Effect of Co3O4 content and compaction pressure on the microstructure and electric properties of SnO2–Sb2O5–Cr2O3 varistor ceramics

The effect of Co3O4 content (1 and 5 mol%) and powder compaction pressure (55, 110, and 166 MPa) on the microstructure and nonlinear electrical behavior of SnO2-based ceramics prepared by high-energy milling was investigated. The results show a correlation between the compaction pressure and Co3O4 content with the electrical properties. At constant oxide concentration and with increase in the compaction pressure, there is a slight decrease in the breakdown electric field and an increase in the nonlinearity coefficient. Conversely, at the highest Co3O4 content the breakdown electric field varies slightly, whilst the nonlinearity coefficient is maintained to be constant. A decrease in grain size with Co3O4 increase is ascribed to the formation of the cobalt/tin oxide spinel (Co2SnO4) at the grain boundaries. The best electrical characteristics were achieved with 1 mol% Co3O4 and 166 MPa, that is, nonlinear coefficient of 13.24 and breakdown electric field of 830 V/cm. Based on thermal analyses, possible reactions for Co2SnO4 formation are outlined.