Improvement of the V-1 characteristic of zinc oxide (ZnO) based metal oxide varistors (MOVs) using Silicon Telluride (SiTe2) and Lanthanum Hexaboride (LaB6) materials

摘要

A study to improve the V-I characteristic of the ZnO-based commercial MOVudusing a characterised chalcogenide material, Silicon Telluride (SiTe2), and a fieldemissiveudmaterial, Lanthanum Hexaboride (LaB6), has been conducted. The needudarises in that the current commercial ZnO-based metal oxide varistors (MOVs)udhave a V-I characteristic that departs substantially from that of the ideal one. As audresult of this shortcoming, they do not offer ideal clamping action, and theudconsequence of this is that the protection they are supposed to offer isudcompromised. The problem behind this shortcoming is the microstructure, whichudis not ideal. An ideal microstructure to constitute an ideal device is not known yet,udhence the problem.udBased on a model, a prototype MOV was fabricated using conventional sinteringudtechniques. The phases and microstructure of this prototype MOV were studiedudusing XRD and SEM with EDS facility. The V-I characteristic was studied usingudthe two point probe method, and the clamping action was studied using anudimpulse generator.udA prototype MOV with a near ideal V-I characteristic, with improvements in theudleakage, active (breakdown) and up-turn regions was developed. In the leakageudregion, leakage currents were reduced by 1.0 %. In the active region, the rate ofudbreakdown was increased and discharge currents were increased by on average 4udtimes those of a dimensionally comparable commercial MOV. The instabilityudresponsible for the breakdown was found to be field dependent. The up-turnudregion was removed. The corresponding surge clamping action of the prototypeudMOV was identical to that that of the studied commercial MOV, but with lowerudsurge current. The improvements are attributed to the usage of characterisedudpowders and new additives, as well as the process method, in the development ofudthe prototype MOV.udOne other related major finding is that the pyrochlore phase, Bi2Zn(Zn4/3 Sb2/3)O6,udand the spinel phase, Zn(Zn4/3 Sb2/3)O4 are not the only phases that can give rise toudthe varistor property which gives rise to the non-linear V-I characteristic in audZnO-based commercial MOV. This is contrary to current know-how.udA prototype ZnO-based MOV with near ideal V-I characteristic can be developed.