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

The effect on microstructual and electrical properties of (Co, Nb)-doped SnO2 varistors upon the addition of Er2O3 was investigated by scanning electron microscopy and I-V and ɛ-f relations' measurements. The threshold electric field of the SnO2-based varistors increased significantly from 305 V mm−1 to 1083 V mm−1, from 384 V mm−1 to 1300 V mm−1, and from 532 V mm−1 to 2270 V mm−1 for samples sintered at 1350 °C, 1300 °C and 1250 °C, respectively, for 1 h separately. The relative dielectric constants of the SnO2-based varistors decreased greatly from 2003 to 451 as Er2O3 concentration was increased up to 2.0 mol% sintered at 1350 °C for 1 h, from 1258 to 339 sintered at 1300 °C for 1 h and from 988 to 185 sintered at 1250 °C for 1 h. The significant decrease of the SnO2 grain size was found with the Er2O3 dopant for the samples sintered at various temperatures, which is the origin for the increase in the threshold voltage and decrease in the dielectric constants. The grain-size reduction is attributed to the segregation of Er2O3 at grain boundaries, which hinders the SnO2 grains from conglomerating into large particles. Varistors were found to have high density and fairly stable nonlinear coefficient α. For all Er2O3-doped samples, relative density was measured to be more than 98.00% except x = 2.0 mol% sintered at 1250 °C during 1 h and nonlinear coefficient α was measured to be around 16. High density and stable nonlinear coefficient α qualify the Er-doped SnO2 varistor as a comparative candidate for high-voltage protection system.