Toward the development of piezoelectric actuators, single-perovskite-structure 0.78Bi_0.5Na_0.5TiO₃-(0.22-x)SrTiO₃-xKNbO₃ (BNT-ST-xKN, withx=0–0.03) lead-free piezoelectric ceramics were prepared by a conventional solid-state reaction method, and the effects of KN substitution on the microstructure and electrical properties were investigated. FE-SEM observations revealed that the grain size of the sintered specimens decreased upon KN replacement. The temperature dependence of the relative dielectric permittivity (ε_r–T) curves showed that the ferroelectric-to-relaxor transition temperature (T_F-R) decreased from 55 ^oC for x=0 to below room temperature for x≥0.01. The hysteresis loop became slim, with the strain initially increasing and then decreasing with an increase in the KN content. Notably, at x= 0.01, a large unipolar field-induced strain (S_max) of up to 0.157% with corresponding normalized strain d₃₃^* (S_max/E_max) of 785 pm/V was achieved at 2 kV/mm. The high strain under an ultra-low electric field was attributed to the local chemical inhomogeneity of the constituent elements, which was further confirmed by backscattered electron (BSE) and EDX mapping analyses.

Keywords: Actuator, Bismuth-based, Electromechanical strain, Lead-free, Piezoelectric.