Degradation of coated impregnated cathode´s surface coating

The author investigates the quantitative temperature dependence on the surface coating degradation of M-type impregnated cathodes. Osmium-ruthenium coated M-type cathodes were continuously heated at four different temperatures for 2000-20000 h in vacuum apparatus with an Auger electron spectrometer (AES). The cathode surface concentration of tungsten increases with time by diffusion from tungsten base metal. The diffusion equation is solved with the boundary condition of the insulated surface and a theoretical equation is derived which represents the tungsten surface concentration. The data calculated from this equation are in very good agreement with the measured data. The activation energy obtained for a tungsten diffusion coefficient in the surface coating (osmium-ruthenium-tungsten) is 8.4 eV. It was found that the surface coating was stable during over 100000 h at cathode temperature of 1050°C_B