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 ( T d ) disobeys the principle of room temperature (RT) cooling. Considerable efforts have been devoted to the shift of T d to near RT via chemical doping, while the corresponding ferroelectric properties of the materials are sacrificed. Herein, we design a relaxor ferroelectric {[Bi 0.5 (Na 0.72 K 0.18 Li 0.1 ) 0.5 ] 1– x Sr x }TiO 3 (BNKLS x T) ceramic system which shows a decreased T d 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 T d . This high ECE performance outperforms most reported lead-free ceramics and even behaves better than 0.9Pb(Mg 1/3 Nb 2/3 )O 3 -0.1PbTiO 3 ceramics. The superior ECE behavior is ascribed to the enhanced maximum polarization ( P max ) resulting from the addition of Sr and the existence of the depolarization process. Piezoresponse force microscopy results verify that the increased P max originates from the decreased domain size and the increased density of the domain in BNKLS x T. 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.