Graphitization of diamond powders of different sizes at high pressure-high temperature

Diamond powders of different sizes ranging from 5 nm to 40 mum were heat treated at temperatures between 973 and 1673 K, at high pressures from 2 to 8 GPa. Elevated temperatures accelerate diamond to graphite phase transition and we studied this process by analyzing X-ray diffraction patterns, Raman spectra and high-resolution transmission electron microscope (HRTEM) images. Increasing activation energy was obtained for diamond powders of increasing sizes, and the critical pressure of the diamond to graphite phase transition decreased. Onion-like carbon was obtained after the graphitization of nano and sub-micron diamond, and attributed to closure of graphite shells. Microcrystalline graphite was formed after graphitization of micron size diamond, mainly due to the good crystallinity of the micron diamond precursor. (C) 2004 Published by Elsevier Ltd.