What is aerogel?
It's a cool space age material formed by driving water out of a gel at a critical heat/pressure point so that the solid structure remains intact. Silica is the most common type. Aerogels are over 99% air, and so have many unusual properties. They are the world's least dense solid, the best thermal insulator, and surprisingly strong (though not very tough). These articles are not
Technical - Berkeley Labs
The formation of aerogels, in general, involves two major steps, the formation of a wet gel, and the drying of the wet gel to form an aerogel. Originally, wet gels were made by the aqueous condensation of sodium silicate, or a similar material. While this process worked well, the reaction formed salts within the gel that needed to be removed by many repetitive washings (a long, laborious procedure). With the rapid development of sol-gel chemistry over the last few decades, the vast majority of silica aerogels prepared today utilize silicon alkoxide precursors. The most common of these are tetramethyl orthosilicate (TMOS, Si(OCH3)4), and tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4). However, many other alkoxides, containing various organic functional groups, can be used to impart different properties to the gel. Alkoxide-based sol-gel chemistry avoids the formation of undesirable salt by-products, and allows a much greater degree of control over the final product. The balanced chemical equation for the formation of a silica gel from TEOS is:
Si(OCH2CH3)4 (liq.) + 2H2O (liq.) = SiO2 (solid) + 4HOCH2CH3 (liq.)
The above reaction is typically performed in ethanol, with the final density of the aerogel dependent on the concentration of silicon alkoxide monomers in the solution. Note that the stoichiometry of the reaction requires two moles of water per mole of TEOS. In practice, this amount of water leads to incomplete reaction and weak, cloudy aerogels. Most aerogel recipes, therefore, use a higher water ratio than is required by the balanced equation (anywhere from 4-30 equivalents). Catalysts
The kinetics of the above reaction are impracticably slow at room temperature, often requiring several days to reach completion. For this reason, acid or base catalysts are added to the formulation. The amount and type of catalyst used play key roles in the microstructural, physical and optical properties of the final aerogel product.
Acid catalysts can be any protic acid, such as HCl. Basic catalysis usually uses ammonia, or, more commonly, ammonia and ammonium fluoride. Aerogels prepared with acid catalysts often show more shrinkage during supercritical drying and may be less transparent than base catalyzed aerogels. The microstructural effects of various catalysts are harder to describe accurately, as the substructure of the primary particles of aerogels can be difficult to image with electron microscopy. All show small (2-5 nm diameter) particles that are generally spherical or egg-shaped. With acid catalysis, however, these particles may appear "less solid" (looking something like a ball of string) than those in base-catalyzed gels.
As condensation reactions progress the sol will set into a rigid gel. At this point, the gel is usually removed from its mold. However, the gel must be kept covered by alcohol to prevent evaporation of the liquid contained in the pores of the gel. Evaporation causes severe damage to the gel and will lead to poor quality aerogels Single-Step vs. Two-Step Aerogels
Typical acid or base catalyzed TEOS gels are often classified as "single-step" gels, referring to the "one-pot" nature of this reaction. A more recently developed approach uses pre-polymerized TEOS as the silica source. Pre-polymerized TEOS is prepared by heating an ethanol solution of TEOS with a sub-stoichiometric amount of water and an acid catalyst. The solvent is removed by distillation, leaving a viscous fluid containing higher molecular weight silicon alkoxy-oxides. This material is redissolved in ethanol and reacted with additional water under basic conditions until gelation occurs. Gels prepared in this way are known as "two-step" acid-base catalyzed gels. Pre-polymerized TEOS is available commercially in the U.S. from Silbond Corp. (Silbond H-5).
These slightly different processing conditions impart subtle, but important changes to the final aerogel product. Single-step base catalyzed aerogels are typically mechanically stronger, but more brittle, than two-step aerogels. While two-step aerogels have a smaller and narrower pore size distribution and are often optically clearer than single-step aerogels. Aging and Soaking
When a sol reaches the gel point, it is often assumed that the hydrolysis and condensation reactions of the silicon alkoxide reactant are complete. This is far from the case. The gel point simply represents the time when the polymerizing silica species span the container containing the sol. At this point the silica backbone of the gel contains a significant number of unreacted alkoxide groups. In fact, hydrolysis and condensation can continue for several times the time needed for gelation. Failure to realize, and to accommodate this fact is one of the most common mistakes made in preparing silica aerogels. The solution is simple--patience. Sufficient time must be given for the strengthening of the silica network. This can be enhanced by controlling the pH and water content of the covering solution. Common aging procedures for base catalyzed gels typically involve soaking the gel in an alcohol/water mixture of equal proportions to the original sol at a pH of 8-9 (ammonia). The gels are best left undisturbed in this solution for up to 48 hours.
This step, and all subsequent processing steps, are diffusion controlled. That is, transport of material into, and out of, the gel is unaffected by convection or mixing (due to the solid silica network). Diffusion, in turn, is affected by the thickness of the gel. In short, the time required for each processing step increases dramatically as the thickness of the gel increases. This limits the practical production of aerogels to 1-2 cm-thick pieces.
After aging the gel, all water still contained within its pores must be removed prior to drying. This is simply accomplished by soaking the gel in pure alcohol several times until all the water is removed. Again, the length of time required for this process is dependent on the thickness of the gel. Any water left in the gel will not be removed by supercritical drying, and will lead to an opaque, white, and very dense aerogel. Supercritical Drying Graph of temperature in degrees centigrade and pressure in psi versus time in hours, for alcohol drying and carbon dioxide drying.
Process conditions for both the carbon dioxide substitution/drying process and the alcohol drying process.
The final, and most important, process in making silica aerogels is supercritical drying. This is where the liquid within the gel is removed, leaving only the linked silica network. The process can be performed by venting the ethanol above its critical point (high temperature-very dangerous) or by prior solvent exchange with CO2followed by supercritical venting (lower temperatures-less dangerous) It is imperative that this process only be performed in an autoclave specially designed for this purpose (small autoclaves used by electron microscopists to prepare biological samples are acceptable for CO2 drying). The process is as follows. The alcogels are placed in the autoclave (which has been filled with ethanol). The system is pressurized to at least 750-850 psi with CO2 and cooled to 5-10 degrees C. Liquid CO2 is then flushed through the vessel until all the ethanol has been removed from the vessel and from within the gels. When the gels are ethanol-free the vessel is heated to a temperature above the critical temperature of CO2 (31 degrees C). As the vessel is heated the pressure of the system rises. CO2 is carefully released to maintain a pressure slightly above the critical pressure of CO2 (1050 psi). The system is held at these conditions for a short time, followed by the slow, controlled release of CO2 to ambient pressure. As with previous steps, the length of time required for this process is dependent on the thickness of the gels. The process may last anywhere from 12 hours to 6 days.
At this point the vessel can be opened and the aerogels admired for their intrinsic beauty. Typical Recipes Single-Step Base Catalyzed Silica Aerogel
This will produce an aerogel with a density of approx. 0.08 g/cm3. The gel time should be 60-120 minutes, depending on temperature.
1. Mix two solutions: 1. Silica solution containing 50 mL of TEOS, 40 mL of ethanol 2. Catalyst solution containing 35 mL of ethanol, 70 mL of water, 0.275 mL of 30% aqueous ammonia, and 1.21 mL of 0.5 M ammonium fluoride. 2. Slowly add the catalyst solution to the silica solution with stirring. 3. Pour the mixture into an appropriate mold until gelation. 4. Process as described above.
Two-Step Acid-Base Catalyzed Silica Aerogel
This will produce an aerogel with a density of approx. 0.08 g/cm3. The gel time should be 30-90 minutes, depending on temperature.
1. Mix two solutions: 1. Silica solution containing 50 mL of precondensed silica (Silbond H-5, or equivalent), 50mL of ethanol 2. Catalyst solution containing 35 mL of ethanol, 75 mL of water, and 0.35 mL of 30% aqueous ammonia. 2. Slowly add the catalyst solution to the silica solution with stirring. 3. Pour the mixture into an appropriate mold until gelation. 4. Process as described above.
Useful tips - 10 year old makes aerogels for science fair
Ten Year Old Child Produces Homemade Aerogels
The most difficult part of the process involves “supercritical drying.” If gels are dried normally the surface tension of the gelling fluid pulls on the solid matrix in the gel and causes it to shrink. Jello left to dry in the air, for example, becomes a thin and hard crust. Aerogels, on the other hand, have all the liquid removed without collapsing the matrix. A little known property of most fluids allows this to happen. For every fluid there is a combination of pressure and temperature that changes it into a supercritical form. In that condition it becomes a fluid that acts like a gas, so it no longer has any surface tension. Under this condition it can be evaporated out of the gel with very little shrinkage.
Older methods of creating aerogels required bringing alcohol to its supercritical temperature of about 275 degrees Celsius (527 F.) and pressure of around 1800 psi. Those conditions are very explosive, and even highly sophisticated laboratories around the world have blown up making aerogels this way.
Fortunately a newer method allows for supercritical drying with liquid carbon dioxide. This gas is not explosive, the pressure required is still a hefty 1050 psi, but the maximum temperature is only 31 degrees Celsius (87.8 F.). This makes a properly designed project totally safe for the amateur experimenter. Nevertheless it seems not to have been previously tried because of the need to carefully control and maintain both pressures and temperatures for 10 to over 100 hours, depending on the thickness of the aerogels being made.
After studying the problem for a full year, William found a simple but elegant way to substitute human attention for expensive laboratory equipment. He would use the thermal properties of water to control the temperature and a hand-operated needle valve to control the pressure and liquid flow.
With the budgetary constraints facing a fifth-grader, William settled on modest ambitions. His homemade aerogels would not be large. For a chamber he settled on a 6" nipple of 2" schedule 80 steel pipe. Schedule 80 pipes are guaranteed to hold 3000 psi, giving William a 200 percent safety margin. The cylinder was closed with 2" to 1/4" schedule 80 adapters that threaded on. This gave a maximum size for the aerogels of about 1-1/2" by 5-1/2", and (for time constraints) about 1 centimeter thick.
Making the “wet” gel
The key to starting a silica aerogel is to find a source of the silicon base. The LBL recommends Silbond H-5, and William found that a phone call to the company netted him a free sample quart. Have an adult make the call, because H-5 is a hazardous chemical, and they need to know it will be safely handled. You also will need absolutely pure ethyl alcohol, and this is the hardest ingredient to find. In William’s case, he finally obtained half a gallon from the local university science department.
Follow the Silbond-based recipe found at http://eande.lbl.gov/ECS/aerogels/ There are a few things they don’t tell you. As soon as the gel has skinned over, cover it with more of the alcohol/water mixture. Let the gel set (covered this way) for about 2 day. This will insure that the gel matrix is fully formed and strong. You will have to get the finished gel out of the mold. This is a problem. Surface adhesion and air pressure will conspire to keep it in. Use a mold without sharp corners. If you can, get a surface with a non-stick coating such as teflon. (Don’t try coating your surface with anything that dissolves in alcohol!) A fine wire, such as a straight pin, pushed down along the edge and moved along the sidecan introduce air and free the gel. Plan to break some gels before you get the technique, and even then up to half of them. If you can make your own molds, consider a rubber plug in the middle of the bottom. You can carefully remove it when your gels are done and slowly introduce air pressure into the hole. Most gels will come out intact.
During this whole process it is vital to keep the gels covered in the alcohol/water bath to keep the surfaces from drying out. Once the gels are removed from the molds they need to be kept separated and protected for the drying process They can be wrapped in filter paper, placed in chambers made from screen, or otherwise protected in a porous medium.
All the water must now be removed from the gel. This is a slow process, because the whole process is done by diffusion. The water in the gel must have time to mix with the alcohol and slowly ooze out of the matrix around it. For gels one centimeter thick this takes about four hours. The process is one of concentration reduced in steps. This means there must be several baths, each one taking out a little more water. Eventually the alcohol will be pure even after soaking. William used 6 baths with twice the volume of his aerogels in each. The gels soaked four hours or more in each bath (the more being due to sleep time). A rule of thumb gleaned from the University of Virginia Aerogel research lab: soak more times than needed just to be sure. The time you invest in making gels will be wasted if they still have water in them.
The Drying Chamber
The foregoing represents the easy part of aerogel making. Now we come to the most difficult phase, removing all liquid from the gel. This process involves bringing the liquid within an aerogel to its “supercritical” condition. For every liquid there is a temperature and pressure combination above which the liquid starts acting like a gas. The important fact is that it no longer has surface tension and no longer adheres to surfaces. For many years this was accomplished by raising the system temperature and pressure high enough to bring the alcohol to supercritical conditions. Under the required temperatures alcohol self-ignites, and many laboratories and production facilities exploded.
Finally someone hit on the plan of replacing the alcohol with liquid CO2. Even then the required pressure is over 1000 psi, though the temperatures are the kind we encounter every day in our homes. To contain that high pressure William turned to schedule 80 steel pipe, rated at 3000 psi. His chamber was constructed out of a 2" ID nipple six inches long. This was capped at both ends with a 2" to 1/4" reducing adapter. His chamber was designed to use in a vertical position so the liquid CO2 could be introduced at the bottom and the alcohol removed at the top since the specific gravity of alcohol is half that of liquid CO2. This is helpful at the first exchange of fluids because most of the soaking alcohol bath can be bled away as the liquid CO2 is introduced.
1/4" schedule 80 steel pipe and fittings were used to bring both ends above the chamber level and then horizontal from the chamber orientation. On the output side he installed a needle valve to control the drain of liquid CO2 from the system. On the input side he used a 2000 psi pressure gauge and a ball valve to seal the chamber system. Beyond that is the standard coupling of brass that hooks up to the liquid CO2 tank. Make sure the tank is equipped with a siphon or no liquid CO2 will ever get into your system.
When putting the small diameter fittings together, double wrap the ends with teflon tape. For the larger pipes William found that four layers worked better. In tightening the pipes avoid clamping the nipples. Use the wrenches on the joints at both ends. This is especially important with the larger chamber piping. We damaged one nipple on one end by using a pipe wrench on the nipple itself. It would then never completely seal at the 1050 psi. We found that putting the wrenches on the adapters and tightening so that both were as tight as we could get them worked best. Then loosen them. One end will loosen first. This will be where you insert the wet gels. Remove the old teflon wrap and put on four fresh layers before putting the gels in.
The Drying Process
When your pressure system is complete and ready, as described above, set the system erect and fill the chamber with ethyl alcohol. Now put the wrapped and separated aerogels into the alcohol bath. Keeping the system erect (in a vise, e.g.), screw on and tighten the top nipple. William did that with the rest of the pressure fittings attached because we pressure tested the assembly beforehand. Once the chamber is tightly sealed the rest of the assembly can be done.
The drying process involves carefully controlling both temperatures and pressures for about a ten hour period. The temperatures vary from maintaining a low of between 41 and 50 degrees Fahrenheit for about 10 hours, to a high of above 90 degrees across an hour or two. The easiest way to accomplish this is to put the whole chamber assembly in a water bath. William used a plastic bucket from the local dollar store. His thermometer was a rapid action cooking thermometer from the grocery market. William needed a large bag of ice to cool the system down to 41 degrees F. I was easy to raise it up to and above the needed 90 degrees during the supercritical stage. Once the system is set up it can be pressurized by introducing CO2 at an ambient temperature of about 70 degrees. This will bring pressure up to about 850 psi. Once the CO2 has been turned on and the pressure is over 800 psi the temperature can be lowered to about 45 degrees F. Fr the next 10 hours you have to exchange the CO2 several times. This can be done in two ways. Either let the CO2 escape in a trickle or in gushes periodically. Professionals told me they prefer the trickle method. Alcohol will be seen dripping from the end of the outlet pipe. This procedure should be continued well beyond the point where no more alcohol drips from the pipe. William gave it an extra three hours. Err on the ie of safety. CO2 is relatively cheap, and at this point you have a sizable investment, and leaving alcohol in the gels will greatly reduce their quality. During this entire period the temperature and pressure must be carefully maintained.
Once you are sure all the alcohol is gone it is time for supercritical drying. Close off the valve supplying liquid CO2. Over a period of 10 to 30 minutes slowly raise the water temperature to about 95 degrees F. This will cause the pressure to rise correspondingly to about 1200 psi. After a few minutes at this pressure you can begin to let the CO2 trickle out. Maintain the pressure as long as you can. If you keep the temperature properly high the pressure should stay up. When it began to drop William increased the temperature to bring it back up while maintaining the trickle of CO2 gas out of the system. After about 45 minutes the pressure would not raise with hot water, and soon dropped down to zero.
We opened the cylinder and found William’s aerogels. They were curved. A scientist from th Berkeley National Laboratory said this was because we had not taken the gels out of the mold prior to drying them. That fact was something not explained in the Berkeley Lab recipe, and we learned it for future experiments.