A procedure to prepare macroporous silica gels from an acid-catalyzed tetraethoxysilane (TEOS)water binary system was developed, and the macroscopic morphology formation was examined at a range of conditions under controlled reaction temperature. This procedure involves two-step mixing of TEOS and water containing pH control agents. The gel morphology is significantly influenced by the solution composition in each mixing step, even if the overall solution composition is unchanged. The macroscopic morphology is formed by phase separation between hydrophobic silica oligomers modified by unhydrolyzed ethoxy groups and hydrophilic solvent mixture consisting of water and ethanol generated from TEOS. The morphology can be described as an aggregation of macroscopic particles, which are most likely formed by the spinodal decomposition and subsequent fragmentation of the silica-rich phase occurring at the solvent-rich side of a miscibility gap. The resultant macroporous gels are dried in a relatively short time to obtain crackfree monolithic silica xerogels, which are useful as silica glass precursors.Solgel synthesis of silicates has been extensively studied, mainly because the precursors (silicon alkoxides) are easy to handle due to their moderate reactivity and their various derivatives including those modified with organic functional groups are commonly available. Solgel method is widely used to form various functional silicates, such as monolithic gels and glasses, films, and powders. In the solgel method silica glasses are obtained by sintering monolithic silica xerogels. The low processing temperature as compared with vapor-phase and conventional fusion methods is beneficial in suppressing devitrification and weight loss during manufacturing of silica glasses. However, the utility of the sol gel method is limited by fracture associated with shrinkage during drying, which originates from evaporation induced capillary force and its inhomogeneous propagation in gel monoliths.1 The capillary force is decreased by decreasing the surface tension of pore liquid and the inhomogeneous shrinkage is suppressed by slowing down the drying. Thus, various techniques, including careful control of drying conditions, 25 incorporation of solvents of low surface tension (often termed "drying control chemical additives," DCCAs), 68 and supercritical drying, 9 have been developed to obtain monolithic silica xerogels. The capillary force is also decreased by increasing the pore size. A notable approach is dispersing particulate silica fillers in the precursor solution. 1015 In this method the pores can be easily enlarged by increasing the size of the fillers. Furthermore, the fillers mechanically strengthen the gel framework, making it possible to greatly alleviate the fracture problem. Another method is to utilize macropores formed by phase separation between silica gel and solvent phases. In alkoxide-based gelling systems macroscopic phase separation is induced by incorporating additives such as organic polymers, 16,17 polar organ...

Amines are used as pH control agents to form macroporous silica gels utilizing macroscopic phase separation of acid-catalyzed tetraethoxysilane–water binary mixture. The wide pKa variety of amines from basic to acidic regions is suitable to buffer the solution at neutral pH range, at which the phase separation becomes prominent. This method allows monolithic silica gels and glasses to be prepared reproducibly from alkoxide-based solutions with minimal use of pH control agents.

Fluorine-doped monolithic silica glasses with low SiOH concentrations were prepared by a solgel method using hydrofluoric acid as a fluorine source. Without employing dehydration of dried gels using halogen-containing gases, or vacuum sintering, the concentration of SiOH groups was reduced to ³10 17 cm ¹3 (³1 ppmw), which is comparable to that of synthetic silica glasses mildly dehydrated during their vapor-phase synthesis. The fluorine concentration evaluated from the intensity of the Raman band of SiF groups (³945 cm ¹1 ) was in the order of ³10 20 10 21 cm ¹3.