J A Aguilar-Martínez et al 2009 J. Phys. D: Appl. Phys. 42 205401
The effect of mechanical treatment in a planetary mill on the microstructure and electrical properties of tin dioxide based varistor ceramics in the system SnO2?Co3O4?Cr2O3?Sb2O5 sintered in the range 1150?1450?C was studied. The mechanical treatment leads to an increase in shrinkage, decrease in porosity, decrease in sample diameter, change in colour of the sintered samples from grey to black and enhancement of nonlinearity. For the sample sintered at 1350?C the mechanical treatment enhances the nonlinearity coefficient from 11 to 31 and decreases the electric field E1 (at 10?3?A?cm?2) from 3500 to 2800?V?cm?1. The observed changes in physical properties are explained in terms of an additional size reduction of oxide particles and a better mixing of oxide powder followed by the formation of potential barriers at the grain boundaries throughout the whole sample. In spite of the low porosity, the low-field electrical conductivity of mechanically treated ceramics is significantly increased with the growth of relative humidity. A higher humidity sensitivity is found for mechanically treated ceramics with higher barrier height and higher nonlinearity coefficient.