Design and electrical properties' investigation of (K0.5 Na0.5) NbO3 - BiMeO3 lead-free piezoelectric ceramics

Based on the discussion on the "origin" of the high piezoelectric properties of Pb-based piezoelectric ceramics, it was predicted that (K0.5 Na0.5) NbO3 - BiMeO3 solid solutions (where Me3+ =Sc, Al, Ga, Y, In, etc.) should possess high piezoelectric properties because of the formation of the morphotropic phase boundary and the hybridization between the Bi 6p and O 2p orbits. (1-x) (K0.5 Na0.5) NbO3 -x BiScO3 [(1-x) KNN-xBS] ceramics were selected as an example to verify this prediction. (1-x) KNN-xBS ceramics were synthesized by conventional solid-state sintering. The phase structure, microstructure, and dielectric and piezoelectric properties of (1-x) KNN-xBS ceramics were investigated. At room temperature, the polymorphic phase transition (PPT) (from the orthorhombic to the tetragonal phase) in (1-x) KNN-xBS ceramics is identified at x=0.0175 by the analysis of x-ray diffraction patterns and dielectric spectroscopy. The ceramics (x=0.0175) with PPT near room temperature exhibit excellent electrical properties (d33 =∼253 pC/N, kp =0.48, kt =0.49, and TC =351 °C). The related mechanisms for the high piezoelectric properties in (1-x) KNN-xBS (x=0.0175) ceramics are also systematically discussed. Compared to the other KNN-based ceramics and hard Pb (Zr,Ti) O3 ceramics, it is concluded that the (1-x) KNN-xBS (x=0.0175) ceramic is a promising lead-free piezoelectric candidate material. In addition, this article could provide an alternative approach for the development of lead-free piezoelectric ceramics.