Nature of Intergranular Phase in Nonohmic ZnO Ceramics Containing 0.5 Mol% Bi 2 O 3

The intergranular phase obtained by sintering a binary mixture of ZnO + 0.5 mol% Bi 2 O 3 was isolated by using a dilute solution of HCIO 4 , which etches ZnO preferentially. The combined results of selected‐area electron diffraction and microscopy, microprobe analysis, and X‐ray diffraction strongly indicate that the intergranular material is a polycrystalline phase of tetragonal β‐Bi 2 O 3 ( P 42 1 c ), rather than the amorphous ZnO‐Bi 2 O 3 phase reported earlier. It appears that the nonohmic behavior in this prototype metal‐oxide varistor must be an interfacial property associated with the semiconducting ZnO grains separated by thin layers of high‐resistivity Bi 2 O 3 .