Effect Of Nitrogen And Oxygen Sintering Atmosphere On Electrical Properties Of Zinc Oxide Varistors

As a crucial device for protecting from damage of thunder and overvoltage,ZnO varistors have extensive applications in power electronic equipment.Thus,the research on its technique and doping for increasing ZnO varistors electrical properties,especially developing properties under high current pulse,has great benefit in engineering.The sintering atmosphere is also one of most essential factors toward varistor electrical properties.In this paper,the relationship between the ratio of oxygen to nitrogen in sintering atmosphere and ZnO electrical properties was discussed.Inspired by method of making p-type ZnO,combining previous study,we go further and study whether the nitrogen in sintering atmosphere will influence ZnO electrical properties.The contents are listed as follow:1.By the experiment of sintering ZnO varistors in different ratio of oxygen to nitrogen?10%O₂ to 100 O₂?,we find that ZnO have little difference in microstructure and crystal phase while ratio of oxygen to nitrogen of sintering atmosphere changes.The electrical fields of ZnO in the same current density are increase at first as ratio of nitrogen to oxygen raise,but,whose electrical fields will decrease as oxygen concentration raise then.The permittivity of ZnO decrease firstly then up as ratio of nitrogen to oxygen raise.With the increase of oxygen/nitrogen ratio in sintering atmosphere,the pulse current resistance of zinc oxide varistor ceramic elements is gradually improved,but the pulse residual voltage ratio will be improved.Based on the above experimental results,it is concluded that both oxygen and nitrogen elements in sintering atmosphere play a corresponding role in conductivity of zinc oxide grains and grain boundary regions.With the increase of oxygen content in sintering atmosphere,oxygen vacancies and zinc intrinsic defects in materials decrease correspondingly,effective donor concentration in zinc oxide grains decreases and grain boundary thickness increases,which leads to the increase of varistor voltage.In addition,the capacitance decreases and the pulse current resistance performance improves.However,with the decrease of nitrogen content to a certain extent,the addition acceptor effect of nitrogen on zinc oxide gradually disappears,and the effective donor concentration increases accordingly,which results in the corresponding reduction of varistor voltage and the corresponding increase of capacitance.2.By the result listed at front,we compare the electrical properties of ZnO sintering in the same oxygen content but mixed with N₂ and Ar respectively,the result shows that the existence of nitrogen in sintering process will enlarge the electrical field in the same current density,decrease the permittivity.Moreover,nitrogen in sintering atmosphere can increase the ratio of pulse voltage to varistor voltage.It is further determined that the nitrogen element in sintering atmosphere will play the role of acceptor addition,thus affecting the performance of zinc oxide varistor ceramic.It means that,we can make better ZnO varistor by lustrated the nitrogen in atmosphere while have no change in the oxygen concentration.