Relation Between Grain Boundary Structure and Electrical Degradation in Zinc Oxide Varistors

Abstract

The electrical characteristics of zinc oxide (ZnO) varistors are closely correlated with the state of impurities, such as Bi2O3, at grain boundaries. To clarify the distributions of these impurities, the fracture surface of a Bi–Mn–Co–Sb–Si‐added ZnO varistor was observed and characterized using scanning electron microscopy, energy dispersive X‐ray spectrometry, and back‐scattered electron detection. It was found that depositing Bi2O3 as an additive yielded two different shapes, spot like and sheetlike, on the surface of the grain boundary fracture for the Bi–Mn–Co–Sb–Si‐added ZnO varistor. By the dissolution of SiO2 in Bi2O3, the surface tension of Bi2O3 decreased and the sheetlike Bi2O3 deposit increased, whereas the spot like Bi2O3 deposit decreased. Moreover, when the sample was annealed at 700°C, a compound of Bi and excess Zn formed a uniform thin deposit or moved to triple points, since the surface free energy (surface tension) decreased. The main factor responsible for the improvement in resistance to electrical degradation by the addition of SiO2 or by annealing is the increase in the surface area of the ZnO grains by the sheetlike deposit of Bi2O3, since this deposit hinders the movement of oxide ions or Zn2+ ions across the grain boundaries under an applied voltage.

Effect of TiO2 doping on microstructural and electrical properties of ZnO–Pr6O11-based varistor ceramics

“…Other oxides (for example, CoO, Sb 2 O 3 , MnO, Cr 2 O 3 , etc.) are doped into varistor ceramics to achieve, hopefully, a larger nonlinear coefficient and higher resistivity [10][11][12][13][14][15].…”

“…Up to now, most of ZnO varistors have been based on bismuth-doped ZnO semiconductor, referred as to ZnO-Bi 2 O 3 -based varistor, which is the commercially commonest composition for ZnO varistor [10][11][12][13][14][15][16][17][18]. However, the ZnO-Bi 2 O 3 -based varistor has a few flaws due to its high volatility and reactivity of Bi 2 O 3 during liquid sintering.…”

Electrical degradation of double-Schottky barrier in ZnO varistors

“…5 It is known that δ-Bi 2 O 3 is a faster oxygen-ion conductor, and the β phase is much more oxygen-conductive than the γ phase, 28 which could lead to the speculation concerning its role as stabilizaing/destabilizing the ZnO varistor. Recently, M. Takada et al 82 have investigated the two types of Bi 2 O 3 deposit (sheet-like and spot-like, as seen in FIG. 10) on fractured surface of ZnO grains using back-scattered electron (BSE) mode in scanning electron spectroscopy.…”

“…The increase in the surface area of the ZnO grains by the sheet-like deposit of Bi 2 O 3 contributes to the resistance to electrical degradation, since this deposit hinders the movement of oxide ions or Zn 2+ ions across the grain boundaries under an applied voltage. 82 Actually, investigation on Bi 2 O 3 of various phases is a subject with broad interest for its wide applications, 83 e.g. Bi 2 O 3 doped with potassium exhibits superconductivity, and hence we cannot drop conclusion on its precise effect on DSB degradation until more atomic-scale investigations are presented which are currently lacked due to the difficulties in theoretical modeling of corresponding systems that are also computationally demanding.…”

Electrical characteristics of SrCoO₃-added zinc oxide varistors

“…It has been reported that there is a correlation between the orientation of ZnO particles, the nonlinearity index, and the resistance to electrical degradation. 9) Changes in the orientation of the (100), (002), and (101) planes of a ZnO particle will manifest themselves as differences in the intensities of the diffraction peaks, as shown in Fig. 5.…”