Design,preparation And Characterization Of Graphite Modified By Carbide Coatings For Collecting Intense Electron Beam

Collector is one of the most important components in high power microwave?HPM?generator,collecting exhausted electrons with intense characteristic.As collector is radiated by intense electron beam?IEB?,complex physical effects such as surface temperature rise,plasma,electron emission and bremsstrahlung radiation,etc.are caused,which are detrimental to beam wave coupling,and leading to a decrease in beam-wave coupling efficiency,microwave pulse shortening,and even the destruction of the collector structure.With the development of higher peak power of HPM technology,the problems of collector will be more and more prominent,and the collector would become the constraint of HPM technology development.Graphite is a commonly used material collecting electrons.However,graphite is outgassing and could lead to carbon pollution of the vacuum system.Carbides such as SiC,TiC and TaC process the properties of high melting point,thermal and electrical conductivity.Therefore,this paper focuses on relieving the problem of graphite when collecting electrons by carbide coating modification.The electron energy deposition law of the material under the irradiation of electron beam with the excitation voltage of 1 MV,the irradiation current density of 1.43kA/cm² and the pulse width of 40 ns was simulated by Monte Carlo software MCNP5,and based on the law of energy conservation,the temperature rise was calculated.The results showed that?1?the energy deposition peak in the material increased with the increase of the electron density of the material;the absorption efficiency of electron energy and the incident depth of electron decreased with the increase of electron density of the material;the temperature rise of the material sub-surface was related with the electron density,density and specific heat capacity of the material,and increased with the increase of the electron density and decrease of and specific heat capacity of the material.?2?SiC/C and TiC/C with a coating thickness of 30?m had the similar electron collection performance with graphite,the electron energy absorption efficiencies of SiC/C,TiC/C and graphite were 0.995,0.993 and 0.996,respectively,and the surface temperature rise of SiC/C,TiC/C and graphite were 190°C,138°C and126°C,respectively.However,the electron collection performance of TaC/C had a big difference with that of graphite,and the electron energy absorption efficiencies and surface temperature rise of TaC/C were 0.866 and 521°C.TiC coating was prepared on the surface of graphite by in-situ reaction of plating coating Ti with the graphite substrate.The influence of electroplating process parameters on the microstructure and deposition rate of Ti coating and the effect of heat treatment on microstructure and phase of Ti coating were studied.The results showed that the coating prepared by pulse electroplating was more compact and fine grained compared with direct current plating;disproportionation reactivity of Ti cation was enhanced with increasing of the molten salts temperature;the stability of Ti³⁺was increased as the atomic ratio of F and Cl increasing,thereby the microstructure and deposition efficiency of the Ti coating were improved.The heat treatment temperature had important influence on the roughness and microstructure of the coating.After the heat treatment at 1400°C,the phase of the coating transited from Ti to TiC,surface roughness of the coating was lowered,and the compactness of the coating was increased.After heat treatment at 2200°C,the coating was composed of high crystallinity TiC particles,and the morphology of the TiC coating was inherited from the morphology of the coating before heat treatment.The influences of carbide coating modification on the gas tightness,electrical conductivity and thermal conductivity of graphite were investigated.The results showed that although the carbide coatings blocked the pores of the graphite,the gas tightness was reduced after carbide coating modification.The surface conductivity of TiC and TaC coating modified graphite was higher than that of graphite,and the surface conductivity of SiC coating modified graphite was obviously lower than that of graphite.The thermal diffusivities of the graphite modified by carbide coatings were almost the same.Graphite and graphites modified by SiC,TiC and TaC carbide coatings material were irradiated by IEB with the energy density of about 9.0 J/cm².The resistance to IEB irradiation damage of graphite,TiC/C,SiC/C and TaC/C,and their influence on the performance of the high power diode were tested.The results showed that there was no irradiation damage on the material surfaces.The diode performances were basically the same,the corresponding voltage pulse width was about 105.0 ns,and the current amplitude and pulse width were about 8.9 kA and 126.7 ns.When the energy density of IEB was increased to about 20.0 J/cm²,the results showed that there were no substantial radiation damages on the surface of graphite,SiC/C and TiC/C,while the surface of TaC/C anode melted.The voltage amplitude and pulse width of graphite anode were about 200.0 kV and 105.0 ns,respectively,and the current amplitude and pulse width were about 8.9 kA and 126.7 ns.Due to the formation of anode plasma on the surface of SiC/C,TiC/C and TaC/C anode,the current increased and pulse width shortened,compared with the graphite anode,their current amplitudes were increased by about12.4%,12.4%and 34.8%,voltage pulse widths were shortened by about 4.5%,7.1%and 19.2%,respectively,and the current pulse widths were shortened by about 4.0%,4.0%and 13.2%.