Portail HAL edf
Not a member yet
11029 research outputs found
Sort by
Amorphous GaN matrix embedded nanocrystals exhibiting bulk bandgap luminescence
No full textInternational audienceThis work reports on the room temperature sputtering growth of gallium nitride thin films, exhibiting band edge luminescence without thermal annealing or post-deposition processing. In particular, we investigate and correlate their luminescence spectra with their structural properties (amorphous or polycrystalline phases), the presence of defects, and the grain features by combining cathodoluminescence and HR-TEM characterizations. The working pressure and Ar/N2 flow ratio are found to have a profound effect on both the structural and optical properties of the films. Notably, the only film with band edge luminescence is grown at the highest investigated pressure (13.3 Pa) and 40% N2 concentration and exhibits single-grained crystallites of small sizes (∼5 nm). Moreover, this polycrystalline film has an almost equal fraction of amorphous and crystalline phases. This suggests that the presence of smooth and nanometer-sized crystals in an amorphous matrix (inducing grain surface passivation) could be a synergetic combination to achieve luminescent films grown at room temperature
Four-dimensional variational data assimilation with a sea-ice thickness emulator
No full textInternational audienceDeveloping operational data assimilation systems for sea-ice models is challenging, especially using a variational approach due to the absence of adjoint models. NeXtSIM, a sea-ice model based on a brittle rheology paradigm, enables high-fidelity simulations of sea-ice dynamics at mesoscale resolution (∼10 km) but lacks an adjoint. By training a neural network as an Arctic-wide emulator for sea-ice thickness based on mesoscale simulations with neXtSIM, we gain access to an adjoint. Building on this emulator and its adjoint, we introduce a four-dimensional variational (4D–Var) data assimilation system to correct the emulator's bias and to better position the marginal ice zone (MIZ). Firstly, we perform twin experiments to demonstrate the capabilities of this 4D–Var system and to evaluate two approximations of the background covariance matrix. These twin experiments demonstrate that the assimilation improves the positioning of the MIZ and enhances the forecast quality, achieving an average reduction in sea-ice thickness root-mean-squared error of 0.8 m compared to the free run. Secondly, we assimilate real CS2SMOS satellite retrievals with this system. While the assimilation of these rather smooth retrievals amplifies the loss of small-scale information in our system, it effectively corrects the forecast bias. The forecasts of our 4D–Var system achieve a similar performance as the operational sea-ice forecasting system neXtSIM-F. These results pave the way to the use of deep learning-based emulators for 4D–Var systems to improve sea-ice modeling
Pressure-Liquefied Ammonia Jet Dispersion: Multi-Model Intercomparison Using Desert Tortoise and FLADIS Field Data
No full textInternational audienceThis paper presents the findings of an international model inter-comparison exercise that was undertaken in the period 2021-2024 to assess the performance of atmospheric dispersion models for simulating releases of pressure-liquefied ammonia. The exercise used data from ammonia field trialsdating from the 1980s and 1990s: the Desert Tortoise and the FLADIS trials. Concentration data from two arcs of sensors in the Desert Tortoise trials and three arcs of sensors in the FLADIS trials were used. Twenty-one independent modelling teams from North America and Europe participated in the exercise and provided in total twenty-seven sets of results from a range of different models, including empirically-based nomograms, integral, Gaussian puff, Lagrangian particle, and Computational Fluid Dynamics (CFD) models. The work is novel in presenting the results from such a large cohort of models,examining specifically the dispersion behaviour of ammonia. This is particularly relevant at the current time, given the growing international interest in using ammonia as a clean energy vector and shipping fuel.The study found that the agreement between model predictions and measurements (as determined by performance measures such as geometric mean bias and geometric variance) varied between different models. At any downwind distance, the range in predicted plume arc-max concentrations spanned arange of up to one or two orders of magnitude about the measurements. Several modelling teams used the same models and, in most cases, their predictions differed. Given appropriate inputs, most models generally predicted concentrations that agreed with the data within commonly-used model acceptance criteria. There was no single class of model that provided superior predictions to others; predictions from several empirically-based nomograms, integral, Gaussian puff, Lagrangian particle, and CFD models were all in close agreement with the data (as defined by the model acceptance criteria).The findings of the exercise are being used to help plan a programme of future ammonia experiments in the USA, called the Jack Rabbit III trials. The results are also useful for assessing the performance of models that may be applied to assess risks at ammonia facilities, and for emergency planning and response
Caractérisation expérimentale du bruit d’un drone en vol stationnaire en environnements réel et contrôlé
No full textDrones et taxi volants en milieu urbain; GAHA - Aéro et Hydro-Acoustique: GABE - Acoustique du Bâtiment et de l'EnvironnementNational audienceLa mesure du bruit des drones dans un environnement réaliste est importante afin d’évaluer leur impact sur les riverains, mais aussi pour comprendre les sources de bruit dominantes en fonction des opérations de vol et ainsi améliorer les outils de modélisation et de simulation du bruit. Cette caractérisation expérimentale est une tâche difficile à cause de la variabilité des conditions météorologiques en milieu extérieur, du bruit de fond, et du bruit du drone. En effet, les différentes hélices du drone voient leur vitesse de rotation varier indépendamment pour assurer la stabilité en vol. C’est pourquoi des mesures en environnement contrôlé comme des chambres anéchoïques sont également réalisées afin de diminuer la variabilité du bruit, même si la taille limitée de ces installations limitent les opérations de vol réalisables. L’objectif de cette étude est de comparer les mesures de bruit en chambre anéchoique et en environnement extérieur pour un drone quadricoptère de petite taille en vol stationnaire, afin de déterminer les similitudes et différences entre les deux types de mesures. Les mesures en chambre anéchoïque ont été réalisées sur une structure fixe, ce qui permet de caractériser les hélices soit de façon isolée, soit par paire soit pour l’ensemble des quatre hélices. Pour les mesures en milieu extérieur, six microphones placés sur des plate-formes rigides ont été utilisées, et des mesures en vol stationnaire à trois hauteurs ont été réalisées. Même si les signaux acoustiques mesurés en extérieur présentent une plus grande complexité, les mêmes composantes spectrales sont retrouvées dans les deux types d’environnement. En particulier, une forte émergence du bruit des moteurs à environ 18 fois la fréquence de passage des pales est observée
Advanced CIGS-mesoporous TiO2 hybrid photocathode functionalized with cobalt quaterpyridine for solar-driven CO₂ reduction
No full textInternational audienceHybrid photocathodes that integrate inorganic semiconductors with molecular catalysts offer a promising strategy for photoelectrochemical CO2 reduction into value-added products. In this work, we present the design and characterization of a high-performance photocathode based on copper indium gallium sulfide (CIGSu), functionalized with a cobalt quaterpyridine (CoQPy) molecular catalyst. The device features a thin (5 nm) TiO2 protective layer deposited by atomic layer deposition (ALD) on CIGSu/CdS, followed by a mesoporous TiO2 layer formed under mild conditions using UV curing and low-temperature annealing (150°C). The mesoporous structure enables high CoQPy loading through chemisorption via phosphonic acid anchoring groups. Under simulated sunlight, the optimized photocathode delivers a photocurrent density of ca. 2 mA/cm² with 95% CO selectivity in carbonate buffer, double the 2 performance of systems using low-porosity TiO2. This work marks progress toward efficient, molecularly functionalized photocathodes for aqueous CO2 reduction
Validation of NEPTUNE_CFD for single-phase natural convection
No full textInternational audienceComputational Fluid Dynamics (CFD) is widely used in nuclear engineering for safety related studies, providing a mechanistic solution approach in support of the system and sub-channel numerical scales applied in the safety demonstration. Co-developed by four of the biggest actors of the French nuclear industry, the NEPTUNE CFD code is specialized in nuclear thermal-hydraulics applications allowing to simulate single and two-phase flows on complex geometries. The increasing protagonism of passive systems in new generation reactors demands specific studies regarding its operation and safety. Passive systems are engineered to rely on natural physical phenomena, such as buoyancy or natural convection and to operate without the need of active mechanical components or external power sources. This paper focuses on the validation of NEPTUNE CFD over experimental data of Qureshi and Gebhart [1] -vertical flat plane immersed in water -and semi-empiric correlations from the literature to simulate single-phase natural convection to validate the use of NEPTUNE CFD for simulating passive systems over a wide range of Rayleigh numbers from laminar to turbulent regimes.</div
La décision d’installation de point de recharge pour véhicule électrique en copropriété : le paradoxe du droit à la prise à l’aube de la diffusion en masse des véhicules électriques
No full textInternational audienceL’électrification de la mobilité individuelle est l’un des moyens de répondre aux impératifs environnementaux de la transition énergétique dans le domaine des transports, qui est le secteur le plus émetteur de dioxyde de carbone en France. La diffusion de véhicules électriques est de ce fait un des éléments promus par les politiques publiques, en remplacement des véhicules conventionnels (bonus écologique). Cet article s’attache à identifier comment les acteurs concernés aboutissent à la décision d’équiper une copropriété d’un point de recharge pour véhicule électrique. Nous montrerons comment s’organise le processus d’installation d’une infrastructure de recharge pour VE en copropriété, en examinant les positionnements des acteurs en présence et les étapes traversées par les copropriétaires demandeurs d’une installation ; puis, distinguant les différents chemins sociotechniques empruntés pour cette infrastructure, nous montrerons ce qui guide le choix pour l’un plutôt que l’autre. Nous verrons que la configuration technique de la recharge en copropriété est corrélée à de complexes stratégies d’installation qui dépendent elles-mêmes du positionnement des trois types d’acteurs en présence
Évaluation des effets d’interférence d’un défaut HVDC sur un pipeline enterré
No full textInternational audienceHVDC transmission systems have potential effects on buried pipelines in their vicinity. Thereby, significant interferences can arise when a fault occurs on an HVDC overhead line, depending on factors such as the distance between the line and the pipeline, the exposure length, the soil resistivity, and the line configuration. This paper investigates the effects of these parameters on the pipeline's induced transient overvoltage. Under normal operating conditions, only small current variations occur in the poles, and their electromagnetic field effects are negligible. However, when a pole-to-ground fault occurs, a transient stage lasting a few milliseconds leads to a significant increase in the electromagnetic field. This paper presents a variety of simulation studies obtained using an electromagnetic transients software (EMTP). Simulations were conducted for different HVDC configurations, specifically symmetric monopolar and bipolar setups. The results show that the induced voltage is significantly higher in bipolar configuration, but in both cases, the effects are not negligible. Precautions must be taken to mitigate adverse effects on the pipeline, particularly regarding its cathodic protection, and to prevent electrocution through contact with the pipeline. The effects on the human body during transient events were analyzed based on various models, considering both the magnitude of the current passing through the body and the duration of the exposure. The findings indicate that with appropriate protection measures in place, there is no risk to human safety and no risk of damage to the pipeline system.</div
Plongement des constituants pour la représentation sémantique des phrases
No full textNational audienceLes méthodes d'apprentissage profond en traitement automatique des langues reposent souvent sur une segmentation des textes en tokens avant leur vectorisation. Cette segmentation produit des sous-unités lexicales offrant une grande flexibilité. Toutefois, la réutilisation de tokens identiques dans des mots de sens différents peut favoriser des représentations basées sur la forme plutôt que sur la sémantique. Ce décalage entre la forme de surface et le sens peut induire des effets indésirables dans le traitement de la langue. Afin de limiter l'influence de la forme sur la sémantique des représentations vectorielles, nous proposons une représentation intermédiaire plus compacte et plus fidèle au sens des mots
A global dialogue on broadening industrial applications of BEPU: outcomes of a panel session at the BEPU-2024 Conference
No full textInternational audienceSince the late 1980s, the Best Estimate Plus Uncertainty (BEPU) methodology has been a cornerstone for deterministic safety analysis of design basis accidents in nuclear power plants. Despite endorsements from the International Atomic Energy Agency (IAEA) and various national regulatory bodies, its industrial application remains limited. At the BEPU-2024 conference in Lucca, Italy, a panel of global experts convened to discuss strategies for expanding BEPU’s industrial use, aiming at:- Enhancing understanding of uncertainty analysis and Input Uncertainty Quantification (IUQ).- Addressing challenges and exploring future advancements in BEPU methodology.- Sharing insights and experiences in BEPU application.- Fostering dialogue on extending BEPU to broader industrial applications.This paper summarizes the main contents and outcomes of the panel discussions