Microstructural Development in SnO2-Doped ZnO–Bi2O3 Ceramics

The effects of adding small quantities of SnO₂ to the basic ZnO–Bi₂O₃ varistor composition were studied in terms of phase reactions, microstructural development, and the formation of inversion boundaries. Scanning and transmission electron microscopy studies showed that the inversion boundaries, triggered by the addition of SnO₂, cause anisotropic grain growth in the early stages of sintering. ZnO grains that include inversion boundaries grow exaggeratedly, at the expense of normal grains, until they dominate the microstructure. Higher additions of SnO₂ lead to an increase in number of grains with inversion boundaries and to a more fine-grained microstructure. The increasing amount of secondary phases is also related to a higher level of SnO₂ addition; however, the influence of these phases on ZnO grain growth is subordinate to the role of inversion boundaries.