New method to study ion-molecule reactions at low temperatures and application to the H2+_2^+ + H2_2 →\rightarrow H3+_3^+ + H reaction

Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the ${\rm H}_2^+ + {\rm H}_2\rightarrow {\rm H}_3^+ + {\rm H}$ reaction with the ${\rm H}_2^+$ ion prepared in a single rovibrational state at collision energies in the range $E_{\rm col}/k_{\rm B} = 5$-60 K. To reach such low collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H$_2$ molecules and the second is a supersonic beam of H$_2$ molecules excited to Rydberg-Stark states of principal quantum number $n$ selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10$^\circ$. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state ${\rm H}_2$ beam and by adapting the electric-potential functions to the electrodes of the deflector. The collision energy is varied down to below $E_{\rm col}/k_{\rm B}= 10$ K, i.e., below $E_{\rm col}\approx 1$ meV, with an energy resolution of 100 $\mu$eV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical-Langevin-capture model in the collision-energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studyingComment: 39 pages, 10 figure

The Monitor project: JW 380 -- a 0.26, 0.15 Msol pre main sequence eclipsing binary in the Orion Nebula Cluster

We report the discovery of a low-mass (0.26 +/- 0.02, 0.15 +/- 0.01 Msol) pre-main-sequence eclipsing binary with a 5.3 day orbital period. JW 380 was detected as part of a high-cadence time-resolved photometric survey (the Monitor project) using the 2.5m Isaac Newton Telescope and Wide Field Camera for a survey of a single field in the Orion Nebula Cluster (ONC) region in V and i bands. The star is assigned a 99 per cent membership probability from proper motion measurements, and radial velocity observations indicate a systemic velocity within 1 sigma of that of the ONC. Modelling of the combined light and radial velocity curves of the system gave stellar radii of 1.19 +0.04 -0.18 Rsol and 0.90 +0.17 -0.03 Rsol for the primary and secondary, with a significant third light contribution which is also visible as a third peak in the cross-correlation functions used to derive radial velocities. The masses and radii appear to be consistent with stellar models for 2-3 Myr age from several authors, within the present observational errors. These observations probe an important region of mass-radius parameter space, where there are currently only a handful of known pre-main-sequence eclipsing binary systems with precise measurements available in the literature.Comment: 11 pages, 9 figures, accepted for publication in MNRA

The skeleton of the staghorn coral Acropora millepora: molecular and structural characterization

15 pagesInternational audienceThe scleractinian coral Acropora millepora is one of the most studied species from the Great Barrier Reef. This species has been used to understand evolutionary, immune and developmental processes in cnidarians. It has also been subject of several ecological studies in order to elucidate reef responses to environmental changes such as temperature rise and ocean acidification (OA). In these contexts, several nucleic acid resources were made available. When combined to a recent proteomic analysis of the coral skeletal organic matrix (SOM), they enabled the identification of several skeletal matrix proteins, making A. millepora into an emerging model for biomineralization studies. Here we describe the skeletal microstructure of A. millepora skeleton, together with a functional and biochemical characterization of its occluded SOM that focuses on the protein and saccharidic moieties. The skeletal matrix proteins show a large range of isoelectric points, compositional patterns and signatures. Besides secreted proteins, there are a significant number of proteins with membrane attachment sites such as transmembrane domains and GPI anchors as well as proteins with integrin binding sites. These features show that the skeletal proteins must have strong adhesion properties in order to function in the calcifying space. Moreover this data suggest a molecular connection between the calcifying epithelium and the skeletal tissue during biocalcification. In terms of sugar moieties, the enrichment of the SOM in arabinose is striking, and the monosaccharide composition exhibits the same signature as that of mucus of acroporid corals. Finally, we observe that the interaction of the acetic acid soluble SOM on the morphology of in vitro grown CaCO3 crystals is very pronounced when compared with the calcifying matrices of some mollusks. In light of these results, we wish to commend Acropora millepora as a model for biocalcification studies in scleractinians, from molecular and structural viewpoints

Transformation-frittage d'alumines de transition nanostructurées (rôle des conditions de mise en forme, du cycle thermique, de l'ensemencement et du dopage)

L'étude du frittage de poudres nanostructurées d'alumine g, abordée avec une approche multiéchelle (comportement macroscopique suivi par dilatométrie et investigations des microstructures (MEB et MET) à différents stades du frittage), permet d'expliciter l'influence de paramètres tels que l'ensemencement, la mise en forme, le traitement thermique, la nature et la quantité de dopants sur la transformation-frittage et de relier les caractéristiques des corps crus à leur aptitude au frittage. Le frittage présente deux régimes de densification associés à la transformation g-a et à la densification de la phase a. Pendant la transformation, un gain de densité supérieur à celui résultant du changement de maille cristallographique est observé et attribué à des mécanismes couplés de transformation-réarrangement- coalescence à partir de "sites de germination" dont la densité est fonction des différents paramètres déjà cités. La microstructure post-transformation, composée de colonies monocristallines poreuses constituées de briques élémentaires de phase a, conditionne la densification ultérieure. L'ajout de dopant modifie le comportement au frittage. Y et Zr, ayant une faible limite de solubilité dans l'alumine a, retardent la transformation g-a et ne favorisent pas la densification ultérieure. Ti et Mg, dont la limite de solubilité est plus élevée, avancent légèrement la transformation pour des taux inférieurs à cette limite, sinon ils la retardent légèrement. Par ailleurs, le frittage des échantillons dopés présente une accélération temporaire de la densification, plus intense pour les échantillons dopés Ti et Y, que pour ceux dopés Mg et Zr. Une transition microstructurale entre ségrégation et précipitation d'une seconde phase aux joints de grains est également observée. Le meilleur comportement au frittage est obtenu pour le dopage au Ti. Il accélère significativement la densification en phase a par une croissance anisotrope des briques.Sintering of g nanostructured transition alumina, studied via a multiscale approach of the sintering behavior (macroscopic behavior investigated by dilatometry and detailed microstructural observations at different steps of sintering by microscopy techniques) clearly points out the influence of parameters such as a-seeding, forming conditions, thermal cycle, nature and concentration of doping element on the transformation-densification of g-Al2O3. Thus, the green bodies characteristics can be linked with their sintering fitness. The densification of g-Al2O3 shows two regimes of densification associated with the g-a phase transformation and the densification of a-phase. During the transformation, an enhanced densification higher that expected for the g-a transition is observed and is brought about particle rearrangement-coalescence during the transformation, the degree of particle rearrangement being influenced by the nuclei density that depends on the above mentioned parameters. The post-transformation microstructure consisting of porous monocrystalline clusters with a elementary bricks promotes the further densification. The doping changes the sintering behavior. Y and Zr, which have a low solubility limit in a-Al2O3, delay the g-a transformation and don't enhance the a-densification. Ti and Mg have a higher solubility limits in a-Al2O3. Consequently, the transformation temperature lightly decreases when the amount of doping elements is lower than the solubility limit; otherwise, it increases. Sintering behavior of doped Al2O3 presents also a transient increase in densification more intense for Ti and Y samples than for Mg and Zr samples. A microstructural transition between segregation at grain boundaries to precipitation of mixed oxide is also observed. The best sintering behavior is obtained with Ti doping which increases the densification rate of Al2O3. Ti induces the formation of elongated elementary bricks, which by coarsening, promotes the densification.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Synthesis, characterization and reinforcing properties of novel, reactive clay/poly(glycidyl methacrylate) nanocomposites

Clay/polymer nanocomposites, of the type montmorillonite/poly(glycidyl methacrylate) [MMT/PGMA], were synthesized via atom transfer radical polymerization (ATRP). An ATRP initiator, consisting of quaternary ammonium salt bearing a 2-bromo-2-methyl propionate moiety was intercalated into the interlayer spacings of the layered silicate. The ammonium salt-modified montmorillonite (MMTBr) served as macroinitiator for the in situ ATRP of glycidyl methacrylate resulting in highly exfoliated MMT/PGMA nanocomposites, as judged by XRD measurements. However, TEM shows the existence of intercalated clay regions, but at a very low extent. TGA and XPS analyses indicate that the nanocomposites have PGMA-rich bulk and surface. Indeed, the mass loading of PGMA reached 61 wt.% whilst XPS spectra, particularly the high resolution C1s region, resemble those of pure PGMA. The MMT/PGMA nanocomposites were found to be soluble in chloroform with a transparent solution, fully compatible with epoxy resin without any sign of phase separation. The primary MMT/PGMA nanocomposites were mixed with DGEBA and DETA in chloroform in order to prepare moulded, ternary MMT/PGMAepoxyDETA nanocomposites by solvent evaporation. The dried ternary systems exhibited superior viscoelastic properties (storage modulus and tan d) compared to the neat crosslinked epoxyDETA adhesive prepared in the absence of any clay nanocomposite

EBSD, XRD and SRS characterization of a casting Al-7wt%Si alloy processed by equal channel angular extrusion: Dislocation density evaluation

International audienceAluminum-silicon (AleSi) alloys of high silicon contents are composite materials; they are used whenever high casting properties are required. They are slightly ductile below 8wt%Si. An increase in ductility can be obtained by refining Si-crystals in elaboration or by a further hot working. In the present work, an Al-7wt%Si alloy was processed by Equal Channel Angular Extrusion (ECAE) at temperatures 20 °C and 160 °C up to three passes. The die was formed by two cylindrical channels with characteristic angles Φ = 110°and Ψ = 0. EBSD, X ray diffraction (XRD) and Strain Rate Sensitivity (SRS) were used to characterize the microstructure and the mechanical properties. High levels of strain were introduced at both temperatures. The activation volume was lowered to 125b 3 and 210b 3 at 20 °C and 160 °C respectively and was considered to be dislocation density dependent. The remaining dislocation densities calculated from EBSD, XRD and SRS experiments are quite different. This was explained by the scale difference and by the sensitivities of the methods to the free surface effect

Genesis of amorphous calcium carbonate containing alveolar plates in the ciliate Coleps hirtus (Ciliophora, Prostomatea).

7 pagesInternational audienceIn the protist world, the ciliate Coleps hirtus (phylum Ciliophora, class Prostomatea) synthesizes a peculiar biomineralized test made of alveolar plates, structures located within alveolar vesicles at the cell cortex. Alveolar plates are arranged by overlapping like an armor and they are thought to protect and/or stiffen the cell. Although their morphology is species-specific and of complex architecture, so far almost nothing is known about their genesis, their structure and their elemental and mineral composition. We investigated the genesis of new alveolar plates after cell division and examined cells and isolated alveolar plates by electron microscopy, energy-dispersive X-ray spectroscopy, FTIR and X-ray diffraction. Our investigations revealed an organic mesh-like structure that guides the formation of new alveolar plates like a template and the role of vesicles transporting inorganic material. We further demonstrated that the inorganic part of the alveolar plates is composed out of amorphous calcium carbonate. For stabilization of the amorphous phase, the alveolar vesicles, the organic fraction and the element phosphorus may play a role

Microstructure et volume d'activation d'un laiton Cu-6%Zn traité par ECAP

International audienceThe generation of a high density of twins few tens nanometers in size can enhance both strength and ductility. The present work aims to produce highly twinned microstructure in Cu-6wt% Zn alloy of a moderate stacking fault energy (SFE) by equal channel angular pressing (ECAP) using a die with angles =110° and =0. X ray diffraction (XRD) shows that a high density of defects is stored up to 2 passes then a recovery takes place. EBSD imaging reveals a high density of twins and a fibrous microstructure. Grains close to 100nm in size are formed. Strain rate sensitivity (SRS) was investigated in compression and nanoindentation (NI). Compression curves show that deformation occurs by slipping with a contribution of twins to hardening. A high contribution of shear bands was revealed during ECAP and compression tests. The activation volume V* in compression and NI is in the range 70b 3-100b 3 , consistent with the emission of dislocations from grain boundaries and twin boundaries The experimental values of V* were compared to those expected from defect densities

D2 and H2 adsorption capacity and selectivity in CHA zeolites: Effect of Si/Al ratio, cationic composition and temperature

International audienceThe work deals with the effect of composition of CHA zeolites on the adsorption and separation of H2 and D2 under cryogenic temperatures. In the first part of this work the effect of Si/Al ratio and cationic composition on single gas adsorption of H2 and D2 was studied at 77.4 K. It was found that the adsorption capacities increase with Al content up to Si/Al = 2.1. Unexpectedly, Na-CHA zeolite with the highest Al content (Si/Al = 1.1) adsorbs only negligible amount because of the collapse of the zeolite structure upon dehydration at 400°C. The Na-and Li-containing chabazites with Si/Al = 2.1 possess similar adsorption capacities. In contrast, progressive replacement of Na + with K + results at 77.4 K in decreasing H2 and D2 adsorbed amount which becomes negligible for K + content higher than ~ 60 mol. %. In the second part of the work the D2/H2 thermodynamic selectivity for Na-, Li-and K-Na chabazites (Si/Al = 2.1) was measured using coadsorption technique at 40-77.4 K in the pressure range 530-650 hPa corresponding to high loading. The cationic composition is found to have only minor effect on the D2/H2 selectivity which increases exponentially with lowering temperature. For example in Na3.9Al3.9Si8.1O24 chabazite it rises from 2.6 at 77.4 K to 13 at 40 K. It is found that variation of D2/H2 selectivity with temperature is in a fair agreement with a simple quantum sieving model making use of the aperture size of the chabazite cavity as a unique adjustable parameter

Immobilization of copper complexes with (1,10-phenanthrolinyl)phosphonates on titania supports for sustainable catalysis

International audienc