From Aquagels to Aerogels
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From Aquagels to Aerogels Aerogels, a new class of low-density materials, were discovered early in this century, but they did not attract extensive research interest until the mid-1960s. In the last three décades, a great deal has been accomplished in their synthesis and characterization. Applications of thèse ultralow-density materials range from extremely efficient thermal insulation and micrometeoroid collectors in the Earth's upper atmosphère to fire-retardants and high-energy particle detectors. Aerogels are a spécial class of highly porous foams with ultrafine pore size (less than 1,000 Â) and an interconnected microstructure with characteristic dimensions less than 100 Â. The porosity of the first aerogels in the 1930s reached 98% and was pushed ail the way to 99.9% late in 1990. The December 1990 issue of the MRS Bulletin described the production methods and material characteristics of many types of low-density microcellular materials. This Historical Note will trace some of the early efforts at developing such substances and the researchers responsible for major developments. T h e first aerogels were created by S.S. Kistler at Stanford University in the early 1930s. Normally when geïatinous substances dry in air, they shrink to about a tenth of their original volume, though they still retain a porosity of about 50%. Kistler found a way to exrract fluid from wet gels under high pressure and température, drying gels without shrinking them. He named the new solid materials—whose densities ranged from 30 to 300 kg/m3— "aerogels." Initially, Kistler worked with silica to make translucent aerogel slabs. He began with a solution of sodium silicate—"water glass"—mixed with hydrochloric acid. After a day, this mixture would gel, and Kistler added water to wash out the hydrochloric acid. He called the resulting water/silica body an "aquagel." Next, he submerged the aquagel in alcohol, making the next phase, an "alcogel." Washing out the acid and replacing the water with alcohol was a time-consuming business, but it needed to be done because water would dissolve the gel structure under the high températures Kistler used in his process. Normally, when liquid évaporâtes from a gel, the pores in the gel begin to empty; the increased surface tension créâtes concave
MRS BULLETIN/MARCH 1991
menisci on the walls of the pores. With continued evaporation, the menisci retreat farther into the body of the material, and the surface tension keeps building until eventually the gel body collapses.
The first aerogels were created by S.S. Kistler at Stanford University in the early 1930s. Kistler, though, dried his gel solutions in an autoclave, where he could monitor the pressure and température. He found that increasing the température and pressure above a certain critical threshold turned the gel into a supercritical fluid, where ail surface tension vanished and, with it, the menisci that caused the collapse of the gel body. An alcogel made with methanol required a pressure of 81 bars and a température of 240°C to convert it i
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