Properties and intergranular phase analysis of a ZnO–CoO–Bi2O3 varistor

The electrical properties, microstructures and phase content of a ZnO–CoO–Bi2O3 varistor have been characterised. High-resolution TEM and selected area electron diffraction provided detailed information on the distribution and structure of the intergranular Bi-rich phase. Triple points between the ZnO matrix grains contained crystallites that could be indexed to tetragonal β-Bi2O3, cubic γ-Bi2O3 and δ-Bi2O3, or corresponding isostructural ZnO-Bi2O3 phases. Thin-film, 1–2 nm grain boundaries were found to be amorphous, but for the first time, crystalline precipitates (with the β-Bi2O3 type structure) were also detected in some thicker, 4–5 nm films.