Rolf Clasen

IR spectroscopy of porous silica gels

IR spectroscopy is a well established tool in chemical and semiconductor industry giving information about the composition and geometry (film thickness) of the samples. The application of this non-destructive method to characterize porous silica gels obviously faces the problem of dealing with inhomogeneities. The samples are basically two-component systems of SiO 2 and air (or vacuum). A rather simple theoretical model is presented being able to describe the effects of the inhomogeneities and to fit experimental spectra quite well. It gives information about the volume fraction and the strength of the SiO 2 network.

IR-spectroscopical investigations on the glass structure of porous and sintered compacts of colloidal silica gels

The forming and sintering of fumed silica powders is an interesting route for the preparation of large, very pure or doped silica glasses with a precise geometry. The processing from the shaping of a porous compact to the sintering of transparent silica glass can be successfully investigated with optical spectroscopy. As only the dielectric function DF (a dielectric function is the square root of the complex refractive index) characterizes the material, the vibrational bands were calculated from reflectance measurements. In compacts of fine particles, the topology cannot be neglected. Therefore, the models describing topological effects are briefly reviewed. With these model calculations it could be proven that new bands in the compacts and the significant shifts in the reflectance spectra during sintering are mainly caused by topological effects and that changes in the glass structure play only a secondary role.