Préparation d'oxydes de manganèse par des procédés de chimie douce (applications pour la combustion catalytique du diméthyléther et évaluation de leurs propriétés électrochimiques)

Cette étude concerne la préparation, la caractérisation, les applications catalytiques et électrochimiques de plusieurs oxydes de manganèse. Différentes stratégies synthétiques ont été utilisées : i) le procédé sol-gel, ii) la voie hydrothermale, iii) les méthodes sous irradiation ultras-sons et iv) le procédé polyol. Les variétés birnessite et cryptomelane ont été préparées avec une grande pureté par le procédé sol-gel et un mécanisme de formation rendant compte de leur formation a été proposé. Une méthode de préparation rapide pour la cryptomelane sous irradiation ultra-sons (UL) a aussi été développée. L'activité catalytique de ces solides a été évaluée pour la combustion du diméthyléther (DME) et comparée à celle de catalyseurs préparés par la voie hydrothermale (HT) et la voie des ultra-sons. L'activité catalytique est principalement liée à la phase cristalline et à la méthode de préparation ; elle suit l'ordre : cryptomelane (UL) > cryptomelane (HT) > cryptomelane (Sol-gel) > birnessite (Sol-gel) > b-MnO (HT) > g-MnO (HT). Qui plus est, l'activité catalytique de la birnessite et de la cryptomelane pour la combustion du DME peut être améliorée via l'incorporation de Ce suivant les méthodes d'échange d'ion et d'imprégnation. Enfin, birnessite et LiMn O ont aussi été préparés via la méthode polyol. Les deux solides présentent de hautes surfaces spécifiques, respectivement 137 et 329 m .g avec de très petites tailles de particules. Des tests électrochimiques préliminaires ont été réalisés et montrent des résultats encourageants.This study concerns the preparation, the characterization, the catalytic applications and the electrochemical properties of different manganese oxides. Several synthetic strategies were used : i) sol-gel process, ii) hydrothermal methods, iii) preparation methods under ultrasonic irradiation and iv) the polyol process. Highly pure birnessite and cryptomelane have been prepared by the sol-gel process and a plausible formation mechanism for the preparation of these solids is proposed. A fast preparation method for the cryptomelane under ultrasonic irradiation assistance (UL) was also developed. The catalytic activity of these solids was evaluated for the dimethylether (DME) combustion and compared to that of manganese oxides prepared using a facile hydrothermal method (HT) and with the ultrasonic assisted method. The catalytic activity is mainly dominated by the cristalline phase and the preparation method, which follows the order : cryptomelane (UL) > cryptomelane (HT) > cryptomelane (Sol-gel) > birnessite (Sol-gel) > b-MnO (HT) > g-MnO (HT). Moreover, the catalytic activity of birnessite and cryptomelane towards DME combustion can be enhanced through Ce incorporation via ion exchange and impregnation methods. Birnessite and LiMn O were also prepared using the polyol process. The two solids display high specific surface areas, respectively 137 and 329 m .g- with very small particle sizes. Preliminary electrochemical tests performed with LiMn O show promising results.PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Solvation Effects Drive the Selectivity in Diels-Alder Reaction Under Hyperbaric Conditions

International audienceHigh pressure effects on the Diels-Alder reaction in condensed phase are investigated by means of theoretical methods, employing advanced multiscale modeling approaches based on physically grounded models. The simulations reveal how the increase of pressure from 1 to 10000 atm does not affect the stability of the reaction products, modifying the kinetics of the process by lowering considerably the transition state energy. The reaction profile at high pressure remarkably differs from that at 1 atm,showing a submerged TS and a pre–TS structure lower in energy. The different solvation between endo and exo pre–TS is revealed as the driving force pushing the reaction toward a much higher preference for the endo product at high pressure

III-V based Heterojunction Tunnel for Multijunction Solar Cells

National audienc

Progress in Traceable Nanoscale Capacitance Measurements Using Scanning Microwave Microscopy

Reference samples are commonly used for the calibration and quantification of nanoscale electrical measurements of capacitances and dielectric constants in scanning microwave microscopy (SMM) and similar techniques. However, the traceability of these calibration samples is not established. In this work, we present a detailed investigation of most possible error sources that affect the uncertainty of capacitance measurements on the reference calibration samples. We establish a comprehensive uncertainty budget leading to a combined uncertainty of 3% in relative value (uncertainty given at one standard deviation) for capacitances ranging from 0.2 fF to 10 fF. This uncertainty level can be achieved even with the use of unshielded probes. We show that the weights of uncertainty sources vary with the values and dimensions of measured capacitances. Our work offers improvements on the classical calibration methods known in SMM and suggests possible new designs of reference standards for capacitance and dielectric traceable measurements. Experimental measurements are supported by numerical calculations of capacitances to reveal further paths for even higher improvements

Fabrication, caractérisation et simulation d’hétérojonctions tunnel et d’un absorbeur à 1eV à base de semiconducteurs III-V pour les cellules solaires multijonctions à haut rendement

National audienc

Modelling of interband transitions in GaAs tunnel diode

International audienceIn this paper, an improved model for non-local band-to-band tunneling carrier transport is presented and compared to experimental measurement from GaAs tunnel junctions devices. By carefully taking into account the coupling between the conduction band and the light holes valence band, the model is able to predict, with realistic material parameters, the amplitude of the current density throughout the whole tunneling regime. The model suggests that elastic band-to-band tunneling instead of trap-assisted-tunneling is the predominant mechanism in GaAs tunnel junctions, which is of great interest for better understanding and improving III–V multijunction solar cells

A femto ampere current amplifier based on a 30 000∶1 cryogenic current comparator

International audienc