Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method

The overall goal of the following experiment is to make monolithic silica aerogels via a rapid one-step extraction process. This is achieved by first mixing the precursor chemicals and pouring the mixture into a metal mold. As a second step, the mold is sealed in a hydraulic hot press in which the chemicals are processed to produce aerogels.

During this process, the chemical precursors gel and age, the solvent within the pores of the salt gel reaches a supercritical state, and the hot press releases the solvent as a supercritical fluid and cools the mold. At the end of the process, monolithic silica aerogels are removed from the mold. The main advantage of this technique over other methods for aerogel preparation is that the entire process from mixing precursors to obtaining monolithic aerogels requires only a few hours.

We came up with the idea for this hot press based method. When an undergraduate student wanted to make aerogels for a senior project, we didn't have access to a critical point dryer, so we improvised and used a hot press. Visual demonstration of this method is critical due to the importance of the proper use of the metal mold and hot press.

If the wells are not filled with a precursor solution or the hot press is not programmed properly, the resulting aerogels will be of subpar quality. First dilute 10.1 milliliters of 14.8 molar, concentrated ammonia to 100 milliliters with deionized water to make a 1.5 molar ammonia solution. After acquiring a square stainless steel mold, wipe it with a clean damp rag to remove any surface oil or dust.

Then spray the inside of each circular well with high temperature mold release spray to ease and arrow gel removal from the mold After processing, prepare three sets of ceiling gaskets by cutting a one 16 inch thick graphite sheet and a 0.0005 inch thick stainless steel foil to the appropriate sizes. To program the hot press ceiling and extraction programs, set up a ceiling program that will be used to seal the bottom of the open mold. Next, set up the extraction program with the correct parameters for the silica aerogels.

Using the mold described to seal the mold bottom, center a graphite sheet on the lower PLA of the hot press and add a sheet of stainless steel foil. Then place the mold on top of the stainless steel foil and add another cent of gasket material on top of the mold. Following this, start the hot press ceiling program using the previously described parameters.

At this point, pipette aliquots of tetraethyl ortho silicate, totaling 17.0 milliliters into a 250 milliliter glass beaker labeled precursor solution. Then pipette aliquots of methanol totalling 55.0 milliliters into the beaker pipette 7.2 milliliters of water into the beaker, and then add 270 microliters of 1.5 molar ammonia. Seal the beaker with plastic paraffin film.

Mix the reagents to ensure that hydrolysis occurs by syndicating the precursor solution for at least five minutes if the solution is not monophasic. After five minutes of mixing, sonicate the mixture for five additional minutes. Once the mold ceiling program has finished and the hot press PLAs have opened, remove the top side gasket material and set aside.

Leave the mold as is in the hot press so that the bottom side of the mold remains sealed. Next, fill each well of the mold completely with the precursor solution. Place fresh gasket material on the top of the mold with a stainless steel foil first followed by a graphite sheet.

Following this, run the hot press extraction program. When the extraction process is complete, remove the mold and gasket material from the hot press. Remove the top gasket material from the mold and set aside.

Then gently loosen the mold from the bottom gasket material. Finally, carefully remove each aerogel from the mold one at a time by firmly pushing them through from one side with a gloved finger. Following the described procedure results in consistent batches of monolithic silica aerogels shown Here are images of typical silica aerogels made via this process.

Each aerogel takes on the shape and size of the well in the processing mold with no shrinkage. The images show that the silica aerogels are translucent. The physical properties of these aerogels are summarized here.

They are comparable to those of silica aerogels produced from similar precursor recipes using low temperature supercritical extraction. Shown here is a typical pore distribution prepared by BJH analysis of the Desorption isotherm acquired with a micro metics accelerated surface area and por symmetry system.2010. The air gels are meso porous with a peak in poor diameter near 20 nanometers Once mastered, this technique can be used to prepare silica aerogels in fewer than 15 minutes of preparation time and eight hours of processing time.

By adapting this procedure to different precursor chemistries, many types of aerogels can be fabricated. These include Illumina, titania, titania, silica, vania, silica, nickel, Illumina, cobalt, Illumina, and hydrophobic silica aerogels. Don't forget that working with tetraethyl, orthos, silicate, methanol, and the hydraulic hot press can be hazardous.

The following precautions should always be taken where safety glasses and gloves, and work in a fume hood for the precursor solution preparation, provide sufficient ventilation of the hot press and avoid wearing loose fitting clothing when performing the procedure.