钼丝表面绝缘层的电泳制备工艺研究

Electrophoretic Preparation of Insulation Layer on Surface of Molybdenum Wire

加热钼丝是内加热型吸气剂的关键材料,但目前电泳制备绝缘层存在烧结温度高、涂层质量差等问题,影响了内加热型吸气剂激活电流的一致性。采用无水乙醇及氧化铝粉体作为悬浮液的主要成分,添加氧化镁改善电泳涂层质量,掺杂烧结助剂二氧化硅降低涂层的烧结温度。通过研究氧化镁的添加量对涂层的影响发现,掺杂氧化镁可以有效地促进氧化铝粉体的电泳沉积,且随氧化镁添加量增加,氧化铝的沉积速率逐渐增加,但过快的沉积速率易导致电泳涂层表面的均匀性变差,甚至开裂。研究二氧化硅的掺杂量对涂层质量的影响,结果发现,过高的掺杂量易导致烧结后涂层的收缩率变大,易产生开裂现象。同时,电泳绝缘层质量受烧结温度影响较大,随烧结温度升高,绝缘层孔隙率逐渐降低,但过高的烧结温度增加了涂层与加热钼丝之间的热应力,导致涂层质量下降。在悬浮液固相成分为Al_(2)O_(3)-1%MgO-4%SiO_(2)(质量分数),烧结温度为1400℃条件下,制备的涂层表面平整,厚度均匀,为最佳工艺参数。优化了现有电泳浆料各原料的比例,降低了烧结温度,并获得了具有高质量绝缘层的加热钼丝,为加热钼丝的发展提供了工艺指导。

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 activated 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 SiO_(2).The suspension was prepared by solvent,oxide powder and charging agent.The solvent was prepared by 0.05%nitrocellulose and 5%butyl acetate added to absolute alcohol.The oxide powder was prepared by Al_(2)O_(3),MgO,and SiO_(2) powders and occupied 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 temperature 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 Al_(2)O_(3) insulation 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 Al_(2)O_(3) powder.The amount of MgO added was positively correlated with the deposition efficiency of Al_(2)O_(3),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,onlyα-Al_(2)O_(3) phase.Analysis showed that Mg2+was generated in the suspension with the addition of MgO,which adsorbed onto the surface of Al_(2)O_(3) particles.Al_(2)O_(3) 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 addition of MgO increased,resulted in a corresponding increase in deposition rate of Al_(2)O_(3).The effect of sintered temperature on the coating with a doping ratio of 1%MgO and 4%SiO_(2) were analyzed.The results showed that a new phase of 3Al_(2)O_(3)·2SiO_(2) was generated in the coating at a sintered temperature of 1200℃.The SiO_(2) phase in the coating decreased at a sintered temperature of 1300℃.The content of 3Al_(2)O_(3)·2SiO_(2) 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 increased.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 Al_(2)O_(3) particles at a sintered temperature of 1200℃.The coating treated at temperature higher than 1200℃showed fewer pores.Moreover,high temperature also caused particle growth of Al_(2)O_(3).The coating reached a dense state with smooth surface and the large pores basically disappeared at a sintered temperature of 1400℃.Obvious micro cracks appeared 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.