In this study, we have investigated the effects of Co doping on I-V curves, bulk trap levels and grain boundary characteristics of ZnO-Bi2O3 (ZB) varistor. From I-V characteristics the nonlinear coefficient (a) and the grain boundary resistivity (ρgb) decreased as 32→22 and 18.4→0.6×10(9) Ωcm with sintering temperature (900∼1,300℃), respectively. Admittance spectra and dielectric functions show two bulk traps of zinc interstitial, Zn(i)·· (0.16∼0.18 eV) and oxygen vacancy, Vo· (0.28∼0.33 eV). The barrier of grain boundaries in ZBCo (ZnO-Bi2O3-Co3O4) could be electrochemically single type. However, its thermal stability was slightly disturbed by ambient oxygen because the apparent activation energy of grain boundaries was changed from 0.93 eV at the 460∼580 K to 1.13 eV at the 620∼700 K. It is revealed that Co dopant in ZB reduced the heterogeneity of the barrier in grain boundaries and stabilized the barrier against the ambient temperature.

In this study, we have investigated the effects of Mn dopant on the bulk trap levels and grain boundary characteristics of Bi2O3-based ZnO (ZB) varistor using admittance spectroscopy and dielectric functions (such as Z*, Y*, M*, ε*, and tanδ). Admittance spectra and dielectric functions show two bulk traps of Zn(i) (0.20 eV) and Vo (0.29~0.33 eV) in ZnO-Bi2O3-Mn3O4 (ZBM). The barrier of grain boundaries in ZBM could be electrochemically single type. However, its thermal stability was slightly disturbed by ambient oxygen because the apparent activation energy of grain boundaries was changed from 0.79 eV at lower temperature to 1.08 eV at higher temperature. The grain boundary capacitance Cgb was decreased slightly with temperature as 1.3~1.8 nF but resistance Rgb decreased exponentially. The relaxation time distribution can result from the heterogeneity of the barriers constituting the varistor. It is revealed that Mn dopant in ZB reduced the heterogeneity of the barrier in grain boundaries and stabilized the barrier against the ambient temperature.