Abstract
A microstructural evaluation was made of experimental non-Ohmic ZnO ceramics by optical, scanning, and transmission electron microscopy complemented with electron beam microprobe and x-ray analyses. Four crystalline phases were identified: a major phase of ZnO grains containing Co and Mn in solid solution and three minor phases of bismuth silicate (12 Bi2O3⋅2SiO2), pyrochlore (Bi2[Zn4/3Sb2/3]O6) and spinel (Zn7Sb2O12) in the intergranular region. Transmission electron microscopy revealed that the ZnO grains are mostly in direct contact with each other and that the intergranular phase is located primarily in the multiple grain junctions. In those regions where ZnO grains are in contact with each other, the measured grain-boundary width was ?25 Å. It is suggested that the barrier to conduction in ZnO varistors resides in the vicinity of the grain boundary within the ZnO grains and not in the intergranular insulating layer.
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