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

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 (Delta T-EC) was obtained around the phase transition temperature (T-FR) by indirect measurements, which was confirmed by direct measurements. The 0.85BNT-0.15ST sample showed a large negative Delta T-EC in the indirect measurements below T-FR where the polarization increases, while the directly measured Delta T-EC 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 T-FR. Published under license by AIP Publishing.