(Pr, Co, Nb)‐Doped SnO ₂ Varistor Ceramics

Abstract

The effect on microstructure and electrical properties of (Co, Nb)‐doped SnO 2 varistors upon the addition of Pr 2 O 3 was investigated by scanning electron microscopy and by determining I – V , ɛ– f , and R – f relations. The threshold electric field of the SnO 2 ‐based varistors increased significantly from 850 to 2280 V/mm, and the relative dielectric constants of the SnO 2 ‐based varistors decreased greatly from 784 to 280 as Pr 2 O 3 concentration was increased up to 0.3 mol%. The significant decrease of the SnO 2 grain size, from 4.50 to 1.76 μm with increasing Pr 2 O 3 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 Pr 2 O 3 at grain boundaries hindering the SnO 2 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% Pr 2 O 3 ‐doped sample, threshold electric field and nonlinear coefficient α were measured to be 1540 V/mm and 61, and for 0.3 mol% Pr 2 O 3 ‐doped sample, V and α were 2150 V/mm and 42, respectively. Superhigh threshold voltage and large nonlinear coefficient α qualify the Pr‐doped SnO 2 varistor as an excellent candidate for a high voltage protection system.