SOLID-STATE SINTERING OF (K0.5Na0.5)NbO3 SYNTHESIZED FROM AN ALKALI-CARBONATE-BASED LOW-TEMPERATURE CALCINED POWDER

Potassium sodium niobate K0.5Na0.5NbO3 (KNN) was synthesized by the double calcination of a homogenized mixture of potassium and sodium carbonates and niobium pentoxide for 4 h at 625 degrees C. The calcination temperature was chosen on the basis of the thermal analyses of the mixture of precursors, where the weight loss being the function of the temperature reaches the plateau. The calcined powder was investigated by X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) and was found to be without unreacted materials or secondary phases. Before sintering, the powder compacts were annealed for 4 h at 450 degrees C, while the sintering was carried out for 2 h at 1115 degrees C using two different configurations: 1) in a closed crucible where the KNN pellets were in close physical proximity to, but not in direct contact with, the KNN packing powder, and 2) in a completely open crucible without any packing powder. The Archimedes' density of the sintered samples was 91.5 % of theoretical density for the first configuration, while it was 93.4 % for the second configuration. The Field-Emission Scanning Electron Microscopy (FE-SEM) and XRD analyses of the sintered ceramics showed that by using a calcination temperature as low as 625 degrees C a typical sintered microstructure of KNN could be achieved with both sintering configurations.