Influence of the preparation process on the electrical properties of high-field co-doped zinc oxide varistors

High-field ZnO/Bi2O3 varistors co-doped with Mn and Co were synthesized using a two-step co-precipitating process. A Zn2+ solution containing the Mn and Co doping elements was first precipitated into hydroxides, further converted into oxalates and finally calcined to create the doped ZnO phase. Bi3+ precipitated at the grain boundaries thanks to an HNO3 treatment of the ZnO grains. The influence of the precursor type (nitrates or chlorides) and the calcination temperature on the properties of the powders and ceramics were investigated. ICP-AES, microstructural analysis and non-linear voltage measurements were used to characterize the samples. The type of precursor solution was found to have a strong influence on the electrical properties. Furthermore, the calcination temperature modified the microstructure of the powder and consequently also that of the varistor.