Direct and indirect measurement of electrocaloric effect in lead-free (100-x)Ba(Hf0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics near multi-phase boundary

In this work, the (100-x)Ba(Hf0.2Ti0.8)O3-x(Ba0.7Ca0.3)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 ΔTEC = 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 ΔTEC = 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 ΔTEC,max/ΔE and a broad ECE peak at a higher electric field, indicating that it is possible for electrocaloric temperature ΔTEC 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.