Shock-sintering of low-voltage ZnO-based varistor ceramics with Bi4Ti3O12 additions

Microstructure development in ZnO ceramics with Bi4Ti3O12 (BIT) additions was studied in dependence of sintering temperature, inversion boundary (IBs) nucleation, heating rate and doping with transition metal oxides (NiO, MnO2 and Co3O4). We demonstrated that one of the essential conditions for homogeneous microstructure development in this system is rapid release and efficient distribution of TiO2, necessary for the formation of Ti-rich (tail-to-tail) IBs in ZnO grains. This can be achieved via the so-called shock-sintering procedure described in this article. Immediate decomposition of BIT to TiO2-rich Bi2O3 liquid phase above 1200 °C leads to nucleation of ZnO grains with IBs. Exploiting the growth of ZnO grains with IBs, microstructure development can be easily controlled via the IB-induced grain growth mechanism, previously described in SnO2-doped and Sb2O3-doped ZnO. In contrast to conventional sintering, where erratic nucleation of IBs leads to bimodal grain size distribution, shock-sintering sintering regime produces microstructures with uniform coarse-grain sizes, required for low-voltage varistor ceramics.