No description available.

No description available.

Thesis (M.S.)--University of Michigan, 2002.

Thesis (M.Sc. (Physics)) -- University of Limpopo, 2015 / While the effect of sulphur on c-ZrO2 is often considered in application of advanced solid oxide fuel cells and biomass gasification cleanup, there has been little study on the effect of sulphur on general structure of c-ZrO2. In this work a study of the structural, energetic, electronic and elastic properties of doped c-ZrO2-xSx, t-ZrO2xSx and m-ZrO2-xSx solid solutions has been carried out using ab-initio total energy calculation of the density functional theory under plane wave pseudopotential method within generalized gradient approximation using the self-consistent virtual crystal approximation (VCA). It has been shown that all the calculated properties obtained after relaxation are in good agreement with available experimental and other calculated values, particularly at x=0. Furthermore, the formation and cohesive energies were calculated to determine the relative stability of all three non-sulphated and sulphated polymorphs of ZrO2. The density of states and band structures have been computed for x = 0.0 - 0.5, and the actual size of the band gap of ZrO2 compounds narrowed with partial replacement of oxygen by sulphur, while peaks above Fermi level move towards the Fermi level. The material changes its insulating properties to semiconductor material as a function of sulphur concentration, which might be useful for potential application. We also investigated and calculated, for the first time, the effect composition variation on mechanical stability, the independent elastic constants and other elastic parameters of the sulphated compounds. The polycrystalline bulk moduli, shear moduli, Young and Poisson’s ratio have been deduced by using Voight-Reuss-Hill (VRH) approximation. In addition we also show the geometric and electronic structure of pure ZrOS and ZrS2 and compare them with the obtained geometric and electronic structures of ZrO2-xSx.

In order to achieve a better understanding of the processes that occur during formation and sintering of zirconia, various chemical and physical techniques were used. Along with Perturbation Angular Correlation spectroscopy, that allowed us to investigate microscopic properties inside the nanometer-size zirconia grains, such techniques as Scanning Electron Microscopy and X-ray diffraction were used for determining the size of particles before and after sintering, and Neutron Activation Analysis was employed for measuring the impurity levels in zirconia powders. By controlling the initial conditions and heat treatment of the powders, we investigated the dependence of formation of the charged defects on the existing molecular structure and morphology of zirconia particles. During the study, it was discovered that at low temperature the PAC frequencies of tetragonal zirconia behave very similarly for all materials that were used in this study. If stabilization is achieved by heavy doping, there are shifts and line-broadening due to the presence of dopants but no obvious differences in the essential physics. One material included in this group is Nb-doped zirconia that has no oxygen vacancies. It was concluded that there are no detectable oxygen vacancies in our pure or lightly doped tetragonal zirconia powders before they are heated into the temperature region where sintering occurs. Vacancies are incorporated as the samples are heated above 1050��C, the temperature at which sintering becomes important. The oxygen vacancies in samples that have been heated to 1200��C remain when cooled. We see no vacancy concentration dependence on the atmosphere for samples not doped with +5 valent elements in order to reduce the vacancy density at 1200��C. In several instances, samples that had been heated to a maximum temperature of 1050��C or 1100��C contained a vacancy density that was small (<100 ppm) but measurable. A reduced oxygen pressure increased the oxygen vacancy density by a measurable amount in these samples. Samples that are tetragonal at 800��C are well-sintered after being heated to 1200��C. Samples that are monoclinic below 1170��C are very poorly sintered at 1200��C and contain few vacancies. Flowing Cl in the system as the samples are sintering retards the densification of the grains. These samples had the smallest density of oxygen vacancies. / Graduation date: 1998

Molecular dynamics (MD) simulations were used in order to investigate structure and mechanical properties of zirconium and zirconium hydride. Calculation of temperature dependent failure of zirconium, diffusion of hydrogen in zirconium, properties of interfaces in zirconium and zirconium hydride and effect of hydrogen on crack nucleation and propagation were in good agreement with available experimental data. These are the first computer simulations where large-scale atomic/molecular massively parallel simulator (LAMMPS) code was used with the Embedded Atom Method (EAM) and Modified Embedded Atom Method (MEAM) to study structure and mechanical properties of zirconium hydrogen system (Zr-H) and zirconium hydride (ZrH2). Verification of methods was done in order to establish the best potential for zirconium and zirconium hydride. EAM and MEAM potentials successfully predicted lattice parameters, mechanical properties and variation of lattice parameters with temperature for α-Zr. MEAM potential was used to predict correctly the face centered structure for ZrH2 and also its mechanical properties. Temperature dependent stress-strain curves were calculated in order to predict yielding point for α-Zr. Results indicate early yielding and failure with increase of temperature in zirconium on application of tensile and compressive strains. Anisotropic stress variation with temperature in α-Zr was calculated. Hydrogen ingress through diffusion of hydrogen in zirconium is a mechanism responsible for formation of hydrides. Temperature-dependent hydrogen diffusion and activation energy for diffusion was calculated and the agreement with experiments was satisfactory. Anisotropy of diffusion of hydrogen is observed for Zr crystal. Hydrogen diffusion was also modeled under tensile and compressive strain and a possible formation of hydrides in the direction perpendicular to applied strain was observed. The effect of strain on orientation of hydride was investigated. Hydride {111} oriented crystal was strained along [1 1 ̅ 0] and [111] direction. Energy as a function of strain is calculated along both directions [111] and [1 1 ̅ 0] and it was found that energy of the system increase with increase in strain along [1 1 ̅ 0] and decrease with increase of strain along [111] direction. Calculated stress and strain curves indicate lower stresses along [111] direction and this causing the hydride to reorient in a direction perpendicular to applied strain. Structure of the interface (0 0 0 1) α-Zr // {1 1 1} δ-ZrH2 is modeled in order to investigate the crack initiation at this interface. Interfacial cracking of hydride under stress is observed. This observation is in good agreement with available experimental studies. Cracks are seen to nucleate earlier at higher temperature. Cracks and voids are common defects in zirconium fuel cladding. A crack is modeled along (0 0 0 1) plane of zirconium with hydrogen. In the presence of hydrogen cracks nucleate in zirconium causing fracture. This observation is in good agreement with previous experimental studies. Bonds surrounding atoms and stress concentration analysis were performed using OVITO and VMD software’s respectively. Weaker bonds and higher stress concentrations are observed in the presence of hydrogen in zirconium. The presented results clearly demonstrate that MD simulation can be used to predict structure and processes that are important for understanding failure in Zr based nuclear materials.

The cubic fluorite-type solid solution of ZrO₂ containing 15 mole % CaO has been prepared by the hot-pressing process. The effects of annealing on the change.of lattice parameter, electrical properties, and density of the solid solution have been investigated. The lattice parameter of the cubic solid solution was found to depend on the heat treatment of the specimens. The decrease of lattice parameter with annealing temperature and time has been interpreted either in terms of the removal of interstitial oxygen ions from the." lattice or in terms of the inhomogeneous distribution of the CaO in the ZrO₂ lattice. The activation energy for conduction was also found to depend on the heat treatment of the specimens. The variation of activation energy with annealing temperature has been interpreted in terms of pairing and clustering of the oxygen vacancies with the substitutional Ca ions in the solid solution. The minimum activation energy obtained in the present investigation corresponded to the theoretically predicted activation energy for the migration of oxygen vacancies. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

<p> The diffusion coefficient of Co has been measured as a function of temperature in the body-centered-cubic phase of 99.99% zirconium. The diffusion rates are extraordinarily large, and follow the relation D = [3.26 ± 2.77] x 10^(-3) exp -(21,820 ± 960/RT) cm^2 /sec over the range 900 to 1600°C. </p> <p> The Arrhenius plot has a positive curvature above 1600°C. The influence of cobalt additions on the diffusion of 95 ZR and 60 Co in dilute Zr-Co alloys was studied from 0 -2 at.% cobalt at 933°C. A theoretical model was developed to relate the effect to the solute correlation factor. It is established that diffusion in these systems cannot be accounted for in terms of a vacancy mechanism alone. Other models are discussed. </p> / Thesis / Doctor of Philosophy (PhD)

Call number: LD2668 .T4 1963 M38 / Master of Science

Two HCP metals, titanium and zirconium, have been modelled using molecular dynamics and recently developed many-body potentials. These two metals have similar lattice parameters, c/a ratios, melting temperatures, elastic and dislocation properties and, more importantly, responses to radiation damage(Griffith 1988,1989 & 1991, Hood 1988 & 1993), but differ by nearly a factor of two in atomic mass, thereby allowing the direct investigation of the effect of mass on radiation damage in the HCP system. Using the MOLDY code, successfully modified for the HCP structure, these two models w re rigorously investigated with respect to their point defect properties, displacement threshold energy response, and cascade processes. A marked preference for interstitial sites within the basal plane was found, in accordance with previous static studies on HCP metals. The displacement threshold energy showed a complex dependence on orientation within the HCP structure, but at higher energies this effect was swamped by structural disruptions during cascade development. The effect of mass was exhibited as a proportional increase in the mean displacement threshold energy, which carries over into cascade generation. Cascade morphology was seen to undergo a transition at energies of -1 keV, associated with the onset of true cascade conditions. This transition was reflected most markedly in the relaxation time for the recombination phase beyond the cascade peak, and explanation is presented for the transition in terms of ballistic, energetic and temporal effects. The dissimilarities between the two models were found to be mainly attributable to the mass difference. The condition of the cascade core at the peak was seen to be close to that of a liquid, with some discrepancies which indicate a lack of true melting, and an absence of the vacancy clustering often associated with a molten cascade core. The approximation of liquid-like structure was supported by the isotropy of the cascade-induced atomic mixing, despite the preference for basal-plane movement in the solid state. In agreement with modelling of other metals, the defect production efficiency for true cascade conditions was well below the NRT estimate, and an empirical relationship between final Frenkel-pair numbers and PKA energy is presented. SIA clustering occurred to a similar extent in both models, and small clusters were highly mobile and confined to single <1120> rows in the basal planes. The implications of these findings for microstructural evolution are discussed, along with comparisons of the results with other systems.