Electrical properties of ZnO varistors prepared by microwave and conventional sintering process

The Bi2O3-based ZnO varistor materials were prepared by 2.45 GHz microwave, 24 GHz millimeter-wave or conventional-sintering processes. Both millimete r-wave and microwave sintering processes are superior to the conventional s intering process in densifying the Bi2O3-based ZnO materials. Large nonline ar coefficient and low leakage current density were attained by sintering t he samples under the 1200 degrees C sintering temperature and 5 min soaking time, A high density as 96.7% T.D. (theoretical density) has been achieved . However, the varistor characteristics, including breakdown voltage (V-bk) , nonlinear coefficient (alpha) and leakage current density (J(L)), degrade d markedly for the samples sintered at too long soaking time (10 min), whic h was ascribed to the occurrence of abnormal grain growth, in accompany wit h the loss of Bi2O3 and ZnO species.