"Several carbon phases have been observed under pressure and they also exhibit extremely high hardness," Zhongwu Wang of Los Alamos Neutron Science Center told nanotechweb.org. "Unfortunately, none of them has been preserved upon the release of pressure to ambient conditions. As a result, we decided to combine a high-pressure approach with several advanced analytical techniques to explore the pressure-induced behaviour of carbon nanotubes."

Wang and colleagues placed carbon nanotubes with a diameter of 1.8 to 5.1 nm in a steel gasket hole that was 70 µm thick and 80 µm in diameter. Compressing the nanotubes caused a hexagonal carbon phase to form at a pressure of about 75 GPa. The material retained its properties on removal of the pressure: structural investigations showed that it was an sp3-rich hexagonal carbon polymorph rather than hexagonal diamond.

The superhard material had a density of about 3.6 g/cm3 and a bulk modulus of 447 GPa.

"The density is higher than that of diamond, and the bulk modulus and hardness are at least compatible to those of diamond," said Wang. "Since such a phase has never been synthesized by using other carbon phases as starting materials, this implies that the particular properties of nanotubes have some undiscovered relation to this new phase."

The researchers believe that, since their superhard carbon phase is stable under ambient conditions, it shows promise for technological applications. They are now trying to understand "why such a stable phase can be synthesized only when carbon nanotubes were used as a starting material".

The researchers reported their work in PNAS.