Investigation of surface heterogeneity in thermally treated controlled porous glasses and systems silica gels/borate crystals by means of the SAXS method

Abstract

Heating of controlled porous glass (CPG) in the temperature range 400–700°C leads not only to a dehydroxylation process of the surface but also brings about a diffusion of the boron atoms remaining in the silica network of CPG towards the glass surface. Some authors assume that a along thermal exposure can cause the formation of B 2O 3 crystals in the pores. In this paper the CPGs possessing different mean pore diameters before and after their thermal treatment were investigated by the SAXS (small angle X-ray scattering) method. The SAXS method is very useful in such cases because there are two sources of X-ray scattering: pores in glass and crystallites of the borate phase. The presence of these two sources is responsible for the change of the intensity of small angle X-ray scattering in comparison to X-ray scattering of the initial porous glass (without borate crystallites). In consequence, the pore size distribution function, calculated from the SAXS data, does not agree with the analogous one calculated from mercury porosimetric data. For the confirmation of the above artificial systems composed of silica gle with either deposited H 3BO 3 or Na 2B 4O 7 were investigated. It appears from the presented data that there is a possibility to investigate the kinetics of the crystallization process of borate crystallites by means of the SAXS method. SAXS also allows one to control the process of the removal of the borate phase from the pores of CPG. It can be concluded from the obtained results that long time thermal heating causes the formation of borate crystallites in pores of CPG and that these crystallites are xNa 2O·yB 2O 3 (where y ⪢ x) rather than B 2O 3.