Graphitization of nanodiamond powder annealed in argon ambient

Nanodiamond powder was annealed at each of the following temperatures: 300, 600, 800, 1000, and 1150 degrees C, for an hour in flowing argon ambient. The variations of x-ray diffraction patterns and Raman spectra of the powder with different annealing temperatures were studied. While being annealed at temperatures higher than 800 degrees C, the powder can undergo a phase-transition process from cubic diamond to graphite. In addition, the size of nanodiamond crystallites decreased from similar to 50 to similar to 25 A. The physical mechanism responsible for the variation in Raman spectra is discussed using a phonon-confinement model. (c) 1999 American Institute of Physics. [S0003-6951(99)03221-0].