Using supercritical fluids to dry aerogels
Aerogels are highly porous materials with large internal surface area and large pore volumes. Their densities are as low as 3 kg/m3 and have porosities as high as 99.9%. This makes them excellent thermal insulators. In fact, aerogels are listed in the Guinness Book of World Records for the being the best insulators and the lowest-density solids.
In addition to their thermal insulating capabilities, aerogels have structural strength and impressive load-bearing ability, exceptional absorptive properties, and acoustic insulating capabilities. A short list of specific applications:
- Thermal insulation to windows and skylights
- Chemical absorber for cleaning up spills
- Thickening agents in paints and cosmetics
- Commercial manufacture of aerogel "blankets"
- NASA used aerogels to trap space dust particles aboard Stardust spacecraft
- NASA also used aerogel for thermal insulation of the Mars Rover space suits
- US Navy is evaluating aerogel undergarments as passive thermal protection for divers
- Use as a drug delivery system due to its biocompatibility. (Due to its high surface area and porous structure, drugs can be adsorbed from supercritical CO2)
Although there are other types of aerogels, such as carbon and alumina, silica aerogels are the most common. They are made with a liquid alcohol like ethanol which is mixed with a silicon alkoxide precursor to form a silicon dioxide sol gel (silica gel). However, removing the aerogels from the solvent bath for common use can be problematic.
Since the structure is so fine, normal drying at atmosphere collapses the network rendering to dust. This is caused by normal capillary pressure at the liquid/vapor interface on the inside of the pore. The energy of vaporization is greater than the wall strength of the pore.
Because a supercritical fluid has no surface tension, it can be used to dry the aerogel without consequence. The end result removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.
There are two methods for drying aerogels:
- Low temperatures
- Liquid CO2 displacement of an organic solvent
- SC-CO2 extraction of an organic solvent
- SC-CO2 venting
- High temperature
- Conversion of a liquid organic solvent to the supercritical state with subsequent venting
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