The effects of adding Sb, Zr, and Y to a Bi–Mn–Co–Si–Cr–Ni-added ZnO varistor (with the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Both the varistor voltage and the nonlinearity index of the voltage–current characteristics a before electrical degradation for samples added 1.0 mol% ZrO2 and 1.0 mol% Y2O3 were maximized at approximately 1.0 mol% Sb2O3 and decreased rapidly when the amount of Sb2O3 exceeded more than about 1.0 mol%. Samples added 1.0 mol% ZrO2, and 1.0 mol% Y2O3, and less than 0.75 mol% Sb2O3 had a comparable leakage current to samples added neither ZrO2 nor Y2O3. Sb is an important element that is necessary to obtain a high varistor voltage, a low leakage current, and an excellent resistance to electrical degradation. However, the same varistor voltage can be obtained by adding Y and reducing the amount of Sb2O3 added.

The effects of the addition of tin oxide (SnO2) and yttrium oxide (Y2O3) to bismuth (Bi)-manganese (Mn)-cobalt (Co)-silicon (Si)-chromium (Cr)-nickel (Ni)-added zinc oxide (ZnO) varistors (a basic varistor) on the varistor voltage, resistance to electrical degradation, and leakage current were investigated. The addition of SnO2increased both the varistor voltage and the resistance to electrical degradation. However, simultaneous addition of both SnO2and Y2O3increased the varistor voltage but the resistance to electrical degradation deteriorated. ZnO varistors with varistor voltage over approximately 520 V/mm, excellent resistance to electrical degradation, and low leakage current could be obtained by adding SnO2with SnO2-to-ZnO molar ratio of approximately 1:10 to the basic varistor.

The effects of adding Sb to a BiMnCoSiCrNiYZr-added ZnO varistor (with the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Bi is incorporated in spinel particles, and δ-Bi2O3eventually disappears with the addition of small amounts of Bi, especially as the amount of Sb2O3added increased. Reduction in both the nonlinearity index and the amount of δ-Bi2O3for small amounts of added Bi with the addition of more than approximately 1.25 mol% Sb2O3demonstrates that Sb inhibits Bi2O3from forming deep interfacial impurity levels at the grain boundaries. The sample containing 1.2 mol% Bi2O3, 1.0 mol% ZrO2, 1.0 mol% Y2O3, and 1.5 mol% Sb2O3added exhibits a high varistor voltage (approximately 630 V/mm), high resistance to electrical degradation and low leakage current.