Effects of additives and secondary phases on the sintering behavior of zinc oxide-based varistors

The influence of additives and secondary phases on the sintering behavior of zinc oxide-based varistors has been investigated in this study. Addition of Bi2O3 and Sb2O3 to ZnO results in rapid shrinkage of the specimens at around 1000°C. This rapid shrinkage is attributed to the reaction between the pyrochlore phase and ZnO. This reaction has been verified by investigating the system of ZnO and a synthesized pyrochlore phase ZnBi1.5Sb1.5O7. This pyrochlore phase has a cubic symmetry, with a lattice parameter ao = 10.4438 Å. ZnBi1.5Sb1.5O7 is found to react with ZnO at 1000°C to lead to the formation of Bi2O3, which accelerates the sintering rate of ZnO through a liquid-phase sintering mechanism. On the other hand, the spinel phase α-Zn7Sb2O12 hinders the sintering process as well as the grain growth of ZnO.