||In this thesis, the powder formation and thermal treament parameters for low temperature firing of zinc oxide(ZnO) based multi-layered varistors are studied. Vanadium oxide(V2O5) was used as electrical former, and manganese oxide(MnOx), coblt oxide(CoOx) and other additives were used as electrical modifiers. The object of this thesis is to investigate the effects of additives on the electrical characteristics of ZnO varistors. Meanwhile, the parameters of thermal treament process of raw materials on the effects of low temperature cofiring of ZnO varistors are also studied.|
As a result, we found that V2O5 can lower the sintering temperature of ZnO-based varistor as low as 900℃. Forthuremore, prolonged sintering duration and reduced ZnO particle size might lower the sintering temperature futher to a given temperature which makes it possible to co-fire the ceramic body, inner electrodes and outer electrodes. The formulation of powder with added 0.3mol%Mn3O4, 0.5mol%CoO and 0.1mol%Bi2O3 can obtain a high αvalue of 24 and leakage current of 2μA. Besides, we also found that the thermal treament of powder can affect the shrinkage or diffusion of inner electrodes in multi-layered ZnO varistor shape dramastically. Without thermal treament or inproper thermal treament conditions will cause the inner electrode over-shrinkaged or even disppeared. According to the results of experiment, 350~400℃and a longger period (4 hours and above) will improve the phenomenon of over -shrinkaged.