Enhancement of the Electrical Stability of ZnO Varistors by a Novel Immersion Process

A novel immersion process was carried out to improve the electrical stability of ZnO varistors under a dc electrical field. It was performed by dipping the conventionally sintered ZnO varistor samples in a bismuth nitrate solution with a subsequent heat treatment. The results showed that this process could improve the electrical stability and the nonlinearity of the ZnO varistors simultaneously. All the immersed specimens exhibited a reduced leakage current and a stable nonlinearity in the dc degradation test. The optimum concentration of the immersion solution was found to be 1.0-2.0M. The microstructure and the electrical performance of the immersed ZnO varistors were investigated. This process generates an effective penetration of molten Bi(2)O(3) from the surface into the interior, which compensates the near-surface bismuth loss that resulted from its vaporization during sintering. The extraneously penetrated molten Bi(2)O(3) plus inherently molten Bi-rich phase facilitate the reconstruction of an intergranular layer and fills more grain boundaries with the gamma-Bi(2)O(3) phase. It is the abundant gamma-Bi(2)O(3) intergranular phase that improves the electrical stability by restraining the oxygen desorption at the grain boundary. The enhanced nonlinearity is related to the modification of the defect structure and the component distribution in the intergranular region.