WHAT, you may ask, is aerogel?  Aerogel is the lightest and lowest-density solid known to exist.  It is typically 50-99.5% air, yet can hold (theoretically) 500 to 4,000 times its weight in applied force. Aerogel can have surface areas ranging from 250 to 3,000 square meters per gram, meaning that a cubic inch (2.5 cm x 2.5 cm x 2.5 cm) of aerogel flattened-out (again theoretically) would have more surface area than an entire football field!   Aerogel's superlow density makes it useful as a lightweight structural material, and its superhigh internal surface area makes it a superinsulating solid material.  For those of you who have always wanted to touch an aerogel, it feels like styrofoam.  Silica aerogel is transparent with a blue cast.

Aerogel has come a long way since this site was launched. For the latest on aerogels, check out Aerogel.org, an open-source aerogel project. On Aerogel.org you will find:
  • A complete encyclopedic reference about aerogels
  • Instructions on how to make aerogels of different types
  • A how-to guide for building a supercritical dryer/autoclave/manuclave
  • Tons of pictures of aerogels
  • A podcast featuring the world's top aerogel scientists
  • and much more!
For the latest on zero-g aerogel research, check out the MIT Zero-G Team and the zero-g aerogel page on Aerogel.org.

Here are some links to the sections of this page.

Starting Out
How are Alcogels Made?
How is Aerogel Made?
How to do Supercritical Drying
Why Aerogels are Blue

Starting Out

Aerogel starts out as a gel, sort of like Jell-OTM, made of silica (silicon oxide, or SiO2,which is what glass and quartz are made of) and a liquid solvent such as ethanol.  A gel is a colloidal system in which a network of interconnected solid particles spans the volume of a liquid medium.  Jell-O is a gel.  Gels are typically free-standing solids but are mostly liquid in volume.

How are Alcogels Made?

Aerogel starts out as a gel, called alcogel.  For you scientists--Alcogels are made by polymerizing a silicon alkoxide (Si(OR)4, where the R is an alkyl group) with water in a mixing solvent (such as ethanol).   The reaction occurs by hydrolysis and water condensation, joining together the alkoxide molecules making silicon-oxygen bonds to form oligomers (mini-polymers).  The oligomers join together and form one giant molecule, which is the solid part of a gel.  Easy, right?  The silica matrix in the alcogel is filled with ethanol, having tiny little pockets 5 to 150 nanometers across.  These tiny pockets of ethanol in the gel are called nanopores. Nano is the metric prefix for one billionth, and a nanometer (abbreviated nm) is one billionth of a meter--close to the size of some atoms.

How is Aerogel Made?

Aerogel is made by drying the alcogel and extracting the liquid from the solid silica component.  If you have ever left Jell-O out of the refrigerator, you probably will have noticed that it shrunk and got fairly disgusting in the matter of a few days.  The same thing happens to alcogel when it is dried by evaporating the solvent off.  The evaporating liquid solvent causes the alcogel's solid silica component to collapse by capillary action.  This means that after the solvent has been completely taken out of the gel, the gel has collapsed and formed a dense solid that is a pitiful 10% of the original volume of the gel.  This solid is called xerogel (xero=dry, gel=gel) and is how they make things like contact lenses and high-purity lenses.

Instead of evaporating the solvent, the gel can be supercrtically dried.  Supercritical drying is a process in which liquid can be removed from a gel without causing the gel to collapse.  Supercritical fluids are semi-liquids/semi-gases that are usually high  pressure and high temperature.  All liquids can be made supercritical.  Supercritical fluids expand like gases, but have density and thermal conductivity closer to liquids.  Supercritical fluids also have lower surface tension than liquids.  Supercritically drying alcogel is a way for the liquid in the gel to slowly sneak out of the solid silica matrix without causing the silica matrix to collapse from capillary action.  This is done by heating the gel past its solvent's critical point. Once the liquid has snuck out of the gel, the solvent can be vented off as a gas.  The remaining solid is made of silica, with tiny pockets (nanopores) filled with air, and is 50-99% of the volume of the original alcogel.  This solid is called an aerogel.

How Do We Do Supercritical Drying?

This is called a manuclave.  It is a manually controlled autoclave.  I made that word up.  It's not in your dictionary.  Stop looking in your dictionary.  Close your dictionary window now please.

The manuclave is a high-pressure vessel made of steel pipes and high-pressure valves.  Here's how we supercritically dry stuff.  First, the manuclave is pressurized with CO2 at 75 atmospheres.  Before anything, we place the alcogels in the manuclave under liquid (so they aren't blown apart when the vessel is pressurzied), pressurize the manuclave, and eventually drain the liquid off.  Then, we soak the gels under liquid CO2.  The alcogels are soaked in CO2 until all of the original solvent in the gels has diffused out. After a few hours, we pressurize the vessel to at least 1,050 psi and 31.1 degrees Celsius (about 88 degrees F).  At that point, the CO2 becomes a supercritical fluid and can safely diffuse out of the alcogels without causing them to collapse.  The CO2 is then vented off and the vessel depressurized. We then have aerogels and can eat (just kidding) I mean study them.  (Aerojell-O is possible to make, by the way). 

Because it is almost completely air and has such high surface areas for heat to deflect against, aerogel could be used in homes, businesses, and industry as clear superinsulation. A piece of aerogel 1 inch thick would be a better insulator than 20 evacuated stacked thermalpane windows.  There is only one problem--aerogel is BLUE not CLEAR.  (Dun dun dun......)

Blue is a great color, I mean, don't get me wrong!!  It's just that people don't want to stare through blue windows all day.

Why Aerogels Are Blue

SO why is aerogel blue?  The same reason the sky is blue--Rayleigh scattering.

Rayleigh scattering is an optical phenomenon that results when white light scatters off of particles smaller than the wavelengths of light, particles typically of the size 5 to 200 nm.  These particles scatter the shorter wavelengths of white light more easily than the longer wavelengths, meaning that blue and violet are scattered the most.  Our eyes are much more sensitive to blue wavelengths than to violet wavelengths, and so we only see blue light.  Aerogels contain nanoparticles of silica (which is what glass is made out of) and nanopores of air that are only a few hundred times larger than atoms.  Some of these nanoparticles made of silica scatter white light and make the aerogel appear blue.

It might be possible, however, to make aerogel clear by reducing the size and spread in diameters of the nanoparticles to a point where very little blue and violet light is scattered, and mostly passes through.

How, then, would we do this?


We believe that it is possible to reduce the sizes of the nanoparticles in aerogel by making it in zero-gravity.  In fact, we have now shown that zero-g does reduce the scattering in silica alcogels, increases the surface area, and decreases the skeletal density (the density of the solid component of the aerogel, which is close to the density of glass).  But, we still have more experiments to do.

How do we do an experiment in zero-gravity?  We do it in NASA's KC-135A, better known as, the Vomit Comet.

The KC-135A flies in parabolic arcs that allow for 23-30 seconds of free-fall per arc, during which, zero-gravity is achieved.  Who saw Apollo 13?  Raise your hands.  Okay, now who knows how they filmed it?  That's how the filmed it.

There is a ton of stuff on Aerogel.org (www.aerogel.org), at NASA (www.nasa.gov), and at Lawrence Berkeley (www.lbl.gov) about aerogels.  For good links to other aerogel sites, go to the links section of our Outreach page.  You can also email me if you want to know more.  If you are intersted in getting your own aerogel, look at our Outreach page.

For more information, please, write us at questions@zerogaerogel.com or write me, the team captain, directly at ssteiner@zerogaerogel.com.

Thanks for stopping by! Check out the Photos page for pictures of aerogel, us in zero-g, and our equipment and the Videos page for videos of us in zero-g and how to make aerogel.

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Copyright (c) 2002, 2009 Stephen Steiner.  All rights reserved.