275 research outputs found
Amélioration du procédé RTM par l'optimisation des paramÚtres d'injection
Quelques chiffres -- Introduction aux matériaux composites -- Le moulage par transfert de résine (RTM) -- Les renforts fibreux -- PhénomÚnes 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.RĂSUMĂLâarticle soutient que la rĂ©pression financiĂšre avec dâautres facteurs institutionnels a jouĂ© un rĂŽle clĂ© dans lâĂ©mergence des coopĂ©ratives financiĂšres (CF) au Costa Rica. Les CF ont profitĂ© de lâoccasion pour servir les emprunteurs dont les demandes avaient Ă©tĂ© refusĂ©es par les banques. Compte tenu des rĂ©formes radicales du systĂšme financier costaricien, visant Ă rĂ©duire lâĂ©tendue de la rĂ©pression financiĂšre, lâarticle pose la question de lâincidence de ces rĂ©formes sur la compĂ©titivitĂ© des CF. La littĂ©rature prĂ©cĂ©dente suggĂšre que la rĂ©forme financiĂšre peut conduire Ă une concentration du secteur financier et non Ă la promotion dâun environnement plus compĂ©titif. Notre article dĂ©montre que les CF du Costa Rica prĂ©sentent une capacitĂ© Ă accroĂźtre leurs parts de marchĂ©. Nous fournissons une explication de la tendance observĂ©e
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 structureâproperty 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 10â20) consecutive cytosine bases through numerous Ï-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 NafionÂź115membranes. 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
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