Microstructure and nonlinear properties of microwave-sintered ZnO-V2O5 varistors:: I, effect of V2O5 doping

The modification of the densification behavior and the grain-growth characteristics of the microwave-sintered ZnO materials, caused by the incorporation of V2O5 additives, have been systematically studied. Generally, the addition of V2O5 markedly enhances the densification rate, such that a density as high as 97.9% of the theoretical density and a grain size as large as 10 mu m can be attained for a sintering temperature as low as 800 degrees C and a soaking time as short as 10 min. Increasing the sintering temperature or soaking time does not significantly change the sintered density of the ZnO-V2O5 materials but it does monotonously increase their grain size. Varying the proportion of V2O5 in the range of 0.2-1.0 mol% does not pronouncedly modify such behavior. The leakage current density (J(L)) of these high-density and uniform-granular-structure samples is still large, which is amended by the incorporation of 0.3 mol% of Mn3O4 in the ZnO materials, in addition to 0.5 mol% of the V2O5 additives. Samples that are obtained using such a method possess good nonohmic characteristics (alpha = 23.5) and a low leakage current density (J(L) = 2.4 x 10(-6) A/cm(2)).