Effects of cobalt doping on nonlinearity of zinc oxide

The microstructures and electrical properties of ZnO–Bi2O3–CoO (ZBC), ZnO–Bi2O3 (ZB) and ZnO–CoO (ZC) ceramics were investigated. Cobalt oxide addition could reduce bismuth loss at high sintering temperature. The same amount of cobalt within ZnO grains was found in both specimens, ZBC and ZC, suggesting that bismuth had no effect on the dissolution of cobalt in ZnO and cobalt substitution for zinc in the ZnO structure. The highest nonlinear coefficient of about 19 was found in the ZBC varistor sintered at 1000–1100 °C. For the ZC specimens, nonlinear properties could also be obtained in this sintering temperature range.