Zouhair Hanani*ab,
Soukaina Merselmiz
a,
Daoud Mezzane
a,
M'barek Amjoud
a,
Andraž Bradeškoc,
Brigita Rožič
c,
Mohammed Lahcini
ad,
Mimoun El Marssie,
Andrey V. Ragulyaf,
Igor A. Luk'yanchukeg,
Zdravko Kutnjakc and
Mohamed Gounéb
aIMED-Lab, Cadi Ayyad University, Marrakesh, 40000, Morocco. E-mail: Zouhair.hanani@edu.uca.ma
bICMCB, University of Bordeaux, Pessac, 33600, France
cJozef Stefan Institute, Ljubljana, 1000, Slovenia
dMohammed VI Polytechnic University, Ben Guerir, 43150, Morocco
eLPMC, University of Picardy Jules Verne, Amiens, 80039, France
fFrantsevich Institute for Problems of Materials Science of NASU, Kyiv, 03142, Ukraine
gPhysics Faculty, Southern Federal University, Rostov-on-Don, 344090, Russia
First published on 20th August 2020
Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) relaxor ferroelectric ceramics exhibit enhanced energy storage and electrocaloric performances due to their excellent dielectric and ferroelectric properties. In this study, the temperature-dependence of the structural and dielectric properties, as well as the field and temperature-dependence of the energy storage and the electrocaloric properties in BCZT ceramics elaborated at low-temperature hydrothermal processing are investigated. X-ray diffraction and Raman spectroscopy results confirmed the ferroelectric–paraelectric phase transition in the BCZT ceramic. At room temperature and 1 kHz, the dielectric constant and dielectric loss reached 5000 and 0.029, respectively. The BCZT ceramic showed a large recovered energy density (Wrec) of 414.1 mJ cm−3 at 380 K, with an energy efficiency of 78.6%, and high thermal-stability of Wrec of 3.9% in the temperature range of 340–400 K. The electrocaloric effect in BCZT was explored via an indirect approach following the Maxwell relation at 60 kV cm−1. The significant electrocaloric temperature change of 1.479 K at 367 K, a broad temperature span of 87 K, an enhanced refrigerant capacity of 140.33 J kg−1, and a high coefficient of performance of 6.12 obtained at 60 kV cm−1 make BCZT ceramics potentially useful coolant materials in the development of future eco-friendly solid-state refrigeration technology.
In 2009, Liu and Ren13 reported a Pb-free ceramic with composition Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT), having extremely high dielectric constant and piezoelectric properties at the Morphotropic Phase Boundary (MPB). This lead-free ceramic possesses a broad frequency-dependent peak corresponding to the temperature-dependent dielectric susceptibility, lower remnant polarization, and slimmer hysteresis loops due to the relaxor ferroelectric nanodomains, which are essential for realizing extremely high energy densities and efficiencies.14–17 Moreover, it was reported that BCZT ceramics possess low dielectric loss (tanδ) (1–3%), which is encouraging for obtaining high-efficiency energy storage density.10,18–21
BCZT relaxor ferroelectric ceramics have been attracting much attention for energy storage applications and electrocaloric cooling devices owing to their outstanding dielectric and ferroelectric properties.22,23 Zhan et al.24 achieved an energy storage density of 590 mJ cm−3 and storage efficiency (η) of 72.8% in Ba0.95Ca0.05Zr0.30Ti0.70O3 ceramics at 160 kV cm−1. Puli et al.25 examined the energy density properties in the (1 − x)BaZr0.20Ti0.80O3–xBa0.70Ca0.30TiO3 (x = 0.10, 0.15, 0.20) system, and observed enhanced energy storage density and high energy storage efficiency of 680 mJ cm−3 and 72.8%, respectively, at x = 0.15, by using an electric field of 170 kV cm−1. The thermal-stability of the recovered energy density plays a crucial role in energy storage technologies. Hence, a wide operating temperature and stable Wrec are essential in this type of application.26
The electrocaloric effect (ECE) in BCZT relaxor ferroelectric ceramics has been widely investigated.15,27–31 Kaddoussi et al.12 studied the ECE in Ba0.85Ca0.15Zr0.10Ti0.90O3 indirectly using the Maxwell relation at 8 kV cm−1, and found a low electrocaloric temperature change (ΔT) of 0.152 K. Using the same approach, Ben Abdessalem et al.28 obtained ΔT = 0.565 K at 30 kV cm−1. Zhou et al.32 studied the compositional dependence of the electrocaloric effect in lead-free (1 − x)Ba(Zr0.20Ti0.80)O3–x(Ba0.70Ca0.30)TiO3 ceramics under a moderate electric field of 28 kV cm−1, and reported a high ECE response of 0.56 K at x = 0.6. For the advanced evaluation of the electrocaloric efficiency, it was highly recommended that the obtained EC properties like ΔT, ΔS (entropy change) be linked to the total energy density (Wtot) of the EC material in question to access the coefficient of performance (COP = input power/output cooling power).33 Nevertheless, few works have simultaneously studied the energy storage performances and electrocaloric effect (ECE) in BT-based ferroelectric materials. Srikanth et al.34 investigated the electrocaloric effect in BaxSr1−xTiO3 ferroelectric ceramics using an indirect approach. ΔT were found to be 0.67, 0.83, and 0.61 K and ΔS was 0.9, 1, and 0.7 J kg−1 K−1 for x = 0.7, 0.8, and 0.9, respectively, under an electric field of 33 kV cm−1. Besides, energy-storage densities of 90, 142, 144 mJ cm−3 were observed for x = 0.7, 0.8, and 0.9, respectively, which correspond to the estimated COP of 17, 5.47, and 5.68 for x = 0.7, 0.8, and 0.9, respectively.34 We previously reported the synthesis of the BCZT ceramic by surfactant-assisted solvothermal processing.15 However, only a small total and recovered energy density and electrocaloric response were obtained due to the low applied electric field of 6.6 kV cm−1. It was stated before that the low-temperature hydrothermal processing allowed the possibility to synthesize BCZT ceramics with enhanced dielectric and ferroelectric properties.35 In this work, the ferroelectric-paraelectric phase transition in the BCZT ceramic was investigated via temperature-dependent X-ray diffraction, Raman spectroscopy and dielectric measurements. Field-dependence and the temperature stability of the energy storage performances of the BCZT ceramic were also investigated at 60 kV cm−1. Moreover, the electrocaloric effect in BCZT ceramic was explored via an indirect approach following the Maxwell relation, and the ECE performances like the refrigerant capacity (RC)33 and COP of the BCZT ceramic were examined and compared to lead-free and lead-based materials.
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Fig. 1 (a) TEM image, (b) grain size distribution (a single BCZT nanoparticle is shown in the inset), (c) TGA curve of the BCZT powder, and (d) SEM micrograph of the BCZT sintered ceramic. |
The structural changes in the BCZT sintered ceramic were examined via the temperature dependence of XRD and Raman spectroscopy. Fig. 2a displays the temperature-dependent XRD patterns of the BCZT ceramic in the temperature range between 300 and 400 K. Pure perovskite structures were formed without any secondary phases, and all peaks were indexed based on the standard BaTiO3 pattern (JCPDS card no. 96-901-4669). At 300 K, the BCZT ceramic exhibited tetragonal symmetry with space group P4mm, due to the peak splitting at around 2θ ∼ 45° (Fig. 2b).13,43
To gain insight into the structural changes in the BCZT ceramic with increasing temperature, the thermal evolution of the peak around 2θ ∼ 45° was followed by in situ XRD (Fig. 2b). On increasing the temperature, the peak at around 2θ ∼ 45° tends to merge into a single peak between 340 and 360 K due to the tetragonal-cubic phase transition,43 and with a further increase in temperature, this peak became sharper. It is worth mentioning that with increasing temperature, all peaks shifted toward lower 2θ, due to the unit cell expansion and the decreasing of the tetragonal phase.12 Besides, Raman spectroscopy is used to have an idea about this structural change in BCZT sintered ceramic in the temperature range 300–400 K (Fig. 2c). The observed Raman peaks around 115, 180, 245, 294, 517, and 716 cm−1 correspond to E(TO1), A1(TO1), A1(TO2), E(TO2), A1(TO3), and A1(LO2)/E(LO), respectively. These peaks are characteristics of perovskite oxides.44,45 With increasing temperature, the peak position and the intensity of the Raman modes present a decreasing trend, alongside with Raman modes broadening, which is similar to other ferroelectric materials.44,46 A 2D colour map of Raman spectra of BCZT ceramic is illustrated in Fig. 2d.
It was observed that the low-temperature data had a much greater signal and lower background as compared to the high-temperature data. Note that the changes in the contour plots between 349 and 361 K (marked by the dashed rectangles) can be attributed to structural changes in the BCZT ceramic. It was stated that the E(TO2) mode is associated with the tetragonal phase transition.47 In order to reveal the origin of the detected structural change in the BCZT ceramic, the wavenumbers around 292–296 cm−1 and temperature between 300 and 400 K were plotted using a 2D colour map as shown in Fig. 2e. It was observed that the intensity of the E(TO2) mode decreased with increasing temperature, and an abrupt intensity change in this mode occurred at 355 K due to the structural change from ferroelectric (tetragonal) to paraelectric (cubic).15,48
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Fig. 3 (a) Room-temperature frequency dependence and (b) temperature-dependence of the dielectric properties of the BCZT ceramic. |
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Room-temperature P–E hysteresis loops of the BCZT ceramic at various electric fields from 10 to 60 kV cm−1 are shown in Fig. 5a. It was observed that the BCZT ceramic exhibited enhanced ferroelectric properties. The maximal polarization (Pmax), remnant polarization (Pr), and coercive field (Ec) increased on increasing the applied electric field. At 60 kV cm−1, the values of Pmax, Pr, and Ec reached 27.21 μC cm−2, 8.59 μC cm−2 and 4.10 kV cm−1, respectively. It should be noted that the hysteresis loops were not fully saturated at 60 kV cm−1; however, the thickness of the sample prevented us from further increasing the electric field.
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Fig. 5 (a) Room-temperature electric field-dependence of P–E loops; (b) energy storage performances in the BCZT ceramic. |
From the first two equations, the increasing applied electric field boosted the obtained energy storage density. The room-temperature electric field-dependence of the energy storage properties of the BCZT ceramic is shown in Fig. 5b. As desired, after increasing the applied electric field from 10 to 60 kV cm−1, the linear behavior of the Wtot and Wrec was observed, where, Wtot and Wrec increased from 44.1 and 31.4 mJ cm−3 to 546.1 and 367.2 mJ cm−3, respectively. However, η dropped from 71.2% to 67.2%, respectively. At 60 kV cm−1, Xu et al.26 and Puli et al.57 obtained Wrec of 121.6 mJ cm−3 (η = 51.3%) and 280 mJ cm−3 (η = 58.3%), respectively. These values are lower than those obtained in the BCZT ceramic (Wrec = 367.2 mJ cm−3, η = 67.2%).
Table 1 compares the energy storage properties of the BCZT ceramic with other lead-free ceramics reported in the literature. Under the high electric field of 160 kV cm−1, Zhan et al.24 achieved a recovered energy density of 590 mJ cm−3 and storage efficiency of 72.8% in the Ba0.95Ca0.05Zr0.3Ti0.7O3 ceramic. Meanwhile, using an electric field of 170 kV cm−1, Puli et al.25 observed a recovered energy density and energy storage efficiency of 680 mJ cm−3 and 72%, respectively, in the 0.85BaZr0.2Ti0.8O3–0.15Ba0.7Ca0.3TiO3 system. In a recent study, Puli et al.57 observed an energy storage density of 1.33 J cm−3 at 106 kV cm−1 but with a reduced energy efficiency of 52.3% in the Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramic. It was stated before that the non-saturated P–E loops in the BCZT ceramic could help to further increase the applied electric field, hence, enhancing the energy density. However, to avoid the electric breakdown at high temperature, the electric field was kept at 60 kV cm−1.
Ceramic | Wtot (mJ cm−3) | Wrec (mJ cm−3) | η (%) | E (kV cm−1) | T (K) | Ref. |
---|---|---|---|---|---|---|
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 546.1 | 367.2 | 67.2 | 60 | 300 | This work |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 535.3 | 398.5 | 74.4 | 60 | 340 | This work |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 526.6 | 414.1 | 78.6 | 60 | 380 | This work |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 436.3 | 302.4 | 69.3 | 50 | 300 | This work |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 278.3 | 197.3 | 70.9 | 35 | 300 | This work |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 136.7 | 97.4 | 71.3 | 20 | 300 | This work |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 44.1 | 31.4 | 71.2 | 10 | 300 | This work |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 236.4 | 121.6 | 51.3 | 60 | RT | 26 |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 480.3 | 280 | 58.3 | 60 | RT | 57 |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 1330 | 695.6 | 52.3 | 106 | RT | 57 |
0.6BaZr0.20Ti0.80O3–0.4Ba0.70Ca0.30TiO3 | 207 | 149 | 72 | 35 | 303 | 63 |
Ba0.85Ca0.15Zr0.10Ti0.90O3 (32.84 μm) | 113.8 | 38.6 | 33.9 | 20 | 298 | 64 |
Ba0.85Ca0.15Zr0.10Ti0.90O3 (32.84 μm) | 90.2 | 71.2 | 78.9 | 20 | 373 | 64 |
Ba0.85Ca0.15Zr0.10Ti0.90O3 (44.37 μm) | 98.1 | 36.4 | 37.1 | 20 | 298 | 64 |
Ba0.85Ca0.15Zr0.10Ti0.90O3 (44.37 μm) | 81.1 | 69.6 | 85.8 | 20 | 373 | 64 |
Ba0.95Ca0.05Zr0.30Ti0.70O3 | 810.4 | 590 | 72.8 | 160 | RT | 24 |
Ba0.95Ca0.05Zr0.20Ti0.80O3 | 569.4 | 410 | 72 | 120 | RT | 65 |
0.85BaZr0.20Ti0.80O3–0.15Ba0.70Ca0.30TiO3 | 940 | 680 | 72 | 170 | RT | 25 |
Ba0.975La0.017(Zr0.05Ti0.90)Sn0.05O3 | 108.3 | 65 | 60 | 12 | 300 | 66 |
BaTi0.89Sn0.11O3 | 92.7 | 84.4 | 91.04 | 25 | 333 | 67 |
BaTiO3 | 1594 | 450 | 28.23 | 110 | RT | 68 |
BaZr0.05Ti0.95O3 | 302 | 218 | 72 | 50 | RT | 69 |
0.90(0.92Bi0.50Na0.50TiO3–0.08BaTiO3)–0.10NaNbO3 | 1082 | 710 | 65.6 | 70 | 298 | 61 |
0.90(0.92Bi0.50Na0.50TiO3–0.08BaTiO3)–0.10NaNbO3 | 954.1 | 790 | 82.8 | 70 | 373 | 61 |
Bi0.48La0.02Na0.40K0.10Ti0.98Zr0.02O3 | 1033 | 630 | 61 | 60 | 298 | 62 |
Bi0.48La0.02Na0.40K0.10Ti0.98Zr0.02O3 | 783.1 | 650 | 83 | 60 | 348 | 62 |
Bi0.48La0.02Na0.40K0.10Ti0.98Zr0.02O3 | 755.5 | 680 | 90 | 60 | 298 | 62 |
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Fig. 6 Temperature-dependence of (a) P–E loops; (b) energy storage performances of the BCZT ceramic at 60 kV cm−1. |
The temperature-dependence of ΔWrec,T/Wrec,300 K values in BCZT ceramics is depicted in Fig. 7. It was observed that ΔWrec,T/Wrec,300 K of BCZT ceramics is always less than 12.7% (Wrec ∼ 367.2–414.1 mJ cm−3) in the temperature range from 300 to 400 K, indicating the excellent energy storage capabilities in a wide temperature range. The energy-storage variation in BCZT ceramics is lower than that reported by Jayakrishnan et al.63 in 0.6BaZr0.20Ti0.80O3–0.4Ba0.70Ca0.30TiO3, where Wrec at 25 kV cm−1 dropped from 121, 115 to 65 mJ cm−3, at 303, 323 and 363 K, respectively, corresponding to ΔWrec,T/Wrec,300 K of 46.3%. Furthermore, the obtained value of the variation in the recovered energy density was also lower than that found by Xu et al.26 in the BCZT–0.5 wt% MgO ceramic (14.18%). Despite the enhanced energy efficiency in BCZT ceramics with different grain sizes found by Cai et al.64 at 20 kV cm−1, the high variation in the recovered energy density was observed in the temperature range of 298 to 393 K (84.4% and 91.2 for BCZT with grain sizes of 32.84 and 44.37 μm, respectively). Besides, Liu et al.70 obtained an ESV value of 15% in the temperature range of 298–373 K in the lead-based Pb0.97La0.02(Zr0.58Sn0.335Ti0.085)O3 ceramic, which is higher than that obtained in the BCZT ceramic. A similar energy-storage variation was obtained by Xu et al.61 in the 0.90(0.92Bi0.50Na0.50TiO3–0.08BaTiO3)–0.10NaNbO3 ceramic at 70 kV cm−1 in the temperature range of 298–423 K.
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Fig. 7 The thermal stability of ΔWrec,T/Wrec,300 K and ΔWrec,T/Wrec,340 K (inset) of the BCZT ceramic at 60 kV cm−1. |
In the case of a high-temperature range, the temperature variation in Wrec values was estimated within the temperature range of 340–400 K by eqn (5), according to Malik et al.,71
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A plot of ΔWrec,T/Wrec,340 K values as a function of temperature in the BCZT ceramic is displayed in the inset of Fig. 7. The temperature variation in the energy storage density values of the BCZT ceramic was estimated to be 3.9% (398.5–414.1 mJ cm−3). These values are considered to be excellent as compared with many lead-free ferroelectric ceramics.26,62,63,71,72
For instance, Butnoi et al.62 found an ESV ratio of 4.6% in Bi0.48La0.02Na0.40K0.10Ti0.98Zr0.02O3 ceramics in the temperature range of 348–423 K. The high energy storage density in relaxor ferroelectric ceramics is due to the diffuse phase transition.73 Consequently, ferroelectricity can be detected in a wide temperature range in relaxor ferroelectrics, which significantly reduces ESV, thus keeping energy storage enhanced in a wide temperature range.15 It is worth recalling that the BCZT powder was elaborated at 160 °C for 24 h in contrast to other lead-free-based powders reported in the literature, including BCZT, which request an inevitable calcination step at high temperature.25,26,57,61–65,69 Hence, the hydrothermal method has better advantages in the elaboration of BCZT ceramics at lower sintering temperatures and with outstanding and thermally-stable energy storage performances.
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Fig. 8 Temperature-dependence of (a) P, (b) ΔS, (c) ΔT and (d) ζ at various applied electric fields in the BCZT ceramic. |
The thermal evolution of the reversible electrocaloric temperature change (ΔT) and the corresponding entropy change (ΔS) under different applied electric fields in the BCZT ceramic are shown in Fig. 8b and c, respectively. ΔT and ΔS increased with increasing the applied electric field, and the temperature at which the maximum was obtained shifted toward higher temperatures. This indicates that the TC of BCZT ceramic shifted to higher temperatures under a higher electric field.74 At 60 kV cm−1, a large ΔT and ΔS of 1.479 K and 1.613 J kg−1 K−1 were found in the BCZT ceramic in the vicinity of the tetragonal-cubic phase transition at 367 K, respectively. Comparable values of ΔS = 1.78 and 1.53 J kg−1 K−1 were reported by Xin et al.6 at 70 kV cm−1 in Ba0.9Ca0.1Zr0.05Ti0.95O3 and Ba0.9Ca0.1Zr0.1Ti0.9O3 ceramics, respectively. Fig. 8d plots the electrocaloric responsivity ζ = ΔT/ΔE as a function of temperature and applied electric field. It was observed that the temperature of the ECE responsivity peak and its magnitude initially increased with increasing the applied electric field then gradually became stable at higher fields. The ECE responsivity saturation could be ascribed to the diffused phase transition and the relaxor behavior in the BCZT ceramic, as observed by Xin et al.6
Table 2 compares the EC responses (ΔT and ζ) in BCZT ceramics with previously published results on lead-free ceramics obtained via the indirect approach under different applied electric fields. At a low electric field of 8 kV cm−1, our ceramics exhibited comparable ECE properties (ΔT = 0.155 K and ζ = 0.194 K mm kV−1 at 347 K) to those found in Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramics synthesized via the solid-state method (ΔT = 0.152 K and ζ = 0.19 K mm kV−1 at 343 K) by Kaddoussi et al.12 Hanani et al.27 reported ECE properties (ΔT = 0.492 K and ζ = 0.289 K mm kV−1 at 360 K) at a low electric field of 17 kV cm−1 in the rod-like Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramic, elaborated by a surfactant-assisted solvothermal route, which are just slightly higher than the present results (ΔT = 0.38 K and ζ = 0.223 K mm kV−1 at 353 K). Nevertheless, the ECE of our sample at 40 kV cm−1 (ΔT = 0.986 K and ζ = 0.245 K mm kV−1 at 363 K) is larger than that obtained for the Ba0.65Sr0.35TiO3 ceramic by a spark plasma sintering process (ΔT = 0.328 K and ζ = 0.164 K mm kV−1 at 303 K) as stated by Liu et al.75 Furthermore, Patel et al.76 elaborated the Ba0.85Ca0.075Sr0.075Ti0.90Zr0.10O3 ceramic by the conventional solid-state method, revealing that the ECE response reached high values of 1.6 K and 0.405 K mm kV−1 for ΔT and ζ, respectively, under 39.5 kV cm−1 at room temperature. Smail et al.66 reported ECE properties (ΔT = 0.24 K and ζ = 0.20 K mm kV−1 at 338 K) at a low electric field of 12 kV cm−1 in Ba0.975La0.017(Zr0.05Ti0.90)Sn0.05O3 ceramics synthesized by solid-state reaction. These differences can be attributed to the synthesis conditions (such as calcination and sintering), chemical doping, grain shape, the number of the coexisting phases as well as the applied external electric field.
Ceramic | ΔT (K) | ΔE (kV cm−1) | T (K) | ζ (K mm kV−1) | Ref. |
---|---|---|---|---|---|
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 1.479 | 60 | 367 | 0.246 | This study |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 0.986 | 40 | 363 | 0.246 | This study |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 0.459 | 20 | 355 | 0.229 | This study |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 0.380 | 17 | 353 | 0.223 | This study |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 0.328 | 15 | 353 | 0.218 | This study |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 0.155 | 8 | 347 | 0.194 | This study |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 0.492 | 17 | 360 | 0.289 | 27 |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 0.4 | 21.5 | 370 | 0.186 | 3 |
Ba0.85Ca0.15Zr0.10Ti0.90O3 | 0.152 | 8 | 373 | 0.19 | 12 |
Ba0.90Ca0.10Zr0.05Ti0.95O3 | 0.465 | 25 | 392 | 0.186 | 28 |
Ba0.91Ca0.09Zr0.14Ti0.86O3 | 0.3 | 20 | 328 | 0.150 | 79 |
Ba0.92Ca0.08Zr0.05Ti0.95O3 | 0.38 | 15 | 410 | 0.253 | 80 |
0.6BZT–0.4BCT | 0.58 | 28 | 398 | 0.21 | 32 |
BZT–30BCT | 0.30 | 20 | 333 | 0.15 | 79 |
Ba0.975La0.017(Zr0.05Ti0.90)Sn0.05O3 | 0.24 | 12 | 338 | 0.20 | 66 |
BaTi0.89Sn0.11O3 | 0.71 | 25 | 325 | 0.284 | 67 |
Ba0.65Sr0.35TiO3 | 0.83 | 40 | 303 | 0.21 | 75 |
Ba0.85Sr0.15Ti0.9Zr0.1O3 | 2.4 | 37 | 303 | 0.65 | 81 |
Ba0.85Ca0.075Sr0.075Ti0.90Zr0.10O3 | 1.60 | 39.5 | 303 | 0.405 | 76 |
The applied electric field-dependence of ΔT and the EC responsivity ζ at the peak temperature are presented in Fig. 9a. ΔT increased with the increasing electric field. In contrast, ζ increased rapidly at low electric fields, then became nearly saturated above 40 kV cm−1, and finally started decreasing above 55 kV cm−1. The EC responsivity ζ, therefore, exhibited a maximum as predicted by theory and confirmed by experimental results in relaxor ferroelectrics.77,78
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Fig. 9 (a) Electric field-dependence of ΔT and ζ at the temperature of peak response; (b) thermal evolution of the COP and ζ in the BCZT ceramic at 55 kV cm−1. |
For practical applications, the temperature range (Tspan) in which a large ECE can be maintained is also critical. Here, Tspan was introduced as the full width at half maximum (FWHM) of the ECE peak at 60 kV cm−1.82,83 The Tspan value of 87 K was found in the BCZT ceramic, indicating that the BCZT ceramic can maintain a high EC response over a relatively broad temperature range as compared to other materials.82,84–86 The suitability of the electrocaloric material for application in new cooling technologies is typically evaluated by a parameter similar to refrigerant capacity, RC = ΔS × δTFWHM = ΔS × Tspan.33,83,87,88 This parameter was estimated to be 140.33 J kg−1 at 60 kV cm−1 in the BCZT ceramic. The obtained value is higher as compared to many other lead-free ceramics.74,84 Another important criterion in evaluating the efficiency of an electrocaloric material is the coefficient of performance (COP) as defined by eqn (8),89,90 where Q is the isothermal heat.
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It was established before that ζ exhibited a maximum at 55 kV cm−1 (Fig. 9a); hence, the COP value was determined at 55 kV cm−1. The COP of the BCZT ceramic reached a maximum of 6.29 at 365 K and 55 kV cm−1 as shown in Fig. 9b. The obtained value is higher than those reported in several works in lead-free materials89,91–94 and some lead-based materials.90,95–98 For instance, Hao et al.97 found a COP value of 2.9 in Pb0.97La0.02Zr0.87Sn0.08Ti0.07O3 thick film.97 The obtained high values of ΔT, ΔS, ζ, RC and COP, and the wide operating temperature range make BCZT ceramics, elaborated by the low-temperature hydrothermal method, a promising candidate for eco-friendly electrocaloric cooling technologies.
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