Preparation Of Nitrogen-doped Ultrananocrystalline Diamond-Coated Graphite Cathode And Its High-Current Pulse Emission Characteristics

This paper takes advantage of the excellent electron emission properties of nitrogen-doped ultrananocrystalline diamond(UNCD), and adopts the microwave plasma chemical vapor deposition(MPCVD) technique by coating the nitrogen-doped UNCD on the graphite cathode edge to modify the graphite cathode and solve the low current, high emission threshold, high degassing and other defects of graphite cathode, which is used in the relativistic backward wave oscillator(RBWO). First of all, by using the triethylamine(TEA) as the doping nitrogen source, the process of preparing the nitrogen-doped UNCD on silicon wafer was explored, and the effects of the liquid source composition, intake and the growth temperature on the quality of the films were studied in great detail. Besides, the filed emission performance of the nitrogen-doped UNCD also got researched. It is necessary to prepare a transition layer of tungsten film on the graphite substrate prior to the deposition of nitrogen-doped UNCD, in order to overcome the problems of low nucleation density and poor integrity of the films deposited on the graphite substrate. What's more, deposited by W(CO)6 as the precursor, the effects of the deposition temperature and the annealing temperature on the preparation of metal tungsten thin films were also studied by using metal organic chemical vapor deposition(MOCVD) technique. Moreover, the morphology, structure, composition and electrical properties of the nitrogen-doped UNCD films prepared and the transition layer of tungsten films were studied at great length by using SEM, multi-wave length Raman, XPS, Hall effect, XRD and other characterization techniques. The technology transfers to the graphite cathode after grasping the fundamental growth rules of preparing the nitrogen-doped UNCD and the tungsten film. Through the optimization of the deposition process, the coated and modified graphite cathode of the high quality nitrogen-doped UNCD film was prepared successfully, and preliminary studies were done on its high current pulsed emission properties and the possible emission mechanisms. Research results show that:(1) By using the methanol triethylamine solution as a liquid source, when the volume ratio of methanol to triethylamine was 1:1, the prepared nitrogen-doped UNCD thin film had the optimal electrical properties. The conductivity rate of the thin film increases with the growth of the liquid source intake. At the same time, the sp2-C content in the films increases and nano-crystalline graphite phase emerges. The growth temperature has a remarkable effect on the morphology, composition and electrical conductivity of the UNCD films. The electrical conductivity of the films increased from 156.1 S/cm to 1853.9 S/cm with the increase of temperature from 760? to 890?. The carrier concentration reached a maximum of 6.9×1020/cm3. DC field emission performance shows that the high conductivity can not guarantee the superior field emission properties. In comparison, the nitrogen-doped UNCD films with dense diamond nanowires structure have the optimal field emission performance, and the emission current density reached 8.0m A/cm2 when the electric field strength was 6.2V/?m.(2) The growth temperature has a significant effect on the surface morphology of the prepared tungsten films. The surface of the tungsten film have a uniform, compact and good coverage when the growth temperature was 360?. With the increase of the annealing temperature, the crystallinity of tungsten film increased, while high annealing temperature didn't have obvious effect on the surface morphology of tungsten film.(3) The tungsten film deposition process can be perfectly copied to the annular graphite cathode, while the preparation process of the nitrogen-doped UNCD can't be copied to the silicon substrate completely. When the deposition temperature was above 770?, the ignition phenomenon will appear. The high-current pulse emission capability of the modified graphite cathode will enhance with the increase of the growth temperature of the coated nitrogen-doped UNCD thin films. The emission property of the modified graphite cathode prepared at 770? was greatly improved, compared with the original graphite cathode. The emission current reached 3.78 k A, an increase of 500 A than the original graphite cathode, when the pulse width was 5ns and voltage was 102.2k V. At the same time, the tungsten transition layer has a certain effect on the suppression of the degassing in the emission process.