Electrical properties of SnO₂ ceramics for low voltage varistors

It is shown that an addition of bismuth oxide Bi₂O₃ (0.5 mol%) to the system SnO₂–CoO–Nb₂O₅–Cr₂O₃ with small amounts of Nb₂O₅ and Cr₂O₃ (0.05 mol%) leads to a significant decrease in the electric field where a strong nonlinearity of the current–voltage characteristic takes place. This effect facilitates manufacture of ceramic materials for low voltage varistors with a relatively high nonlinearity coefficient of 15–20 and an electric field of 430−580Vcm⁻¹ (at a sintering temperature of 1300 °C) and, respectively, 11 and 287Vcm⁻¹ (at 1400 °C). The obtained experimental data indicate that ceramic materials in the system SnO₂–CoO–Nb₂O₅–Cr₂O₃–Bi₂O₃ exhibit the typical structure of varistor ceramics with conductive grains of tin dioxide (SnO₂) and highly resistive grain boundaries. In particular, the capacitance at 10 Hz and high dc bias becomes negative as in other SnO₂ and ZnO varistors