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.