How Things Work: Aerogel

Welcome to a whole new year of How Things Work! Being mindful of your break-induced lack of mental acuity, I’ve decided to write about something simple: nothing! Specifically, I’m referring to aerogels, the world’s least dense solids. To put that into perspective, the newspaper you are holding is about 200 times denser than most aerogels.

Aerogels are solids resembling a tinted glass or transparent plastic. Though they are rightfully gels, their name is misleading: Instead of a jiggly, wobbling mass of Cosby-endorsed deliciousness, they behave more like hard foam. Their physical properties are broadly classified under colloids, fine particles suspended in a continuous medium. If that sounds confusing, just visualize aerogels as microscopic mixtures of a solid structure and air, like that of styrofoam.

The most common aerogels are made of silica, a compound of silicon and oxygen. Other continuous media include carbon, metal oxides, cellulose, and agar. Though diverse, these substances have the common ability to form rigid, three-dimensional networks of atoms. The choice of medium, then, depends simply on what physical characteristics are desired.

The physical characteristics of aerogels are truly remarkable. When Steven Kistler invented the first silica aerogels in the 1930s, he found that they had amazing potential as insulators. Aerogels can effectively insulate many forms of energy, including physical force, acoustic energy, electricity, and heat. They also have a very large microscopic surface area. One measly gram of silica aerogel has a surface area four times the size of the Cut! These properties have potential in many obvious and not-so-obvious applications.

Of course, before a product’s potential can be realized, it has to be produced! When Kistler invented aerogel, he wasn’t searching for a substance with such amazing properties. Instead, he was looking for a method to remove the liquid component of a gel. If the liquid component is allowed to evaporate, the gel will significantly shrink and crack. The tough skin on old cafeteria Jell-O is an early stage of this evaporation. These negative effects are caused by the liquid forcing its way out of the gel, through a type of surface tension called capillary action.

Kistler found a way to avoid this: supercritical drying. If a liquid is heated and pressurized beyond a certain point, it becomes a supercritical fluid. Such fluids are important to

the production of aerogels because they diffuse through solids like a gas, preventing the gel from shrinking and cracking. What remains is a torturous maze of branching molecules with many pores and voids that fill with air. So much air, in fact, that some aerogels are 99.8 percent gas. And you thought I was full of hot air!

The actual processing steps vary and are chemistry-intensive, so describing them here would make this article about 2000 times denser than aerogel. Nevertheless, most processes follow a general procedure. First, the continuous medium is mixed with water and allowed to bond, or polymerize. This solution is left alone to set into an alcogel. This step is analogous to making Jell-O.

After the structure of the alcogel has strengthened, it is placed in a pressure vessel known as an autoclave. The autoclave is connected to a liquid capable of becoming supercritical at a certain temperature and pressure. Carbon dioxide is often used because it is safe, cheap, and convenient. The liquid is pumped into the chamber, submersing the alcogel. Over time, the liquid carbon dioxide soaks into the gel, and the previous continuous medium is replaced.

The autoclave is then heated and pressurized over the critical temperature and pressure, where the superfluid carbon dioxide escapes easily from the gel. The newly formed aerogel is ready for processing into forms like pure blocks, applicable coatings, or even woven fabrics.

The versatility of aerogels has led to some interesting applications. In the 1940s, they were marketed as thickening agents for paints, cosmetics, and even napalm. As competing thickeners became cheaper, aerogels were largely forgotten until the 1970s. It was then that research was revived and many applications were marketed.

Aerogels are well-suited to insulating objects in extreme conditions, such as the electronics of the Mars rovers and winter gear for scientists in the Antarctic. Recently, aerogel was successfully employed in the Stardust probe to collect the particles from the tail of a comet. Soon, we may even be seeing aerogel in our homes. One aerogel window is so insulating that it is equivalent to about 20 traditional windows stacked together.

Commercial products are becoming smaller, lighter, and cheaper. Therefore, the need for them to be efficient becomes larger and larger. Aerogels are sure to be at the forefront of this drive. Never since Seinfeld has nothing been more important!