Direct and indirect measurements of the electro-caloric effect in (Bi,Na)TiO3-SrTiO3 ceramics

Abstract

The electrocaloric effect (ECE) in lead-free (Bi0.5Na0.5)TiO3-SrTiO3 (BNT-ST) ceramics was investigated by direct and indirect measurements. Polarization-electric field hysteresis loops of the BNT-ST ceramics changed from well-saturated shapes at room temperature to constricted shapes around the depolarization temperature due to an electric field-induced phase transition. A large adiabatic temperature change due to ECE (ΔTEC) was obtained around the phase transition temperature (TFR) by indirect measurements, which was confirmed by direct measurements. The 0.85BNT-0.15ST sample showed a large negative ΔTEC in the indirect measurements below TFR where the polarization increases, while the directly measured ΔTEC displayed positive values. It suggests that the indirect method using the Maxwell relation is not valid in the temperature range where the polarization increases with temperature. The X-ray diffraction structural refinements showed that the poled 0.85BNT-0.15ST sample has mixed phases of a rhombohedral R3C and a tetragonal P4bm in the temperature range where the polarization increases below TFR.

Achieving Ultrahigh Electrocaloric Response in (Bi0.5Na0.5)TiO3-Based Ceramics through B-Site Defect Engineering

“…Direct measurement was employed with PT100 in this study as indirectly calculated results tend to deviate from reality, especially for ferroelectrics with hysteresis. 7 However, there is often a significant heat loss during temperature measurement using thermal resistors, and a calibration factor (K) is required to be calculated to obtain the intrinsic temperature change of the material. Figure S1 and Table S1 in the Supporting Information (SI) present the specific method of calibration.…”

On the barocaloric properties of non-magnetic materials: application to K 2 TaF 7 and AgI

“…However, the largest values of the magnetocaloric quantities in the most studied materials are associated with magnetic field variation larger than 2 T, which is not so easy to generate with small magnets. Due to this fact, alternative cooling processes such as the barocaloric effect [8][9][10][11][12][13][14][15][16][17][18][19] electrocaloric effect [20][21][22][23][24] and elastocaloric effect [25][26][27][28][29] have gained more attention. In this background, the barocaloric effect which is the heating/cooling process of material by varying an external hydrostatic pressure has been proved to be a very good alternative to yield large values of the caloric quantities, with pressure variation that is easy to get in the laboratory.…”