Abstract: The protuberance phenomenon in shape of conic or cylinder was discovered for the first time. It was found by EDS that the protuberance is entity formed by ZnO, not the air bubble. And it was proved that the polar growth of ZnO leads to the protuberance phenomenon. The detailed reasons were probed from internal and external courses. The internal course is the polar structure and morphology of ZnO crystal. The external course is the physical and chemical environment made by Bi₂O₃ liquid phase, and this conclusion was proved by the comparisons of the samples which fabricated by 97mol%ZnO+3mol%Bi₂O₃ (in mole) compound and 100mol% (in mole) ZnO compound.

Key words: ZnO varistor, protuberance, polar growth, crystal structure, Bi2O3 liquid phase