Effects of Sr on the microstructure and electrical properties of (Co, Ta)-doped SnO2 varistors

The effect on the microstructure and electrical properties of (Co, Ta)-doped SnO2 varistors upon the addition of SrCO3 was investigated. The threshold electric field of the SnO2-based varistors increased significantly from 318 to 3624 V/mm, the relative dielectric constants of the SnO2-based varistors decreased greatly from 1509 to 69 as SrCO3 concentration was increased up to 2.5 mol%. The significant decrease of the SnO2 grain size, from 8.19 to 1.03 μm with increasing SrCO3 concentration over the range of 0–2.5 mol%, is the origin for increase in the threshold voltage and decrease of the dielectric constants. The grain size reduction is attributed to the segregation of SrCO3 at grain boundaries hindering the SnO2 grains from conglomerating into large particles. Varistors were found to have superhigh threshold voltage and comparatively large nonlinear coefficient α. For 1.5 mol% SrCO3-doped sample, threshold electrical field E and nonlinear coefficient α were measured to be 1738 V/mm and 23.7, for 2.0 mol% SrCO3-doped sample, E and α were 2204 V/mm and 24, and for 2.5 mol% SrCO3-doped sample, E and α were 3624 V/mm and 22. Superhigh threshold voltage and large nonlinear coefficient qualify the Sr-doped SnO2 varistor as an excellent candidate in use for high voltage protection system. The reasons why the grain resistivity of the SnO2-based varistors increased with increasing SrCO3 concentration and the boundary resistivity of the SnO2-based varistors gradually decreased with increasing SrCO3 concentration were explained.