89 research outputs found
Vitreous silica distends in helium gas: acoustic vs. static compressibilities
Sound velocities of vitreous silica are measured under He compression in the
pressure range 0-6 GPa by Brillouin light scattering. It is found that the
well-known anomalous maximum in the pressure dependence of the compressibility
is suppressed by He incorporation into the silica network. This shows that the
elastic anomaly relates to the collapse of the largest interstitial voids in
the structure. The huge difference between the static and the acoustic
compressibilities indicates that the amount of incorporated helium still
increases at 6 GPa.Comment: 5 pages, 4 figure
Pressure-induced densification of vitreous silica: insight from elastic properties
\textit{In situ} high-pressure Brillouin light scattering experiments along
loading-unloading paths are used to investigate the compressibility of vitreous
silica. An accurate equation of state is obtained below \SI{9}{GPa} using sound
velocities corrected for dispersion. Conversely, huge inelastic effects are
observed in the range \SIrange{10}{60}{GPa}, unveiling the reversible
transformation from the fourfold-coordinated structure to the sixfold one. We
find that the associated density changes fully correlate with the average Si
coordination number. Decompression curves from above \SI{20}{GPa} reveal abrupt
backward coordination changes around \SIrange{10}{15}{GPa} and significant
hysteresis. Further, contrary to common wisdom, the residual densification of
recovered silica samples can be figured out from the pressure cycles.Comment: 5 pages, 4 figures, revised versio
Small Angle Neutron Scattering of Aerogels: Simulations and Experiments
A numerical simulation of silica aerogels is performed using
diffusion-limited cluster-cluster aggregation of spheres inside a cubic box
(with periodic boundary conditions). The volume fraction is taken to be
sufficiently large to get a gel structure at the end of the process. In the
case of monodisperse spheres, the wavevector dependent scattered intensity
is calculated from the product of the form factor of a sphere by
the structure factor , which is related to the Fourier transform of
, where is the pair correlation function between sphere centers.
The structure factor exhibits large- damped oscillations
characteristics of the short range (intra-aggregate) correlations between
spheres. These oscillations influence the curve in the -region
between the fractal regime and the Porod regime and quantitative comparisons
are made with experiments on colloidal aerogels. Moreover, at small- values,
goes through a maximum characteristic of large range (inter-aggregate)
correlations. Quantitative fits of the maximum in the experimental
curves of base-catalyzed aerogel are presented. In the case of polydisperse
spheres, is calculated directly from a single aggregate simulation. It
is shown that increasing polydispersity shifts the location of the cross-over
between the fractal and Porod regimes towards low -value.Comment: RevTex, 9 pages + 11 postscript figures, compressed using "uufiles".
Proceeding of the 4th International Simposium on Aerogels (To appear in J. of
Non-Cryst. Solids
Anharmonic vs. relaxational sound damping in glasses: II. Vitreous silica
The temperature dependence of the frequency dispersion in the sound velocity
and damping of vitreous silica is reanalyzed. Thermally activated relaxation
accounts for the sound attenuation observed above 10 K at sonic and ultrasonic
frequencies. Its extrapolation to the hypersonic regime reveals that the
anharmonic coupling to the thermal bath becomes important in
Brillouin-scattering measurements. At 35 GHz and room temperature, the damping
due to this anharmonicity is found to be nearly twice that produced by
thermally activated relaxation. The analysis also reveals a sizeable velocity
increase with temperature which is not related with sound dispersion. This
suggests that silica experiences a gradual structural change that already
starts well below room temperature.Comment: 13 pages with 8 figure
Anharmonic vs. relaxational sound damping in glasses: I. Brillouin scattering from densified silica
This series discusses the origin of sound damping and dispersion in glasses.
In particular, we address the relative importance of anharmonicity versus
thermally activated relaxation. In this first article, Brillouin-scattering
measurements of permanently densified silica glass are presented. It is found
that in this case the results are compatible with a model in which damping and
dispersion are only produced by the anharmonic coupling of the sound waves with
thermally excited modes. The thermal relaxation time and the unrelaxed velocity
are estimated.Comment: 9 pages with 7 figures, added reference
Corticosteroid therapy is associated with a decrease in mortality in a multicenter cohort of mechanically ventilated COVID-19 patients
peer reviewedRetrospectively analyzing the data of a multicenter cohort,
we observed that mortality of patients with SARS-CoV-2 pneumoniatreated
with mechanical ventilation was as high as 45% and median
survival time was 82 days. In this series, the risk factors for mortality
included age, renal and circulatory dysfunction, lymphopenia and the
absence of corticosteroid use during the first week of mechanical ventilation.
Corticosteroid therapy during the first week of mechanical ventilation
was associated with a lower mortality (34% vs 48%) (p = 0,01)
Adsorption de gaz rares dans la silice sous pression
Des travaux récents ont mis en évidence la très forte solubilité de l’hélium dans la silice vitreuse à haute pression : jusqu’à 1 molécule d’hélium par molécule de SiO2 à la pression de 5 GPa. Ces concentrations élevées inattendues remettent en question les mécanismes, acceptés jusqu’à maintenant, d’adsorption physique des gaz dans le volume libre du réseau vitreux. Je présenterai ici les résultats d’une étude, par diffusion Brillouin de la lumière, de l’élasticité de la silice sous pression hydrostatique des gaz : hélium, néon et argon. Nous montrons que le module de compression de v-SiO2 dépend du fluide de pressurisation et qu’il diffère de celui obtenu par des méthodes statiques [1]. Dans le cas de l’hélium et du néon, nous mettons en évidence la déformation volumique macroscopique associée à l’adsorption du fluide au sein de la structure vitreuse. Nous montrerons que ce comportement peut être décrit par un modèle de poroélasticité généralisé
- …