| Zinc oxide varistors are often used in the fields of electronic circuits and power systems as voltage surge protection devices due to their highly nonlinear characteristics and excellent surge capability.Metallization is the key link to the production process of varistor.At present,most of the manufacturers use the screen printing method to prepare silver electrode,which is complex,costs more and polluted seriously.The base metal electrode of zinc oxide varistor prepared by spraying will be studied,to reduce the production costs,simplify the operation process and solve the pollution problem.The main research contents and research results are listed as follows:The combination of spraying electrodes and ceramics,the principle of ohmic contact,the conductive properties and welding properties of the electrode layer were discussed.The effects of the above factors on the electrical properties of varistor ceramics were discussed and the solutions were given.A design scheme of electrode structure called "connection layer/welding layer" was proposed,and a variety of metal electrode materials were screened.It was determined that aluminum was used as the connection layer electrode and copper as the welding layer electrode.The aluminum-copper composite electrode was prepared by arc spraying equipment,and the spreading areas of the six kinds of customized solder paste on the surface of copper electrode and the tensile strength of the tin-coated copper sheet were measured and compared.It was found that the three kinds of high-temperature solder paste could achieve a good welding effect between spraying electrode layer and tin-coated copper sheet.The silver electrode,spraying aluminum electrode and spraying aluminum-copper composite electrode were prepared on the surface of Zinc oxide varistor ceramics.The bonding strength between the electrode and the ceramic,the welding performance of the surfaces of electrodes,the electrical properties and the microstructure of the electrode layers were compared and analyzed.The results showed that the bonding strength between the electrode and the ceramic plate was the highest,and the bonding strength between the aluminum electrode layer and the ceramic plate was slightly worse,but it could still meet the production standard.All the electrodes had passed the test of high temperature solderability,indicating that the copper electrode layer could prevent the dissolution of high-temperature solder effectively;Compared with the silver electrode,the breakdown voltage of the aluminum-copper composite electrode increased,the leakage current decreased,and the nonlinear coefficient increased,which meant that the small current characteristic was improved.It was mainly because the spraying electrode did not need to be sintered,which avoided the influence of heat treatment process on electrical properties of varistor ceramics.Compared with the silver electrode,the residual voltage ratio of the aluminum-copper composite electrode increased,and the change rate of breakdown voltage after the current pulse of 40 KA 8/20μs waveform increased,which were mainly attributed to the existence of interspaces(floating voltage)between spraying aluminum electrode layer and the ceramic surface.After EDX analysis,it was proved that the oxide film formed on the surface of aluminum electrode could prevent the diffusion of aluminum element or copper element into ceramic effectively.The parameters of the spray process are adjusted by orthogonal test,so that the residual voltage ratio and surge absorption capability of the spray electrode varistor are close to the level of silver electrode varistor.The preliminary results showed that compared with the screen printing silver process,spraying electrode technology had overall superiority,which could reduce the production cost greatly.Littelfuse Electronics Co.,Ltd.is about to achieve the production of the varistor products of spraying electrode,which is expected to save material costs of 1.8 million US dollars per year. |
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