Published on Apr 17, 2020
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.
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.
The kinetics of the above reaction is impractically slow at room temperature, often requiring several days to reach completion. For this reason, acid or base catalysts are added to the formulation. The amount and type of catalyst used play key roles in the microstructural, physical and optical properties of the final aerogel product . Acid catalysts can be any protic acid, such as HCl. Base-catalysis usually uses ammonia, or ammonia buffered with ammonium fluoride.
Aerogels prepared with acid catalysts often show more shrinkage during supercritical drying and may be less transparent than base catalyzed aerogels. The microstructural effects of various catalysts are harder to describe accurately, as the substructure of the primary particles of aerogels can be difficult to image with electron microscopy (section 3.7). All have small (2-5 nm diameter) particles that are generally spherical or egg-shaped. With acid catalysis, however, these particles may appear less defined than those in base-catalyzed gels.
Typical acid or base catalyzed gels are often classified as "single-step" gels, referring to the "one-pot" nature of this reaction. A more recently developed approach uses pre-polymerized TEOS as the silica source. Pre-polymerized TEOS is prepared by heating an ethanol solution of TEOS with a sub-stoichiometric amount of water and an acid catalyst. The solvent is removed by distillation, leaving a viscous fluid containing higher molecular weight silicon alkoxy-oxides. In a second step, this material is redissolved in ethanol and reacted with additional water under basic conditions until gelation occurs.
Gels prepared in this way are known as "two-step" acid-base catalyzed gels. Pre-polymerized TEOS is available commercially from Silbond Corp. (Silbond H-5). These slightly different processing conditions impart important changes to the final aerogel product. Single-step base catalyzed aerogels are typically more brittle than two-step aerogels. Moreover, two-step aerogels have a smaller and narrower pore size distribution and are often optically clearer than single-step aerogels .
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