Direct and indirect measurement of electrocaloric effect in lead-free (100-x)Ba(Hf_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ ceramics near multi-phase boundary

In this work, the (100-x)Ba(Hf_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BHT–xBCT) ferroelectric ceramics were fabricated by a conventional solid-state reaction method. The electrocaloric effect (ECE) of these ceramics was studied by both indirect and direct methods over a wide temperature range with the composition changing from x = 20 to 50. The most excellent direct ECE occurs in the BHT–50BCT with a good direct ΔT_EC = 0.27 K (at 92 °C) under the electric field of 10 kV/cm. What's more, the direct EC effect peaks not only locates at near the Curie temperature, but also at the phase-transition temperatures corresponding to the transitions between different ferroelectric phases. In particular, it is found that BHT–50BCT exhibits a direct ECE anomaly ΔT_EC = 0.1 K at 30 °C under an electric field of 10 kV/cm, which can be ascribed to the orthorhombic to tetragonal phase transition, proposing that one can achieve a broad temperature window by tuning the temperature gap between ferroelectric (FE) to paraelectric (PE) boundary and FE-FE boundary. In addition, BHT–30BCT shows good stability of indirect ΔT_EC,max/ΔE and a broad ECE peak at a higher electric field, indicating that it is possible for electrocaloric temperature ΔT_EC to be tuned linearly by electric field. Overall, our results imply that the BHT– xBCT ceramics are promising candidates for lead-free cooling refrigeration applications.