Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method

Summary

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.

Abstract

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.

Introduction

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 t....

Protocol

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 press, and to ensure that there are no ignition sources within the vent path of the hot press. In addition, we recommend installation of a safety shield....

Results

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 .......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Tetramethylorthosilicate  (TMOS)	Sigma Aldrich	218472-500G	98% purity, CAS 681-84-5
Methanol  (MeOH)	Fisher Scientific	A412-20	Certified ACS Reagent Grade, ≥99.8%
Ammonium Hydroxide (aqueous ammonia)	Fisher Scientific	A669S212	Certified ACS Plus, about 14.8 N, 28.0-20.0 w/w%
Deionized Water	on tap in house
Flexible Graphite Sheet	Phelps Industrial Products	7500.062.3	1/16 in thick
Stainless Steel Foil	Various		0.0005 in thick, 304 Stainless Steel
High Temperature Mold Release Spray	various (for example, CRC Industrial Dry PTFE Lube)		Should be able to withstand high temperatures.