In this research, full density, single phase microstructure (in the XRD and SEM detection limits), and fine-grained SnO2 varistors with an average grain size of 0.65 μm was acquired at low temperature by the two-step sintering technique. The sintering temperature was successfully decreased from 1300 °C in the normal sintering to 1150 °C and 1100 °C of respectively the first and the second stage in the two-step sintering. The mechanism of grain growth suppression of the two-step sintered samples was discussed and it was suggested that in addition to triple point junction, solute drag and/or nano-secondary phases contribute to cease the grain growth. These SnO2 varistors exhibited ultra-high breakdown electric field of 26 kV/cm with a nonlinear coefficient of about 120 in the 1–10 mA/cm2 standard. The exact evaluation showed that this high value of the nonlinear coefficient is artificial and was the result of joule-self heating phenomenon. The true value of the nonlinear coefficient (28) is measured within the 0.1–1 mA/cm2 standard. Due to the joule self-heating phenomenon, the thermal runaway was observed for ultra-high voltage SnO2 varistors in DC-accelerated aging test conditions (0.85 E1mA/cm2 and room temperature). By changing DC-accelerated aging electric field to 0.85 E0.1 mA/cm2, TSS3-20h presented excellent stability after 10 h from room temperature to 75 ᵒC.
