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Relation Between Grain Boundary Structure and Electrical Degradation in Zinc Oxide Varistors
Author(s) -
Takada Masayuki,
Sato Yuuki,
Yoshikado Shinzo
Publication year - 2012
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2012.05212.x
Subject(s) - varistor , grain boundary , materials science , annealing (glass) , scanning electron microscope , zinc , impurity , analytical chemistry (journal) , oxide , mineralogy , metallurgy , composite material , microstructure , chemistry , voltage , electrical engineering , organic chemistry , chromatography , engineering
The electrical characteristics of zinc oxide ( ZnO ) varistors are closely correlated with the state of impurities, such as Bi 2 O 3 , 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 Bi 2 O 3 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 SiO 2 in Bi 2 O 3 , the surface tension of Bi 2 O 3 decreased and the sheetlike Bi 2 O 3 deposit increased, whereas the spot like Bi 2 O 3 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 SiO 2 or by annealing is the increase in the surface area of the ZnO grains by the sheetlike deposit of Bi 2 O 3 , since this deposit hinders the movement of oxide ions or Zn 2+ ions across the grain boundaries under an applied voltage.