The 1990’s: New Techniques and the Final Frontier
The 1990’s was the decade when people started to hear about aerogels and the incredible things they can do, largely thanks to NASA and the Internet. Along with increased publicity, aerogel synthesis evolved to include ways to speed up or even eliminate supercritical drying and to extend aerogels to metal oxides across the periodic table.
Efforts to Recommercialize Aerogels
Researchers at the University of New Mexico, lead by Prof. C. Jeffrey Brinker and Doug Smith, and researchers at other institutions became increasingly successful at eliminating the supercritical drying step used in aerogel production. In these techniques, the skeletal framework of the gel is first rendered hydrophobic (waterproof) by chemical modification. The modified gel is then dried ambiently and partially collapses. However, unlike an unmodified gel, the chemical modification to the famework prevents the framework from sticking to itself as the gel collapses. Once dry, the partially collapsed xerogel (dry gel) is then heated under vacuum where it “springs back” to ~85% of its original volume. This work lead to the founding of Nanopore, aimed at commercialization of lower-cost aerogels, which today makes extremely high R-value (50 per inch) vacuum insulating panels (VIPs). The technique was also later licensed by Cabot Corporation, which makes translucent silica aerogel granules under the tradename Nanogel. In 1992, Hoechst Corp. in Frankfurt, Germany also began a program in low cost granular aerogels.
Carbon Dioxide Drying
Hunt and Ayers at Lawrence Berkeley National Laboratory again moved towards commercialization of aerogels in the 1990’s:
The Aerojet Corp. in Sacramento, California began a cooperative project with Berkeley Lab, LLNL, and others to commercialize aerogels using the carbon dioxide substitution process in 1994. Aerojet obtained the 300 liter autoclave formerly operated by Thermalux and produced various forms of silica, resorcinol-formaldehyde, and carbon aerogels. However, this program was abandoned in 1996.
New Techniques and New Flavors
Many important innovations soon followed, largely driven by efforts at Lawrence Livermore National Laboratory
Transparent, Ultra-low Density Silica Aerogels
In 1991, Dr. Tom Tillotson at Lawrence Livermore National Laboratory developed the two-step sol-gel process for preparing ultralow density silica aerogels-the technique used to prepare the aerogels that hold the Guinness World Record for lowest density solid. The technique involves first synthesizing a silica oil by refluxing alkoxide with a substoichiometric amount of water and then diluting the oil with alcohol and adding catalyst. The technique has produced highly transparent almost completely non-blue aerogels just 0.0031 g cm-3 in density that are actually lighter than air when the air is removed from them!
Metal Oxide Aerogels and Epoxide-Assisted Gelation of Metal Salts
In 1993, Tillotson demonstrated the ability to make aerogels of any of the lanthanide oxides using propylene oxide and lanthanide chloride salts. This important technique-epoxide-assisted gelation of metal salts-soon enabled production of a large array of transition, main group, lanthanide, and actinide metal oxide aerogels, including many beautifully colored aerogels and catalytically active aerogels. Dr. Alex Gash at Livermore expanded upon this work starting in 1999, digging down at the mechanism of how epoxide-assisted gelation works and finding that a slow, gradual pH rise facilitated by irreversible ring opening of the epoxide enables gelation. Dr. Ted Baumann at Livermore later demonstrated the applicability of the technique to alumina and crystalline tin oxide aerogels. Because of Gash’s efforts in demonstrating the mechanism and expanding the technique to a wide array of metal oxides, the technique is commonly referred to as the “Gash prep”.
Rapid Supercritical Extraction
In 1996, Dr. John Poco, Dr. Paul Coronado, Dr. Rick Pekala, and Dr. Larry Hrubesh (say ROObish) developed a technique for speeding up aerogel production. A sol mixture, not yet gelled, is injected into a metal mold with a lid that is closed but not sealed. The mold is then placed inside a high-temperature supercritical dryer partially filled with methanol or ethanol. The vessel is heated rapidly to supercritical conditions, during which time the sol in the mold will gel from an increase in temperature. Together the mechanical restraint from the mold (which prevents positive strains from the liquid expanding in volume) and the rapid heating (which minimizes negative strains from the gel contracting as it sets) minimize stress throughout the monolith, enabling much faster production with damaging the aerogel. Once supercritical, the fluid in the gel and outside the mold can then freely exchange through the mold’s lid and the system can be isothermally depressurized to produce an aerogel, all in about 4 hours start to finish.
The Final Frontier: Aerogel and Space
On the applications side, NASA used aerogel on three missions that really began to bring attention to aerogels to the world: Mars Pathfinder, the Mars Exploration Rovers, and Stardust. Around the same time, experiments in making aerogels in zero-gravity were undertaken towards preparing transparent, non-blue aerogel for window insulation.
Mars Pathfinder and Mars Exploration Rovers
First in 1997, NASA employed aerogel insulation to protect the electronics of the Sojourner rover in exploring the Red Planet as part of the Mars Pathfinder mission. The success of aerogel in the mission then merited its use again for the twin Mars Exploration Rovers Spirit and Opportunity, which used aerogel to insulate the battery, electronics, and computer in the chassis.
As early as the late 1980’s, Dr. Peter Tsou at NASA’s Jet Propulsion had set forth on a mission to understand the formation of the solar system and was interested in analyzing the composition of comets as a way to gain insights into the composition of the early solar system. To do this, he wanted to execute a sample return mission to bring dust back from a comet, but need some sort of medium that could softly capture high-velocity particles ejected from the comet but at the same time be transparent so that the captured particles could be later extracted. Tsou circumstantially came across silica aerogel and identified it as a possible medium. Tsou quickly picked up the chemistry he needed to make aerogels and oversaw the development of a novel gradient-density aerogel material that could both softly accept impacts and slow down the particles within a reasonable thickness for the spacecraft design. The mission was eventually called “Stardust” and on February 7, 1999 blasted off for comet Wild 2 (say Vilt 2). On January 2, 2004 Stardust successfully engaged Wild 2, captured dust from the comet, and headed back to Earth. On January 15, 2006 the probe reentered Earth’s atmosphere and was successfully captured. Particles from the comet were successfully extracted from the aerogel tiles and analyzed thoroughly. A special issue of the journal Science featured discoveries from the mission later that year. An interesting spin-off of this project was the development of a method for precisely machining fragile aerogels using a high-speed vibrating glass needle, which was developed at Johnson Space Center.
Zero-Gravity Aerogel Production
In the 1990’s, Dr. Arlon Hunt et al. at Lawrence Berkeley National Laboratory became interested in developing ways of eliminating Rayleigh scattering from silica aerogels to develop transparent, non-blue window insulation. Hunt hypothesized that non-uniformity in the size of particles that make up the aerogel framework was the origin of the of the Rayleigh scattering in aerogels that gives them their characteristic blue cast. It had been shown previously that buoyancy-driven fluid flow affects the formation of micro- and nanostructures in a gel. Prior to gelation, buoyancy-induced eddies and sedimentation significantly perturb the substructures that make up the gel’s framework. These perturbations may then become “frozen” as imperfections in the gel’s structure. In conducting a space-flight experiment on the growth of Stöber particles (unagglomerated nanosized particles suspended in a liquid), Doug Smith along with Hunt and others found that chemistry intended to produce such particles actually resulted in low-density gels in space, while samples prepared in 1 G remained in suspension. Thus it appeared that gravity may have an effect on the formation of an aerogel’s nanostructured framework, and that forming aerogels in microgravity might enable production of aerogels that don’t scatter visible light.
Hunt proceed to conduct a series of light scattering experiments aboard NASA’s KC-135A aircraft (affectionately referred to as the “Vomit Comet”) towards understanding particle growth in solutions used to prepare silica gels used for aerogel production. This was followed by an experiment in 1998 flown on STS-95 (notably executed by John Glenn) to actually prepare gels suitable for silica aerogel production, however due to an experimental error the results of this experiment were inconclusive.
Prior to 2001, silica gels suitable for transparent aerogel production could only reliably be formed on long-duration spaceflights, as the time required for the gels to form (the “gel time”) greatly exceeded the 18-23 seconds of microgravity readily attainable by parabolic flight. Between 2001 and 2004, Stephen Steiner et al. at the University of Wisconsin conducted a series of experiments demonstrating a rapid gelation method for preparing gels suitable for silica aerogel production within the microgravity time afforded by a single parabola on the KC-135A. In 2003, the group reported aerogels derived from gels prepared in microgravity exhibited higher surface areas and lower skeletal densities when compared to aerogels derived from gels prepared in 1 G.
The results of these experiments provided new insights into the fundamentals of the formation of sol-gel-derived nanomaterials. As far as aerogel windows go, though, Tillotson’s ultra-low density two-step sol-gel process works pretty well, although the materials are really too fragile for practical use. However, a new class of mechanically robust “x-aerogels” would emerge in the next decade, making possible use of aerogels as structural materials.
A New Century, New Opportunities
With the technological developments and publicity generated in the 1990’s, aerogel seemed poised to become a rapidly growing field. Despite this, enthusiasm for aerogels began to wane by the end of the 1990’s, however the potential of aerogel had just begun to emerge.