Grain resistivity in zinc oxide and tin dioxide varistor ceramics

In this paper the grain resistivity was measured for commercial ZnO-based and laboratory-made SnO2-based varistor ceramics exhibiting highly nonlinear current-voltage characteristics at low current (1 mA cm−2). The high-current study was performed in the current range 2-500 A cm−2 using recently developed technique based on a concept of the differential resistance and application of a single exponential voltage pulse. At 295 K commercial ZnO varistors exhibit some lower values of the grain resistivity 0.59-0.61 Ω сm in comparison with higher values 2.4-28 Ω сm for SnO2 varistors. Respectively, the high-current nonlinearity coefficients for ZnO varistors are higher than for SnO2 varistors. The capacitive currents did not affect the obtained results. The activation energy of electrical conduction at low fields (in the range of Ohm's law) for SnO2 varistors (0.82-0.98 eV) is larger than for ZnO varistors (0.69-0.76 eV). The observed correlation between the high-current and the low-current properties of ZnO-based and SnO2-based varistors is discussed.