The effects of zinc oxide microstructure on the electrical characteristics of low-voltage ceramic varistors
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Date
2004
Authors
Shahrom, Mahmud
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Abstract
The effects of ZnO microstructure, on the electrical characteristics of low-voltage ceramic varistors, have been investigated in this work. A wide-shape distribution of ZnO particle size has been found to cause a large increase in the leakage current, reduction in the dielectric constant, increase in the donor density, reduction in the interfacial trap density, rise in the junction capacitance density and big gain in grain resistivity. At a ZnO specific surface area (SSA) of 4.7-4.8 m2/g, maximum nonlinear coefficient (α=36.68) was achieved, accompanied by a donor density of 1.3x1014 cm-3, a boundary Schottky barrier of 1.8V and an average ZnO particle size of 0.23 micron. Increasing SSA of ZnO tend to raise the leakage current, reduce the interfacial trap density, reduce the junction capacitance density, decrease the grain boundary resistivity, increase degradation and raise the grain resistivity. Intrinsic defects were introduced by over-milling the ZnO powder for 100 hours prior to mixing with the additive oxides. The resulting increase of zinc interstitials and lattice defects raised the donor concentration by an amazing 63,400% (1.3x1017 cm-3) and the interfacial trap density by 3,200% (5.9x1013 cm-2). Other effects due to the increased intrinsic defects were large increase in leakage current, large drop in nonlinear coefficient, big jump in the Schottky barrier, large reduction in junction capacitance density and big increase in degradation. Large concentration of bismuth was detected at the grain boundary, from the EDX data, which explains the big rise in the Schottky barrier. The immense increase in interfacial trap density could be due to the higher concentration of dangling bonds that could have originated from the bigger lattice defect concentration.
Description
Master
Keywords
Engineering , Zinc Oxide , Electrical Characteristic