Colossal negative electrocaloric effects in lead-free bismuth ferrite-based bulk ferroelectric perovskite for solid-state refrigeration†
The electrocaloric effect (ECE) provides a novel route to realize eco-friendly cooling, whose cooling efficiency could be further enhanced by integrating the positive and negative ECE into one device. In contrast to the positive ECE that has been extensively achieved in ferroelectrics, few works have reported the negative ECE in lead-free polycrystalline solutions. In this work, a colossal negative ECE in lead-free bismuth ferrite-based systems is demonstrated for the first time. A record ECE in lead-free ferroelectric ceramic bulks, a ΔT of −2.0 °C and a ΔS of 1.9 J kg−1 K−1 measured directly by a heat flux sensor, is achieved, as well as large electrocaloric strength (ΔT/ΔE = −0.325 °C mm kV−1). The presence of short-range antiferrodistortive regions exhibiting non-polar states with average cubic symmetry may rationalize the large negative ECE in the system. The findings not only introduce a brand-new ferroelectric material into the negative ECE family but also may contribute to the design of phase structures of ferroelectrics for a giant negative ECE.