Regional manifestation of the widespread disruption of soil-landscapes by the 4kyr BP impact-linked dust event using pedo-sedimentary micro-fabrics

The co-occurrence of a sharp dust peak, low lake levels, forest reduction, and ice retreat at ca. 4-kyr BP throughout tropical Africa and West Asia have been widely explained as the effect of an abrupt climate change. The detailed study of soils and archaeological records provided evidence to re-interpret the 4 kyr BP dust event linked rather to the fallback of an impact-ejecta, but not climate change. Here we aim to further investigate the exceptional perturbation of the soil-landscapes widely initiated by the 4 kyr BP dust event. Results are based on soil data from the eastern Khabur basin (North-East Syria), the Vera Basin (Spain), and the lower Moche Valley (West Peru) compared with a new study at the reference site of Ebeon (West France). The quality of the 4 kyr BP dust signal and the related environmental records are investigated through a micromorphological study of pedo-sedimentary micro-fabrics combined with SEM-microprobe, mineralogical, and geochemical analyses.In the four regions studied, the intact 4 kyr BP signal is identifi ed as a discontinuous burnt soil surface with an exotic dust assemblage assigned to the distal fallout of an impact-ejecta. Its unusual two-fold micro-facies is interpreted as (1) flash heating due to pulverization of the hot ejecta cloud at the soil surface, and (2) high energy deflation caused by the impact-related air blast. Disruption of the soil surface is shown to have been rapidly followed by a major de-stabilisation of the soil cover. Local factors and regional settings have exerted a major control on the timing, duration, and magnitude of landscape disturbances. Studies showed how a high quality signal allows to discriminate the short-term severe landscape disturbances linked to the exceptional 4 kyr BP dust event from more gradual environmental changes triggered by climate shift at the same time

Effects of the bias enhanced nucleation hot-filament chemical-vapor deposition parameters on diamond nucleation on iridium

The effects of the bias current density and the filament-to-substrate distance on the nucleation of diamond on iridium buffer layers were investigated in a hot-filament chemical-vapor deposition (HFCVD) reactor. The nucleation density increased by several orders of magnitude with the raise of the bias current density. According to high-resolution field-emission gun scanning electron microscopy observation, diamond nuclei formed during bias-enhanced nucleation (BEN) did not show any preferred oriented growth. Moreover, the first-nearest-neighbor distance distribution was consistent with a random nucleation mechanism. This occurrence suggested that the diffusion of carbon species at the substrate surface was not the predominant mechanism taking place during BEN in the HFCVD process. This fact was attributed to the formation of a graphitic layer prior to diamond nucleation. We also observed that the reduction of the filament sample distance during BEN was helpful for diamond growth. This nucleation behavior was different from the one previously reported in the case of BEN-microwave chemical-vapor deposition experiments on iridium and has been tentatively explained by taking into account the specific properties and limitations of the HFCVD technique

CVD diamond coated silicon nitride self-mated systems : tribological behaviour under high loads

Friction and wear behaviour of self-mated chemical vapour deposited (CVD) diamond films coating silicon nitride ceramics (Si3N4) were investigated in ambient atmosphere. The tribological tests were conducted in a reciprocal motion ball-on-flat type tribometer under applied normal loads up to 80 N (~10 GPa). Several characterisation techniques - including scanning electron microscopy (SEM), atomic force microscopy (AFM) and micro-Raman studies - were used in order to assess the quality, stress state and wear resistance of the coatings. In addition, a novel method is presented to estimate the wear coefficient of the diamond coated flat specimens from AFM and optical microscopy (OM) observations of the wear tracks

Impact fingerprints of the 4 kyr BP dust event based on archaeological, soil, lake and marine archives

The 4 kyr BP impact event has been identified from deep-sea, soil and archaeological records as the worldwide pulverisation of a volatile-rich debris jet(1). High resolution sequences show two stages of ejecta fallout linked to the impact-triggered doublet gaseous regime(2): scattered solid debris at the ground surface and spray of the vaporized hot fireball with thermal blast and local ignition. Ejecta debris consist of flow-textured impact glass, impact breccia and incompletely melted marine clasts: volcano-clastic sandy mudstone, calcareous mud, granite-gneiss, schists, volcanic breccia, kerogen and algal mud. Marine microfossils, organo-mineral markers, and the debris-fallout spatial pattern indicate two potential impact craters: an Antarctic source with an admixture of volcanic glass and ice rafted debris, from the vicinity of Heard Island and the Kerguelen plateau; a low latitude, shallow water one with hydrocarbons possibly from the Gulf of Mexico. Fine mixing of target materials from far distant source craters emphasizes a unique impact-ejecta. This matches the theoretical view of a debris jet channelled along the corridor cut through the atmosphere by the incoming projectile, raised upward, and dispersed widely(3). The isotopic anomaly of the sulphur phase in the kerogen volatile-component, indicating mass independent fractionation due to photolytic transformation, suggests launching at great heights, beyond the O2-O3 UV shield, responsible for climate disturbances. The incomplete melting of target rocks and global dispersion of impact breccia out of the craters would result from splash of small-sized projectile at rather great water depth and a low angle impact (10-15 degrees) into porous, highly compressible marine sediments. The spatially variable distribution of the organo-mineral and melt components, and the wide range of phase transformation reflect non-equilibrium shock-melting and micro-scale thermal processes in the heterogeneous vapor plume ejected from the impact sites. The 4 kyr BP event provides the first opportunity to compare on a variety of scales the environmental effects of globally dispersed ejecta from a multiple-site oceanic impact and the complex responses of human societies to frightening manifestations

The use of micro-Raman imaging to measure 18O tracer distribution in thermally grown zirconia scales

International audienceHigh temperature Zircaloy-4 oxidation has been studied through two-stage oxidation experiments using 18O isotope. 18O distribution in the oxidation scales was investigated by micro-Raman imaging. First, zirconia standards with known 18O content were prepared and analyzed for calibration purposes. Then, to assess the method on a simple case, 18O tracer penetration patterns were recorded after a 800°C two-stage oxidation. Finally, oxidation at 850°C of a Zircaloy4 cladding tube pre-corroded at 425°C to simulate corrosion during normal reactor operation was studied. These preliminary results give insights on the protective nature of the first formed zirconia scale in case of an accident. © 2015 Elsevier Ltd

High temperature Zircaloy-4 oxidation in water vapour-containing environments examined with Raman imaging and labelled oxygen

International audienceHigh Temperature oxidation of Zircaloy-4 was studied in N2-O2-H2O gas mixtures using labelled 18 O2, thermogravimetry, Raman imaging and mass spectrometry. Samples were either exposed to N2-18 O2-H2O gas mixtures, or two-stage oxidised first in 18 O2, then in H2 16 O-N2 or 16 O2-N2 atmospheres. In the pre-transition, diffusion-controlled regime, mostly steam contributes to the zirconia formation, while in the nitrogen-catalysed post transition regime both oxygen and steam contribute to the oxidation. These results were tentatively explained by competitive adsorption of oxygen and steam at the gas/solid interfaces, and the formation of porosity in the oxide layers

Intercalation of manganese chloride into mesophase pitch-based graphite fibers via gaseous complexes

International audienceThe intercalation of certain metal chlorides into carbon fibers presents some problems, which depend on both their structure (three-dimensional ordering in particular) and the intercalation techniques. In this work, the intercalation of manganese chloride into pitch-based carbon fibers was investigated. The use of manganese chloride in the presence of iron(III) chloride or copper(II) chloride enhances the vapor pressure of the manganese salt and thus makes easier the intercalation process. The results of this work confirm the existence of FeCl3–MnCl2 complexes in the gaseous phase during intercalation. Stage-I intercalation compounds have been obtained for different graphite fibers. Several experimental techniques were used to characterize both the pristine and the intercalated graphite fibers, including X-ray diffraction, Raman spectroscopy and electrical resistivity measurements

Review of stress fields in Zirconium alloys corrosion scales

A review of the data available in the literature on the residual stress determination in zirconia scales has been undertaken. Deflexion method and curvature measurement as well as Raman spectroscopy and X-ray diffraction have been used. Stress magnitudes obtained by different authors vary between a few hundred of MPa and more than 5. Gpa, depending on the parameters investigated such as oxidized alloy, oxidation atmosphere and temperature. The influence of the later on both the average stress magnitudes and also on its evolution has been determined, and the stress release processes have been discussed. © 2015 Elsevier Ltd