Near room temperature giant negative and positive electrocaloric effects coexisting in lead–free BaZr0.2Ti0.8O3 relaxor ferroelectric films

BaZr_0.2Ti_0.8O₃ (BZT) relaxor ferroelectric (FE) films with tetragonal structure were deposited on Pt/TiO₂/SiO₂/Si(100) substrates via a sol-gel technique. A giant electrocaloric effect (ECE) was observed in the compact films with ΔT = 43.6 K and ΔS = 61.9 J/K kg at near room temperature of 366.5 K, which was attributed to the high breakdown electric field of E = 1010 kV/cm. Meanwhile, large negative ECE (ΔT = ? 5.2 K and ΔS = ? 8.8 J/K kg at room temperature of 305 K, ΔT = ? 5.1 K and ΔS = ? 7.2 J/K kg at near room temperature of 375 K) were achieved. The abnormal negative ECE is originated from structural transformation in relaxor ferroelectrics. The coexisting of giant negative and positive electrocaloric effects makes it more effective to realize the refrigeration during the application or removal of an electric field. The maximum electrocaloric coefficient (ζ_max = 0.043 K cm/kV) and refrigeration efficiency (COP = 35.18) of the films were obtained at near room temperature of 366.5 K. The giant electroelectric effect makes BZT films promising as lead-free materials for application in environment-friendly refrigeration equipment at near room temperature.