Diffusion of molten Bi2O3 into the grain boundaries of sintered, alumina-doped (0.23 and 0.7mol%) ZnO pellets resulted in varistors with breakdown voltages in the 3 to 5V range and non-linearity coefficients of 10 to 24. The varistors were fabricated by spreading a thin layer of Bi2O3 powder on the surface of ZnO pellets and heating the combination to various temperatures (860 to 1155C) and different times. The highest non-linearity coefficients (20 to 24) and lowest breakdown voltages (3 to 5V) were recorded in samples annealed at 860C for 35min. Longer annealing times and/or higher temperatures resulted in progressively higher breakdown voltages. Eventually the devices became insulating, which was attributed to the formation of an insulating Bi2O3 layer between the grains. Separate wetting experiments have shown that the penetration of Bi2O3 into ZnO grain boundaries was a strong function of alumina doping; the penetration rate was decreased by a factor of 5 to 7 as the ZnO was doped with as little as 0.2mol% of alumina. It was this slowing down of the penetration of the ZnO grain boundaries that was believed to be critical in the development of the low breakdown voltages observed.

Low Breakdown Voltage Varistors by Grain Boundary Diffusion of Molten Bi2O3 in ZnO. Barsoum, M.W., Elkind, A., Selim, F.A.: Journal of the American Ceramic Society, 1996, 79[4], 962–6