The Effects Of Zinc Oxide Microstructure On The Electrical Characteristics Of Low-Voltage Ceramic Varistors
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Date
2004-01
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 (a=36.68)
was achieved, accompanied by a donor density of 1.3xlOl4 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, oxygen vacancies and lattice defects raised the donor concentration by
an amazing 63,400% (1.3xlO I7 cm-3
) and the interfacial trap density by 3,200% (5.9xlO I3
cm-\ 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.
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Keywords
The effects of ZnO microstructure on the , electrical characteristics of low-voltage ceramic varistors