Escaping the Refractory Limit: Enabling new VED operational paradigms through mechanism-based re-engineering of thermionic cathodeemitting materials

A fundamental research effort integrating synthesis, characterization, testing, modeling and design will be used to develop fundamental understanding to guide the engineering of enhanced thermionic cathodes. Thin film deposition techniques (primarily sputtering, but possibly including powder processes) will be used to fabricate materials for cathode coatings. Initial efforts will sample the composition and phase space available in Sc-containing W alloys, examining the role of crystal structure, microstructure, grain size and distribution, grain boundaries, and surface structure. Emitter materials will be characterized for structure, composition, and thermionic properties. In parallel, atomistic calculations will be used to determine atomic and electronic structure of synthesized materials, focusing on alloying, crystal orientation, crystal size, and interface/surface effects. State-of-the-art quantum mechanical techniques will be used to directly predict thermionic emission on the basis of structure and composition. Computational and experimental results will be combined to reveal structure-processing-performance relationships governing synthesized materials (e.g., how composition impacts grain size and shape, and how grain size and shape influence thermionic emission), and the fundamental mechanisms connecting structure and composition to emitted current densities. New empirical multi-scale models linking atomic and electronic structure to observed emitted current densities will be derived and parameterized based on identified mechanisms. Experimental and modeling results will inform fabrication of full cathodes (~2/year) and electron guns (~1/year). Testing of cathodes and electron guns will validate emission/performance models and, combined with experimental and computational materials characterization results, allow confirmation of mechanisms controlling cathode performance. These results will be used to first design manufacturing processes that yield reliable cathodes with performance equivalent to that claimed in lab-scale tests of Sc-containing ("scandate") cathodes and reported in the scientific literature, and subsequently to allow computational screening for and identification of completely new "beyond scandate" materials.