R. Sempere

Sol–gel derived porous silica films

Abstract A modified sol–gel process, involving gelation of drops of a starting solution pressed on a substrate by a non-wetting glass, has been employed to obtain porous silica coatings on glass substrates at room temperature and pressure. Deposited films are homogeneous and crack-free. Films having a thickness higher than 5 μm can be obtained. These aerogel like coatings show a refractive index lower than 1.2 and a dielectric constant lower than 2 with a pore volume of about 60%. Films exhibit hydrophobic properties because of surface modification.

Tribological Properties of Ormosil Coatings

Transparent plastics are not scratch resistant. The damage leads to a loss of optical properties. Coatings prepared using either tetraethoxysilane or colloidal silica particles embedded in “glymo” is a way to avoid these disadvantages. Tribological experiments are carried out to better understand the surface modification due to a sliding friction. It is shown that the wear track is not directly related to usual mechanical properties such as Youngs modulus and the hardness of the coating. The different stages leading to material loss are discussed in term of particle removal and debris circulation (accumulation or elimination) through the friction track. The mechanical properties of the film combined with the film to substrate adhesion are expected to play an important role as it can be deduced from results obtained as a function of the coating composition.

Mechanical properties of nanocomposite organosilicate films

Nanocomposite coatings have been deposited on plastic substrates by the dipping–drawing technique. The coatings were constituted by a matrice of a hybrid organomineral gel and a reinforcement made of amorphous silica. Two methods by which increase the silica content were investigated: silica was added via a silicon alkoxide compound or via dense silica particles of 10 nm size. Youngs modulus and the hardness of the coating were measured using home-built equipment, and results compared to literature models. It is shown that the agreement between models and experimental values depends on the method of preparation of the nanocomposite coating. On the other hand, deviations appear when the volume fraction of reinforcement surpasses the three-dimensional percolation threshold.

Densification kinetics and structural evolution during sintering of silica aerogel

A silica aerogel was sintered at 1050°C, and the effect of densification on the pore structure was determined by nitrogen sorption. Based on the initial pore size distribution (psd) of the aerogel, the kinetics of densification and the evolution of the psd were calculated using the theory of viscous sintering. If the psd is ignored, and the sintering behavior is predicted using the average pore size, then the agreement with measurements is not very good. However, when the actual psd is used in the calculation, then the predictions are excellent for density versus time, surface area and mean pore size versus density, and psd versus time. The quality of the fit depends on the constitutive laws describing the response of the porous material to stresses; the best results are obtained using the laws originally derived for the cylinder model. The power-laws that describe the density dependence of the elastic modulus of aerogels do not seem to apply to the viscosity; reasons for this dependence are discussed.

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.

Textural properties of densified aerogels

Silica aerogels were subjected to isostatic pressure using a Hg porosimeter. Nitrogen adsorption experiments were performed to estimate specific surface area, porous volume and pore size distribution. In a same way, gas permeability measurements have been carried out. During the first stages of compression, the specific surface area seems to be constant. The difference between the total pore volume and that measured by the BJH method (using the desorption part of the isotherm) is less important in compressed aerogels than in sintered ones. Permeability decrease with densification is faster in partially compressed aerogels. This behavior is correlated to disappearance of largest pores and has been associated to the evolution of the pore size distribution which evolves toward smaller pore diameters.

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.

Mechanical characterization of Ormosil films on plastics: Young's modulus determination by three points bending

A protective layer has been deposited to improve the scratching properties of polycarbonate organic glasses. The starting solution consists of a mixture of organosilicon compounds, silicon alkoxides or silica colloidal solutions. The deposition is carried out using the dip coating technique. The thickness is about 2–5 µm for the film and 1 mm for the substrate.Youngs moduli of the films are obtained by a new three points bending apparatus. Youngs modulus of coating depends both on silica content and on the nature of the silica used as a filler.

Raman study of structural defects in SiO2 aerogels

The structure of the silica aerogels was studied by Raman spectroscopy. The spectra of the solid network resembles that of bulk silica with additional bands related to organic groups and a large amount of OH groups.The typical bands due to ring breathing also called defect bands D1 and D2 located at 490 and 610 cm−1 are present. However, the evolution of the D2 band compared to that of OH band (980 cm−1) seems apparently, in contradiction with the results previously reported in the literature. During heat treatments between 25 and 300°C the D2 and the OH bands increase simultaneously. Generally, in silica glass the defect band D2 grows at the expense of the OH groups.This result is explained by the oxidation of the organic compounds which, in this temperature range, leads to the formation of the both species (OH) and those related to siloxane rings. 29Si MAS NMR results are in agreement with the Raman study.