Electrophoretic Preparation of Insulation Layer on Surface of Molybdenum Wire
Getter was a material used to maintain the internal vacuum environment of electric vacuum devices.Usually,the getter was prepared by Zr alloys or Ti alloys through physical vapor deposition,powder metallurgy and other methods.The getter was activat-ed by heating,baking or induction heating before work.Molybdenum wire was the key material for internal heating type getter with a recrystallization temperature more than 800 ℃.There was an insulating layer between the getter and the molybdenum wire to ensure the activation of the getter.The electrophoretic preparation of the insulating layer had problems such as high sintered temperature and poor coating quality,which affects the high temperature mechanics performance of the molybdenum wire and activation current of the internal heating type getter.Anhydrous ethanol and alumina powder were used as the main components of the suspension.The quality of the electrophoretic coating improved with the addition of MgO.The sintered temperature of the coating was reduced with the doping sintered aid of SiO2.The suspension was prepared by solvent,oxide powder and charging agent.The solvent was prepared by 0.05％ni-trocellulose and 5％butyl acetate added to absolute alcohol.The oxide powder was prepared by Al2O3,MgO,and SiO2 powders and oc-cupied 40％of the suspension.An appropriate amount of cerium ammonium nitrate was added as a charging agent.The suspension was ball milled in an agate tank at a speed of 150 r·min-1 for 24 h,resulting in a well dispersed suspension.The spring wire with an outer diameter of 8 mm was prepared by molybdenum wire after winding and shaping treatment.Impurities such as organic matter,oxides,and graphite on the surface of the spring wire was removed through processes such as alkali cleaning,acid pickling,and high tempera-ture heat treatment under hydrogen atmosphere.At a temperature of 20～25 ℃,graphite sheet was used as the anode to connect the positive electrode of the power supply,and the spring wire with surface treatment was used as the cathode.Electrophoretic deposition was carried out under a certain voltage and time,and then the sample was sintered in a hydrogen atmosphere to obtain an Al2O3 insula-tion layer.The microstructure of the coating was observed by scanning electron microscope(SEM).The composition of the coating was analyzed by energy dispersive spectroscopy(EDS).The crystal structure type of the coating was analyzed by using X-ray diffraction(XRD).The appearance quality of the coating was observed by optical microscope(OM).The results showed that the addition of MgO can effectively promote the electrophoretic deposition of Al2O3 powder.The amount of MgO added was positively correlated with the de-position efficiency of Al2O3,however,an excessive deposition rate tended to cause surface uniformity or crack on the coating.When the proportion of MgO added was 1％,3％,and 6％,the Mg content in the prepared coating was 0.3％,0.5％,and 0.8％respectively,which was significantly lower than the MgO content in the suspension.The results showed that there were no diffraction peaks of MgO in the coating with a doping ratio of 1％MgO,only α-Al2O3 phase.Analysis showed that Mg2+was generated in the suspension with the addition of MgO,which adsorbed onto the surface of Al2O3 particles.Al2O3 particles with positive charges moved towards the cathode under the driving force of an electric field,thus deposited on the surface of the molybdenum wire.More Mg2+was generated as the addi-tion of MgO increased,resulted in a corresponding increase in deposition rate of Al2O3.The effect of sintered temperature on the coat-ing with a doping ratio of 1％MgO and 4％SiO2 were analyzed.The results showed that a new phase of 3Al2O3·2SiO2 was generated in the coating at a sintered temperature of 1200 ℃.The SiO2 phase in the coating decreased at a sintered temperature of 1300 ℃.The content of 3Al2O3·2SiO2 phase further increased at a sintered temperature of 1400 ℃.The shrinkage rate of the coating was less than 10％at a sintered temperature of 900 ℃.As the sintered temperature increased,the shrinkage rate of the coating also gradually in-creased.The shrinkage rate of the coating was more than 50％at a sintered temperature of 1400 ℃.Analysis showed that there was a significant sintered fusion phenomenon of Al2O3 particles at a sintered temperature of 1200 ℃.The coating treated at temperature high-er than 1200 ℃ showed fewer pores.Moreover,high temperature also caused particle growth of Al2O3.The coating reached a dense state with smooth surface and the large pores basically disappeared at a sintered temperature of 1400 ℃.Obvious micro cracks ap-peared on the surface of the coating at a sintered temperature of 1500 ℃.This study optimized the proportion of various raw materials in the existing electrophoretic slurry,reduced the sintering temperature to,and obtained heating molybdenum wire with high quality insulating layer,provided process guidance for the development of heating molybdenum wire.
Mo heating wireelectrophoresisinsulating layerAl2O3sintering temperature
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