Giant Negative and Positive Electrocaloric Effects Coexisting in Lead-Free Na0.5Bi4.5Ti4O15 Films Over a Broad Temperature Range

Giant negative and positive electrocaloric effects (ECEs) coexist in lead-free Na0.5Bi4.5Ti4O15 (NBTO) ferroelectric films with layered perovskite structure over a broad temperature range, with T peaks of -14.8 degrees C (at 273K) in the negative electrocaloric (EC) region (273-328K), of 3.45 degrees C (at 373K) in the positive EC region (328-418K) and of -6.76 degrees C (at 483K) in another negative EC region (418-513K), respectively, under the electric field of 900kVcm(-1). Such giant ECEs are attributed to both the high break-down electric field and prominent ferroelectricity. And the coexistence originates in the competition of the dipole polarization between both Na0.5Bi0.5TiO3 (NBT) and Bi4Ti3O12 (BTO) cells of NBTO films over the whole temperature region. Furthermore, the outstanding anti-fatigue features with temperature stability for NBTO films ensure the durability of EC refrigeration. This work will prompt a novel technology for next generation of environmental-friendly solid-state cooling devices.