An aerogel is an ultra-low density, highly-porous material derived from a gel in which the liquid component has been replaced by gas. Practical applications of aerogels include ultra-light structural materials, radiation detectors and thermal insulators.
Most aerogels to date have been made from silica or organic polymers, and are produced by subjecting the wet-gel precursor to critical-point-drying (CPD) or freeze-drying.
Arjun Yodh and his colleagues used these same techniques, and created carbon nanotube aerogels with well-defined and regular pore structures, with control over the nanotube density, microscopic structure and macroscopic shape of the material. Pure aerogels produced in this way are fragile, but reinforced with polyvinyl alcohol (PVA), the material is rendered stronger and more stable. Carbon nanotube aerogels are also electrically-conductive.
“Though critical-point drying has given us better results thus far, it should be possible to modify the freeze-drying process to improve the aerogel properties for large-scale applications,” says Yodh. “For applications such as sensors, where only small amounts of the aerogel are needed, critical point drying should present few difficulties.” Large-scale CPD techniques are also in development.
The researchers are now studying in detail the properties of their new aerogels. “The fact that we can backfill the aerogels with other stuff (like epoxy) suggests that we might find applications in conductive epoxies,” says Yodh. “I think sensors and electrodes might be the most easily attainable applications. We are also involved with a small company to explore possible applications and scale-up.”
David Weitz, a physicist at Harvard University in Cambridge, US, is impressed: “The authors have achieved fine control over the structure of the gel, and been able to dry it without destroying the structure. They have created a connected, strong structure made of carbon nanotubes in air, and apparently produced a material that is both remarkably tough and quite conductive.”
Carbon nanotube aerogels, Bryning et al., Advanced Materials 19, 661 (2007) (subscription required)
Images of carbon nanotube aerogels. (a) left: pure aerogel; right: PVA-reinforced aerogel; (b) 13 mg of aerogel supporting a 100 g mass; (c) scanning electron miscroscope image of reinforced aerogel pore structure; (d) transmission electron microscope image of un-reinforced aerogel.
Image: © Wiley-VCH.
Article first published in Nanomaterials News.