Microstructure, Phase Transition, and Electrical Properties of KxNa1−xNbO3 Lead-Free Piezoceramics
The microstructure, phase transition, and electrical properties of lead-free KxNa1−xNbO3 (x = 0.46 to 0.51, abbreviated as KNN) piezoceramics prepared by a conventional solid-state reaction method were investigated with an emphasis on the influence of the K/Na ratio. Scanning electron microscopy (SEM) images showed that the grain growth was slightly improved by increasing x. However, all ceramic samples possessed high densifications with fine grains from several hundred nanometers to several micrometers. X-ray diffraction (XRD) results showed that a discontinuous change of lattice parameters appeared between x = 0.49 and 0.50, which suggested a typical morphotropic phase boundary (MPB) separating two different orthorhombic phases OI and OII. The sample with composition x = 0.49 had the peak values of the piezoelectric constant d33 of 146 pC/N and the planar electromechanical coupling coefficient kp of 43%. These results indicated that improving the piezoelectric properties in KNN ceramics could be achieved by optimizing the K/Na ratio.