Aerogel: Thinner, lighter and stronger

Aerogel: Thinner, lighter and stronger

Imagine preparing a bowl of sweet gelatine dessert. The gelatin powder is mixed with hot water and then cooled in the refrigerator until it sets. Now it's gel. If you put this rickety gel in the oven, all the moisture dries up and you're left with a pile of powder.

But imagine if the dried gelatin kept its shape, even after the liquid was removed. The structure of the gel will remain the same, but it will be very light due to its low density. That's exactly how aerogel is made.

Aerogel is one of the lightest solid materials known to man. They're made of polymers and solvents that combine to form a gel, and then you take the liquid out of the gel and replace it with air. Aerogel is very porous and very low density. They are solid to the touch. This translucent material is considered one of the best insulation materials.

Although aerogel was first invented in the 1930s, NASA's Glenn Research Center in Cleveland has developed a ground-breaking way to make new aerogels that will change the way we think about insulation.

Aerogel porous material

Since their invention, aerogel has been made mainly of silica. The silica combines with the solvent to form a gel. The gel is then extracted with a supercritical fluid. This supercritical fluid extraction involves the introduction of liquid carbon dioxide into the gel. Carbon dioxide passes its supercritical point, where it can be either a gas or a liquid, and is expelled. This exchange is performed several times to ensure that all liquid is removed from the gel. The resulting material is aerogel.

This is a key step in differentiating aerogels from other porous materials, "said MaryAnnMeador, chemical engineer at Glenn and leader of the aerogels team. "Keeping the gel structure is the most important thing."

Aerogels provide very effective insulation because they are very porous and pore in the nanoscale. Nanopores are invisible to the human eye. The presence of these pores makes aerogel very good at insulation.

"The pores are too small and the gas-phase heat transfer is poor," Dr. Meador said. "Molecules in the air can't travel through aerogel, so the material is very poor at heat transfer."

Traditional silicon-based aerogels have been successfully used in many applications, such as providing insulation on Mars probes. They are also used in many commercial products. When aerogels are used commercially, they usually come in the form of pellets or in combination with other materials. Aerogel has been combined with percussion to create an insulating "blanket", as well as filling the glass between to create translucent panels for fluorescent lamp applications.

Silicon-based aerogels are very light because they are 95% porous. Silica aerogels are very useful, but they have limitations -- they're very fragile.

Aerogel innovation

The National Aeronautics and Space Administration (NASA), working with industry partners, investigated the use of different types of aerogel for multiple uses. With funding from NASA's Basic Aeronautics Program (HyPersonics and Subsonic Fixed Wing programs) and the Exploration Systems Mission Administration, NASA's Glenn Research Center has developed two cutting-edge approaches to revolutionizing aerogel technology.

The first innovation is a polymer-reinforced approach to aerogel. This changes the surface of the gel as it reacts with the polymer. The results show that a thin layer of polymer is formed on the inner surface of the aerogel, which greatly enhances the performance of the aerogel.

"If you compare a polymer-reinforced silicone to a silicone of the same density, the strength of the polymer-reinforced aerogel is about two orders of magnitude higher," Medole says.

These polymer-reinforced aerogels have the same insulating properties as typical aerogels and can be translucent. They have the same advantages as silicon-based aerogels and are less fragile. Glenn's team used their patented method to create a number of different aerogels with different polymer properties. Glenn also works with Aspen Aerogel in Northborough, Massachusetts. Make a polymer-reinforced aerogel, combine it with the fiber, and create a new product.

The second innovation is a way to make aerogel entirely from polymers. These polymer-based aerogels are very powerful and flexible. They can also be made into flexible films.

Aerogel in flight

Glenn's team is currently working on a NASA project called the Hypersonic Inflatable Aerodynamic Accelerator (HIAD). HIAD is an inflatable re-entry vehicle that is folded and stored inside a launch vehicle. Before entering the atmosphere, the HIAD is inflated and stiffened. This helps spacecraft slow down, safely land, and land on Earth, Mars, or any other planet with an atmosphere.

HIAD allows more mass to pass through the atmosphere more slowly and safely, and reduces the heat absorbed by vehicles. HIAD is covered by a flexible thermal protection system that uses aerogel as an insulator to protect the payload.

Thin film polymer aerogel is well suited to the needs of HIAD. HIAD(funded by the Basic Aeronautics Program's HyPersonics program) is scheduled for flight testing in 2012. An important component will be flexible thermal protection systems (funded by the HyPersonics project and the Space Technology Programme under the leadership of NASA's Chief Technologist). The flexible thermal protection system uses a baseline aerogel insulation blanket manufactured by Aspen Aerogel. Subsequent tests may include new thin film polymer-based aerogels as improvements to baseline insulation.

"The hope is that aerogel will be more flexible, more foldable, dust-free and will not shed insulation particles, so it will not cause harm and it will not cause trouble," Medoll said. To that end, we started looking at different polymers and technologies that could make this aerogel more flexible."

The team determined that aerogel could not be flexible because it contained silica, so they began exploring ways to make it entirely from polymers. They developed a way to make polymer-based aerogel that is completely flexible and can be made into a very thin film -- an ability that has not been available before. These aerogels are stable even at high temperatures.

Polymer aerogels have the same advantages as traditional aerogels with a porosity of 85 to 95%. It weighs just as little and has the same thermal conductivity as silicon-based aerogel. But these aerogels offer unprecedented flexibility, as well as their durability and strength, and the ability to be made into thin films.

"I was very surprised when we determined that it could be made into a flexible film," Medoll said. "It was a 'wow' moment! It's better than we expected."

Aerogel application

The films were made in collaboration with the UniversityofAkron in Ohio. The films were also sent to centers at other government agencies and NASA, which attracted interest in the technology.

"Usually when people see them, they say 'Wow, that's aerogel? '" Meador said.

Other NASA centers have expressed interest in further exploring these thin polymer aerogels for cryogenic technology or applications in the next space suit. Polymer aerogel is ideal for use in vacuum, such as in space, and in different gravity situations, such as the moon or other planets.

Government agencies are also interested in exploring applications for thin polymer aerogel, such as insulating tents. Industry is also attracting attention for possible applications in refrigeration, building and construction, renewing historic structures and many other insulation needs, especially where space is scarce and smaller, more efficient insulation materials are needed.

Aerogel and the future

Polymer-reinforced aerogels and polymer aerogels have many potential applications in space, on distant planets, and on our own Earth. They are lightweight, durable and very effective in insulating and preventing heat transfer. NASA has taken aerogel to a higher level than previously imagined and discovered a world of possibilities for this versatile material.