6,303 research outputs found
Understanding the negative thermal expansion in planar graphite–metal composites
The addition of graphitic fibers and flakes as fillers is commonly used to control the thermal expansion of metals. Sintered metal matrix composites with a planar distribution of graphite flakes show a low or negative thermal expansion coefficient perpendicular to the orientation plane of the graphite (z-CTE). Since the metal matrix has a positive isotropic expansion and graphite has a high z-CTE, this effect cannot be explained by a simple model of stapled metal–graphite layers. Instead, a mechanical interaction between graphite and matrix must be considered. With neutron scattering measurements, we show that there is little or no strain of the graphite flakes caused by the matrix, which can be explained by the high modulus of graphite. Instead, we suggest that a macroscopic crumpling of the flakes is responsible for the low z-CTE of the composite. The crumpled flakes are thicker at low temperature and get stretched and flattened by the expanding matrix at high temperature, explaining the reduction in the thermal expansion across the orientation plane
On the Erigone family and the secular resonance
The Erigone family is a C-type group in the inner main belt. Its age has been
estimated by several researchers to be less then 300 My, so it is a relatively
young cluster. Yarko-YORP Monte Carlo methods to study the chronology of the
Erigone family confirm results obtained by other groups. The Erigone family,
however, is also characterized by its interaction with the secular
resonance. While less than 15% of its members are currently in librating states
of this resonance, the number of objects, members of the dynamical group, in
resonant states is high enough to allow to use the study of dynamics inside the
resonance to set constraints on the family age.
Like the and secular resonances, the resonance is
characterized by one stable equilibrium point at in the
resonance plane , where is the
resonant angle of the resonance. Diffusion in this plane occurs on
timescales of My, which sets a lower limit on the Erigone family
age. Finally, the minimum time needed to reach a steady-state population of
librators is about 90 My, which allows to impose another, independent
constraint on the group age.Comment: This paper has 11 pages, 12 figures, and 1 table. Accepted for
publication in MNRA
Towards Dead Time Inclusion in Neuronal Modeling
A mathematical description of the refractoriness period in neuronal diffusion
modeling is given and its moments are explicitly obtained in a form that is
suitable for quantitative evaluations. Then, for the Wiener, Ornstein-Uhlenbeck
and Feller neuronal models, an analysis of the features exhibited by the mean
and variance of the first passage time and of refractoriness period is
performed.Comment: 12 pages, 1 figur
Pansharpening techniques to detect mass monument damaging in Iraq
The recent mass destructions of monuments in Iraq cannot be monitored with the terrestrial survey methodologies, for obvious reasons
of safety. For the same reasons, it’s not advisable the use of classical aerial photogrammetry, so it was obvious to think to the use of
multispectral Very High Resolution (VHR) satellite imagery. Nowadays VHR satellite images resolutions are very near airborne
photogrammetrical images and usually they are acquired in multispectral mode. The combination of the various bands of the images
is called pan-sharpening and it can be carried on using different algorithms and strategies. The correct pansharpening methodology,
for a specific image, must be chosen considering the specific multispectral characteristics of the satellite used and the particular
application. In this paper a first definition of guidelines for the use of VHR multispectral imagery to detect monument destruction in
unsafe area, is reported.
The proposed methodology, agreed with UNESCO and soon to be used in Libya for the coastal area, has produced a first report
delivered to the Iraqi authorities. Some of the most evident examples are reported to show the possible capabilities of identification of
damages using VHR images
Longer aftershocks duration in extensional tectonic settings
Aftershocks number decay through time, depending on several parameters peculiar to each seismogenic regions, including mainshock magnitude, crustal rheology, and stress changes along
the fault. However, the exact role of these parameters in controlling the duration of the aftershock sequence is still unknown. Here, using two methodologies, we show that the tectonic setting primarily controls the duration of aftershocks. On average and for a given mainshock magnitude (1) aftershock sequences are longer and (2) the number of earthquakes is greater in extensional tectonic settings than in contractional ones. We interpret this difference as related to the different type of energy dissipated during earthquakes. In detail, (1) a joint effect of gravitational forces and pure elastic stress release governs extensional earthquakes, whereas (2) pure elastic stress release controls contractional earthquakes. Accordingly, normal faults operate in favour of gravity, preserving inertia for a longer period and seismicity lasts until gravitational equilibrium is reached. Vice versa, thrusts act against gravity, exhaust their inertia faster and the elastic energy dissipation is buffered by the gravitational force. Hence, for seismic sequences of comparable magnitude and rheological parameters, aftershocks last longer in extensional settings because gravity favours the collapse of the hangingwall volumes
Are quantization rules for horizon areas universal?
Doubts have been expressed on the universality of holographic/string-inspired
quantization rules for the horizon areas of stationary black holes or the
products of their radii, already in simple 4-dimensional general relativity.
Realistic black holes are not stationary but time-dependent. Using two examples
of 4D general-relativistic spacetimes containing dynamical black holes for at
least part of the time, it is shown that the quantization rules (even counting
virtual horizons) cannot hold, except possibly at isolated instants of time,
and do not seem to be universal.Comment: One example and one figure added, two figures improved, bibliography
expanded and updated. Matches the version accepted for publication in Phys.
Rev.
Submillimeter Spectrum of Formic Acid
We have measured new submillimeter-wave data around 600 GHz and around 1.1
THz for the 13C isotopologue of formic acid and for the two deuterium
isotopomers; in each case for both the trans and cis rotamer. For cis-DCOOH and
cis-HCOOD in particular only data up to 50 GHz was previously available. For
all species the quality and quantity of molecular parameters has been increased
providing new measured frequencies and more precise and reliable frequencies in
the range of existing and near-future submillimeter and far-infrared
astronomical spectroscopy instruments such as Herschel, SOFIA and ALMA
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