2,835 research outputs found
Embedded arrays of vertically aligned carbon nanotube carpets and methods for making them
According to some embodiments, the present invention provides a system and method for supporting a carbon nanotube array that involve an entangled carbon nanotube mat integral with the array, where the mat is embedded in an embedding material. The embedding material may be depositable on a carbon nanotube. A depositable material may be metallic or nonmetallic. The embedding material may be an adhesive material. The adhesive material may optionally be mixed with a metal powder. The embedding material may be supported by a substrate or self-supportive. The embedding material may be conductive or nonconductive. The system and method provide superior mechanical and, when applicable, electrical, contact between the carbon nanotubes in the array and the embedding material. The optional use of a conductive material for the embedding material provides a mechanism useful for integration of carbon nanotube arrays into electronic devices
Separation of Single-Walled Carbon Nanotubes with DEP-FFF
A process using a modified dielectrophoresis device separates single-walled carbon nanotubes (SWNTs) according to their polarizability in electric fields. This depends on the size and dielectric constant of individual nanotubes and easily separates metallic from semiconducting nanotubes. Separation by length has also been demonstrated. Partial separation (enrichment) according to bandgap (which is linked to polarizability) has also been shown and can be improved to full separation of individual types of semiconducting SWNTs with better control over operational parameters and the length of SWNT starting material. This process and device can be scaled affordably to generate useful amounts of semiconducting SWNTs for electronic device development and production. In this study, a flow injection dielectrophoresis technique was used with a modified dielectrophoresis device. The length, width, and height of the modified chamber were 28, 2.5, and 0.025 cm, respectively. On the bottom of the chamber, there are two arrays of 50-m-wide, 2-m-thick gold electrodes, which are connected to an AC voltage generator and are alternately arranged so that every electrode is adjacent to two electrodes of the opposite polar. There is an additional plate electrode on the top of the chamber that is negatively biased. During the experiment, a syringe pump constantly pumps in the mobile phase, 1-percent sodium dodecylbenzene sulfonate (SDBS) solution, into the chamber. The frequency and voltage are set to 1 MHz and 10 V peak-to-peak, respectively. About 150 micro-L of SWNTs in 1- percent SDBS decanted solution are injected to the mobile phase through a septum near the entrance of the chamber. The flow rate of the mobile phase is set to 0.02 cu cm/min. The injected SWNTs sample flows through the chamber before it is lead into a fluorescence flow-through cell and collected for further analysis. The flow-through cell has three windows, thus allowing the fluorometer to collect fluorescence spectrum and visible absorption spectrums simultaneously. Dielectrophoresis field-flow fractionation (DEP-FFF) generally depends on interaction of a sedimentation force and DEP force for particle separation, and SWNTs are neutrally buoyant in water. In this innovation, the third electrode was added to create a sedimentation force based on DC electrophoresis. This makes this particular device applicable to separations on any neutrally buoyant particles in solution and a more general process for a broad range of nanomaterials sorting and separations
Spectral Equivalence of Bosons and Fermions in One-Dimensional Harmonic Potentials
Recently, Schmidt and Schnack (cond-mat/9803151, cond-mat/9810036), following
earlier references, reiterate that the specific heat of N non-interacting
bosons in a one-dimensional harmonic well equals that of N fermions in the same
potential. We show that this peculiar relationship between specific heats
results from a more dramatic equivalence between bose and fermi systems.
Namely, we prove that the excitation spectrums of such bose and fermi systems
are spectrally equivalent. Two complementary proofs are provided, one based on
an analysis of the dynamical symmetry group of the N-body system, the other
using combinatoric analysis.Comment: Six Pages, No Figures, Submitted to Phys. Rev.
Bulk Cutting of Carbon Nanotubes Using Electron Beam Irradiation
According to some embodiments, the present invention provides a method for attaining short carbon nanotubes utilizing electron beam irradiation, for example, of a carbon nanotube sample. The sample may be pretreated, for example by oxonation. The pretreatment may introduce defects to the sidewalls of the nanotubes. The method is shown to produces nanotubes with a distribution of lengths, with the majority of lengths shorter than 100 tun. Further, the median length of the nanotubes is between about 20 nm and about 100 nm
Star Formation Activity in the Galactic HII Complex S255-S257
We present results on the star-formation activity of an optically obscured
region containing an embedded cluster (S255-IR) and molecular gas between two
evolved HII regions S255 and S257. We have studied the complex using optical,
near-infrared (NIR) imaging, optical spectroscopy and radio continnum mapping
at 15 GHz, along with Spitzer-IRAC results. It is found that the main exciting
sources of the evolved HII regions S255 and S257 and the compact HII regions
associated with S255-IR are of O9.5 - B3 V nature, consistent with previous
observations. Our NIR observations reveal 109 likely young stellar object (YSO)
candidates in an area of ~ 4'.9 x 4'.9 centered on S255-IR, which include 69
new YSO candidates. Our observations increased the number of previously
identified YSOs in this region by 32%. To see the global star formation, we
constructed the V-I/V diagram for 51 optically identified IRAC YSOs in an area
of ~ 13' x 13' centered on S255-IR. We suggest that these YSOs have an
approximate age between 0.1 - 4 Myr, indicating a non-coeval star formation.
Using spectral energy distribution models, we constrained physical properties
and evolutionary status of 31 and 16 YSO candidates outside and inside the gas
ridge, respectively. The models suggest that the sources associated within the
gas ridge are of younger population (mean age ~ 1.2 Myr) than the sources
outside the gas ridge (mean age ~ 2.5 Myr). The positions of the young sources
inside the gas ridge at the interface of the HII regions S255 and S257, favor a
site of induced star formation.Comment: 46 pages, 14 figures, 5 tables. Accepted for publication in The
Astrophysical Journa
HATS-8b: A Low-Density Transiting Super-Neptune
HATS-8b is a low density transiting super-Neptune discovered as part of the
HATSouth project. The planet orbits its solar-like G dwarf host (V=14.03
0.10 and T =5679 50 K) with a period of 3.5839 d. HATS-8b is the
third lowest mass transiting exoplanet to be discovered from a wide-field
ground based search, and with a mass of 0.138 0.019 M it is
approximately half-way between the masses of Neptune and Saturn. However
HATS-8b has a radius of 0.873 (+0.123,-0.075) R, resulting in a bulk
density of just 0.259 0.091 g.cm. The metallicity of the host star
is super-Solar ([Fe/H]=0.210 0.080), arguing against the idea that low
density exoplanets form from metal-poor environments. The low density and large
radius of HATS-8b results in an atmospheric scale height of almost 1000 km, and
in addition to this there is an excellent reference star of near equal
magnitude at just 19 arcsecond separation on the sky. These factors make
HATS-8b an exciting target for future atmospheric characterization studies,
particularly for long-slit transmission spectroscopy.Comment: 11 pages, 7 figures, accepted for publication in A
New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables
Discoveries of two new white dwarf plus M star binaries with striking optical
cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to
six the total number of X-ray faint, magnetic accretion binaries that accrete
at rates < 10^{-13} Msun/yr, or <1% of the values normally encountered in
cataclysmic variables. This fact, coupled with donor stars that underfill their
Roche lobes and very cool white dwarfs, brand the binaries as post
common-envelope systems whose orbits have not yet decayed to the point of
Roche-lobe contact. They are pre-magnetic CVs, or pre-Polars. The systems
exhibit spin/orbit synchronism and apparently accrete by efficient capture of
the stellar wind from the secondary star, a process that has been dubbed a
``magnetic siphon''. Because of this, period evolution of the binaries will
occur solely by gravitational radiation, which is very slow for periods >3 hr.
Optical surveys for the cyclotron harmonics appear to be the only means of
discovery, so the space density of pre-Polars could rival that of Polars, and
the binaries provide an important channel of progenitors (in addition to the
asynchronous Intermediate Polars). Both physical and SDSS observational
selection effects are identified that may help to explain the clumping of all
six systems in a narrow range of magnetic field strength around 60 MG.Comment: 25 pages, 13 figures, Accepted to Ap
HATS-7b: A Hot Super Neptune Transiting a Quiet K Dwarf Star
IW ../submit_V2/abstract.txt ( Row 1 Col 1 6:48 Ctrl-K H for help We report
the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune
with a mass of 0.120+/-0.012MJ, a radius of 0.563+/-(0.046,0.034)RJ, and an
orbital period of 3.1853days. The host star is a moderately bright
(V=13.340+/-0.010mag, K_S=10.976+/-0.026mag) K dwarf star with a mass of
0.849+/-0.027Msun , a radius of 0.815+/-(0.049,-0.035)Rsun, and a metallicity
of [Fe/H]=+0.250+/-0.080. The star is photometrically quiet to within the
precision of the HATSouth measurements and has low RV jitter. HATS-7b is the
second smallest radius planet discovered by a wide-field ground-based transit
survey, and one of only a handful of Neptune-size planets with mass and radius
determined to 10% precision. Theoretical modeling of HATS-7b yields a
hydrogen-helium fraction of 18+/-4% (rock-iron core and H2-He envelope), or
9+/-4% (ice core and H2-He envelope), i.e.it has a composition broadly similar
to that of Uranus and Neptune, and very different from that of Saturn, which
has 75% of its mass in H2-He. Based on a sample of transiting exoplanets with
accurately (<20%) determined parameters, we establish approximate power-law
relations for the envelopes of the mass-density distribution of exoplanets.
HATS-7b, which, together with the recently discovered HATS-8b, is one of the
first two transiting super-Neptunes discovered in the Southern sky, is a prime
target for additional follow-up observations with Southern hemisphere
facilities to characterize the atmospheres of Super-Neptunes (which we define
as objects with mass greater than that of Neptune, and smaller than halfway
between that of Neptune and Saturn, i.e. 0.054 MJ<Mp<0.18 MJ).Comment: 11 pages, accepted for publication by Ap
HATS-15 b and HATS-16 b: Two massive planets transiting old G dwarf stars
We report the discovery of HATS-15 b and HATS-16 b, two massive transiting
extrasolar planets orbiting evolved ( Gyr) main-sequence stars. The
planet HATS-15 b, which is hosted by a G9V star ( mag), is a hot
Jupiter with mass of and radius of
, and completes its orbit in nearly 1.7 days.
HATS-16 b is a very massive hot Jupiter with mass of and radius of ; it orbits around
its G3 V parent star ( mag) in days. HATS-16 is slightly
active and shows a periodic photometric modulation, implying a rotational
period of 12 days which is unexpectedly short given its isochronal age. This
fast rotation might be the result of the tidal interaction between the star and
its planet.Comment: 16 pages, 8 figures, submitted to PAS
Transit Detection in the MEarth Survey of Nearby M Dwarfs: Bridging the Clean-First, Search-Later Divide
In the effort to characterize the masses, radii, and atmospheres of
potentially habitable exoplanets, there is an urgent need to find examples of
such planets transiting nearby M dwarfs. The MEarth Project is an ongoing
effort to do so, as a ground-based photometric survey designed to detect
exoplanets as small as 2 Earth radii transiting mid-to-late M dwarfs within 33
pc of the Sun. Unfortunately, identifying transits of such planets in
photometric monitoring is complicated both by the intrinsic stellar variability
that is common among these stars and by the nocturnal cadence, atmospheric
variations, and instrumental systematics that often plague Earth-bound
observatories. Here we summarize the properties of MEarth data gathered so far,
and we present a new framework to detect shallow exoplanet transits in wiggly
and irregularly-spaced light curves. In contrast to previous methods that clean
trends from light curves before searching for transits, this framework assesses
the significance of individual transits simultaneously while modeling
variability, systematics, and the photometric quality of individual nights. Our
Method for Including Starspots and Systematics in the Marginalized Probability
of a Lone Eclipse (MISS MarPLE) uses a computationally efficient semi-Bayesian
approach to explore the vast probability space spanned by the many parameters
of this model, naturally incorporating the uncertainties in these parameters
into its evaluation of candidate events. We show how to combine individual
transits processed by MISS MarPLE into periodic transiting planet candidates
and compare our results to the popular Box-fitting Least Squares (BLS) method
with simulations. By applying MISS MarPLE to observations from the MEarth
Project, we demonstrate the utility of this framework for robustly assessing
the false alarm probability of transit signals in real data. [slightly
abridged]Comment: accepted to the Astronomical Journal, 21 pages, 12 figure
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