Review of porous tungsten matrix materials for diffusion cathodes
In microwave vacuum electronic devices, cathode as an electron emission source, its performance directly affects the performance and life of the device, which is referred to as the "heart" of vacuum electronic devices. The impregnation diffusion cathode is the most widely used hot cathode at present. The porous tungsten matrix as the framework of the cathode has the following functions: (1) the pores store a large amount of active emission materials, providing a huge contact surface for the reaction; (2) the connected holes provide a migration channel for the reactive barium formed by the reaction; (3) the porous tungsten surface and the active barium form a surface electron emission system. Therefore, it is of great significance to study the cathode porous tungsten matrix material with high current, high reliability, long life and high performance to meet the requirements of high power, high efficiency, high frequency and wide band microwave source, so as to promote the development of satellite communication and high power microwave devices.
How to improve the emission performance, reliability and life of cathode materials has been the focus of cathode material researchers. A lot of experimental work has been carried out in the aspects of material composition, structural characteristics and preparation technology. In this paper, the preparation and characterization methods of porous tungsten matrix materials for diffusion cathodes are reviewed systematically in order to provide material support and process guidance for manufacturing high-performance diffusion cathodes.
The design of tungsten matrix material requires that the characteristics of tungsten matrix material include pore size, pore size distribution and tungsten skeleton strength, which have a significant impact on the emission current of cathode, evaporation dissipation and diffusion compensation of emission active materials, anti poisoning and ion bombardment resistance. Therefore, the design has the following requirements: (1) a certain porosity. The pore size of porous tungsten matrix is too large, and the strength is insufficient; if the hole is too small, the quality of the stored emitter is not much, which affects the cathode life. If the space long-life application, the porosity is generally required to be 16%~20%. (2) The pore size should be suitable and the distribution of holes should be uniform. When the pore size is fixed, the pore size is large, and the distribution of the holes is not easy to be uniform; if the pore size is small, it is easy to obtain the uniformly distributed holes, but it is difficult to fill in the emitting material. (3) The material is not filled because the hole is closed. (4) Suitable orifice diameter. When the cathode is too large, the evaporation of the emission material will be large, which will affect the cathode life; if the cathode is too small, it will not only be difficult to fill the emitting material, but also affect the area of electron emission, which is not conducive to improving the apparent emission current density. For example, the orifice diameter is generally required to be 0.8 ~ 1.5 um for space long-life applications.
