Zinc oxide semiconductor ceramic with nonlinear electric properties obtained by the coprecipitation method [for varistors]

The paper presents a wet synthesis method of doped zinc oxide by coprecipitation in order to obtain a semiconductor ceramic with nonlinear electric properties. The manufacturing of varistor with high electric performances involves the using of high purity materials synthesised by successive purification processes at 99.995% purity level. The solutions purification that contains Zn²⁺ ions depends on type of starting zinc salts. The experiments were made with nitrate and sulphate solutions. After doping of these solutions for 0.5% Bi₂O₃-1% Sb₂O₃-0.5% MnO₂-0.5% Cr₂O₃-0.5% CoO composition, the simultaneous precipitation of all metallic ions was made. The hydroxides precipitation being different at same pH, the corrections for dopant quantities, on base of chemical analysis, were necessary. The resulted precipitates were analysed by DTA. The hydroxides precipitated from zinc sulphate solution show weight losses with 10% grater than hydroxides precipitated from zinc nitrate solution. The thermal decomposition of hydroxides from nitrate is faster and at approximately 400°C was finished, comparatively with hydroxides from sulphates, at which this process is not over, up to 880°C. The decomposition of hydroxides mixture precipitated from nitrates has an exothermic effect at 250°C comparatively with hydroxides mixture from sulphate, which has endothermic effects only. These differences of thermal decomposition functions of synthesised hydroxides have major effects on oxide powders, from which will be formed ceramic for varistors. In this way, the oxide powders are finer than that obtained by normal way and can be sintered at low temperature. The ceramic microstructure realised from these oxides is very fine, the microcrystals grain size having 1-2 μm, without porosity and with good chemical homogeneity