KSEF RDE: Microstructure-Property Relations in Osmium-Ruthenium Coatings for Porous Tungsten Dispenser Cathodes

Abstract Dispenser cathodes serve as electron sources in numerous vacuum devices, including traveling wave tubes and cathode ray tubes. These devices find use in commercial, military and space applications, requiring a long and reliable operating lifetime, especially for space-based operation. Semicon Associates, in Lexington, KY, leads this market, producing over 20,000 dispenser cathodes annually. The cathodes comprise several materials, including platinum group metal coatings that add significant production cost. Surprisingly, there is almost no data on optimizing film microstructure for maximum cathode performance. Instead, osmium-ruthenium coatings have a standard thickness of 500 nm, which may not deliver optimal performance and may also waste some of this very expensive material. It is not understood how the microstructure of these precious metal films affects cathode performance. Fundamental understanding of microstructure-property relationships in the coating could improve device performance and allow more economical use of the precious metals. The objective of this research is to investigate the effects of microstructure, including film thickness, texture and grain size, on the performance of osmium-ruthenium coatings added to porous tungsten. Osmium-ruthenium films on porous tungsten substrates will be investigated systematically. Films will be deposited in a range of thicknesses using magnetron sputtering. Electrical biasing will be varied during sputtering, to control texture and density of as-deposited films. Grain size will also be varied, as this affects diffusion through the film. Electron emission will be measured, and changes in these properties will be explained in relation to microstructure. This work will be performed in close collaboration with Semicon Associates, who will provide materials and technological guidance. Success of this research project will benefit the Commonwealth economically by helping Semicon remain on the cutting edge of dispenser cathode fabrication, and will lead to additional grants for longer-term scientific studies of noble metal thin films.