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

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.