Excellent Electrocaloric and Energy Storage Capabilities in (Na_0.8K_0.2)_0.5Bi_0.5TiO₃ Ceramic

Abstract

A temperature-dependent polarization switching study reveals a normal ferroelectric P – E loop to an antiferroelectric double hysteresis loop in the vicinity of depolarization temperature (400 K, T d ), which is believed to show a strong electrocaloric effect (ECE) and electrical energy-storage density ( W rec ) in (Na 0.8 K 0.2 ) 0.5 Bi 0.5 TiO 3 (NKBT) ceramic when measured by an indirect method. Structural analysis using X-ray diffraction, Raman spectroscopy, and transmission electron microscopy studies confirmed the coexistence of tetragonal ( P 4 mm ) and rhombohedral ( R 3 c ) phases. A temperature-dependent dielectric study has revealed a frequency dispersion around 400 K, which hints at the presence of a frustrated interaction resulting in the slowing down of polar domain dynamics below a diffuse phase transition. The high value of ECE in the lead-free NKBT ceramic is also investigated by a direct method, confirming the multifunctional nature of NKBT and its usefulness for applications in refrigeration and energy storage. A direct method of electrocaloric (EC) measurement in NKBT ceramic exhibits an adiabatic temperature change (Δ T ) ∼ 1.10 K and an EC strength (ξ) ∼ 0.55 Kmm/kV near T d at an externally applied field of 20 kV/cm, which is consistent with the literature. The recoverable energy ( W rec ) ∼ 0.78 J/cm 3 and electrical storage efficiency (η) ∼ 86% are also observed near T d when an applied field of 20 kV/cm is switched at 1 Hz. This behavior is ascribed to the delicate balance between the field-induced normal ferroelectric–antiferroelectric transition and the thermal energy needed to disrupt the frustrated interaction.