Effect of In2O3 doping and sintering on the electrical properties and the microstructure of (Co,Ta)-doped SnO2 varistors

The effects of In2O3 doping and sintering temperature on the microstructure and electrical properties of (Co,Ta)-doped SnO2 varistors were investigated and the maximal non-linear co-efficient of α = 51 of the sample doped with 0.08 mol% In2O3 sintered at 1350 °C was obtained. It was found that the breakdown electrical field and resistivity of grain boundaries increased significantly with increasing In2O3 concentration or decreasing sintering temperature. The sample doped with 0.12 mol% In2O3 sintered at 1350 °C has the highest breakdown electrical field of 2592 V/mm. The relative dielectric constant decreased with increasing In2O3 concentration or decreasing sintering temperature. The decrease or increase of breakdown electrical field, increase or decrease of relative dielectric constant with altering In2O3 concentration and sintering temperature is mainly attributed to the increase or decrease of the grain size. The mechanism of the variation of SnO2 grain size was discussed.