Sintered ZnO ceramics painted with a mixture of $Bi_2O_3-PbO$ were annealed at various temperatures as a function of time.
The electrical breakdown voltage and nonohmic coefficient increase with annealing time, which certainly shows the diffusion phenomena of these additives into the grain boundaries of ZnO ceramics.
As the diffusivities of Bi and Pb are considered to be larger in the grain boundary than in the ZnO grain, it is believed that these additives make the roles of the electrical barrier formation, where the breakdown voltage per barrier reaches about 2.6 V at equilibrium.
It is also found through the electron microscopic analysis that the nonzero dihedral angle formed at three or four grain junctions in $ZnO-Bi_2O_3-PbO$ system does not correspond to complete wetting.
It is, therefore, supposed that the varistor breakdown phenomena is attributed to electrical barrier formed by the continuous, compositionally altered, thin grin boundary.
