CHARACTERIZATION OF GELS PREPARED FROM SILICON ETHOXIDE IN PRESENCE OF HCL AND HF

Xerogels and aerogels prepared from homogeneous mixtures of TEOS, H2O and ethanol in the presence or absence of N,N,dimethyl formamide were investigated. The dried gels were characterized using pore size distribution, thermal analysis, X-ray diffraction, and scanning electron microscopy. In the presence of HCl, high ethanol content yields finer texture and smaller pores not accessible to mercury while in the presence of HF, high ethanol content produces coarser texture with large pores. With low water content, linear chains grow forming spheres with large free volume with tiny pores within the particulate structure. With higher water content, dense spheres grow and interlock producing large connected porosity. On heating, three regions were identified: (a) evaporation of solvent retained after drying, (b) decomposition and oxidation of residual organic matter, and (c) dehydration of silanol groups. The weight loss, which corresponds to the amount of solvent, retained and unreacted organic matter, decreases as the pore size increases. Crystals of cristobalite were detected by heating to a high temperature. Glass transition and crystallization temperatures were correlated to the amount of water and ethanol in presence of HCl or HF.