4,499 research outputs found
Energetics and dynamics of H adsorbed in a nanoporous material at low temperature
Molecular hydrogen adsorption in a nanoporous metal organic framework
structure (MOF-74) was studied via van der Waals density-functional
calculations. The primary and secondary binding sites for H were confirmed.
The low-lying rotational and translational energy levels were calculated, based
on the orientation and position dependent potential energy surface at the two
binding sites. A consistent picture is obtained between the calculated
rotational-translational transitions for different H loadings and those
measured by inelastic neutron scattering exciting the singlet to triplet (para
to ortho) transition in H. The H binding energy after zero point energy
correction due to the rotational and translational motions is predicted to be
100 meV in good agreement with the experimental value of 90 meV.Comment: 5 pagers, 4 figures. added reference
Preparation of porous stainless steel hollow-fibers through multi-modal particle size sintering towards pore engineering
The sintering of metal powders is an efficient and versatile technique to fabricate porous metal elements such as filters, diffusers, and membranes. Neck formation between particles is, however, critical to tune the porosity and optimize mass transfer in order to minimize the densification process. In this work, macro-porous stainless steel (SS) hollow-fibers (HFs) were fabricated by the extrusion and sintering of a dope comprised, for the first time, of a bimodal mixture of SS powders. The SS particles of different sizes and shapes were mixed to increase the neck formation between the particles and control the densification process of the structure during sintering. The sintered HFs from particles of two different sizes were shown to be more mechanically stable at lower sintering temperature due to the increased neck area of the small particles sintered to the large ones. In addition, the sintered HFs made from particles of 10 and 44 μm showed a smaller average pore size (<1 μm) as compared to the micron-size pores of sintered HFs made from particles of 10 μm only and those of 10 and 20 μm. The novel HFs could be used in a range of applications, from filtration modules to electrochemical membrane reactors
X-ray and Optical Study of Low Core Density Globular Clusters NGC6144 and E3
We report on the Chandra X-ray Observatory and Hubble Space Telescope
observation of two low core density globular clusters, NGC6144 and E3. By
comparing the number of X-ray sources inside the half-mass radius to those
outside, we found 6 X-ray sources within the half-mass radius of NGC6144, among
which 4 are expected to be background sources; 3 X-ray sources are also found
within the half-mass radius of E3, of which 3 is expected to be background
source. Therefore, we cannot exclude that all our sources are background
sources. However, combining the results from X-ray and optical observations, we
found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be
cataclysmic variables and that 1 source in NGC6144 is an active binary, based
on the X-ray and optical properties. The number of faint X-ray sources in
NGC6144 and E3 found with Chandra and HST is higher than a prediction based on
collision frequency, but is closer to that based on mass. Our observations
strongly suggest that the compact binary systems in NGC6144 and E3 are
primordial in origin.Comment: 28 pages, 9 figures, 6 tables, Accepted for publication in Ap
Polyurea-Functionalized Multiwalled Carbon Nanotubes
An in situ polycondensation approach was applied to functionalize multiwalled carbon nanotubes (MWNTs), resulting in various linear or hyperbranched polycondensed polymers [e.g., polyureas, polyurethanes, and poly(urea-urethane)-bonded carbon nanotubes]. The quantity of the grafted polymer can be easily controlled by the feed ratio of monomers. As a typical example, the polyurea-functionalized MWNTs were measured and characterized in detail. The oxidized MWNTs (MWNT-COOH) were converted into acyl chloride-functionalized MWNTs (MWNT-COCl) by reaction with neat thionyl chloride (SOCl2). MWNT-COCl was reacted with excess 1,6-diaminohexane, affording amino-functionalized MWNTs (MWNT-NH2). In the presence of MWNT-NH2, the polyurea was covalently coated onto the surfaces of the nanotube by in situ polycondensation of diisocyanate [e.g., 4,4‘-methylenebis(phenylisocyanate)] and 1,6-diaminohexane, followed by the removal of free polymer via repeated filtering and solvent washing. The coated polyurea content can be controlled to some extent by adjusting the feed ratio of the isocyanato and amino groups. The structure and morphology of the resulting nanocomposites were characterized by FTIR, NMR, Raman, confocal Raman, TEM, EDS, and SEM measurements. The polyurea-coated MWNTs showed interesting self-assembled flat- or flowerlike morphologies in the solid state. The signals corresponding to that of the D and G bands of the carbon nanotubes were strongly attenuated after polyurea was chemically tethered to the MWNT surfaces. Comparative experiments showed that the grafted polymer species and structures have a strong effect on the Raman signals of polymer-functionalized MWNTs
First-Principles Calculation of the Superconducting Transition in MgB2 within the Anisotropic Eliashberg Formalism
We present a study of the superconducting transition in MgB2 using the
ab-initio pseudopotential density functional method and the fully anisotropic
Eliashberg equation. Our study shows that the anisotropic Eliashberg equation,
constructed with ab-initio calculated momentum-dependent electron-phonon
interaction and anharmonic phonon frequencies, yields an average
electron-phonon coupling constant lambda = 0.61, a transition temperature Tc =
39 K, and a boron isotope-effect exponent alphaB = 0.31 with a reasonable
assumption of mu* = 0.12. The calculated values for Tc, lambda, and alphaB are
in excellent agreement with transport, specific heat, and isotope effect
measurements respectively. The individual values of the electron-phonon
coupling lambda(k,k') on the various pieces of the Fermi surface however vary
from 0.1 to 2.5. The observed Tc is a result of both the raising effect of
anisotropy in the electron-phonon couplings and the lowering effect of
anharmonicity in the relevant phonon modes.Comment: 4 pages, 3 figures, 1 tabl
Detecting, Quantifying, and Discriminating the Mechanism of Mosaic Chromosomal Aneuploidies Using MAD-seq
Current approaches to detect and characterize mosaic chromosomal aneuploidy are limited by sensitivity, efficiency, cost, or the need to culture cells. We describe the mosaic aneuploidy detection by massively parallel sequencing (MAD-seq) capture assay and the MADSEQ analytical approach that allow low
Chandra and XMM-Newton observations of the merging cluster of galaxies PLCK G036.7+14.9
We present Chandra and XMM-Newton observations of PLCK G036.7+14.9 from the
Chandra-Planck Legacy Program. The high resolution X-ray observations reveal
two close subclusters, G036N and G036S, which were not resolved by previous
ROSAT, optical, or recent Planck observations. We perform detailed imaging and
spectral analyses and use a simplified model to study the kinematics of this
system. The basic picture is that PLCK G036.7+14.9 is undergoing a major merger
(mass ratio close to unity) between the two massive subclusters, with the
merger largely along the line-of-sight and probably at an early stage. G036N
hosts a small, moderate cool-core, while G036S has at most a very weak
cool-core in the central 40 kpc region. The difference in core cooling times is
unlikely to be caused by the ongoing merger disrupting a pre-existing cool-core
in G036S. G036N also hosts an unresolved radio source in the center, which may
be heating the gas if the radio source is extended. The Planck derived mass is
higher than the X-ray measured mass of either subcluster, but is lower than the
X-ray measured mass of the whole cluster, due to the fact that Planck does not
resolve PLCK G036.7+14.9 into subclusters and interprets it as a single
cluster. This mass discrepancy could induce significant bias to the mass
function if such previously unresolved systems are common in the Planck cluster
sample. High resolution X-ray observations are necessary to identify the
fraction of such systems and correct such a bias for the purpose of precision
cosmological studies.Comment: 23 pages, 8 figures (low resolution) with additional 12 figures in
the Appendix, accepted for publication in Ap
Structural and Superconducting Transitions in Mg_{1-x}Al_{x}B_2
From systematic ab initio calculations of the alloy system Mg_{1-x}Al_{x}B_2,
we find a strong tendency for the formation of a superstructure characterized
by Al-rich layers. We also present a simple model, based on calculated energies
and an estimate of the configurational entropy, which suggests that the alloy
has two separate concentration regimes of phase separation, with critical
points near x = 0.25 and x = 0.75. These results, together with calculations of
electronic densities of states in several ionic arrangements, give a
qualitative explanation for the observed structural instabilities, as well as
the x-dependence of the superconducting T_c for x<0.6.Comment: 4 pp./4 figs.; revisions in responce to Referee comment
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