905 research outputs found
Insights on finite size effects in Ab-initio study of CO adsorption and dissociation on Fe 110 surface
Adsorption and dissociation of hydrocarbons on metallic surfaces represent
crucial steps to carburization of metal. Here, we use density functional theory
total energy calculations with the climbing-image nudged elastic band method to
estimate the adsorption energies and dissociation barriers for different CO
coverages with surface supercells of different sizes. For the absorption of CO,
the contribution from van der Waals interaction in the computation of
adsorption parameters is found important in small systems with high
CO-coverages. The dissociation process involves carbon insertion into the Fe
surface causing a lattice deformation that requires a larger surface system for
unrestricted relaxation. We show that, in larger surface systems associated
with dilute CO-coverages, the dissociation barrier is significantly decreased.
The elastic deformation of the surface is generic and can potentially
applicable for all similar metal-hydrocarbon reactions and therefore a dilute
coverage is necessary for the simulation of these reactions as isolated
processes.Comment: 12 pages, 6 figures. Submitted to Journal of Applied Physic
Les interactions entre l'homme et le poulain de la naissance au sevrage : synthèse bibliographique
Les Hommes de chevaux cherchent à avoir les chevaux les plus dociles possibles. Les scientifiques ont étudié la période juvénile du cheval car une période dite « critique a été mise en évidence chez de nombreuses autres espèces animales. En effet, pendant celle-ci, le jeune est plus sensible et réceptif aux stimuli extérieurs, aux interactions avec d’autres individus, ainsi qu’aux manipulations. La bonne gestion de cette période aboutit à une meilleure socialisation. Qu’en est-il du poulain ? Ce travail porte donc sur les études menées auprès des poulains entre leur naissance et leur sevrage, le but étant d’évaluer l’impact des différentes manipulations précoces sur leur comportement futur. A travers l’analyse des résultats relevés, ce travail vise également à dégager les attitudes optimales qu’il faudrait adopter auprès du poulain pour le socialisé à l’Homme et le rendre plus maniable
Dislocation interaction with C in alpha-Fe: a comparison between atomic simulations and elasticity theory
The interaction of C atoms with a screw and an edge dislocation is modelled
at an atomic scale using an empirical Fe-C interatomic potential based on the
Embedded Atom Method (EAM) and molecular statics simulations. Results of atomic
simulations are compared with predictions of elasticity theory. It is shown
that a quantitative agreement can be obtained between both modelling techniques
as long as anisotropic elastic calculations are performed and both the
dilatation and the tetragonal distortion induced by the C interstitial are
considered. Using isotropic elasticity allows to predict the main trends of the
interaction and considering only the interstitial dilatation will lead to a
wrong interaction
Pressure effect on diffusion of carbon at the 85.91◦ 100 symmetric tilt grain boundary of α-iron
The diffusion mechanism of carbon in iron plays a vital role in carburization processes, steel fabrication, and metal dusting corrosion. In this paper, using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo algorithm with on-the-fly catalog building that allows to obtain diffusion properties over large time scales taking full account of chemical and elastic effects coupled with an EAM potential, we investigate the effect of pressure on the diffusion properties of carbon in
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symmetric tilt grain boundaries (GB) of
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-iron up to a pressure of 12 kbar at a single temperature of 600 K. We find that, while the effect of pressure can strongly modify the C stability and diffusivity in the GB in ways that depend closely on the local environment and the nature of the deformation, isotropic and uniaxial pressure can lead to opposite and nonmonotonous effects regarding segregation energy and activation barriers. These observations are relevant to understanding of the evolution of heterogeneous materials, where variations of local pressure can alter the carbon diffusion across the material
Valorisation des polymères styréniques issus des déchets d'équipements électroniques et électriques
L objectif de cette thèse est de valoriser les polymères styréniques issus des déchets d équipements électroniques et électriques. Les polymères considérés sont le polystyrène (PS), le polystyrène choc (HIPS), le poly(acrylonitrile-butadiène-styrène) (ABS), l'ABS couplé avec du polycarbonate (ABS/PC) et le poly(styrène-acrylonitrile) (SAN). Une étude préliminaire a permis de définir deux mélanges à compatibiliser : le mélange PS/ABS et le mélange PS/PC. Pour le mélange PS/ABS, la voie de compatibilisation ionique est étudiée. L ajout d un copolymère présentant une structure ionique et la création d un réseau in-situ sont tentés. Ces deux voies n ont pas donné de résultats concluants en termes d amélioration des propriétés finales du mélange. Le mélange PS/PC est lui compatibilisé par ajout d un copolymère polystyrène bloc-poly(éthylène-butylène)-bloc-polystyrène greffé PC. Ce copolymère est tout d abord créé en mélangeur interne, afin d étudier différents catalyseurs susceptibles d initier le greffage, puis l extrusion réactive est utilisée pour synthétiser ce copolymère à grande échelle. Plusieurs taux de copolymère sont alors introduits dans le mélange PS/PC et l amélioration des propriétés et des microstructures prouve son effet compatibilisantThe aim of this work consists in the recycling of the styrenic polymers coming from waste of electric and electronic equipments. Polystyrene (PS), high impact polystyrene (HIPS), poly(acrylonitrile-butadiene-styrene) (ABS), its alloy with the polycarbonate (ABS/PC) and poly(styrene-acrylonitrile) (SAN) are considered. A preliminary study permits to choose two blends to study: PS/ABS and PS/PC. In the case of PS/ABS blend, the ionic way of compatibilization was studied. The addition of a copolymer containing an ionic structure and the creation of an ionic network in-situ are investigated. Both these attempts are not conclusive about the improvement of the final properties of the blend. The PS/PC blend is then compatibilized by adding a polystyrene-block-poly(ethylene-butylene)-block-polystyrene grafted polycarbonate. This copolymer is first created in an internal mixer, in order to study different catalysts able to initiate the grafting, and then the reactive extrusion is used to synthesize it on a large scale. Several amounts are introduced in the PS/PC blend and the improvement of the properties and microstructures proves its compatibilizing effectST ETIENNE-Bib. électronique (422189901) / SudocSudocFranceF
Acute dengue virus 2 infection in Gabonese patients is associated with an early innate immune response, including strong interferon alpha production
<p>Abstract</p> <p>Background</p> <p>Dengue is now a leading cause of morbidity and mortality throughout the tropics. We conducted the first <it>ex vivo </it>study of dengue fever (DF) in African patients infected during the first Gabonese dengue virus 2 (DENV-2) outbreak in 2007, in order to investigate cytokine production, including the antiviral cytokine IFN-α, reported to be a potent inhibitor of DENV replication <it>in vitro</it>.</p> <p>Methods</p> <p>Levels of 50 cytokines, chemokines and growth factors were measured in plasma from 36 patients with DENV-2 infection, and in uninfected controls, using Luminex multiplex technology. The results were interpreted according to the day of sampling after symptom onset. PBMC from six patients were also studied for T lymphocyte cell surface marker expression by flow cytometry.</p> <p>Results</p> <p>Acute DENV-2 infection elicited high levels of several pro-inflammatory cytokines (IL-6 and IL-17), chemokines (MIF, RANTES, IP-10 and MCP-1) and growth factors (G-CSF, GM-CSF and VEGF-A). We also observed high levels of IFN-α for the first time in adult DF patients, and CD4+ and CD8+ T cell activation at symptom onset.</p> <p>Conclusion</p> <p>Acute DENV-2 infection in African patients elicits a strong innate response involving IFN-α production, as well as an adaptive immune response.</p
Threshold concentration for H blistering in defect free W
Lattice distortion induced by high concentration of H is believed to be
precursor of H blistering in single crystalline W (SCW) during H isotope
irradiation. However, the critical H concentration needed to trigger
bond-breaking of metal atoms presents a challenge to measure. Using density
functional theory, we have calculated the formation energy of a vacancy and a
self-interstitial atom (SIA) in supersaturated defect-free SCW with various H
concentrations. When the ratio of H:W exceeds 1:2, the formation of both
vacancies and self-interstitials becomes exothermic, meaning that spontaneous
formation of micro-voids which can accommodate molecular H2 will occur.
Molecular H2 is not allowed to form, and it is not needed either at the very
initial stage of H blistering in SCW. With supersaturated H, the free volume at
the vacancy or SIA is greatly smeared out with severe lattice distortion and
more H can be trapped than in the dilute H case.Comment: 13 pages, 4 figure
Study of hydrogen isotopes behavior in tungsten by a multi trapping macroscopic rate equation model
International audienceDensity functional theory (DFT) studies show that in tungsten a mono vacancy can contain up to six hydrogen isotopes (HIs) at 300 K with detrapping energies varying with the number of HIs inthe vacancy. Using these predictions, a multi trapping rate equation model has been built and used to model thermal desorption spectrometry (TDS) experiments performed on single crystaltungsten after deuterium ions implantation. Detrapping energies obtained from the model to adjust temperature of TDS spectrum observed experimentally are in good agreement with DFTvalues within a deviation below 10%. The desorption spectrum as well as the diffusion of deuterium in the bulk are rationalized in light of the model results
Carbon adsorption on and diffusion through the Fe(110) surface and in bulk: Developing a new strategy for the use of empirical potentials in complex material set-ups
Oil and gas infrastructures are submitted to extreme conditions
and off-shore rigs and petrochemical installations require
expensive high-quality materials to limit damaging failures.
Yet, due to a lack of microscopic understanding, most of these
materials are developed and selected based on empirical
evidence leading to over-qualified infrastructures. Computational
efforts are necessary, therefore, to identify the link
between atomistic and macroscopic scales and support the
development of better targeted materials for this and other
energy industry. As a first step towards understanding
carburization and metal dusting, we assess the capabilities of
an embedded atom method (EAM) empirical force field as well
as those of a ReaxFF force field using two different parameter
sets to describe carbon diffusion at the surface of Fe, comparing
the adsorption and diffusion of carbon into the 110 surface and
in bulk of a-iron with equivalent results produced by density
functional theory (DFT). The EAM potential has been
previously used successfully for bulk Fe-C systems. Our study
indicates that preference for C adsorption site, the surface to
subsurface diffusion of C atoms and their migration paths over
the 110 surface are in good agreement with DFT. The ReaxFF
potential is more suited for simulating the hydrocarbon reaction
at the surface while the subsequent diffusion to subsurface and
bulk is better captured with the EAM potential. This result
opens the door to a new approach for using empirical potentials
in the study of complex material set-up
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