Thesis (Ph. D.)--University of North Texas, Aug., 2007. / Title from title page display. Includes bibliographical references.

Nanostructured noble metals exhibit novel physical, mechanical and chemical behavior, and hold promise for applications such as gas sensing and electron emission. A strong emphasis was placed on the processing and characterization of these materials, in the form of nanoporous or nanocrystalline thin films. Palladium-based and osmium-ruthenium alloys were investigated in this dissertation research and will be presented as follows: (1) Preparation and Characterization of Nanoporous Metal Thin Films (2) Characterization of Osmium-Ruthenium Coatings Nanoporous palladium (np-Pd) thin films were prepared by dealloying co-sputtered palladium-nickel precursor alloys. Nanoporous structures were created with 3-D interconnected ligaments and open pores. Size of ligaments and pores was ~5 nm, achieved with a novel processing method developed in this study. Hydrogen cycling tests performed with np-Pd films demonstrated a significant improvement in sensitivity to hydrogen and response time for sensing. Effects of alloying element (Ni), film thickness, local stress and pore/ligament size on hydrogen cycling behavior were investigated in detail. Additionally, nanoporous gold and gold-palladium thin films were studied to clarify the evolution of microstructure during dealloying, including the formation of nanoporous structure and effects of substrate curvature on dealloying behavior. The results from this project have yielded a new understanding of dealloying as well as an ideal coating material for hydrogen sensing. Nanocrystalline osmium-ruthenium (Os-Ru) thin films were deposited on porous tungsten substrates with varied sputtering parameters. These parameters were mapped to microstructure, film texture and film composition in samples that were comparable to commercial devices. Using this map, Os-Ru films can be produced with higher stability during annealing and/or high-temperature operation. These results should lead to Os-Ru top coatings that increase the lifetime and emission performance of dispenser cathodes.

Thesis (Ph.D.) - James Cook University, 2005. / Journal publications by the author contained on CD-ROM. Typescript (photocopy) Includes bibliographical references.

Thesis (M. Phil.)--University of Hong Kong, 1992.

Dispenser cathodes emit electrons through thermionic emission and are a critical component of space-based and telecommunication devices. The emission of electrons is enhanced when coated with a refractory metal such as osmium (Os), osmium-ruthenium (Os-Ru), or iridium (Ir). In this work the microstructure, thermionic emission, and work function of thin film Os-Ru coatings were studied in order to relate microstructural properties and thermionic emission. Os-Ru thin film coatings were prepared through magnetron sputtering and substrate biasing to produce films with an array of preferred orientations, or texture. The effect of texture on thermionic emission was studied in detail through closely-spaced diode testing, SEM imaging, and x-ray diffraction. Results indicated that there was a strong correlation with emission behavior and specific preferred orientations. An ultra-high vacuum compatible Kelvin Probe was used to measure the work function of W-Os-Ru ternary alloy films to determine the effect W interdiffusion has on work function. The results indicated that a high work function alloy coating corresponded to low work function cathodes, as expected. It was inferred that a high work function alloy coating results in a low work function cathode because it aligns more closely with ionization energy of Ba. The results also proved that this method of evaluating dispenser cathode coatings can distinguish small variations in microstructure and composition and may be a beneficial tool in the development of improved dispenser cathode coatings. A novel experimental apparatus was constructed to measure the work function of dispenser cathode coatings in-vacuo using the ultra-high vacuum Kelvin Probe. The apparatus is capable of activating cathodes at high temperature and measuring the work function at elevated temperature. The design of this apparatus allows for more rapid evaluation of dispenser cathode coatings.