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The dynamics of oxygen vacancies in zirconia : an analysis Of PAC dataAlves, Mauro A. 13 March 2003 (has links)
Nuclear techniques such as perturbed angular correlation (PAC) sample the
hyperfine interactions of a large number of probe atoms in specific crystallographic
sites. Real crystals contain static defects producing a distribution
of electric field gradients (EFGs) that add to the ideal EFG of the crystal at
any given probe site. Also, dynamic defects like moving vacancies and interstitial
atoms can be present in the crystal and contribute to the distribution
of EFGs. The distribution of EFGs leads to line broadening and a change in
the observed asymmetry parameter η since the total EFG no longer has the
symmetry of the perfect crystal. When both defects are present in a material,
obtaining quantitative information from the analysis of PAC spectra is usually very difficult since great care has to be taken to ensure that the source
of line broadening is identified correctly. In order to relate the relationship
between the static line broadening and changes in the asymmetry parameter
η, a uniform random distribution of point charges was used to simulate the
static defect EFG. PAC spectra collected on cubic niobium metal, cubic stabilized
zirconia and Nb-doped tetragonal zirconia were fitted with this model.
Although the quality of the fits is good, more work is needed to clarify the
relationship between the new model parameters and the line broadening and
asymmetry parameter derived from conventional model fits. The PAC spectra
of Nb-doped tetragonal zirconia were fitted with a conventional static model
to establish a reliable relationship between line broadening and the asymmetry
parameter when only static defects are present in a sample. To account for effects
of dynamic defects, a four state stochastic model for vacancy motion was
adapted in order to include the line broadening and changes in the asymmetry
produced by static defects. As a result, the activation energies corresponding
to the rates at which a oxygen vacancy is trapped by, detraps from, and hops
among equivalent sites about a PAC probe atom were calculated. The values
that were found are physically reasonable, indicating that the dynamics of an
oxygen vacancy around a PAC probe atom are satisfactorily described. / Graduation date: 2003
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Doping and Defect Structure of Mixed-conducting Ceramics for Gas SeparationZuo, Chendong 21 November 2006 (has links)
My main objective is to gain a firm understanding of the correlation between the defect chemistry and the properties of Ba-based perovskite structure proton-conducting ceramics, especially B-site doped BaCeO3, so as to allow the engineering of these compounds with the desired properties for the application in devices; develop membranes of mixed protonic-electronic conductors suitable for hydrogen separation from gas mixtures; and further advance hydrogen separation technology by gaining fundamental understanding about electrochemical separation mechanism.
BZCY proton conductors with various compositions have been synthesized and characterized. The absence of low-angle supercell reflections indicates a random B-site cation distribution. The substitution of Zr led to a decrease in cell volume and an enhanced structural stability against reactions with CO2. The total conductivity for BZCY pellets increased with temperature increased and decreased as the zirconium content increased at each fixed temperature. Dense Ni-BZCY composite membranes have been successfully fabricated for evaluating hydrogen permeability and stability. Doping Zirconium in the B-site only slightly reduced the hydrogen permeation at high temperatures, but dramatically increased the chemical stability in CO2- and H2O-containing gases. Among the compositions studied, the Ni-BZCY7 exhibited both highest H2 permeation rate and good chemistry stability, thus having potential for practical applications.
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