Ultradisperse-Diamond Nanoclusters. Fractal Structure and Diamond–Graphite Phase Transition

A 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. X-ray characterization showed the ultradisperse-diamond, irrespective of the cooling rate used, to be a cluster material possessing a diamond structure with a characteristic nanocrystal size of 44 Å. The diamond–graphite structural phase transition in ultradisperse diamond is shown to start from the cluster surface inwards at Tpt≈1200 K, i.e., at substantially lower temperatures than is the case with bulk diamond single crystals. Ultradisperse-diamond clusters are shown to be fractal objects and the character of variation of the fractal dimension in the course of the diamond–graphite phase transition is studied.