Study of sol-gel processing for fabrication of hollow silica-aerogel spheres

Preliminary results were previously reported indicating that uniform, hollow silica-aerogel spheres of controlled size and thickness can be fabricated by controlled hydrolysis and condensation of tetraethylorthosilicate (TEOS). The method consists of first producing a stream of hollow drops of a mixture of TEOS, H₂O, ethanol, and a catalyst using a dual-nozzle system and then introducing the drops into a gelation chamber where the drops solidify into a rigid form while being levitated by a gelation gas mixture. A detailed further study designed to understand and control the kinetics of the sol-gel processing that is responsible for the hollow silica-aerogel sphere formation is described. Specifically, the optimal rheology and stoichiometry of the reactant solution, the make-up of the gelation/levitation gas mixture, and the characteristics of the resulting silica-aerogel spheres, such as the size, thickness, porosity, pore size, and density, are investigated.