Relationship between grain boundaries and electrical degradation of ZnO varistors

The effects of the electrical degradation characteristics and microstructure of Sb 2 O 3 ‐added ZnO varistors were investigated by field emission scanning electron microscopy (FE‐SEM), energy dispersion X‐ray spectroscopy (EDX), optical microscopy, X‐ray diffraction (XRD), and voltage– current ( V − I ) characteristic measurement. The nonlinearity index α of the V − I characteristic for Bi‐Mn‐Co‐Sb 2 O 3 ‐added ZnO varistors decreased with increasing amount of Sb 2 O 3 after electrical degradation. Twin crystals of ZnO were formed by the addition of Sb 2 O 3 . The number of twin crystals with two c ‐axes perpendicular to the twin plane increased and the number of twin crystals with two c ‐axes parallel to the twin plane decreased with increasing addition of Sb 2 O 3 . It is suggested that electrical degradation is affected by a combination of orientations of ZnO grains containing twin planes and that a double Schottky barrier may not be formed in the twin plane. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(3): 1– 9, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20493