Densification and grain growth of microwave sintered ZnO varistors

The densification and grain growth kinetics of microwave and conventional sintered ZnO varistors were studied. It reveals that there are no differences in the components of microwave and conventional sintered samples. Microwave is beneficial to the densification and can lower down the sintering temperature. Both densification and anti-densification processes affect the densities of the final products and the evaporation of Bi component is the main cause. The grain growth exponent values (n) are 2.9-3.4 for the ZnO varistor sintered by microwave, and lower than the ones of conventional sintered samples. And the apparent activation energies (Q) of microwave and conventional sintered ZnO varistors are 225 and 363kJ/mol, respectively. Bi2O3 liquid phase, spinel phase and the "nonthermal effect" of microwave are the main factors to co-influent the grain growth of microwave sintered ZnO varistors.