Ceramic varistors based on ZnO-SnO2

Model ceramic varistor formulations based on 98% ZnO-SnO2 (plus 2% oxides of Bi, Co and Mn) were prepared by conventional powder processing routes; specimens were sintered at 1150-1275 degreesC. The product density increased with SnO2 content, but decreased with increase in sintering temperature. The microstructures contained ZnO, Zn2SnO4 spinet and very small bismuth rich phases; with increasing Sn content the spinet became the dominant phase. From I V measurements, the non-linear coefficients were found to be in the range 24-32. For all compositions, the alpha values decreased with increasing sintering temperature; the maximum alpha value was obtained with samples containing 20 mol% SnO2 sintered at 1200 degreesC. Breakdown fields were in the range 3000-11000 V/cm and increased with increasing tin content. Leakage currents were in the range 1-11 muA. Tin substitution for Zn appears to cause a strong donor effect. (C) 2003 Elsevier Ltd. All rights reserved.