The 2000’s: New Possibilities and Commercialization
At the turn of the century, aerogel research had begun to lose momentum. But the exciting developments were just around the corner. Mechanically strong aerogels, aerogels of strange new materials, and finally, commercially-available aerogels would hit the scene and change the way we think of everything from energy to lightweight structures to sensors.
Mechanically Strong and Flexible Aerogels
In 2002, Prof. Nicholas Leventis at the University of Missouri-Rolla (now Missouri University of Science and Technology) asked a question: if you can dope polymers with nanomaterial fillers, could you dope a filler material with a polymer? Thinking about a common filler material, silica, he identified the lowest-density form of silica available-silica aerogel-and proceeded to see what kinds of materials you could make if you doped them, that is, crosslinked them, with polymers. In the process he discovered x-aerogels, a new class of flexible, mechanically robust aerogels. He found that by diffusing crosslinking agents such as diisocyanates (used in making polyurethane) into the pores of a silica aerogel, he could use the hydroxyl groups on the surface of the particles that make up the aerogel framework to create polymer bridges between particles-the structural weak spots in the aerogel structure. Leventis along with Dr. Mary Ann Meador at NASA Glenn Research Center in Cleveland, OH then began the NASA research program on x-aerogels and further optimized the crosslinking technology and extended the types of polymer crosslinks possible to a wide variety of polymers. Leventis later returned to Rolla and has since prepared x-aerogels from many more substances than just silica, including most of the transition metal and lanthanide oxides and many organic aerogels.
Quantum Dot Aerogels and Beyond
In 2003, Prof. Stephanie Brock at Wayne State University in Detroit, Michigan wondered if aerogels of metal oxides could be made, why other chalcogenides, such as sulfides and selenides? Brock looked to the literature and found gels of cadmium sulfide and selenide had been prepared, and so she proceeded to prepare metal chalcogenide of her own and supercritically dry these gels. The result was a novel class of photoluminescent, semiconducting aerogels with unusual and exciting properties. Brock later expanded on the work and prepared aerogels of many metal sulfides and selenides as well as nitrides and phosphides. Brock has demonstrated applications of these types of aerogels in photoelectrolysis of water and chemical sensors.
Metal Aerogels
The long-sought goal of creating metal aerogels (that is actual metallic aerogels, not just metal oxides) began to come into focus beginning around 2005 when Dr. Juergen Biener and Dr. Greg Nyce at Lawrence Livermore National Laboratory began development of a technique for producing monoliths of nanoporous gold. In their technique, Biener and Nyce cast bimetallic alloys of gold and another metal (such as Cu or Ag) over micron-diameter polystyrene spheres, which served as a template for a foam structure. After dissolving the template, the less-noble metal in the alloy was then etched away through a technique called dealloying. The remaining structure was a macroporous foam with nanoporous walls, closely approximating a “gold aerogel”. The foams had two levels of porosity, micron-diameter pores leftover from the template and nanometer-size pores from dealloying. However, these materials did not exhibit significant mesoporosity, that is, pores between 2 and 50 nm in diameter, which is more typical of aerogels.
Around the same time, Dr. Bryce Tappan and colleagues at Los Alamos National Laboratory came across an unexpected related discovery. Tappan’s group, one of the world’s leading groups in high explosives, was testing the combustion properties energetic transition metal complexes with the bis(tetrazole)amine ligands, originally designed for pyrotechnic applications. Tappan found that ignition of pressed pellets of these compounds under an inert atmosphere left behind incredibly low-density metallic foams. Characterizing the foams further, he discovered that the foams, although largely macroporous, contained significant mesoporosity as well, approximating the expected structure of a metal aerogel. The group eventually extended the technique to prepare foams of Fe, Co, Ni, and Cu, among other metals.
Then in 2009, Prof. Nicholas Leventis, inventor of x-aerogels, took the aerogel world by storm again by announcing the discovery of the first real metal aerogel-an iron aerogel. Leventis was working with hybrid organic-inorganic aerogels combining resorcinol-formaldehyde polymer with iron oxide in high concentration. In an attempt to make carbon-metal oxide hybrid aerogels, upon pyrolyzing these aerogels he found the surprising result the polymer smelted the iron oxide component of the aerogel, leaving behind an aerogel of “pig iron”-iron rich in carbon content but magnetic and metallic as characterized by X-ray diffraction. The technique shows great promise for enabling production of a large array of metal aerogels.
Carbon Nanotube Aerogels
In 2007, Prof. Arjun Yodh et al. at the University of Pennsylvania showed that carbon nanotubes could be assembled into a gel structure using polyvinyl alcohol as a “binder”. The group then supercritically dried these gels to prepare carbon nanotube aerogels. Then in 2008, Dr. Marcus Worsley and Dr. Sergei Kucheyev at Lawrence Livermore developed a surprising material, a hybrid of carbon nanotubes and carbon aerogel, with the amazing ability to compress 80% and elastically return undeformed. The materials also showed high electrical conductivity.
Commercialization of Aerogels
In 2001, a start-up called Aspen Aerogels spun off from Aspen Systems, a contract research and development firm. Aspen Aerogels developed a novel flexible aerogel composite blanket by casting silica gel onto a fibrous batting then supercritically drying the blanket. The resulting material was surprisingly robust-unlike monolithic silica aerogel made without fibrous batting. Aspen has since developed a range of products optimized for high-temperature and cryogenic applications capitalizing on the incredible insulating abilities of silica aerogel. Their products are now found in subsea oil pipelines, refineries, winter apparel, and even shoe insoles.
Also in 2001, Cabot Corporation began production of subcritically-dried, waterproof silica aerogel granules under the tradename Nanogel. These granules were first used as insulating in skylights, and are now being used to insulate subsea oil pipelines as well.
A number of companies are now using aerogels for a wide range of applications but almost all exclusively obtain their materials from Aspen or Cabot.
Acid-Catalyzed Carbon Aerogels
In 2001, Prof. Jochen Fricke introduced acid-catalyzed carbon aerogels-an intriguing variation on carbon aerogels with incredibly enhanced strength. Although technically not aerogels because of their primarily macroporous bone-like structure, these materials exhibit compressive strength and stiffness much higher typical of most aerogels and different from any other carbon material. In 2008, Dr. Ted Baumann at Lawrence Livermore demonstrated a way to make these materials nanoporous by etching them with CO2 at high temperatures, resulting in ultrahigh surface area carbon aerogels with promise for hydrogen storage applications.
The Open Source Aerogel Project: Aerogel.org
In 2001, Stephen Steiner at the University of Wisconsin registered the last available top-level domain for “aerogel” with the hopes of one day creating a central reference about aerogels that would bring people interested in aerogels together and teach the world about the exciting possibilities of aerogels. Several years later in 2007, in collaboration with aspiring aerogel artist Will Walker and with the support of the world’s leading aerogel scientists, the effort to create Aerogel.org began. Walker, a staunch believer in open-source philosophy, proposed expanding the effort to an open-source effort that would not only inform people about aerogels but empower them to create aerogels of their own. Together Steiner and Walker established Aerogel.org with the mission “to catalyze the evolution of the next generation of aerogels, inspire the next generation of scientists and engineers, and explore unfulfilled artistic and technological possibilities”. In Fall 2008 the site launched in beta phase and in March 2009 officially opened to the public. Today Aerogel.org works to foster a community spanning amateur enthusiasts, students, and research scientists where aerogel ideas can thrive. The project aims to solve global problems by catalyzing breakthroughs in aerogel technologies through education, research, and enabling independent aerogel-related creative endeavors.