Lead-free bilayer thick films with giant electrocaloric effect near room temperature

Electrocaloric refrigeration utilizing ferroelectrics has recently gained tremendous attention because of the urgent demand for solid-state cooling devices. However, the low room-temperature electrocaloric effect and narrow operation temperature window hinder the implementation of lead-free ferroelectrics in high-efficiency cooling applications. In this work, chemical engineering and thick-film architecture design strategies were integrated into a BaTiO₃-based system to resolve this challenge. Novel environmental-friendly Ba(Zr_0.20Ti_0.80)O₃-Ba(Sn_0.11Ti_0.89)O3 (BZT-BST) bilayer films of ~13 μm in single-layer thickness were prepared by the tape casting process. A giant adiabatic temperature change, ΔT ~ 5.2 K, and a large isothermal entropy change, ΔS ~ 6.9 J kg^-1 K^-1, were simultaneously achieved at room temperature based on the direct measurements, which are much higher than those reported previously in many lead-free ferroelectrics. Moreover, the BZT-BST thick films exhibited a remarkably widened operation temperature range from about 10 to 60 °C. These outstanding properties were mainly attributed to the multiphase coexistence near room temperature, relaxor ferroelectric characteristics, and improved electric-field endurance of the bilayer thick films. This work provides a guideline for the development of environment-friendly electronic materials with both ultrahigh and stable electrocaloric performance and will broaden the application areas of lead-free ferroelectrics.