218 research outputs found
Structural properties of a calcium aluminosilicate glass from molecular-dynamics simulations: A finite size effects study
We study a calcium aluminosilicate glass of composition
(SiO)-(AlO)-(CaO) by means of
molecular-dynamics (MD) simulations, using a potential made of two-body and
three-body interactions. In order to prepare small samples that can
subsequently be studied by first-principles, the finite size effects on the
liquid dynamics and on the glass structural properties are investigated. We
find that finite size effects affect the Si-O-Si and Si-O-Al angular
distributions, the first peaks of the Si-O, Al-O and Ca-O pair correlation
functions, the Ca coordination and the oxygen atoms environment in the smallest
system (100 atoms). We give evidence that these finite size effects can be
directly attributed to the use of three-body interactions.Comment: 36 pages, 14 figures. Journal of Chem. Phys., in pres
Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography
We propose an original adaptive wavefront holographic setup based on the
photorefractive effect (PR), to make real-time measurements of acousto-optic
signals in thick scattering media, with a high flux collection at high rates
for breast tumor detection. We describe here our present state of art and
understanding on the problem of breast imaging with PR detection of the
acousto-optic signal
Theoretical study of Acousto-optical coherence tomography using random phase jumps on US and light
Acousto-Optical Coherence Tomography (AOCT) is variant of Acousto Optic
Imaging (called also ultrasonic modulation imaging) that makes possible to get
z resolution with acoustic and optic Continuous Wave (CW) beams. We describe
here theoretically the AOCT e ect, and we show that the Acousto Optic tagged
photons remains coherent if they are generated within a speci c z region of the
sample. We quantify the z selectivity for both the tagged photon eld, and for
the M. Lesa re et al. photorefractive signal
Biochar a partir de biomassa de Miscanthus e Quebracho, produzido em forno tubular em sistema aberto e sob fluxo de nitrogênio.
bitstream/item/141831/1/Comunicado-Tecnico-375-2015.pd
Strain mapping at the nanoscale using precession electron diffraction in transmission electron microscope with off axis camera
International audiencePrecession electron diffraction is an efficient technique to measure strain in nanostructures by precessing the electron beam, while maintaining a few nanometre probe size. Here, we show that an advanced diffraction pattern treatment allows reproducible and precise strain measurements to be obtained using a default 512 x 512 DigiSTAR off-axis camera both in advanced or non-corrected transmission electron microscopes. This treatment consists in both projective geometry correction of diffraction pattern distortions and strain Delaunay triangulation based analysis. Precision in the strain measurement is improved and reached 2.7 x 10(-4) with a probe size approaching 4.2 nm in diameter. This method is applied to the study of the strain state in InGaAs quantum-well (QW) devices elaborated on Si substrate. Results show that the GaAs/Si mismatch does not induce in-plane strain fluctuations in the InGaAs QW region. (C) 2014 AIP Publishing LLC
The importance of major mergers in the build up of stellar mass in brightest cluster galaxies at z=1
Recent independent results from numerical simulations and observations have
shown that brightest cluster galaxies (BCGs) have increased their stellar mass
by a factor of almost two between z~0.9 and z~0.2. The numerical simulations
further suggest that more than half this mass is accreted through major
mergers. Using a sample of 18 distant galaxy clusters with over 600
spectroscopically confirmed cluster members between them, we search for
observational evidence that major mergers do play a significant role. We find a
major merger rate of 0.38 +/- 0.14 mergers per Gyr at z~1. While the
uncertainties, which stem from the small size of our sample, are relatively
large, our rate is consistent with the results that are derived from numerical
simulations. If we assume that this rate continues to the present day and that
half of the mass of the companion is accreted onto the BCG during these
mergers, then we find that this rate can explain the growth in the stellar mass
of the BCGs that is observed and predicted by simulations. Major mergers
therefore appear to be playing an important role, perhaps even the dominant
one, in the build up of stellar mass in these extraordinary galaxies.Comment: 15 pages, 6 figures, accepted for publication in MNRAS. Reduced data
will be made available through the ESO archiv
Miscanthus x giganteus and Aspidosperma quebracho-blanco as feedstock sources for biochar production in Argentina.
Shear instabilities of freely standing thermotropic smectic-A films
In this Letter we discuss theoretically the instabilities of thermotropic
freely standing smectic-A films under shear flow\cite{re:wu}. We show that, in
Couette geometry, the centrifugal force pushes the liquid crystal toward the
outer boundary and induces smectic layer dilation close to the outer boundary.
Under strong shear, this effect induces a layer buckling instability. The
critical shear rate is proportional to , where is the thickness
of the film.Comment: 12 pages, 2 figure
Self-healing capacity of nuclear glass observed by NMR spectroscopy
Safe management of high level nuclear waste is a worldwide significant issue for which vitrification has been selected by many countries. There exists a crucial need for improving our understanding of the ageing of the glass under irradiation. While external irradiation by ions provides a rapid simulation of damage induced by alpha decays, short lived actinide doping is more representative of the reality. Here, we report radiological NMR experiments to compare the damage in International Simplified Glass (ISG) when irradiated by these two methods. In the 0.1 mole percent 244Cm doped glass, accumulation of high alpha decay only shows small modifications of the local structure, in sharp contrast to heavy ion irradiation. These results reveal the ability of the alpha particle to partially repair the damage generated by the heavy recoil nuclei highlighting the radiation resistance of nuclear glass and the difficulty to accurately simulate its behaviour by single ion beam irradiations
Shear induced instabilities in layered liquids
Motivated by the experimentally observed shear-induced destabilization and
reorientation of smectic A like systems, we consider an extended formulation of
smectic A hydrodynamics. We include both, the smectic layering (via the layer
displacement u and the layer normal p) and the director n of the underlying
nematic order in our macroscopic hydrodynamic description and allow both
directions to differ in non equilibrium situations. In an homeotropically
aligned sample the nematic director does couple to an applied simple shear,
whereas the smectic layering stays unchanged. This difference leads to a finite
(but usually small) angle between n and p, which we find to be equivalent to an
effective dilatation of the layers. This effective dilatation leads, above a
certain threshold, to an undulation instability of the layers. We generalize
our earlier approach [Rheol. Acta, vol.39(3), 15] and include the cross
couplings with the velocity field and the order parameters for orientational
and positional order and show how the order parameters interact with the
undulation instability. We explore the influence of various material parameters
on the instability. Comparing our results to recent experiments and molecular
dynamic simulations, we find a good qualitative agreement.Comment: 15 pages, 12 figures, accepted for publication in PR
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