Microstructural observations in (Na_0.5K_0.5)NbO ₃ ceramics with CuO and ZnO additives

The characterizations of CuO and ZnO pockets which had formed in (Na _0.5K_0.5)NbO₃ (NKN) matrix sintered at 920 °C with CuO of 1.5 mol% and ZnO of 1.5 and 3.0 mol% as the additives were investigated from a microstructural point of view using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Two types of pockets, composed of CuO and ZnO as a dominant component, were observed in the microstructure as new microstructure constituents. The abnormal grain growth has occurred by the liquid phase sintering. The pockets were melted partially or completely by the interactions with element Na in the matrix which has formed a eutectic compound whose melting point is lower than the sintering temperature. The reaction starts at the interfaces between the pocket and matrix and the kinetics depends not only on the size of the pocket but also on the environments where the pockets are located. When the additive content of ZnO was increased to 3.0 mol %, there are interactions between CuO and ZnO and both elements are found in the compound pocket. The sintering kinetics was much enhanced by the presence of both additives.