26-05-2012, 05:10 PM
Aerogel
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INTRODUCTION
Aerogel is defined as a group of extremely light and porous solid materials. Silica-based aerogels are among the lightest ones, can be less than four times as dense as dry air, and some are nearly transparent, its nickname is “solid smoke” or “frozen smoke”.
Since this definition is good for most porous materials, the term aerogels became reserved for the porous gels obtained by removing solvent from highly swollen gels at the conditions that no or minimal collapse occurs, which causes the liquid in the gel to become supercritical (in a state between a liquid and a gas) and lose its surface tension. The result is an open porous material with a backbone morphology that can be modeled in terms of three dimensionally interconnected strings of nanoscopic pearls. The length scale of both the “pearls” as well as the interconnected voids can be independently tailored over a wide range, i.e. from a few nanometers to several microns.One of the striking advantages of aerogels compared to other porous materials is that both porosity and inner surface area can be tuned independently. Porosities of up to 99.9 % are achievable; when microporosity is present, the specific surface area can exceed 1500 m2/g. Because of their unique properties, i.e., large surface area, very small pores and very low bulk density, aerogels are potentially important candidates for a wide range of applications.
HISTORY
Steven. S. Kistler of the College of the Pacific in Stockton, California set out to prove that a "gel" contained a continuous solid network of the same size and shape as the wet gel. It is believed that Kistler's interest was stimulated by a friendly wager with fellow worker Charles Learned. They competed to see if one of them could replace the liquid inside a jelly jar with gas without causing any shrinkage. Kistler won the bet, and published his findings in a 1931 edition of the journal Nature.
As is often the case, the obvious route included many obstacles. If a wet gel were simply allowed to dry on its own, the gel would shrink, often to a fraction of its original size. This shrinkage was frequently accompanied by severe cracking of the gel. Kistler surmised, correctly, that the solid component of the gel was microporous, and that the liquid-vapor interface of the evaporating liquid exerted strong surface tension forces that collapsed the pore structure. Kistler then discovered the key aspect of aerogel production:
Aerogels had been largely forgotten when, in the late 1970s, the French government approached Stanislaus Teichner at Universite Claud Bernard, Lyon seeking a method for storing oxygen and rocket fuels in porous materials. There is a legend passed on between researchers in the aerogel community concerning what happened next. Teichner assigned one of his graduate students the task of preparing and studying aerogels for this application. However, using Kistler's method, which included two time-consuming and laborious solvent exchange steps, their first aerogel took weeks to prepare. Teichner then informed his student that a large number of aerogel samples would be needed for him to complete his dissertation. Realising that this would take many, many years to accomplish, the student left Teichner's lab with a nervous breakdown. Upon returning after a brief rest, he was strongly motivated to find a better synthetic process. This directly lead to one of the major advances in aerogel science, namely the application of sol-gel chemistry to silica aerogel preparation. This process replaced the sodium silicate used by Kistler with an alkoxysilane, (tetramethyorthosilicate, TMOS). Hydrolyzing TMOS in a solution of methanol produced a gel in one step (called an "alcogel"). This eliminated two of the drawbacks in Kistler's procedure, namely, the water-to-alcohol exchange step and the presence of inorganic salts in the gel. Drying these alcogels under supercritical alcohol conditions produced high-quality silica aerogels.