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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

High-field transport in semiconducting material and devices

Ahmad, Nisar January 1990 (has links)
No description available.
2

Electronic and optical properties of perfect and imperfect II-VI superlattices

Beavis, A. W. January 1990 (has links)
No description available.
3

Turbulences et ondes en milieu naturel stratifié : deux études de cas : étude du mélange turbulent et des ondes internes du lac Pavin (Auvergne, France) ; influence des ondes de Rossby sur la concentration en chlorophylle de surface dans l'upwelling du Pérou / Turbulence and waves in natural stratified environment

Bonhomme, Céline 15 February 2008 (has links)
Dans les milieux aquatiques, les facteurs liés à l’hydrodynamique et au mélange aux échelles de la turbulence sont déterminants pour le transport des substances chimiques et des micro-organismes. Ce travail s’intéresse à ces phénomènes physiques et se situe à la frontière entre la limnologie physique et l’océanographie physique, au travers de l’étude de deux cas : le lac Pavin (Auvergne, France) et la région d’upwelling au large du Pérou. Un des intérêts de ce travail réside dans le changement d’échelle de l’observation de milieux naturels aquatiques allant de l’échelle millimétrique (étude de la microstructure dans le Lac Pavin) à l’échelle de la centaine de kilomètres (étude des ondes de Rossby). Le Lac Pavin est le seul lac méromictique français, ce qui signifie que le compartiment inférieur du lac ne se mélange pas annuellement. La méromicticité concerne pourtant bien d’autres bassins, plus connus et plus vastes comme la Mer Noire, la Mer Caspienne, le lac Baïkal, d’où son importance écologique. Sa méromicticité lui confère des propriétés physiques particulières, étudiées dans le cadre de cette thèse. L’étude de la variabilité du mélange diapycnal à différentes échelles de temps et d’espace dans le lac Pavin au travers de mesures de microstructure et de surveillance en continu des principaux paramètres physiques a conduit à mettre en évidence la variabilité de la stabilité de la colonne d’eau à l’échelle interannuelle. Par ailleurs, les propagations d’ondes internes de gravité sont étudiées dans ce cadre particulier à l’aide d’un modèle linéaire bidimensionnel. Elles génèrent des mouvements advectifs verticaux importants dans toute la colonne d’eau dont il faut tenir compte dans les protocoles d’échantillonnage chimiques ou biologiques. Enfin, une intrusion sous lacustre semble avoir une grande importance dans la détermination de la profondeur du mélange hivernal et le maintien de la méromicticité. La zone de l’upwelling du Pérou est parmi les plus poissonneuses au monde. Elle est caractérisée par une forte productivité primaire, observable par satellite. L’observation des ondes de Rossby a permis de mettre en évidence qu’elles génèrent plus de 50% de la variabilité observée à l’échelle intra saisonnière sur la concentration en chlorophylle de surface, à distance des côtes. Les ondes transportent la chlorophylle vers le large et participent à la détermination de l’extension de la zone de forte productivité primaire. A travers des mesures variées allant de la microstructure à l’observation satellite et grâce à l’utilisation de modèles conceptuels, la thèse met en évidence l’influence certaine des facteurs hydrodynamiques sur l’organisation des paramètres biogéochimiques de ces deux écosystèmes / In aquatic environments, hydrodynamics and turbulent mixing are key parameters which determine the transport of chemical substances and microorganisms. This work focuses on these phenomena and comes within the scope of today’s physical limnology and oceanography issues as seen through two case studies: Lake Pavin (Auvergne, France) and the upwelling region off Peru. One major interest of this work is the change in observation scales of the aquatic environment ranging from millimetre (microstructure investigations) to kilometre scales (Rossby wave study). Lake Pavin is the only meromictic lake in France, which means that the bottom layer of the lake does not undergo seasonal mixing. Meromixis concerns other wider basins than Lake Pavin, such as the Black Sea, the Caspian Sea and Lake Baïkal for example, and can be of great ecological importance. Meromixis creates particular physical conditions in Lake Pavin, that are studied in the framework of this PhD. In particular, this work highlights diapycnal mixing and water column stability variability at different spatial and time scales in Lake Pavin using microstructure and continuous measurements. Moreover, internal gravity wave propagations in the water column are studied in the particular framework of meromixis by using a two dimensional model. These waves generate significant vertical advective water movements, which must be taken into account for sampling protocols in the lake. At last, a sublacustrine intrusion seems to have great importance in the determination of mixing depth during seasonal overturn and in the maintaining of meromixis. The Peru upwelling zone is one of the most fish aboundant ocean region in the world. It is characterized by strong primary productivity, which can be detected by satellite observations. The satellite observation of Rossby waves shows that they influence more than 50% of the intra-annual sea surface chlorophyll concentration variability at a few hundreds of kilometres from the shore. Waves transport chlorophyll offshore and contribute to the extension of the high productivity zone. Through various measurement techniques and satellite observations, and using conceptual models, this work shows the influence of hydrodynamic factors on the organisation of biogeochemical parameters in these two ecosystems
4

The optical properties of InAs/GaInSb superlattices in the infrared

Jenner, Christopher January 1998 (has links)
No description available.
5

Etude expérimentale et modélisation des microstructures de déformation plastique intragranulaires discrètes / Experimental study and modelling of the intra granular deformation microstructure

Perrin, Camille 29 October 2010 (has links)
L’amélioration des techniques de caractérisation (EBSD, MET, AFM) permet actuellement une meilleure compréhension des mécanismes plastiques intra-granulaires pour des poly-cristallin déformé. Les observations expérimentales montrent que les processus de plastification sont fortement hétérogènes et intermittent à l’intérieur des grains. Les modèles micromécaniques à champs moyens développés ces dernières décennies ne considèrent pas proprement les hétérogénéités intra-granulaires du glissement plastique. Or il est aujourd’hui démontré (simulations de Dynamique des Dislocations Discrètes par exemple) que la prise en compte de l’auto-organisation des dislocations à l’intérieur des grains est fondamentale pour mieux comprendre et expliquer les effets de taille de grains sur le comportement mécanique des polycristaux. Dans cette étude, deux approches complémentaires ont été développées : Une approche théorique qui consiste aux calculs des champs élastiques (contrainte interne et rotation de réseau) dus à une distribution discrète de boucles de dislocations contraintes par le joint de grains, et une approche expérimentale dont le but est de caractériser quantitativement les longueurs caractéristiques (espacements inter-bandes, et niveau de plasticification dans les bandes) pour des polycristaux à plusieurs tailles de grains se déformant plastiquement et de mesurer les rotations de réseau locales associées (mesure EBSD de désorientation de réseau cristallin) en vue de les comparer au champs de rotations élastiques calculés par le modèle. Le modèle a également été étendu pour permettre l’étude à des microstructures plus complexes, comme par exemple, les cellules de dislocations / The improvement of the materials characterization techniques in the last years has given access to new important information about the microstructure of polycrystalline metals. From experimental studies of deformed polycrystals, plastic strain within grains is known to be strongly heterogeneous and intermittent. As a consequence of the collective motion of dislocations, sample surfaces are indeed characterized by the presence of slip lines and slip bands (as slip traces). In the present study, a new micromechanical approach is developed to derive the mechanical fields (stresses, distortion, lattice curvature, elastic energy) arising from the presence of an inelastic strain field representing a typical internal "microstructure" as the one observed during the plastification of metallic polycrystals. This "microstructure" is due to the formation of discrete (spatial-temporal) intra-granular plastic slip heterogeneities which are modelled using discrete distributions of circular glide dislocation loops for a grain embedded in an infinite elastic matrix. Then, field equations have been solved using the method of Fourier Transforms. In contrast with the mean field approach based on the Eshelby formalism, it is then found that stress and lattice curvature fields are not more uniform inside the grain. A grain boundary layer actually appears where strong gradients occur and whose thickness depends on the introduced internal lengths. These results are compared with experimental measurements of local lattice rotation fields obtained by orientation imaging mapping (OIM). The model is able to capture different behaviours between near grain boundary regions and grain interior. The model was also develop to allow the study of more complex microstructures like the dislocation cells
6

Design of the Light Guide Plate with Stepped Micro Structures

Lien, Yen-Po 26 January 2008 (has links)
In this thesis, we utilized CCFL as the sidelight of backlight module. At the same time, we designed many um-size and 45 degree stepped microstructures on the bottom of LGP to reach the goal of uniform plane light due to its diffraction effect. Then mostly incident light can be induced to the ejected surface by once of total reflection. This way can avoid light energy being wasted, which always happens in traditional LGP design because of several times of diffraction and reflection occurred in front of light being ejected out of LGP. Moreover, stepped microstructures LGP combines the functions of reflector, diffuser, and prism film, so it can lower material cost. Relating to geometrical optics, we simulated our design with TracePro software to verify the effect we expect; relating to wave motion theory, we verified it by Definite Difference Method and programming with Matlab language. At last, we made samples to verify the above simulations and theories.
7

Titaniq Thermobarometry of Fabric Development in the Strafford Dome, Vermont: Linking Microstructures to Orogenic Processes

Ashley, Kyle T. 15 July 2011 (has links)
Abstract Geochemical, microstructural and petrological analyses were conducted on metapelites from the Strafford Dome, Vermont. Samples record metamorphic conditions from biotite to peak kyanite/staurolite grade and preserve microstructures related to two Acadian nappe emplacement events. The purpose of this study was to test the validity and application of the Ti-in-quartz (“TitaniQ”) thermobarometer to constraining pressure-temperature-timing-deformation (P-T-t-D) paths in metamorphic tectonites. Due to the nearly ubiquitous presence of quartz in continental rocks, the ability to apply this method would have significant implications for improving our ability to resolve tectonic histories. Cathodoluminescence (CL) imaging on quartz was conducted to qualitatively assess the distribution of Ti in a single grain and/or compare neighboring crystals. X-ray mapping of garnet porphyroblasts was conducted to estimate P-T conditions during garnet growth to provide a framework for included quartz grains. P-T-X contour diagrams (used in P-T calculations for garnet growth) were constructed from data obtained by X-ray fluorescence analysis on bulk-rock chemistries. Secondary ion mass spectrometry analysis was conducted to constrain Ti concentrations in quartz due to the low [Ti] present in the Strafford samples (<10 ppm). Analysis of the samples revealed [Ti] in zoned quartz grains that can be grouped and associated with certain P-T-D conditions. A majority of quartz grains have dark cores in CL images with low [Ti] (~2.5–3.5 ppm) in both matrix quartz and inclusions. Quartz inclusions in garnets that grew syn-tectonically with D2 have bright rims ~5.5 ppm. Matrix quartz, on the other hand, has rims with much higher [Ti] (~7.5–9.5 ppm). Comparing these Ti concentrations to summary P-T paths from previous studies suggests: quartz inclusions have rims recrystallized during the end of D1 deformation, matrix grains have rims re-equilibrated at peak P-T conditions post-D2 deformation, and the dark cores observed in CL images must be from early prograde or relics of the protolith. The evaluation of the TitaniQ thermobarometer’s application to constrain P-T-t-D histories has highlighted some potential problems and significant benefits. To use the thermobarometer, either T or P must be independently constrained, which is often difficult to do given the many microstructural contexts of quartz in a single sample. This study capitalized on the ability to determine the relative timing of quartz (re)crystallization relative to garnet growth. Using another trace element thermobarometer would be ideal (e.g. Zr-in-rutile) for greater precision, although the relevant accessory phases may not be present and constraining the timing of re-equilibration is challenging. The abundance of quartz in continental rocks and the various microstructural occurrences of quartz in a single metamorphic tectonite provides additional opportunities to constrain points on the P-T-D path than conventional thermobarometers. The TitaniQ thermobarometer has the potential to yield deeper insights into the tectonic history of crustal rocks than previously available. These findings further elucidate the potential of the method for use in studies of metamorphic tectonites, continental tectonics and rheology.
8

Optimised part programs for excimer laser-ablation micromachining directly from 3D CAD models

Mutapcic, Emir, n/a January 2006 (has links)
Fabrication of a 3D structure and surface texture using excimer laser mask projection ablation processes typically requires the machine operator to develop a specific NC part program for the desired structure geometry, and also incorporate appropriate machine parameters to achieve the desired surface finish. The capability of the laser ablation process could therefore be significantly improved by developing a CAD/CAM system that automatically generates the NC part program using the 3D information of the CAD model of the desired structure. Accordingly, the focus of this research was to develop such a system that is, an effective CAD/CAM system specifically for excimer laser mask projection micromachining tools. To meet these requirements, a unique combination of commercially available systems was used to develop the new CAD/CAM system. The systems used comprised of a computer aided, feature based parametric design system (SolidWorks), together with its extended programming capabilities based on Automated Programming Interface (API) functions for Windows applications, and Visual Basic (VB) 6.0 programming utilities. The system's algorithms use a novel methodology to extract the 3D geometry of a microstructure. Two different techniques have been developed to extract the 3D data. First, where 3D geometry information from a CAD model was defined as a Stereolithography (STL) file, and second, where this information has been contained in a set of bit-map (BMP) files that represent a sliced or layered structure of a CAD model. Based on this, first an algorithm to create NC part programs to support Step-and-repeat micromachining technique was developed and then successfully extended to be applicable for another commonly used micromachining method, Workpiece-Dragging technique. The systems algorithms for both techniques are based on the raster-colour programming technique, resulting in substantially reduced mathematical complexity and computational time. This is the first time this approach has been used to support direct conversion of 3D geometry from a CAD model into an NC part program compatible with the excimer laser CNC controller. 2D mathematical models for controlling edge and stitching errors were also implemented in the system. An additional technique, named as 'Common Nest' has been developed with the aim to enable automatic NC part programming when microstructure design to be completed successfully, requires use of multiple complex mask patterns as a projection tool instead of just a single square aperture. The effectiveness of the system was verified by NC part program generation for several 3D microstructures and subsequent machining trials using polycarbonate (PC) and Polyethylene terephthalate (PET), and optimised processing parameters. Excellent agreement was obtained between the laser machined geometries and the microstructure CAD models. The Laser Scanning Confocal Microscope (LSCM) measured the lateral dimensions tolerance of 2m. The system was also successfully applied for a practical micro-engineering application, for the development of a microfluidics cell transportation device.
9

Turbulences et ondes en milieu naturel stratifié : deux études de cas : étude du mélange turbulent et des ondes internes du lac Pavin (Auvergne, France) ; influence des ondes de Rossby sur la concentration en chlorophylle de surface dans l'upwelling du Pérou

Bonhomme, Céline 15 February 2008 (has links) (PDF)
Dans les milieux aquatiques, les facteurs liés à l'hydrodynamique et au mélange aux échelles de la turbulence sont déterminants pour le transport des substances chimiques et des micro-organismes. Ce travail s'intéresse à ces phénomènes physiques et se situe à la frontière entre la limnologie physique et l'océanographie physique, au travers de l'étude de deux cas : le lac Pavin (Auvergne, France) et la région d'upwelling au large du Pérou. Un des intérêts de ce travail réside dans le changement d'échelle de l'observation de milieux naturels aquatiques allant de l'échelle millimétrique (étude de la microstructure dans le Lac Pavin) à l'échelle de la centaine de kilomètres (étude des ondes de Rossby). Le Lac Pavin est le seul lac méromictique français, ce qui signifie que le compartiment inférieur du lac ne se mélange pas annuellement. La méromicticité concerne pourtant bien d'autres bassins, plus connus et plus vastes comme la Mer Noire, la Mer Caspienne, le lac Baïkal, d'où son importance écologique. Sa méromicticité lui confère des propriétés physiques particulières, étudiées dans le cadre de cette thèse. L'étude de la variabilité du mélange diapycnal à différentes échelles de temps et d'espace dans le lac Pavin au travers de mesures de microstructure et de surveillance en continu des principaux paramètres physiques a conduit à mettre en évidence la variabilité de la stabilité de la colonne d'eau à l'échelle interannuelle. Par ailleurs, les propagations d'ondes internes de gravité sont étudiées dans ce cadre particulier à l'aide d'un modèle linéaire bidimensionnel. Elles génèrent des mouvements advectifs verticaux importants dans toute la colonne d'eau dont il faut tenir compte dans les protocoles d'échantillonnage chimiques ou biologiques. Enfin, une intrusion sous lacustre semble avoir une grande importance dans la détermination de la profondeur du mélange hivernal et le maintien de la méromicticité. La zone de l'upwelling du Pérou est parmi les plus poissonneuses au monde. Elle est caractérisée par une forte productivité primaire, observable par satellite. L'observation des ondes de Rossby a permis de mettre en évidence qu'elles génèrent plus de 50% de la variabilité observée à l'échelle intra saisonnière sur la concentration en chlorophylle de surface, à distance des côtes. Les ondes transportent la chlorophylle vers le large et participent à la détermination de l'extension de la zone de forte productivité primaire. A travers des mesures variées allant de la microstructure à l'observation satellite et grâce à l'utilisation de modèles conceptuels, la thèse met en évidence l'influence certaine des facteurs hydrodynamiques sur l'organisation des paramètres biogéochimiques de ces deux écosystèmes
10

Light driven microactuators : design, fabrication, and mathematical modeling

Han, Li-Hsin 24 January 2011 (has links)
This dissertation is concerned with design, fabrication, and mathematical modeling of three different microactuators driven by light. Compared to electricity, electromagnetic wave is a wireless source of power. A distant light source can be delivered, absorbed, and converted to generate a driving force for a microactuator. The study of light-driven microsystems, still at its early stage, is already expanding the horizon for the research of microsystems. The microactuators of this dissertation include micro-cantilevers driven by pulsed laser, photo-deformable microshells coated with gold nanospheres, and a nano-particles coated micro-turbine driven by visible light. Experimental investigation and theoretical analysis of these microactuators showed interesting results. These microactuators were functioned based on cross-linked, multiple physics phenomenon, such as photo-heating, thermal expansion, photo-chemistry effect, plasomonics enhancement, and thermal convection in rarefied gas. These multiple physics effects dominate the function of a mechanical system, when the system size becomes small. The modeling results of the microactuators suggest that, to simulate a microscale mechanical system accurately, one has to take account the minimum dimension of the system and to consider the validity of a theoretical model. Examples of the building of different microstructures were shown to demonstrate the capacity of a digital-micromirror-device (DMD) based apparatus for three-dimensional, heterogeneous fabrication of polymeric microstructures. / text

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