Nanodiamond nucleation below 2273 K at 15 GPa from carbons with different structural organizations

Five precursors covering the whole range of carbon structural organization, i.e. a quasi-amorphous soot (QAS), a raw carbon black (CB), a carbon black heat-treated at 2600 °C (HTCB), a polycrystalline graphite (PCG) and a highly oriented pyrolytic-graphite (HOPG) were run at 15 GPa in the 1500-1900 °C range between 15 and 60 min. Full transformation into nano-diamonds was not always achieved and the corresponding run products preserved the tracks of the transformation mechanisms which led to diamond formation. These mechanisms and their kinetics were characterized combining X-ray powder diffraction, Raman micro-spectroscopy and high-resolution TEM. Globally, the disordered precursors react faster than the crystalline ones: they achieve higher transformation rates and become transparent more easily. For the spherical CB particles, nano-diamond preferentially nucleates in their centre. The graphitic layers in the QAS directly transform into diamond without any prior graphitization. The crystalline organization is even found to decrease for the graphitized precursors (HTCB and PCG) as evidenced by HRTEM images showing graphite delamination. These precursors mostly evolve according to a diffusion-limited reconstructive mechanism which initiates at the precursor structural defects. HOPG behaves differently since it mostly transforms into lonsdaleite rather than cubic diamond.