Study And Application Of Mo-La₂O₃-Sc₂O₃ Cathode

The Mo-La2O3 cathode has been systematically studied to substitute for ordinary Th-W cathode with the problems of radioactive pollution and elevated temperature in operation. The precursor results show the 6T51-type electronic tube equipped with La-Mo cathode suffers from good emission stability when conditioned by work voltage of 4.5V. This paper aims to develop a new type cathode in order to make a series of rare-earth Molybdenum cathode applied in tubes.The preparations of La-Sc-Mo cathode material are studied at first. The Mo powder doped with La2O3 and Sc2O3 was pressed in a steel die and sintered by middle-frequency sintering technology. The sintered bars were swaged and drawn to wires whose diameters meet the requirements of cathode wires for electronic tubes. The drawn wires occur from excellent mechanical properties.According to the size and shape of La-Mo cathode in the 6T51 tube conditioned by work voltage of 4.5 V, the cathode wires for tubes conditioned by work voltage of 6.3 V is designed. In this paper, three kinds of 6T51 tubes are prepared. The tests for these tubes show that the cathodes are affected with the anode. So, the exhausting technology of the tubes is adjusted. The 6T51 tubes prepared with the optimal technology conditioned by work voltage of 6.3V suffer from good emission properties and are made a tryout in the industry.This paper makes an effort to apply the La-Sc-Mo cathode into TM928 tube for understanding more about the new cathode. Through doing many experiments, the carbonizing process of TM928 equipped with the La-Sc-Mo cathode is obtained. The work voltage of the tube is considered as 4.5V after testing the tubes. The test results also show that the properties of TM928 tube equipped with the La-Sc-Mo cathode reached the requirements of the standard TM928 tube.In this paper, the microstructures and components of the carbonized La-Sc-Mo cathode are analyzed through SEM with EDXA. The carbonized layer of the cathode well carbonized is composed coarse Mo2C particles with lots of grain boundary crevices and mini-holes arranging perpendicular to the wire axle and up to surface.The micrometer La2o3 and Sc2O3 particles existed at the grain boundary crevices and mini-holes in the carbonized layer. With the prolongation of the emission time, the grain of the cathode surface becomes big, and the boundary and the crevices decrease. It is also founded that the active matters decrease more in the carbonized layer, while keeping steadiness in the core molybdenum of the wire.According to the investigation of microstructure and the experiments of tubes, the emission behavior of the La-Sc-Mo cathode is discussed. The active matters firstly are produced in the carbonized layer, then diffuse along the boundary to the surface, and form the thin-film at the surface. At the same time, the active matters evaporate from the surface.The experiment results show that the La-Sc-Mo cathode suffers from good electron emission properties. The reasons may be as the following two sides. The La-Sc atoms produced by the solution of Sc into La overlay on the surface, which reduces the work function of Mo. The decreases of the work function make more electrons emitting from Mo. On the other hand, The LaScC3 particles lie in the Mo grains together with LOs, or assemble at the boundary. The structure like above makes more defects congregate around La2O3 particles, which is beneficial to the diffusion of La2O3. It will keep more long balance between the evaporation and diffusion of the active matters.