In this technique, a gel containing an organic solvent such as methanol, ethanol, acetone, or acetonitrile is soaked under liquid carbon dioxide to replace the liquid in the gel with liquid CO 2. This is compared with, say, methanol, which is very flammable and has a critical point of One drawback, however, is that unlike methanol or other organic solvents, CO 2 does not exist as a liquid at ambient conditions.
In fact, dry ice, the solid form of CO 2 which you can buy at some gas stations and grocery stores , sublimes directly to gaseous CO 2 at atmospheric pressure instead of melting. As a result, in order to work with liquid CO 2 so that we can soak a gel in it, we have to use CO 2 at a pressure where it can exist as a liquid around 58 times atmospheric pressure at room temperature. To perform CO 2 exchange, a gel is placed in a pressure vessel which is then sealed and slowly pressurized with a tank of liquid CO 2 equipped with a siphon tube like a liquid soap dispenser.
Liquid CO 2 siphon tanks are common, and can be found in almost any restaurant or bar as the source of carbonation in a soda fountain system. At that point, liquid CO 2 will siphon into the vessel and cover the gel. Depending on the size of the vessel and the gel, it is common to pre-fill the vessel with organic solvent whatever is in the gel to prevent the gel from drying out while waiting for CO 2 to siphon in.
This organic solvent is then drained off as soon as CO 2 starts to siphon in. After liquid CO 2 has siphoned in, the gel is simply allowed to soak for a number of hours. The liquid in the vessel is drained out and replaced with new liquid every few hours for a period of time of days for small samples and up to a week or two for large samples.
As the gel soaks in the liquid CO 2 , the organic solvent held within its pores diffuses out, and liquid CO 2 diffuses in its place.
Learn how to build a supercritical dryer of your own and find a fully-illustrated step-by-step process of performing supercritical drying with CO 2 under the Make section. Just as you can only bake a pie as big as your oven, you can only supercritically dry an aerogel as large as your pressure vessel.
This means one of three things—either you need a big supercritical dryer, you limit yourself to making small aerogels, or you use a non-supercritical drying technique see below. Additionally, large continuous volumes such as cubes or spheres are generally difficult to make since it takes exponentially longer for solvent from the interior of the gel to diffuse out of the gel as the gel thickness is increased.
For folks interested in getting an aerogel sample disc, block or tile for engineering projects or to experiment with, BuyAerogel. I am descended from 5, generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine. Our websites use cookies to improve your browsing experience.
Aerogel Recipes Procedures for making aerogels of various types. Appreciating Your Work Articles about how to measure the properties of aerogels, how to photograph them, and how to set up demos and stupid tricks with aerogels.
Build a Supercritical Dryer Learn how to build a supercritical dryer for making aerogels. Gel Drying Procedures How to perform supercritical and subcritical drying of gels to make aerogels.
Repeat this process a few times over three days. Transfer the gel into the supercritical drying chamber, and fill the chamber with methanol. Add liquid CO2, then open the chamber's bottom valve to remove the methanol.
Make sure the gels are always covered with liquid CO2. Wait a day for methanol to diffuse into the liquid CO2. Open the bottom valve and remove more methanol.
Repeat the methanol draining procedure while making sure the gels stay submerged in liquid CO2. Raise the chamber temperature to cause the CO2 to become supercritical. Slowly vent the chamber while applying heat to ensure the CO2 moves from the supercritical phase to the gas phase. Continue venting the chamber slowly, then remove the finished aerogels. Posted by Ben Krasnow at PM.
Labels: aerogel , aerogel at home , aerogel production , supercritical , supercritical drying , tetramethoxysilane , tetramethyl orthosilicate , xerogel. Joshua Stults November 13, at PM. Anonymous February 4, at AM. Ben Krasnow November 13, at PM. Will Walker November 14, at AM. Greg Courville November 14, at PM. Jean-Michel November 15, at PM. Anonymous November 16, at PM. Snowcamo November 17, at AM.
Anonymous November 20, at AM. John November 29, at PM. Anonymous January 8, at PM. Ben Krasnow January 8, at PM. Ben Krasnow January 15, at PM.
Unknown April 9, at AM. Ben Krasnow April 9, at AM.
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