GOALI: Understanding the Microstructural Evolution and High-Temperature Behavior of Osmium-Ruthenium Coatings for Dispenser Cathodes

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, is a leader in 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 understanding of how to tailor the 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. Fundamental understanding of microstructure-property relationships in the coating will lead to improved device performance and allow more economical use of precious metals. This project will help advance the dispenser cathode industry from an empirical field with lots of experience, to a field with fundamental understanding and the ability to tailor new coatings for improved performance. The research objective of this proposal is an improved understanding of osmium-ruthenium coatings that are subjected to high temperatures and undergo significant microstructural changes. Topics of fundamental scientific importance that will be explored here include the effects of thin film microstructure on the diffusion of refractory metals, as well as the effects of film texture on electron work function. In addition to the scientific goals of this project, there are several industrially relevant issues that will be explored, including how to achieve enhanced emission, longer lifetime and/or lower operating temperature. By systematically investigating the effects of microstructure (including film thickness, texture and grain size) on the compositional stability and emission properties of annealed and conditioned cathodes, the PI and co-PI will build a fundamental scientific, microstructure-based understanding of osmium-ruthenium film coatings. Equipped with an improved understanding of how microstructure evolves in osmium-ruthenium films and how microstructure affects electron emission, we will explore alternative coating microstructures that could lead to significantly improved performance while still being feasible from a production standpoint. This work will be performed in close collaboration with Semicon Associates, who will provide materials and technological guidance. The intellectual merit of this proposal lies in a strong fundamental research component that will address an issue that has not yet been investigated: what are the microstructure-property relationships in osmium-ruthenium coatings used in dispenser cathodes? The knowledge gained in this project will be directly applied to the design of improved thin film coatings for Semicon's products, and will also uncover important microstructural details relevant to coatings intended for demanding environments (vacuum, high temperature, high current density). This project will establish the knowledge needed for a fundamental shift in how cathodes are made and mitigate the effects of skyrocketing material costs.