Microstructure of sodium-potassium niobate ceramics sintered under high alkaline vapor pressure atmosphere

One of the most challenging steps in processing sodium potassium niobate (KNN) ceramics is sintering. At temperatures close to the solidus line, the high volatility of the alkaline becomes an issue of major concern for the sintering process. While alkaline evaporation is frequently related to difficulties in densification, few work on the effects of alkaline vapor pressure on microstructure have been reported. KNN materials with alkaline/niobium ratios ranging from 1.02 to 0.98 were sintered at 1105 °C. Two different sintering setups were used. An alkaline rich sintering atmosphere was provided when sintering the materials embedded in (K0.5Na0.5)1.02NbO3 powder, while reference ceramics were sintered in loosely covered crucibles. Resulting from the alkaline content in the sintering atmosphere a shift toward microstructures considered typical for batch compositions with higher alkaline content was detected. Densities decrease for KNN with alkaline excess and stoichiometric KNN, whereas they tend to increase for niobium excess material.