Coexistence of giant positive and large negative electrocaloric effects in lead-free ferroelectric thin film for continuous solid-state refrigeration

The solid-state cooling technique utilizing electrocaloric (EC) materials is an alternative approach to tackle the greenhouse effect caused by traditional vapor-compression refrigeration. However, such a promising technique is severely hampered by the lack of proper materials considering that most existing EC materials with a single positive/negative EC effect exhibit a limited cooling effect. Here, the coexistence of a positive and negative EC effects has been achieved in a lead-free Na 0.5 Bi 0.5 (Ti 0.97 W 0.01 Fe 0.02 )O 3 ferroelectric film . A state-of-the-art positive adiabatic temperature change (Δ T ) of ~56 K accompanied by an isothermal entropy change (Δ S ) of ~64 J K −1 kg −1 at 143 °C and a large negative Δ T of ~ − 17 K with a Δ S of ~ − 24 J K −1 kg −1 at 55 °C are obtained under a strong electric field strength of 2692 kV cm −1 . Oxygen vacancy-related defect dipoles play a critical role in the negative EC effect at lower temperatures, while the phase transition is responsible for the positive EC effect at higher temperatures. Meanwhile, the film exhibits a high EC strength with a maximum Δ T /Δ E of 0.021 K cm kV −1 , together with a Δ S /Δ E of 0.024 J cm K −1 kg −1 kV −1 . This work ensures a giant total temperature change by utilizing and combining both the negative and positive EC effects in a dual cooling process.