42,950 research outputs found
Mechanical properties of polycrystalline graphene based on a realistic atomistic model
Graphene can at present be grown at large quantities only by the chemical
vapor deposition method, which produces polycrystalline samples. Here, we
describe a method for constructing realistic polycrystalline graphene samples
for atomistic simulations, and apply it for studying their mechanical
properties. We show that cracks initiate at points where grain boundaries meet
and then propagate through grains predominantly in zigzag or armchair
directions, in agreement with recent experimental work. Contrary to earlier
theoretical predictions, we observe normally distributed intrinsic strength (~
50% of that of the mono-crystalline graphene) and failure strain which do not
depend on the misorientation angles between the grains. Extrapolating for grain
sizes above 15 nm results in a failure strain of ~ 0.09 and a Young's modulus
of ~ 600 GPa. The decreased strength can be adequately explained with a
conventional continuum model when the grain boundary meeting points are
identified as Griffith cracks.Comment: Accepted for Physical Review B; 5 pages, 4 figure
Acoustical evaluation of the NASA Langley full-scale wind tunnel
Determining types of acoustical measurements suitable for test section of NASA Langley wind tunne
Lithological maps of selected Apollo 14 Breccia samples
A booklet of mapped surfaces of some Apollo 14 samples was prepared as an intermediate step towards the preparation of a new Apollo 14 sample catalog. It contains recently obtained observations and pictures of some of the largest and less well documented Apollo breccia samples. Some of the samples (14303, 14305, 14306, and 14311) were chosen because they have large sawn surfaces. These were dusted and mapped using a binocular microscope through the window of the nitrogen cabinet
The Economic Rationale for Agricultural Regeneration and Rural Infrastructure Investment in South Africa
This paper informs government policy insofar as it relates to the agricultural and rural development sectors and infrastructure investment within these sectors. The paper first quantifies the role of agriculture in the South African economy. This is done within the context of, inter alia, food security, agriculture’s contribution to gross domestic product (GDP), economic linkages and multipliers with respect to the agricultural sector, as well as agriculture’s employment creation and external stabilisation capacity. Investment in the agricultural and rural sectors are then analysed with a view of supporting the argument that agriculture’s role in the economy is sufficiently important to warrant regenerative strategies, including renewed emphasis on agricultural and rural infrastructure investment by South African policy makers. The quantification of the agricultural sector in relation to the total economy and that of agricultural and rural infrastructure investment are investigated against the backdrop of declining government support, increasing production risks due to a variety of exogenous events like climate change, and increasing dynamic trade impacts. In this paper, the authors offer both supporting arguments in terms of current economic policy and recommendations for more decisive policy measures aimed at agricultural regeneration and rural infrastructure investment.
Spectral Models of Convection-Dominated Accretion Flows
For small values of the dimensionless viscosity parameter, namely
, the dynamics of non-radiating accretion flows is
dominated by convection; convection strongly suppresses the accretion of matter
onto the central object and transports a luminosity from small to large radii in the flow. A fraction of this convective
luminosity is likely to be radiated at large radii via thermal bremsstrahlung
emission. We show that this leads to a correlation between the frequency of
maximal bremsstrahlung emission and the luminosity of the source, . Accreting black holes with X-ray luminosities are expected to
have hard X-ray spectra, with photon indices , and sources with
are expected to have soft spectra, with
. This is testable with {\it Chandra} and {\it XMM}.Comment: final version accepted by ApJ; significant modifications from
previous versio
A Na I Absorption Map of the Small-Scale Structure in the Interstellar Gas Toward M15
Using the DensePak fiber optic array on the KPNO WIYN telescope, we have
obtained high S/N echelle spectra of the Na I D wavelength region toward the
central 27" x 43" of the globular cluster M15 at a spatial resolution of 4".
The spectra exhibit significant interstellar Na I absorption at LSR velocities
of +3 km/s (LISM component) and +68 km/s (IVC component). Both components vary
appreciably in strength on these scales. The derived Na I column densities
differ by a factor of 4 across the LISM absorption map and by a factor of 16
across the IVC map. Assuming distances of 500 pc and 1500 pc for the LISM and
IVC clouds, these maps show evidence of significant ISM structure down to the
minimum scales of 2000 AU and 6000 AU probed in these absorbers. The
smallest-scale N(Na I) variations observed in the M15 LISM and IVC maps are
typically comparable to or higher than the values found at similar scales in
previous studies of interstellar Na I structure toward binary stars. The
physical implications of the small and larger-scale Na I features observed in
the M15 maps are discussed in terms of variations in the H I column density as
well as in the Na ionization equilibrium.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter
Wavelets: mathematics and applications
The notion of wavelets is defined. It is briefly described {\it what} are
wavelets, {\it how} to use them, {\it when} we do need them, {\it why} they are
preferred and {\it where} they have been applied. Then one proceeds to the
multiresolution analysis and fast wavelet transform as a standard procedure for
dealing with discrete wavelets. It is shown which specific features of signals
(functions) can be revealed by this analysis, but can not be found by other
methods (e.g., by the Fourier expansion). Finally, some examples of practical
application are given (in particular, to analysis of multiparticle production}.
Rigorous proofs of mathematical statements are omitted, and the reader is
referred to the corresponding literature.Comment: 16 pages, 5 figures, Latex, Phys. Atom. Nuc
Advection-dominated Inflow/Outflows from Evaporating Accretion Disks
In this Letter we investigate the properties of advection-dominated accretion
flows (ADAFs) fed by the evaporation of a Shakura-Sunyaev accretion disk (SSD).
In our picture the ADAF fills the central cavity evacuated by the SSD and
extends beyond the transition radius into a coronal region. We find that,
because of global angular momentum conservation, a significant fraction of the
hot gas flows away from the black hole forming a transsonic wind, unless the
injection rate depends only weakly on radius (if , ). The Bernoulli number of the inflowing gas is negative
if the transition radius is Schwarzschild radii, so matter
falling into the hole is gravitationally bound. The ratio of inflowing to
outflowing mass is , so in these solutions the accretion rate is
of the same order as in standard ADAFs and much larger than in
advection-dominated inflow/outflow models (ADIOS). The possible relevance of
evaporation-fed solutions to accretion flows in black hole X-ray binaries is
briefly discussed.Comment: 5 pages Latex with 2 ps figures. Accepted for publication in ApJ
Letter
An Incoherent Dynamo in Accretion Disks
We use the mean-field dynamo equations to show that an incoherent alpha
effect in mirror-symmetric turbulence in a shearing flow can generate a large
scale, coherent magnetic field. We illustrate this effect with simulations of a
few simple systems. In accretion disks, this process can lead to axisymmetric
magnetic domains whose radial and vertical dimensions will be comparable to the
disk height. This process may be responsible for observations of dynamo
activity seen in simulations of dynamo-generated turbulence involving, for
example, the Balbus-Hawley instability. In this case the magnetic field
strength will saturate at times the ambient pressure in real
accretion disks. The resultant dimensionless viscosity will be of the same
order. In numerical simulations the azimuthal extent of the simulated annulus
should be substituted for . We compare the predictions of this model to
numerical simulations previously reported by Brandenburg et al. (1995). In a
radiation pressure dominated environment this estimate for viscosity should be
reduced by a factor of due to magnetic buoyancy.Comment: 23 pages, uses aaste
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