Designing nanoparticles during the drawing step

International audienceNanoparticles in the core of optical fibres are widely studied due to the opportunity they give to tailor spectroscopic properties. Such fibres are usually obtained by drawing at high temperature a preform containing nanoparticles. This study focuses on the effect of the fibre drawing on nanoparticles. We fabricated an MCVD optical preform by doping the porous layer with nanoparticles. The optical fibre was studied by a FIB/SEM tomography.Figure 1 is the volume reconstruction of the core of the optical fibre. The yellow phase represents nanoparticles inside the core of the optical fibre. This reconstruction shows evidences of break-up, elongation and coalescence of particles. These features will be discussed according to phenomena well known from the rheology of emulsions and polymers. It comes from a competition between viscous stresses of the flow and surface tension.Observation of these size-controlling phenomena occuring during fibre drawing offer new perspectives to tailor the size of nanoparticles and are therefore of great interest for light scattering issues

Structure of the carbon layers deposited on the toroidal pump limiter of Tore Supra

International audienceScanning and transmission electron microscopy analyses have been performed for tiles extracted from the toroidal pump limiter of Tore Supra for erosion- and deposition-dominated zones. Deposit thicknesses have been estimated for the plasma-facing top and the gap side lateral surfaces. Deposit thickness profiles have been measured inside gaps, showing that deposition mainly occurs in the first millimetre and that both poloidal and toroidal gap deposition is asymmetric. Quantitative information on the deposit volume and on D-retention are thus obtained from these measurements. Carbon probed at the tile top surfaces is mainly amorphous carbon, due either to the amorphization induced by ion bombardment in the erosion dominated zone, or to deposit formation processes in the deposition-dominated zones. Deposits are tip-shaped and are oriented, which should give information on transport processes

Determination of the pressure in micrometric bubbles in irradiated nuclear fuels

In oxide nuclear fuels, at high burn-up or during high temperature periods such as ramp tests, out-of- pile heating tests, or any irradiations at high linear heat rates, fission gases can form micrometric or quasi-micrometric bubbles. During nominal operations, these bubbles participate to the pellet swelling, to the decrease of the fuel thermal conductivity and are involved in the mechanisms leading to fission gas release. During events involving a temperature increase, the resulting increase in the internal pres- sure of the bubbles might play a role in fuel fragmentation and in the opening of grain boundaries. The gas densities inside these bubbles are therefore one of the useful experimental information for the un- derstanding of the fuel behaviour, and for the fuel behaviour code progress and validation. Two methods were developed to evaluate the gas density in the quasi-micrometric bubbles, using electron probe micro analyser, secondary ion mass spectrometry and focused ion beam scanning electron microscope together. The first method provides a mean gas density for all quasi-micrometric bubbles in a given area. The sec- ond method provides a gas density in a single selected bubble. In addition to the gas density, the 3D size and shape of the selected bubble is measured and can be related to the gas density result. In this work, these methods were applied to the bubbles formed in the centre of a PWR Cr doped UO 2 at 38.8 GWd/t U after a ramp test in the Osiris reactor, with a 12 h plateau at 470 W/cm, and to the bubbles formed in a PWR Cr doped UO 2 at 62.8 GWd/t U in the centre of the pellet and on the bubbles of the high burn-up structure on the rim. Both show the high pressures reached in these bubbles.CEA-DES, EDF and Framatom

Effect of annealing temperature on the microstructure of Pr2Co7 alloy and its hydrogen absorption-desorption kinetics

International audienc

A comparative study of the dissolution mechanisms of amorphous and crystalline feldspars at acidic pH conditions

International audienceAbstract The dissolution of amorphous and crystalline oligoclase, a Na-feldspar with traces of Ca and K, was investigated at 90 °C and acidic pH of 1.5 and 3 to unravel the impact of long- and short-range orders on silicate dissolution mechanisms. Experiments were conducted in solutions spiked with 29 SiO 2 (aq) and saturated with respect to SiO 2 (am). Through morphology, structural, and composition characterizations, we showed that on the amorphous samples (glass samples), the altered layer was mostly formed by leaching, while a combination of both interfacial precipitation and leaching explains the layers formed on the crystalline sample. As expected, the altered layer was thicker at the most acidic pH and it became passivating on crystalline sample at pH 3. The alteration was faster on amorphous oligoclase than on its crystalline equivalent due to the more open structure of the glass. The preferential release of Al was suggested to play a key role, by weakening the silicate network of both substrates. Finally, in this study, a large overestimation of the global alteration of the materials was noticed based on the solution analyses. Discrepancies with solid analyses were attributed to an underestimation (≥2-fold factor) of the total reactive surface area

Gold coverage and faceting of MBE grown silicon nanowires

International audienc

TEM characterisation of helium platelets in implanted uranium dioxide

International audienc