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Thesis--Wisconsin. / Includes bibliographical references (leaves 76-77).

Thesis (Ph. D.)--West Virginia University, 2000. / Title from document title page. Document formatted into pages; contains xiv, 110 p. : ill. (some col.). Includes abstract. Includes bibliographical references.

No description available.

Fluorescence spectra and UV-Vis absorption spectra are collected on cerium oxide nanocrystalline particles. While CeO2 is the stable form of bulk cerium oxide, ceria nanoparticles exhibit a nonstoichiometric composition, CeO2-γ, due to the presence of oxygen vacancies and the formation of Ce2O3 at the grain boundaries. The Ce(III) ions, which are more reactive and therefore more desirable for various applications, are created by oxygen vacancies, which act as defects in the CeO2-γ crystal lattice. These defects form trap states in the band gap of CeO2, which can be seen in the absorption spectra. Ce(III) is required for fluorescence of the ceria nanoparticles while Ce(IV) is involved in only nonradiative transitions. The optical spectroscopy results show that the ceria samples have different ratios of Ce(III) ions to Ce(IV) ions, which is verified by x-ray photoemission spectroscopy (XPS). / Master of Science

Nanomaterials possess unusual chemical and physical properties than their bulk counterparts because of their large surface to volume ratio. This benefit has found applications in the fields of optics, electronics catalysis and biomedicine. Over the past two decades cerium oxide based materials have been extensively studied and used in applications such as glass and ceramics, phosphor/luminescence and in various catalysis and chemical applications. Nanocrystalline cerium oxide materials can benefit not only these applications, but they also possess some unique properties such as blue shift in ultraviolet absorption spectra, shifting and broadening of Raman allowed modes and lattice expansion. Unfortunately, the high specific surface area of the nanocrystalline particles also results in a stronger tendency of the particles to agglomerate. The problem of agglomeration is of prime concern for the particles smaller than 5nm and the beneficial effects of the nanosized particles are usually lost due to the agglomeration problem. Therefore synthesis of non-agglomerated nanocrystalline cerium oxide particles is highly important in improvement of properties for various applications. The present study investigates the use of microemulsion for synthesis of monodispersed, non-agglomerated nanocrystalline cerium oxide particles. Sodium bis(2-ethylhexyl sulphosuccinate (AOT) was used as a surfactant in the microemulsion system used in this study. It was found that the use of hydrogen peroxide as a precipitating agent gives a very stable sol of cerium oxide containing nanocrystalline particles of 3nm in size. The particle morphology and chemical state study was done for these particles and it was found that cerium oxide nanoparticles consist of both Ce(+3) and Ce(+4) valence states while the micron sized cerium oxide particles consist of only Ce(+4) valence state. Different applications of the synthesized cerium oxide nanoparticles were also studied. The beneficial effects of the synthesized nanocrystalline ceria to improve the high temperature oxidation resistance of stainless steel were investigated using oxidation kinetics measurements. For comparing the size effect on the improvement, comparative coatings of 10 μm and 20nm-sized cerium oxide were also studied. It was found that the 3nm-sized ceria gave the best results in improving high temperature oxidation resistance of stainless steel even in cyclic heating conditions. It results in a fine grained scale morphology with improved scale adhesion to the substrate and changed the scale growth mechanism from cation outward to oxygen inward. The role of Ce(+3) valence state in nanocrystalline cerium oxide particles to improve the oxidation resistance is proposed and discussed. Another application of the synthesized nanocrystalline ceria was found in improving lifespan of in vitro cell cultures in collaboration with Molecular Biology and Microbiology Department. Although this is the not main part of this thesis, however, it is worth mentioning that cerium oxide nanoparticles prolonged brain cell longevity by 2-3 fold. Further, these nanoparticles reduced hydrogen peroxide and UV light induced cell injury by over 60%. It is hypothesized that the cerium oxide nanoparticles act as free radical scavengers due to their unique structure, with respect to valence and oxygen defects, to promote cell longevity. Thus nanotechnology plays a vital role at the interface of materials science and molecular and microbiology.

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In order to understand better the defect structure and dynamics associated with lower valent dopants complexed with native defects in group IV oxides, In/Cd perturbed angular correlation spectroscopy was performed in ceria and zirconia. Examining the orientation symmetry axis of defects in ceria single crystals at low temperature has allowed the identification of a cadmium with a bound near-neighbor oxygen-vacancy complex as well as a complex involving a cadmium with two opposing, near-neighbor oxygen vacancies. The orientation of the symmetry axis of a third complex is reported; however, this information is not sufficient to identify it. Complementing these low temperature studies, the dynamics of the cadmium/oxygen-vacancy interaction in zirconia at high temperatures was studied. The motion of the oxygen vacancy at high temperatures results in a damping of the PAC signal. This damping is not well characterized by the heuristic Marshall-Meares PAC fitting function, and a model is proposed to fit the data in terms of three physical parameters associated with the vacancy's motion. These parameters are the rate at which a bound oxygen vacancy hops among equivalent sites about the probe, the rate at which a bound vacancy detraps, and the rate at which a vacancy is trapped by cadmium. Fits of individual spectra using this model give respective activation energies of 0.3-0.6 eV, 0.9-1.6 eV, and 0.4-0.6 eV. The uncertainty in these energies can most likely be reduced by fitting spectra from multiple temperatures simultaneously. Despite the large uncertainty in the fitted energies, the values are physically reasonable and indicate that the model adequately describes the motion of the oxygen vacancy about cadmium. / Graduation date: 1997

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references. Also available in electronic version.