Dissertations / Theses on the topic 'SiGe, Segregation'

Understanding thermal transport in nanoscale is important for developing nanostructured thermolelectric materials and for heat management in nanoelectronic devices. This dissertation is devoted to understand thermal transport through SiGe based superlattices. First, we systematically studied the cross-plane thermal conductivity of SiGe superlattices by varying the thickness of Si(Ge) spacers thickness. The observed additive character of thermal resistance of the SiGe nanodot/planar layers allows us to engineer the thermal conductivity by varying the interface distance down to ~1.5 nm. Si-Ge intermixing driven by Ge surface segregation is crucial for achieving highly diffusive phonon scattering at the interfaces. By comparing the thermal conductivity of nanodot Ge/Si superlattices with variable nanodot density and superlattices with only wetting layers, we find that the effect of nanodots is comparable with that produced by planar wetting layers. This is attributed to the shallow morphology and further flattening of SiGe nanodots during overgrowth with Si. Finally, the experiments show that the interface effect on phonon transport can be weakened and even eliminated by reducing the interface distance or by enhancing Si-Ge intermixing around the interfaces by post-growth annealing. The results presented in this dissertation are expected to be relevant to applications requiring optimization of thermal transport for heat management and for the development of thermoelectric materials and devices based on superlattice structures
Verständnis des thermischen Transport auf Nanoskala ist sowohl grundlegend für die Entwicklung nanostrukturierter Materialien, als auch für Temperaturkontrolle in nanoelektronischen Bauteilen. Diese Dissertation widmet sich der Erforschung des thermischen Transports durch SiGe basierenden Übergittern. Variationen, der Si(Ge) Schichtdicken, wurden zur systematischen Untersuchung der Normalkomponente zur Wachstumsrichtung der Wärmeleitfähigkeit, von SiGe Übergittern, genutzt. Die Beobachtung des additiven Charakters, des thermischen Widerstands, der SiGe Schichten, mit oder ohne Inselwachstum, ermöglicht die Erstellung von Strukturen mit bestimmter Wärmeleitfähigkeiten durch die Variation der Schichtdicken bis zu einer Minimaldistanz zweier Schichtübergänge von ~1.5nm. Die Ge Segregation führt zu einer Vermischung, von Si und Ge, welche eine essentielle Rolle zur diffusen Phononenstreuung spielt. Unsere Untersuchungen, von planaren Übergittern und Übergittern mit variabler Inseldichte, zeigen, dass Inseln und planare Schichten zu einer vergleichbaren Reduktion, der Wärmeleitfähigkeit, führen. Diese Beobachtung lässt sich, sowohl auf die flache Morphologie als auch die Abplattung der SiGe Inseln, aufgrund der Überwachsung mit Si, zurückführen. Die Experimente zeigen außerdem, dass sich der Barriereneffekt, der Schichtgrenzen, durch Reduktion der Schichtabstände und durch verstärkte Vermischung im Bereich der Schichtgrenzen, durch Erhitzung, eliminieren lässt. Die präsentierten Messungen sind sowohl, für die Entwicklung jener Bauteile, die eine Optimierung des thermischen Transports oder Temperaturmanagment erfordern, als auch von thermoelektrischen Matieralien und Bauteilen, basierend auf Übergittern, relevant

Understanding thermal transport in nanoscale is important for developing nanostructured thermolelectric materials and for heat management in nanoelectronic devices. This dissertation is devoted to understand thermal transport through SiGe based superlattices. First, we systematically studied the cross-plane thermal conductivity of SiGe superlattices by varying the thickness of Si(Ge) spacers thickness. The observed additive character of thermal resistance of the SiGe nanodot/planar layers allows us to engineer the thermal conductivity by varying the interface distance down to ~1.5 nm. Si-Ge intermixing driven by Ge surface segregation is crucial for achieving highly diffusive phonon scattering at the interfaces. By comparing the thermal conductivity of nanodot Ge/Si superlattices with variable nanodot density and superlattices with only wetting layers, we find that the effect of nanodots is comparable with that produced by planar wetting layers. This is attributed to the shallow morphology and further flattening of SiGe nanodots during overgrowth with Si. Finally, the experiments show that the interface effect on phonon transport can be weakened and even eliminated by reducing the interface distance or by enhancing Si-Ge intermixing around the interfaces by post-growth annealing. The results presented in this dissertation are expected to be relevant to applications requiring optimization of thermal transport for heat management and for the development of thermoelectric materials and devices based on superlattice structures.
Verständnis des thermischen Transport auf Nanoskala ist sowohl grundlegend für die Entwicklung nanostrukturierter Materialien, als auch für Temperaturkontrolle in nanoelektronischen Bauteilen. Diese Dissertation widmet sich der Erforschung des thermischen Transports durch SiGe basierenden Übergittern. Variationen, der Si(Ge) Schichtdicken, wurden zur systematischen Untersuchung der Normalkomponente zur Wachstumsrichtung der Wärmeleitfähigkeit, von SiGe Übergittern, genutzt. Die Beobachtung des additiven Charakters, des thermischen Widerstands, der SiGe Schichten, mit oder ohne Inselwachstum, ermöglicht die Erstellung von Strukturen mit bestimmter Wärmeleitfähigkeiten durch die Variation der Schichtdicken bis zu einer Minimaldistanz zweier Schichtübergänge von ~1.5nm. Die Ge Segregation führt zu einer Vermischung, von Si und Ge, welche eine essentielle Rolle zur diffusen Phononenstreuung spielt. Unsere Untersuchungen, von planaren Übergittern und Übergittern mit variabler Inseldichte, zeigen, dass Inseln und planare Schichten zu einer vergleichbaren Reduktion, der Wärmeleitfähigkeit, führen. Diese Beobachtung lässt sich, sowohl auf die flache Morphologie als auch die Abplattung der SiGe Inseln, aufgrund der Überwachsung mit Si, zurückführen. Die Experimente zeigen außerdem, dass sich der Barriereneffekt, der Schichtgrenzen, durch Reduktion der Schichtabstände und durch verstärkte Vermischung im Bereich der Schichtgrenzen, durch Erhitzung, eliminieren lässt. Die präsentierten Messungen sind sowohl, für die Entwicklung jener Bauteile, die eine Optimierung des thermischen Transports oder Temperaturmanagment erfordern, als auch von thermoelektrischen Matieralien und Bauteilen, basierend auf Übergittern, relevant.

Dense flows of grains are commonplace throughout natural and industrial environments, from snow-avalanches down the sides of mountains to flows of cereal down chutes as it is transported from one part of a factory to another. A ubiquitous feature in all of these flows is their ability to separate the different grain types when shaken, stirred, sheared or vibrated. Many flows are sheared through gravity and these flows are particularly efficient at segregating particles based on their size, with small particles percolating to the bottom of the flow and large particles collecting at the top. Within this mechanism, an asymmetry between the large and small particles has been observed, with small particles percolating downwards through many large particles at a faster rate than large particles rise upwards through many small particles. This alternative format thesis presents a revised continuum model for segregation of a bidisperse mixture that can account for this asymmetry. A general class of asymmetric segregation flux functions is introduced that gives rise to asymmetric velocities between the large and small grains. Exact solutions for segregation down an inclined chute, with homogenous and normally graded inflow conditions, show that the asymmetry can significantly enhance the distance for complete segregation. Experiments performed using a classical shear-box with refractive index matched scanning are able to quantify the asymmetry between large and small particles on both bulk and particle scales. The dynamics of a single small particle indicate that it not only falls down faster than a single large particle rises, but that it also exhibits a step-like motion compared to the smooth ascent of the large grain. This points towards an underlying asymmetry between the different sized constituents. The relationship between the segregation-time and the volume fraction of small grains is analysed, and solutions presented for the steady-state balance between segregation and diffusive remixing. These help to show the good agreement between the asymmetric model and experimental data. Segregation at the front of natural avalanches produces a recirculation zone, known as a `breaking size-segregation wave', in which large particles are initially segregated upwards, sheared towards the front of the flow, and overrun before being resegregated again. Solutions for the structure of this recirculation zone are derived using the asymmetric flux model, revealing a novel `lens-tail' structure. Critically, it is seen that a few large particles starting close to the bottom of the flow are swept a long way upstream and take a very long time to recirculate. The breaking size-segregation waves highlight the important interplay between segregation and the bulk velocity field. The properties of flowing monodisperse grains are explored through experiments on a cone that produce a beautiful radial fingering pattern. Equations developed in a conical coordinate system reproduce the measured linear relationship between fingering radius and initial flux, whilst also predicting the slowing and thinning dynamics of the flow. Overall, these results illustrate the complex nature of the granular rheology and provide perspectives for future modelling of segregation in dense granular flows.

Thesis: M.C.P., Massachusetts Institute of Technology, Department of Urban Studies and Planning, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 61-66).
The striking contrast between two Chicago neighborhoods Hyde Park and Englewood which exist side by side is a prime example of what Edward Soja calls socially produced geographies of institutionalized racial segregation and what David Harvey terms as territorial injustice. Hyde Park with about 26,705 mostly white residents is a thriving economic center that has realized gains in property values and commercial investment. Nearby Woodlawn and Englewood have experienced declining populations, lower densities, lower property values, and increased vacancies that border Hyde Park, creating clear spatial lines of uneven development. Englewood and Woodlawn have one asset that Hyde Park does not: two public transit rail lines, the Green Line and the Red Line. In this thesis, I ask the question, can rapid transit be used to challenge uneven development and segregation in low income communities? I argue that public transit does not create growth, it merely redistributes it, and without the necessary development preconditions, the maximized benefits of public rapid transit in segregated communities will be hindered by persistent racial and residential segregation. I provide evidence to support that unless there is an intentional effort in conjunction with the proposed Red Line Extension to minimize residential and economic segregation, the expected benefits of transit-oriented development and economic revitalization in Chicago's African-American neighborhoods will be greatly limited. Such a plan might include developing transit stations into economic anchors through public and private partnerships, creating a coalition of community partnerships to develop land use plans that respond to the needs of the neighborhood, and working to secure investment for rapid transit infrastructure alongside investment for economic development.
by Alaa Mukahhal.
M.C.P.

Durant ces dernières années, l’apparition de nouvelles architectures (FDSOI, FinFETs ou NW-FETs) et l’utilisation de nouveaux matériaux (notamment SiGe) ont permis de repousser les limites des performances des dispositifs MOS et de contourner l’effet canal court inhérent à la miniaturisation des composants. Cependant, pour toutes ces nouvelles architectures, la résistance de contact se dégrade au fil des nœuds technologiques. Celle-ci dépend fortement de deux paramètres physiques : la concentration de dopants actifs proches de la surface du semi-conducteur et de la hauteur de barrière Schottky du contact siliciuré. De multiples procédés avancés ont été proposé pour améliorer ces deux paramètres physiques (pré-amorphisation, recuit laser, ségrégation de dopants, etc…). Afin d’optimiser les conditions expérimentales de ces nouvelles techniques de fabrication, il est primordial de pouvoir caractériser avec fiabilité leur impact sur les deux grandeurs physiques citées. Dans le cadre de cette thèse, deux thématiques dédiées à l’étude de chacun des paramètres sont abordées, explicitant les méthodes de caractérisation développées ainsi que des exemples concrets d’applications. La première partie concerne l’étude de la concentration de dopants actifs proches de la surface du semi-conducteur. Dans cet axe, nous avons mis en place une méthode d’Effet Hall Différentiel (DHE). Cette technique combine gravures successives et mesures par effet Hall conventionnel afin d’obtenir le profil de concentration de dopants actifs en fonction de la profondeur. Nous avons développé et validé une méthode de gravure chimique et de mesure électrique pour des couches ultra-minces de SiGe et de Si dopées. Les profils de concentration générés ont une résolution en profondeur inférieure à 1 nm et ont permis d’étudier de façon approfondie dans les premiers nanomètres proches de la surface de couches fabriquées grâce à des techniques d’implantation et de recuit avancées comme par exemple, la croissance en phase solide activée par recuit laser. La deuxième partie porte sur la mesure de hauteurs de barrière Schottky pour des contacts siliciurés. Durant cette étude, nous avons transféré une technique se basant sur des diodes en tête bêche pour caractériser l’impact de la ségrégation de différentes espèces à l’interface siliciure/semi-conducteur sur la hauteur de barrière Schottky d’un contact en siliciure de platine. Cette méthode de mesure associée à des simulations physiques a permis d’une part, d’extrairer avec fiabilité des hauteurs de barrières avec une précision de 10meV et d’autre part, d’effectuer une sélection des meilleures conditions de ségrégation de dopants pour la réduction de la hauteur de barrière Schottky. Pour conclure, ce projet a rendu possible le développement de méthodes de caractérisation pour l’étude de matériaux utilisés en nanoélectronique. De plus, nous avons pu apporter des éclaircissements concernant l’impact de techniques d’implantation ionique alternatives sur des couches de Si et SiGe ultrafines, et ce, dans le but de réduire la résistance de contact entre siliciure et semi-conducteur dans le module source-drain de transistors ultimes
During the past few decades, the emergence of new architectures (FDSOI, FinFETs or NW-FETs) and the use of new materials (like silicon/germanium alloys) allowed to go further in MOS devices scaling by solving short channel effect issues. However, new architectures suffer from contact resistance degradation with size reduction. This resistance strongly depends on two parameters: the active dopant concentration close to the semi-conductor surface and the Schottky barrier height of the silicide contact. Many solutions have been proposed to improve both of these physical parameters: pre-amorphisation, laser annealing, dopant segregation and others. In order to optimize the experimental conditions of these fabrication techniques, it is mandatory to measure precisely and reliably their impact on cited parameters.Within the scope of this thesis, two parts are dedicated to each lever of the contact resistance, each time precising the developed characterization method and concrete application studies. The first part concerns the study of the active dopant concentration close to the semi-conductor surface. In this axis, we developed a Differential Hall Effet method (DHE) which can provide accurate depth profiles of active dopant concentration combining successive etching processes and conventional Hall Effect measurements. To do so, we validated layer chemical etching and precise electrical characterization method for doped Si and SiGe. Obtained generated profiles have a sub-1nm resolution and allowed to scan the first few nanometers of layers fabricated by advanced ion implantation and annealing techniques, like solid-phase epitaxy regrowth activated by laser annealing. In the second part, we focused on the measurement of Schottky barrier height of platinum silicide contact. We transferred a characterization method based on back-to-back diodes structure to measure platinum silicide contacts with different dopant segregation conditions. The electrical measurements were then fitted with physical models to extract Schottky barrier height with a precision of about 10meV. This combination between measurements and simulations allowed to point out the best ion implantation and annealing conditions for Schottky barrier height reduction.To conclude, thanks to this project, we developed highly sensitive characterization methods for nanoelectronics application. Moreover, we brought several clarifications on the impact of alternative ion implantation and annealing processes on Si and SiGe ultra-thin layers in the perspective of contact resistance reduction in FDSOI source-drain module

Thesis (M.A.)--Dept. of Economics. University of Missouri--Kansas City, 2007.
"A thesis in economics." Typescript. Advisor: Erik Olsen. Vita. Title from "catalog record" of the print edition Description based on contents viewed Dec. 18, 2007. Includes bibliographical references (leaves 142-148). Online version of the print edition.

Heterojunction bipolar transistors (HBT) based on SiGeC havebeen investigated. Two high-frequency architectures have beendesigned, fabricated and characterized. Different collectordesigns were applied either by using selective epitaxial growthdoped with phosphorous or by non-selective epitaxial growthdoped with arsenic. Both designs have a non-selectivelydeposited SiGeC base doped with boron and a poly-crystallineemitter doped with phosphorous.

Selective epitaxial growth of the collector layer has beendeveloped by using a reduced pressure chemical vapor deposition(RPCVD) technique. The incorporation of phosphorous and defectformation during selective deposition of these layers has beenstudied. A major problem of phosphorous-doping during selectiveepitaxy is segregation. Different methods, e.g. chemical orthermal oxidation, are shown to efficiently remove thesegregated dopants. Chemical-mechanical polishing (CMP) hasalso been used as an alternative to solve this problem. The CMPstep was successfully integrated in the HBT process flow.

Epitaxial growth of Si1-x-yGexCy layers for base layerapplications in bipolar transistors has been investigated indetail. The optimization of the growth parameters has beenperformed in order to incorporate carbon substitutionally inthe SiGe matrix without increasing the defect density in theepitaxial layers.

The thermal stability of npn SiGe-based heterojunctionstructures has been investigated. The influence of thediffusion of dopants in SiGe or in adjacent layers on thethermal stability of the structure has also been discussed.

SiGeC-based transistors with both non-selectively depositedcollector and selectively grown collector have been fabricatedand electrically characterized. The fabricated transistorsexhibit electrostatic current gain values in the range of 1000-2000. The cut-off frequency and maximum oscillation frequencyvary from 40-80 GHz and 15-30 GHz, respectively, depending onthe lateral design. The leakage current was investigated usinga selectively deposited collector design and possible causesfor leakage has been discussed. Solutions for decreasing thejunction leakage are proposed.

Key words:Silicon-Germanium-Carbon (SiGeC),Heterojunction bipolar transistor (HBT), chemical vapordeposition (CVD), selective epitaxy, non-selective epitaxy,collector design, high-frequency measurement, dopantsegregation, thermal stability.

Twin roll casting (TRC) offers a promising route for the economic production of Mg sheet, but unfortunately, it produces strip with coarse and non-uniform microstructures and severe centre line segregation. Recently, a novel magnesium strip casting process termed melt conditioned twin roll casting (MC-TRC) was developed that, compared with the conventional TRC process, emphasizes solidification control at the casting stage rather than hot rolling. This was achieved by melt conditioning under intensive forced convection prior to twin roll casting resulting in enhanced heterogeneous nucleation followed by equiaxed growth. In this study the development of TRC and MC-TRC processes and a microstructural comparison of the MC-TRC Mg-alloy strip with that of conventional TRC strip, have been investigated. Emphasis has been focused on the solidification behaviour of the intensively sheared liquid metal, and on the mechanisms for microstructural refinement and compositional uniformity in the MCTRC process. The results of the process development indicate that the MC-TRC process reduces considerably or eliminates defects such as the centre line segregation, voids and cracks at or near the strip surface that are always present in conventional TRC strip. The newly-designed homogenization treatment investigated for TRC and MC-TRC magnesium alloy strips was based on microstructural evolution obtained during heat treatment. The results of the MC-TRC strips showed a much faster recrystallization rate with finer recrystallized grains, which are due to more homogeneous and a finer grain size of the as-cast MC-TRC strips compared with the as-cast TRC strips. During down-stream processing, the effects of MC-TRC process on microstructural evolution of hot-rolled magnesium strips have been understood thoroughly by accurate control of the hot-rolling procedure during each step of strip thickness reduction. This study indicates that the MC-TRC strip requires fewer rolling steps when compared to TRC strip, thus offering reduced processing cost and carbon footprint. Mechanical properties at room temperature of MC-TRC as-cast and rolled sheets are much improved when compared with the conventional TRC as-cast and rolled sheets which can result in a higher quality of final components. The mechanical properties at elevated temperature shows for the first time that the higher elongation and lower yield strength of MC-TRC as-cast strips at a temperature close to its optimised hot-rolling temperature results in better ability for rolling and higher ductility of MC-TRC Mg strip compared with the TRC Mg strip.

La compréhension du tri granulométrique des particules est un enjeu majeur pour l’étude des évolutions morphologiques des rivières de montagne. La prédiction des flux de transport reste difficile avec des écarts de plusieurs ordres de grandeurs entre les valeurs prédites et mesurées. L’une des raisons principales de cette difficulté est la ségrégation, phénomène granulaire de tri granulométrique des particules constituant le lit sédimentaire. La ségrégation est donc un phénomène à l’échelle du grain ayant un impact à l’échelle morphologique.Cette thèse présente une étude numérique de la ségrégation en tant que phénomène granulaire dans le cas du charriage et son impact sur le transport sédimentaire. Un modèle aux éléments discrets (DEM) couplé à un modèle fluide turbulent unidimensionnel est utilisé. A l’instant initial, des petites particules sont déposées au dessus de particules plus grosses. Le fluide s’écoule par gravité et transporte les particules du lit sédimentaire. Cette configuration, proche d’un lit érodable, est caractérisée dans la profondeur par des profils exponentiellement décroissant de la vitesse particulaire, du taux de cisaillement et du nombre inertiel et présente une phénoménologie de ségrégation particulière. Les petites particules s’infiltrent en couche, sous forme d’onde progressive, dont la vitesse est contrôlée par le nombre inertiel en bas de la couche. On observe aussi que la vitesse de ségrégation est dépendante de la concentration locale en petites particules et du ratio de taille. Le problème de ségrégation est ensuite analysé à partir d’un modèle d’advection-diffusion. Avec un coefficient d’advection proportionnel au nombre inertiel, le modèle continu reproduit parfaitement la dynamique de la phase des petites particules. Enfin on démontre que pour reproduire l’onde progressive observée dans les simulations DEM, le coefficient de diffusion doit avoir la même dépendance avec le nombre inertiel que le coefficient d’advection.Très récemment, un nouveau modèle d’advection-diffusion a été proposé dans la littérature à partir de forces inter-particulaires, notamment une force de portance (ou force de ségrégation) et de traînée, apportant de nouvelles paramétrisations physiques aux coefficients d’advection et de diffusion. Ce nouveau modèle est analysé ici dans la configuration du charriage. La dépendance en nombre inertiel, observée dans les résultats DEM, peut être retrouvée à partir de ces nouvelles paramétrisations. Pour reproduire quantitativement les simulations DEM, de nouvelles dépendances en nombre inertiel et concentration en petites particules sont proposées pour la force de ségrégation et le coefficient de traînée.Enfin, l’impact de la ségrégation sur le transport sédimentaire est étudié en s’intéressant à la mobilité d’un lit bi-disperse déjà ségrégé. Les grosses particules sont placées au dessus des petites et on observe que, pour la même contrainte fluide et pour le même état granulaire de surface, le transport est plus élevé dans le cas bi-disperse que dans le cas mono-disperse. Pour la gamme de ratio de taille étudié (r<4), on montre que l’augmentation de mobilité n’est pas un effet de rugosité mais un effet rhéologique. À partir d’une analyse dans le cadre de la rhéologie mu(I), il est démontré que les petites particules en profondeur sont plus mobiles que les grosses particules, jouant le rôle d’un tapis roulant pour les grosses particules de surface et augmentant ainsi la mobilité globale du lit sédimentaire. Basé sur des arguments rhéologiques, un modèle simple de prédiction de l’augmentation du flux sédimentaire est proposé, reproduisant correctement les résultats DEM pour une large gamme de nombre de Shields et pour des ratio de taille inférieurs à 4. Les résultats du modèle sont exploités pour identifier quatre régimes de transport différent selon les mécanismes responsables de la mobilité des petites particules
Understanding particle size segregation is one of the great challenge in fluvial geomorphology. It is still notoriously difficult to predict sediment transport more accurately than within one order of magnitude. One of the main origin of this difficulty is particle size segregation, a granular process of particle sorting in the sediment bed. Size segregation is therefore a grain scale process impacting the morphological scale.This PhD presents a numerical study of size segregation as a granular process during bedload transport. A coupled fluid discrete element method (DEM) is used to study the infiltration of small particles in a large particle bed. This configuration, close to granular flows on erodible beds, is characterized by a particle velocity profile, a shear rate profile and an inertial number profile exponentially decreasing into the bed. It presents a particular segregation phenomenology with small particles infiltrating the bed as a travelling wave, the velocity being controlled by the inertial number at the bottom of the layer. The segregation velocity is observed dependent on the local small particle concentrations and on the size ratio. The segregation problem is also analyzed with an advection diffusion model. With advection and diffusion coefficients both proportional to the inertial number, the continuum model perfectly reproduces the dynamics observed in the DEM results.Very recently, a new segregation advection diffusion model has been derived based on particle scale forces, in particular a granular buoyancy force (or segregation force) and an inter-particle drag force. This provides new physically based parametrisations for the advection and diffusion coefficients. This new model is analysed in the bedload configuration, and reproduces qualitatively the DEM results. To improve the model, new dependencies on the inertial number and small particle concentration are proposed for the segregation and drag forces.Finally, the impact of size segregation on sediment transport is studied through the mobility of bidisperse already segregated particle beds. Large particles are placed above small ones, and it is observed that, in the same fluid and surface bed conditions, the transport rate is higher in the bidisperse configuration than in the monodisperse one. For the range of studied size ratio (r<4), it is showed that it is not a rugosity but a granular effect. This is analyzed within the framework of the mu(I) rheology and it is demonstrated that the buried small particles are more mobile than larger particles and play the role of a conveyor belt for the large particles at the surface. Based on rheological arguments, a simple predictive model for the additional transport in the bidisperse case is proposed, which reproduces quite well the DEM results for a large range of Shields numbers and for size ratios smaller than 4. The results of the model were used to identify four different transport regimes of bidisperse mixtures, depending on the mechanisms responsible for the mobility of the small particles.This work represents an important improvement in the understanding of size segregation during bedload transport and questions our understanding of bidisperse granular media, which have not been much studied. It also represents a first step in an upscaling process towards the morphological scale through continuum models

Debris flows, avalanches and other geophysical mass flows pose a significant hazard to settlements in or near mountainous regions. Understanding the physical processes that govern these flows is an essential part of hazard assessment and mitigation strategies. This thesis addresses two aspects of geophysical mass flows: flow self-channelisation due to the formation of lateral levees, and granular shocks, which occur when a rapidly-moving debris flow or avalanche collides with an obstacle. We present the results of large-scale debris flow experiments in which the flow is channelised by coarse-particle levees that form at the flow margins. The flow surface velocities are measured with high speed overhead photography, and the deposits both sampled to obtain the grain size distribution and excavated to recover the deposited locations of tracer pebbles that were introduced in to the flow. We propose a model, supported by evidence from the large-scale experiments, that describes in detail the size segregation and kinematic transport processes responsible for the deposition of lateral levees. The second problem addressed in the thesis concerns granular shocks, or jumps, which are rapid changes in the depth and velocity of granular avalanches. We investigate these through experiments in which a falling jet of granular material impacts on an inclined plane, generating a steady granular jump, which is either teardrop-shaped or 'blunted'. Numerical solutions of a depth-averaged flow model agree quantitatively with many of the observed flow features. We use this model show that the transition between the teardrop-shaped and blunted jump regimes corresponds to a transition between two shock reflection structures, known as a regular and a Mach shock reflection. On planes inclined at a shallow angle, we demonstrate a wide variety of unsteady and channelised flows, which occur due to the complex interaction between flowing and stationary regions of granular material.

Clusters are aggregates of a finite number of atoms or molecules. In the present work, free clusters out of metallic parent materials have been created and studied by synchrotron radiation-based photoelectron spectroscopy. The clusters have been formed and studied in a beam and the electronic structure of the clusters has been investigated. Conclusions have been drawn about the spatial distribution of atoms of different elements in bi-component clusters, about the development of metallicity in small clusters, and about the excitation of plasmons. Bi-component alloy clusters of sodium and potassium and of copper and silver have been produced. The site-sensitivity of the photoelectron spectroscopy technique has allowed us to probe the geometric distribution of the atoms of the constituent elements by comparing the responses from the bulk and surface of the clusters. In both cases, we have found evidence for a surface-segregated structure, with the element with the largest atoms and lowest cohesive energy (potassium and silver, correspondingly) dominating the surface and with a mixed bulk. Small clusters of tin and lead have been probed to investigate the development of metallicity. The difference in screening efficiency between metals and non-metals has been utilized to determine in what size range an aggregate of atoms of these metallic parent materials stops to be metallic. For tin this has been found to occur below ~40 atoms while for lead it happened somewhere below 20-30 atoms. The excitation of bulk and surface plasmons has been studied in clusters of sodium, potassium, magnesium and aluminium, with radii in the nanometer range. The excitation energies have been found to be close to those of the corresponding macroscopic solids. We have also observed spectral features corresponding to multi-quantum plasmon excitation in clusters of Na and K. Such features have in macroscopic solids been interpreted as due to harmonic plasmon excitation. Our observations of features corresponding to the excitation of one bulk and one surface plasmon however suggest the presence of sequential excitation in clusters.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
This paper aims at developing a better understanding of what has set up the new role of the medium-size cities in Sao Paulo in their urban network as well as how their forms of expansion have contributed to the continuity of the construction of cities increasingly unequal as regards the occupation of the territory. The addressed settings concern the production of Housing Projects of Social Interest promoted by the housing policy aimed at low-income families and the production of Closed Housing Projects associated with land and real estate interests. These modalities help in the spatialization of socio-territorial segregation and supports the understanding of the logic city center x outskirts. The research on two empirical objects aimed to illustrate how these settings appropriate the peripheral areas of the cities of Sao Carlos and Sao Jose do Rio Preto. In order to strengthen the argument, the application of three analyzing variables guided the reading of the processes that have been reinforced as well as the processes that have been provoked in the intra-urban scale. Although the origin of the phenomena is the result of disputes over the occupation of urban space, the analytical approach indicated the similarities and peculiarities which were found in each city.
Este trabalho pretende desenvolver um melhor entendimento do que têm configurado o novo papel das cidades médias paulistas em sua rede urbana, e como suas formas de expansão têm contribuído para dar continuidade na construção de cidades cada vez mais desiguais em relação à ocupação do território. As configurações abordadas referem-se à produção de Empreendimentos Habitacionais de Interesse Social, alavancados pela política habitacional destinada às famílias de menor renda, e à produção de Empreendimentos Habitacionais Fechados, associados aos interesses fundiários e imobiliários. Essas modalidades auxiliam na espacialização da segregação de âmbito socioterritorial e dá suporte para compreensão da lógica centro x periferia. A pesquisa de dois objetos empíricos buscou ilustrar como essas configurações se apropriam dos espaços periféricos da cidade de São Carlos e de São José do Rio Preto. Para fortalece a discussão, a aplicação de três variáveis de análise orientou a leitura dos processos que têm sido reforçados, e dos processos que têm sido provocados na escala intraurbana. Embora a origem dos fenômenos seja resultado de disputas pela ocupação do espaço urbano, a abordagem analítica indicou as semelhanças e as peculiaridades encontradas em cada cidade.

Geophysical research has long been interdisciplinary, with many phenomena on the Earth's surface involving multiple, linked processes that are best understood using a combination of techniques. This is particularly true in the case of grain flows on sand dunes, in which the sedimentary stratification with which geologists are concerned arises from the granular processes investigated by physicists and engineers, and the water permeation that interests hydrologists and soil scientists determines the seismic velocities of concern to exploration geophysicists. In this dissertation, I describe four projects conducted for the degree of Doctor of Philosophy, using a combination of laboratory experimentation, fieldwork, numerical simulation, and mathematical modelling to link avalanching on dunes to its effects on stratification, on the permeation of water, and on seismic surveys. Firstly, I describe experiments on erodible, unbounded, grain piles in a channel, slowly supplied with additional grains, and I demonstrate that the behaviour of the consequent, discrete avalanches alternates between two regimes, typified by their size statistics. Reconciling the `self-organised criticality' that several authors have predicted for such a system with the hysteretic behaviour that others have observed, the system exhibits quasi-periodic, system-spanning avalanches in one regime, while in the other avalanches pass at irregular intervals and have a power-law size distribution. Secondly, I link this power-law size distribution to the strata emplaced by avalanches on bounded grain piles. A low inflow rate of grains into an experimental channel develops a pile, composed of strata in which blue-dyed, coarser grains overlie finer grains. Associating stopped avalanche fronts with the `trapped kinks' described by previous authors, I show that, in sufficiently large grain piles, mean stratum width increases linearly with distance downslope. This implies the possibility of interpreting paleodune height from the strata of aeolian sandstones, and makes predictions for the structure of avalanche-associated strata within active dunes. Thirdly, I discuss investigations of these strata within active, Qatari barchan dunes, using dye-infiltration to image strata in the field and extracting samples across individual strata with sub-centimetre resolution. Downslope increases in mean stratum width are evident, while measurements of particle size distributions demonstrate preferential permeation of water along substrata composed of finer particles, explaining the strata-associated, localised regions of high water content discovered by other work on the same dunes. Finally, I consider the effect of these within-dune variations in water content on seismic surveys for oil and gas. Having used high performance computing to simulate elastic wave propagation in the vicinity of an isolated, barchan sand dune, I demonstrate that such a dune acts as a resonator, absorbing energy from Rayleigh waves and reemitting it over an extensive period of time. I derive and validate a mathematical framework that uses bulk properties of the dune to predict quantitative properties of the emitted waves, and I demonstrate the importance of internal variations in seismic velocity, resulting from variations in water content.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
As atuais formas de produção e de apropriação do espaço urbano têm acentuado a tendência de segregação socioespacial, promovendo alterações na vida urbana contemporânea e nas práticas socioespaciais dos citadinos. Como consequência, a redefinição das relações entre o público e o privado tem modificado o significado do que é a cidade enquanto lócus da diversidade e confronto entre as diferenças. Essa nova estruturação, marcada por novas práticas socioespaciais, tem gerado dinâmicas de fragmentação socioespacial, o que exige que se observe a articulação entre formas, conteúdos e valores, para se compreender as novas lógicas da produção desigual do espaço urbano e as dimensões do controle social que lhes são atinentes. Com base nestas premissas, esta pesquisa propõe-se a contribuir para o estudo da fragmentação socioespacial, a partir de duas cidades paulistas de porte médio: Marília e São Carlos. Recolheu-se elementos para avaliar em que medida os sujeitos autossegregados destas cidades optam por espaços residenciais fechados e pelo consumo seletivo da cidade, nos quais, estejam presentes aspectos de uma sociabilidade segmentada
The current production forms and of appropriation of the urban space has accentuated the tendency of socio-spatial segregation, promoting alterations in the contemporary urban life and in the socio-spatial practices of the city dwellers. Also, the redefinition of the relation between the public and the private has changed the meaning of what is the city, while place of the diversity and confrontation among the differences. This new structuring, marked by new socio-spatial practices, has generated dynamics of socio-spatial fragmentation. This requires an examination the articulation between forms, contents and values, to understand the new logics of the unequal production of the urban space and the dimensions of the social control that are them concerning. Based on this, this research intends to contribute with the study of socio-spatial fragmentation, starting from two middle-size cities, located in São Paulo State: Marília and São Carlos. It was collected elements to evaluate in that measured the subjects self-segregated, in these cities, opt for residential spaces closed and for the selective consumption of the city where present aspects of a segmented sociability be. This research articulates to project: Diffuse Urbanization, Public Space and Urban Insecurity, supported by FAPESP

Chez les bactéries, la ségrégation du chromosome est initiée durant la phase de réplication. Des expériences de time lapse, utilisées pour observer que la dynamique des loci frères durant le cycle cellulaire, montrent que, chez Escherichia coli, les régions sœurs restent colocalisées pour une période significative dans les régions des macrodomaines du chromosome et pour une courte période dans les régions non-structurées. Nous nous sommes posés la question suivante: est ce que l'étape de colocalisation révèle une réelle cohésion entre les chromatides sœurs ? Pour y répondre, nous avons développé un outil génétique, alternatif aux outils de biologie cellulaire, permettant de mesurer la distance entre les chromatides sœurs de manière directe. La fréquence de recombinaison intermoléculaire médiée par la recombinase Cre entre les sites loxP positionnés sur les chromatides sœurs est mesurée pour différentes positions. De cette fréquence, nous avons pu déduire la proximité entre les chromatides sœurs. Nous révélons que les loci frères restent proche l'un de l'autre pour une courte période après la réplication. Nous appelons cette étape la cohésion moléculaire, celle-ci est dépendante du locus considéré. Nous montrons que les facteurs qui favorisent la colocalisation des foci frères n'augmentent pas nécessairement l'habilité des loci frères à recombiner. En effet, la protéine MatP, un acteur de la colocalisation des macrodomaines Ter, n'affecte pas la cohésion entre les deux copies de cette région. La Topoisomérase IV est un facteur essentiel à la ségrégation des chromosomes. En son absence, les chromosomes ne peuvent se ségréger et restent colocalisés dans la cellule. Nous révélons par le test de recombinaison que l'absence de Topoisométase IV dans les cellules provoque une augmentation des interactions entre chromatides sœurs. Au final, nous avons montré que l'étape de cohésion est différente de la colocalisation, que les mécanismes moléculaires diffèrent d'une étape à l'autre et que les liens de précaténation moduleraient la cohésion post-réplicative entre chromatides sœurs.

Entre 2011 et avril 2017, La Réunion a connu 22 attaques de requin, dont 9 mortelles. Cette intensification des Conflits Hommes-Requins (CHR) à La Réunion a donné naissance au programme de recherche, CHARC, dont le but est de mieux comprendre la biologie et l'écologie des requins bouledogue (Carcharhinus leucas). C'est dans ce contexte et au sein de ce programme que s'est réalisé ce projet de thèse. Représentant la première étude de ce type sur cette espèce, l'objectif principal de la thèse a été de déterminer les caractéristiques et les particularités de la population observée, de délimiter son habitat et ses micro-habitats, d'en étudier son occupation spatiale et temporelle et de comprendre le rôle de ces micro-habitats dans le cycle de vie de l'espèce. Pour atteindre ces objectifs, l'étude repose sur une campagne de marquage et le suivi acoustique passif des 35 requins bouledogue le long de la côte ouest pendant 18 mois. La quasi-totalité des requins marqués sont des adultes de grande taille, avec un sex-ratio en faveur des femelles. De part ces caractéristiques, cette population de requins bouledogue se distingue de la majorité des autres populations observées à travers le monde.L'occupation spatiale et temporelle des requins n'était pas homogène, avec l'utilisation de zones préférentielles et une forte saisonnalité. On observe également une variabilité individuelle, influencée par la taille et le sexe des individus : les jeunes adultes, des deux sexes, sont essentiellement localisés dans le sud de la zone, au cours de deux pics de présence, un hivernal et un estival. A l'inverse, les grandes femelles sont majoritairement localisées dans le nord de la zone, avec un pic de présence hivernal. Les grands mâles sont peu présents et essentiellement localisés dans le nord et à l'extrémité sud de la zone d’étude au cours de la période creuse de juillet à octobre. Indépendamment du sexe et de la taille des individus, les zones préférentielles sont utilisées comme zone de repos sur l'ensemble des périodes nycthémérale et comme zone de chasse nocturne. Certaines zones préférentielles pourraient également jouer un rôle dans la reproduction. Les zones secondaires représentent principalement des zones de passages entre les zones préférentielles ou des zones de chasse nocturne secondaire. Si l'ensemble de ces résultats devra être confirmé par d'autres études à plus grande échelle spatio-temporelle, il montre toute la complexité du comportement et des déplacements des requins bouledogue dans les eaux réunionnaises. Cette thèse ouvre de nouvelles perspectives d'étude, notamment sur l'influence des facteurs environnementaux dans l'occupation spatio-temporelle des requins ou sur l'existence de phénomènes liés à la reproduction comme la philopatrie et la polyandrie
A serie of 22 shark attacks occurred at Reunion Island between 2011 and April 2017, causing nine deaths. Following the rise of the Human Shark Conflict (HSC), a research program, CHARC, was launched to better understand the biology and ecology of bullshark (Carcharhinus leucas). This thesis was carried out within this program. Representing the firststudy on this specie at Reunion Island, the main objective was to determine characteristics of the observed population, to define it habitat and its micro-habitats, to study its spatial and temporal occupation and to understand the role of these micro-habitats in the life cycle of the specie. This study is based on a tagging campaign and passive acoustic telemetry of 35 bullsharks along the west coast for 18 months. Almost all tagged sharks are adults, large in size and with a sex ratio in favor of females. Base of these characteristics, this bullshark population differs from the majority of other observed populations throughout the world. The spatial and temporal occupation was not homogeneous, with use of preferential areas and strong seasonality. Individual variability is also observed, influenced by size and sex: young adults, of both sexes, are mainly located in the southern part of the study area, during two peaks of presence, one in winter and one in summer. Conversely, large females are mostly located in the north of study area, with a peak of presence in winter. Large males are rarely present and mostly located at the ends of the north and south of the study area from July to October. Apart from sex and size of individuals, preferential areas are used as resting areas, during all nycthemeral periods and as hunting area at night. Some preferential areas may also play a role in breeding behaviour. The secondary areas are mainly used to move from a preferential area to another or as secondary hunting area at night. If all these results have to be confirmed by larger-scale spatio-temporal studies, it put in light the complexity of bullshark behavior and movements along the coasts of Reunion Island. This thesis gives perspectives for new studies, including studies on environmental factors influence on spatio-temporal occupation of sharks or on phenomena related to reproduction such as philopatry and polyandry

Orientador: Maria Encarnação Beltrão Sposito
Banca: Nécio Turra Neto
Banca: Oscar Alfredo Sobarzo Miño
Resumo: As atuais formas de produção e de apropriação do espaço urbano têm acentuado a tendência de segregação socioespacial, promovendo alterações na vida urbana contemporânea e nas práticas socioespaciais dos citadinos. Como consequência, a redefinição das relações entre o público e o privado tem modificado o significado do que é a cidade enquanto lócus da diversidade e confronto entre as diferenças. Essa nova estruturação, marcada por novas práticas socioespaciais, tem gerado dinâmicas de fragmentação socioespacial, o que exige que se observe a articulação entre formas, conteúdos e valores, para se compreender as novas lógicas da produção desigual do espaço urbano e as dimensões do controle social que lhes são atinentes. Com base nestas premissas, esta pesquisa propõe-se a contribuir para o estudo da fragmentação socioespacial, a partir de duas cidades paulistas de porte médio: Marília e São Carlos. Recolheu-se elementos para avaliar em que medida os sujeitos autossegregados destas cidades optam por espaços residenciais fechados e pelo consumo seletivo da cidade, nos quais, estejam presentes aspectos de uma sociabilidade segmentada
Abstract: The current production forms and of appropriation of the urban space has accentuated the tendency of socio-spatial segregation, promoting alterations in the contemporary urban life and in the socio-spatial practices of the city dwellers. Also, the redefinition of the relation between the public and the private has changed the meaning of what is the city, while place of the diversity and confrontation among the differences. This new structuring, marked by new socio-spatial practices, has generated dynamics of socio-spatial fragmentation. This requires an examination the articulation between forms, contents and values, to understand the new logics of the unequal production of the urban space and the dimensions of the social control that are them concerning. Based on this, this research intends to contribute with the study of socio-spatial fragmentation, starting from two middle-size cities, located in São Paulo State: Marília and São Carlos. It was collected elements to evaluate in that measured the subjects self-segregated, in these cities, opt for residential spaces closed and for the selective consumption of the city where present aspects of a segmented sociability be. This research articulates to project: Diffuse Urbanization, Public Space and Urban Insecurity, supported by FAPESP
Mestre

Depuis la fin du 19ème siècle, l’espace minier de Gafsa (Sud-Ouest de Tunisie) s’est spécialisé dans la mono-activité de l’extraction et l’enrichissement du phosphate pour des raisons de profits économiques certains. Avec ces exportations, l’espace a subi une dégradation intense des ressources du sol, de la couverture végétale et une détérioration avancée des zones agricoles. La gestion des infrastructures minières n’a pas su répondre aux normes environnementales et a provoqué ainsi des inégalités sociales et spatiales et a accentué le niveau des risques environnementaux.L’activité minière a été à l’origine de la naissance et de la genèse des villes minières dans le Bassin Minier de Gafsa (BMG). La formation de cités spontanées a provoqué la désarticulation du tissu urbain. Les extensions urbaines notamment l’expansion des quartiers anarchiques, s’étendant vers les laveries et vers les mines, ont créé ainsi des conflits avec le domaine phosphatier. Tenant compte de la complexité des multiples aspects sociaux, spatiaux et environnementaux du BMG, nous avons utilisé à la fois l’approche géographique classique et l’approche géomatique qui nous a semblé appropriée pour aider à la prise de décision. L’intérêt de cette dernière approche permet d’identifier, d’analyser, et de modéliser l’état des lieux, les mutations urbaines et environnementales en relation avec la croissance spatiale-minière. Les principaux résultats permettent d’avoir une banque de données statistiques et cartographiques, à différentes échelles, pour une vision synthétique et une meilleure compréhension du BMG et de réaliser un modèle d’aide à la décision
Since the end of the 19th century, the mining area of Gafsa (south-west Tunisia) has specialized in the single-activity extraction and enrichment of phosphate for reasons of economic profits. With these exports, this area has suffered an intense degradation of soil resources, vegetation cover and an advanced deterioration of agricultural areas. The management of the mining infrastructure has failed to meet environmental standards, causing social and spatial inequalities and increasing the level of environmental risks.The mining activity was at the origin of the birth and genesis of the mining towns in the Basin of Gafsa (BMG). The formation of spontaneous cities has caused the disarticulation of the urbanization. Urban extensions, including the expansion of anarchic cities, extending to laundries and to mines, thus created conflicts with the phosphatier domain. All these factors make costly and complicated attempts at development and rehabilitation that mining companies do not dare to commit.Taking into account the complexity of the many social, spatial and environmental aspects of the BMG, we used both the classical geographical approach and the geomatics approach that seemed appropriate for better decision-making. The interest of this latter approach makes it possible to identify, analyze, and model the state of the sites, the urban and environmental mutations in relation to the spatial-mining growth

Les nano-objets unidimensionnels alliés présentent des propriétés physiques spécifiques qui résultent à la fois de leur morphologie, de leur taille et de la répartition chimique des atomes. Nous exploitons un modèle d'Ising sur réseau qui rend compte en particulier des effets de ségrégation au sein de nanofils bimétalliques pour obtenir une compréhension fine des effets gouvernant cette répartition à l'équilibre.Dans une première section, nous détaillons l'équilibre d'un nanofil en fonction de sa taille et de sa composition, de manière à mettre en évidence le rôle des effets de taille finie sur la thermodynamique d'équilibre d'objets bimétalliques 1D. Contrairement aux systèmes infinis, l'équilibre dépend de l'ensemble statistique considéré. Ainsi la ségrégation est plus marquée dans l'ensemble canonique, où la concentration du nanofil est imposée, que dans l'ensemble pseudo-Grand Canonique (p-GC) où le nanofil est en équilibre avec un réservoir qui fixe la différence de potentiel chimique entre les espèces. De même, la contrainte de composition dans l'ensemble canonique induit des corrélations chimiques d'occupation des sites qui favorisent davantage les paires hétéroatomiques. Nous montrons que l'écart observé entre les isothermes des deux ensembles croît avec la courbure de l'isotherme canonique et avec l'amplitude des fluctuations de la concentration nominale dans l'ensemble p-GC. Ces fluctuations diminuant avec la taille du nanofil considéré, l'écart entre les ensembles s'annule à la limite thermodynamique. Les effets de taille finie se traduisent par ailleurs par l'apparition, à basse température et pour de petits nanofils, d'une coexistence d'un mode pur en l'espèce ségrégeante et d'un mode de faible concentration nominale constitué principalement de configurations de type cœur-coquille et Janus. Nous développons alors un formalisme permettant de caractériser cette bimodalité.Alors que les résultats évoqués précédemment concernent un nanofil considéré seul, nous étudions dans la deuxième section l'équilibre de l'ensemble des nanofils formant un co-dépôt unidimensionnel inférieur à la mono-couche. Nous montrons que la distribution en taille de ces nanofils varie globalement selon une loi de puissance, quelle que soit la composition du codépôt, de sorte que la ségrégation n'a que peu d'influence sur la microstructure observée. Par contre, en raison du rapport surface/volume et des corrélations chimiques dans ces objets, la composition des nanofils du co-dépôt varie très fortement selon leur taille, les petits nanofils étant plus riches en l'espèce ségrégeante que les plus grands. Enfin, nous étendons le diagramme de bimodalité d'un nanofil seul à l'ensemble des nanofils du co-dépôt et montrons que cette bimodalité est difficilement observable car elle ne concerne que des amas de petite taille qui sont très minoritaires du fait de la cohésion atomique.

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the Degree of Master of Science in Engineering. May, 2016
Particle size and size range are among the characteristics that affect the segregation of particles in a jig hence they affect the separation efficiency. The effects of these variables on segregation of particles are not fully understood. This work aimed at contributing to knowledge in this area. To better understand how particle size and size range influence segregation, tests were conducted in which the effects of the density and shape of the particles on segregation were minimized by using as the feed material spherical glass beads of uniform shape and density. Batch experiments of two components systems of various particle sizes were conducted under the same set of jigging conditions: the jigging frequency and jigging time were respectively maintained at 60 cycles per minute and 999 seconds (16.65 minutes). The effect of these operating conditions on segregation was not investigated. At the end of each test run, the jig bed was split into horizontal slices and the composition of each slice was determined. The experimental results showed that below a particle size ratio of 1.50:1, the driving force for the segregation of particles, i.e. the particle size difference, was small hence a low degree of segregation was obtained. The degree of segregation increased above this ratio. However, above the size ratio of 2.00:1, interstitial trickling occurred. With the smaller particles tested (8, 6 and 4mm) poor segregation was observed when the size ratios were of 1.50:1 or less along with what is believed to have been remixing due to convective currents within the jig chamber. It was found that the particle size range had a more pronounced effect on size segregation than the particle size. From the results, it can be said that above a size ratio of about 1.50:1, size segregation is very pronounced. This suggests that density separations of real ores, where both the density and size of particles vary, would be impaired if the particle size range of the material fed to the jig exceeds this ratio. However, this needs further confirmation by testing multiple component systems.

Dissertação de Mestrado em Ecologia apresentada à Faculdade de Ciências e Tecnologia
Apesar de se tratar de um comportamento amplamente descrito e estudado no reino animal, especialmente em ungulados, a segregação sexual carece de uma definição consensual entre autores. Ao longo dos anos, várias hipóteses foram sugeridas e estudadas com o objetivo de explicar quer a componente social quer a componente de habitat deste complexo comportamento. No entanto, nenhuma foi capaz de fornecer uma justificação universal para este fenómeno, o que sugere a contribuição simultânea de vários fatores para a origem do mesmo. Segundo a hipótese da seletividade alimentar, machos e fêmeas possuem diferentes eficiências digestivas e necessidades energéticas, decorrentes do seu dimorfismo sexual em termos de tamanho, pelo que selecionam habitats distintos em detrimento da qualidade de alimento. Este estudo tem, então, como objetivo avaliar o pressuposto que serve de base à hipótese da seletividade alimentar. Através da análise de excrementos de veado, recolhidos por observação direta e em animais abatidos em montarias na Serra da Lousã, comparou-se o tamanho dos fragmentos de plantas excretados por machos e fêmeas de forma a avaliar diferenças na eficiência digestiva entre sexos. Nos excrementos recolhidos na época reprodutiva foi possível encontrar diferenças significativas no tamanho dos fragmentos, o que levou a concluir que os machos possuem maior eficiência digestiva do que as fêmeas quando a qualidade da dieta e os padrões de atividade são idênticos entre sexos, o que só acontece na época reprodutiva. Deste modo, os nossos resultados apoiam a hipótese da seletividade alimentar, não sendo as diferenças obtidas suficientes para explicar na totalidade o padrão de segregação sexual nesta população de veados. Sendo também um pressuposto da hipótese da sincronização de atividades, as diferentes eficiências digestivas de machos e fêmeas poderão ser o ponto de partida para encontrar uma explicação para a segregação sexual, relacionando as componentes de habitat e social.
Despite being a behaviour widely described and studied in the animal kingdom, especially in ungulates, the sexual segregation lacks a consensual definition between authors. Over the years, many hypotheses have been suggested and studied with the objective of explaining either the social or the habitat component of this complex behaviour. However, none was able to provide a universal explanation for this phenomenon, which suggests a simultaneous contribution of various factors to be in its origin. According to the forage selection hypothesis, males and females present different digestive efficiencies and energetic requirements, due to their sexual body size dimorphism, which results in the selection of distinct habitats as consequence of the forage quality. Therefore, this study aims to evaluate the assumption in which the forage selection hypothesis is based. Through the analysis of red deer faeces, collected by direct observation and in hunted animals in the Lousã Mountain, the size of the plant fragments excreted by males and females was compared to evaluate differences between the digestive efficiencies of both sexes. It was possible to identify significative differences in the size of the fragments present in the faeces collected during the rut season, which leads to the conclusion that males have a higher digestive efficiency than females when the food quality and the activity patterns are similar between sexes, which only happens during the rut season. As so, our results provide support to the forage selection hypothesis, but the differences obtained do not seem enough to explain the full patterns of sexual segregation of this red deer population. The fact that the differential digestive efficiency of males and females is also an assumption of the activity budget hypothesis might suit as a basis to find an explanation for sexual segregation, relating the habitat and social components.

Inconel 718 (IN 718) is widely used in applications, such as aircraft and power turbine components. Recently, fiber laser welding has become an attractive joining technique in industry for fabrication and repair of service-damaged components. However, a major limitation in the laser welding of IN 718 is that liquation cracking occurs. In the present work, autogenous fiber laser welding of IN 718 was used to study the effects of welding parameters and different pre-weld heat treatments on liquation cracking. Contrary to previous studies, a dual effect of heat input on cracking is observed. A rarely reported effect of heat input is attributed to process instability. Liquation cracking increases with pre-weld heat treatment temperatures that increase grain size and/or, possibly, intregranular boron segregation. The study shows that pre-weld heat treatment at 950oC can be used for repair welding of IN 718 without significant loss in cracking resistance.

Pipeline steels that carry oil and natural gas in severe environments suffer from two important modes of failure: stress corrosion cracking (SCC) and hydrogen-induced cracking (HIC). The SCC has been studied extensively in the literature; however, HIC phenomenon in pipeline steels is less investigated by researchers. Nevertheless, HIC is recognized as the most important damage mode in sour environment. Hydrogen atoms produced due to surface corrosion of the steel diffuse into it through microstructural defects. When a critical amount of hydrogen is accumulated in such defects, HIC cracks initiate and propagate. The main objectives of this thesis are to find the HIC crack nucleation and propagation sites, evaluate a role of texture and grain boundary character distribution in crack growth and finally establish the effect of different microstructural parameters contributing to the HIC related failure in pipeline steel. In this thesis, HIC standard test and electrochemical hydrogen-charging experiments were used to induce HIC cracks in pipeline steels. HIC cracks at the cross section of tested samples were observed using scanning electron microscope (SEM). The SEM observations clearly indicate that the investigated X60 and X70 steels are susceptible to HIC while the X60SS steel showed a higher resistance to HIC. This experiment also proved that the X70 steel has higher susceptibility to HIC than the other investigated steel. Energy dispersive spectroscopic (EDS) analyses indicated that two types of inclusion namely manganese sulfide and carbonitiride precipitates serve as crack nucleation sites. HIC cracks were observed to propagate at the center of cross section where the segregation of some elements such as carbon and manganese occurred. Moreover, two other experiments were carried out in order to evaluate the capability of pipeline steels for hydrogen-trapping. The first test, hydrogen-permeation experiment, showed that all pipeline steel specimens, such as X70, X60 and X60SS steels, contain both reversible and irreversible hydrogen traps. However, the density of traps at the center of cross section was higher than other regions in all tested specimens. The hydrogen-discharging experiments also showed that all specimens keep a considerable amount of hydrogen inside their traps. The hydrogen traps, based on their binding energy with the metal matrix, are categorized as reversible (weak) and irreversible (strong) traps and the roles of each type of traps are explained. Electron backscatter diffraction (EBSD) measurements were done along the HIC crack in X70 steel after standard HIC test. The results showed that the {100} texture was strong while the {111} texture was weak. Some special texture components, such as the {110}, {332} and {112}, were observed after the HIC crack-stoppage. EBSD results also documented that fine grain colonies were prone to intergranular HIC crack propagation and IPF and PF, calculated in both sides of HIC cracks, showed the preferences of ND||<100> orientation. Both susceptible X60 and non-susceptible X60SS steel to HIC were compared based on the EBSD results. It was observed that the high amount of recrystallization fraction with no stored energy is one of the main reasons for a higher HIC resistance of X60SS steel to HIC. Moreover, Kernel Average Misorientation (KAM) data showed that the deformation is more concentrated in the as-received and HIC tested X60 specimens. The effect of hydrogen-charging during tensile/fatigue loading of X60SS steel was studied and it was observed that some HIC cracks at the cross section of X60SS steel were appeared after hydrogen-charging at stresses below the yield stress. Experiments were carried out to understand the effect of cold-rolling and annealing on HIC susceptibility in pipeline steels. The results documented that the {100} dominant texture is more pronounced in 50% and 90% cold-rolled and annealed specimens. The effect of different factors such as KAM degree and recrystallized fraction affecting HIC susceptibility on cold-rolled and annealed specimens was investigated. The obtained results showed that the cold-rolling and annealing process may not be considered as an effective method to increase HIC resistance in pipeline steels.

The present dissertation reports the findings of a study of cracking behavior of a newly developed superalloy, Allvac 718Plus during high power beam welding and post-weld heat treatment. Microstructures of the base alloy, heat affected zone (HAZ) and fusion zone (FZ) of welded and post-weld heat treated (PWHT) coupons were examined by the use of standard metallographic techniques involving optical microscopy, analytical scanning electron microscopy (SEM) and analytical transmission electron microscopy. Moreover, grain boundary segregation behavior of boron atoms during pre-weld heat treatments was evaluated using secondary ion mass spectroscopic system. In the first phase of the research, 718Plus was welded using a low and high heat input CO2 laser to assess its weld cracking response. Detailed examination of the welds by analytical electron microscopic technique revealed the occurrence of cracking in the HAZ of low heat input welds, while their FZ was crack free. However, both the FZ and HAZ of high heat input welds were crack-free. Resolidified constituents were observed along the cracked grain boundaries of the lower heat input welds, which indicated that HAZ cracking in this newly developed superalloy was associated with grain boundary liquation. However, despite a more extensive liquation of grain boundaries and grain interior in the HAZ of high heat input welds, no cracking occurred. This was attributed to the combination of lower welding stresses generated during cooling, and relaxation of these stresses by thick intergranular liquid. Although HAZ cracking was prevented by welding with a high heat input laser, it resulted in a significant damage to the parent microstructure through its extensive liquation. Thus, the use of low heat input welding is desirable. However, this resulted in HAZ cracking which needs to be minimized or eliminated. Therefore, during the second phase of this research, the effects of pre-weld thermal processing on the cracking response of 718Plus were investigated. Results from the quantification of the cracking of the alloy showed that HAZ cracking may be significantly reduced or eliminated through an adequate selection of pre-weld thermal cycle. In the third stage of this research, crack-free welds of 718Plus were post-weld heat treated using standard thermal schedules. A significant solid state cracking of the alloy occurred during the PWHT. The cracking was attributed to the presence of embrittling phases on HAZ grain boundaries, coupled with aging contraction stresses that are generated by a considerable precipitation of gamma prime phase during aging.