Diamond-graphite phase transition in ultradisperse-diamond clusters

systematic study of the diamond-graphite structural phase transition in ultradisperse-diamond clusters obtained by the detonation technique is reported. Samples of two types, differing in the kinetics of detonation-product cooling, were investigated. The phase transition was achieved under heating in an inert atmosphere in the temperature range 720-1400 K. The transition was identified by Raman scattering and x-ray diffraction data. Raman and x-ray characterization showed the ultradisperse diamond, irrespective of the cooling rate used, to be cluster material possessing diamond structure with a characteristic nanocrystal size of 43 Angstrom. The diamond-graphite phase transition in ultradisperse diamond is shown to start from the cluster surface inwards at T-pt approximate to 1200 K, i.e. at substantially lower temperatures than is the case with bulk diamond single crystals. (C) 1997 American Institute of Physics.