Study On Tungsten Matrix Material For Diffusion Barium Tungsten Cathode

Modern vacuum electronic devices,especially high-end terahertz devices,require highly reliable cathodes with stable emission,high emission current density,and long emission life.However.the conventional cathode tungsten matrix exists the problems of large fluctuations in pore size,large average pore size,high closed-porosity,and low skeleton strength.Therefore,improving the pore characteristics and mechanical properties of the porous tungsten matrix is critical to the improvement of the quality of the highly reliable cathode and even the entire device.In this paper,medium-grain size tungsten powder was used as raw material,and jet grading technology was adopted to control the particle size of raw material powder,after cold isostatic press molding,the tungsten compact was sintered at high temperature in hydrogen automosphere to obtain tungsten sintered body,then by the process of copper infiltration,machining and decoppering,a porous tungsten matrix was obtained.The tap density,particle morphology,chemical composition,particle size,and distribution of tungsten powder were characterized by using a tap density meter,FE-SEM,XPS,and a laser particle size analyzer,respectively;Universal tensile testing machine,Vickers hardness tester,and Mercury injection apparatus were adopted to characterize the mechanical properties of tungsten-copper materials,the mechanical properties and pore characteristics of tungsten matrix,correspondingly;And test emission properties of prototype cathodes are also obtained.The main conclusions are as follows:(1)For GW045 and GW050 raw material tungsten powders,after particle size regulation,their particle size distribution became narrower and tap density increased significantly,indicating that it would have better particle stacking and suppression properties;XPS results showed that oxidative contamination such as WO3,WO2,etc,which are unfavorable to cathode emission appears in graded tungsten powder.Therefore,the purification treatment before press molding is very important.(2)The GW045 and GW050 grade tungsten powders were shaped by a cold isostatic pressing(200 MPa,10 min)to obtain a tungsten compact with a relatively regular shape,and the relative densities of the rod blanks were 66.4%and 68.3%,respectively,which corresponded well with the tap density.In order to achieve a high uniformity of the density of tungsten rods in cathode applications,it is necessary to overcome the difference in the compaction density caused by the high aspect ratio.The rod blank anatomical experiment supported the interception of 20 to 30 mm from the bottom end of the tungsten rod blanks which have a large fluctuation in density.(3)The porosity tungsten rods pressed by GW045 classified tungsten powders decreased linearly from 22.8%at 1900? to 17.8%at 2050?;Accordingly,the porosity of tungsten rods pressed by GW050 classified tungsten powders gradually decreased from 24.4%at 1900? to 20.8%at 2050?.Since the sintering of tungsten rods is a typical unit system sintering,with the sintering temperature increases,the diffusion of tungsten atoms increases and the sintering neck grows.(4)The tensile strength of tungsten copper infiltrated material which was pressed by GW045 graded tungsten powder and sintered at 2000? is 864 MPa,its Vickers hardness HV10 is 308.5±10.5 MPa,and it has good processing performance.The tensile fracture morphology appears crystal sugar-like intergranular fractures and skeletal particles resembling river-like cleavage fractures.The overall fractures are dominated by cleavage fractures.(5)Mercury injection and mechanical properties test results showed that this paper has prepared a porous tungsten matrix with a porosity of(18?25)%,an average pore size of 1.0?1.6 ?m,a closed porosity(<1%),and a matrix strength of no less than 100MPa.The emission and underheating characteristics of the prototype cathode were tested,which showed that when the cathode was operating at 1050?,the pulse current density was 28.14 A/cm2,with the slope was 1.35,and the characteristic operating point Tso(834?b)was low,indicating that the prepared cathode has good emission performance.