SUPERCRITICAL DRYING OF GELS

Crack-free dry gels provide a possibly advantageous route to glasses for, for example, optical fibers. Cracking advantageous condensed alkoxide silica gels occurs during drying when strains are produced because some capillaries are liquid filled while others have dried. If drying is carried out at or near supercritical conditions such strains are reduced. Gels formed from ethylorthosilicate were autoclave dried by various pressure-temperature-time paths. To avoid cracks, the initial fill of the autoclave had to be well above the critical fill for ethyl alcohol and the temperature had to be brought above the critical temperature, T//c, before the pressure was released.