Long-term Leakage Current Characteristics and Conduction Mechanism for ZnO Varistors

Long-term leakage current characteristics of ZnO varistors are evaluated current flowing behavior by applying a DC voltage of 70 to 80% of the operation starting voltage (V1mA) or an AC voltage of 85 to 90% of V1mA/√2 for 1000 hours in a thermostatic oven at 115°C. However, due to thermal runaway in as-sintered specimens, measurements are conducted after heat treatment at 500-600°C. The Schottky energy barrier model fails to consider the factors that cause thermal runaway or the effect of heat treatment on leakage current suppression. Conversely, the proposed composite barrier model can elucidate the aforementioned phenomena by considering the depletion layer in the “holes” (contact areas between ZnO grains) in the intergranular insulating layer as the dominant factor in the leakage current properties. As a result, it reveals that the composite barrier model is the most consistent with the electrical properties as a conduction mechanism model for ZnO varistors that comprehensively covers non-Ohmic behavior and long-term leakage current characteristics.