Academic literature on the topic 'Selective dispersion'

Oxide dispersion strengthened (ODS) steels contain a fine dispersion of nano-sized, typically Y based, oxide particles which result in the material displaying significantly better creep, irradiation and oxidation resistance when compared to conventional alloys. Thus, such materials are considered as candidate structural materials for a number of applications in the fossil and nuclear energy sectors and in other high-temperature applications. ODS steels are currently produced by powder metallurgy which includes mechanical alloying (MA) of master alloys or elemental powder, hot extrusion or hot isostatic pressing (HIP) followed by a final heat treatment. Recent studies revealed that Y added during MA in the form of yttria (Y2O3) breaks down and the elements go into supersaturated solution in the Fe matrix; and Y based dispersoids form during fabrication of the alloy. In this work, an additive manufacturing method, selective laser melting (SLM), was applied to as-MA ODS-PM2000 (Fe-19.0wt.%Cr-5.5Al-0.5Ti-0.5Y2O3) powder. SLM produces almost fully dense solid freeform components by successively melting thin layers of metal powder. In order to investigate the feasibility of SLM in an ODS alloy environment, a number of builds were fabricated. These included a complex thin walled structure, coatings on Inconel IN939 (Ni-22.5wt.%Cr-19.0Co-3.7Ti-2W-1.9Al-1.0Nb-1.4Ta-0.15C), a nickel based superalloy, and optimised wall and solid builds. A wide range of microstructural and mechanical characterisation techniques were carried out on these builds with the focus to study the fundamentals of SLM in ODS environment. The most important finding of this work was that a fine homogeneous dispersion of globular shaped nanoscopic particulates could be retained in the SLM build configurations investigated. Indications were found that there is a very low number of dispersoids in the deposited layer after it was put down. Repeated heating cycles during SLM deposition of further slices resulted in coarsening and growth of existing precipitates, but probably also in nucleation and growth of new dispersoids in the α-Fe matrix. Such heating cycles and post-build annealing trials resulted in modification of initially multiphased dispersoid compounds including originally a number of elements, such as O, Al, Si, Ti, Cr, Fe and Y, into structures having significantly increased concentrations of Al and Y. After post-build annealing, the particles were most frequently of the compound type yttrium aluminium monoclinic, Y4Al2O9. SLM processing parameters were developed leading to a relative density of >99.5 % for wall builds having different thicknesses and of >98.5 % for solid builds. Electron backscatter diffraction (EBSD) was conducted and revealed a strong [001] fibre texture along the growth direction of a wall build. For annealed walls, values of the 0.2% offset yield strength YS0.2 up to those of recrystallized conventional produced PM2000 could be achieved. Fracture behaviour and the individual key parameters determined, YS0.2 and Young's modulus were anisotropic due to this texture. In coatings, Y-rich dispersoids could be retained. When oxidised isothermally in laboratory air at 1100°, the SLM deposit PM2000 formed a mainly α-Al2O3 (alumina) scale, which was similar to conventionally fabricated PM2000. Oxidation at 870°C, however, resulted in different scale morphologies between both variants. Those were Al-rich equiaxed structures and nodules and Ti-rich needles for conventional PM2000. On the other hand, the SLM material exhibited Al-rich platelet structures and Al-rich equiaxed crystals in pores. The observations of this work confirmed the feasibility of SLM in ODS alloy environment, which may motivate further studies in this field.

The main aim of this thesis has been to contribute to the ongoing work with development of new diode-laser-based spectroscopic techniques and metho­do­lo­gies for sensitive detection of molecules in gas phase. The techniques under scrutiny are tunable diode laser absorption spectrometry (TDLAS) and Faraday modulation spectrometry (FAMOS). Conventional distributed-feedback (DFB) tele­communication diode lasers working in the near-infrared (NIR) region have been used for detection of carbon monoxide (CO) and temperature in hot humid media whereas a unique frequency-quadrupled external-cavity diode laser producing mW powers of continuous-wave (cw) light in the ultra violet (UV) region have been used for detection of nitric oxide (NO). A methodology for assessment of CO in hot humid media by DFB-TDLAS has been developed. By addressing a particular transition in its 2nd overtone band, and by use of a dual-fitting methodology with a single reference water spectrum for background correction, % concentrations of CO can be detected in media with tens of percent of H2O (≤40%) at T≤1000 °C with an accuracy of a few %. Moreover, using an ordinary DFB laser working in the C-band, a technique for assessment of the temperature in hot humid gases (T≤1000 °C) to within a fraction of a percent has been developed. The technique addresses two groups of lines in H2O that have a favorable temperature dependence and are easily accessed in a single scan, which makes it sturdy and useful for industrial applications. A technique for detection of NO on its strong electronic transitions by direct absorption spectrometry (DAS) using cw UV diode laser light has been deve­loped. Since the electronic transitions are ca. two or several orders of magnitude stronger than of those at various rotational-vibra­tional bands, the system is capable of detecting NO down to low ppb∙m concentrations solely using DAS. Also the FAMOS technique has been further developed. A new theoretical description expressed in terms of both the integrated line strength of the transition and 1st Fourier coefficients of a magnetic-field-modulated dispersive lineshape functions is presented. The description has been applied to both ro-vib Q-transitions and electronic transitions in NO. Simulations under different pressures and magnetic field conditions have been made that provide the optimum conditions for both cases. A first demonstration and characteri­zation of FAMOS of NO addressing its electronic transitions in the UV-region has been made, resulting in a detection limit of 10 ppb∙m. The characterization indicates that the technique can be significantly improved if optimum conditions can be obtained, which demonstrates the high potential of the UV-FAMOS technique.

L’objectif de cette thèse est d’ouvrir la voie à la démonstration de l’amplification parabolique à 2 μm en format tout fibré. La théorie sur l’amplification d’impulsions auto-similaires est d’abord présentée. Il en découle que le meilleur amplificateur tout fibré à 2 μm devra s’appuyer sur une fibre dopée aux ions Tm3+ présentant à la fois une forte dispersion normale et une grand aire effective. L’amplification parabolique ne peut pas être réalisée dans des fibres monomodes à base de silice à 2 μm du fait de la forte dispersion anormale présentée par le matériau. Afin de surmonter cette limitation, une fibre dont le mode LP02 présente à la fois une forte dispersion normale et une grande aire effective à 2 μm a été conçue et réalisée. Un convertisseur spatial de lumière dédié à la génération du mode LP02 a également été conçu et réalisé. La pureté d’excitation par ce convertisseur du mode LP02 de la fibre passive a été évaluée à 99,9% par la technique interférométrique d’imagerie spatialement et spectralement résolue. Une source d’impulsions ultrabrèves (100 fs) et accordable en longueur d’onde de 1,6 μm à 2 μm a été créée pour générer le signal de l’amplificateur. Cette source a en outre permis de mesurer la dispersion du mode LP02 de la fibre passive à la longueur d’onde de 1,95 μm, celle-ci vaut -106 ps/(nm.km) en excellent accord quantitatif avec les résultats numériques. Des modélisations numériques de l’amplificateur nonlinéaire basé sur la version active de cette fibre indiquent que des impulsions présentant une puissance crête de l’ordre du MW à 1,9 μm sont envisageables en sortie de l’amplificateur parabolique<br>The goal of this PhD thesis is to pave the way towards the demonstration of parabolic amplification at 2 μm in an all-fiber format. The physical theory on self-similar pulse amplification is first presented. The best all-fiber amplifier at 2 μm would need a special Tm-doped fiber with both high normal dispersion and large effective area. Parabolic amplification is however prohibited in silica based singlemode fiber working at 2 μm due to the large anomalous dispersion of silica. To overcome this limitation, a four-mode fiber in which the LP02 mode exhibits high normal dispersion combined with large effective area at 2 μm was designed and manufactured. A dedicated mode converter has also been designed and manufactured. Using the spatially- and spectrally-resolved imaging technique the purity of excitation of the LP02 mode in the passive fiber was evaluated to 99.9%. An ultrashort pulse (100 fs) source tunable from 1.6 μm to 2 μm was implemented to seed the amplifier. This source allowed to measure the LP02 mode dispersion of the passive fiber at the wavelength of 1.95 μm: D = -106 ps/(nm.km) in excellent quantitative agreement with the simulations. Numerical modeling of a nonlinear amplifier based on the realized active fiber shows that MW peak power class pulses centered at 1.9 μm can be obtained at the output of the parabolic amplifier

Antimoine (Sb) et tellure (Te), sont des contaminants peu étudiés (isotopes stables) et leurs radionucléides artificiels peuvent être rejetés dans le milieu aquatique lors des accidents nucléaires. La connaissance de leurs comportements biogéochimiques respectifs est nécessaire à l'évaluation du risque radiologique post-accidentel.Ce travail présente des données originales sur le comportement biogéochimique de Sb et de Te dans les systèmes de transition continent-océan, tels que l'estuaire de la Gironde et la rivière du Rhône. Un suivi de 14 ans et des campagnes océanographiques dans le bassin versant de l’estuaire de la Gironde ont permis d’identifier des concentrations, des flux, et des réactivités (variabilités spatio-temporelles et distribution solide/liquide) plus élevés pour Sb que pour Te, mettant en évidence un comportement additif pour Sb et de soustraction pour Te le long des gradients de salinité et de turbidité estuariennes. Des expériences couplant l’adsorption d’isotopes marqués sur des matières en suspension (MES) et des extractions sélectives des phases porteuses, suggèrent que les formes apportées de Sb et de Te sont plus mobiles et potentiellement plus biodisponibles que leurs équivalents naturels. De plus, l’observation de la bioaccumulation non-négligeable de Sb et de Te naturels dans les huîtres sauvages à l’embouchure de l’estuaire permet d’envisager une absorption potentielle de leurs homologues radioactifs.Ainsi, le développement de scenarios de dispersion de radionucléides rejetés dans les zones de transition dépendra (i) de la position géographique de la source (Rhône) et/ou de la zone de turbidité maximale (ZTM; système fluvio-estuarien de Gironde), (ii) de la situation hydrologique pendant et post accident, ainsi que (iii) de la réactivité biogéochimique et des temps de demi-vies des radionucléides. Les premiers scénarios de dispersion de radionucléides dans l'estuaire de la Gironde suggèrent (i) un transport préférentiel de Sb dissous vers la zone côtière, et (ii) une forte rétention de Te radioactif dans la ZTM si la dernière est présente en aval du site d’accident, impliquant le risque de migration saisonnière de la radioactivité vers la ville de Bordeaux pendant l’étiage suivant. Ainsi, la dynamique intra estuarienne (marée, débit et migration de la ZTM) sera le facteur prédominant dans le devenir de Te radioactif, depuis son rejet jusqu’à sa désintégration complète en iode radioactif. L’ensemble de ce travail met en évidence la nécessité d’une évaluation plus approfondie de la radiotoxicité potentielle de Sb et Te lors de leurs rejets en milieu aquatique<br>Antimony (Sb) and tellurium (Te) are relatively uncommon contaminants (stable isotopes) and may form short-lived fission products (radionuclides) released into the environment during nuclear power plants accidents. Little is known about their respective biogeochemical behaviours, necessary for general contamination studies and post-accidental radiological risk assessment.This work provides original knowledge on Sb and Te biogeochemical behaviour in highly dynamic continent-ocean transition systems: the Gironde Estuary and the Rhône River. Concentrations, spatial/temporal variations, solid/liquid partitioning (Kd), and fluxes are studied from long-term records at the watershed scale. Four estuarine sampling campaigns during contrasting hydrological conditions show higher Sb solubility and Te particle affinity in the estuary than in the upstream fluvial reaches. Historical records (1984-2017) in wild oysters from the estuary mouth do not show clear trends of past or recent contamination, but measurable bioaccumulation suggests that potential uptake of radionuclides is likely to occur. Combined adsorption experiments using isotopically-labelled (spiked) Sb and Te, and subsequent selective extractions of carrier phases from suspended particulate matter (SPM) suggest that spiked Sb and Te are more mobile and potentially bioaccessible than their environmental (inherited) equivalents. Radiotracer adsorption experiments using environmentally representative concentrations of both Gironde and Rhône systems underpin that highly soluble elements may show contrasting reactivity between inherited and spiked forms.Radionuclide dispersion will greatly depend on (i) the geographical position of the source (Rhône) and/or the maximum turbidity zone (MTZ; Gironde fluvial-estuarine system), (ii) the succession of hydrological situations during and after the accident, and (iii) the biogeochemical reactivity and half-lives of the radionuclides. First scenarios on hypothetical dissolved radionuclide dispersion in the Gironde Estuary suggest (i) low sorption of Sb to the SPM, implying a transport of radionuclides in dissolved phase towards the coast, and (ii) high retention of Te within the MTZ, especially for accidental releases during flood conditions, linking the fate of radioactive Te to long estuarine SPM residence times (1-2 years). Potential upstream migration of Te radionuclides in the MTZ towards the city of Bordeaux during the following summer season and Te decay into radioactive iodine warrants further evaluation of the associated potential radiotoxicity

Knowledge about dispersion is of utmost importance for understanding populations’ reaction to changes in the environment. Expansion of a population range brings with it both spatial sorting and over time, spatial selection. This means that dispersion rates increases over time at the expanding edge. Most studies have so far been performed on continuously expanding populations. This study aims to bring more knowledge about dispersal biology in dynamic systems. I studied dispersal traits in two permanent and two seasonal vegetation habitats of an isopod (Asellus aquaticus), for which differentiation between habitat types has previously been shown. I quantified differences in displacement (dispersal rate) and three morphological traits, head angle (body streamline) and leg of the third and seventh pair of legs. Isopods from the seasonal vegetation had higher displacement rates than animals from permanent vegetation. This inclines that mechanisms driving spatial selection in expanding population ranges also exist in dynamic systems. The more streamlined isopods found in seasonal sites further points towards spatial sorting by dispersion capability. Because no effect of permanence was found on leg length and there was no correlation between streamlining and displacement, the higher dispersion among animals from seasonal habitats most likely derives from behavioral differences.

The thesis focuses on exploring the sub-grain structure in stainless steel 316L prepared by additive manufacturing (AM). Two powder-bed based AM methods are involved: selective laser melting (SLM) and electron beam melting (EBM). It is already known that AM 316L has heterogeneous property and hierarchy structure: micro-sized melt pools, micro-sized grains, nano-sized sub-grain structure and nano-sized inclusions. Yet, the relation among these structures and their influence on mechanical properties have not been clearly revealed so far. Melt pool boundaries having lower amount of sub-grain segregated network structures (Cellular structure) are weaker compared to the base material. Compared with cell boundaries, grain boundaries have less influence on strength but are still important for ductility. Cell boundaries strengthen the material without losing ductility as revealed by mechanical tests. Cellular structure can be continuous across the melt pool boundaries, low angle sub-grain boundaries, but not grain boundaries. Based on the above understanding, AM process parameters were adjusted to achieve customized mechanical properties. Comprehensive characterization were carried out to investigate the density, composition, microstructure, phase, magnetic permeability, tensile property, Charpy impact property, and fatigue property of both SLM and EBM SS316L at room temperature and at elevated temperatures (250°C and 400°C). In general, SLM SS316L has better strength while EBM SS316L has better ductility due to the different process conditions. Improved cell connection between melt pools were achieved by rotating 45° scanning direction between each layer compared to rotating 90°. Superior mechanical properties (yield strength 552 MPa and elongation 83%) were achieved in SLM SS316L fabricated with 20 µm layer thickness and tested in the building direction. Y2O3 added oxide dispersed strengthening steel (ODSS) were also prepared by SLM to further improve its performance at elevated temperatures. Slightly improved strength and ductility (yield strength 574 MPa and elongation 90%) were obtained on 0.3%Y2O3-ODSS with evenly dispersed nanoparticles (20 nm). The strength drops slightly  but ductility drops dramatically at elevated temperatures. Fractographic analysis results revealed that the coalescence of nano-voids is hindered at room temperature but not at elevated temperatures. The achieved promising properties in large AM specimens assure its potential application in nuclear fusion. For the first time, ITER first wall panel parts with complex inner pipe structure were successfully fabricated by both SLM and EBM which gives great confidence to application of AM in nuclear industry.<br><p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.</p>

L'apprentissage statistique cherche à modéliser un lien fonctionnel entre deux variables X et Y à partir d'un échantillon aléatoire de réalisations de (X,Y ). Lorsque la variable Y prend un nombre binaire de valeurs, l'apprentissage s'appelle la classification (ou discrimination en français) et apprendre le lien fonctionnel s'apparente à apprendre la frontière d'une variété dans l'espace de la variable X. Dans cette thèse, nous nous plaçons dans le contexte de l'apprentissage actif, i.e. nous supposons que l'échantillon d'apprentissage n'est plus aléatoire et que nous pouvons, par l'intermédiaire d'un oracle, générer les points sur lesquels l'apprentissage de la variété va s'effectuer. Dans le cas où la variable Y est continue (régression), des travaux précédents montrent que le critère de la faible discrépance pour générer les premiers points d'apprentissage est adéquat. Nous montrons, de manière surprenante, que ces résultats ne peuvent pas être transférés à la classification. Dans ce manuscrit, nous proposons alors le critère de la dispersion pour la classification. Ce critère étant difficile à mettre en pratique, nous proposons un nouvel algorithme pour générer un plan d'expérience à faible dispersion dans le carré unité. Après une première approximation de la variété, des approximations successives peuvent être réalisées afin d'affiner la connaissance de celle-ci. Deux méthodes d'échantillonnage sont alors envisageables : le " selective sampling " qui choisit les points à présenter à un oracle parmi un ensemble fini de candidats et l'" adaptative sampling " qui permet de choisir n'importe quels points de l'espace de la variable X. Le deuxième échantillonnage peut être vu comme un passage à la limite du premier. Néanmoins, en pratique, il n'est pas raisonnable d'utiliser cette méthode. Nous proposons alors un nouvel algorithme basé sur le critère de dispersion, menant de front exploitation et exploration, pour approximer une variété.

There are many regularization parameter selection methods that can be used when solving inverse problems, but it is not clear which one is best suited for the inverse dispersion problem. The suitability of three different methods for solving the inverse dispersion problem are evaluated here in order to pick a suitable method for these kinds of problems in the future. The regularization parameter selection methods are used to solve the separable non-linear inverse dispersion problem which is adjusted and solved as a linear inverse problem. It is solved with Tikhonov regularization and the model is a time integrated Gaussian puff model. The dispersion problem is used with different settings and is solved with the three methods. The three methods are generalized cross-validation, L-curve method and quasi-optimality criterion. They produce rather different solutions and the results show that generalized cross-validation is the best choice. The other methods are less stable and the errors are sometimes orders of magnitude larger than the errors from generalized cross-validation.