The Building And Studying Of The Plane Grain Boundary Barrier Of ZnO Varistor

ZnO varistors are widely used to control the surge current in the electrical and electronics circuits.It is generally believed that the high nonlinearity of ZnO varistors are derived from the double shottky potential barrier formed in the grain boundary.On the basis of the double shottky barrier model,this paper aim to study the different process conditions of the sol-gel spin coating and magnetron sputtering method,which will have an influence on micromorphology and electrical properties of thin films.The purpose of this paper is to bulid the single plane grain boundary,whose nonlinearity is similar to the of Zn O varistors ceramic.First of all,the process conditions of the sol-gel spin coating method are studyed,the influence of annealing temperature of ZA(ZnO(doped Al3+))layer and ZCMN(ZnO(doped Co2+?Mn2+?Ni2+))layer and pretreatment temperature of each layer on micromorphology and electrical properties of the thin films are studyed.The results show that the nonlinearity of the five layer thin film(ZA-ZCMN-Bi2O3-ZCMN –ZA)is increasing with the increasing annealing temperature,when the three annealing temperature are 500??550??600?.From the analysis of the XRD and SEM,it is concluded that the grain of the ZnO thin films grows big and uniform with increasing annealing temperature,thus making the better barrier because of good contaction,which cause the increasing of nonlinearity.The influence of different kinds of pretreatment temperature(150??200??250??300?)on the film structure and electrical properties are studyed,When fixing the annealing temperature to 600?.The nonlinearity of the five layer thin film first increase and then decrease.The optimal pretreatment temperature is 200 ?.Appropriate pretreatment temperature can make the solvent and volatile impurities evaporate fully and it won,t ruin film quality because of evaporating too fast.The different electrical properties of a single grain boundary barrier under positively and reverse biased voltage is studyed briefly.The voltage-dependent nonlinearity are approximately consistent with that of ZnO Varistors.Preparing the multilayer ZnO thin films through the magnetron sputtering method are also studyed.The influence of the process conditions of the ZnO layer and Bi2O3 layer on micromorphology and electrical properties of the thin films are studyed.When the ZnO layer prepared with ZnO target,50 W of operating power,O2:Ar=0.6:12,2.0Pa of operating pressure and 1000 s of sputtering time,we can prepare the uniform ZnO layer with the thickness of about 300 nm.Preparing the Bi2O3 layer by the Bi target and Bi2O3 target,the results show that the uniform Bi2O3 phase with stoichiometric ratio can be easily prepared by Bi2O3 target.We prepare the three-layer structure(ZnO layer-Bi2O3 layer –ZnO)thin films with the ZnO target and Bi2O3 target.When ZnO layer use the preparation condition above,comparing the influence of the different operating power(30W?45W?65W?100W?175W?215W)on micromorphology and electrical properties.The three-layer thin films have better nonlinearity when 45 W.From the analysis of the SEM,it goes against forming the barrier because of the small and nonuniform grain size when the operating power is too small.When the operating power is too high,the high-energy particles sputtering onto the surface will ruin the film,causing more defects,which resulting in the decreasing of properties of the thin films.In conclusion,the multilayer ZnO thin film varistors prepared by the sol-gel spin coating and magnetron sputtering method both have nonlinearity,and the three-layer structure(ZnO layer-Bi2O3 layer –ZnO)thin films prepared by magnetron sputteri ng method has better nonlinearity.