INFRARED AND THERMOGRAVIMETRIC STUDY OF HIGH-PRESSURE CONSOLIDATION IN ALKOXIDE SILICA-GEL POWDERS

High density, transparent, crack-free and hard compacts of silica gel were produced by compaction under nearly hydrostatic environment at 4. 5 GPa, at room temperature. The starting material was used three days after synthesis by hydrolysis of alkoxides without additional treatmen t. Fourier transform infrared spectroscopy (FTIR), using the KBr techn ique and a high vacuum cell at temperatures up to 450 degrees C, and t hermogravimetric analysis (TGA) up to 900 degrees C was conducted. A r eduction up to 60% in the adsorbed water content of the compacted sili ca gel was observed. Changes in the 3000 to 3800 cm(-1) region indicat e that the surface silanol groups became bridged, which promotes conde nsation reactions of dehydroxylation. Those results suggest that the m echanism for consolidation under high-pressure, is 'cold sintering' pr ocess, where silanol groups at the surface of the nanoparticles conden se to form siloxane bonds between the particles and water, resulting i n a stiff body with closed nanopores containing trapped water. (C) 199 7 Elsevier Science B.V.