The effects of adding small quantities of SnO2 to the basic ZnO-Bi2O3 varis
tor composition were studied in terms of phase reactions, microstructural d
evelopment, and the formation of inversion boundaries. Scanning and transmi
ssion electron microscopy studies showed that the inversion boundaries, tri
ggered by the addition of SnO2, cause anisotropic grain growth in the early
stages of sintering. ZnO grains that include inversion boundaries grow exa
ggeratedly, at the expense of normal grains, until they dominate the micros
tructure. Higher additions of SnO2 lead to an increase in number of grains
with inversion boundaries and to a more fine-grained microstructure, The in
creasing amount of secondary phases is also related to a higher level of Sn
O2 addition; however, the influence of these phases on ZnO grain growth is
subordinate to the role of inversion boundaries.