**3.2. Electrical properties**

electric field (P–E) hysteresis loops of the samples at various temperatures were measured using a combination of a programmable signal generator and a charge amplifier (POEL 101). The samples were cut into 3–4 mm squares, and their temperatures were changed by immers‐ ing them in a heated or a cooled oil bath [21,22]. Strain–electric field (s–E) hysteresis loops of the samples at room temperature were measured using a combination of a programmable signal generator and a strain gauge. Triangular waves of 0.1 Hz with 30 kV/cm were applied to the samples in P–E and s–E measurements. The sample temperatures during the application of triangular waves of 0.1 Hz with 30kV/cm field were measured using a platinum thermom‐ eter. The sample temperatures changed periodically in accordance with the external field. The polarization reversals of the samples were monitored on the basis of signals from the charge amplifier (POEL 101). By synchronization of electric field to sample temperature, temperature–

electric field (T–E) hysteresis loops were obtained.

**Figure 3.** The operation mechanism of the ECE cooler.

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Figure 6 shows the P-E hysteresis loops at 10°C, 27°C, and 100°C, s–E hysteresis loops at room temperature, and the T–E hysteresis loops of the PLZT(7/65/35) and PLZT(9.1/65/35) ceramics

50m

Fig. <sup>5</sup> **Figure 5.** SEM micrographs of the surface of the BaTiO3 ceramics sintered at 1300°C, 1350°C, and 1400°C.

sintered at 1225°C. Those of the BaTiO3 ceramics sintered at 1300°C, 1350°C, and 1400°C are shown in Fig. 7. The electrical properties of these ceramics are summarized in Table 1. The change to "soft" ferroelectrics with La content increase yields the increase in dielectric constant, the decrease in remanent polarization (Pr) and coercive force (Ec), the slanted and slim P-E hysteresis loops, and the parabolic s-E loops in the PLZT(9.1/65/35) ceramics, compared with the PLZT(7/65/35) ceramics. In the case of BaTiO3 ceramics, ferroelectricity increases with the grain growth accompanying the higher sintering temperature. The increase in Pr and d33, the more distinct shrink around Ec in s-E loops with sintering temperature would be due to the increase of ferroelectricity. The low Ec in the BaTiO3 sintered at 1300°C is probably due to the slim P-E loop in weaker ferroelectricity and the low Ec in the BaTiO3 sintered at 1400°C is due to the high domain mobility in large grain ceramics. The higher dielectric constant in the BaTiO3 sintered at 1300°C compared with those in the BaTiO3 sintered at 1350°C and 1400°C is characteristic of BaTiO3 ceramics, and the similar results that BaTiO3 with grains with at around 1μm size have been reported thus far [23-25].


**Table 1.** Electrical properties of PLZT and BaTiO3 ceramics

#### **3.3. Indirect estimation**

The dP/dT between 10°C and 100°C for the PLZT and BaTiO3 ceramics were calculated using P-E hysteresis loops. Estimated ∆T for from Equation (1) for these ceramics are shown in Table 2. Among the ceramics, PLZT(9.1/65/35), which contains relaxor behavior by introducing Lanthanum substitution, estimated the largest temperature change. Among the BaTiO3 ceramics, the BaTiO3 sintered at 1400°C with large grains and accompanying strong ferroe‐ lectricity estimated the largest temperature change.


**Table 2.** Electrocaloric properties of PLZT and BaTiO3 ceramics

#### **3.4. Direct measurement**

sintered at 1225°C. Those of the BaTiO3 ceramics sintered at 1300°C, 1350°C, and 1400°C are shown in Fig. 7. The electrical properties of these ceramics are summarized in Table 1. The change to "soft" ferroelectrics with La content increase yields the increase in dielectric constant, the decrease in remanent polarization (Pr) and coercive force (Ec), the slanted and slim P-E hysteresis loops, and the parabolic s-E loops in the PLZT(9.1/65/35) ceramics, compared with the PLZT(7/65/35) ceramics. In the case of BaTiO3 ceramics, ferroelectricity increases with the grain growth accompanying the higher sintering temperature. The increase in Pr and d33, the more distinct shrink around Ec in s-E loops with sintering temperature would be due to the increase of ferroelectricity. The low Ec in the BaTiO3 sintered at 1300°C is probably due to the

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BT 1300°C

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BT 1350°C

BT 1400°C

Fig. <sup>5</sup> **Figure 5.** SEM micrographs of the surface of the BaTiO3 ceramics sintered at 1300°C, 1350°C, and 1400°C.

Figures 6 and 7 contain s-E loops and T-E loops of the PLZT and BaTiO3 ceramics. The similar shapes between s-E loops and T-E loops are observed in these samples. The similar results were reported by J. Wang et al. and our previous report. Field-induced displacement derives from the change in the polarization, and the appearance of similar loops is reasonable. The

Fig. 6

**Figure 6.** Polarization–electric field (P–E) loops (above), strain–electric field (s–E) loop (middle), and temperature–elec‐ tric field (T–E) loop (below) of the PLZT(7/65/35) and PLZT(9.1/65/35) ceramics sintered at 1225°C.

Fig. 7 **Figure 7.** Polarization–electric field (P–E) loops (above), strain–electric field (s–E) loop (middle), and temperature–elec‐ tric field (T–E) loop (below) of the BaTiO3 ceramics sintered at 1300°C, 1350°C, and 1400°C.

temperature change ∆T of the samples was calculated from the slope beginning with maximum field and ending at the zero field. The temperature change, ∆T, in PLZT(9.1/65/35) ceramics induced by bipolar switching field of 30 kV/cm was 0.26K, and that in the BaTiO3 sintered at 1400 °C by bipolar switching field of 30 kV/cm was 0.29K. The round T-E and s-E shapes around polarization switching observed in the loop from PLZT(9.1/65/35) attributes characteristic of relaxor ferroelectric materials. The decreasing transition temperature and increasing the polarization movements in relaxor ferroelectrics provide larger temperature change.

The direct measurement shows smaller values, compared with the estimation, generally. The reasons are unknown at present; heat dissipation may play a role in real systems. Although quantitative consistency is not obtained, it is safe to say that the materials with large dP/dT provided large temperature change generally.

(a) PLZT(7/65/35) ceramics (b) PLZT(9.1/65/35) ceramics

**Figure 6.** Polarization–electric field (P–E) loops (above), strain–electric field (s–E) loop (middle), and temperature–elec‐

tric field (T–E) loop (below) of the PLZT(7/65/35) and PLZT(9.1/65/35) ceramics sintered at 1225°C.

Fig. 6

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