Amélioration du procédé RTM par l'optimisation des paramètres d'injection

Quelques chiffres -- Introduction aux matériaux composites -- Le moulage par transfert de résine (RTM) -- Les renforts fibreux -- Phénomènes entourant l'imprégnation d'un renfort fibreux -- Objectifs de la recherche -- Organisation du mémoire -- Porosity reduction using optimized flow velocity in resin transfer molding part I : selecting the proper injection conditions -- Porosity reduction using optimized flow velocity in resin transfer molding part II : voids and mechanical properties

NMR imaging of water flow in packed beds

Measurements by magnetic resonance imaging (MRI) of water flow within granular porous media are presented in this study. Our goal was not only to obtain visualizations of velocity field in porous media but rather to make accurate measurements of interstitial and averaged velocities in bead packs. Two situations were examined: the first for a packed bed with a large beads diameter where it was possible to visualize the interstitial velocities and the second with a packed bed with a small beads diameter where only averaged interstitial velocities were measured

Long range coherent magnetic bound states in superconductors

The quantum coherent coupling of completely different degrees of freedom is a challenging path towards creating new functionalities for quantum electronics. Usually the antagonistic coupling between spins of magnetic impurities and superconductivity leads to the destruction of the superconducting order. Here we show that a localized classical spin of an iron atom immersed in a superconducting condensate can give rise to new kind of long range coherent magnetic quantum state. In addition to the well-known Shiba bound state present on top of an impurity we reveal the existence of a star shaped pattern which extends as far as 12 nm from the impurity location. This large spatial dispersion turns out to be related, in a non-trivial way, to the superconducting coherence length. Inside star branches we observed short scale interference fringes with a particle-hole asymmetry. Our theoretical approach captures these features and relates them to the electronic band structure and the Fermi wave length of the superconductor. The discovery of a directional long range effect implies that distant magnetic atoms could coherently interact leading to new topological superconducting phases with fascinating properties

Effects of mass transfers and heating rates during bark and wood hydrothermal treatment

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The competitive edge of credit unions in Costa Rica: From financial repression to the risks of a new financial environment

This article argues that financial repression played a key role in the emergence of credit unions (CUs) in Costa Rica, along with other institutional factors. Credit unions took advantage of the opportunity to serve borrowers whose requests had been refused by banks. Given the sweeping reforms of the Costa Rican financial system aimed at reducing the scope of financial repression, this article poses the question of how those reforms impacted the competitiveness of CUs. Previous literature suggests that financial reform may lead to concentration in the financial sector, and not to the promotion of a more competitive environment. This article presents data showing that CUs in Costa Rica exhibited an enhanced ability to gain market share and also provides an explanation for the observed trend.RÉSUMÉL’article soutient que la répression financière avec d’autres facteurs institutionnels a joué un rôle clé dans l’émergence des coopératives financières (CF) au Costa Rica. Les CF ont profité de l’occasion pour servir les emprunteurs dont les demandes avaient été refusées par les banques. Compte tenu des réformes radicales du système financier costaricien, visant à réduire l’étendue de la répression financière, l’article pose la question de l’incidence de ces réformes sur la compétitivité des CF. La littérature précédente suggère que la réforme financière peut conduire à une concentration du secteur financier et non à la promotion d’un environnement plus compétitif. Notre article démontre que les CF du Costa Rica présentent une capacité à accroître leurs parts de marché. Nous fournissons une explication de la tendance observée

Study of dispersion by NMR: comparison between NMR measurements and stochastic simulation

Dispersion remains, today, a highly topical subject. Our group has been interested in characterizing this phenomenon by pulsed-field-gradient NMR technique. Direct measurement of the dispersion coefficient can be done with a Pulsed Gradient Spin Echo (PGSE) sequence by assuming that the asymptotic regime is reached. In unsteady state, the propagator formalism is used. To better understand these measurements, the NMR experiment is modeled using a stochastic simulation (random walks) and compared with experimental results. The comparison is made for the simple case of Poiseuille flow in a circular tube (Taylor-Aris dispersion)

Study of dispersion by NMR: comparison between NMR measurements and stochastic simulation

Dispersion remains, today, a highly topical subject. Our group has been interested in characterizing this phenomenon by pulsed-field-gradient NMR technique. Direct measurement of the dispersion coefficient can be done with a Pulsed Gradient Spin Echo (PGSE) sequence by assuming that the asymptotic regime is reached. In unsteady state, the propagator formalism is used. To better understand these measurements, the NMR experiment is modeled using a stochastic simulation (random walks) and compared with experimental results. The comparison is made for the simple case of Poiseuille flow in a circular tube (Taylor-Aris dispersion)

Structural and photophysical templating of conjugated polyelectrolytes with single-stranded DNA

A promising approach to influence and control the photophysical properties of conjugated polymers is directing their molecular conformation by templating. We explore here the templating effect of single-stranded DNA oligomers (ssDNAs) on cationic polythiophenes with the goal to uncover the intermolecular interactions that direct the polymer backbone conformation. We have comprehensively characterized the optical behavior and structure of the polythiophenes in conformationally distinct complexes depending on the sequence of nucleic bases and addressed the effect on the ultrafast excited-state relaxation. This, in combination with molecular dynamics simulations, allowed us a detailed atomistic-level understanding of the structure−property correlations. We find that electrostatic and other noncovalent interactions direct the assembly with the polymer, and we identify that optimal templating is achieved with (ideally 10−20) consecutive cytosine bases through numerous π-stacking interactions with the thiophene rings and side groups of the polymer, leading to a rigid assembly with ssDNA, with highly ordered chains and unique optical signatures. Our insights are an important step forward in an effective approach to structural templating and optoelectronic control of conjugated polymers and organic materials in general

Non-hyperbolic ergodic measures and horseshoes in partially hyperbolic homoclinic classes

We study a rich family of robustly non-hyperbolic transitive diffeomorphisms and we show that each ergodic measure is approached by hyperbolic sets in weak$*$-topology and in entropy. For hyperbolic ergodic measures, it is a classical result of A. Katok. The novelty here is to deal with non-hyperbolic ergodic measures.Comment: 25 pages and 1 figur

NMR study of the anisotropic transport properties of uniaxially stretched membranes for fuel cells

We used NMR techniques to probe the anisotropic properties of stretched Nafion®115membranes. The alignment of the polymeric structure under a uniaxial load is at the origin of a strong anisotropy of both the water self-diffusion coefficient and the proton conductivity. The determination of these two important membrane properties may lead to new fundamental information on the nature of the proton transport mechanisms in such oriented weaklycharged systems