Large room-temperature electrocaloric effect in lead-free BaHf_xTi_1-xO₃ ceramics under low electric field

Ferroelectrics are promising candidate materials for electrocaloric refrigeration. Materials with a large electrocaloric effect (ECE) near room temperature and a broad working temperature range are getting closer to practical applications. However, the enhanced ECE is always achieved under high electric field, which limits their wide cooling applications. In this paper, the phase diagram of lead-free BaHf_xTi_1-xO₃ (BHT) ferroelectric ceramics was established. A large ECE under relatively low electric field (ΔE = 10 kV/cm) is firstly reported in BHT ferroelectric ceramics. The direct temperature change (ΔT = 0.35 °C under 10 kV/cm) in BHT ceramics is comparable with those reported in the literature under high electric fields. Meanwhile, the electrocaloric efficiency (ΔT/ΔE = 0.35 K mm kV^-1 under 10 kV/cm) is thirteen percent higher than the best value reported previously under high electric field (ΔE = 145 kV/cm). We demonstrate that the ECE can be greatly enhanced by tuning the composition of the lead-free BHT ceramics to its first-order phase transition (FPT), invariant critical point (ICP) or diffuse phase transition (DPT). It is shown that the enhancement in ECE is strongly dependent on the nature of structural phase transition and electric field coupling effect, which has been confirmed by both the indirect and direct ECE measurements. A phenomenological explanation based on Landau model was also proposed to understand this phenomenon. Our findings in this work may provide a better understanding and design methodology for developing more practically useful electrocaloric materials.