Scaling Theory and Numerical Simulations of Aerogel Sintering

A simple scaling theory for the sintering of fractal aerogels is presented. The densification at small scales is described by an increase of the lower cut-off length $a$ accompanied by a decrease of the upper cut-off length $\xi$, in order to conserve the total mass of the system. Scaling laws are derived which predict how $a$, $\xi$ and the specific pore surface area $\Sigma$ should depend on the density $\rho$. Following the general ideas of the theory, numerical simulations of sintering are proposed starting from computer simulations of aerogel structure based on a diffusion-limited cluster-cluster aggregation gelling process. The numerical results for $a$, $\xi$ and $\Sigma$ as a function of $\rho$ are discussed according to the initial aerogel density. The scaling theory is only fully recovered in the limit of very low density where the original values of $a$ and $\xi$ are well separated. These numerical results are compared with experiments on partially densified aerogels.Comment: RevTex, 17 pages + 6 postscript figures appended using "uufiles". To appear in J. of Non-Cryst. Solid

Conductivité électrique des verres et fontes d'oxydes (effet de l'incorporation de particules de RuO2)

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Solubilité d'éléments aux degrés d'oxydation III et IV dans les verres de borosilicate

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Séparation de phase et cristallisation induites par l'ajout de molybdène et de phosphore dans un verre silico-sodo-calcique

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Etude et contrôle des réactions d'oxydoréduction au cours de l'élaboration des matrices vitreuses de confinement des déchets nucléaires

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Aerogel Sintering: From Optical Glasses to Nuclear Waste Containment

International audienceAerogels are ultraporous materials with a very low permeability. They exhibithigh specific surface area but, due to their huge porous volume, their mechanical properties are quite poor. Aerogels belong to two main families: inorganic materials as silica, silicates, and a few aluminates and organic aerogels which are easily transformed into carbon aerogels by a simple pyrolysis thermal treatment. In this chapter, we mainly report experiments performed on alcohol-dried aerogels. Due to their broad range of porosity, aerogels canfitin very different applications: from analysis of cosmic dust to nuclear waste containment glasses. Advantages of large porosity are sometimes used directly like in thermal and acoustic insulation, or in catalyzers, but a too high pore volume can also be a drawback like in glass precursor and host matrix. Fortunately, aerogel porosity can be tailored using sintering or room isostatic compression or eventually a combination of both methods. Knowledge in this area allows now to synthesize aerogels with a broad range of porosity and also a very broad range of texture

THE PERMEABILITY OF SILICA AEROGELS

Silica aerogels have many potential applications, including as host matrices for chemical species or nuclear waste storage. For these applications, the permeability is a key parameter and data show that the silica aerogels have poor permeability (~10-60 nm 2). In this chapter we review the method to measure the liquid and gas permeability in gels, aerogels and composite aerogels. Ideally, permeability does not depend on the type of pore fluid, therefore permeability measured using gas should be the same as that measured using water. We measured gas and water permeability in sets of nano composite silica aerogels. Experimental results show that gas permeability in aerogels was larger than water permeability by almost two orders of magnitude. The observed difference in gas and water permeability was analyzed from the point of view of the slip regime (Klinkenberg correction), and transition regime (Knudsen correction), the slip flow of gas at pore walls enhance the gas flow when pore sizes are small. This work addresses the problem of estimating permeability with high porosity materials such as aerogels. The effects of structural parameters of porous media (pore volume, tortuosity, fractal features) on the Klinkenberg and Knudsen corrections are discussed and the different models proposed in the literature are tested

Aggressive angiomyxoma of the perineal region

Aggressive angiomyxoma is a rare tumour that frequently involves the perineal region with a high risk of local recurrence. This is a case report of a 24-year-old female patient with a genital prolapse. We performed a surgical treatment. Histological examination found an aggressive angiomyxoma. The tumour recurred 1 year after surgery. Long-term follow-up is necessary