Fabrication and characterization of zinc phosphate passivation layers for ZnO-based varistor

The chemical and morphological modification of zinc phosphates as a protection layer for ZnO-based varistor has been made through the addition of Mg or Ca species to the conversion solution combined with the calcinations of zinc phosphates. The results showed that the aspect ratio of as-coated zinc phosphates grains can be greatly reduced through the addition of Mg or Ca species. Moreover, the introduction of calcination to zinc phosphate not only makes the coating layer more dense, smooth, and resistive, but also results in the increase in capacitance, non-linear coefficient (α). On the other hand, loss tangent and breakdown voltage are both slightly decreased with increasing calcination temperatures. The increase in capacitance after calcinations may result from both the re-crystallization of Bi₂O₃-containing grain boundary phase and the reduction of oxygen vacancies.