No description available.

No description available.

Crystals with a hexagonal close-packed crystal structure are inherently anisotropic, and have a limited number of independent slip systems, which lead to strong deformation textures and reduced formability in polycrystalline products. In magnesium (Mg), all of the easy slip systems have a Burgers vector in the <a> direction making twinning necessary for arbitrary shape changes. The most common twinning system which allows extension along the c-axis is {10-12}<10-11>. A good predictor of slip is the global Schmid factor, which resolves the externally applied force onto the slip plane and direction of a crystal. The critically resolved shear stress (CRSS) at which a grain twins is not readily measured by experiment and the CRSS for twin initiation often appears larger than for twin propagation. In polycrystals, twin variants with both low and high Schmid factors have been observed indicating that this Schmid factor is inappropriate to predict twinning and more local effects play an important, though still uncertain role. In this work, experiments were devised to dynamically study extension twinning in a polycrystalline Mg alloy AZ31B with a strong basal rolling texture by tensile deformation parallel to the plate normal. Three-dimensional X-ray diffraction using a synchrotron source was used to map the centre-of-mass positions, orientations, and grain-resolved elastic strain tensors of over 1000 grains in-situ up to a true strain of 1.4%. The majority of twins formed in grains with a high local Schmid factor; however, low-ranked twin variants were common. The average grain-resolved stress did not always select the highest twin variant and resulted in some negative Schmid factors. The internal stress state of parent grains and twinned grains did not differ significantly within the large measurement uncertainties. The misorientations between grains ideally oriented for twinning and their nearest neighbours could not explain cases of no twin activity. Results suggest that the controlling factors for twin formation are much more local and not captured within the spatial resolution of the technique. Complimentary measurement of the strain rate sensitivity during twinning, by instantaneous strain rate change tests, suggest that basal slip is a part of this local process. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-03-12 13:40:26.039

In the past sixty years, X-ray, neutron and electron dffraction have emerged as the structural techniques of choice in the solid state. However, despite many advances in theory and instrumentation, these diffraction methods are still reliant on a number of assumptions. Chief amongst these is that the atoms in the crystal vibrate in a harmonic fashion. This thesis is concerned with understanding the effects of anharmonic motion on crystal structure determination and developing new ways of moving beyond the harmonic approximation used in crystallography. A method has been developed, using molecular dynamics simulations, to correct experimental structures to equilibrium structures. This has been applied to the crystal structures of phase-I deutero-ammonia, deutero-nitromethane and benzophenone. Path-integral molecular dynamics simulations have been used to obtain meaningful comparison with experimental data collected at low temperatures. The simulations also offer information on the probability density functions that describe thermal motion in solids. Using data from simulations of nitromethane and other compounds it has been demonstrated that the molecular dynamics-derived data can be used to assess and develop new functions for modelling thermal motion in crystal structure refinements. Finally, similar molecular dynamics techniques have been applied to determine the equilibrium structures of some polyhedral oligomeric silsesquioxanes in the gas phase. Some members of this class of compounds feature such strong anharmonic motion that refinement of the structures using gas electron diffraction is impossible without taking into account the effects of the anharmonicity.

A state-of-the-art gas electron diffraction (GED) apparatus has been reassembled in the school of chemistry at the University of Edinburgh. This combines molecularbeam and telefocus-electron-gun technologies and the alignment of the electron beam produced by the latter has been discussed. A new custom-made CCD detector has also been installed and electron diffraction patterns for a few small molecules have been recorded. In analogy to the rotating sector in a conventional GED apparatus, the new camera contains an optical filter and a procedure for its calibration is outlined and followed step by step to produce an estimate of the filter transmittance. The data have been shown to be of less than ideal quality and the probable root of the problem is discussed. GED refinements of two pairs of compounds (arachno-6,9-decaboranes, and a covalent sulfonate and thiosulfonate) are presented, using data collected with the conventional Edinburgh GED apparatus.

by Lee Yau-choi. / Title in Chinese: / Includes bibliographical references / Thesis (M.Ph.)--Chinese University of Hong Kong, 1986

Le multiferroїque BiFeO3 est l'un des matériaux ferroïques les plus étudiés à ce jour du fait de la coexistence à température ambiante d'un état ferroélectrique et antiferromagnétique. Il présente de plus une réponse photovoltaïque dont l'origine précise n'est actuellement pas comprise. Le but principal de cette thèse est donc d'étudier les propriétés photovoltaïques de films épitaxiés BiFeO3. Préalablement à l'investigation des propriétés photovoltaïques une étude des mécanismes de conduction a été entreprise. Un transport polaronique de saut via les défauts les plus proches voisins a été mis en évidence et une transition est observée à 253K. En dessous de cette transition les états associés aux défauts proches du niveau de Fermi contribuent principalement et une longueur de saut variable émerge. Cette observation semble être corrélée à la réponse photovoltaïque avec un changement de régime de la tension photo-induite autour de 253K. Cette réponse photovoltaïque est provoquée par l'état ferroélectrique et peut être basculée à l'aide d'un champ électrique. Afin de reproduire artificiellement l'état en domaines associé à l'effet photovoltaïque de BiFeO3 des super-réseaux BiFeO3/SrRuO3 ont été fabriqués et une étude préliminaire de la structure a été entreprise. Nous observons un changement structural d'une phase rhomboédrique vers une phase pseudo quadratique dans ces super-réseaux de période variable et attribuons cette transition à l'influence prépondérante des contraintes élastiques induites dans le plan / The multiferroic BiFeO3 is one of the most studied material because of the room temperature coexisting ferroelectric and antiferromagnetic state. It also shows a photovoltaic response not yet understood. The main objective of this thesis is therefore to investigate the the photovoltaic properties of epitaxial BiFeO3 thin films. Preliminary to photovoltaic studies an investigation of the conduction mechanism has been performed. A polaronic transport with next nearest hopping mechanism is evidenced with a change of regime below 253K. Below 253K variable range hopping transport is observed and involves defects states near the Fermi level. This transport behavior seems connected to the photovoltaic response and change observed at 253K in the photo-induced voltage. Interestingly the photovoltaic response is induced by the ferroelectric state and we demonstrate a switchable photovoltaic effect by an applied electric field. In order to artificially reproduce the domain structure involved in the photovoltaic effect in BiFeO3 BiFeO3/SrRuO3 superlattices have been fabricated and a preliminary structural investigation is presented. A structural change is evidenced from a rhombohedral structure to pseudo-tetragonal state in the superlattices with variable periodicities and we attribute this transition to the influence of the induced in-plane elastic strain

Les éléments de diffraction sont largement utilisés aujourd'hui dans un nombre grandissant d'applications grâce à la progression des technologies de microstructuration dans le sillage de la micro-électronique. Pour un design optimal de ces éléments, des méthodes de modélisation précises sont nécessaires. Plusieurs méthodes ont été développées et sont utilisées avec succès pour des réseaux de diffraction unidimensionnel de différents types. Cependant, les méthodes existantes pour les réseaux deux dimensionnel ne couvrent pas tous types de structures possibles. En particulier, le calcul de l'efficacité de diffraction sur les réseaux métalliques à deux dimensionnel avec parois verticales représente encore une grosse difficulté pour les méthodes existantes. Le présent travail a pour objectif le développement d'une méthode exacte de calcul de l'efficacité de diffraction de tels réseaux qui puisse servir de référence. La méthode modale développée ici - dénommée ,,true-mode" en anglais - exprime le champ électromagnétique sur la base des vrais modes électromagnétiques satisfaisant les conditions limites de la structure 2D à la différence d'une méthode modale où les modes sont ceux d'une structure approchée obtenue, par exemple, par développement de Fourier. L'identification et la représentation de ces vrais modes à deux dimensions restait à faire et ce n'est pas le moindre des résultats du présent travail que d'y avoir conduit. Les expressions pour la construction du champ sont données avec des exemples de résultats concrets. Sont aussi fournies les équations pour le calcul des intégrales de recouvrement et des éléments de la matrice de diffusion / Diffractive elements are widely used in many applications now as the microstructuring technologies are making fast progresses in the wake of microelectronics. For the optimization of these elements accurate modeling methods are needed. There exists well-developed and widely used methods for one-dimensional diffraction gratings of different types. However, the methods available for solving two-dimensional periodic structures do not cover all possible grating types. The development of a method to calculate the diffraction efficiency of two dimensional metallic gratings represents the objective of this work. The one-dimensional true-mode method is based on the representation of the field inside the periodic element as a superposition of particular solutions, each one of them satisfying exactly the boundary conditions. In the developed method for the two-dimensional gratings the representation of the field within the grating in such way is used. In the present work, the existing modal methods for one-dimensional gratings can be used as the basis for the construction of the modal field distribution functions within two-dimensional gratings. The modal function distributions allow to calculate the overlap integrals of the fields outside the grating with those within the structure. The transition matrix coefficients are formed on the basis of these integrals. The final stage is the calculation of the scattering matrix based on two transition matrices. The equations for the field reconstruction are provided and accompanied by examples of results. Further equations used to calculate the overlap integrals and scattering matrix coefficients are provided

Abstract not available

This paper briefly describes the theory and methods of x-ray residual stress measurements. Residual stresses can be defined as the stresses which remain in a material in the absence of any external forces. There are many stress determination methods. Some of those methods are destructive and some are nondestructive. X-ray residual stress measurement is considered as a nondestructive method. X-ray diffraction together with the other diffraction techniques of residual stress measurement uses the distance between crystallographic planes as a strain gage. The deformations cause changes in the spacing of the lattice planes from their stress free value to a new value that corresponds to the magnitude of the residual stress. Because of Poisson’s ratio effect, if a tensile stress is applied, the lattice spacing will increase for planes perpendicular to the stress direction, and decrease for planes parallel to the stress direction. This new spacing will be the same in any similarly oriented planes, with respect to the applied stress. Therefore the method can only be applied to crystalline, polycrystalline and semi-crystalline materials. The diffraction angle, 2&#952;, is measured experimentally and then the lattice spacing is calculated from the diffraction angle, and the known x-ray wavelength using Bragg's Law. Once the d-spacing values are known, they can be plotted versus 2 sin &#968;, ( &#968; is the tilt angle). In this paper, stress measurement of the samples that exhibit a linear behavior as in the case of a homogenous isotropic sample in a biaxial stress state is included. The plot of d vs. 2 sin &#968; is a straight line which slope is proportional to stress. On the other hand, the second set of samples showed oscillatory d vs. 2 sin &#968; behavior. The oscillatory behavior indicates the presence of inhomogeneous stress distribution. In this case the xray elastic constants must be used instead of E and &#957; values. These constants can be obtained from the literature for a given material and reflection combination. It is also possible to obtain these values experimentally. Calculation of the residual stresses for these samples is beyond the scope of this paper and will not be discussed here.