Thesis (Ph. D.)--Washington State University, May 2007. / Includes bibliographical references (p. 130-136).

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La zone de transition est située entre 410 et 660 km de profondeur dans le manteau terrestre. Bien qu'il s'agisse d'une zone assez petite en volume du manteau terrestre, son rôle peut être important pour déterminer le mode, la vigueur et l'échelle de la convection globale, par exemple par le devenir de la subduction des plaques lithospheriques. Cette convection résulte de la déformation plastique des minéraux, qui elle-même résulte du mouvement des défauts cristallins. Parmi ces défauts, les dislocations sont souvent considérées comme les agents les plus efficaces de la plasticité intracristalline. C'est pourquoi nous proposons d'étudier les mouvements des dislocations dans les principales phases de la zone de transition: la wadsleyite et la ringwoodite. Par une approche de modélisation numérique, nous avons déterminé la mobilité thermiquement activée du glissement des dislocations que l’on trouve dans les polymorphes haute-pression de l'olivine. A partir de l'échelle atomique, nous avons modélisé les propriétés de coeur des dislocations. La déformation plastique est formulée en rendant compte de la dépendance intrinsèque du taux de déformation sur la mobilité des dislocations. Pour mieux comprendre les mécanismes de fluage dans la wadsleyite et la ringwoodite en conditions naturelles, on a utilisé les résultats précédemment définis sur le glissement que l’on a combiné avec la mobilité des dislocations en montée. Les résultats montrent que le glissement des dislocations est inefficace par rapport au fluage par montée dans des minéraux majeurs de la zone de transition. Cela suggère l'importance potentielle du fluage par montée, ce qui rendrait la zone de transition rhéologiquement distincte du manteau supérieur. / The transition zone is the region in the Earth's mantle between 410 and 660 km depth that separates the upper from the lower mantle. In spite of its small volume, it may play a role in constraining the style, vigour and scale of global mantle convection through, for instance, the fate of subducting slabs. Mantle convection is governed by plastic flow that occurs through the motion of crystal defects. Line defects or dislocations are considered to be one of the most efficient defects contributing to intracrystalline deformation. That is why in this work, we concentrate on the motion of dislocations in relation to the major phases of the mantle transition zone: wadsleyite and ringwoodite. A theoretical mineral physics approach is used to model the thermally activated glide motion of dislocations at appropriate pressure conditions in both high-pressure polymorphs of olivine. The intrinsic properties of dislocation core structures are modelled and parametrized by atomic scale calculations to take into account the effect of pressure on atomic bonding. Plastic deformation is finally described by taking into account the instrinsic strain rate dependence on the mobility of the defects.Since plastic deformation by the motion of dislocations is associated with creep, we use the above results and a climb mobility law to address the effective creep process in wadsleyite and ringwoodite under natural conditions. We show the inefficiency of dislocation glide as a strain producing deformation mechanism and suggest the potential importance of pure climb creep in the main minerals constituting the Earth's transition zone. This would imply the mantle transition zone to be rheologically distinct from the upper mantle.

Test pieces of GaAs were cut from Czochralski grown <100> wafers. Prior to deformation the dislocation configuration was established by cathodoluminescence (CL). Etch pits produced by molten KOH on examined crystal surfaces coincided with the CL images. The test pieces were capped with Si₃N₄, heated to between 950 and 1050°C, and plastically deformed by bending. The dislocation configuration after bending was then compared to that of the undeformed crystal. It was observed that heating to 1050°C did not significantly change the as grown cellular dislocation arrays in the crystal. With strain the dislocation configuration changed appreciably. New bands of dislocations were formed, parallel to the bend axis with dislocation free regions between them. Increasing the strain increased the number of bands. Observations were made on undoped crystals with high and low dislocation densities, and Si doped crystals. The luminescent properties of the dislocations were observed to change with heating and strain. As grown, a dislocation imaged as a dark spot surrounded by a bright halo, giving bright dislocation networks. After heating to 950°C samples showed only the dark spots without halos. After deformation, all the new dislocations appeared as dark spots or lines without halos. At very low strains, the original dislocations were still evident but were distinct from the new arrays. In an attempt to correlate the dislocation images with impurity segregation some observations of the samples were made using secondary ion mass spectroscopy (SIMS). The results suggest the possibility of the dark areas in the CL images being associated with the presence of carbon. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The temperature fields and resulting stress fields have been calculated for a growing GaAs crystal produced by the LEC process. The calculations are based in a finite element numerical thermoelastic stress analysis. The calculated temperature fields have been compared to reported experimental measurements with good agreement. The stress fields have been used to calculate the resolved shear stresses, in the growing crystal, from which the dislocation density and distribution were determined. Using the model the effects of a range of growth and environmental parameters on the dislocation density and distribution were determined. Theses parameters include crystal length, crystal diameter, cone taper, boron oxide thickness, gas pressure, solid/liquid interface shape, vertical temperature gradients and others. The results show that the temperature distribution in the gas surrounding the crystal, and the boron oxide thickness, were critical factors in determining the dislocation density and distribution in the crystal. The crystal radius, crystal length and interface curvature also strongly influenced the dislocation configuration. After crystal growth, the dislocation density at the end of the crystal was strongly influenced by the cooling procedure adopted. The dislocation distribution on cross-sections of the crystal exhibited two-fold, four-fold and eight-fold symmetry depending on growth and cooling conditions and position in the crystal. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

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