Influence of Alkaline and Niobium Excess on Sintering and Microstructure of Sodium-Potassium Niobate (K0.5 Na0.5)NbO3

One of the major factors limiting the application of lead-free sodium potassium niobate-based materials are difficulties in establishing stable processing procedures. The effects of alkaline and niobium excess on the preparation and properties of (K-0.5 Na-0.5)NbO3 (KNN) ceramics have been investigated. KNN ceramics with varying A/B site ratios were prepared from alkaline carbonates and niobium oxide. The properties of the powders after calcination and milling were monitored, sintering behavior was analyzed, and microstructures of the ceramics were investigated using scanning electron microscopy during several stages of the sintering process. Compared with the stoichiometric KNN, sintering shrinkage is shifted to lower temperatures by A-site excess and toward higher temperatures by B-site excess. When sintering close to the solidus, the stoichiometric KNN shows large 10-40-mu m-sized, nonpolyhedral-shaped grains with intragranular porosity. The grains of A-site excess KNN are similar in size, but growing in cuboid forms without the presence of intragranular pores. In B-site excess KNN, the grains remain almost one order of magnitude smaller. Attempts are made to specify the mechanisms that influence the marked differences in sintering behavior between the compositions and to relate them to the microstructural development. The results indicate the importance of the A/B stoichiometry for the microstructure of the ceramics and the development of appropriate processing procedures.