The effects of SnO
2 additon on the electrical degradation characteristics of Bi
2O
3-MnO
2-Co
3O
4-added ZnO varistors were investigated by field-emission scanning electron microscopy (FE-SEM), energy dispersion X-ray spectroscopy (EDX), X-ray diffraction (XRD), voltage-current (
V-I) characteristics, and capacitance-voltage (
C-V) characteristics. The microstructure containing both twins of ZnO and Zn
2SnO
4-type spinel particles was observed at the grain boundary similar to Bi-Mn-Co-Sb
2O
3-added ZnO varistors. Spinel particles began to appear by the addition of SnO
2 over approximately 0.5mol%. Before electrical degradation, value of nonlinearity index α of
V-I characteristic for Bi-Mn-Co-SnO
2-added samples was approximately 50 and varistor voltage was 120∼140V/mm. Varistor voltage increased with increasing the amount of SnO
2. Value of α after the electrical degradation showed local maximum at approximately 0.1mol% with the increase of the amount of SnO
2 and then showed local minimum at approximately 0.5mol%
similar to the relative integral intensity of XRD diffraction peak for (004) plane at small amount of SnO
2. It is suggested that the diffusion of oxygen ions through the grain boundary is affected by the change of crystal orientation of ZnO grains at the grain boundary by addition of small amount of SnO
2. It is found that the increase of the varistor voltage by addition of SnO
2 is due to the increase of the barrier height of Schottky barriers.
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