Investigation of electrical properties of tantalum doped SnO2 varistor system

Ta2O5 doped SnO2 varistor systems containing 0.5 mol% ZnO and 0.5 mol% CoO were prepared by mixed oxide method. Considering that ZnO and CoO oxides are densification additives only the SnO2·ZnO·CoO ceramics cannot exhibit electrical nonlinearity. A small amount of Ta2O5 improves the nonlinear properties of the samples greatly. The height and width of the defect barriers were calculated. It was found that samples doped with 0.05 mol% Ta2O5 exhibit the highest density (98.5%), the lowest electric breakdown field (Eb = 1100 V/cm) and the highest coefficient of nonlinearity (α = 11.5). The effect of Ta2O5 dopant could be explained by the substitution of Ta5+ by Sn4+. A grain-boundary defect barrier model for the SnO2·ZnO·CoO·Ta2O5 varistor system was also introduced.