Synthesis and characterization of lead-free piezoelectric (K0.50Na0.50)NbO3 powder produced at lower calcination temperatures: A comparative study with a calcination temperature of 850 °C

To prevent the evaporation of alkali elements during the calcination of the lead-free piezoelectric K0.50Na0.50NbO3 (KNN), lower temperatures for the solid-state synthesis of the KNN from a mixture of Na2CO3, K2CO3 and Nb2O5 were investigated with a variety of powder-characterization techniques. The collected data suggested 550 °C as a possible low calcination temperature, as well as being sufficiently high for a completed synthesis of the KNN. The XRD and TEM analyses of double-calcined KNN powder at 550 °C revealed nanocrystalline, cube-shaped particles with a perovskite structure, but without noticeable unreacted precursors or secondary phases. Our investigations revealed that the morphology of the calcined powders underwent a dramatic change from small, differently shaped particles at 600 °C/6 h, to fully rounded, large, spherical agglomerates produced at 1000 °C/6 h. To explain this behavior, a mechanism involving the formation of a liquid phase during the reaction of the alkali elements with the moisture and CO2 in the atmosphere is proposed.