(Pr, Co, Nb)-Doped SnO2 Varistor Ceramics

The effect on microstructure and electrical properties of (Co, Nb)-doped SnO₂ varistors upon the addition of Pr₂O₃ was investigated by scanning electron microscopy and by determining I – V , ?– f , and R – f relations. The threshold electric field of the SnO₂-based varistors increased significantly from 850 to 2280 V/mm, and the relative dielectric constants of the SnO₂-based varistors decreased greatly from 784 to 280 as Pr₂O₃ concentration was increased up to 0.3 mol%. The significant decrease of the SnO₂ grain size, from 4.50 to 1.76 μm with increasing Pr₂O₃ concentration over the range of 0–0.3 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 Pr₂O₃ at grain boundaries hindering the SnO₂ grains from conglomerating into large particles. Varistors were found to have a superhigh threshold voltage and a comparatively large nonlinear coefficient α. For 0.15 mol% Pr₂O₃-doped sample, threshold electric field and nonlinear coefficient α were measured to be 1540 V/mm and 61, and for 0.3 mol% Pr₂O₃-doped sample, V and α were 2150 V/mm and 42, respectively. Superhigh threshold voltage and large nonlinear coefficient α qualify the Pr-doped SnO₂ varistor as an excellent candidate for a high voltage protection system.