Often called "frozen smoke", aerogels are among the amazing materials of our time, with fifteen Guinness Book of World Records entries to their name. However, despite their list of extreme properties, traditional aerogels are brittle, crumbling and fracturing easily enough to keep them out of many practical applications. A new class of mechanically robust polymer aerogels discovered at NASA's Glenn Research Center in Ohio may soon enable engineering applications such as super-insulated clothing, unique filters, refrigerators with thinner walls, and super-insulation for buildings.
First synthesized in 1931, aerogels were the result of a bet between two chemists. Knowing that jellies are mostly pectin gelled with water, they challenged each other to remove the water without shrinking the jelly. Now aerogels are among the least dense solids, possess compressive specific strength similar to aerospace grade graphite composite, and provide the smallest thermal conductivity for any solid. With this array of amazing properties, why don't we see more aerogel applications?
Mary Ann B. Meador, Ph.D., a chemist at NASA Glenn, explains that despite these amazing properties, traditional aerogels made from silica (silicon dioxide, or beach sand) are brittle, and break and crumble easily. Not so when newer polymer aerogels are considered. Meador and her team have developed a particularly encouraging form of polymer aerogel, which is strong, flexible, and robust against folding, creasing, crushing, and being stepped upon. Their new class of polymer aerogels won a 2012 R&D100; award.
“The new aerogels are up to 500 times stronger than their silica counterparts,” says Meador. “A thick piece actually can support the weight of a car. And they can be produced in a thin form, a film so flexible that a wide variety of commercial and industrial uses are possible.”
So just how did Meador and her colleagues approach the problem of synthesizing robust, flexible aerogels? Early attempts to produce stronger and more durable aerogels focused on taking a silica aerogel, and depositing a thin layer of a polymer on the surface of the aerogel structures. This can be done using chemical vapor deposition, for example, but the process is quite slow. (Such coating can also be accomplished by putting a silica aerogel in a small container with a pool of super glue, just as exposure to superglue vapors can reveal fingerprints by coating their grease.) In addition, most of the polymers that could be deposited in this manner have rather low melting temperatures, whereas many of the potential applications require some degree of thermal tolerance.
A new idea was called for. As the only role of the silica aerogel was to give shape to the conformal polymer coating, why not see if a polymer aerogel can be directly formed? Polyimides such as Kapton generally show resistance to temperatures of 400 C (750 F) or higher, are structurally very strong, and have high glass transition temperatures, so were an obvious candidate for such applications.
Unfortunately, standard methods for forming aerogels ran into serious problems. When polyimides in a dilute solution were gelled and then subjected to supercritical drying, the gels shrank by up to 40%, leading to unacceptably dense materials. A number of variations have been tried, primarily based on altering the properties of the polyimides with a range of additives, but these were unsatisfactory in various ways.
The NASA group tried a cross-linking approach, where linear polyamides were reacted with a bridging compound to form a three-dimensional covalent polymer. Such polymers are far more stiff than linear polymers, rather like an I-beam compared to a solid round rod of the same weight. They formed the gel at room temperature, and were able to achieve virtually total coupling between the various three-dimensional polymers. When this gel was subjected to supercritical drying, they were able to form polymer aerogels with densities as small as 0.14 g/cc and having 90% porosity – far from a record, but light enough to provide useful properties such as very low thermal conductivity.
The above micrograph of the nanocellular structure of the aerogel shows pores averaging about ten nanometers in size. A quarter-inch (6 mm) sheet of this aerogel would provide as much insulation as three inches of fiberglass.
The new class of polymer aerogels also have superior mechanical properties. For example silica aerogels of a similar density have a resistance to comperession and tensile limit more than 100 times smaller than the new polymer aerogels.
Silica aerogels would crush to powder if placed under a car tire. As seen above, the same is not true of the new polymer aerogels, even if the car is only a Smart car. Overall, the mechanical properties are rather like those of a synthetic rubber, save that the aerogel has the same properties (and far smaller thermal conductivity) with only about 10 percent of the weight.
Applications in clothing as well as insulation of pipes, buildings, water heaters, and the like are enabled by these materials. Tents and sleeping bags can also benefit from the combination of light weight and thermal insulation. NASA is even considering the new polymer aerogels for use as inflatable heat shields. The practicality of many such applications will depend on the cost of polymer aerogel in commercial quantities. In any case, these types of products now have another dimension of design flexibility.
Source: NASA Glenn Research Center
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