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Overfitting Bayesian Mixture Models with an Unknown Number of Components
No full textInternational audienc
Capillary stretching of fibers
No full textInternational audienceWe study the interaction of a finite volume of liquid with two parallel thin flexible fibers. A tension along the fibers is imposed and may be varied. We report two morphologies, i.e. two types of wet adhesion: a weak capillary adhesion, where a liquid drop bridges the fibers, and a strong elastocapillary adhesion where the liquid is spread between two collapsed fibers. We show that geometry, capillarity and stretching are the key parameters at play. We describe the collapse and detachment of the fibers as a function of two nondimensional parameters, arising from the geometry of the system and a balance between capillary and stretching energies. In addition, we show that the morphology, thus the capillary adhesion, can be controlled by changing the tension within the fibers. Copyright (C) EPLA, 201
Laser-driven platform for generation and characterization of strong quasi-static magnetic fields
No full textInternational audienceQuasi-static magnetic-fields up to 800 T are generated in the interaction of intense laser pulses (500 J, 1 ns, 10(17) W cm(-2)) with capacitor-coil targets of different materials. The reproducible magnetic-field peak and rise-time, consistent with the laser pulse duration, were accurately inferred from measurements with GHz-bandwidth inductor pickup coils (B-dot probes). Results from Faraday rotation of polarized optical laser light and deflectometry of energetic proton beams are consistent with the B-dot probe measurements at the early stages of the target charging, up to t approximate to 0.35 ns, and then are disturbed by radiation and plasma effects. The field has a dipole-like distribution over a characteristic volume of 1 mm(3), which is consistent with theoretical expectations. These results demonstrate a very efficient conversion of the laser energy into magnetic fields, thus establishing a robust laser-driven platform for reproducible, well characterized, generation of quasi-static magnetic fields at the kT-level, as well as for magnetization and accurate probing of high-energy-density samples driven by secondary powerful laser or particle beams
Effect of argon ion energy on the performance of silicon nitride multilayer permeation barriers grown by hot-wire CVD on polymers
No full textInternational audiencePermeation barrier Polymer substrate Permeation barriers for organic electronic devices on polymer flexible substrates were realized by combining stacked silicon nitride (SiN x) single layers (50 nm thick) deposited by hot-wire chemical vapor deposition process at low-temperature (~100°°C) with a specific argon plasma treatment between two successive layers. Several plasma parameters (RF power density, pressure, treatment duration) as well as the number of single layers have been explored in order to improve the quality of permeation barriers deposited on polyethylene tere-phthalate. In this work, maximum ion energy was highlighted as the crucial parameter making it possible to minimize water vapor transmission rate (WVTR), as determined by the electrical calcium test method, all the other parameters being kept fixed. Thus fixing the plasma treatment duration at 8 min for a stack of two SiN x single layers, a minimum WVTR of 5 × 10 −4 g/(m 2 day), measured at room temperature, was found for a maximum ion energy of ~30 eV. This minimum WVTR value was reduced to 7 × 10 −5 g/(m 2 day) for a stack of five SiN x single layers. The reduction in the permeability is interpreted as due to the rearrangement of atoms at the interfaces when average transferred ion energy to target atoms exceeds threshold displacement energy
A review of adjoint methods for sensitivity analysis, uncertainty quantification and optimization in numerical codes
No full textNational audienceThe goal of this paper is to briefly recall the importance of the adjoint method in many problems of sensitivity analysis, uncertainty quantification and optimization when the model is a differential equation. We illustrate this notion with some recent examples. As is well known, from a computational point of view the adjoint method is intrusive, meaning that it requires some changes in the numerical codes. Therefore we advocate that any new software development must take into account this issue, right from its inception
Mesures et modélisation du comportement de panneaux photovoltaïques en environnement réel
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Etude de nanofils de silicium comme matériau d'électrode négative de batterie lithium-ion
No full textSilicon is a promising anode material to increase the capacity of Li-ion battery. Unfortunately, it undergoes huge volume variations upon cycling, leading to fast capacity decay of the electrode. One of the keys to overcome this issue is to use silicon nanowires (SiNWs), synthesized by chemical vapor deposition, which have the ability to better accommodate the volume changes. The thesis presented here aims at studying the electrochemical performances of such electrodes and the possible improvement in the perspective of a use in a Li-ion battery. First of all, analysis by nuclear magnetic resonance (NMR), electron energy loss spectroscopy and electronic tomography were performed along the first cycle in order to study lithium insertion and extraction in this material. Different strategies were then addressed to reach a better cycle life, and ex-situ analysis by electron microscopy and NMR allowed a better understanding of the electrode ageing, by notably pointing out the continuous electrolyte degradation. This is at the origin of irreversible lithium consumption which is not compatible with a cycling in a full Li-ion cell. The silicon electrode prelithiation appears as a promising way to overcome this issue. Another important barrier for the elaboration of full Li-ion batteries with high energy density is the low surfacic capacity of SiNWs electrodes. New electrodes based on a silicon nano-tree structure were then synthesized to reach higher silicon loading and allowed to increase the surfacic capacity of the electrodes by a factor 6. Hence, this work gives new insights for the elaboration of high energy density Li-ion battery using a nanostructured silicon anode.Le silicium est un matériau d’électrode prometteur pour augmenter la capacité de stockage des batteries Li-ion. Il subit malheureusement d’importantes variations de volume lors du cyclage qui diminuent la durée de vie des électrodes. Une des clés pour remédier à ce problème est l’utilisation de nanofils de silicium, synthétisés par dépôt chimique en phase vapeur, pouvant accommoder plus facilement ces variations de volume. La thèse présentée ici s’attache à étudier les performances de ce nouveau type d’électrode et les possibilités d’amélioration en vue d’une utilisation en batterie Li-ion. Dans un premier temps, des analyses par résonance magnétique nucléaire (RMN), spectroscopie de perte d’énergie des électrons et tomographie électronique ont été effectuées à différents stades du premier cycle afin d’étudier l’insertion et l’extraction du lithium dans ce matériau. Différentes stratégies de cyclage ont ensuite été abordées pour augmenter la durée de vie. Des analyses ex-situ d’électrodes par microscopie électronique et RMN ont alors permis de mieux comprendre le vieillissement, mettant notamment en évidence la dégradation continue de l’électrolyte sur le silicium qui est à l’origine d’une consommation irréversible de lithium. La prélithiation des électrodes apparaît comme une voie prometteuse pour remédier à ce problème. Enfin, de nouvelles structures plus denses à base de nano-arbres de silicium ont été synthétisées et ont permis de multiplier par plus d’un facteur 6 la capacité surfacique. Ce travail ouvre ainsi des perspectives intéressantes pour l’élaboration de cellules lithium-ion complètes de grande densité d’énergie utilisant une anode de silicium nanostructurée
Analyse statistique des modèles de croissance-fragmentation
No full textThis work is concerned with growth-fragmentation models, implemented for investigating the growth of a population of cells which divide according to an unknown splitting rate, depending on a structuring variable – age and size being the two paradigmatic examples. The mathematical framework includes statistics of processes, nonparametric estimations and analysis of partial differential equations. The three objectives of this work are the following : get a nonparametric estimate of the division rate (as a function of age or size) for different observation schemes (genealogical or continuous) ; to study the transmission of a biological feature from one cell to an other and study the feature of one typical cell ; to compare different populations of cells through their Malthus parameter, which governs the global growth (when introducing variability in the growth rate among cells for instance).Cette étude théorique est pensée en lien étroit avec un champ d'application : il s'agit de modéliser la croissance d'une population de cellules qui se divisent selon un taux de division inconnu, fonction d’une variable dite structurante – l’âge et la taille des cellules étant les deux exemples paradigmatiques étudiés. Le champ mathématique afférent se situe à l'interface de la statistique des processus, de l’estimation non-paramétrique et de l’analyse des équations aux dérivées partielles. Les trois objectifs de ce travail sont les suivants : reconstruire le taux de division (fonction de l’âge ou de la taille) pour différents schémas d’observation (en temps généalogique ou en temps continu) ; étudier la transmission d'un trait biologique général d'une cellule à une autre et étudier le trait d’une cellule typique ; comparer la croissance de différentes populations de cellules à travers le paramètre de Malthus (après introduction de variabilité dans le taux de croissance par exemple)