Core growth of detonation nanodiamonds under high-pressure annealing

Abstract

The present paper reports the results of a study of the X-ray and Raman spectra of detonation nanodiamonds after high-pressure–high-temperature (HPHT) annealing at different temperatures. The experiments were carried out using a multi-anvil high-pressure apparatus of the “split-sphere” type (BARS) at 5 GPa and 1100–1500 °C. It was found that HPHT annealing at 1500 °C led to the growth of nanodiamond cores sized from 4.2 to 6.9 nm. It was determined that transformation of the detonation nanodiamond particles includes the formation of both a new diamond phase as well as ordered graphite around the initial diamond core. It was suggested that the gasification of adsorbed impurities from the nanodiamond shell results in the formation of a supercritical fluid that stimulates the growth of the diamond phase. The appearance of newly formed graphite led to covering the exposed areas of the nanodiamond core capable of contacting with neighboring nanodiamonds during annealing, which explains the earlier established heterogeneity of the hardness increase within the sample volume after HPHT sintering.