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Detailed analysis of the microstructure of grain boundaries, especially triple-grain and multiple-grain junctions, in ZnO varistor materials has been performed using transmission electron microscopy. Different polymorphs of Bi 2 O 3 are shown to exhibit different wetting properties on ZnO interfaces. Recent investigations suggest that the equilibrium configuration consists of crystalline Bi 2 O 3 in the triple-grain and multiple-grain junctions and an amorphous bismuth-rich film in the ZnO/ZnO grain boundaries. The present investigation supports this suggestion for ␦-Bi 2 O 3 and also adds to the microstructural image and wetting properties of ␣-Bi 2 O 3 .

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