to be related to the polarization and strain-coupling effect [29–31]. Under these conditions, 0.78Bi0.5Na0.5TiO3-0.06BaTiO3-0.16(Sr0.7Bi0.2)TiO3 RF is added into a BNT-0.06BT matrix to explore how phase/local structure and ECE evolve as functions of RF mass. 2. Experimental procedure (BNT-0.06BT) and The 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 0.78Bi0.5Na0.5TiO3-0.06BaTiO3-0.16(Sr0.7Bi0.2)TiO3 (BNT-BT-0.16SBT) powders were calcined separately by using the following raw chemicals: Bi2O3, Na2CO3, TiO2, SrCO3 and BaCO3 (≥99.0%). Different mass fraction of BNT-BT-0.16SBT calcined powders were added into the BNT-0.06BT calcined powders (10%, 20%, 30% and 40%, denoted as 2
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Fig. 1. (a & c) The unpoled and poled XRD patterns for x = 0.1–0.4 composites, respectively; (b & d) the enlarged (111) and (200) diffractions for the four compositions, respectively.
denoting that the ferroelectric order vanishes when the electric field is removed, which could explain their ergodic relaxor nature. These results are also in line with the results of our XRD analysis. The existence of a ferroelectric-to-relaxor transition leads to a striking structural transition and thus influences the ECE behaviors of these composites, which will be discussed in further sections. To clearly understand structural evolution in the poled composites at the nanoscale level, the in-situ temperature dependence of their
PNRs with rhombohedral and tetragonal symmetry rather than phase transformation [35]. Interestingly, another TFR anomaly could be observed in x = 0.1 and 0.2 composites. With an increase in RF inclusions, TFR decreases from 76 °C at x = 0.1–55 °C at x = 0.2. The presence of a TFR indicates that a ferroelectric order is induced under the action of an external electric field; these TFR values are maintained even removing the electric field, suggesting that these composites are non-ergodic RF ceramics. This anomaly is absent in x = 0.3 and 0.4 composites,
Fig. 2. (a–d) The SEM images of x = 0.1–0.4 composites, the grain size distribution for each composite is displayed in their insets. 3
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Fig. 3. (a–d) The dielectric constant and loss for the poled composites (x = 0.1–0.4), respectively.
Fig. 4. (a–d) The Raman spectra for the poled composites (x = 0.1–0.4) within the temperature range of 25–200 °C, respectively. The Raman spectra are deconvoluted using the Lorentzian function, as shown at the bottom of each figure.
explore phase transitions with respect to increasing temperature, the diffuse Raman spectra are deconvoluted into seven Lorentzian peaks, as shown at the bottom of each figure. The deconvolution of Raman peak in BNT-based material is elaborately discussed in previous study by J. Kreisel et al. [37]. Firstly, room-temperature Raman spectra corresponding to the poled x = 0.1–0.4 composites (dark lines) are significantly different. The Raman peaks corresponding to Ti–O bonds and TiO6 octahedra in x = 0.1 and 0.2 composites are much sharper than
Raman spectra at poled x = 0.1–0.4 is analyzed from 25 to 200 °C, as shown in Fig. 4(a–d). All the Raman spectra are broad and diffuse, which can be ascribed to the random occupation of Bi, Na, Ba, and Sr cations at A-sites. Generally, Raman spectra in the wavenumber range of 100–700 cm-1 can be divided into three regions. 1) peaks in the 100–200 cm-1 range are ascribed to mixed A-site vibrations, 2) peaks in the 200–400 cm-1 range are related to Ti–O bonds, and 3) peaks in the 400–700 cm-1 range are ascribed to TiO6 octahedral vibrations [36]. To
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Fig. 5. (a–d) The evolution of wavenumber and full-width at half maximum (FWHM) for PA and PB band as a function of temperature (25–200 °C) for x = 0.1–0.4 composites, respectively.
At x = 0.3 and 0.4, d33 shows a significant decrease, indicating that they evolve into incipient piezoelectric ceramics with negligible piezoelectric properties. Fig. 7(a, h, m & n) show the PFM images of x = 0.1–0.4 composites at room temperature, respectively. The central area (5 × 5 μm2) is prepoled at a dc voltage of 40 V. Images are then recorded within a scanning area of 10 × 10 μm2 after removing the external electric field. A clear contrast could be seen in x = 0.1 and 0.2 composites, illustrating that domains are easily written into the composite matrix. However, no clear contrast could be detected in x = 0.3 and 0.4 composites, indicating that the domains could not be maintained after removing the electric field. Fig. 8(a–d) exhibit piezoresponse distributions within the scanning area [indicated by the line in Fig. 7(a)]. A clear valley can be observed in the distributions of x = 0.1 and 0.2 composites, indicating that a downward polarization is induced by the electric field; this polarization is maintained after removing it. However, only fluctuating lines could be observed for x = 0.3 and 0.4 composites, suggesting that a local random field disrupts the induced ferroelectric order. To further explore domain evolution with increasing temperature, in-situ temperature-dependent PFM images are collected, as shown in Fig. 7(a–g & h–l). It can be seen that with an increase in temperature, the contrast in piezoresponse gradually become dim and unclear; this may be attributed to thermal agitation. When the temperature rises to 70–80 °C for x = 0.1 and 50–60 °C for x = 0.2 composites, there is a significant attenuation in the contrast, which indicates disappearance of the induced downward ferroelectric order. Fig. 9(a1–d1) map the directly-measured temperature change (ΔT) as a function of time for poled x = 0.1–0.4 composites in the temperature range of 25–120 °C. Exothermic peaks are induced upon the application of an electric field and endothermic peaks appears when the electric field is removed. These observations strongly suggest positive ECE characteristics for x = 0.1–0.4 composites. Though several inverse ECE properties have been reported for BNT-based ceramics, they are generally calculated based on Maxwell equations [14,15,40,41]. The adiabatic calorimeter employed in this investigation can more actually evaluate ECE in non-equilibrium BNT-based systems. Variations in temperature-dependent ΔT are plotted in Fig. 9(a2–d2). In x = 0.1 and 0.2 composites, the maximum ΔT peak could be clearly observed at specific temperature, i.e., 0.58 K at 80 °C for x = 0.1 and 0.5 K at 65 °C for x = 0.2. These specific temperatures are slightly higher than the corresponding TFR temperatures. This difference may be attributed to an increase in the transition temperature due to the electric field. Changes in entropy (ΔS) are proportional to lnΩ and the square of difference in polarization (ΔP) (ΔS ∝ lnΩ × ΔP2, where Ω denotes the
those of x = 0.3 and 0.4 composites, which can be ascribed to the greater polarity of x = 0.1 and 0.2 samples. It can be clearly seen that the wavenumber corresponding to PA and PB bands increases abruptly at x = 0.3 at room temperature, as shown by the arrows in Fig. 5(a–d). This further clarifies their non-ergodic characteristics (x = 0.1 and 0.2) and a long-range order appears upon the application of an electric field. The evolution of wavenumber and full-width at half maximum (FWHM) of PA and PB peaks as functions of temperature is plotted in Fig. 5(a–d); structural evolution is correlated with changes in Ti–O bonds and TiO6 octahedral vibrations in Bi-based solid solutions [38]. It is known that anomalies in Raman spectra reflect phase transition. The wavenumbers as well as FWHM values of PA and PB bands exhibit anomalies in the vicinity of 70 °C for x = 0.1 and 50 °C for x = 0.2, which signifies phase transition in these two composites. These observations are also in accordance with the TFR values in dielectric curves. However, such anomalies are not observed in x = 0.3 and 0.4 composites and the wavenumbers corresponding to their PA and PB bands reduce gradually while their FWHM increase as temperature increases. These changes indicate a gradual enhancement in relaxor properties. Fig. 6(a–b) shows the P-E loops of x = 0.1–0.4 composites with the corresponding current-density (I-E) loops. It can be seen that the P-E loops gradually transform from a square shape to a slanted shape with an increase in the RF content, indicating an enhanced relaxor degree. This can also be inferred from the I-E loops. There is only one current peak in the first and third quadrants, indicating that a long-range ferroelectric order is induced in the x = 0.1 sample upon the application of an electric field. In the case of x = 0.2 and 0.3 composites, there are two I-E peaks in each quadrant, indicating the coexistence of non-ergodic and ergodic relaxor phases (but with different proportions). Nevertheless, the underlying mechanism behind double I-E peaks is still a topic of debate. One particular theory based on transmission electron microscopy (TEM) suggests that a high-electric peak corresponds to nano-to-lamellar domain growth and a low-field peak indicates the disruption of the induced macro-ferroelectric domains [39]. As for the x = 0.4 composite, only one peak could be observed in the fourth quadrant with a current platform in the third quadrant, indicating that it is a pure ergodic relaxor phase. As shown in Fig. 6(c), the maximum polarization (Pmax) varies slightly, indicating that the electric field overcomes the local random field and domains can switch fully at E = 6 kV/mm. In contrast, remnant polarization (Pr) drastically decreases from x = 0.2 to 0.3, suggesting that a long-range order is maintained at x = 0.1 and 0.2 and disappears at x = 0.3 and 0.4. Their macroscopic piezoelectric performance also indicates that d33 increases from 168 pC/N (x = 0.1) to the maximum value of 204 pC/N (x = 0.2). 5
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Fig. 6. (a) The P-E loops and (b) the corresponding I-E loops for x = 0.1–0.4 composites; (c) the variation of maximal polarization (Pmax), remnant polarization (Pr) and d33 as a function of the relaxor phase content.
Fig. 7. The PFM images of (a–g) x = 0.1 in the temperature range of 25–100 °C; (h–l) x = 0.2 in the temperature range of 25–70 °C and (m & n) x = 0.3 and 0.4 at room temperature, respectively. The scanning area is 10 × 10 μm2 and the poled area in central region is 5 × 5 μm2 for each figure.
promising for cooling devices [9,43].
number of polar states, ferroelectric domains for the ferroelectric state, and PNRs for the relaxor phase) [42]. Below the transition temperature, the polar states are ferroelectric domains and Ω is quite low; however, when the temperature increases to TFR, PNRs act as polar states and hence Ω increases. Therefore, ΔT is maximum at the transition temperature. Further increasing the temperature increases the local random field, retards PNRs alignment, and weakens polarization and the ECE. In the case of x = 0.3 and 0.4 composites, the absence of a phase transition leads to a flat ΔT vs. temperature curve without inflection points [0.25 K for x = 0.3 and 0.21K for x = 0.4 at room temperature (RT)], which is useful for increasing the thermal stability of ECE. Another point that should be noted is that the maximum value of ΔT decreases gradually with an increase in RF content, as shown by the arrow in Fig. 9. This is due to the pseudo-first-order to second-order transition. For example, as discussed in BaTiO3 single crystals, a large ECE is obtained near sharp first-order phase transitions with a large latent heat at the expense of thermal stability. When ZrO2 is incorporated in a BaTiO3 ceramic matrix, a large ECE with a good thermal stability can be obtained due to diffuse dielectric properties; thus, these materials are
4. Conclusions Different amounts of 0.78Bi0.5Na0.5TiO3-0.06BaTiO30.16(Sr0.7Bi0.2)TiO3 inclusions with a relaxor phase are incorporated into 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ferroelectric ceramic matrices to modulate their electrocaloric effect. Their phase/local structure and macroscopic properties, such as piezoelectric, ferroelectric, and dielectric properties, change drastically with an increase in the RF content. The maximum ECE of 0.58 K for x = 0.1 and 0.5 K for x = 0.2 composites is obtained at specific temperatures; however, there is a significant variation in the ECE beyond these temperature points. ECE is observed to become stable with respect to temperature at x = 0.3 and 0.4 sample due to the absence of a pseudocubic-first-order to secondorder phase transition. Thus, we successfully demonstrate that the incorporation of a relaxor phase into a ferroelectric matrix can effectively tune the electrocaloric effect of the latter and improve its thermal stability, which is necessary in cooling materials. 6
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Fig. 8. The room-temperature piezoresponse for x = 0.1–0.4 composites along the line, as indicated in Fig. 7(a).
Declaration of competing interest
Acknowledgements
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was supported by the National Natural Science Foundation of China, China (under grant no. 51772211, 51502001 and 51872001). This work was also supported by the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics, China (grant no. KF201803) and the start-up funding of Anhui University, Anhui University, China (no. S020118002/098).
Fig. 9. (a1–d1) The appearance of exothermic and endothermic peaks with application/removal of electric field (E = 4 kV/mm) at selected temperatures; (a2–d2) ΔT as a function of temperature at E = 4 kV/mm. 7
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Tuning the electrocaloric effect in 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by relaxor phase blending Feng Lia,d,∗, Jiahao Lib, Shandong Lib, Tianyu Lic, Renjun Sic, Chunchang Wangc,∗∗, Jiwei Zhaid,∗∗∗ a
Institutes of Physical Science and Information Technology, Anhui University, Hefei, 230601, PR China College of Physics, Qingdao University, 308 Ningxia Road, Qingdao, 266071, PR China c Laboratory of Dielectric Functional Materials, School of Physics and Material Science, Anhui University, Hefei, 230601, PR China d Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai, 201804, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrocaloric effect Dielectric property PFM images Composites
The pseudo-first-order phase transition in 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics leads to a sharp increase in temperature change (ΔT) in the vicinity of the ferroelectric-to-relaxor transition temperature TFR (~100 °C) [Appl. Phys. Lett. 110 (2017) 182904]. In this study, we add the 0.78Bi0.5Na0.5TiO3-0.06BaTiO30.16(Sr0.7Bi0.2)TiO3 relaxor phase to the 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ferroelectric matrix to tune its electrocaloric effect. The results show that the addition of the relaxor phase plays a vital role in phase and localstructure evolution. A transition occurs between the ferroelectric and ergodic relaxor phases when the mass fraction of the latter increases to 30% (x = 0.3), as verified by X-ray diffraction analysis, Raman spectroscopy, and polarization-electric field (P-E) hysteresis loops. Furthermore, addition of the relaxor phase reduces the TFR from 76 °C at x = 0.1–55 °C at x = 0.2; however, this transition disappears at x = 0.3 and 0.4 composite. In-situ piezo-force microscopy (PFM) images illustrate that domains can be written into x = 0.1 and 0.2 ceramics with a valley in the piezoresponse curves. Increasing the temperature agitates the domain arrangement and decreases the contrast for PFM images; this indicates a gradual phase transition in the composite. The temperature corresponding to maximum ΔT exhibits a downward shift (0.58 K at 80 °C for x = 0.1 and 0.5 K at 65 °C for x = 0.2), while the temperature-ΔT curves are flat when x = 0.3 and 0.4. Moreover, the maximum ΔT shows a decrease with an increase in the relaxor phase content; this is believed to be related to a decrease in the latent heat due to a pseudo-first-order to second-order transition. Thus, we suggest that the incorporation of a relaxor phase into ferroelectric matrices is an effective technique to tune their electrocaloric effect and improve the thermal stability of ceramic composites.
1. Introduction Zero-global-warming techniques are gradually gaining importance in the modern world as the earth continues to witness a steady increase in temperature, which in turn is responsible for extreme weather condition [1]. It is well known that hydrofluorocarbon gases, often used as refrigerants, are major contributors to the green-house effect. To overcome this drawback, several environmentally-friendly and novel techniques have been developed, such as the magnetocaloric effect and elastocaloric effect. However, these two caloric-effects can be only induced under large magnetic/stress fields and exhibit low cooling efficiency, thus restricting miniaturization and integration when
assembling cooling devices. The electrocaloric effect (ECE, including positive ECE and inverse ECE), defined as a change in entropy/temperature upon applying/removing an electric field in polar dielectrics under adiabatic conditions [2,3], has been extensively explored in the past decade for refrigeration applications ever since a giant ECE (ΔT ~ 12 K) was discovered in antiferroelectric inorganic thin films of PbZr0.95Ti0.05O3 near its antiferroelectric-paraelectric transition temperature [4]. Subsequently, a giant ECE was also detected in P(VDFTrFE-CFE) ferroelectric polymers at room temperature [5]. More recently, a high cooling-power device is designed by adopting flexible EC polymer films. The result show that when the cooling device is connected to a battery, its surface temperature decreases drastically by 8 °C
∗
Corresponding author. Institutes of Physical Science and Information Technology, Anhui University, Hefei, 230601, PR China. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (F. Li), [email protected] (C. Wang), [email protected] (J. Zhai). ∗∗
https://doi.org/10.1016/j.ceramint.2019.10.171 Received 5 October 2019; Received in revised form 16 October 2019; Accepted 18 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Feng Li, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.171
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x = 0.1, 0.2, 0.3 and 0.4). The mixed powders were milled again for another 8 h. The detailed experimental procedures can be found in Ref. [26]. The phase structure was detected by X–ray diffractometer (Rigaku Smart-lab Beijing Co, Beijing, China). Raman spectra were measured by a Horiba Lab-Ram iHR550 spectrometer with a Linkam THMSE 600 heating stage. The morphology of ceramics was observed using fieldemission SEM (Regulus 8230; Hitachi Co, Tokyo, Japan). The dielectric properties were obtained by using an LCR meter (Agilent E4980A, Santa Clara, CA) within temperature range of 25–400 °C. Polarizationelectric field (P-E) hysteresis loops were measured at room temperature with a frequency of 10 Hz by ferroelectric testing system (Precision LC, Radiant Technologies, Inc. Albuquerque, NM). The composites were poled under E = 4 kV/mm at room temperature and the d33 was measured by a quasi-static d33 m (ZJ-6A, Institute of Acoustics, Beijing, China). The piezoresponse force microscopy (PFM) images were obtained based on a commercial atomic force microscopy (AFM 5300E, Hitachi, Japan). The central area (5 × 5 μm2) was pre-poled at a dc voltage of 40 V. The images were then recorded within the area of 10 × 10 μm2 after removing the external electric field. The home–made adiabatic calorimeter was made to characterize the ECE behaviors for composites, and the measurement device is shown in Ref. [26].
within a very short period of time (~5s), thus demonstrating that ECE is a promising method for refrigeration applications [6]. The ECE has been investigated in lead-free relaxor ferroelectric (RF) ceramics and antiferroelectric ceramics, including those based on BaTiO3- [7–10], K0.5Na0.5NbO3- [11–13], Bi0.5Na0.5TiO3- (BNT-) [14–18], and NaNbO3 [19,20]; most of these ceramics exhibit excellent machinability and thermal stability. Among these ceramics, ECE in BNT-based compositions is of special interest. Due to the particular dielectric relaxor characteristics of BNT-based ceramics, a large ECE occurs over a wide temperature window. Furthermore, the existence of polar nanoregions (PNRs) contributes to ECE due to PNRs alignment in the presence of electric field [21]. For example, Weyland suggested that the existence of a diffuse critical point contributed to an increase in ECE and EC responsivity of 0.75BNT-0.25SrTiO3 ceramics [22]. Goupil illustrated that the maximal directly-measured ECE (~0.73 K) was located in the vicinity of the ferroelectric-to-relaxor transition temperature (TFR ~ 160 °C) in 0.82BNT-0.18(Bi0.5K0.5)TiO3 ceramics [23]. Later, Goupil also demonstrated that the shoulder temperature, Ts, in the dielectric permittivity curves of BNT-KNbO3 ceramics represented the upper temperature limit of the ECE peak, which facilitated the selection of novel lead-free ceramics for ECE cooling [24]. Recently, Li stated that ECE in 0.94BNT-0.06BaTiO3 (BNT-0.06BT) ceramics was maximum at the pseudo-first-order phase transition temperature [25]. Moreover, the addition of a relaxor ferroelectric (Sr, Bi)TiO3 member into BNT-0.06BT binary systems can lower their TFR and a large ECE may be observed at ambient temperature [26]. These studies indicate that modifying the ECE behavior of BNT-based ceramics makes them promising solid-state materials for several applications. With respect to the exploration of ECE in BNT-based ceramics, both direct and indirect methods have been adopted. Direct ECE methods refer to directly measuring entropy/temperature change, such as modified differential scanning calorimetry (DSC) and adiabatic calorimetry. In indirect methods, ΔT is calculated according to the Maxwell 1
E2
3. Results and discussion Fig. 1(a and c) shows the room-temperature X-ray diffraction (XRD) patterns of fresh and poled x = 0.1–0.4 composites. It can be seen that at all compositions, a pure perovskite-type structure is obtained without the inclusion of any other phase. To clearly detect differences in the phase structure of the fresh and poled composites, locally magnified (111) and (200) diffraction peaks are displayed in Fig. 1(b and d). Clear differences could be observed in phase structures of the two composites. In fresh composites, only a single diffraction peak could be detected, which indicates a pseudocubic phase. However, splitting occurs in the (111) and (200) peaks at poled x = 0.1 and 0.2, as shown by the arrows, demonstrating the coexistence of rhombohedral (R) and tetragonal (T) phases. With an increase in the RF content (x = 0.3 and 0.4), the splitting of diffraction peaks reduces; instead, a hump emerges at the lower angle of (200) peaks, as shown by diamond markers, owing to the presence of tetragonal PNRs. Notably, the macroscopic structure of poled x = 0.3 and 0.4 composites is still pseudocubic. This phenomenon was described in previous studies as well [32,33]. Therefore, it may be deduced that electric field application changes the phase structure of the matrix from pseudocubic to a co-existing R and T ferroelectric phase at x = 0.1 and 0.2. Meanwhile, the pseudocubic phase is maintained in the x = 0.3 and 0.4 composites, regardless of the application of an electric field. In fact, transition between non-ergodic relaxor and ferroelectric phases is irreversible, while the transition between ergodic relaxor and ferroelectric phase is reversible [34]. Therefore, compositions with x = 0.1 and 0.2 are classified as non-ergodic relaxors while those with x = 0.3 and 0.4 are ergodic relaxors. Fig. 2(a–d) show the SEM images of x = 0.1–0.4 composites, respectively. A dense microstructure without porosity and voids can be observed at all compositions, which indicates that the RF inclusions form a tight interface with the ferroelectric matrix. Notably, such a compact microstructure is advantageous in withstanding high-voltage pulses in ECE measurement. Meanwhile, grain size decreases slightly with an increase in the RF content. The grain size decreases from 1.73 μm for x = 0.1–1.48 μm for x = 0.4 composite, respectively, as clearly shown in their insets. Fig. 3(a–d) illustrate variations in dielectric constant (εr) and loss tangent (tanδ) of poled (x = 0.1–0.4) composites, respectively, in the temperature range of 25–400 °C and at frequencies of 1, 10, and 100 kHz. At all compositions, Tm (the temperature corresponding to dielectric constant maxima) at high temperatures could be clearly observed along with a dielectric shoulder, Ts, at low temperatures. These two dielectric anomalies are related to the mutual transitions between
∂P
equation using temperature-dependent P-E loops: Δ T= − Cρ ∫ T( ∂T )E dE E1
(C is the heat capacity and ρ is the density of ceramic pellets). Therefore, it is not difficult to analyze why both positive and inverse ECE are reported for BNT-based ceramics in previous reports [14,15,25,27]. For example, large discrepancies are observed in BNT0.18BKT ceramics using indirect and direct (DSC) methods, which can be ascribed to an increase in polarization and a decrease in the coercive field with an increase in temperature, leading to an inverse ECE [23]. Such large discrepancies are also observed in RF polymers. Lu et al. emphasize that the validity of the non-ergodic RF analyzed using Maxwell relation is a concern [28]. Therefore, the Maxwell equation is not suitable for non-equilibrium systems such as the nonergodic relaxor phase in BNT-based ceramics; in such condition, it is essential to calculate the actual ECE [27]. Hence, in this study, a home-made adiabatic calorimeter is designed to explore the evolution of ECE. Recently, ferroelectric/relaxor composites have been developed to tune the electrostrain properties of BNT-based ceramics. The results show that a large strain performance accompanied by a low hysteresis may be obtained at a relatively low electric field, which is thought to be related to the polarization and strain-coupling effect [29–31]. Under these conditions, 0.78Bi0.5Na0.5TiO3-0.06BaTiO3-0.16(Sr0.7Bi0.2)TiO3 RF is added into a BNT-0.06BT matrix to explore how phase/local structure and ECE evolve as functions of RF mass. 2. Experimental procedure (BNT-0.06BT) and The 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 0.78Bi0.5Na0.5TiO3-0.06BaTiO3-0.16(Sr0.7Bi0.2)TiO3 (BNT-BT-0.16SBT) powders were calcined separately by using the following raw chemicals: Bi2O3, Na2CO3, TiO2, SrCO3 and BaCO3 (≥99.0%). Different mass fraction of BNT-BT-0.16SBT calcined powders were added into the BNT-0.06BT calcined powders (10%, 20%, 30% and 40%, denoted as 2
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Fig. 1. (a & c) The unpoled and poled XRD patterns for x = 0.1–0.4 composites, respectively; (b & d) the enlarged (111) and (200) diffractions for the four compositions, respectively.
denoting that the ferroelectric order vanishes when the electric field is removed, which could explain their ergodic relaxor nature. These results are also in line with the results of our XRD analysis. The existence of a ferroelectric-to-relaxor transition leads to a striking structural transition and thus influences the ECE behaviors of these composites, which will be discussed in further sections. To clearly understand structural evolution in the poled composites at the nanoscale level, the in-situ temperature dependence of their
PNRs with rhombohedral and tetragonal symmetry rather than phase transformation [35]. Interestingly, another TFR anomaly could be observed in x = 0.1 and 0.2 composites. With an increase in RF inclusions, TFR decreases from 76 °C at x = 0.1–55 °C at x = 0.2. The presence of a TFR indicates that a ferroelectric order is induced under the action of an external electric field; these TFR values are maintained even removing the electric field, suggesting that these composites are non-ergodic RF ceramics. This anomaly is absent in x = 0.3 and 0.4 composites,
Fig. 2. (a–d) The SEM images of x = 0.1–0.4 composites, the grain size distribution for each composite is displayed in their insets. 3
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Fig. 3. (a–d) The dielectric constant and loss for the poled composites (x = 0.1–0.4), respectively.
Fig. 4. (a–d) The Raman spectra for the poled composites (x = 0.1–0.4) within the temperature range of 25–200 °C, respectively. The Raman spectra are deconvoluted using the Lorentzian function, as shown at the bottom of each figure.
explore phase transitions with respect to increasing temperature, the diffuse Raman spectra are deconvoluted into seven Lorentzian peaks, as shown at the bottom of each figure. The deconvolution of Raman peak in BNT-based material is elaborately discussed in previous study by J. Kreisel et al. [37]. Firstly, room-temperature Raman spectra corresponding to the poled x = 0.1–0.4 composites (dark lines) are significantly different. The Raman peaks corresponding to Ti–O bonds and TiO6 octahedra in x = 0.1 and 0.2 composites are much sharper than
Raman spectra at poled x = 0.1–0.4 is analyzed from 25 to 200 °C, as shown in Fig. 4(a–d). All the Raman spectra are broad and diffuse, which can be ascribed to the random occupation of Bi, Na, Ba, and Sr cations at A-sites. Generally, Raman spectra in the wavenumber range of 100–700 cm-1 can be divided into three regions. 1) peaks in the 100–200 cm-1 range are ascribed to mixed A-site vibrations, 2) peaks in the 200–400 cm-1 range are related to Ti–O bonds, and 3) peaks in the 400–700 cm-1 range are ascribed to TiO6 octahedral vibrations [36]. To
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Fig. 5. (a–d) The evolution of wavenumber and full-width at half maximum (FWHM) for PA and PB band as a function of temperature (25–200 °C) for x = 0.1–0.4 composites, respectively.
At x = 0.3 and 0.4, d33 shows a significant decrease, indicating that they evolve into incipient piezoelectric ceramics with negligible piezoelectric properties. Fig. 7(a, h, m & n) show the PFM images of x = 0.1–0.4 composites at room temperature, respectively. The central area (5 × 5 μm2) is prepoled at a dc voltage of 40 V. Images are then recorded within a scanning area of 10 × 10 μm2 after removing the external electric field. A clear contrast could be seen in x = 0.1 and 0.2 composites, illustrating that domains are easily written into the composite matrix. However, no clear contrast could be detected in x = 0.3 and 0.4 composites, indicating that the domains could not be maintained after removing the electric field. Fig. 8(a–d) exhibit piezoresponse distributions within the scanning area [indicated by the line in Fig. 7(a)]. A clear valley can be observed in the distributions of x = 0.1 and 0.2 composites, indicating that a downward polarization is induced by the electric field; this polarization is maintained after removing it. However, only fluctuating lines could be observed for x = 0.3 and 0.4 composites, suggesting that a local random field disrupts the induced ferroelectric order. To further explore domain evolution with increasing temperature, in-situ temperature-dependent PFM images are collected, as shown in Fig. 7(a–g & h–l). It can be seen that with an increase in temperature, the contrast in piezoresponse gradually become dim and unclear; this may be attributed to thermal agitation. When the temperature rises to 70–80 °C for x = 0.1 and 50–60 °C for x = 0.2 composites, there is a significant attenuation in the contrast, which indicates disappearance of the induced downward ferroelectric order. Fig. 9(a1–d1) map the directly-measured temperature change (ΔT) as a function of time for poled x = 0.1–0.4 composites in the temperature range of 25–120 °C. Exothermic peaks are induced upon the application of an electric field and endothermic peaks appears when the electric field is removed. These observations strongly suggest positive ECE characteristics for x = 0.1–0.4 composites. Though several inverse ECE properties have been reported for BNT-based ceramics, they are generally calculated based on Maxwell equations [14,15,40,41]. The adiabatic calorimeter employed in this investigation can more actually evaluate ECE in non-equilibrium BNT-based systems. Variations in temperature-dependent ΔT are plotted in Fig. 9(a2–d2). In x = 0.1 and 0.2 composites, the maximum ΔT peak could be clearly observed at specific temperature, i.e., 0.58 K at 80 °C for x = 0.1 and 0.5 K at 65 °C for x = 0.2. These specific temperatures are slightly higher than the corresponding TFR temperatures. This difference may be attributed to an increase in the transition temperature due to the electric field. Changes in entropy (ΔS) are proportional to lnΩ and the square of difference in polarization (ΔP) (ΔS ∝ lnΩ × ΔP2, where Ω denotes the
those of x = 0.3 and 0.4 composites, which can be ascribed to the greater polarity of x = 0.1 and 0.2 samples. It can be clearly seen that the wavenumber corresponding to PA and PB bands increases abruptly at x = 0.3 at room temperature, as shown by the arrows in Fig. 5(a–d). This further clarifies their non-ergodic characteristics (x = 0.1 and 0.2) and a long-range order appears upon the application of an electric field. The evolution of wavenumber and full-width at half maximum (FWHM) of PA and PB peaks as functions of temperature is plotted in Fig. 5(a–d); structural evolution is correlated with changes in Ti–O bonds and TiO6 octahedral vibrations in Bi-based solid solutions [38]. It is known that anomalies in Raman spectra reflect phase transition. The wavenumbers as well as FWHM values of PA and PB bands exhibit anomalies in the vicinity of 70 °C for x = 0.1 and 50 °C for x = 0.2, which signifies phase transition in these two composites. These observations are also in accordance with the TFR values in dielectric curves. However, such anomalies are not observed in x = 0.3 and 0.4 composites and the wavenumbers corresponding to their PA and PB bands reduce gradually while their FWHM increase as temperature increases. These changes indicate a gradual enhancement in relaxor properties. Fig. 6(a–b) shows the P-E loops of x = 0.1–0.4 composites with the corresponding current-density (I-E) loops. It can be seen that the P-E loops gradually transform from a square shape to a slanted shape with an increase in the RF content, indicating an enhanced relaxor degree. This can also be inferred from the I-E loops. There is only one current peak in the first and third quadrants, indicating that a long-range ferroelectric order is induced in the x = 0.1 sample upon the application of an electric field. In the case of x = 0.2 and 0.3 composites, there are two I-E peaks in each quadrant, indicating the coexistence of non-ergodic and ergodic relaxor phases (but with different proportions). Nevertheless, the underlying mechanism behind double I-E peaks is still a topic of debate. One particular theory based on transmission electron microscopy (TEM) suggests that a high-electric peak corresponds to nano-to-lamellar domain growth and a low-field peak indicates the disruption of the induced macro-ferroelectric domains [39]. As for the x = 0.4 composite, only one peak could be observed in the fourth quadrant with a current platform in the third quadrant, indicating that it is a pure ergodic relaxor phase. As shown in Fig. 6(c), the maximum polarization (Pmax) varies slightly, indicating that the electric field overcomes the local random field and domains can switch fully at E = 6 kV/mm. In contrast, remnant polarization (Pr) drastically decreases from x = 0.2 to 0.3, suggesting that a long-range order is maintained at x = 0.1 and 0.2 and disappears at x = 0.3 and 0.4. Their macroscopic piezoelectric performance also indicates that d33 increases from 168 pC/N (x = 0.1) to the maximum value of 204 pC/N (x = 0.2). 5
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Fig. 6. (a) The P-E loops and (b) the corresponding I-E loops for x = 0.1–0.4 composites; (c) the variation of maximal polarization (Pmax), remnant polarization (Pr) and d33 as a function of the relaxor phase content.
Fig. 7. The PFM images of (a–g) x = 0.1 in the temperature range of 25–100 °C; (h–l) x = 0.2 in the temperature range of 25–70 °C and (m & n) x = 0.3 and 0.4 at room temperature, respectively. The scanning area is 10 × 10 μm2 and the poled area in central region is 5 × 5 μm2 for each figure.
promising for cooling devices [9,43].
number of polar states, ferroelectric domains for the ferroelectric state, and PNRs for the relaxor phase) [42]. Below the transition temperature, the polar states are ferroelectric domains and Ω is quite low; however, when the temperature increases to TFR, PNRs act as polar states and hence Ω increases. Therefore, ΔT is maximum at the transition temperature. Further increasing the temperature increases the local random field, retards PNRs alignment, and weakens polarization and the ECE. In the case of x = 0.3 and 0.4 composites, the absence of a phase transition leads to a flat ΔT vs. temperature curve without inflection points [0.25 K for x = 0.3 and 0.21K for x = 0.4 at room temperature (RT)], which is useful for increasing the thermal stability of ECE. Another point that should be noted is that the maximum value of ΔT decreases gradually with an increase in RF content, as shown by the arrow in Fig. 9. This is due to the pseudo-first-order to second-order transition. For example, as discussed in BaTiO3 single crystals, a large ECE is obtained near sharp first-order phase transitions with a large latent heat at the expense of thermal stability. When ZrO2 is incorporated in a BaTiO3 ceramic matrix, a large ECE with a good thermal stability can be obtained due to diffuse dielectric properties; thus, these materials are
4. Conclusions Different amounts of 0.78Bi0.5Na0.5TiO3-0.06BaTiO30.16(Sr0.7Bi0.2)TiO3 inclusions with a relaxor phase are incorporated into 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ferroelectric ceramic matrices to modulate their electrocaloric effect. Their phase/local structure and macroscopic properties, such as piezoelectric, ferroelectric, and dielectric properties, change drastically with an increase in the RF content. The maximum ECE of 0.58 K for x = 0.1 and 0.5 K for x = 0.2 composites is obtained at specific temperatures; however, there is a significant variation in the ECE beyond these temperature points. ECE is observed to become stable with respect to temperature at x = 0.3 and 0.4 sample due to the absence of a pseudocubic-first-order to secondorder phase transition. Thus, we successfully demonstrate that the incorporation of a relaxor phase into a ferroelectric matrix can effectively tune the electrocaloric effect of the latter and improve its thermal stability, which is necessary in cooling materials. 6
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Fig. 8. The room-temperature piezoresponse for x = 0.1–0.4 composites along the line, as indicated in Fig. 7(a).
Declaration of competing interest
Acknowledgements
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was supported by the National Natural Science Foundation of China, China (under grant no. 51772211, 51502001 and 51872001). This work was also supported by the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics, China (grant no. KF201803) and the start-up funding of Anhui University, Anhui University, China (no. S020118002/098).
Fig. 9. (a1–d1) The appearance of exothermic and endothermic peaks with application/removal of electric field (E = 4 kV/mm) at selected temperatures; (a2–d2) ΔT as a function of temperature at E = 4 kV/mm. 7
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