Material analysis and characterization of cesium iodide (CsI) coated C fibers for field emission applications

Field emission cathodes consisting of cesium iodide (CsI) coated graphite fibers have been investigated as promising electron sources for high power microwave (HPM) devices. After conditioning for removing adsorbed water vapor, these cathodes have operated stably for more than one million pulses and release little or no plasma. One of the most useful properties of the Csl coatings appears to be a significant reduction in turn-on electric fields compared to un-coated graphite fiber cathodes. Computational investigations of the modification of the electronic properties of a C surface upon adsorption of both isolated and crystalline thin layers of Csl have been performed, indicating that these adsorbed layers are capable of significantly improving the emission properties of the C fibers by forming surface dipole layers . These layers are predicted to alter the electrostatic potential barrier for electron emission of the surface and lower the overall work function of the system from ~4.5 eV (pure graphite) down to ~1.2 - 1.4 eV. In order to further correlate the microscopic properties of the material system with the computational results obtained and the macroscopic performance characteristics of these cathodes, a series of materials characterizations are being carried out on the cathodes. These studies will facilitate the determination of both bulk and surface elemental composition of the cathodes, provide the value of the surface work function, determine the micro-morphological characteristics of the cathode's surface as well as the micro-structural characteristics of the bulk, and identify the role of any interface regions on the emission properties of the cathode. A more complete understanding of the mechanism(s) responsible for the superior emission characteristics of Csl coated C fibers and the nano-physics of the system will be obtained.