Controlled-Atmosphere Sintering of KNbO3

The effect of sintering atmosphere (O2, air, N2, N2-5% H2, and H2) on the densification, grain growth, and structure of KNbO3 was studied. KNbO3 powder was prepared by solid state reaction, and samples were sintered at 1040 °C for 1–10 h. The sample microstructure was studied using Scanning Electron Microscopy (SEM). The sample structure was studied using X-Ray Diffraction (XRD). H2-sintered samples showed reduced density, whereas other sintering atmospheres did not affect density much. Samples sintered in N2-5% H2 showed abnormal grain growth, whereas sintering in other atmospheres caused stagnant (O2, air, N2) or pseudo-normal (H2) grain growth behavior. Samples sintered in reducing atmospheres showed decreased orthorhombic unit cell distortion. The grain growth behavior was explained by the mixed control theory. An increase in vacancy concentration caused by sintering in reducing atmospheres led to a decrease in the step free energy and the critical driving force for appreciable grain growth. This caused grain growth behavior to change from stagnant to abnormal and eventually pseudo-normal.