Microstructures, phase evolution and electrical properties of (1-x)K0.40Na0.60Nb0.96Sb0.04O3-xBi0.5K0.5HfO3 lead-free ceramics

An investigation was carried out on the surface morphologies, crystal structures and electrical properties of lead-free piezoelectric (1-x)K0.40Na0.60Nb0.96Sb0.04O3-xBi(0.5)K(0.5)HfO(3) ceramics, prepared by conventional solid-state reaction route. Based on X-ray diffraction results and temperature-dependent dielectric properties, we found that the ceramics experienced a structural evolution from orthorhombic phase to multi-phase coexistence and then to rhombohedral phase, with the increase of Bi0.5K0.5HfO3 content. Furthermore, a rhombohedral-tetragonal phase boundary was identified at x = 0.035, which was considered to be caused by the merging of rhombohedral-orthorhombic and orthorhombic-tetragonal phase transition temperatures. The ceramics exhibited an enhanced piezoelectric activity near the rhombohedral-tetragonal phase boundary, with a piezoelectric constant d(33) of 230 pC/N and a planar electromechanical coupling coefficient k(p) of 0.340. In addition, a clear composition dependence was observed for the microstructure of the ceramics, which could be explained by the combined effect of vacancies and phase segregations.