Development of the electrical properties of KNN-based lead-free piezoceramics based on the phase boundary building and ternary composition designing

Abstract

Two kinds of ternary KNN-based lead-free piezoceramics: (0.985-x)K0.5Na0.5NbO3-0.015Bi0.5Na0.5TiO3-xBiScO3 and (0.985-y)K0.5Na0.5NbO3-0.015Bi0.5Na0.5TiO3-yBiFeO3 have been prepared by a conventional solid-state sintering. For these KNN-based ceramics, the increasing doping of BiScO3 or BiFeO3 changed the phase structure from orthorhombic symmetry to tetragonal symmetry and then to pseudocubic symmetry, while a typical bimodal grain size distribution was observed in their rectangular grains at x=0– 0.015 or y=0.06–0.012. With increasing x or y, TO-T gradually decreases to room temperature, and an orthorhombic-tetragonal (O-T) morphotropic phase boundary was formed at 0.009≤x≤0.015 or 0.009≤y≤0.012. Around the phase boundary, both a good piezoelectric activity and a high Curie temperature were obtained, such as d33–320pC/N, kp–0.45,TC=327°C at x=0.015, and d33–304pC/N, kp–0.42,TC=350°C at y=0.012. In addition, the test for the piezoelectric constant following the annealing temperature indicate that these two compositions also have a good thermal stability.