817 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
Lectures on curvature flow of networks
We present a collection of results on the evolution by curvature of networks of planar curves. We discuss in particular the existence of a solution and the analysis of singularities
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
On the global structure of conformal gradient solitons with nonnegative Ricci tensor
In this paper we prove that any complete conformal gradient soliton with
nonnegative Ricci tensor is either isometric to a direct product
, or globally conformally equivalent to the Euclidean
space or to the round sphere . In particular,
we show that any complete, noncompact, gradient Yamabe-type soliton with
positive Ricci tensor is rotationally symmetric, whenever the potential
function is nonconstant.Comment: Minor correction
Probable nonradial g-mode pulsation in early A-type stars
A survey for line profile variability in early A-type stars has been
performed in order to detect nonradial pulsation signatures. The star HR 6139,
with spectral type A2V and estimated T_eff=8800 K, shows evident line profile
variations that can be explained by oscillations in prograde g-modes. This
feature and the known photometric variability are similar to those observed in
the Slowly Pulsating B-type stars. However HR 6139 is much cooler than the cool
border of the instability strip of such variables, and it is hotter than the
blue edge of the delta Scuti instability strip. There are indications of a tiny
variability also in other four objects, whose nature is not yet clear.Comment: 4 pages, 5 figures; accepted for publication in A&A (letter
Pretransitional behavior in a water-DDAB-5CB microemulsion close to the demixing transition. Evidence for intermicellar attraction mediated by paranematic fluctuations
We present a study of a water-in-oil microemulsion in which surfactant coated
water nanodroplets are dispersed in the isotropic phase of the thermotropic
liquid crystal 5CB. As the temperature is lowered below the isotropic to
nematic phase transition of pure 5CB, the system displays a demixing transition
leading to a coexistence of a droplet rich isotropic phase with a droplet poor
nematic. The transition is anticipated, in the high T side, by increasing
pretransitional fluctuations in 5CB molecular orientation and in the
nanodroplet concentration. The observed phase behavior supports the notion that
the nanosized droplets, while large enough for their statistical behavior to be
probed via light scattering, are also small enough to act as impurities,
disturbing the local orientational ordering of the liquid crystal and thus
experiencing pretransitional attractive interaction mediated by paranematic
fluctuations. The pretransitional behavior, together with the topology of the
phase diagram, can be understood on the basis of a diluted Lebwohl-Lasher model
which describes the nanodroplets simply as holes in the liquid crystal.Comment: 64 pages, 16 figures, J. Chem. Phys. in pres
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