Improvement of the V-1 characteristic of zinc oxide (ZnO) based metal oxide varistors (MOVs) using Silicon Telluride (SiTe2) and Lanthanum Hexaboride (LaB6) materials
Date
2009-06-29T08:12:43Z
Authors
Hove, Miidzo
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Abstract
A study to improve the V-I characteristic of the ZnO-based commercial MOV
using a characterised chalcogenide material, Silicon Telluride (SiTe2), and a fieldemissive
material, Lanthanum Hexaboride (LaB6), has been conducted. The need
arises in that the current commercial ZnO-based metal oxide varistors (MOVs)
have a V-I characteristic that departs substantially from that of the ideal one. As a
result of this shortcoming, they do not offer ideal clamping action, and the
consequence of this is that the protection they are supposed to offer is
compromised. The problem behind this shortcoming is the microstructure, which
is not ideal. An ideal microstructure to constitute an ideal device is not known yet,
hence the problem.
Based on a model, a prototype MOV was fabricated using conventional sintering
techniques. The phases and microstructure of this prototype MOV were studied
using XRD and SEM with EDS facility. The V-I characteristic was studied using
the two point probe method, and the clamping action was studied using an
impulse generator.
A prototype MOV with a near ideal V-I characteristic, with improvements in the
leakage, active (breakdown) and up-turn regions was developed. In the leakage
region, leakage currents were reduced by 1.0 %. In the active region, the rate of
breakdown was increased and discharge currents were increased by on average 4
times those of a dimensionally comparable commercial MOV. The instability
responsible for the breakdown was found to be field dependent. The up-turn
region was removed. The corresponding surge clamping action of the prototype
MOV was identical to that that of the studied commercial MOV, but with lower
surge current. The improvements are attributed to the usage of characterised
powders and new additives, as well as the process method, in the development of
the prototype MOV.
One other related major finding is that the pyrochlore phase, Bi2Zn(Zn4/3 Sb2/3)O6,
and the spinel phase, Zn(Zn4/3 Sb2/3)O4 are not the only phases that can give rise to
the varistor property which gives rise to the non-linear V-I characteristic in a
ZnO-based commercial MOV. This is contrary to current know-how.
A prototype ZnO-based MOV with near ideal V-I characteristic can be developed.