Modified citrate gel routes to ZnO-based varistors

Chemical routes, based on a modified citrate gel synthesis, were developed for the production of ZnO-based varistor powders and compared with the conventional mixed-oxide route. Green bodies of powders prepared by the chemical routes were found to undergo densification in one or two discrete steps (at 770 and 910°C) compared to multiple-step densification of the conventional powder. In addition, it was found that the microstructures and electrical properties of the sintered pellets, although of nominally similar chemical composition, varied according to the synthesis procedure. Pellets from the chemical routes showed smaller grain sizes, higher densities, more evenly distributed intergranular phases and higher non-linearity coefficients than those from the conventional mixed-oxide route.