Thierry Lours

A SAXS study of kinetics of aggregation of silica sols

Abstract SAXS in situ experiments on the evolution of TMOS solutions during hydrolysis and polycondensation lead to power laws with scaling exponents α ≈ 2. It is suggested that this could be the result of the polydispersity of the samples and that only an apparent fractal dimension can be obtained in this way. Kinetic studies tend to indicate that agglomeration in the sol is the result of a diffusion-controlled process.

SAXS and BET studies of aging and densification of silica aerogels

Aerogels prepared by hypercritical drying of gels obtained by hydrolysis of TMOS/methanol 50% solution in acid and basic conditions were studied combining SAXS and BET methods. Diffraction results do not seem to reveal a fractal character as the PORODs limit I(q) q4 → Cte was obtained in all cases without well characterised portions in log I(q) vs log q graphs. Aging produces a small increase in the density of the matrix. Densification studies indicate that for moderate temperatures 530°–810°C the poresshow a shrinkage due to a diffusion controlled mechanism with an activation energy ΔE ~ 9 Kcal/mole. For higher temperatures expansion due to bloating was observed. The density of the matrix undergoes only a slight increase.

Textural characteristics of silica aerogels from SAXS experiments

Abstract Small angle X-ray scattering measurements on silica aerogels give systematically low values for the solid phase density. In order to explain these results, a hierarchical structure is proposed. Textural parameters such as specific surface area, size and density of clusters, and pore size are a function of the number of levels in the structure. Experimental values are consistent with values calculated for a hierarchy of two to four levels, depending on the initial sol concentration. The model can explain the fractal features for lower density aerogels.

Structural study of fractal silica humid gels

Abstract The structures of gels of silica obtained from solutions of TMOS, methanol and water have been studied by small angle X-ray scattering (SAXS) using synchrotron radiation. The SAXS results indicate that all the gels studied exhibit fractal structures. It is inferred that gels obtained under basic conditions are volume fractals built up by surface and volume fractal units for high and low water content, respectively. The gels obtained under acidic conditions are all volume fractals having structural units much smaller than those of basic gels. The influence of aging on the structure of basic gels has also been established.

SAXS study of isothermal sintering of silica aerogel

Abstract Isothermal sintering of SiO 2 aerogels was followed by small angle X-ray scattering using synchrotron radiation. An improved treatment of SAXS data which takes into account the presence of fluctuations explains the discrepancy previously observed with He-pycnometric methods. The matrix density of the gel is shown to increase in several steps, first by a diffusion-controlled mechanism and then by viscous flow.

Structural studies of fractal silica aerogels

The structural properties of silica aerogels of various compositions and prepared in acidic, neutral and basic conditions are studied by SAXS, BET, density and TEM techniques. All structures can be described as mass fractals (D 2 matrix having average size of - 13.5 A (basic), - 8.5 A (neutral) and

Light scattering dynamic study of the gelation process

Abstract Light scattering is a choice method for studying the dynamic properties of fractal structures and growth processes. We present preliminary light scattering measurements to characterize the formation of small silica particles prepared in highly basic TMOS/methanol/water sols and the transformation of sol into gel employing digital clipped autocorrelation spectroscopy. The light scattering is due to gel clusters.