Name: preparing silica aerogel monoliths via a rapid supercritical extraction method
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Description: Watch this Scientific Journal Video about Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method at JoVE.com

The authors thank undergraduate students Lutao Xie, for physical characterization of the aerogel materials, and Aude Bechu, for testing the draft procedure. We are grateful to the Union College Engineering Laboratory for machining the stainless steel mold. The Union College Aerogel Laboratory has been funded by grants from the National Science Foundation (NSF MRI CTS-0216153, NSF RUI CHE-0514527, NSF MRI CMMI-0722842, NSF RUI CHE-0847901, NSF RUI DMR-1206631, and NSF MRI CBET-1228851). This material is based upon work supported by the NSF under Grant No. CHE-0847901.

Safety Considerations: Safety glasses or goggles should be worn at all times during the preparative work with solutions and the hydraulic hot press. Laboratory gloves should be worn when preparing the chemical reagent solution and when pouring the solution into the mold in the hot press. TMOS, methanol and concentrated ammonia, and solutions containing these reagents, must be handled within a fume hood. The supercritical extraction process releases hot methanol, so it is necessary both to vent the hydraulic hot pr...

<ol>
	<li>Aegerter, M. A., Leventis, N., Koebel, M. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Aerogels Handbook. , (2011).</li><li>Gurav, J. L., Jung, I. -. K., Park, H. -. H., Kang, E. S., Nadargi, D. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silica+aerogel:+Synthesis+and+applications.">Silica aerogel: Synthesis and applications.</a> J. Nanomater. , .</li><li>Kistler, S. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coherent+expanded+aerogels.">Coherent expanded aerogels.</a> J. Phys. 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The RSCE method produces consistent batches of monolithic silica aerogels using an automated and simple process. The method as presented here requires an eight-hour processing step. It is possible to speed up the heating and cooling steps to make monolithic aerogels in as little as 3 hr22; however, when an 8 hr procedure is employed, more consistent batches of aerogel monoliths result. Small variations in the process parameters do not affect the physical properties of the resulting aerogels, indicating that th...

Silica aerogel materials have low density, high surface area, and low thermal and electrical conductivity combined with a nanoporous structure with excellent optical properties. The combination of these properties in one material makes aerogels attractive in a large number of applications1. In a recent review article, Gurav et al. describe in detail the current and potential applications of silica aerogel materials, both in scientific research and in development of industrial products2. For example, silica aerogels have been used as absorbents, as sensors, in low-dielectric materials, as storage media for fuels, and for a wide array of thermal insulating applications2.
Aerogels are typically fabricated using a two-step process. The first step involves mixing appropriate chemical precursors, which then undergo condensation and hydrolysis reactions to form a wet gel. To prepare silica gels, the hydrolysis reactions occur between water and a silica-containing precursor, in this case tetramethylorthosilicate (TMOS, Si(OCH3)4), in the presence of acid or base catalyst. 
Si(OCH3)4 + H2O <img alt="arrow" fo:content-width=".2in" src="/files/ftp_upload/51421/arrow.jpg" width="20" /> Si(OCH3)4-n(OH)n + n CH3OH
TMOS is insoluble in water. In order to facilitate hydrolysis, it is necessary to include another solvent, in this case methanol (MeOH, CH3OH), and to stir or sonicate the mixture. Base-catalyzed polycondensation reactions then occur between the hydrolyzed silica species:
R3SiOH + HOSiR3 <img alt="arrow" fo:content-width=".2in" src="/files/ftp_upload/51421/arrow.jpg" width="20" /> R3Si-O-SiR3 + H2O
R3SiOH + CH3OSiR3 <img alt="arrow" fo:content-width=".2in" src="/files/ftp_upload/51421/arrow.jpg" width="20" /> R3Si-O-SiR3 + CH3OH
The polycondensation reactions result in the formation of a wet gel, comprised of a porous SiO2 solid matrix, in which the pores are filled with the solvent byproducts of the reaction, in this case methanol and water. The second step involves drying the wet gel to form an aerogel: removing the solvent from the pores without altering the solid matrix. The drying process is critically important to the formation of the aerogel. If not carried out correctly the fragile nanostructure collapses and a xerogel is formed as illustrated schematically in Figure 1.
There are three basic methods for drying sol-gel materials to produce aerogels: supercritical extraction, freeze drying and ambient pressure drying. The supercritical extraction methods avoid crossing the liquid-vapor phase line so that surface tension effects do not cause the nanostructure of the gel to collapse. Supercritical extraction methods can be performed at high temperature (250-300 &#176;C) and pressure with direct extraction of the alcohol solvent byproduct of the condensation and hydrolysis reactions3-7. Alternatively, one can perform a set of exchanges and replace the alcohol solvent with liquid carbon dioxide, which has a low supercritical temperature (~31 &#176;C). The extraction can then be performed at relatively low temperature8,9, albeit at high pressure. Freeze drying methods10,11 first freeze the wet gel at low temperature and then allow the solvent to sublimate directly to a vapor form, again avoiding crossing the liquid-vapor phase line. The ambient pressure method uses surfactants to reduce the surface tension effects or polymers to strengthen the nanostructure, followed by drying of the wet gel at ambient pressure12-16.
The Union College Rapid Supercritical Extraction (RSCE) process is a one-step (precursor to aerogel) method17-19. The method employs high-temperature supercritical extraction, which allows fabrication of monolithic aerogels in hours, rather than the days to weeks required by other methods. The method utilizes a confined metal mold and a programmable hydraulic hot press. Chemical precursors are mixed and poured directly into the mold, which is placed between the platens of the hydraulic hot press. The hot press is programmed to close and apply a restraining force to seal the mold. The hot press then heats the mold at a specified rate to a temperature, Thigh, above the critical temperature of the solvent (see Figure 2 for a plot of the process). During the heatup period the chemicals react to form a gel and the gel strengthens and ages. As the mold is heated the pressure also rises, eventually reaching a supercritical pressure. Upon reaching Thigh, the hot press dwells at a fixed state while the system equilibrates. Next the hot press force is decreased and the supercritical fluid escapes, leaving behind a hot aerogel. The press then cools the mold and its contents to room temperature. At the end of the process (which can take 3-8 hr) the press opens and monolithic aerogels are removed from the mold.
This RSCE method offers significant advantages over other aerogel fabrication methods. It is fast (&#60;8 hr total) and not very labor intensive, typically requiring only 15-20 min preparation time followed by 3-8 hr processing time. It does not require solvent exchanges, which means that relatively little solvent waste is generated during the process.
In the section that follows, we describe a protocol for preparing a set of cylindrical silica aerogel monoliths via the Union RSCE method from a precursor mixture comprised of TMOS, methanol, and water with aqueous ammonia employed as the catalyst for the hydrolysis and polycondensation reactions (with a TMOS:MeOH:H2O:NH3 molar ratio of 1.0:12:3.6:3.5 x 10-3). We note that the Union RSCE method can be used to prepare aerogels of various different sizes and shapes, depending on the metal mold and hydraulic hot press employed. This RSCE method has also been used to prepare other types of aerogels (titania, alumina, etc.) from different precursor recipes20.

<table border="0" cellpadding="0" cellspacing="0" width="1344">
	<colgroup>
		<col />
		<col span="2" />
		<col />
	</colgroup>
	<tbody>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Tetramethylorthosilicate&#160; (TMOS)</td>
			<td style="width:252px;">Sigma Aldrich&#160;&#160; www.sigmaaldrich.com</td>
			<td style="width:252px;">218472-500G</td>
			<td style="width:336px;">98% purity, CAS 681-84-5&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Methanol&#160; (MeOH)</td>
			<td style="width:252px;">Fisher Scientific&#160; www.fishersci.com</td>
			<td style="width:252px;">A412-20</td>
			<td style="width:336px;">Certified ACS Reagent Grade, &#8805;99.8%</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Ammonium Hydroxide (aqueous ammonia)</td>
			<td style="width:252px;">Fisher Scientific&#160; www.fishersci.com</td>
			<td style="width:252px;">A669S212</td>
			<td style="width:336px;">Certified ACS Plus, about 14.8N, 28.0-20.0 w/w%</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Deionized Water</td>
			<td style="width:252px;">On tap in house</td>
			<td style="width:252px;"></td>
			<td style="width:336px;"></td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Flexible Graphite Sheet</td>
			<td style="width:252px;">Phelps Industrial Products</td>
			<td style="width:252px;">7500.062.3</td>
			<td style="width:336px;">1/16&#34; thick</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Stainless Steel Foil</td>
			<td style="width:252px;">Various</td>
			<td style="width:252px;"></td>
			<td style="width:336px;">.0005&#34; thick, 304 Stainless Steel</td>
		</tr>
		<tr height="110">
			<td height="110" style="height:111px;width:504px;">High Temperature Mold Release Spray</td>
			<td style="width:252px;">Various&#160; (for example, CRC Industrial Dry PTFE Lube)</td>
			<td style="width:252px;"></td>
			<td style="width:336px;">Should be able to withstand high temperatures.</td>
		</tr>
	</tbody>
</table>

preparing silica aerogel monoliths via a rapid supercritical extraction method

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The procedure requires 15-20 min of preparation time, during which a liquid precursor mixture is prepared and poured into wells of a metal mold that is placed between the platens of a hydraulic hot press, followed by several hours of processing within the hot press. The precursor solution consists of a 1.0:12.0:3.6:3.5 x 10-3 molar ratio of tetramethylorthosilicate (TMOS):methanol:water:ammonia. In each well of the mold, a porous silica sol-gel matrix forms. As the temperature of the mold and its contents is increased, the pressure within the mold rises. After the temperature/pressure conditions surpass the supercritical point for the solvent within the pores of the matrix (in this case, a methanol/water mixture), the supercritical fluid is released, and monolithic aerogel remains within the wells of the mold. With the mold used in this procedure, cylindrical monoliths of 2.2 cm diameter and 1.9 cm height are produced. Aerogels formed by this rapid method have comparable properties (low bulk and skeletal density, high surface area, mesoporous morphology) to those prepared by other methods that involve either additional reaction steps or solvent extractions (lengthier processes that generate more chemical waste).The rapid supercritical extraction method can also be applied to the fabrication of aerogels based on other precursor recipes.

Following the procedure described here results in consistent batches of monolithic silica aerogels. Figure 4 shows 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 in Table 4. They are comparable to those of silica aerogels produced from similar ...

Watch this Scientific Journal Video about Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method at JoVE.com

Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method

This article describes a rapid supercritical extraction method for fabricating silica aerogels. By utilizing a confined mold and hydraulic hot press, monolithic aerogels can be made in eight hours or less.

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The ...

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