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 (V_1mA) or an AC voltage of 85 to 90% of V_1mA/√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.

Cr₂N and CrN were synthesized using chromium oxides and boron nitride (BN) as starting materials. Cr₂N powder with particle sizes of 0.1–5 μm was obtained together with Na₃BO₃ (melting point (m. p.): 675°C) by heating a mixture of Cr₂O₃, BN, and Na in an Ar atmosphere at 500–1100°C. CrN powder with particle size of 1 μm and NaBO₂ (m. p.: 968°C) were obtained as the main phases by heating NaCrO₂ and BN in an Ar atmosphere at 900–1000°C, while Cr₂N crystal grains with sizes of 10–100 μm and NaBO₂ were obtained at 1100°C. The formation reactions of the chromium nitrides are metathesis reactions in which the O in chromium oxides and N in BN are exchanged, resulting in the formation of sodium borates (Na₃BO₃ and NaBO₂). The melts of these sodium borates were found to accelerate the formation reactions of chromium nitrides, and the NaBO₂ melt, in particular, contributes significantly to the grain growth of Cr₂N.