Electrical and dielectric properties of ZrO2 doped ZnO varistor

Summary

ZnO varistor ceramic has been widely applied in electronic circuit and power grid for transient overvoltage protection because of its excellent nonlinear voltage-current characteristic and energy absorption capability. Thermal buffer materials can reduce the thermal-induced stress and consequently prevent the fracture failure. In the paper, ZrO2 is doped into ZnO-Bi2O3 based varistor ceramic as the thermal buffer material, and the effects of ZrO2 dopant on the microstructure, electrical property, and dielectric property of ZnO varistor ceramic are investigated. The polynary ZnO ceramic samples with 0.0, 0.4, 0.8, 1.2 mol% ZrO2 were prepared by the semi-solution method, and sintered at 1200oC for 2h. Current-Voltage characteristic test show that with the increasing ZrO2 content, the breakdown field E1mA show a growing tendency, while the nonlinear coefficient a increase firstly, and then drop sharply after 0.4 mol%. EDS and XRD results reveal that ZrO2 is a second stable phase existing in grain boundaries, and is non-reactive to host ZnO. SEM results indicate that owing to the pinning effect, the average grain size decreases with the ZrO2 content, resulting in the enhancement of the breakdown field E1mA. Dielectric spectrum test reveal that ZrO2 can reversely influence the concentrations of intrinsic point defects. Two loss peaks originated from two intrinsic defects (oxygen vacancy and zinc interstitial) are observed in the frequency dependence curve of dielectric loss e" at low temperatures. With peak separation of dielectric loss e" plot, the conduction process and two relaxation processes are analysed, and it is found that as the ZrO2 doping content is increased, the defect concentration of intrinsic oxygen vacancy is decreased, while that of intrinsic zinc interstitial is enhanced. Intrinsic zinc interstitial with inferior stability is responsible for the deterioration of Schottky barrier in depletion layer, thus ZrO2 dopant is against the long-term stability of ZnO varistor ceramic. Above all, the appropriate doping content of ZrO2 is suggested to be 0.4 mol% in the present formula.

Additional informations

Authors

J. LIN, W. Liu, S. LI

Keywords

ZnO varistor ceramic, ZrO2 dopant, Dielectric property