Graphitization of diamond powders of different sizes at high pressure–high temperature Original Research Article

Diamond powders of different sizes ranging from 5 nm to 40 μm 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.