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Micromechanical experimental investigation of mudstones
No full textInternational audienceThis paper reports on a micromechanical investigation of mudstones using a novel experimental method, based on a combination of environmental scanning electron microscopy (ESEM) and digital image correlation techniques. A specifically designed rig was developed, allowing in situ tests under combined hydric and mechanical loadings in the ESEM chamber. Observations were performed on the scale of the composite microstructure of the involved rocks (i.e. numerous grains of carbonate and quartz embedded in a clay matrix). Preliminary results from a uniaxial compression test on a sample with 5?4% water content are presented. Heterogeneous strain fields that correlate well with the microstructure of mudstones are illustrated, enabling different deformation modes (particularly shear bands and tensile microcracks) and their interactions to be identified
Fabrication, caractérisation et modélisation de couches minces d'alliages silicium-carbone microcristallins
No full textDespite continuous effort, thin-film silicon multi-junction solar cells are still limited by the light-induced degradation of amorphous materials that they employ − hydrogenated amorphous silicon layers (a-Si:H) or amorphous silicon-germanium (a-SiGe:H) layers. To survive, this technology must fully benefit from the ease with which it allows multi-band gap photovoltaic (PV) devices to be assembled. To this end, materials that are stable under light soaking and have an electronic band gap between that of hydrogenated microcrystalline silicon (µc-Si:H, 1.1 eV) and that of a-Si:H (1.7 eV) are needed. The goal of this PhD thesis was to develop a new class of materials satisfying all these requirements by alloying carbon and silicon. Indeed, hydrogenated microcrystalline silicon-carbon alloys (µc-Si1−xCx:H) are a promising candidate for expanding the toolbox of useful materials for thin-film photovoltaics. The interest in these alloys lies in the possibility of easily varying their effective band gap by changing the amount of carbon in their composition. In this thesis, the usefulness of such materials in thin-film PV devices was probed using a broad range of deposition and characterization techniques. Using thin-film growth techniques at low temperatures (175−300° C), the range in which such electronically useful materials can be grown has been explored. It was confirmed that even in the condition of small crystallites, no stable sub-stoichiometric Si-C crystalline phase exists (i.e. no parallel for silicon-rich c-SiGe has been observed). Under all deposition techniques utilized, these materials were composed of submicron-size silicon crystallites embedded in an amorphous silicon-carbon (a-Si1−xCx:H) matrix. However, while the presence of the crystallites assures a higher conductivity compared to a-Si1−xCx:H, the carbon incorporation leads to an effective energy gap larger than that of microcrystalline silicon, supporting our investigation of these materials as promising optoelectronic layers. In the first part of this work, different Plasma Enhanced Chemical Vapor Deposition strategies have been investigated to achieve the widest range of processing conditions and to learn the most about the growth conditions required to produce a high quality µc-Si1−xCx:H material. Material properties were extensively characterized both on the structural side and also from an electrical point of view, in order to establish a correlation between the deposition parameters and the microstructural, transport and defect-related properties. The extensive set of results has allowed the proposal of a coherent growth model for such µc-Si1−xCx:H thin films. Exploiting these results, PV devices using these alloys as active layers were made. Although the absolute levels of efficiency (around 3.5 %) are not as high as state-of-the-art microcrystalline silicon, this work showed that it is possible to obtain variations in the open circuit voltage by varying the amount of carbon incorporated in such µc-Si1−xCx:H alloys. This important result shows that a process parameter other than silane dilution can be used to control this aspect of device performance. PV performances are modest so far, which is expected as these are the first ever results concerning the application of such a new class of materials as the active layer in thin-film solar cells. However, with further advancements in such materials, their replacement of the less stable a-SiGe:H is not unforeseeable.Malgré les efforts de la communauté scientifique, les cellules solaires multijonctions à base de matériaux amorphes, sont limitées par la dégradation sous lumière des matériaux actifs qu'elles emploient - notamment, le silicium amorphe hydrogéné (a-Si:H) ou le silicium-germanium amorphe (a-SiGe:H). Compte tenu de la facilité avec laquelle les dispositifs multijonctions peuvent être fabriqués dans cette filière couches minces, pour que cette filière reste compétitive sur le marché photovoltaïque, il est nécessaire de déposer des matériaux présentant les meilleures propriétés de transport possible, avec une énergie de gap variable comprise entre celle du silicium microcristallin hydrogéné (µc-Si:H, 1.1 eV) et celle du a-SiH (1.7 eV), et peu sensibles au vieillissement sous lumière. Le but de ces travaux de thèse était de développer une nouvelle classe de matériaux satisfaisant les critères précédemment cités, alliant carbone et silicium. En effet, les alliages silicium-carbone microcristallins hydrogénés (µc-Si1−xCx:H), sont des candidats prometteurs pour la réalisation de cellules photovoltaïques (PV): d'une part, la structure microcristalline devrait les rend moins sensible au vieillissement sous lumière, et d'autre part, il est possible de faire varier l'énergie de gap en modifiant le taux de carbone de l'alliage. Dans cette thèse, nous avons étudié les propriétés structurales et électriques de ces alliages à l'aide de nombreuses techniques de caractérisation complémentaires, et en faisant varier de nombreux paramètres de dépôt afin de déterminer celles permettant d'obtenir les meilleures propriétés possibles. Nous avons montré en particulier que ces alliages sont composés de cristallites de taille sous-micrométrique, enrobé dans une matrice de silicium-carbone amorphe (a-Si1−xCx:H). En plus, ces matériaux sont caractérisés par une croissance colonnaire, typique du µc-Si:H, avec la taille moyen des grains qui est contrôler en prévalence par la puissance RF utilisé pour le dépôt. L'incorporation de carbone, qui ne peut pas se passer dans la phase cristalline, cause la formation d'un tissue amorphe interstitielle qui enrobe les grain et en empêche la croissance. Ainsi, l'analyse de l'ensemble de nos résultats nous a permis de proposer un modèle cohérent de la croissance de ces alliages. Les propriétés électriques de nos matériaux ont été étudiées à l'aide des mesures de courant d'obscurité et de photocourant en régime stationnaire, de spectroscopie par interférométrie laser et de photocourant modulé. Nous avons pu établir une corrélation directe entre les propriétés électriques et les conditions de dépôt, mettant notamment en évidence que la présence des cristallites assure une conductivité plus élevée par rapport à du a-Si1−xCx:H, et que l'incorporation de carbone amène à des énergies de gap plus grandes que celle du µc-Si:H. Nous avons également montré que les meilleurs matériaux étaient obtenus pour des puissances RF faibles, de l'ordre de 113 mW/cm2. Des résultats de caractérisation de cellules photovoltaïques de type p-i-n ou n-i-p réalisées à partir de nos alliages, ont été aussi présentés. Les rendements obtenus restent encore modestes (de l'ordre de 3,5%) mais nous avons mis en évidence qu'il est possible de faire varier la tension de circuit ouvert (Voc) des cellules en changeant le taux de carbone incorporé dans les alliages. Ainsi, un paramètre autre que la dilution de silane lors du dépôt peut être utilisé pour contrôler Voc. Ces dispositifs sont les tout premiers déposés et nécessitent encore des étapes d'optimisation. Néanmoins, l'étude approfondie que nous avons réalisée sur ces alliages nous laisse penser qu'ils ont un potentiel intéressant pour les applications PV
Modélisation de la structure et des spectres IR d’ions micro-hydratés
No full textIon micro-hydration has been modeled by a combination of classical and quantum methods. The structures, dynamics and IR spectra of micro-hydrated cluster ions ranging from 1 to 216 water molecules have been considered with special emphasis on the comparison to recent experimental data. Quantum modeling has been used to provide reference values against which the polarizable AMOEBA force field could be calibrated. Extension of the parameter set was carried out in some cases, in particular when improvement of the electrostatic and polarization energy terms was deemed necessary. Classical molecular dynamics was then used with two different aims: (1) extensive exploration of potential energy surfaces for large cases with numerous low energy minima, for which statistical sampling is required, and (2) computation of IR spectra through Fourier transform of the Dipole moment AutoCorrelation Function (DACF), at various temperatures. The hydration of Zn2+ was studied in detail. While the coordination number (CN) of zinc cation is firmly established to be six in bulk solution, recent experimental and computation studies in the gas phase have unveiled a complex pattern for small clusters with up to 12 water molecules with a CN more near 5 or maybe even 4. Our calculations, performed from 6 to 216 water molecules, allow a precise description of CN evolution with cluster size.Sodiated tryptamine with one or two water molecules attached was investigated in order to interpret gas phase IR spectra of these species. Particular care was taken of electrostatics - especially atomic charges - and of starting structures in molecular dynamics simulations. It was then possible to investigate temperature-dependent dynamics in detail and to obtain good agreement with IRPD spectra. Finally, the hydration of the sulfate anion with up to 100 water molecules was analyzed focusing on the existence of dangling O-H bonds on the cluster surface as a function of cluster size. In agreement with IRPD experiments, we find that small clusters are compact with no dangling bond. The latter appear when ca. 25 water molecules are present with a characteristic vibrational band. Overall, these results underline the requirements for accurate modeling of ion hydration. These involve refined treatment of electrostatic and polarization interactions, extensive sampling of the potential energy surfaces, careful treatment of cluster boundaries including a confining repulsive wall, and long-time simulations to attain converged radial distribution functions. With such developments at hand, it is now possible to obtain a realistic picture of structures and dynamics of large micro-hydrated cluster ions, as can be judged from the comparison to experiment, especially vibrational spectroscopy.La micro-hydratation d’ions a été modélisée par une combinaison de méthodes classiques et quantiques. Les structures, la dynamique et les spectres IR d’agrégats ioniques micro-hydratés allant de 1 à 216 molécules d'eau ont été examinés en mettant l’accent sur la comparaison avec des résultats expérimentaux récents. La modélisation quantique a été utilisée pour fournir des valeurs de référence pour la calibration du champ de force polarisable AMOEBA. L’extension du jeu de paramètres a été réalisée dans certains cas, en particulier lorsqu’il a été nécessaire d’améliorer les termes d’interaction électrostatique et de polarisation. La dynamique moléculaire classique a ensuite été utilisée avec deux objectifs distincts: (1) l'exploration extensive des surfaces d'énergie potentielle pour des systèmes de grande taille avec de nombreux minima de basse énergie, pour lesquels un échantillonnage statistique est nécessaire, et (2) le calcul des spectres IR par transformée de Fourier de la fonction d’autocorrélation du moment dipolaire (DACF) à différentes températures.L'hydratation du cation Zn2+ a été étudiée en détail. Alors que le nombre de coordination (CN) de Zn2+ est clairement établi à six en solution aqueuse, des études expérimentales et théoriques récentes en phase gazeuse ont mis en évidence un modèle complexe pour les petits agrégats jusqu'à 12 molécules d'eau avec un CN de l’ordre de 5 ou peut-être même 4. Nos calculs allant de 6 à 216 molécules d'eau permettent une description précise de l’évolution du CN en fonction de la taille de l’agrégat. La tryptamine complexée aux ion et potassium et à une ou deux molécules d'eau a été étudiée afin d'interpréter des spectres IR en phase gazeuse de ces espèces. Une attention particulière a été portée à l'électrostatique, en particulier les charges atomiques, et sur le rôle des structures de départ des simulations de dynamique moléculaire. Il a alors été possible d'étudier en détail les phénomènes dynamiques dépendant de la température et d’obtenir une bonne concordance avec les spectres IRPD. Enfin, l'hydratation de l'anion sulfate avec un nombre maximum de 100 molécules d'eau a été analysée en mettant l'accent sur l'existence de liaisons O-H libres sur la surface de l’agrégat en fonction de sa taille. En accord avec les expériences IRPD, nous constatons que les petits agrégats sont compacts et sans liaison pendante. Ces dernières apparaissent avec une bande devibration caractéristique quand environ 25 molécules d'eau sont présentes.Globalement, ces résultats mettent en évidence les conditions requises pour une modélisation précise de l’hydratation d'ions. Il s'agit notamment de prendre en compte un traitement raffiné des interactions électrostatiques et de polarisation, un échantillonnage étendu des surfaces d'énergie potentielle, le traitement adapté des conditions aux limites des agrégats, y compris d’un mur de confinement répulsif, et des simulations de longue durée pour atteindre la convergence des fonctions de distribution radiale. Avec de tels développements, il est maintenant possible d'obtenir une description réaliste des structures et de la dynamique d’agrégats ioniques microhydratés de grande taille, ainsi que leur dépendance en température, comme le montre la comparaison avec l’expérience, en particulier la spectroscopie vibrationnelle
Observation of tropospheric δD by IASI and comparison with LMDZiso over the Western Siberia
No full textInternational audienceWestern Siberia has undergone a sharp increase in temperature during the last decades, modifying the biogeochemical and the hydrological cycles. Water vapor plays an important role in the atmosphere including in the radiative transfer, cloud formation and precipitation. Information about evaporation/condensation processes of water vapor can be provided by water stable isotopologues such as H216O and HDO. This study presents the joint H216O and HDO retrievals from Infrared Atmospheric Sounding Interferometer (IASI) spectra over Siberia. IASI is an instrument on board the MetOp-A European satellite launched in October 2006. The global coverage of this instrument and the good signal-to-noise ratio allow us to provide information on δD over this region. IASI measurements may be used to estimate integrated δD between the surface and 3 km altitude or from 1 to 5 km depending on the thermal contrast between the surface and the low troposphere. The retrieved data are compared to simulations from an isotopic GCM, LMDZiso, for 2011. The data show variations that are well correlated with the model at seasonal (r up to 0.8) and day-to-day (r≡0.6) time scales. The IASI-based retrievals and the model capture also well the seasonal variation of the specific humidity in the [0-3km] and the [1-5km] altitude ranges
How well is black carbon in the Arctic atmosphere captured by models?
No full textInternational audienceA correct representation of the spatial distribution of aerosols in atmospheric models is essential for realistic simulations of deposition and calculations of radiative forcing. It has been observed that transport of black carbon (BC) into the Arctic and scavenging is sometimes not captured accurately enough in chemistry transport models (CTM) as well as global circulation models (GCM). In this study we determine the discrepancies between measured equivalent BC (EBC) and modeled BC for several Arctic measurement stations as well as for Arctic aircraft campaigns. For this, we use the output of a set of 5 models based on the same emission dataset (ECLIPSE emissions, see eclipse.nilu.no) and evaluate the simulated concentrations at the measurement locations and times. Emissions are separated for different sources such as biomass burning, domestic heating, gas flaring, industry and the transport sector. We focus on the years 2008 and 2009, where many campaigns took place in the framework of the International Polar Year. Arctic stations like Barrow, Alert, Station Nord in Greenland and Zeppelin show a very pronounced winter/spring maximum in BC. While monthly averaged measured EBC values are around 80 ng/m^3, the models severely underestimate this with some models simulating only a small percentage of the observed values. During summer measured concentrations are a magnitude lower, and still underestimated by almost an order of magnitude in some models. However, the best models are correct within a factor of 2 in winter/spring and give realistic concentrations in summer. In order to get information on the vertical profile we used measurements from aircraft campaigns like ARCTAS, ARCPAC and HIPPO. It is found that BC in latitudes below 60 degrees is better captured by the models than BC at higher latitudes, even though it is overestimated at high altitudes. A systematic analysis of the performance of different models is presented. With the dataset we use we capture remote, polluted and fire-influenced conditions. We estimate the impact of model deficiencies on calculated BC radiative forcing by introducing scaling factors based on the model-measurement comparisons
Recent developments in femtosecond filamentation
No full textInternational audienceWe review recent developments in the field of femtosecond laser filamentation
Third Harmonic Generation from Perturbed Femtosecond Filaments in Air
No full textInternational audienceWe investigate third harmonic generation from femtosecond laser filaments. A two-order-of-magnitude enhancement of third harmonic light is observed when the (signal) filament is perturbed by a second laser (pump) filament. This enhancement is studied as function of laser polarization, time delay between the pump and signal filaments, the pump filament energy, etc. Based on these systematic results, we attribute the enhancement to a quenching of the interference effects of the third harmonic generated in the first and second half of the signal filament. Numerical simulations based on a two-color propagation model reproduce well the experimental observations and confirms our explanation
Study of laser induced plasma grating dynamics in gases
No full textInternational audienceThe relaxation of a plasma grating resulting from the interference of two crossing laser filaments in molecular and atomic gases is studied experimentally. Dissipation of the grating fringes is dominated by ambipolar diffusion in atomic gases and by a combination of ambipolar diffusion and collision-assisted free electron recombination in molecular gases. A theoretical model of the grating evolution is developed and compared to experimental results. Good agreement with simulations allows extracting plasma properties such as electron density, diffusion and recombination coefficients in Ne, Ar, Kr, Xe, N_2 , O_2 , CO_2 and air at atmospheric pressure
Solution to the many-body problem in one point
No full textCf erratum :https://hal.archives-ouvertes.fr/hal-01117926International audienc
Microfluidic in situ mechanical testing of photopolymerized gels
No full textInternational audienceGels are a functional template for micro-particle fabrication and microbiology experiments. The control and knowledge of their mechanical properties is critical in a number of applications, but no simple in situ method exists to determine these properties. We propose a novel microfluidic based method that directly measures the mechanical properties of the gel upon its fabrication. We measure the deformation of a gel beam under a controlled flow forcing, which gives us a direct access to the Young's modulus of the material itself. We then use this method to determine the mechanical properties of poly(ethylene glycol) diacrylate (PEGDA) under various experimental conditions. The mechanical properties of the gel can be highly tuned, yielding two order of magnitude in the Young's modulus. The method can be easily implemented to allow for an in situ direct measurement and control of Young's moduli under various experimental conditions