**1. Introduction**

The electrocaloric effect(ECE) is a phenomenon in which a material shows a reversible temperature change under an applied electric field [1, 2]. There has been some problem in the conventional refrigerator. Since the conventional refrigerator operates by using a compressor, vibration generation is inevitable. The conventional refrigerator uses Freon as refrigerants; however, Freon acts implicated in ozone depletion. The other disadvantage includes the difficulty in down-scaling. Thermoelectric cooling using the Peltier device has been considered as a solid state cooling device; however, low efficiency has been a hindrance to the wide applications. In addition, common thermoelectric materials used as semi-conductors include bismuth telluride, lead telluride, silicon germanium, and bismuth-antimony alloys. Some of them are toxic. Although new high-performance materials for thermoelectric cooling are being actively researched, the good results have not been obtained. From the viewpoint of the refrigerator innovation, new refrigerators based on the new mechanism are expected. ECE is

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considered to be one of the new cooling mechanisms [1, 3, 4]. By using ECE, the application to compact a high energy-effective, inexpensive, and safe refrigerator would be considered, as shown in Fig. 1. ECE was discovered in 1930 by Kobeko and Kurchakov [5]. The research activities on ECE have been not active until the year 2006. In that year, "giant" temperature change in Pb(Zr,TiO3 (PZT) thin films were activated at one sweep [6]. Figure 2 shows the relation between the numbers of the published papers and the published year. After 2006, the number of papers on ECE increased rapidly [7-17]. The operation principle of the refrigerator using ECE is shown in Fig. 3. By applying the electric field, the ferroelectrics are heated by ECE. This process corresponds to the compression process in the compressor type refrigerator. By removing the electric field, the directions of the polarization become random. This process is endothermic, corresponds to the expansion process in the compressor type refrigerator, and the object is cooled. The electrocaloric effect (ECE) is a phenomenon in which a material shows a reversible temperature change under an applied electric field. In order to create ECE cooling devices, materials with large ECEs are required. The electrocaloric temperature change ∆T due to applied ∆E is calculated from the following equation [6]:

$$
\Delta \mathbf{T} = -\frac{T}{\rho \mathbf{C}} \int\_{E\_1}^{E\_2} \left(\frac{\partial P}{\partial T}\right)\_E dE \tag{1}
$$

Here, C and *ρ* are the specific heat and density, respectively. Based on equation (1), a large (∂P/ ∂T)<sup>E</sup> (i.e., a large polarization change with temperature under high electric field) is desirable. With respect to achieving large (∂P/∂T)E, relaxor materials have recently attracted attention [1, 3, 4]. For direct measurement of the ∆T, there are some difficulties. Most temperature changes are less than 1K. And heat dissipation from ferroelectric materials through electrode, wire, and/or the supporting jig for field application occurs. Most probably due to these difficulties, the reports on the direct measurement of ∆T are limited thus far [13, 17, 18]. In this study, the electrocaloric temperature change, ∆T, due to applied ∆E, of the PLZT ceramics and BaTiO3 ceramics is estimated and directly measured. Concerning direct measurement of temperature– electric field (T–E) hysteresis loops, the reports have been limited. Detailed measurements of various measurements are required to clarify the insights of the ECE [4, 18, 19, 20].
