D. Bourret

Gas permeability of partially densified aerogels

Abstract Gas flows through partially densified aerogels were measured for samples of various densities (0.4–1.2 g cm −3 ). Measurements were done at various pressure drops with argon and nitrogen. The gas flow in these partially densified aerogels is always molecular at the pressures and temperature examined. The permeability and conductivity of two sets of samples obtained from aerogels having different initial densities were studied. The conductivity always decreases when the density increases. The difference of gas conductivity between samples of the same density but prepared from aerogels having different initial density is explained using a model previously reported for the sintering of fractal silica aerogels. These results were analyzed using classical models for porous materials and are linked to pore size distribution obtained by thermoporometry and Hg porosimetry.

Ultraporous materials with low permeability

Physical and chemical properties of silica aerogels are influenced by their texture. Silica aerogels exhibit both high porosity and low permeability. This low permeability results from the small size of the pores. The behaviour of gels subjected to a stress is linked to permeability and low rigidity. It is shown that the interaction of the solid network with the fluid within the pores is responsible for stress development which in turn gives rise to easily measured strains. Interactions between the solid and a liquid, a supercritical fluid and finally a gas are investigated. Experiments are reported to demonstrate the solid-fluid interaction.

Diffusion phenomena in partially densified silica gels and doped silica glasses elaboration

Abstract Silica gels have been obtained in our laboratory by the hydrolysis and polycondensation reaction of a silicon organometallic compound in an alcoholic medium. Solvent is then released from gel under hypercritical evacuation in an autoclave. Firing time is adjusted to obtain samples whose density varies between 0.3 and 2.2. So that, materials with various textural properties (specific surface area, porosity, average pore radius) are available. Experimental conditions which allow gels free of cracks to be obtained after filling by various solvents have been studied as a function of solvent interfacial energy and gel density. Diffusion of aqueous solution in cylindrical samples is then studied. The water-gas boundary in the sample is followed using a video-camera coupled with a computer. Finally, the elaboration of doped silica glasses from filled samples is described.

Scaling theory and numerical simulations of aerogel sintering

Abstract 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, ξ, in order to conserve the total mass of the system. Scaling laws are derived which predict how a , ξ and the specific pore surface area, Σ, should depend on the density, ρ. 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 , ξ and Σ as a function of ρ 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 ξ are well separated. These numerical results are compared with experiments on partially densified aerogels.

Phase separation during isothermal sintering of (1 − x)SiO2-xLn2O3 aerogels

Rare earth silicate aerogels (1 − x)SiO2-xLn2O3, with Ln = Dy or Er, have been prepared by the sol-gel process, starting from Dy or Er nitrates and TMOS. After an isothermal treatment of the aerogels, transparent glasses resulted when starting from aerogels with an Ln2O3 mole fraction x

Crystallization of lead titanate PbTiO3 in gel-derived solder glasses

Abstract Homogeneous precursors of ‘solder glasses’ used in electronic device packaging have been synthesized by gel process. Experimental details and structural modifications of starting alkoxides during gel synthesis and aging are presented. Crystallization of lead titanate, PbTiO3, during the sintering of amorphous and porous lead borate matrix is described. Al2O3 and/or SiO2 addition effects on metastable lead titanate crystallization and on grain size distribution are reported.

Gas transport in porous media: simulations and experiments on partially densified aerogels

The experimental density dependence of gas (argon and nitrogen) permeability of partially densified silica aerogels in the Knudsen regime is quantitatively accounted for by a computer model. The model simulates both the structure of the sintered material and the random ballistic motion of a point particle inside its voids. The same model is also able to account for the density dependence of the specific pore surface as measured from nitrogen adsorption experiments.

Faraday effect of sol-gel derived glasses

Abstract A Faraday device has been constructed in our laboratory. A second, fixed, analyzer was placed behind a rotating analyzer thus a reference signal was not required to achieve measurements. Automatic control, data acquisition and data processing were performed by a personal computer. Preliminary results for doped silica glasses elaborated by the sol-gel process are presented and discussed.

SiO2Nb2O5 thin films using sol-gel technology

Xerogel and aerogel SiO2Nb2O5 thin films were obtained and characterized by UV, visible and IR microscopy. The aerogel sheet shows a λ−4 evolution close to monolithic one. Water and silanol contents are also analyzed for each type of thin sheets. Glass surface treatment before dip-coating was investigated using SEM, ellipsometry, reflectance measurement and contact angle. The best washing schedule was deduced. One to eight successive coatings were usually done and analyzed. Refractive indexes and thickness of the coatings are presented.

Mixed silica-iron oxide aerogels

This work deals with the synthesis of silica-iron oxide aerogels from gels obtained by hydrolysis and polycondensation of tetramethoxysilane and iron acetylacetonate sols in methyl alcohol. Aerogels with various iron contents were studied by DTA, TGA, X-ray and dilatometry. The sol to gel evolution was studied by UV-visible spectroscopy. Spectral evolution observed during gelation is related to the change of iron surroundings.