Synergistically optimizing electrocaloric effects and temperature span in KNN-based ceramics utilizing a relaxor multiphase boundary

Abstract

A combination of large adiabatic temperature change (ΔT) and wide temperature span (T_span) in lead-free ceramics is highly desired to develop eco-friendly and highly efficient solid-state refrigeration based on electrocaloric effects. However, the lack of success in synergistically optimizing both large ΔT and wide T_span in one material impedes the practical applications of electrocaloric refrigeration technology. Here, a relaxor multiphase boundary (RMPB) is structured in (1 − x)(K_0.49Na_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃–xCaZrO₃ (KNLNT–xCZ) ceramics through intentional composition engineering, successfully achieving a region of multiphase coexistence accompanied by diffuse phase transition. A moderate ΔT (0.63 K under 80 kV cm⁻¹) with a wide T_span (43 K within 90% ΔT attenuation) is obtained in a relaxor KNLNT–5CZ ceramic with the coexistence of an R–O–T phase. The results indicate that a coupling effect originally from multi-directional polarization and polar nano-regions (PNRs) in RMPB contributes to the excellent performance combining large ΔT and wide T_span. This work not only expands the applications of KNN-based ceramics from electric energy storage and piezoelectric applications to the refrigeration field, but also opens up a new design avenue for the development of high performance electrocaloric materials with broad temperature ranges.