Element doping is an effective method to improve the performance of ZnO varistors. Previous studies mainly focused on the variation of microstructures and Schottky barriers. In this study, the effects of Co dopant on electrical properties are investigated from the aspect of multiscale defect structures, including intrinsic point defects, the heterogeneous interface of depletion/intergranular layers, and interface states at grain boundaries. Combining with analysis of phase composition and energy dispersive spectroscopy, it is found that Co tends to dissolve into ZnO grains when slightly doped. It substitutes Zn 2+ with the same valence and affects little on densities of donors. Segregation of Co at grain boundaries would result in the formation of spinel phase Co(Co 4/3 Sb 2/3)O 4 and transformation of the intergranular phase from α-Bi 2 O 3 to δ-Bi 2 O 3. Meanwhile, densities of point defects are indirectly affected by oxygen ambient during sintering, resulting in abnormal variation of grain resistivity. And interface states are enhanced, leading to improved barriers at grain boundaries. Therefore, reduced leakage current, enhanced grain resistivity, and improved non-linear coefficient in Co-doped ZnO varistor blocks are understood from the underlying multiple defect structures. This presents a potential approach to explore short-term performance and long-term stability of ZnO varistors from the aspect of defect responses.

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In numerical solutions of Hanai equation widely used in heterogeneous systems, the uncertainty of conductivity at high frequency affects the accuracy of phase parameters results (permittivity and conductivity of components). To overcome this limitation, broadband dielectric spectroscopy of eight groups of silicone rubber (SR) was measured at a frequency from 10−1 Hz to 105 Hz and at a temperature from 0 °C to −40 °C. An obvious relaxation peak related to the interface between aluminium hydroxide (ATH) and SR was observed and confirmed. Based on the interfacial polarization, phase parameters of ATH and SR were calculated by modified numerical solutions, which was verified by other information. It was found that with the increase of ATH concentration, polarization strength increased and relaxation time decreased, which indicated that the number of charges accumulated at the interface increased in interfacial polarization process and the Debye length of interface decreased. Because of the surface conductivity around ATH, the conductivity of ATH is higher than that of SR. This paper is beneficial for the studies on the interface properties of ATH filled silicone rubber and provides supports for studying the electrical parameters of particles dispersed in continuous medium systems without physical transition or chemical changes with enhanced accuracy.