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June 1st, 2011

Transformed aerogels: From amorphous carbon to nanocrystalline diamond

Published on June 1st, 2011 | By: Eileen De Guire
Laser-heated diamond anvil cell allows very large hydrostatic pressure to be applied to amorphous carbon aerogels. Credit: LLNL

Laser-heated diamond anvil cell allows very large hydrostatic pressure to be applied to amorphous carbon aerogels. Cavity dimensions are approximately 100–170 μm wide by 35 μm thick. Credit: LLNL.

While there has been considerable interest in capturing the properties of diamond in a low-density form, converting an amorphous carbon aerogel to a crystalline phase without collapsing the porous network has proven tricky — but not impossible.

In a paper published in the May 9 online edition of the Proceedings of the National Academy of Sciences, a team of Lawrence Livermore researchers describe the successful synthesis of a diamond aerogel from an amorphous carbon aerogel precursor (“Synthesis and characterization of a nanocrystalline diamond aerogel,” doi: 10.1073/pnas.1010600108). The density of the amorphous precursor aerogel in this study is 0.04 g/cm3, which according to a LLNL press release, is about the density of the nanocrystalline diamond aerogel (pure diamond has a density of 3.52 g/cm3).

The team, led by former LLNL fellow, Peter Pauzauskie (now at the University of Washington, Department of Materials Science & Engineering), created an amorphous carbon precursor material using sol-gel processing (see pdf describing details, here) and used a diamond anvil to subject the aerogel to pressures where diamond is the stable phase of carbon. Laser heating was used to overcome the kinetic barriers of the phase transformation to nanocrystalline diamond.

A key goal of the experiment was to maintain the porous structure of the sample. The precursor aerogel is self-supporting, but the researchers note that it is still delicate – finger pressure is enough to crush it. Supercritical neon was used to apply hydrostatic pressures of 21.0 GPa, 22.5 GPa and 25.5 GPa (that’s about 3,000-3,700 ksi), and the samples were laser heated to approximately 1850 oC. The study made no attempt to optimize the pressure and temperature parameters.

Using Raman spectroscopy, the researchers concluded that there was not a superhard graphite phase helping to prevent pore collapse, which had been suggested as a possible mechanism for mesoporous carbon. TEM showed that diamond aerogel is a network of nanocrystals (2.5-100 nm) that appear to be connected by thin surface coatings of graphitic carbon and that the porous morphology seems to have been preserved.

It’s likely a long road from diamond anvil synthesis to a bulk processing method, but the study shows that the phase transformation from amorphous carbon to diamond can be achieved nondestructively while maintaining the porous morphology. Materials of this type are expected to have applications as tunable and optically effficient antireflective coatings, optical quantum bits, and cellular biomarkers. The unique optical, thermal, and chemical properties of a nanocrystalline diamond, porous material will lead quickly to other applications, some novel and some fairly pedestrian (but important), such as  water desalination.

As a proof-of-concept, the study showed that nondestructive phase transformation from amorphous to nanocrystalline morphologies is possible from sol-gel precursor materials, which, the authors note, opens the possibility of producing other highly porous, nanocrystalline materials such as SiO2.

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