Nanocrystalline diamond directly transformed from carbon nanotubes under high pressure

Multiwalled carbon nanotubes are heated in a diamond anvil cell by a CO2 laser above 17 GPa and 2500 K. The recovered product consists of nano-sized octahedral crystals of less than 50 nm. The tubular structure completely changed to granular and grain sizes corresponded to the diameter of nanotubes. Electron energy loss spectra from the products coincide with that of diamond. No metastable phase or amorphous carbon was detected in the products. The grain size of the diamond suggests that the transformation took place by direct conversion of nanotubes. This result suggests that diamond grain size may be controlled. The products are thought to be sintered compacts of nanocrystalline diamond. If larger sintered compacts are synthesized from nanotubes it may have an advantage in mechanical strength similar to natural polycrystalline diamond such as carbonado. (C) 2002 Elsevier Science B.V. All rights reserved.