Wet acid-catalysed gels prepared by hydrolysis and condensation of tetramethylorthosilicate (TMOS) can be dried without the occurrence of fractures or cracks when drying conditions are appropriate. Furthermore, the resultant dried gel monoliths can be converted to silica glass on heating to 1100°C and holding there for 30 min when the composition of the starting solution is appropriate.

The effects of heat-treatments on silica xerogels prepared from base catalyzed hydrolysis and condensation of TMOS are studied by low frequency Raman scattering, scanning electron microscopy, small angle X-ray scattering and BET measurements. Raman scattering in the range 3-700 cm-1 reveals a low-frequency band that arises from the discrete particulate character of base-catalysed gels at very small scale. A continuous evolution of the structure inside particles is observed during the thermal treatment and the Raman spectra is similar to the fused silica one after treatment at 1050°C. Electron microscopy observations of the nanostructure of the xerogels correlate with the particle Raman band maximum. Complementary SAXS and BET measurements allows us to propose a model for the gel-to-glass transformation.

We compare the crystallographic morphology of epitaxial polycrystalline silicon layers (9 10-3 03A9.cm resistivity, 450 nm thickness, L.P.C.V.D. at 670 °C by silane decomposition) which undergo for 6 to 180 min a high temperature treatment (900 °C to 1 050 °C) in inert gas ambiance (argon) or oxidizing ambiance (dry oxygen). The crystallite size evolution of the polycrystalline layer has been studied by means of X-Ray diffraction. In both cases, we find a grain size increase (faster in oxygen ambiance) (from 20 to more than 100 nm), corresponding to the crystallization of amorphous zones enhanced by dopant diffusion (phosphorus) at grain boundaries and by mechanical stresses. As compared to single crystal, the oxide formation kinetics is controlled by the random crystallographic orientation of grains and by grain boundaries (intergrain diffusion and dopant segregation). The destructive breakdown of oxide is not direct. We often observe a pre-breakdown followed by self-curing. Dielectric rigidity depends on doping rate and varies from 2 to 5 MV/cm when the oxidation temperature steps from 900 °C to 1000 °C. Revue Phys. Appl. 24 (1989) 133-141 FEVRIER 1989, 1

The effect of long heat treatments on the ZBA fluorozirconate glass (BaF2, ZrF4, AlF3) at a temperature close to the glass transition (Tg) was studied. The crystallization of ß-BaZrF6 was detected by Raman scattering in transparent samples after heating for 400 h at 310°C. A Porod regime in SAXS was observed after the same heat treatment. Raman scattering and fluorescence of Pr3+ ions showed a continuous modification of the glass during the heat treatment. This observation is in agreement with continuous decomposition of the glass before crystallization.

Very low frequency Raman scattering is observed for the early stages of the nucleation of Zerodur glass. From the relation ω ~ 0.8 v/2ac the diameter 2a of the nuclei is deduced from the position ω of the Raman very low frequency line, taking into account the sound velocity v in the material. For a sample heat-treated 24 h at 710 °C, diameters of nuclei of (60 ± 20) Å are observed. These results are correlated with small angle X-ray scattering results. Along with characterisation by X-ray diffraction analysis, a description of the early stages of nucleation is proposed.