Overcoming non-radiative losses with AlGaAs PIN junctions for near-field thermophotonic energy harvesting

In a thermophotonic device used in an energy-harvesting configuration, a hot light-emitting diode (LED) is coupled to a photovoltaic (PV) cell by means of electroluminescent radiation in order to produce electrical power. Using fluctuational electrodynamics and the drift-diffusion equations, we optimise a device made of an AlGaAs PIN LED and a GaAs PIN PV cell with matched bandgaps. We find that the LED can work as an efficient heat pump only in the near field, where radiative heat transfer is increased by wave tunnelling. A key reason is that non-radiative recombination rates are reduced compared to radiative ones in this regime. At 10 nm gap distance and for 100 cm.s --1 effective surface recombination velocity, the power output can reach 2.2 W.cm --2 for a 600 K LED, which highlights the potential for low-grade energy harvesting

Quantifying Degree of Goal Directedness in Document Navigation: Application to the Evaluation of the Perspective-Drag Technique

International audienceThis article pursues a two-fold goal. First we introduce degree of goal directedness (DGD), a novel quantitative dimension for the taxonomy of navigation tasks in general. As an attempt to operationalize the DGD concept in the context of electronic documents navigation, we introduce the serial target-acquisition (STA) experimental paradigm. We suggest that DGD and the STA paradigm may usefully enrich the conceptual toolkit of HCI research for the evaluation of navigation techniques. Our second goal is to illustrate the utility of the DGD concept by showing with a concrete example, Perspective Drag, the refinement it allows in evaluating navigation techniques. We report data obtained from two experiments with the STA paradigm that cast light on what Perspective Drag is specifically good for: it is particularly suitable in realistic task contexts where navigation is less than 100% directed by its terminal goal, that is, where the user wants not only to reach a particular item but also to pick up information from the document during document traversal

Influence des conditions de mouillage sur les déplacements quasi-statiques eau-air et l'évaporation en milieux poreux modèles. Application à la gestion de l'eau dans les piles à combustibles de type PEMFC

Cette étude est motivée par le problème de la gestion de l'eau des piles à combustible à membrane échangeuse de protons (PEMFC). Elle a pour objectif principal une étude expérimentale et le développement d'outils de simulation d'écoulements quasi-statiques eau-air avec ou sans changement de phase en milieu poreux modèles dans le but d'améliorer les connaissances, notamment en ce qui concerne l'effet de la mouillabilité du milieu poreux sur les lois d'invasion et les cinétiques de séchage. L'étude est effectuée pour deux types de milieux poreux modèles bidimensionnels : - Réseau carré de liens, chaque lien étant de section rectangulaire variable. - Réseau de cylindres (de façon à s'approcher de la structure des milieux fibreux généralement rencontrés dans les couches de diffusion des piles). Nous développons une modélisation de l'imbibition et du drainage quasi-statiques dans des milieux de mouillabilité homogène, soit hydrophobe (pour des gammes d'angle de contact comparables à ceux des milieux poreux rencontrés dans les PEMFC), soit hydrophile. Ensuite, nous montrons comment les règles d'imbibition et du drainage établies peuvent être utilisées pour modéliser l'évaporation lente dans de tels milieux par des approches de type réseau de pores. Des comparaisons entre le drainage et l'imbibition montrent une influence tout à fait significative de la mouillabilité sur la saturation à la percée, la percée étant atteinte beaucoup plus tôt dans le cas hydrophile. Cette influence est également très nette lorsqu'on compare les distributions des phases lors de l'évaporation ou les temps de séchage, le milieu hydrophile séchant plus rapidement que le milieu hydrophobe. Ces temps sont caractérisés à partir d'une étude statistique en fonction de la taille du réseau. Le cas de milieux de mouillabilité hétérogène comprenant une région hydrophobe et une région hydrophile est également considéré. L'implication des résultats sur la problématique de la gestion de l'eau dans les piles à combustibles est discutée. ABSTRACT : This study is motivated by the water management problem in PEM fuel cells (PEMFC). Experiments and numerical simulations of quasi-static air-water flows and evaporation in model porous media are developped in order to study the effect of wettability on fluid invasion patterns and evaporation kinetics. The study is performed for two diffrent types of 2D model porous media : - square network of bonds of rectangular cross section. - network of cylinders (considered as a model fibrous medium) Modelling of quasi-static imbibition and drainage is developped in model of uniform wettability, either hydrophilic or hydrophobic for contact angles similar to those expected in the gas diffusion layer of PEMFCs. Then, it is shown how the local invasion rules developed for the modelling of the drainage and imbibition processes can be used to model the evaporation process. Comparisons between drainage and imbibition show a strong influence of wettability on the saturation at breakthrough as well as on the drying times. The drying time is characterized as a function of network size from a statistical study. Media of heteregeneous wettability made of two layers, one hydrophobic, the other hydrophilic are also considered. In a last part, implications of the results on the water management problem in the gas diffusion layers (GDLs) of PEMFC are discussed

L’École Polytechnique et les hydrographes de la Marine

Olivier Chapuis, docteur en Histoire et journaliste à « Voiles et voiliers », est l’auteur d’« À la mer comme au ciel - Beautemps-Beaupré & la naissance de l’hydrographie moderne ou L’émergence de la précision en navigation et dans la cartographie marine (1700-1850) » (Presses de l’université de Paris-Sorbonne, 1999), primé par l’Académie de marine et Grand-prix de la Mer 2000 de l’Association des écrivains de langue française. Le présent article est tiré de cet ouvrage, lui-même issu de sa ..

L’École Polytechnique et les hydrographes de la Marine

Olivier Chapuis, docteur en Histoire et journaliste à « Voiles et voiliers », est l’auteur d’« À la mer comme au ciel - Beautemps-Beaupré & la naissance de l’hydrographie moderne ou L’émergence de la précision en navigation et dans la cartographie marine (1700-1850) » (Presses de l’université de Paris-Sorbonne, 1999), primé par l’Académie de marine et Grand-prix de la Mer 2000 de l’Association des écrivains de langue française. Le présent article est tiré de cet ouvrage, lui-même issu de sa ..

Strong tip-sample coupling in thermal radiation scanning tunneling microscopy

We analyze how a probing particle modifies the infrared electromagnetic near field of a sample. The particle, described by electric and magnetic polarizabilities, represents the tip of an apertureless scanning optical near-field microscope (SNOM). We show that the interaction with the sample can be accounted for by ascribing to the particle dressed polarizabilities that combine the effects of image dipoles with retardation. When calculated from these polarizabilities, the SNOM signal depends only on the fields without the perturbing tip. If the studied surface is not illuminated by an external source but heated instead, the signal is closely related to the projected electromagnetic local density of states (EM-LDOS). Our calculations provide the link between the measured far-field spectra and the sample's optical properties.We also analyze the case where the probing particle is hotter than the sample and evaluate the impact of the dressed polarizabilities on near-field radiative heat transfer. We show that such a heated probe above a surface performs a surface spectroscopy, in the sense that the spectrum of the heat current is closely related to the local electromagnetic density of states. The calculations agree well with available experimental data.Comment: Soumis \`a JQSRT. arXiv admin note: substantial text overlap with arXiv:1201.483

Increase of Thermal Resistance Between a Nanostructure and a Surface due to Phonon Multireflections

The thermal resistance between a nanostructure and a half-body is calculated in the framework of particle-phonons physics. The current models approximate the nanostructure as a thermal bath. We prove that the multireflections of heat carriers in the nanostructure significantly increase resistance in contradiction with former predictions. This increase depends on the shape of the nanostructure and the heat carriers mean free path only. We provide a general and simple expression for the contact resistance and examine the specific cases of nanowires and nanoparticles

Trade-offs Between a Vertical Shared Display and Two Desktops in a Collaborative Path-Finding Task

International audienceLarge vertical displays are considered well adapted for collaboration, due to their display surface and the space in front of them that can accommodate multiple people. However, there are few studies that empirically support this assertion, and they do not quantitatively assess the differences of collaboration in front of a shared display compared to a non-shared setup, such as multiple desktops with a common view. In this paper, we compare a large shared vertical display with two desktops, when pairs of users learn to perform a path-planning task. Our results did not indicate a significant difference in learning between the two setups, but found that participants adopted different task strategies. Moreover, while pairs were overall faster with the two desktops, quality was more consistent in the vertical shared display where pairs spent more time communicating, even though there is a-priori more implicit collaboration in this setu