Large Electrocaloric Effect in (Bi0.5Na0.5)TiO3-Based Relaxor Ferroelectrics.

The electrocaloric effect (ECE) is significantly critical for environmentally friendly cooling technologies to replace vapor compression-based refrigeration. Lead-free bismuth sodium titanate-based ferroelectrics have attracted considerable attention because of their depolarization process, which can induce a high ECE. However, their high depolarization temperature (Td) disobeys the principle of room temperature (RT) cooling. Considerable efforts have been devoted to the shift of Td to near RT via chemical doping, while the corresponding ferroelectric properties of the materials are sacrificed. Herein, we design a relaxor ferroelectric {[Bi0.5(Na0.72K0.18Li0.1)0.5]1-xSrx}TiO3 (BNKLSxT) ceramic system which shows a decreased Td toward RT, and a high ECE (adiabatic temperature change of 2.51 K and electrocaloric coefficient of 0.386 K mm kV-1) is achieved when the measurement temperature approaches the Td. This high ECE performance outperforms most reported lead-free ceramics and even behaves better than 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 ceramics. The superior ECE behavior is ascribed to the enhanced maximum polarization (Pmax) resulting from the addition of Sr and the existence of the depolarization process. Piezoresponse force microscopy results verify that the increased Pmax originates from the decreased domain size and the increased density of the domain in BNKLSxT. This work not only offers us an attractive lead-free ferroelectric material for ECE coolers but also opens a new direction toward the rational design of ceramics for ECE.