Kinetic laws of the chemical process in the CVD of SiC ceramics from CH3SiCl3-H2 precursor

On the basis of growth rate measurements as a function of temperature and initial gas phase composition, the kinetic process of the CVD of silicon carbide from MTS-H2 is investigated, particularly under conditions of rate-control by the chemical reactions. Within specific ranges of dilution ratio 3 ≤ α ≤ 5 KPa and total pressure 3 ≤ P ≤ 10 KPa, two kinetic regimes are brought out depending on the temperature. At about 825°C, the deposition of SiC could be rate-limited by the homogeneous decomposition of MTS with an activation energy higher than 300 kJ.mol-1 and an apparent reaction order of 2.5 with respect to MTS. At about 925°C, surface reactions could control the overall kinetic process with an activation energy of only 160 kJ.mol-1 and an inhibitor chemisorption of Cl species which involves a negative reaction order with respect to HCl

Kinetic Processes in the CVD of SiC from CH3SiCl3-H2 in a Vertical Hot-Wall Reactor

The chemical vapour deposition of SiC-based ceramics from the CH3SiCl3-H2 precursor is investigated on the basis of large scale experimental and theoretical approaches. The use of a vetirtical cylindrical hot-wall LPCVD reactor permits to get a wide isothemal reaction zone with a creeping laminar flow around the substrate and a largely chemical control of the kinetics, which favours a high supersaturation and a nucleation regime. A calculation of the coverage of C (111) or Si(111) planes of SiC points out the importance of the chemisorption of SiCl3 and H radicals on C atoms and of CH3 and Cl radicals on Si atoms. On the basis of kinetic experiments and of chemical and structural investigations of the deposits, several domains of conditions are defined with different rnechanisms for the formation of SiC-based ceramics. For low temperatures and low pressures, a regime of growth of stoichiometric SiC microcrystals occurs from reaction of CH3 and SiCl3 intermediates. Higher pressures in the chemical control knetic domain, favour a regime of nucleation of nanocrystals with an excess of silicon resulting from SiCl3/SiCl2 silicon precursor

Le projet européen AIMWATER : utilisation de l'humiditésuperficielle des sols mesurée par radar embarqué (ERS/SAR)pour la modélisation pluie-débit

International audienceSoil moisture is a key hydrological variable in flood forecasting because it determines the partition of the rain between runoff and infiltration and thus governs the flows at the outlet of a catchment. The aim of the AIMWATER project is to improve the commonly used hydrological tools in an operational context for reservoir management by assimilating soil moisture data into rainfall-runoff models. The surface soil moisture is derived from ERS/SAR spaceborne data at basin scale. The project has validated the methodology set up to derive the spaceborne information showing its consistency both in time (3 years of consecutive data) and in space (3 Seine sub catchments). Different assimilation methodologies have been put forward for use and improved in a forecasting context: the variational method is presented there. It updates the parameters values of hydrological models during the forecast. We present in this paper the results of these methodologies applied on the Seine sub-catchments.L'humidité des sols est une variable clé pour la prévision des débits, car elle détermine la partition de la pluie entre ruissellement et infiltration et de ce fait elle conditionne l'écoulement à l'exutoire du bassin. L'objectif du projet AIMWATER est d'améliorer la performance des outils communément utilisés par les hydrologues dans un contexte opérationnel de gestion de barrages-réservoirs, grâce à l'assimilation de données d'humidité du sol dans des modèles pluie-débit. Les humidités superficielles sont obtenues à l'échelle d'un bassin versant à partir de données SAR du satellite ERS. Le projet a permis de valider la méthodologie d'inversion de l'information satellitale en montrant la robustesse à la fois dans le temps ( trois années de données consécutives) et dans l'espace ( trois sous-bassins versants de la Seine). Différentes méthodologies d'assimilation des données d'humidité des sols ont été testées et améliorées dans un contexte de prévision : une méthode variationnelle est présentée ici. Elle consiste à faire varier les valeurs de paramètres des modèles hydrologiques en tenant compte des données récentes de débit et d'humidité des sols. On présente ici les résultats obtenus sur des sous-bassins de la Seine

When does higher spatial resolution rainfall information improve streamflow simulation? An evaluation using 3620 flood events

Precipitation is the key factor controlling the high-frequency hydrological response in catchments, and streamflow simulation is thus dependent on the way rainfall is represented in a hydrological model. A characteristic that distinguishes distributed from lumped models is the ability to explicitly represent the spatial variability of precipitation. Although the literature on this topic is abundant, the results are contrasting and sometimes contradictory. This paper investigates the impact of spatial rainfall on runoff generation to better understand the conditions where higher-resolution rainfall information improves streamflow simulations. In this study, we used the rainfall reanalysis developed by Météo-France over the whole country of France at 1 km and 1 h resolution over a 10 yr period. A hydrological model was applied in the lumped mode (a single spatial unit) and in the semidistributed mode using three unit sizes of subcatchments. The model was evaluated against observed streamflow data using split-sample tests on a large set of French catchments (181) representing a variety of sizes and climate conditions. The results were analyzed by catchment classes and types of rainfall events based on the spatial variability of precipitation. The evaluation clearly showed different behaviors. The lumped model performed as well as the semidistributed model in western France, where catchments are under oceanic climate conditions with quite spatially uniform precipitation fields. By contrast, higher resolution in precipitation inputs significantly improved the simulated streamflow dynamics and accuracy in southern France (Cévennes and Mediterranean regions) for catchments in which precipitation fields were identified to be highly variable in space. In all regions, natural variability allows for contradictory examples to be found, showing that analyzing a large number of events over varied catchments is warranted

Diversité des approches du changement d'échelle en hydrologie

The scale change is a concrete and epistemologic challenge which can be tackled in various ways, but it is also a solution or at least a reflexion axis for the design of models. One of these methods, which strongly influences the models, is to impose a scale invariance. This method has been able to reflect an extended scale range, and to sort out phenomena which seem to transfer to all scales from those which seem " forgotten ", aspects which are put forward to respectively emphasize or rebut the scale transfer possibility. / Le changement d'échelle est un problème aux enjeux concrets et épistémologiques qui peut être abordé de façon multiple, mais c'est aussi une solution, ou du moins un axe de réflexion pour la conception des modèles. L'une de ces méthodes, fortement structurante pour les modèles, est d'imposer une invariance d'échelle. Cette méthode s'est avérée capable de rendre compte d'une gamme d'échelle étendue, et de faire la part des phénomènes qui semblent se transférer à toutes les échelles et de ceux qui semblent "oubliés", aspects respectivement mis en avant pour magnifier ou féfuter la possibilité du transfert d'échelle

Physicochemical properties of SiC-based ceramics deposited by low pressure chemical vapor deposition from CH3SiCl3H2

International audienceThe physicochemical properties of SiC-based deposits, obtained from the thermal decomposition of CH3SiCl3(MTS) in hydrogen, under conditions of reduced pressure (P < 20 kPa) and low temperature (T < 1400 K), are studied. The morphology, the structure, the nainostructure and the chemical composition of the deposits are discussed from a kinetic and a thermodynamic point of view. They are closely related to the kinetic control regime (either mass transfer or chemical reactions). The silicon excess has a pronounced influence on the morphology and the nanostructure of the deposits

Soil moisture estimation from ERS/SAR data: toward an operational methodology

International audiencePrevious studies have shown the possibility of using ERS/SAR data to monitor surface soil moisture from space. The linear relationships between soil moisture and the SAR signal have been derived empirically and thus were a priori specific to the considered watershed. In order to overcome this limit, this study focused on 2 objectives. The first one was to validate over 2 years of data the empirical sensitivity of the radar signal to soil moisture, in the case of 3 agricultural watersheds with different soil compositions and land cover uses. The slope of the observed relationship was very consistent. The second one was to propose an operational methodology for soil monitoring based on ERS/SAR data. The implementation of this methodology is based on 2 steps: the calibration period and the operational period. During the calibration period ground truth campaigns are performed to measure vegetation parameters and the ERS/SAR data is processed only once a field land cover map is established. In contrast during the operational period no vegetation field campaigns are performed and the images are processed as soon as they are available. The results confirm the relevance of this operational methodology since no loss of performance is observed between the calibration and operational periods.Il a été préalablement établi, la possibilité d`estimer un indice hydrologique à l`échelle du bassin à partir des données satellites ROS, telles que celles issues des images du satellite ERS. Les relations linéaires entre l` état hydrique et le signal radar étant établies empiriquement elles sont donc spécifiques d`un bassin donné. Afin de généraliser ces relations cette recherche s`est centrée sur 2 objectifs. Le premier devait permettre de valider sur 2 ans de données la sensibilité du signal à l`état hydrique du sol dans le cas de 3 bassins versants agricoles avec des compositions de sols et des couverts végétaux différents. La pente des relations est apparue très robuste. Le second objectif était de proposer une méthodologie opérationnelle pour suivre l`état hydrique du sol à partir de données ROS. La mise en oeuvre de cette méthodologie s`est appuyée sur deux étapes une période de calage et une période opérationnelle . Pendant la période de calage il est nécessaire de mesurer les paramètres de végétation ainsi que d`établir une carte précise de l`occupation du sol. Par ailleurs, pendant la période opérationnelle aucune campagne de terrain n`est nécessaire et les images sont traitées au fur et à mesures qu`elles arrivent. Les résultats obtenus confirment l`intérêt de cette méthode opérationnelle puisque aucune diminution de performance n`est constatée entre les périodes de calage et opérationnelles