Morphology and formation mechanism of the pyrochlore phase in ZnO varistor materials

The secondary phases in ZnO varistor materials, having Bi₂O₃, Sb₂O₃, MnCO₃ and CoO as additives, were investigated using transmission electron microscopy with an energy dispersive X-ray spectrometer and X-ray diffractometer scans. The information about the morphology of the pyrochlore phase and its formation reaction were obtained. The pyrochlore phase is formed from the reaction between the spinel and Bi₂O₃ phase. Below 1200° C, the spinel phase reacts with the liquid Bi₂O₃ phase present at the multiple grain junction and forms the pyrochlore phase. The pyrochlore phase nucleates at the corners of multiple grain junction adjacent to the spinel grain and grows towards the centre of the grain junction.