Dielectric properties of Mn-doped ZnO-Bi₂O₃ varistors below liquid nitrogen temperature

ZnO-based varistors have been widely applied in surge protection devices because of their excellent nonlinear current-voltage (I-V) characteristic and energy absorption ability. Manganese is always added into ZnO-based varistors in order to enhance the nonlinear characteristic of materials. In the present work, the dielectric properties of Mn-doped ZnO-Bi₂O₃ varistors at extremely low temperatures (20K∼100K) were investigated. According to the XRD results, all the samples exhibit the same overall phase structures, comprising regularly ZnO grain, Bi-rich phase. The SEM photos show that the average grain size of ZnO grain increases at first and then decreases. For the dielectric properties of samples in the temperature range of 20K-100K, the complex permittivity show different variation trends with the increase of frequency. The doping of Mn element (1 mol%) will lead to the appearance of relaxation processes. Two relaxation processes with the activation energies of 0.03eV and 0.007eV are observed in the sample. However, only one relaxation process with the activation energy of 0.03eV is detected from the sample with 1.5 mol% Mn. Further increase of Mn amount (2 mol%) can eliminate all the relaxation process. The results demonstrate that dielectric spectroscopy measurement below liquid nitrogen temperature can provide abundant information about the shallow defects in ZnO-based varistors. At the temperature region between 20K and 100K, new relaxation process can be detected which offering new understanding to the defect structures of materials. Moreover, the doping amount of Mn element will influence the dielectric properties and defect structures of ZnO-based varistors.