Effects of alumina surrounding in sintering process on ZnO-Bi2O3varistors doped with CoO

Nonlinear current-voltage properties of zinc oxide-bismuth oxide varistors doped with cobalt oxide were investigated. The composition of ZnO doped with 0.5 mol% Bi2O3 and 0.75 mol%CoO was prepared and sintered in two different surroundings, zinc oxide-bismuth oxide-cobalt oxide powder and alumina powder. Microstructures and element tracing were studied using scanning electron microscopy with X-ray microanalysis, electron probe microanalysis and X-ray diffraction. Results clearly showed that the alumina surrounding led to bismuth loss from the green body and degraded the nonlinear property of varistors. At sintering temperatures higher than 1150 °C, the alumina-surrounding sintered specimens became ohmic and the electrical resistivity of 200 Ω cm was found in the specimen sintered at 1200 °C.