108 research outputs found
Structural selective charge transfer in iodine-doped carbon nanotubes
We have investigated iodine intercalated carbon nanostructures by extended X-ray absorption fine structure (EXAFS) and Raman spectroscopies. We discuss here the charge transfer and the iodineâcarbon interaction as a function of the carbon nanostructures (graphite, multi-walled, double-walled and single walled nanotubes). The results show that iodine is weakly adsorbed on the surface of all multi-walled nanotubes. By contrast, a charge transfer between iodine and single walled nanotubes is evidenced
XMCD under pressure at the Fe K edge on the energy dispersive beamline of the ESRF
The present paper demonstrates the feasibility of X-ray Absorption
Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) at high
pressure at the Fe-K edge on the ID24 energy dispersive beamline of the ESRF.
In 3d transition metals, performing experiments at the hard X-ray K-edge rather
than at the magnetically interesting soft X-ray L-edges represents, the only
way to access the high pressure regime obtainable with Diamond Anvil Cells. The
simultaneous availability of a local structure (XAS) and of a magnetic (XMCD)
probe on the sample in identical thermodynamical conditions is essential to
study correlations between local structural and magnetic properties. We briefly
summarize the state of the art theoretical understanding of K-edge XMCD data,
then illustrate the setup of beamline ID24 for high pressure XMCD experiments
and underline the conditions required to perform measurements at the K-edges of
3d transition metals. Finally, we present two examples of recent high pressure
results at the Fe-K edge in pure Fe and Fe3O4 powder
Facile synthesis of high-surface area platinum-doped ceria for low temperature CO oxidation
International audienceUsing a simple slow decomposition method of nitrate precursors, high-surface area platinum-doped ceria with a crystallite size of 9 nm can be prepared. The catalytic performance of the compound can be tuned by changing the reduction temperature under hydrogen (300°C, 500°C and 700°C). The catalyst treated at 300°C shows the best catalytic performance, being active at room temperature. The materials were analysed using a combination of structural characterization methods (X-ray diffraction (XRD), nitrogen physisorption, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM)), surface sensitive methods (X-ray photoelectron spectroscopy (XPS), H 2-chemisorption and H 2-temperature-programmed reduction (TPR)) and X-ray absorption fluorescence spectroscopy (XAFS). HAADF-STEM and XAFS analysis suggests successful doping of platinum in the ceria lattice. After pretreatment at 300°C, the situation is slightly different. While no defined platinum nanoparticles can be identified on the surface, some platinum is in a reduced state (XPS, H 2-chemisorption)
EXAFS investigations on the pressure induced local structural changes of GeSe2glass under different hydrostatic conditions
Pressure-induced transformations in glassy GeSe2 have been studied using the x-ray absorption spectroscopy. Experiments have been carried out at the scanning-energy beamline BM23 (European Synchrotron Radiation Facility) providing a micrometric x-ray focal spot up to pressures of about 45 GPa in a diamond anvil cell. Both Se and Ge K-edge experiments were performed under different hydrostatic conditions identifying the metallization onsets by accurate determinations of the edge shifts. The semiconductor-metal transition was observed to be completed around 20 GPa when neon was used as a pressure transmitting medium (PTM), while this transition was slightly shifted to lower pressures when no PTM was used. Accurate double-edge extended x-ray absorption fine structure (EXAFS) refinements were carried out using advanced data-analysis methods. EXAFS data-analysis confirmed the trend shown by the edge shifts for this disordered material, showing that the transition from tetrahedral to octahedral coordination for Ge sites is not fully achieved at 45 GPa. Results of present high pressure EXAFS experiments have shown the absence of significant neon incorporation into the glass within the pressure range up to 45 GPa
CAGIRE: a wide-field NIR imager for the COLIBRI 1.3 meter robotic telescope
The use of high energy transients such as Gamma Ray Bursts (GRBs) as probes
of the distant universe relies on the close collaboration between space and
ground facilities. In this context, the Sino-French mission SVOM has been
designed to combine a space and a ground segment and to make the most of their
synergy. On the ground, the 1.3 meter robotic telescope COLIBRI, jointly
developed by France and Mexico, will quickly point the sources detected by the
space hard X-ray imager ECLAIRs, in order to detect and localise their
visible/NIR counterpart and alert large telescopes in minutes. COLIBRI is
equipped with two visible cameras, called DDRAGO-blue and DDRAGO-red, and an
infrared camera, called CAGIRE, designed for the study of high redshift GRBs
candidates. Being a low-noise NIR camera mounted at the focus of an
alt-azimutal robotic telescope imposes specific requirements on CAGIRE. We
describe here the main characteristics of the camera: its optical, mechanical
and electronics architecture, the ALFA detector, and the operation of the
camera on the telescope. The instrument description is completed by three
sections presenting the calibration strategy, an image simulator incorporating
known detector effects, and the automatic reduction software for the ramps
acquired by the detector. This paper aims at providing an overview of the
instrument before its installation on the telescope.Comment: Accepted by Experimental Astronom
Frequency of extreme Sahelian storms tripled since 1982 in satellite observations
The hydrological cycle is expected to intensify under global
warming, with studies reporting more frequent extreme rain
events in many regions of the world, and predicting increases in future flood frequency. Such early, predominantly mid-latitude observations are essential because of shortcomings within climate models in their depiction of convective rainfall. A globally important group of intense stormsâmesoscale convective systems (MCSs)âposes a particular challenge, because they organize dynamically on spatial scales that cannot be resolved by conventional climate models. Here, we use 35 years of satellite observations from the West African Sahel to reveal a persistent increase in the frequency of the most intense MCSs. Sahelian storms are some of the most powerful on the planet, and rain gauges in this region have recorded a rise in âextremeâ daily rainfall totals. We find that intense MCS frequency is only weakly related to the multidecadal recovery of Sahel annual rainfall, but is highly correlated with global land temperatures. Analysis of trends across Africa reveals that MCS intensification is limited to a narrow band south of the Sahara desert. During this period, wet-season Sahelian temperatures have not risen, ruling out the possibility that rainfall has intensified in response to locally warmer conditions. On the other hand, the meridional temperature gradient spanning the Sahel has increased in recent decades, consistent with anthropogenic forcing driving enhanced Saharan warming. We argue that Saharan warming intensifies convection within Sahelian MCSs through increased wind shear and changes to the Saharan air layer. The meridional gradient is projected to strengthen throughout the twenty-first century, suggesting that the Sahel will experience particularly marked increases in extreme rain. The remarkably rapid intensification of Sahelian MCSs since the 1980s sheds new light on the response of organized tropical convection to global warming, and challenges conventional projections made by general circulation models
The SuperCam Instrument Suite on the Mars 2020 Rover: Science Objectives and Mast-Unit Description
On the NASA 2020 rover mission to Jezero crater, the remote determination of the texture, mineralogy and chemistry of rocks is essential to quickly and thoroughly characterize an area and to optimize the selection of samples for return to Earth. As part of the Perseverance payload, SuperCam is a suite of five techniques that provide critical and complementary observations via Laser-Induced Breakdown Spectroscopy (LIBS), Time-Resolved Raman and Luminescence (TRR/L), visible and near-infrared spectroscopy (VISIR), high-resolution color imaging (RMI), and acoustic recording (MIC). SuperCam operates at remote distances, primarily 2-7 m, while providing data at sub-mm to mm scales. We report on SuperCam's science objectives in the context of the Mars 2020 mission goals and ways the different techniques can address these questions. The instrument is made up of three separate subsystems: the Mast Unit is designed and built in France; the Body Unit is provided by the United States; the calibration target holder is contributed by Spain, and the targets themselves by the entire science team. This publication focuses on the design, development, and tests of the Mast Unit; companion papers describe the other units. The goal of this work is to provide an understanding of the technical choices made, the constraints that were imposed, and ultimately the validated performance of the flight model as it leaves Earth, and it will serve as the foundation for Mars operations and future processing of the data.In France was provided by the Centre National d'Etudes Spatiales (CNES). Human resources were provided in part by the Centre National de la Recherche Scientifique (CNRS) and universities. Funding was provided in the US by NASA's Mars Exploration Program. Some funding of data analyses at Los Alamos National Laboratory (LANL) was provided by laboratory-directed research and development funds
ELABORATION D'UN SUBSTITUT OSSEUX HYBRIDE ASSOCIANT DES CELLULES PERIOSTEES ET UN SUPPORT PHOSPHOCALCIQUE (EVALUATION CHEZ LE MOUTON)
TOULOUSE-EN Vétérinaire (315552301) / SudocSudocFranceF
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