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A cathode structure was demonstrated that utilizes aligned carbon nanotubes (CNTs) to improve the thermionic electron emission by increasing the field enhancement of the cathode surface. Aligned CNTs were grown on the surface of a tungsten substrate by plasma enhanced chemical vapor deposition. The tungsten-CNT structure was further coated with a thin film of low work function emissive materials by magnetron sputtering. Numerous cathodes with varying CNT morphology and oxide layer thickness were created. The field and thermionic emission of the cathodes were tested in order to study the effects of the surface properties on the emission characteristics. It was observed that the introduction of CNTs into an oxide cathode structure improves both the thermionic and field emission, even in cathodes with relatively low field enhancement factors. Because of the high field enhancement factors that are available for CNTs, there remains a potential for dramatically improved electron emission. / Department of Physics and Astronomy

We have developed a thermionic cathode capable of high emission currents. The structure of this cathode is oxide coated carbon nanotubes (CNTs) on a tungsten (W) substrate. This cathode was superior in emission due to the combination of the field enhancement effect from the CNTs and the lowered work function from the semiconducting oxide surface. Such oxide coated CNTs were excellent electron emitters. Conventional electron emission theories, such as Richardson's and Fowler-Nordheim's, did not accurately describe the field enhanced thermionic emission from such emitters. A unified electron emission theory was adopted and numerical simulations were performed to explain the deviation of electron emission from conventional field and thermionic emission theories. Also, the thermionic measurement system and measurement methods were improved in order to measure and characterize the strong electron emission from this new cathode. Large electron emission current from such structures also made a new thermionic cooling device a possibility. Cooling due to the electron emission was measured in terms of temperature drop, and a large temperature drop was observed from this cathode structure. Finally, applications of this cathode in plasma discharge devices were explored. This new cathode was tested in a plasma environment and initial results were obtained. / Department of Physics and Astronomy