To accommodate new uses, the cathodes in travelling wave tubes need to operate at higher frequencies and output powers; thus they have to function at higher temperatures and current densities. The life of coated tungsten cathodes is limited by tungsten diffusion; to prevent this the cathodes operate at 950°CB (brightness temperature).
Therefore the degradation of a porous tungsten cathode impregnated with barium calcium aluminate and coated with an osmium-ruthenium film has been studied (N. Mita, IEEE Trans. Electron Devices, 1991, 38, (11), 2554). During tests tungsten diffused into the surface, converting it to osmium-ruthenium-tungsten. The activation energy for the tungsten diffusion coefficient was 8.4 eV, giving a stable period for the coating of over 100,000 hours at a cathode temperature of 1050°CB, and the coating could be operated at a current density of 30 A/cm2.