442 research outputs found
Coefficient of thermal expansion of nanostructured tungsten based coatings assessed by thermally induced substrate curvature method
The in plane coefficient of thermal expansion (CTE) and the residual stress
of nanostructured W based coatings are extensively investigated. The CTE and
the residual stresses are derived by means of an optimized ad-hoc developed
experimental setup based on the detection of the substrate curvature by a laser
system. The nanostructured coatings are deposited by Pulsed Laser Deposition.
Thanks to its versatility, nanocrystalline W metallic coatings,
ultra-nano-crystalline pure W and W-Tantalum coatings and amorphous-like W
coatings are obtained. The correlation between the nanostructure, the residual
stress and the CTE of the coatings are thus elucidated. We find that all the
samples show a compressive state of stress that decreases as the structure goes
from columnar nanocrystalline to amorphous-like. The CTE of all the coatings is
higher than the one of the corresponding bulk W form. In particular, as the
grain size shrinks, the CTE increases from 5.1 10 K for
nanocrystalline W to 6.6 10 K in the ultra-nano-crystalline
region. When dealing with amorphous W, the further increase of the CTE is
attributed to a higher porosity degree of the samples. The CTE trend is also
investigated as function of materials stiffness. In this case, as W coatings
become softer, the easier they thermally expand.Comment: The research leading to these results has also received funding from
the European Research Council Consolidator Grant ENSURE (ERC-2014-CoG No.
647554
Capacitive mixing for the extraction of energy from salinity differences: Survey of experimental results and electrochemical models
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Mean curvature flow with triple junctions in higher space dimensions
We consider mean curvature flow of n-dimensional surface clusters. At
(n-1)-dimensional triple junctions an angle condition is required which in the
symmetric case reduces to the well-known 120 degree angle condition. Using a
novel parametrization of evolving surface clusters and a new existence and
regularity approach for parabolic equations on surface clusters we show local
well-posedness by a contraction argument in parabolic Hoelder spaces.Comment: 31 pages, 2 figure
V39: an unusual object in the field of IC 1613
The variable star V39 in the field of IC 1613 is discussed in the light of
the available photometric and new spectroscopic data. It has strong emission
Balmer lines, and the observed characteristics could be explained by a W Vir
pulsating star with a period of 14.341 d, located at more than 115 kpc, that is
in the very outer halo of our Galaxy. It should have an apparent companion, a
long period (1118d) red variable, belonging to IC 1613. The main uncertainty in
this interpretation is an emission feature at 668.4 nm, which we tentatively
identified as a He I line.Comment: 5 pages; accepted for publication in Astronomy & Astrophysic
Variable stars in nearby galaxies. VI. Frequency-period distribution of Cepheids in IC 1613 and other galaxies of the Local Group
The frequency--period distribution and other properties of Cepheids in IC
1613 are discussed and compared with those of stars in our Galaxy (Milky Way),
LMC, SMC, M31 and M33. Taking into account the observational limitations and
related incompleteness, it is concluded that the frequency-period distribution
of Cepheids in IC 1613 is similar to that of SMC; we suspect that a much larger
number of stars exist in IC 1613 with a period of less than 2 d that have not
yet been detected. A discussion of the deficiency of fundamental mode Cepheids
with periods in the range 8 - 10 d in the Milky Way, M31 and M33 is reported.
The present data are not sufficient to verify if this is produced by a real
bimodal frequency--period distribution or whether depends on the lack of
pulsating stars in such a period range due to pulsational stability reasons.
Some arguments are presented in favor of a bimodal distribution that is a
function of the average metallicity. The Milky Way, M31 and M33 have the two
maxima located at the same periods, about 5 and 13 d, respectively. A comment
on very long period Cepheids is also given.Comment: 6 pages; accepted for publication in Astronomy and Astrophysic
Variable stars in nearby galaxies. V. Search for Cepheids in Field A of NGC 6822
The results of a CCD survey for variability of stars in the nearby galaxy NGC
6822 are presented. The goal of the survey was to obtain good light curves of
Cepheids for Fourier decomposition and to detect shorter period Cepheids. Since
the program was carried out with a relatively small telescope, the Dutch 0.9 m
at ESO-La Silla, the observations were unfiltered (white light, or Wh-band).
The analysis revealed the presence of more than 130 variable stars. 21
population I Cepheids are detected; 6 of them were already known from previous
works (Kayser, 1967). For at least three Cepheids, however, the previous
identification or period was wrong. Some probable population II (W Vir) stars
are also identified. The dispersion of the fundamental mode Cepheid PL relation
appears to be small.Comment: 5 pages; accepted for publication in Astronomy and Astrophysic
Characterization of anisotropic nano-particles by using depolarized dynamic light scattering in the near field
Light scattering techniques are widely used in many fields of condensed and
sof t matter physics. Usually these methods are based on the study of the
scattered light in the far field. Recently, a new family of near field
detection schemes has been developed, mainly for the study of small angle light
scattering. These techniques are based on the detection of the light intensity
near to the sample, where light scattered at different directions overlaps but
can be distinguished by Fourier transform analysis. Here we report for the
first time data obtained with a dynamic near field scattering instrument,
measuring both polarized and depolarized scattered light. Advantages of this
procedure over the traditional far field detection include the immunity to
stray light problems and the possibility to obtain a large number of
statistical samples for many different wave vectors in a single instantaneous
measurement. By using the proposed technique we have measured the translational
and rotational diffusion coefficients of rod-like colloidal particles. The
obtained data are in very good agreement with the data acquired with a
traditional light scattering apparatus.Comment: Published in Optics Express. This version has changes in bibliograph
Regular Oscillation Sub-spectrum of Rapidly Rotating Stars
We present an asymptotic theory that describes regular frequency spacings of
pressure modes in rapidly rotating stars. We use an asymptotic method based on
an approximate solution of the pressure wave equation constructed from a stable
periodic solution of the ray limit. The approximate solution has a Gaussian
envelope around the stable ray, and its quantization yields the frequency
spectrum. We construct semi-analytical formulas for regular frequency spacings
and mode spatial distributions of a subclass of pressure modes in rapidly
rotating stars. The results of these formulas are in good agreement with
numerical data for oscillations in polytropic stellar models. The regular
frequency spacings depend explicitly on internal properties of the star, and
their computation for different rotation rates gives new insights on the
evolution of mode frequencies with rotation.Comment: 14 pages, 10 figure
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