Size-dependent phase transition of diamond to graphite at high pressures

Size-dependent phase transition of nanosize diamond particles (NDPs) to graphite was found in the course of studies of high pressure high temperature (HPHT) induced transformations of NDPs at 8 GPa and in the temperature range of 873-1623 K. Analysis of the products of HPHT treatment of powder NDPs (with the mean size of 4.5 nm) by X-ray diffraction (XRD), scanning (SEM) and transmission electron (TEM) microscopies, and Raman spectroscopy have shown that at 8 GPa the increase of the treatment temperature results in a gradual enlargement of NDPs. According to XRD data, the temperature increase from 873 to 1473 K at the treatment time of 60 s gives rise to NDP mean size enlargement from initial 4.5 to 8.8 nm. However, the largest NDPs can reach the sizes of 13.5-14.0 nm at 1473 K, as measured directly from TEM images. At 1623 K, size-dependent phase transition of diamond to graphite has been observed. By applying the Arrenius equation to the particle mean size versus treatment temperature plot, activation energy for the diamond solid phase growth at 8 GPa was calculated to be 112 8 kJ/mol. The critical size of NDP corresponding to the point of phase transition at 8 GPa and 1623 K was estimated to be similar to 18 run. The likely effect of the size-dependent diamond-to-graphite phase transition on the process of diamond synthesis and, in particular, the problem of formation of critical seeds for crystal growth of macroscale diamond are discussed in the paper.