ELECTRICAL CHARACTERISTICS AND REHEAT-TREATMENT EFFECTS IN A ZNO VARISTOR FABRICATED BY 2-STAGE HEAT-TREATMENT

Conventional ZnO varistors are generally fabricated by sintering ZnO powder mixed with additives such as Bi2O3, Sb2O3, Cr2O3, Co2O3, and MnO2. To reduce abnormal grain growth and change in electrical characteristics in the conventional ZnO varistors caused by volatilization of Bi2O3, the ZnO powder with all additive oxides except Bi2O3 was pressed into disc form and sintered. The disc was then painted with metal oxide paste containing Bi2O3 and again fired. The ZnO varistor fabricated by this process, i.e. a two-stage heat-treatment process, showed typical non-linear I-V characteristics with higher breakdown voltage exceeding 800 V mm-1. It was also observed that the non-linear I-V coefficient change rate, DELTA-alpha, in the ZnO varistor due to reheat-treatment is almost linearly proportional to the sintered density.