Enhancement of the electrocaloric effect in the 0.4BCZT-0.6BTSn ceramic synthesized by sol-gel route

The lead-free ferroelectric 0.4Ba0.85Ca0.15Zr0.10Ti0.90O3–0.6BaTi0.89Sn0.11O3 (0.4BCZT–0.6BTSn) ceramics were successfully prepared by the sol–gel process. Raman spectroscopy was used to examine the structural properties of the 0.4BCZT-0.6BTSn sample. The findings indicate that the sample was well crystallized into a single perovskite structure. The phase transitions of the studied sample have been investigated using the DSC technique. The electrocaloric effect (ECE) properties were indirectly determined using the Maxwell approach. Under a relatively low applied electric field of 30 kV cm−1, the results show enhanced electrocaloric temperature change and entropy change of ΔT = 1.32 K and ΔS = 1.41 J/kg.K, respectively. Besides, the electrocaloric responsivity (ξ max = 0.45 K·mm/kV) obtained is among the highest reported values in pb-free ferroelectrics near room temperature. These findings demonstrate that the lead-free 0.4BCZT–0.6BTSn ceramic is a promising candidate for solid-state cooling applications.


Introduction
Recently, the electrocaloric effect (ECE) has emerged as a promising route for the development of advanced cooling and refrigeration technologies that are more energy-efficient and environmentally friendly than traditional methods [1][2][3].This phenomenon arises from the coupling between the electric polarization and the thermal behavior of the material [4].Researchers are actively investigating various electrocaloric materials, such as ferroelectric materials, relaxor ferroelectric materials, and organic materials, to develop new and improved cooling technologies.Lead-free BaTiO 3 -based materials (BT) have attracted significant research attention due to their enhanced ferroelectric and ECE performances [5,6].The development of electrocaloric materials with larger electrocaloric responses and improved performance will be crucial for practical application [2,7].Hence, doping with other metal oxides or forming solid solutions with other compounds can be used to improve the electrocaloric effect properties of BT as well as other properties at near-room temperatures (RT) [8,9].Among these systems, BaTiO 3 -CaZrO 3 (BCZT) systems have received a lot of attention because of their improved dielectric, ferroelectric, piezoelectric, and energy storage performances due to the presence of several sequences of structural transitions [10][11][12][13][14].For instance, Abdessalem et al [9] reported an electrocaloric temperature change (ΔT) of 0.565K in Ba 0.9 Ca 0.1 Zr 0.05 Ti 0.95 O 3 under an electric field of 30 kV cm −1 .Merselmiz et al [11] recently reported an electrocaloric temperature change (ΔT) of 0.57 K in Ba 0.85 Ca 0.15 Zr 0.10 Ti 0.90 O 3 under 25 kV cm −1 .
On the other hand, the lead-free ferroelectric BaTi 1-x Sn x O 3 (BTSn) system, constitutes one of the BT-based materials that tend to achieve significant energy storage and electrocaloric properties around the ferroelectricparaelectric phase transition [15,16].The combination of BCZT and BTSn solid solutions improved piezoelectric and energy storage properties [17,18].This approach has been used in various BT-based ceramics [19][20][21][22][23][24].
In particular, our earlier research [18] highlighted a significant energy-storage capacity (energy storage density and energy-storage efficiency) in the composite 0.4Ba 0.85 Ca 0.15 Zr 0.10 Ti 0.90 O 3 -0.6BaTi0.89 Sn 0.11 O 3 (named as 0.4BCZT-0.6BTSn)ceramic under a moderate applied electric field of 30 kV cm −1 .Therefore, the current work focuses on the electrocaloric effect of the designed 0.4BCZT-0.6BTSncomposite ceramic synthesized via sol-gel method.

Experimental details
The polycrystalline lead-free 0.4BCZT-0.6BTSnceramic was prepared using the sol-gel method.A detailed description of the synthesis process can be found elsewhere in reference [18].The circular pellets with 6 mm diameter and 0.4 mm thickness were pressed at 2.5 ton/cm using a uniaxial hydraulic press and then sintered at 1350 °C for 7 h.Raman spectroscopy measurements of the sintered 0.4BCZT-0.6BTSnceramics were conducted to support the x-ray diffraction (XRD) analysis using the HORIBA LabRAM HR800 spectrometer.Differential scanning calorimetry (PerkinElmer Jade DSC) was used to point out the different phase transitions in a temperature range from 30 to 90 °C.
The electrodes were placed on both sides of the ceramic samples for electrical measurements using silver paster that was dried at a temperature of 150 °C.Temperature-dependent P-E hysteresis loops were measured at 200 Hz using a ferroelectric test system (PolyK Technologies State College, PA, USA).The J-E hysteresis loops were recorded using an AixACCT TF 2000 analyzer with a SIOS Meβtechnik GmbH laser interferometer and a TREK model 609E-6 high-voltage amplifier.

Raman spectroscopy analysis
Because of its sensitivity to crystal lattice vibration, Raman spectroscopy is a powerful tool for studying the ferroelectric phase transition and local crystal symmetry [25,26].Figure 1 displays Raman spectra of 0.4BCZT-0.6BTSnceramic recorded at room temperature.One can see that all Raman active mode assignments of the studied system are thought to be similar to those of pure BaTiO 3 .Two transverse optical modes [A 1 (TO 1 )] and [A 1 (TO 2 )] were detected in the range of 100-250 cm −1 of 0.4BCZT-0.6BTSnceramic.The presence of an interference effect at 178 cm −1 , the peak at 297 cm −1 assigned to the combined mode [B 1 , E(TO, LO)], and asymmetric bands at 517 and 720 cm −1 are considered particularly distinctive of the tetragonal symmetry [27,28].The clear mode observed at 208 cm −1 is the A 1 (LO) mode, which is also a feature of Rhombohedral (R) and Orthorhombic (O) symmetries [29,30].These findings support the coexistence of orthorhombic and tetragonal phases in 0.4BCZT-0.6BTSnceramic, consistent with previous x-ray and dielectric studies reported in our earlier work [18].

DSC analysis
Figure 2 depicts the DSC curve of our 0.4BCZT-0.6BTSnsample, which reveals two distinct and significant endothermic peaks at temperatures close to 42 and 56 °C.The first peak around room temperature (RT) corresponds to the Orthorhombic-Tetragonal (O-T) phase transition, while the second one characterizes the Tetragonal-Cubic (T-C) phase transition at Curie temperature.These results corroborate those obtained in dielectric measurements published in our previous work [18].Moreover, the O-T phase transition around RT confirms the results discussed in the Raman spectroscopy analysis section above.However, as stated by Gao et al [31], the transition enthalpy at the Curie temperature drops progressively as the composition x of the Sn dopant in BTSx grows, and when it changes to the multi-phase point (x = 10.5), the heat flow peak almost disappears.

Ferroelectric and electrocaloric properties
Figure 3(a) shows the thermal evolution of P-E hysteresis loops of 0.4BCZT-0.6BTSnceramic under an electric field of 30 kV cm −1 .These non-linear curves prove the ferroelectric nature of 0.4BCZT-0.6BTSnceramic.Besides, the room-temperature J-E hysteresis loops of 0.4BCZT-0.6BTSnsample at different electric fields are depicted in figure 3(b).Such peaks can be observed for both positive and negative cycles of the applied electric  field E.Moreover, these peaks are detected around the coercive field (E C ) before the high electric field, revealing domain switching and the ferroelectric nature of the 0.4BCZT-0.6BTSnceramic [32,33].
The ECE in eco-friendly 0.4BCZT-0.6BTSnceramic is determined using the indirect Maxwell method.The EC effect is computed using the measured ferroelectric order parameter P (E,T), extracted from the upper branches of the corresponding P-E hysteresis loops (at 200 Hz).The reversible adiabatic temperature (ΔT) and the isothermal entropy change (ΔS) as the applied electric field changes from E 1 = 0 to E 2 of an EC material are given as follows [34]: here, r is the density of the sintered samples which was measured by Archimede's method (5.87 g cm −3 ).C p is the specific heat capacity of the sintered 0.4BCZT-0.6BTSn,which was taken as 380 JKg −1 K −1 in the studied range temperature as commonly used for BCZT and BTSn in the literature [35].E 1 and E 2 are the starting and final applied fields, respectively.The critical factor (∂P/∂T) E value is calculated using a seven-order polynomial fitting of raw P-T data at various external electric fields extracted from the hysteresis loops.The EC temperature change (ΔT) and the isothermal entropy change (ΔS) for these ceramics are determined from equations (2) and (3), and the results are depicted in figures 4(a) and (b).The sample 0.4BCZT-0.6BTSnexhibits large values of ΔT = 1.32 K and ΔS = 1.41J/kg.K.The thermal variation of ΔT and ΔS shows the same varying tendency and maximum around the RT corresponding to the ferroelectric-paraelectric phase transition.Moreover, electrocaloric responsivity (ξ max ) refers to a material's ability to change its EC temperature in response to an applied electric field and can be defined as ξ max = (ΔT max /ΔE max ) [36]. Figure 4(c) depicts the plots of ΔT and the corresponding ξ max versus the applied electric field.One can see that ΔT and ξ max increase as the applied electric field increases.The corresponding EC responsivity in our 0.4BCZT-0.6BTSnceramic is about 0.45 K•mm/kV, which is one of the highest EC response values reported for BT-lead-free based ceramics.A broad ΔT peak was observed for 0.4BCZT-0.6BTSndue to the diffused phase transition and excellent EC strength was obtained.When the external electric field is applied, the local regions become relatively ordered, which increases isothermal changes in entropy and electrocaloric effect [37,38].
Table 1 compares the EC response of 0.4BCZT-0.6BTSnceramic with previously published works for leadfree ferroelectrics ceramics.Hanani et al [12] reported a high ΔT and ξ max of 0.986 K and 0.246 K•mm/kV under an electric field of 40 kV cm −1 in Ba 0.95 Ca 0.05 Zr 0.1 Ti 0.9 O 3 ceramic.Furthermore, Patel et al [39] recorded a high ΔT and ξ max of 1.5 K and 0.455 K•mm/kV in the Sr and Sn doped BCZT (Ba 0.85 Ca 0.075 Sr 0.075 Zr 0.1 Ti 0.88 Sn 0.02 O 3 ) ceramic under 33 kV cm −1 .Under a high electric field of 60 kV cm −1 , Hanani et al [12] reported a large ΔT of 1.479 K in Ba 0.85 Ca 0.15 Zr 0.10 Ti 0.90 O 3 ceramic.As a result, multiphase coexistence can enhance EC properties of EC materials.Such behavior indicates there is a very low energy barrier exists in the multiphase coexisting region (Tetragonal and Orthorhombic).In the multiphase coexistence region, the low energy barrier between the ferroelectric phases increases the equilibrium orientations of polar states in the presence of an external electric field.It has been determined that the number of equilibrium orientations of the polar state increases with the theoretical maximum ΔT change under an electric field [40][41][42].The electrocaloric results depend on the synthesis conditions, the applied electric field, and the grain size.
Another significant additional factor in evaluating an electrocaloric material's efficiency is the coefficient of performance (COP), given by the following equation [43]: where Q and W rec are the isothermal heat and recovered density, respectively.W rec was calculated by numerically integrating the area between P-E loops discharge curve and polarization axis.More details on the energy storage capacities of the studied system are reported elsewhere [18].As shown in figure 5

Conclusion
In summary, the sol-gel method was used to prepare 0.4BCZT-0.6BTSnceramic successfully.The structural properties of the 0.4BCZT-0.6BTSnsample were confirmed using room-temperature Raman spectroscopy.Using the indirect Maxwell's approach, a high EC performances was demonstrated in 0.4BCZT-0.6BTSnceramic under a moderate electric field of 30 kV cm −1 , with values of ΔT = 1.32 K, ΔS = 1.41J/kg.K, ξ max = 0.45 K•mm/kV and a coefficient of performance of about 23.The results indicate that the 0.4BCZT-0.6BTSnsample could be a promising material for environmentally friendly refrigeration applications.

Figure 4 .
Figure 4. Temperature-dependence of (a) ΔT and (b) ΔS at different applied electric fields.(c) Electric field-dependence of ΔT and ξ max near the T C .(d) Thermal-evolution of COP and ξ max in 0.4BCZT-0.6BTSnceramic.
[46] the COP of the 0.4BCZT-0.6BTSnceramicreachedamaximum of 23 at 345 K and 30 kV cm −1 .The obtained value exceeds those reported in various published works on lead-free and lead-based materials.For instance, Kumar et al[44]obtained a COP value of 4.16 under 50 kV cm −1 in 0.97K 0.5 Na 0.5 NbO 3 -0.03LaNbO 3 ceramic.Peng et al[45]reported a COP value of 3.37 in Nb-doped Pb 0.99 (Zr 0.65 Sn 0.3 Ti 0.05 ) 0.98 O 3 antiferroelectric thin film by using a sol-gel method.In addition, Merselmiz et al[11]stated a COP value of 11 under 25 kV /cm in Ba 0.85 Ca 0.15 Zr 0.10 Ti 0.90 O 3 ceramic prepared by the solid-state method.Recently, Khardazi et al[46]reported a COP value of 25 under 20 kV cm −1 in Ba 0.9 Sr 0.1 Ti 0.9 Sn 0.1 O 3 ceramic synthesized by the sol-gel technique.