3,726 research outputs found
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Innovative Selective Laser Sintering Rapid Manufacturing using Nanotechnology
The objective of this research is to develop an improved nylon 11 (polyamide 11) polymer
with enhanced flame retardancy, thermal, and mechanical properties for selective laser sintering
(SLS) rapid manufacturing (RM). A nanophase was introduced into nylon 11 via twin screw
extrusion to provide improved material properties of the polymer blends. Atofina (now known
as Arkema) RILSAN® nylon 11 injection molding polymer pellets was used with three types of
nanoparticles: chemically modified montmorillonite (MMT) organoclays, nanosilica, and carbon
nanofibers (CNF) to create nylon 11 nanocomposites. Wide angle X-ray diffraction (WAXD)
and transmission electron microscopy (TEM) were used to determine the degree of dispersion.
Fifteen nylon 11 nanocomposites and control nylon 11 were fabricated by injection molding.
Flammability properties (using a cone calorimeter with a radiant flux of 50 kW/m2
) and
mechanical properties such as tensile strength and modulus, flexural modulus, elongation at
break were determined for the nylon 11 nanocomposites and compared with the baseline nylon
11. Based on flammability and mechanical material performance, five polymers including four
nylon 11 nanocomposites and a control nylon 11 were cryogenically ground into fine powders
for SLS RM. SLS specimens were fabricated for flammability, mechanical, and thermal
properties characterization. Nylon 11-CNF nanocomposites exhibited the best overall properties
for this study.Mechanical Engineerin
OH (1720 MHz) Masers: A Multiwavelength Study of the Interaction between the W51C Supernova Remnant and the W51B Star Forming Region
We present a comprehensive view of the W51B HII region complex and the W51C
supernova remnant (SNR) using new radio observations from the VLA, VLBA,
MERLIN, JCMT, and CSO along with archival data from Spitzer, ROSAT, ASCA, and
Chandra. Our VLA data include the first 400 cm (74 MHz) continuum image of W51
at high resolution (88 arcsec). The 400 cm image shows non-thermal emission
surrounding the G49.2-0.3 HII region, and a compact source of non-thermal
emission (W51B_NT) coincident with the previously-identified OH (1720 MHz)
maser spots, non-thermal 21 and 90 cm emission, and a hard X-ray source.
W51B_NT falls within the region of high likelihood for the position of TeV
gamma-ray emission. Using the VLBA three OH (1720 MHz) maser spots are detected
in the vicinity of W51B_NT with sizes of 60 to 300 AU and Zeeman effect
magnetic field strengths of 1.5 to 2.2 mG. The multiwavelength data demonstrate
that the northern end of the W51B HII region complex has been partly enveloped
by the advancing W51C SNR and this interaction explains the presence of W51B_NT
and the OH masers. This interaction also appears in the thermal molecular gas
which partially encircles W51B_NT and exhibits narrow pre-shock (DeltaV 5 km/s)
and broad post-shock (DeltaV 20 km/s) velocity components. RADEX radiative
transfer modeling of these two components yield physical conditions consistent
with the passage of a non-dissociative C-type shock. Confirmation of the
W51B/W51C interaction provides additional evidence in favor of this region
being one of the best candidates for hadronic particle acceleration known thus
far.Comment: Accepted to Ap
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Polyamide 11-Carbon Nanotubes Nanocomposites: Preliminary Investigation
The objective of this research is to develop an improved polyamide 11 (PA11) polymer with
enhanced flame retardancy, thermal, and mechanical properties for selective laser sintering
(SLS) rapid manufacturing. In the present study, a nanophase was introduced into polyamide 11
via twin screw extrusion. Arkema Rilsan® polyamide 11 molding polymer pellets were used
with 1, 3, 5, and 7 wt% loadings of Arkema’s GraphistrengthTM multi-wall carbon nanotubes
(MWNTs) to create a family of PA11-MWNT nanocomposites.
Transmission electron microscopy and scanning electron microscopy were used to determine
the degree and uniformity of dispersion. Injection molded test specimens were fabricated for
physical, thermal, mechanical properties, and flammability measurements. Thermal stability of
these polyamide 11-MWNT nanocomposites was examined by TGA. Mechanical properties such
as ultimate tensile strength, rupture tensile strength, and elongation at rupture were measured.
Flammability properties were also obtained using the UL 94 test method. All these different
methods and subsequent polymer characteristics are discussed in this paper.Mechanical Engineerin
Adiabatic Domain Wall Motion and Landau-Lifshitz Damping
Recent theory and measurements of the velocity of current-driven domain walls
in magnetic nanowires have re-opened the unresolved question of whether
Landau-Lifshitz damping or Gilbert damping provides the more natural
description of dissipative magnetization dynamics. In this paper, we argue that
(as in the past) experiment cannot distinguish the two, but that
Landau-Lifshitz damping nevertheless provides the most physically sensible
interpretation of the equation of motion. From this perspective, (i) adiabatic
spin-transfer torque dominates the dynamics with small corrections from
non-adiabatic effects; (ii) the damping always decreases the magnetic free
energy, and (iii) microscopic calculations of damping become consistent with
general statistical and thermodynamic considerations
Mid-infrared Identification of 6 cm Radio Source Counterparts in the Extended Groth Strip
A new 6-cm survey of almost 0.6 square degrees to a limit of 0.55-mJy/beam
(10-sigma) finds 37 isolated radio sources and 7 radio source pairs (not
necessarily physical companions). IRAC counterparts are identified for at least
92% of the radio sources within the area of deep IRAC coverage, which includes
31 isolated sources and 6 pairs. This contrasts with an identification rate of
<74% to R<23.95 in visible light. Eight of the IRAC galaxies have power law
spectral energy distributions, implying that the mid-infrared emission comes
from a powerful AGN. The remaining 26 IRAC galaxies show stellar emission in
the mid-infrared, probably in most of these galaxies because the stellar
emission is bright enough to outshine an underlying AGN. The infrared colors
suggest that the majority of these galaxies are bulge-dominated and have
redshifts between approximately 0.5 and 1.0. Visible spectra from the DEEP2
redshift survey, available for 11 galaxies, are consistent with this
suggestion. The IRAC galaxies fall into two distinct groups in a
color-magnitude diagram, one group (the "stripe") includes all the AGN. The
other group (the "blue clump") has blue 3.6 to 8 micron colors and a small
range of 8 micron magnitudes. This separation should be useful in classifying
galaxies found in other radio surveys.Comment: Accepted by A
Discovery of a Quadruple Lens in CANDELS with a Record Lens Redshift z=1.53
Using spectroscopy from the Large Binocular Telescope and imaging from the
Hubble Space Telescope we discovered the first strong galaxy lens at z(lens)>1.
The lens has a secure photometric redshift of z=1.53+/-0.09 and the source is
spectroscopically confirmed at z=3.417. The Einstein radius (0.35"; 3.0 kpc)
encloses 7.6 x 10^10 Msol, with an upper limit on the dark matter fraction of
60%. The highly magnified (40x) source galaxy has a very small stellar mass
(~10^8 Msol) and shows an extremely strong [OIII]_5007A emission line (EW_0 ~
1000A) bolstering the evidence that intense starbursts among very low-mass
galaxies are common at high redshift.Comment: accepted for publication in ApJ Letter
Localization-delocalization transition of a reaction-diffusion front near a semipermeable wall
The A+B --> C reaction-diffusion process is studied in a system where the
reagents are separated by a semipermeable wall. We use reaction-diffusion
equations to describe the process and to derive a scaling description for the
long-time behavior of the reaction front. Furthermore, we show that a critical
localization-delocalization transition takes place as a control parameter which
depends on the initial densities and on the diffusion constants is varied. The
transition is between a reaction front of finite width that is localized at the
wall and a front which is detached and moves away from the wall. At the
critical point, the reaction front remains at the wall but its width diverges
with time [as t^(1/6) in mean-field approximation].Comment: 7 pages, PS fil
Variational perturbation approach to the Coulomb electron gas
The efficiency of the variational perturbation theory [Phys. Rev. C {\bf 62},
045503 (2000)] formulated recently for many-particle systems is examined by
calculating the ground state correlation energy of the 3D electron gas with the
Coulomb interaction. The perturbation beyond a variational result can be
carried out systematically by the modified Wick's theorem which defines a
contraction rule about the renormalized perturbation. Utilizing the theorem,
variational ring diagrams of the electron gas are summed up. As a result, the
correlation energy is found to be much closer to the result of the Green's
function Monte Carlo calculation than that of the conventional ring
approximation is.Comment: 4 pages, 3 figure
Magnetic-field-induced collapse of charge-ordered nanoclusters and the Colossal Magnetoresistance effect in Nd(0.3)Sr(0.3)MnO(3)
We report synchrotron x-ray scattering studies of charge/orbitally ordered
(COO) nanoclusters in NdSrMnO. We find that the COO
nanoclusters are strongly suppressed in an applied magnetic field, and that
their decreasing concentration follows the field-induced decrease of the sample
electrical resistivity. The COO nanoclusters, however, do not completely
disappear in the conducting state, suggesting that this state is inhomogeneous
and contains an admixture of an insulating phase. Similar results were also
obtained for the zero-field insulator-metal transition that occurs as
temperature is reduced. These observations suggest that these correlated
lattice distortions play a key role in the Colossal Magnetoresistance effect in
this prototypical manganite.Comment: 5 pages, 3 embedded eps figures; to appear in PRB Rapid
Commumication
The definability criterions for convex projective polyhedral reflection groups
Following Vinberg, we find the criterions for a subgroup generated by
reflections \Gamma \subset \SL^{\pm}(n+1,\mathbb{R}) and its finite-index
subgroups to be definable over where is an integrally
closed Noetherian ring in the field . We apply the criterions for
groups generated by reflections that act cocompactly on irreducible properly
convex open subdomains of the -dimensional projective sphere. This gives a
method for constructing injective group homomorphisms from such Coxeter groups
to \SL^{\pm}(n+1,\mathbb{Z}). Finally we provide some examples of
\SL^{\pm}(n+1,\mathbb{Z})-representations of such Coxeter groups. In
particular, we consider simplicial reflection groups that are isomorphic to
hyperbolic simplicial groups and classify all the conjugacy classes of the
reflection subgroups in \SL^{\pm}(n+1,\mathbb{R}) that are definable over
. These were known by Goldman, Benoist, and so on previously.Comment: 31 pages, 8 figure
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