Enhanced electrocaloric effect over a broad temperature range in lead-free Na0.5Bi0.5TiO3-based relaxor ferroelectrics via doping with Bi(Mg0.5Zr0.5)O3

With an increasing demand for environmentally friendly refrigeration, the solid-state refrigeration based on the electrocaloric effect (ECE) has been drawn extensive attention. It is a challenge to maintain a large adiabatic temperature change (∆T) over a broad temperature span. Herein, the authors designed and synthesised (0.74-x) Na_0.5Bi_0.5TiO₃-0.06BaTiO₃-0.2SrTiO₃-xBi(Mg_0.5Zr_0.5)O₃ (abbreviated as NBT-xBMZ) (x = 0, 0.02, 0.04, 0.06 and 0.08) lead-free relaxor ferroelectrics. Their microstructures, dielectric properties, ferroelectric properties, ECEs and the structure-property relationships were investigated. Via doping with BMZ, an enhanced relaxor feature and a wider temperature range where multi-phases coexist were achieved. The relaxor ferroelectric characteristics were illustrated using the Vogel-Fulcher relation. The indirectly calculated ECE results showed that the optimal ΔT of 1.11 K was obtained for the x = 0.02 sample at 90°C and 70 kV/cm over a wide T_span of 120°C, providing a potential ECE material. The direct ECE results procured using thermocouple indicated that the maximal ∆T of 2.14 K and ∆T/∆E of 0.31 K m/MV were achieved in the same sample at 70°C and 7 MV/m and the variation trend of ECE results was consistent with the indirect results. Moreover, the multi-phases coexistent strategy can be extended to other materials system to generate a large ΔT over a wide temperature range.