Liquid phases in ZnO varistors cause more complex phase development and microstructure, which makes the control of electrical properties and reliability more difficult. Therefore, we have investigated 2 mol% CaCO3 doped ZnO-Co3O4-Cr2O3-La2O3 (ZCCLCa) bulk ceramics as one of the compositions without liquid phase sintering additive. The results were as follows: when CaCO3 is added to ZCCLCa (644 Ωcm) acting as a simple ohmic resistor, CaO does not form a secondary phase with ZnO but is mostly distributed in the grain boundary and has excellent varistor characteristics (high nonlinear coefficient α＝78, low leakage current of 0.06 μA/㎠, and high insulation resistance of 1×1011 Ωcm). The main defects Zni·· (AS: 0.16 eV, IS & MS: 0.20 eV) and V˙o (AS: 0.29 eV, IS & MS: 0.37 eV) were found, and the grain boundaries had 1.1 eV with electrically single grain boundary. The resistance of each defect and grain boundary decreases exponentially with increasing the measurement temperature. However, the capacitance (0.2 nF) of the grain boundary was ~1/10 lower than that of the two defects (~3.8 nF, ~2.2 nF) and showed a tendency to decrease as the measurement temperature increased. Therefore, ZCCLCa varistors have high sintering temperature of 1,200℃ due to lack of liquid phase additives, but excellent varistor characteristics are exhibited, which means ZCCLCa is a good candidate for realizing chip type or disc type commercial varistor products with excellent performance.
