| The cathode and collector,respectively,act as the starting and ending points of the intense electron beams,which have an important impact on the performance of the high power microwave source.Graphite is a preferable material used for explosive emission cathode and intense electron beams collector,but it still has the disadvantages of easy gas release and easy to cause carbon pollution of the system,which affects the performance of graphite cathode and collector in high power microwave source.At present,there are few studies on improving the performance of graphite cathode and collector from the perspective of materials,and few researches on the influence of compositions and micro-morphologies of the cathode surface on the process and properties of explosive electron emission,results in a relativelypoorer understanding of this problem.Meanwhile,the development of high power microwave technology requires higher performance and service life of graphite cathode and collector.In this paper,the graphite cathodes were modified by carbides with three different compositions of SiC,TiC and TaC,respectively.And the carbidemodified graphite cathodes with different micro-morphologies were obtained by controlling the modification process.Then the influence of the composition and microstructure of the carbide modified graphite cathode surface on the emission characteristics of the cathode were investigated,to provide references for the development of the graphite-based medium modified cathode.In addition,the performances of TiC and TaC modified graphite materials in collecting intense electron beams were studied,providing references for improving the performance of collector and extending the collector materials.The controllable preparation of SiC modified graphite materials with different micro-morphologies was realized by chemical vapor infiltration method.It has been shown that the growth morphologies of SiC on graphite surface can be controlled by adjusting the concentration(or partial pressure)of CO gas,and the length,density of SiC whiskers and size of SiC particles can be controlled by adjusting reaction temperature.Reducing the total pressure of the system,or reducing the reaction temperature,is conducive to increase the infiltrating depth ofSiC into the graphite.Metal carbides can be formed in-situ on the surface of graphite by disproportionation of the unstable intermediate metal ions in molten salt.The coverage of metal carbides on the graphite surface can be changed by adjusting the reaction temperature.After the post-processing technology with two steps heat treatment of vacuumat 1600 °C and flowing Ar at 2200°C,the residual salt and residual oxygen in the graphite substrate can be effectively eliminated,to ensure the purity of the metal carbides modified graphite material.Based on these,SiC modified graphite cathodes with different surface micro-morphologies,TiC and TaC modified graphite cathodes with different coverage,TiC and TaC modified graphite collector samples were respectively prepared.The starting speed of field emission and the growth rate of field emission current density of carbide modified graphite cathode materials were studied by combining the field emission experiments and the simulation calculation of the electric field distribution on the surface of microprotrusions with different compositions and morphologies.The starting speed of explosive electron emission,explsovie emission current and the uniformity of explosive emission of carbide modified graphite cathodes were investigated by combining the explosive electron emission experiments and the simulation calculation of explosive electron emission on the surface of microprotrusions with different compositions and morphologies.The results show that the starting speed of field emission of the graphite is slowed after modified by carbides,while the growth rate of field emission current density decreases.The starting speed of field emission of graphite cathode full covered with TiC and TaC is faster than that of the partially covered graphite cathode,and the growth rate of field emission current density is larger than that of the partially covered graphite cathode.Among the SiC,TiC and TaC modified graphite cathodes with similar morphology,the SiC modified graphite cathode has the slowest field emission initiatingspeed and the smallest field emission current density growth rate.The faster the field emission starting speed of the cathode is,the faster the explosion emission initiating speed is.However,due to the influence of the field emission current density growth rate and the compositions on the cathode surface on the process of joule heating,the starting speed of the explosive emission is not completely dependent on the starting speed of the field emission of the cathode.Because of the lower electron mobility of the dielectric,the effective emission area of the dielectric micro-protrusion surface is larger than that of conductor with the same morphology,resulting in the emission uniformity of the dielectric modified graphite cathode is better compared with the conductor modified graphite with the similar micro-morphology.The explosive emission threshold,growth rate of electron beam current,and starting time,pulse duration,amplitude of the generated microwave of carbide modified graphite cathodes were respectively researched.The results show that the growth rate of the electron beam current does not always depend on the explosive emission threshold of the cathode,but when the explosive emission threshold of the cathode is larger,a faster growth rate of the electron beam current will be obtained.The growth rate of the electron beam current have an important influence on the starting time and the amplitude of the generated microwave.When the growth rate of the electron beam current is faster,it either causes the microwave to start earlier or cause the microwave amplitude to increase,which are beneficial to improve the energy of the microwave.The studies of emission stabilities of carbide modified graphite cathodes show that the intense electron emission of the annular cathode in a backward wave oscillator is concentrated on the edge of the blade,especially the outer edge.Although the carbide on the outer edge of the carbide modified graphite cathodeis obviously consumed after the multiple pulse explosive electron emission,as a result of the carbides exist tens or even hundreds of micrometers inside the graphite,so the carbide will be bare to continue to participate in intense electron emission even if the carbide of the cathode surface is completely consumed as the number of pulses increases.Therefore,the explosive electron emission stability of the carbide modified graphite cathode can be maintained.The modification of SiC,TiC and TaC on graphite surface can improve the intense electron emission stability and the service life of graphite cathode.Starting from the three basic properties of thermal conductivity,surface electrical conductivity and flatness,as well as the two application properties of diode characteristics and resistance of the electron beam bombardment,the performance of collecting intense electron beams of TiC and TaC modified graphite materials were studied.The results show that the surface conductivity and flatness of graphite are improved obviously after carbide modification while their thermal conductivities are almost the same.Under the experimental condition of lower-voltage longer-pluse,the composition of the anode target material has a significant effect on the diode characteristics.The reason is that the characteristics of the anode plasma generated by the different targets after being affected by the intense electron beam are distinguishing.The earlier the anode plasma is generated or the faster the anode plasma expands,the faster the diode gap is shortened,resulting in rapid decrease of diode impedance and increase of diode perveance.Under the action of single lower-voltage longer-pulse intense electron beam,the damage of graphite,TiC and Ta C modified graphite materials mainly appears on their surface layers,and the damage is mainly attributed to the gas release,particle oxidation of graphite,coating melting and shedding,which are all caused by temperature rise.Nevertheless,under the action of repetitive multiple higher-voltage shorter-pulse intense electron beam,the damage of graphite,TiC and TaC modified graphite materials mainly appears inside the graphite,which is imputed to the super-large stress formed by thethermal shock wave. |
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