Bi2O3 vaporization from ZnO-based varistors

Bi2O3-doped ZnO ceramic varistors are usually sintered at temperatures near to 1200 °C in the presence of a Bi-rich liquid phase, which is partially vaporized during the sintering process. Volatilization of bismuth oxide depends on the total surface area in direct contact to the reaction atmosphere and this in turn is related to the area/volume ratio of the ceramic compact. This loss of Bi2O3 has a significant role on the development of the varistor microstructure and more specifically ZnO grain growth, which is strongly enhanced by the presence of the liquid phase, should be particularly affected. In the present paper, X-ray fluorescence analysis is performed to describe the Bi2O3 vaporization profile as a function of distance to the outer surface, taking into account its influence on microstructural evolution.