Silica Glass from Aerogels
In order however to preserve the expected sol-gel advantages, it was obvious that gels should be obtained in a monolithic form of appreciable size and maintain their integrity throughout all subsequent processing steps (drying, sintering ) which transform a gel to a glassy material.
Freshly prepared silica gels contain an appreciable amount of solvent (usually
70 to 90 wt %) which must be eliminated.
The liquid inside the pores (which size is similar to ultimate silica
particles) exert during drying a stress in the "walls" of the capillaries which is inversely proportional to the pores diameter
This is high enough to break the gel into useless
pieces of a few mm3 in size.
The size of these gels never exceeded 2 to 4 cm diameter disks with 1 to 2 cm in thickness.
A variety of techniques have been suggested (solvent exchange or use of surfactants to minimize surface tension effects, gel aging to reinforce the silica network, in situ chemical modification of the capillary surface etc.) None of the investigated techniques were able to reproducibly provide large size, cracks free dry monoliths.
In 1931 however Samuel Kistler a genius researcher at the University of Illinois had demonstrated
practically what thermodynamics teach us. Above
its critical pressure and temperature any substance is present only in one phase neither liquid nor gas. No more two phase and therefore no
reason for capillary forces to appear.
The first attempt of using hypercritical drying to obtain monolithic piece of porous silica, as glass precursors, was made at the Glass laboratory of the University of Montpellier, France by two researchers M. Prassas and J. Phalippou in 78. The technique used previously by Nicolaon et Teichner at the University of Lyon to elaborate high surface area catalysts from alkoxides based gels was improved and adapted to easy produce monoliths of more than 30 cm in size, a dimension which today seems ridiculous but at that time was a significant improvement over the existing methods.
The typical apparatus used to obtain a monolithic gel by hypercritical solvent evacuation is illustrated below:
An autoclave from inoxidized steel is used to bring the initial solution (in this case Si(OCH3)4 - CH3OH - H2O) above the critical point of the solvent (CH3OH/H2O) present within the pores of the silica gel. To avoid crossing the liquid-gas equilibrium an additional amount of solvent is added inside the autoclave and the system allowed to reach and exceed the critical point (for pure methanol Tcr = 240°C , Pcr = 79.7 bars). As soon as the critical temperature is reached the vapors of the solvent are slowly evacuated, keeping the temperature constant. When the pressure reach atmospheric pressure, the autoclave is flushed with dry Argon and then cooled to room temperature. The entire operation (sol preparation, heating, solvent evacuation, cooling take less than 10 h.)
porous gel so-called aerogel (from the greek aera
= air ) is monolithic, extremely fragile, it shows no shrinkage
and is one
of the lightest solid materials in earth. Typical density of silica aerogels
is in the range of 0.1 to 0.3 g/cc.
Later, the procedure was further improved with the use of CO2 (which certainly needs solvent exchange but is much more safe than the alcoholic hypercritical evacuation). For more details see the Aerogel web site at Laurence Berkeley National Laboratory