A new method for studying the conduction mechanism of ZnO-based varistor ceramics

The I-V characteristics of ZnO-based varistor ceramics were investigated in a wide temperature range of 93 K-373 K. From the relationship between overall conductance and temperature, individual conductance component having different temperature dependence can be separated and analyzed. In addition, the equivalent circuits can be developed as well. As to ZnO-Bi₂O₃ system, the overall conductance is equivalent to three parts in series, due to ionic impurity scattering, neutral impurity scattering and optical scattering; as to ZnO-Bi₂O₃-MnO system, it comprises four parts in parallel series; for commercial ZnO-based varistor ceramics, it involves a thermionic emission component in parallel with a tunneling component. In commercial ZnO-based varistor ceramics, the calculated nonlinear coefficient corresponding to tunneling effect is as high as 33, which is close to the measured value of 43. Furthermore, it is found that the tunneling effect is active even in small current region, especially at low temperatures.