Glasses from colloids

A process is described for preparation of glass from colloidal dispersions of particles produced by flame oxidation. Particles of SiO₂ made from SiCl₄ (BET diameter ∼ 60 nm) are dispersed in a hydrophobic solvent by an adsorbed layer of alcohol molecules. The adsorption sites are shown by photoacoustic spectroscopy to be isolated silanol (SiOH) groups on the surface of the particles. The alcohol presents a steric barrier to coagulation of the colloid, so that a suspension of 13 vol.% of SiO₂ retains a viscosity below 50 cP for weeks. The silanol is acidic, so more basic alcohols adsorb more strongly; e.g. 1-propanol disperses silica, but methanol does not, nor does more acidic fluorinated propanol (C₃F₇OH). The colloid is readily gelled by exposure to a base, such as an alkyl amine or NH₃, which deprotonates the silanol. The resulting charges cause gelling, in qualitative agreement with the theory of Féat and Levine. Evaporation of the solvent is readily achieved to obtain a hydrophobic gel containing ∼ 65-75% porosity. A cylindrical monolith ∼ 1.3 × 19 cm can be dried in 3 days, then sintered to clear glass at 1300-1400°C.