Carbon nanofoam

Carbon nanofoam is an allotrope of carbon discovered in 1997 by Andrei V. Rode and co-workers at the Australian National University in Canberra. It consists of a cluster-assembly of carbon atoms strung together in a loose three-dimensional web. The fractal-like bond structure consists of sp² graphite-like clusters connected by sp³ bonds. The sp³ bonds are located mostly on the surface of the structure and make up 15% to 45% of the material, making its framework similar to diamond-like carbon films. The material is remarkably light, with a density of 2-10 × 10⁻³ g/cm³ (0.0012 lb/ft³) and is similar to an aerogel. Other remarkable physical properties include the large surface area of 300–400 m²/g (similar to zeolites). of nanofoam weighs about .

Each cluster is about 6 nanometers wide and consists of about 4000 carbon atoms linked in graphite-like sheets that are given negative curvature by the inclusion of heptagons among the regular hexagonal pattern. This is the opposite of what happens in the case of buckminsterfullerenes in which carbon sheets are given positive curvature by the inclusion of pentagons.

The large-scale structure of carbon nanofoam is similar to that of an aerogel, but with 1% of the density of previously produced carbon aerogels—or only a few times the density of air at sea level. Unlike carbon aerogels, carbon nanofoam is a poor electrical conductor. The nanofoam contains numerous unpaired electrons, which Rode and colleagues propose is due to carbon atoms with only three bonds that are found at topological and bonding defects. This gives rise to what is perhaps carbon nanofoam's most unusual feature: it is attracted to magnets, and below −183 °C can itself be made magnetic.

Carbon nanofoam is the only form of pure carbon known to be ferromagnetic which is unusual for a carbon allotrope. Ferromagnetism is an intrinsic property observed in the carbon nanofoam and may be accounted for by its complex structure. Impurities in the material are excluded as the source of magnetism as they are not sufficient for the strong magnetization observed. Researchers postulate that embedded carbon atoms with unpaired electrons carry enough of a magnetic moment to lead to strong magnetization.