Preparation Of Graphite-Tungsten-Diamond Composite Cathode And Study Of Its Electron Emission Performance

In order to improve the low cathode emission current and material deflation of high-power microwave relativistic backward-wave oscillator?RBWO?,microwave plasma chemical vapor deposition?MPCVD?was used to coat the surface of the shaped graphite cathode with nitrogen-doped ultra-nano crystalline diamond?UNCD?to modify the original graphite cathode.Firstly,on the planar silicon substrate,liquid diethylamine and hydrogen were used as reaction gas sources to explore the influence of various growth factors on the microstructure and composition of the prepared films,and their electrical properties and electron emission performance were analyzed.Then,the planar substrate preparation process reference was applied to the shaped graphite cathode,and the reaction gas source was changed to a mixed solution of triethylamine and methanol and argon,and a nitrogen-doped UNCD film was prepared by preliminarily coating a tungsten transition graphite cathode surface.Therefore graphite-tungsten-diamond composite cathodes were formed.Finally the strong current pulse emission performance of composite cathodes were analyze systematically.The results show that:?1?When nitrogen-doped UNCD film is prepared on a planar silicon substrate,the liquid source flux,film deposition pressure,and growth temperature all change the morphology and composition of the film.The growth temperature is the main factor which effect micromorphology and composition of the prepared films.?2?FESEM,Raman and XRD were used to characterize the films prepared at different growth temperatures.The electrical properties and electron emission performance of the films were analyzed by Hall effect test and DC field emission test.With the increase of the growth temperature,the micro-morphology of the film transforms from very fine particles to a composite of multi-graphene and diamond particles perpendicular to the substrate.At the same time,the TPA,presenting in the grain boundaries,which is not benefit to the conductive,is converted into the sp²-C phase with better conductivity,and accordingly the conductivity of the film increases from 17?^-1·cm^-1 to 760?^-1·cm^-1.DC field emission test results showed that when the substrate growth temperature was raised from 730°C to 830°C,the current density increased from 0.2 mA/cm² to 1.6 mA/cm² at an electric field strength of9.9 V/?m.?3?A nitrogen-doped ultra-nano-diamond-coated graphite-tungsten-diamond composite cathode was prepared by predepositing a tungsten transition layer on the graphite cathode surface.The comparison test results of high-current pulsed electron emission performance between the composite cathode that deposited at different deposition time,microwave power,and growth temperature and the original graphite cathode show that the electron emission performance and gas release performance of the composite cathode obtained under the optimized process conditions are significantly improved;at the same time,the performance of the composite cathode is changed with different process conditions.Among different conditions,the composite cathode prepared under the growth power at 1800W,the substrate temperature of 800°C and a growth time of 2 h has the best high-current pulse performance.The beam density is 29.06 kA/cm²,which is 25%higher than that of the original graphite cathode.The deflation rate can be reduced by up to 62.5%.The application of nitrogen-doped UNCD film on the cathode surface of graphite effectively improves the cathode emission current and improves the gas liberation of the graphite cathode,providing new research ideas for finding new cathode materials for high-power relativistic backward-wave oscillator.