(Yb,Co,Nb)-doped SnO2 varistor ceramics

The effect on microstructure and electrical properties of (Co,Nb)-doped SnO2 varistors upon the addition of Yb2O3 was investigated by scanning electron microscopy and by determining J-E (current density vs. electric field), ɛ-f (relative permittivity vs. frequency) and Z′′-Z′ (reactance vs. resistance) relations. The threshold electric field of the SnO2-based varistors increased significantly from 412 V/mm to 1430 V/mm, and the relative dielectric constants of the SnO2-based varistors decreased greatly from 1571 to 230 as Yb2O3 concentration was increased up to 0.98 mol%. The significant decrease of the SnO2 grain size, from 13.68 μm to 6.88 μm with increasing Yb2O3 concentration over the range of 0-0.98 mol%, is the origin for the increase in the threshold voltage and decrease of the dielectric constants. The grain size reduction is attributed to the segregation of Yb2O3 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 0.05 mol% Yb2O3-doped sample, threshold electric field EB and nonlinear coefficient α were measured to be 593 V/mm and 42.4, and for 0.98 mol% Yb2O3-doped sample, EB and α were 1430 V/mm and 37, respectively. Superhigh threshold voltage and large nonlinear coefficient α qualify the Yb-doped SnO2 varistor as an excellent candidate for high voltage protection system.