Traveling Wave Tube Cathode Microscopy Studies

As the heart of military electronic equipment, high-power broadband traveling wave tube (TWT) play an important role in the modern high-tech war. The gap of TWT between home and abroad mainly embodies in life and reliability. As a key part that influents the TWT life,the basic relation between performance and structure must be clarified in order to develop high-performance cathode. Currently the research on the cathode is usually based on the process, theory, materials and others, but it is few based on the microstructure. By using scanning electron microscopy and electron microprobe spectrometer combined with cathode process, the comparative analysis of raw material composition and structure, the contrast analysis of different processes experiment sample and the failure analysis is carried out. The main contents and results are summarized as follows.(1) The investigation of the secondary electron image of composition. According to the difference of atomic number of the elements that compose material, a new method was proposed to characterize the composition using the second electron image. The method has been applied in the analysis of the cathode material. Compared to the backscattered electrons image of the cathode, it can obtain the information of both composition and morphology.(2) The comparative analysis of cathode substrate. Through the microanalysis to investigate the degree of oxidation of the tungsten powder after long-term storage in air and comparison of the tungsten powder before and after reduction with hydrogen, the results showed that oxygen content decreased significantly after the reduction, which means significant reduction occurs. According to the change of microstructure of tungsten powder after reduction, the decline of the cathode substrate adhesion was explained. Through the surface micro-structure comparative analysis between home and the Russia cathode, the best way to prepare perfects substrate is found, the results show that choosing the 5-5.5um tungsten powder before reduction can prepare the cathode substrate most similar to Russia production in the porosity and microstructure.(3) Aimed at the situations of life exhaust with no toxic gas in the tube and the failure phenomenon after the on-off test of cathode, by using microscopic technology to analyze the cathode, the results show the life end of the cathode showed the depletion of the active material in the microstructure and the holes of surface; the on-off process don't cause significant changes in microstructure of the cathode but the EDS analysis showed that the active substance was also a trend to surface diffusion.(4) Research on three cathode failure phenomenon by microanalysis. The results showed there were three toxic gas atmospheres in the tube which are water vapor, oxygen and anode plasma. The black spot existing on the cathode surface is the salting out phenomenon of the active material because of water vapor in the traveling wave tube. The drop down of the cathode emission properties results from oxygen in the tube that has no significant effect on the cathode surface structure but will cause composition change of active material. The formation of the cathode surface contaminants is mainly due to low vacuum in the tube. When cathode and positive pole are put on certain voltage, positive pole will bring ignition phenomenon, and positive material will turn towards cathode, which sputters to the cathode surface and destroys the film structure, and also brings black area into the cathode surface.(5) During the process of sintering for positive pole, there was phenomena of new white sediments appeared in the inner wall of horizontal annealing furnace .Using the SEM and EDS to analyzed the difference of microstructures and component between white sediments and fuscous sediments that had been in existence in the process of anneal in the inner wall of horizontal annealing furnace. The experimental results revealed the white sediments were not evident for deposited electricity and mainly composed of element C, N and O. The difference was due to volatile rudimental electroplating liquid in hot temperature that was not cleaned completely after prophase electroplating arts and crafts. According to the analysis we suggested that it would to be better to improve the cleaning technique, experiment results significantly avoided the phenomena.(6) Aimed at the phenomena of cathode emission properties lowering in use, the Ta leaf which is applied to heat shield tube and the black material in the copper positive pole surface are researched. The results show that the black material is the active material of cathode, and combined with cathode temperature, a large number of active materials evaporates because of the high cathode temperature. These results have important guiding significance in the choice of cathode raw materials, process optimization, failure analysis and so on, which would also play important roles in improving the life and reliability of the cathode.