Microstructure and current–voltage characteristics of ZnO–V2O5–MnO2 varistor system

The effects of sintering temperature and time on the microstructure and the electrical properties have been studied in a ZnO–0.5 mol%V2O5–2 mol%MnO2 system. The microstructure of the samples consists mainly of ZnO grains with Zn3(VO4)2 as the minority secondary phase. This minor secondary phase disappears for samples sintered at 1200 °C and above. An increase in sintering temperature also brings about a deleterious effect on the non-linear coefficient and the breakdown electric field. An optimum sintering time of 4 h at 900 °C gives the best electrical properties, with the non-linear coefficient α attaining a highest value of 31.8. Grain growth process is also investigated in terms of the phenomenological kinetic grain growth expression: Gn=Kot exp(−QRT). The kinetic grain growth exponent n is observed to be 4, while the apparent activation energy Q is 193±19 kJ/mol.