1,009 research outputs found
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Imaging the atomic structure of activated carbon
The precise atomic structure of activated carbon is unknown, despite its huge commercial importance in the purification of air and water. Diffraction methods have been extensively applied to the study of microporous carbons, but cannot provide an unequivocal identification of their structure. Here we show that the structure of a commercial activated carbon can be imaged directly using aberration-corrected transmission electron microscopy. Images are presented both of the as-produced carbon and of the carbon following heat treatment at 2000 degrees C. In the 2000 degrees C carbon clear evidence is found for the presence of pentagonal rings, suggesting that the carbons have a fullerene-related structure. Such a structure would help to explain the properties of activated carbon, and would also have important implications for the modelling of adsorption on microporous carbons
Exchange biasing of single-domain Ni nanoparticles spontaneously grown in an antiferromagnetic MnO matrix
Exchange biased composites of ferromagnetic single-domain Ni nanoparticles
embedded within large grains of MnO have been prepared by reduction of
NiMnO phases in flowing hydrogen. The Ni precipitates are 15-30
nm in extent, and the majority are completely encased within the MnO matrix.
The manner in which the Ni nanoparticles are spontaneously formed imparts a
high ferromagnetic- antiferromagnetic interface/volume ratio, which results in
substantial exchange bias effects. Exchange bias fields of up to 100 Oe are
observed, in cases where the starting Ni content in the precursor
NiMnO phase is small. For particles of approximately the same
size, the exchange bias leads to significant hardening of the magnetization,
with the coercive field scaling nearly linearly with the exchange bias field.Comment: 6 pages PDFLaTeX with 9 figure
Volatility dynamics of nymex natural gas futures prices
Despite their importance in pricing futures and other derivative contracts, seasonalvariations in mean and variance of energy prices have not been fully captured inprevious studies of energy prices. We examine the volatility dynamics of daily naturalgas futures traded on the NYMEX via the partially overlapping time-series (POTS) modelof Smith (2005, Journal of Applied Econometrics). We illustrate that the volatility of dailyprice changes of natural gas exhibits strong seasonality, even as the volatility increases asa contract approaches its expiration, a time-to-maturity effect. Our analysis reveals thatthe persistence of price shocks and, hence, the correlations among concurrently tradedcontracts, also exhibit substantial seasonal and cross-sectional variation. These volatilitypatterns we estimate are closely related to the seasonal cycle of US natural gas storage ina way consistent with the theory of storage. We demonstrate that, by ignoring theseasonality in the volatility dynamics of natural gas futures prices, previous studies havesuggested sub-optimal hedging strategies
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Imaging the structure of activated carbon using aberration corrected TEM
The precise atomic structure of activated carbon is unknown, despite its commercial importance. Here we show that the structure of a commercial activated carbon can be imaged directly using aberration corrected transmission electron microscopy. Images are presented both of the as-produced carbon and of the carbon following heat- treatment at 2000°C. In the 2000°C carbon clear evidence is found for the presence of pentagonal rings, suggesting that the carbons have a fullerene-related structure
Self-field effects upon the critical current density of flat superconducting strips
We develop a general theory to account self-consistently for self-field
effects upon the average transport critical current density Jc of a flat
type-II superconducting strip in the mixed state when the bulk pinning is
characterized by a field-dependent depinning critical current density Jp(B),
where B is the local magnetic flux density. We first consider the possibility
of both bulk and edge-pinning contributions but conclude that bulk pinning
dominates over geometrical edge-barrier effects in state-of-the-art YBCO films
and prototype second-generation coated conductors. We apply our theory using
the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba)
as a function of a perpendicular applied magnetic induction Ba and show how
Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to
JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux
density zero at the strip's edge. However, Jc(Ba) is suppressed relative to
JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring
when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference
The usefulness of diagnostic imaging for the assessment of pain symptoms in temporomandibular disorders
SummaryThe causes of pain symptoms in the temporomandibular joint (TMJ) and masticatory muscle (MM) regions may not be determined by clinical examination alone. In this review, we document that pain symptoms of the TMJ and MM regions in patients with temporomandibular disorders (TMDs) are associated with computed tomography and magnetic resonance (MR) findings of internal derangement, joint effusion, osteoarthritis, and bone marrow edema. However, it is emphasized that these imaging findings must not be regarded as the unique and dominant factors in defining TMJ pain. High signal intensity and prominent enhancement of the posterior disk attachment on fat saturation T2-weighted imaging and dynamic MR imaging with contrast material are closely correlated with the severity of TMJ pain. Magnetic transfer contrast, MR spectroscopy, diffusion tensor imaging, and ultrasonography findings have helped identify intramuscular edema and contracture as one of the causes of MM pain and fatigue. Recently, changes in brain as detected by functional MR neuroimaging have been associated with changes in the TMJ and MM regions. The thalamus, the primary somatosensory cortex, the insula, and the anterior and mid-cinglate cortices are most frequently associated with TMD pain
Atomic Configuration of Nitrogen Doped Single-Walled Carbon Nanotubes
Having access to the chemical environment at the atomic level of a dopant in
a nanostructure is crucial for the understanding of its properties. We have
performed atomically-resolved electron energy-loss spectroscopy to detect
individual nitrogen dopants in single-walled carbon nanotubes and compared with
first principles calculations. We demonstrate that nitrogen doping occurs as
single atoms in different bonding configurations: graphitic-like and
pyrrolic-like substitutional nitrogen neighbouring local lattice distortion
such as Stone-Thrower-Wales defects. The stability under the electron beam of
these nanotubes has been studied in two extreme cases of nitrogen incorporation
content and configuration. These findings provide key information for the
applications of these nanostructures.Comment: 25 pages, 13 figure
Microstructure and Structural Defects in MgB2 Superconductor
We report a detailed study of the microstructure and defects in sintered
polycrystalline MgB2. Both TEM and x-ray data reveal that MgO is the major
second-phase in our bulk samples. Although MgB2 and MgO have different crystal
symmetries, being P6/mmm and Fm-3m, respectively, their stacking sequence of Mg
and B (or O) and lattice spacings in certain crystallographic orientations are
very similar. The size of MgO varies from 10~500nm, and its mismatch with the
MgB2 matrix can be a source for dislocations. Dislocations in MgB2 often have a
Burgers vector of . 1/3 and 1/3 partial dislocations and their
associated stacking faults were also observed. Since both dislocations and
stacking faults are located in the (001) basal plane, flux pinning anisotropy
is expected. Diffuse scattering analysis suggests that the correlation length
along the c-axis for defect-free basal planes is about 50nm. (001) twist
grain-boundaries, formed by rotations along the c-axis, are major grain
boundaries in MgB2 as a result of the out-of-plane weak bonding between Mg and
B atoms. An excess of Mg was observed in some grain boundaries. High-resolution
nano-probe EELS reveals that there is a difference in near edge structure of
the boron K-edge acquired from grain boundaries and grain interiors. The change
at the edge threshold may be suggestive of variation of the hole concentration
that would significantly alter boundary superconductivity.Comment: 20 pages and 12 figure
Towards atomically precise manipulation of 2D nanostructures in the electron microscope
Despite decades of research, the ultimate goal of nanotechnology—top-down manipulation of individual atoms—has been directly achieved with only one technique: scanning probe microscopy. In this review, we demonstrate that scanning transmission electron microscopy (STEM) is emerging as an alternative method for the direct assembly of nanostructures, with possible applications in plasmonics, quantum technologies, and materials science. Atomically precise manipulation with STEM relies on recent advances in instrumentation that have enabled non-destructive atomicresolution imaging at lower electron energies. While momentum transfer from highly energetic electrons often leads to atom ejection, interesting dynamics can be induced when the transferable kinetic energies are comparable to bond strengths in the material. Operating in this regime, very recent experiments have revealed the potential for single-atom manipulation using the Ångströmsized electron beam. To truly enable control, however, it is vital to understand the relevant atomicscale phenomena through accurate dynamical simulations. Although excellent agreement between experiment and theory for the specific case of atomic displacements from graphene has been recently achieved using density functional theory molecular dynamics, in many other cases quantitative accuracy remains a challenge. We provide a comprehensive reanalysis of available experimental data on beam-driven dynamics in light of the state-of-the-art in simulations, and identify important targets for improvement. Overall, the modern electron microscope has great potential to become an atom-scale fabrication platform, especially for covalently bonded 2D nanostructures. We review the developments that have made this possible, argue that graphene is an ideal starting material, and assess the main challenges moving forward
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