117 research outputs found
The plasticity size effect in replicated microcellular aluminium
Microcellular aluminium can be produced by replication in porous material with pores across a wide range of sizes but with otherwise identical structures. Compressive tests reveal a plasticity size effect, with samples showing higher strengths and higher rates of work hardening for smaller pore diameters. This size effect is shown to be dislocational, its main origin being dislocation emission during the composite stage of foam processing. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Continuous Casting of Multi-Alloy Metal Products and Related Methods
Described are multi-alloy metal products formed by a continuous casting system, as well as methods of continuously casting multi-alloy metal products. The disclosed multi-alloy metal product includes a plurality of layers of metallurgically bonded together metal alloys. The layers can be distributed along the thickness of the metal product or along the width of the metal product. The disclosed continuous casting system includes a plurality of injectors configured to simultaneously inject a plurality of metal alloys into a casting cavity to form the multi-alloy metal product
Djerba
SituĂ©e Ă lâextrĂ©mitĂ© sud-est du Golfe de GabĂšs, la Petite Syrte des Anciens, Djerba est la plus grande â avec 538 km2 de superficie â et surtout la plus cĂ©lĂšbre des Ăźles du littoral dâAfrique du Nord. Elle nâest sĂ©parĂ©e du continent que par deux Ă©troits passages â celui entre Adjim et Djorf Ă lâouest et celui dâEl Kantara Ă lâest -qui encadrent la mer de Bou Grara*. Une zone de hauts-fonds qui lâenveloppe entiĂšrement, une marĂ©e dont lâamplitude est ici de 1,30 m en pĂ©riode de vives eaux, rend..
Djerba
SituĂ©e Ă lâextrĂ©mitĂ© sud-est du Golfe de GabĂšs, la Petite Syrte des Anciens, Djerba est la plus grande â avec 538 km2 de superficie â et surtout la plus cĂ©lĂšbre des Ăźles du littoral dâAfrique du Nord. Elle nâest sĂ©parĂ©e du continent que par deux Ă©troits passages â celui entre Adjim et Djorf Ă lâouest et celui dâEl Kantara Ă lâest -qui encadrent la mer de Bou Grara*. Une zone de hauts-fonds qui lâenveloppe entiĂšrement, une marĂ©e dont lâamplitude est ici de 1,30 m en pĂ©riode de vives eaux, rend..
Interferometric imaging of carbon monoxide in comet C/1995 O1 (Hale-Bopp): evidence for a strong rotating jet
Observations of the CO J(1-0) 115 GHz and J(2-1) 230 GHz lines in comet
C/1995 O1 (Hale-Bopp) were performed with the IRAM Plateau de Bure
interferometer on 11 March, 1997. The observations were conducted in both
single-dish (ON-OFF) and interferometric modes with 0.13 km s-1 spectral
resolution. Images of CO emission with 1.7 to 3" angular resolution were
obtained. The ON-OFF and interferometric spectra show a velocity shift with
sinusoidal time variations related to the Hale-Bopp nucleus rotation of 11.35
h. The peak position of the CO images moves perpendicularly to the spin axis
direction in the plane of the sky. This suggests the presence of a CO jet,
which is active night and day at about the same extent, and is spiralling with
nucleus rotation. The high quality of the data allows us to constrain the
characteristics of this CO jet. We have developed a 3-D model to interpret the
temporal evolution of CO spectra and maps. The CO coma is represented as the
combination of an isotropic distribution and a spiralling gas jet, both of
nucleus origin. Spectra and visibilities (the direct output of interferometric
data) analysis shows that the CO jet comprises ~40% the total CO production and
is located at a latitude ~20 degrees North on the nucleus surface. Our
inability to reproduce all observational characteristics shows that the real
structure of the CO coma is more complex than assumed, especially in the first
thousand kilometres from the nucleus. The presence of another moving CO
structure, faint but compact and possibly created by an outburst, is
identified.Comment: 20 pages, 26 figures. Accepted for publication in Astronomy &
Astrophysic
Detection of Cold Atomic Clouds in the Magellanic Bridge
We report a detection of cold atomic hydrogen in the Magellanic Bridge using
21-cm absorption spectroscopy toward the radio source B0312-770. With a column
density of N_HI=1.2E20 cm^-2, a maximum absorption optical depth of tau=0.10
and a maximum 21-cm emission brightness temperature of 1.4 K, this line of
sight yields a spin temperature, T_s, between 20 K and 40 K. H I 21-cm
absorption and emission spectroscopy toward 7 other low column density
sightlines on the periphery of the LMC and SMC reveal absorption toward one
additional background radio source behind the SMC with tau=0.03. The data have
typical sensitivities of sigma_tau=0.005 to 0.070 in absorption and
sigma_{T_B}=0.03 K in emission. These data demonstrate the presence of a cold
atomic phase which is probably accompanied by molecular condensations in the
tenuous interstellar medium of the Bridge region. Young OB stars observed in
the Magellanic Bridge could form "in situ" from these cold condensations rather
than migrate from regions of active star formation in the main body of the SMC.
The existence of cold condensations and star formation in the Magellanic Bridge
might be understood as a small scale version of the mechanism that produces
star formation in the tidal tails of interacting galaxies.Comment: 25 pages, uses AASTeX and psfig; Accepted for Publication in the
Astronomical Journa
Interferometric imaging of the sulfur-bearing molecules H2S, SO and CS in comet C/1995 O1 (Hale-Bopp)
We present observations of rotational lines of H2S, SO and CS performed in
comet C/1995 O1 (Hale-Bopp) in March 1997 with the Plateau de Bure
interferometer (IRAM). The observations provide informations on the spatial and
velocity distributions of these molecules. They can be used to constrain their
photodissociation rate and their origin. We use a radiative transfer code which
allows us to compute synthetic line profiles and interferometric maps, to be
compared to the observations. Both single-dish spectra and interferometric
spectral maps show a day/night asymmetry in the outgassing. From the analysis
of the spectral maps, including the astrometry, we show that SO and CS present
in addition a jet-like structure that may be the gaseous counterpart of the
dust high-latitude jet observed in optical images. A CS rotating jet is also
observed. Using the astrometry provided by continuum radio maps obtained in
parallel, we conclude that there is no need to invoke of nongravitational
forces acting on this comet, and provide an updated orbit. The radial extension
of H2S is found to be consistent with direct release from the nucleus. SO
displays an extended radial distribution. Assuming that SO2 is the parent of
SO, the photodissociation rate of SO is measured to be 1.5 E-4 s-1 at 1 AU from
the Sun. This is lower than most laboratory-based estimates and may suggest
that SO is not solely produced by SO2 photolysis. From the observations of
J(2-1) and J(5-4) CS lines, we deduce a CS photodissociation rate of 1 to 5 E-5
s-1. The photodissociation rate of CS2, the likely parent of CS, cannot be
constrained due to insufficient resolution, but our data are consistent with
published values. These observations illustrate the cometary science that will
be performed with the future ALMA interferometer.Comment: Accepted for publication in Astronomy & Astrophysic
13Câmethyl formate : observations of a sample of high mass starforming regions including OrionâKL and spectroscopic characterization
We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic centre for
methyl formate, HCOOCH3, and its isotopologues H13COOCH3 and HCOO13CH3. The observations were carried out with the
APEX telescope in the frequency range 283.4-287.4 GHz. Based on the APEX observations, we report tentative detections of
the 13C-methyl formate isotopologue HCOO13CH3 towards the following four massive star-forming regions: Sgr B2(N-LMH),
NGC 6334 IRS 1, W51 e2 and G19.61-0.23. In addition, we have used the 1 mm ALMA science verification observations of
Orion-KL and confirm the detection of the 13C-methyl formate species in Orion-KL and image its spatial distribution. Our
analysis shows that the 12C/13C isotope ratio in methyl formate toward Orion-KL Compact Ridge and Hot Core-SW
components (68.4±10.1 and 71.4±7.8, respectively) are, for both the 13C-methyl formate isotopologues, commensurate with the
average 12C/13C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with
the 12C/13C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the
complex H13COOCH3 and HCOO13CH3 species. New spectroscopic data for both isotopomers H13COOCH3 and HCOO13CH3,
presented in this study, has made it possible to measure this fundamentally important isotope ratio in a large organic
molecule for the first time.This work was supported by the National Science Foundation under grant 1008800. We are grateful to the Ministerio de Economia y Competitividad of Spain for the financial support through grant No. FIS2011-28738-C02-02 and to the French Government through grant No. ANR-08-BLAN-0054 and the French PCMI (Programme National de Physique Chimie du Milieu Interstellaire). This paper makes use of the following ALMA data: ADS/JAO. ALMA#2011.0.00009.SV.ALMAis a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. C.F. thanks Dahbia Talbi, Eric Herbst, and Anthony Remijan for enlightening discussions. Finally, we thank the anonymous referee for helpful comments
The effect of density and feature size on mechanical properties of isostructural metaffic foams produced by additive manufacturing
Simple models describing the relationship between basic mechanical properties and the relative density of various types of porous metals (such as foams, sponges and lattice structures) are well established. Carefully evaluating these relationships experimentally is challenging, however, because of the stochastic structure of foams and the fact that it is difficult to systematically isolate density changes from variations in other factors, such as pore size and pore distribution. Here a new method for producing systematic sets of stochastic foams is employed based on electron beam melting (EBM) additive manufacturing (AM). To create idealised structures, structural blueprints were reverse-engineered by inverting X-ray computed tomographs of a randomly packed bed of glass beads. This three-dimensional structure was then modified by computer to create five foams of different relative density Ïr, but otherwise consistent structure. Yield strength and Youngâs modulus have been evaluated in compression tests and compared to existing models for foams. A power of 3 rather than a squared dependence of stiffness on relative density is found, which agrees with a recent model derived for replicated foams. A similar power of 3 relation was found for yield strength. Further analysis of the strength of nominally fully dense rods of different diameters built by EBM AM suggest that surface defects mean that the minimum size of features that can be created by EBM with similar strengths to machined samples is âŒ1 mm
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