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.