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 moi%V2O5-2 mol%MnO2 system. The microstructure of the samples consists mainly of ZnO grains with Zn-3 (VO4)(2) as the minority secondary phase. This minor secondary phase disappears for samples sintered at 1200 degreesC 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 degreesC gives the best electrical properties, with the non-linear coefficient a attaining a highest value of 31.8. Grain growth process is also investigated in terms of the phenomenological kinetic grain growth expression: G(n) = K(o)t exp(-Q/RT). The kinetic grain growth exponent n is observed to be 4, while the apparent activation energy Q is 193 +/- 19 kJ/mol. (C) 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved.