Polymorphic phase transition dependence of piezoelectric properties in (K_0.5Na_0.5)NbO₃-(Bi_0.5K _0.5)TiO₃ lead-free ceramics

Lead-free ceramics (1 - x)(K_0.5Na_0.5)NbO ₃-x(Bi_0.5K_0.5)TiO₃ [(1 - x)KNN-xBKT] were synthesized by conventional solid-state sintering. The phase structure, microstructure and electrical properties of (1 - x)KNN-xBKT ceramics were investigated. At room temperature, the polymorphic phase transition (from the orthorhombic to the tetragonal phase) (PPT) was identified at x = 0.02 by the analysis of x-ray diffraction patterns and dielectric spectroscopy. Enhanced electrical properties (d₃₃ = 251 pC N^-1, k_p = 0.49, k_t = 0.50, , tan δ = 0.03 and T_C = 376 °C) were obtained in the ceramics with x = 0.02 owing to the formation of the PPT at 70 °C and the selection of an optimum poling temperature. The related mechanisms for high piezoelectric properties in (1 - x)KNN-xBKT (x = 0.02) ceramics were discussed. In addition, the results confirmed that the selection of the optimum poling temperature was an effective way to further improve the piezoelectric properties of KNN-based ceramics. The enhanced properties were comparable to those of hard Pb(Zr, Ti)O₃ ceramics and indicated that the (1 - x)KNN-xBKT (x = 0.02) ceramic was a promising lead-free piezoelectric candidate material for actuator and transducer applications.