Large electrocaloric effect under electric field behavior in potassium sodium niobate ceramics with incompletely overlapped phase boundaries

Abstract

Solid state refrigeration technology based on the electrocaloric effect (ECE) has been a promising candidate for replacing vapor-compression refrigeration. Environmentally friendly lead-free ferroelectric materials draw much attention because of the potential of the ECE from their large ferroelectricity and plentiful phase structure. To realize high ECE around room temperature with a broad temperature span (T_span), a typical strategy involves shifting ferroelectric–paraelectric phase boundaries to room temperature along with inevitable relaxor behavior. However, ferroelectricity is always severely deteriorated while lowering the Curie temperature (T_C) to room temperature, limiting the temperature change (ΔT). Herein, a novel strategy based on phase engineering of the incompletely overlapped phase boundaries with rhombohedral–tetragonal (R–T) and tetragonal–cubic (T–C) phase boundaries is successfully constructed in potassium sodium niobate (KNN)-based ceramics to enhance the ECE for the first time. Combined with the contribution of electric field behavior, facilitated polarization rotation under electric fields originating from the lowered polarization energy barrier and the strengthening reversion after removing electric fields are revealed along with strong ferroelectricity. Subsequently, a strongly enhanced ECE is achieved, i.e. ΔT =1.14 K at 100 kV cm⁻¹ and 70 °C along with a broad T_span of 34 °C, demonstrating improved comprehensive ECE and conforming to the demands of cooling systems of electric devices. Meanwhile, an unusual trend is presented for the electric field dependence of the ECE coefficient (ΔT/ΔE) with the existence of the critical electric field (E_CE), which is relevant to the temperature. This work proposes an environmentally friendly ECE material with high performance for solid-state cooling applications.