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A demonstration of an aerogel's exceptional insulating properties
An ounce of aerogel has the surface area of 10 football fields, and this is only one of its interesting properties. Aerogel has great potential in a wide range of applications that include energy-efficient insulation and windows, acoustics, gas-phase catalysis, battery technology and microelectronics. The Microstructure Materials Group of LBNL's Energy Conversion and Storage Program has been studying both the basic properties of aerogel and techniques to refine desirable qualities like transparency and insulating efficiency. Another goal of the group is making the manufacture of aerogel safer and less expensive.
As implied by the name, aerogel is mostly air. It is the lightest existing solid material, and it can have a surface area as high as 1,000 m2 per gram. Aerogel is one of the few existing materials that is both transparent and porous. It can be formed into almost any useful shape and makes an excellent insulator. Although silica aerogel is the most familiar form, metal oxides such as iron and tin oxide, organic polymers, natural gels, and carbon can all form aerogels. Discovered more than 60 years ago, they are being developed in the Microstructure Materials Group with an eye to commercial application.
Steven Kistler, at what is now the University of the Pacific, in Stockton, California, first experimented with aerogels in the 1930s. He proved experimentally that the more familiar liquid-based gels or jellies were an open solid network of cells permeated by liquid. Kistler made the first aerogel by soaking a water-based gel in alcohol to replace the water. Then he heated the alcohol and gel in a closed container to a high temperature and pressure (80 atmospheres and 240°C) and slowly depressurized the vessel. This allowed the alcohol, now a vapor, to escape, leaving an air-filled cellular matrix.
Today, researchers typically prepare metal oxide aerogels by reacting metal alkoxide with water to form an alcosol, a suspension of metal oxide particles in alcohol that link together to form an alcogel (alcohol-permeated gel). The alcogel is then dried at high temperature and pressure to produce aerogel.
The Microstructure Materials Group developed a process for producing aerogel at lower temperature and pressure by substituting liquid carbon dioxide for the alcohol in the gel under pressure, then drying the aerogel with carbon dioxide at 40°C and 70 atmospheres—considerably reducing the risk of explosion and fire compared to the high-pressure alcohol process. The new method also decreases the energy use and manufacturing time thereby lowering the costs.
Aerogel's primary building-related application is as a transparent or high- performance thermal insulator. An obvious choice for superinsulating windows, skylights, solar collector covers, and specialty windows, aerogels are transparent because their microstructure is small (average pore size is 10 to 20 nanometers) compared to the wavelength of light (400 to 700 nanometers). Their slightly hazy blue appearance is a deviation from transparency that is caused by the occasional appearance of large pores, a happenstance revealed by the Microstructure Materials Group's light scattering and transmission electron microscope studies. Thus, current research to improve aerogel clarity at LBNL is focused on decreasing the number of larger pores.
Arlon Hunt studies a sample using a light scattering instrument.
Aerogels are efficient thermal insulators as well. Silica aerogel has a higher thermal resistance than the polyurethane foams that are widely used in refrigerators, boilers and building insulation. Since these foams are blown with ozone-depleting CFCs, aerogels could be an excellent CFC-free alternative. Aerogels in a partial vacuum are even better insulators, because removing most of the air from their pores eliminates half to two-thirds of the material's thermal conductivity (the portion due to gas conduction). Silica aerogel in a 90% vacuum, which is simply and inexpensively produced, has a thermal resistance of R-20/in. Thus, a one-inch-thick aerogel window has the same thermal resistance as a window with ten double panes of glass. LBNL researchers have improved their performance to R-32/inch by adding carbon, to absorb infrared radiation in the material, another mechanism of heat transfer. Carbon-doped aerogels are perfect candidates for opaque insulators such as those used in refrigerators and pipes.
Current LBNL research is focused on developing new nanocomposite materials based on chemical vapor infiltration and reaction of gases in the aerogel. The resulting materials may have a wide range of applications in electronics, optics, and sensors. A cooperative research and development agreement with Aerojet Corp. will transfer the production methods into the commercial sector and refine the current aerogel process for large-scale production. The group is also working with Maytag on refrigerator insulation application, with General Motors and Bentley on automotive insulation, and with Boeing on acoustic and thermal insulation.
—Arlon Hunt and Allan Chen
To Aerogels web site
Energy Conversion and Storage Program
(510) 486-5370; (510) 486-4260 fax
This work is supported by DOE's Office of Industrial Technologies and the Advanced Research Project Agency's Technology Reinvestment Project.
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