Effect Of Ag₂O, B₂O₃Respective Addition And Pre-heat Treatment Of B₂O₃on The Properties Of ZnO Based Varistors

Varistors all have the function of voltage suppression, they can prevent the damagecaused by ESD, surge and other transient current, their application fields covers almostall of electrical-equipment and electronic devices. Compared with other commonpressure sensor, ZnO varistors has the clear advantage in non-linear, surge capacity,voltage range, and many other technical parameters. Japan, Russia and the United Statesall have had the technology to product UHV ZnO surge arrester and varistors withultrahigh voltage gradient. Among them, Japan has been manufacturing high-endvaristors with strong protection property in2011. Now, advanced core technology ofZnO varistors is monopolized by electric power industry in the developed country, suchas Japan. ZnO varistor industry in China has developed for decades, but there are stillsome gaps with international advanced level.In this article, I added Ag₂O and B₂O₃in practical formulation respectively, andcomplemented with advanced burn-in process of adding powder, and at last, IResearched the properties and micro-structure of HDC ZnO Varistors. Research resultsshow that: on the premise of B₂O₃powder’s added weight do not exceed0.12wt%, Ag2Oand B₂O₃improve the relative density.However, due to the large difference between the ionic radius of B and Ag, mixedwith Ag₂O and B₂O₃have different effects on other electrical properties of ZnO varistors.The leakage current ILdecreases with added of Ag₂O, but it increases with added of B₂O₃;the varistor voltage U1mAincreases with added of Ag₂O, but it decreases with added ofB₂O₃; nonlinear coefficient α decreases with added of Ag₂O, but it increases with addedof B₂O₃. The mechanism is significantly different between Ag2O and B₂O₃. Ag ions,which inthe Ag₂O, enter in lattice of the ZnO grain, improve the acceptor density of samples; andAg ions can replace some of the interstitial Zn ions as gender dopant doping. Only extraAg ions are located in Crystal territories, it can inhibit the growth of ZnO grain duringsintering progress. As the donor dopant, the ionic radius of B is so small that it diffusesinto the ZnO crystal lattice as interstitial ion. Because the melting point of B₂O₃is557℃,it is melting in sintering process, so crystal liquid phase increases. Different mechanismof these two added powder makes different effects on the varistor.The different microscopic structure between samples with Ag₂O and samples withB₂O₃confirms their different mechanism. Acroding to the SEM, samples with Ag₂O havesmaller grain volume of ZnO with clearly corners, grain boundary phase is uniform in thearea, the sample was broken with mixed section and intracrystalline failure phenomenon,and this indicates that combinative force between grains weaker, but combinative forcebetween intergranular phases and grains stronger; samples with B₂O₃have bigger grainvolume of ZnO with round grain edges, grain boundary phase concentrate in the area, thesample was broken with flat section, and this indicates that combinative force betweengrains stronger, but combinative force between intergranular phases and grains weaker.