Effects of vanadium and manganese concentrations on the composition, structure and electrical properties of ZnO-rich MnO2-V2O5-ZnO varistors

The composition, structure and electrical properties of ZnO-rich MnO2-V2O5-ZnO varistors have been analysed. Samples were prepared by a conventional powder route with 0.25-0.75 V2O5 mol% and 0.1-1.5 MnO2 mol% concentrations. All the microstructures consisted of ZnO grains with zinc vanadates as minority secondary phases. The quantity and type of zinc vanadates found depended on the cooling rate. alpha-Zn-3(VO4)(2) and Zn4V2O9 were found in air-cooled samples, whereas gamma-Zn-3(VO4)(2) and Zn4V2O9 were found in samples cooled relatively slowly at 5 degreesC min(-1). All samples exhibited non-linear current-voltage varistor behavior, with non-linear coefficients, a, ranging from 9.7 to 27.3. The electrical behaviour was relatively insensitive to the different rates of cooling. The highest values of a were obtained in slowly cooled samples when the MnO2 concentration was 0.25 mol% and when the V2O5 concentration was 0.5 mol%. (C) 2003 Elsevier Ltd. All rights reserved.