The Mechanism Of Suppressing Grid Emission Using Carbon Or Hafnium Thin Film

In this dissertation, the effect that carbon and hafnium film have on the suppressing grid emission in pulsed-controlled grid traveling wave tubes has been studied, while the working mechanism of these films has been discussed. In our experiments, the carbon films and hafnium films were deposited on molybdenum substrates by dual ion beam sputtering and radio frequency magnetron sputtering technology, respectively, and the barium oxide layers were prepared by the chemical method to simulate the grid structure of traveling wave tubes. Then, the surface phase and compositional change in the grid was studied under high temperature by annealing from 900K to 1300K, which can be used in explaining the suppression mechanism of carbon and hafnium film. The DLC films can be prepared by dual ion beam sputtering deposition technology. After 973K annealing, the films changed into graphite structure from that of DLC. So it is believed that carbon film exists in the form of graphite under high grid temperature. Moreover, to investigate the interaction of carbon film and grid surface contamination (i.e. barium oxide), two types of samples were prepared as follows: BaO/Mo and BaO/C/Mo. By comparing the peak intensity of Ba3d5/2 of the XPS spectra under the different annealing temperatures, one believes that carbon has effect on the removal of barium oxide and the effect is dependent on the temperature. And the removal is caused by the chemical interaction of carbon and barium oxide under high temperature, which decreases the content of BaO and maintains the high surface work function. Therefore, the grid emission is suppressed effectively. Two types of samples were also prepared by radio frequency magnetron sputtering technology and the chemical method as follows: Hf/Mo and BaO/Hf/Mo. The SEM and XPS results show the great change in the state of hafnium with barium oxide caused by the chemical interaction of barium and hafnium, which breaks the structure of electron emission on the BaO/Mo surface and suppresses the grid emission.