On first sight, silica aerogels cause most people to do a double take. Observers perceive a ghost-like substance, what looks like fog that somehow has been molded into a distinct form yet fog which is encased by no evident means. Almost like solid smoke, an aerogel resembles a hologram, appearing to be a projection rather than a solid object.
Aerogels produced at LBL are 96-percent air mixed with a wispy matrix of silica. Despite their lack of substance, these materials are the world's best solid insulator, transmitting only one hundredth the heat of normal glass. See the Lab's Microstructured Materials Group website for additional information.
Aerogels are advanced materials yet also are literally next to nothing. They consist of more than 96 percent air. The remaining four percent is a wispy matrix of silica (silicon dioxide), a principal raw material for glass. Aerogels, consequently, are one of the lightest weight solids ever conceived.
Arlon Hunt was working in LBL's solar energy and energy conversion research program in 1981 when he first saw an aerogel which had been brought to the Laboratory by a visiting Swedish professor. Hunt says that immediately, he was fascinated by the material and its manifest possibilities.
"I was intrigued by how lightweight, transparent, and amazingly porous the stuff was," recalled Hunt. "Porous materials scatter light and almost always are opaque or whitish. The near transparency of the material implied extremely fine pore structure. Later, I found out just how fine."
As Hunt learned of the unique thermal, optical, and acoustical properties of aerogels, he became further intrigued. Since 1982, the Applied Science Division researcher has explored fundamental questions about the properties of aerogels and developed processes for creating thermally and optically-enhanced versions. Over the past decade, Hunt also has evaluated aerogels for many applications and developed chemical production methods suitable for commercial manufacturing.
Made of inexpensive silica, aerogels can be fabricated in slabs, pellets, or most any shape desirable and have a range of potential uses. By mass or by volume, silica aerogels are the best solid insulator ever discovered. Aerogels transmit heat only one hundredth as well as normal density glass. Sandwiched between two layers of glass, transparent compositions of aerogels make possible double-pane windows with high thermal resistance. Aerogels alone, however, could not be used as windows because the foam-like material easily crumbles into powder. Even if they were not pulverized by the impact of a bird, after the first rain they would turn to sludge and ooze down the side of the house.
Aerogels are a more efficient, lighter-weight, and less bulky form of insulation than the polyurethane foam currently used to insulate refrigerators, refrigerated vehicles, and containers. And, they have another critical advantage over foam. Foams are blown into refrigerator walls by chlorofluorocarbon (CFC) propellants, the chemical that is the chief cause of the depletion of the earth's stratospheric ozone layer. The ozone layer shields life on Earth from ultraviolet light, a cause of human skin cancer. According to the Environmental Protection Agency, 4.5 to 5 percent of the ozone shield over the United States was depleted over the last decade. Based on the current levels of ultraviolet exposure, the agency projects that more than 12 million Americans will develop skin cancer and more than 200,000 will die of the disease over the next 50 years.
Replacing chlorofluorocarbon-propelled refrigerant foams with aerogels could help reduce this toll. Exchanging refrigerant foams with aerogels reportedly would reduce CFC emissions in the U.S. by 16 million pounds per year.
Aside from their insulating properties, aerogels have other promising characteristics. Sound is impeded in its passage through an aerogel, slowed to a speed of 100 to 300 meters per second. This could be exploited in a number of ways, as for example, improving the accuracy and reducing the energy demand of the ultrasonic devices used to gauge distances in autofocus cameras and robotic systems. A layer of aerogel on a camera's ceramic piezoelectric transducer could considerably improve the efficiency with which it generates ultrasonic waves.
Aerogels also have a number of novel applications. Currently, they are components of Cerenkov radiation detectors used in high- energy physics research at CERN near Geneva, Switzerland. Another scientific application currently under consideration involves utilizing aerogels in space like a soft, spongy net to capture fast-moving micrometeroids without damaging them.
The new generation of aerogels that Hunt is creating is based on the groundwork laid down in the 1930s by Stanford University's Steven Kistler. Kistler worked with gels and in a 1932 paper published in Nature, resolved many of the basic questions about this odd form of matter. Kistler showed that a gel is an open structure composed of a matrix of solid pore walls and a liquid fill. Subsequently, he invented a way to dry a gel of its liquid contents without collapsing or shrinking it. Kistler called his new material an aerogel.
Aerogels -- the name pays tribute to the near paradoxical accomplishment of creating a hybrid between a gel and thin air -- are not known to exist in nature. Jellyfish are a non-manmade example of a gel, and a dead jellyfish washed up on the beach and baked by the sun is an illustration of what happens to a gel when it is dried in nature. Unlike aerogels which start out as a gel and do not lose volume as they dry out, a dead, desiccated jellyfish ultimately shrinks to 10 percent of its former size.
After Kistler brought aerogels into the world, they remained a forgotten phenomenon for three decades. Briefly, they reemerged in the scientific literature in the 1960s but aerogels were not fully resurrected as an object of significant scientific curiosity until the 1980s. That was when Hunt first encountered aerogels and immediately, he saw their potential. Hunt also realized that Kistler's aerogels had drawbacks. As they were at the time, aerogels were commercially worthless.
Aerogels were cloudy rather than totally transparent. Before they could be used in double-pane windows or skylights, clarity had to be improved. Aerogels were splendid insulators but in order for them to become a cost-effective alternative to existing products, they had to be made even more thermally resistant. From the standpoint of fabrication, several obstacles emerged. The extant chemistry and processing technology was too expensive and it was potentially explosive. Finally, processing required toxic compounds which presented yet another impediment. Taken together, a formidable phalanx of technological barriers prevented aerogels from making the leap from the laboratory to the consumer.
Over the past eight years, Hunt has confronted each of these obstacles. Fundamental studies he has conducted have resulted in applied advances toward resolving each of the major shortcomings. Along the way, Hunt founded a private firm which has been licensed by the Laboratory to manufacture and sell aerogels. Thermalux, L.P. is the only U.S. aerogel firm and has set up a development-stage pilot plant in Richmond, California. Currently, a Swedish company that produces aerogels for use in radiation counters, is the only other commercial aerogel manufacturer in the world.
Whether they are the commercial aerogels Thermalux is fabricating for tests and assessment by the refrigeration industry or the experimental compounds Hunt is producing in his laboratory, all aerogels start out as a gel. A gel consists of chains of linked particles or polymers permeated by a liquid. To transform a gel into an aerogel, the liquid must be removed without collapsing the solid framework. This is a tricky proposition.
The gel lattice consists of solid pore walls filled by a liquid. When liquid is evacuated from a gel, normally surface tension overwhelms the porous network, causing it to collapse. As air replaces the liquid inside each pore, surface tension inexorably pulls the sides of the pores together and the gel shrinks.
Kistler discovered the secret to drying a gel without collapsing it. He dried his gels at elevated temperatures and pressures, transforming the liquid to a supercritical state wherein there is no longer a distinction between a liquid and a gas. After cranking up the temperature and pressure to create supercritical conditions, pressure is slowly released. The supercritical fluid is vented out of the gel matrix without any surface tension effects. What remains is an aerogel that is more than 96 percent air.
Aerogels were exquisite structures but they were formulated with a standard starting compound known to damage the cornea of the eye. The toxic material, tetramethylorthosilicate (TMOS), had been introduced in the 1960s as a means of reducing the preparation time for aerogels from several weeks to a few hours. Hunt and his colleagues Rick Russo, Mike Rubin, Kevin Lofftus, Paul Berdahl, and Param Tewari experimented, looking for safer preparations and processes.
One known alternate compound favored by Russo was tetraethylorthosilicate (TEOS). However, the only aerogels ever made with TEOS were less transparent and more shrunken than the aerogels made with TMOS. The LBL group conducted a number of experiments with TEOS and focused on the base catalysis process. Ultimately, they tried ammonium fluoride, an acid catalyst. Voila, the result was a clearer aerogel and less shrinkage.
Sven Henning, one of the few other scientists in world then doing aerogel work, was visiting Hunt's group in 1984 when word arrived that his small aerogel manufacturing facility in Sweden, the world's first, had exploded. Gases escaping the autoclave aerogel drying apparatus had ignited, blown the roof off the plant, demolished the building, and injured several employees who were hospitalized.
Hunt was motivated to explore alternate aerogel drying processes.
The drying process in use at the time relied on alcohol. When the gel was ready for drying, it was loaded into a pressure vessel, alcohol was added, and heat was applied. At 280 C and 1800 pounds per square inch of pressure, the alcohol was a supercritical fluid. After reaching that plateau, pressure was slowly released and the supercritical alcohol gradually was vented from the vessel.
In addition to the evident potential for explosions, Hunt realized that this process was too costly for successful commercialization. The high pressures and temperatures required massive, expensive fabrication chambers, and beyond that, it was an energy-hungry process. Hunt looked for a substitute for alcohol. The surrogate substance had to become supercritical at a lower temperature and pressure and it had to be nonflammable.
Liquid carbon dioxide proved to be the ideal supercritical fluid. Under pressure, it becomes liquid at near room temperature. And whereas alcohol can be bomb-like, carbon dioxide is fire- quenching. Hunt's carbon-dioxide aerogel drying process has been patented.
From scratch, Hunt's aerogel process begins with the mixing of TEOS and water. To allow these two immiscible fluids to loosen up and mix, alcohol is added. The water breaks apart the TEOS, attacking the silicon bonds, and creating an intermediate ester that condenses into pure silica particles. With the assist of a catalyst, ammonium fluoride, and a solution of ammonium hydroxide to control the pH, the silica particles grow and link, forming an alcogel. A clear gel, the alcogel is sufficiently strong so that when a bottle is half filled with it and turned upside down, it will not flow.
The gel is then inserted into a pressure vessel where liquid carbon dioxide flushes out and replaces the alcohol in the gel, reducing potential fire risks in the process. Pressure is increased, the carbon dioxide becomes supercritical, and as it is slowly vented, the alcogel dries into an aerogel.
Hunt says he continues to fine-tune the drying process. "In principle," he says, "the carbon dioxide process is straight- forward, but you have to practice the process. At 600-800 pounds per square inch, there are a whole world of things going on inside that pressure vessel. It's like driving a sports car on a mountain road. You have to slow down, speed up, make adjustments in the pressure and temperature. You can crack-up in a car and you can fracture aerogels or, you can make them crack-free."
With these multiple refinements, Hunt and company had created a safer, more energy-efficient process that required less massive and costly equipment. He turned next to the problem of clarity.
Aerogels were transparent but they were not transparent enough to be used in double-paned windows. They scatter light through a natural process first described by Lord Rayleigh in the late 19th century. This phenomenon -- Rayleigh scattering -- is why the sky looks blue against the dark background of outer space and why the same sky looks yellow when viewed in the direction of a setting sun. Hunt's aerogels scatter light in a similar manner. Placed against a dark background, they appear bluish whereas against a light background, they are yellowish.
Hunt decided to tweak his recipe, altering the quantities of the five compounds that go into the gel plus the variable of temperature in an effort to increase clarity. Some 500 formulations were tested and additional variations were evaluated using a powerful experimental technique called factorial design analysis that helps pinpoint the roles that different ingredients play.
Additionally, Hunt drew on his doctoral thesis work, employing a beloved, mothballed device he had devised to measure light scattering. The scientist retrieved his trusty, old scanning polarization modulated nephelometer. The nephelometer measured several of the 16 separate elements of the light scattering matrix of various experimental formulations of aerogels, allowing Hunt to isolate and identify the structures responsible for scattering.
"My measurements revealed that the largest of the pores was responsible for the scattering and the haziness in aerogels. The cross-linked silica particles are extremely fine, 20-40 angstroms in diameter. That is smaller than the wavelengths of visible light and too small to cause scattering, which is good news. The average pore size was 200 angstroms but the largest in our TEOS gels were 3,000 angstroms. The large pores are the problem."
By filtering out impurities in the starting solution, improving the overall cleanliness of operations, and providing more uniform gelling conditions, pores larger than 500 angstroms have been eliminated. This has considerably improved the clarity of Hunt's aerogels, making them suitable for use in skylights or atrium coverings. But further research and development is necessary before aerogels are totally transparent. Until then, the promise of aerogel-insulated double-paned windows will remain just out of sight.
On the other hand, aerogels could make their debut as insulation in refrigerators within several years.
Refrigerators and freezers account for about 20 percent of residential electricity use in the U.S. Because of a vast potential for energy savings through the use of available, cost-effective technology, Congress passed the National Appliance Energy Conservation Act in 1987. Implementing the act, the Department of Energy (DOE) has announced rules requiring improved energy efficiencies in appliances with the new standards for refrigerators taking effect January 1, 1993. Only a handful of the 2,000 models now on the American market meet the 1993 standards. The DOE has pledged it will impose yet more stringent standards in the future as soon as new and affordable technology makes this practical. (The 1993 DOE standards are based on technical and economic analysis performed by Isaac Turial and Jim McMahon in LBL's Applied Science Division.)
Every year in the U.S., 300 million square feet of insulation are used in new refrigerators. Currently, the insulation of choice is polyurethane foam which is expanded into refrigerator walls by chlorofluorocarbons (CFCs). Refrigerators account for an estimated two percent of the U.S.' annual CFC usage. The U.S., through an international treaty and the Clean Air Act of 1990, has committed to halt its production of ozone-destroying CFCs by the end of the century.
Three insulating materials, all silica-based, are the leading candidates to replace foams. The competition pits aerogels against silica powder and glass beads which are sealed inside steel sheets. All three insulating systems would be sealed in a partial vacuum to increase their thermal resistance.
In a partial vacuum, aerogels outperform silica powder and glass beads. Inch-thick aerogels have the same R value (a measure of thermal resistance) as inch-thick foams. But when 90 percent of the air is evacuated from a plastic-sealed aerogel packet, the R-7 value nearly triples to R-20 per inch. To match the R-value of aerogels at this vacuum of one-tenth of an atmosphere, silica powder has to be evacuated to a few thousandths of an atmosphere. Glass beads require one-billionth of an atmosphere.
Achieving a vacuum of one-tenth of an atmosphere and sustaining it for the lifetime of a refrigerator is a piece of cake. Existing plastic vacuum packing techniques can do the job. Maintaining a vacuum of one-thousandth of an atmosphere or better is a major technological challenge.
Whereas Hunt doesn't have to worry about vacuum sealant technology, he is under pressure to reduce the cost of aerogels. About $20 of foam goes into a 1991 model refrigerator using 40- square-feet of polyurethane insulation. Insulating the same refrigerator with aerogels would cost in excess of $80. The aerogels, however, would have double the R-value of foam and in two years, the energy saved would recoup the $60 in additional costs.
Hunt has conducted fundamental studies on how heat is transmitted through aerogels in an effort to improve the material. The less aerogel necessary for a given application, the lower the cost.
Research shows that the little remaining heat which is conducted through an aerogel under vacuum is attributable to solid conduction through the silica lattice and to radiant heat transfer. The solid and the radiative component each account for about half of the heat that passes. Focusing on neutralizing the radiative element, Hunt conducted analysis which pinpointed the spectra of infrared energy which aerogels conduct. Whereas aerogels block the passage of most wavelengths, they are transparent to infrared radiation between the wavelengths of three and eight microns.
Hunt began a Cinderella-like search for an additive that would block the infrared energy in this wavelength region. The substance had to fit the job at hand and no less than a perfect fit would do. The perfectly-proportioned additive would absorb infrared radiation in the three to eight micron region, be available in small particle sizes, not interfere with the gelation or drying process, disperse uniformly without clumping, and be non-toxic and inexpensive.
Hunt tried carbon black. The slipper fit. Doped (mixed) with carbon, aerogels turn black and become better insulators. Inch- thick carbon-doped aerogels have been tested and rated at R-25 per inch.
All these years later, Hunt remains entranced by aerogels. They were the best solid insulator known when he first saw them and he has made them even more impervious to heat. Today, Hunt continues to work on improving aerogels. Currently, he is contriving to fabricate them using still less raw material so that they are yet cheaper and lighter, just a wisp of solid within a filigree of air. Beautiful as Hunt finds the new aerogels, the scientist intends to create ever more elegant aerogels, materials that consumers and manufacturers will find absolutely irresistible.
Additional Information on Aerogels: The Lab's Microstructured Materials Group