Abstract
Aerogels are unique materials with many fascinating properties. A scientific curiosity since being prepared in the 1930’s, today’s research has focused on potential applications and more economic production routes. In this paper, we present a historic review of aerogels while bringing the reader up-to-date on the latest technological developments.
Introduction
Aerogels are transparent, highly porous, open cell, low density foams. The microstructure, comprised of nano-sized pores and linked primary particles, as well as the elemental composition can be tailored by solution chemistry via a process known as the sol–gel method. As a result of this unique microstructure, these light-weight materials exhibit many interesting and unusual properties. In fact, aerogels have the lowest thermal conductivity, refractive index, sound velocity and dielectric constant of any solid ever tested. Applications for these materials are potentially vast in number.
A scientific curiosity since the early 1930’s, today’s research has focused on possible applications and more economic production routes. In this paper, we present a historic review of aerogels while bringing the reader up-to-date on the latest technological developments. We conclude with a brief description of some interesting scientific applications where aerogels are finding a niche and describe some potential commercial applications that are currently under investigation.
Section snippets
Historic review
It began in the 1930’s with the scientific community’s interest in gels and gel structure. Kistler reasoned, correctly, that a gel was composed of a solid and liquid phase which were independent of each other. In other words, if the liquid phase was removed from the gel in a non-destructive manner, a solid porous material would be left with approximately the same shape and volume as the original gel.
However, in practice, this was difficult to achieve. Evaporation of liquids from gels resulted
Aerogel production
While the applications for aerogels are virtually unlimited, they will have to be cheaply produced in order to have an impact in the commercial marketplace. As such, researchers have now turned their attention to achieving these goals. Some groups have sought ways to eliminate the SCE process and the high capital cost associated with it, while others have sought to make it more efficient. Studies have shown that the high cost of the raw materials contributes significantly to the final cost of
New classes of aerogels
Although, in theory, any multifunctional monomer that can be cross-linked to form a gel could be converted into an aerogel via SCE, very little work has been done outside of the silica and RF organic systems. Some groups have prepared aerogels from the transition metal alkoxides but their fast hydrolysis rates and low cross-linking densities have made monolithic aerogels hard to achieve. For catalytic applications, however, monolithicity is not important and work on zirconia and titania
Aerogel characterization
Several methods are currently being used to probe the structure and properties of aerogels with varying degrees of success. Here, we describe the most common laboratory practices employed by sol–gel scientists noting the limitations of the technique, if any.
Applications
To date, the use of aerogels has been primarily centered on scientific applications. But the potential now exists for aerogels to move into the commercial marketplace for a number of reasons: scientists are now finding more cost effective methods for their production; fundamental chemistry–structure–property relationships are now known and can be used to optimize the aerogel microstructure for specific applications; aerogels are inert, non-toxic, environmentally friendly insulation