1300www.MaterialsViews.com wileyonlinelibrary.com . IntroductionElectrocaloric effect (ECE), an electric fi eld induced temperature and/or entropy change of an insulating material, has attracted a great deal of attention after the recent fi ndings of large ECE in ceramic thin fi lms, polymers, and a dielectric fl uid. [1][2][3][4][5][6][7][8][9][10] Several recent works have also demonstrated potential of realizing high performance cooling devices exploiting large ECE in new electrocaloric (EC) materials. [11][12][13][14] In these newly developed EC materials, the giant ECE, i.e., large adiabatic temperature change Δ T and isothermal entropy change Δ S, were obtained under high applied electrical fi elds. For example, a | Δ T| = 35 K and Δ S = 160 Jkg −1 K −1 were induced in a high energy electron irradiated P(VDFTrFE) relaxor polymer under an electric fi eld change Δ E of 180 MVm −1 . [ 15 ] Analogously, in ferroelectric BaTiO 3 (BT) thick fi lms, a | Δ T| = 7.1 K and Δ S = 10.1 Jkg −1 K −1 were generated under the application of high electric fi elds change Δ E = 80 MVm −1 . [ 16 ] As one starts to address these EC materials for practical cooling devices applications, additional parameters should be considered besides a large Δ T and Δ S to measure the performance of EC materials. For instance, in order for cooling devices to be operated at low voltage such as <200 volts which is the normal voltage range for most cooling devices, an EC material with a large ECE induced under small electric fi elds is also important. In addition, a wide operational temperature range near room temperature is highly desired in order to develop high performance and practical EC cooling devices. [11][12][13][14] Therefore, one critical question is how to design and develop dielectric materials which are capable of generating giant ECE over a broad operation temperature with relatively low applied electric fi eld change Δ E.As a lead-free ferroelectric material which is environmentally friendly and is in fact the most widely used ferroelectric material, ECE in BT has been studied quite extensively in the past several years by many groups and in various forms including thin fi lms, bulk ceramics (including thick fi lms multilayer ceramic capacitors (MLCC)), and single crystals. [16][17][18][19][20][21][22] Large Δ T and Δ S have been reported in BT ceramics at temperatures near the ferroelectric-paraelectric (FE-PE) transition. [ 16 ] It is noted that for BT, besides the high temperature FE-PE transition (the transition temperature T FE-PE > 100 ° C) between the tetragonal ferroelectric (Tet) and cubic paraelectric phase, also exhibits an orthorhombic (O) and a rhombohedral (Rhom) phases as illustrated in Figure 1 (a). [ 23 ] As ECE is directly related to the entropy change of an insulation dielectric under the change of the applied electric fi eld, a dielectric with a polar phase which contains a large number of polar-states with similar energy levels can be induced under a reasonable electric fi eld from a nonpolar phase (i.e., belo...

We report the giant electrocaloric effect (ECE) of BaZr0.2Ti0.8O3 (BZT) thick film near room temperature. The BZT thick film was fabricated by the tape casting method with the thickness of 12.0 μm. Due to the near invariant critical point composition, relaxor behavior, and the stress generated between the film and the substrate, the thick film exhibits a large adiabatic temperature drop ΔT = −7 °C under 19.5 MV/m electric field, large EC coefficient ΔT/ΔE = 0.50 x 10−6 K • m • V−1, ΔS/ΔE = 0.88 x 10−6 J • m • kg−1 • K−1 • V−1 over a wide temperature range near room temperature, where ΔS is the isothermal entropy change and ΔE is the applied field. These high EC properties and possibility of fabrication of the EC ceramics into multilayer ceramic capacitor configuration provide solution for the application of the EC material for practical cooling device applications.

Regeneration processes are commonly used in cooling devices to improve the device performance. However, irreversible heat loss within the regenerators in many earlier designs of magnetocaloric and electrocaloric (EC) based cooling devices reduces the device performance. In this paper, an electrocaloric based refrigerator without external regenerators is proposed and studied. The regeneration process in this device is realized by direct heat exchange between contacting EC elements which are moving in opposite directions with different applied fields. Simulation results show that a 37 W/cm3 cooling power density is obtained for a Tspan of 20 K while the refrigerator still maintains 57% of Carnot efficiency for a cooling device made of an EC polymer.

The relaxor ferroelectric materials, because of their large and reversible electric fi eld induced polarization, have been demonstrated to possess giant electrocaloric effect (ECE) over a broad temperature range, which are attractive for refrigeration with high energy effi ciency and environmental friendliness. However, high electric fi elds are required to generate the giant ECE in these materials, posing challenge for these materials in practical cooling devices which also require high reliability and low cost. Here, a general approach is reported, for example, establishing an internal bias fi eld in these relaxor ferroelectric polymers, to signifi cantly improve ECE which can be induced at low electric fi elds. It is demonstrated that in a polymer blend (nanocomposite) with a properly controlled normal ferroelectric in nanophase dispersion in the relaxor polymer matrix, the charge neutrality in the blends can cause an internal biasing fi eld, leading to more than 45% enhancement in the ECE at low electric fi eld (≈50 MV m −1 ). This internal biasing approach provides a universal strategy to enhance other low fi eld responses such as the electromechanical response in relaxor ferroelectrics.