Precipitation mechanisms of micro-crystal graphite powder during type IIa large diamonds crystallization process

•For the first time, we put forward the micro crystal graphite powder in the synthesis of type IIa large diamond crystals.•The additive Ti (Cu) and related reaction-products in metal-carbon system could change solvent property of the catalyst.•The “relatively high temperature and relatively low pressure” condition lead to the precipitation of micro crystal graphite.

Type IIa large diamond crystals were synthesized in FeNiCo-C system with Ti (Cu) as nitrogen getter by temperature gradient method (TGM) under high pressure and high temperature (HPHT) conditions. Diamonds grew upon (111) facet under 5.4 GPa and 1620 K while black powder appeared around the growing diamond crystal. And the black powder covered the growing crystals in cap shape and restrained the diamond growth drastically with growth time prolonged. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to characterize the black powder described above and the regrown graphite obtained from the growth process of traditional type Ib diamonds. The black powder was proved to be micro-crystal graphite powder and the powder was similar to the regrown graphite. However, there were obvious differences between the micro-crystal graphite powder and the regrown graphite. The present discussion about precipitation mechanisms of micro-crystal graphite powder implied that the added Ti (Cu) and related reaction-products in FeNiCo-C system could change solvent property of the catalyst, which affected solubility curve of graphite. Furthermore, the practice of diamond growth indicated that the relatively high temperature and relatively low pressure conditions were remarkably responsible for the precipitation of micro-crystal graphite powder.

Graphical abstractThe black graphite powder covered the growing type IIa large diamond crystals in cap shape and restrained the diamond growth drastically with growth time prolonged. It was observed as platelets, several micron in diameter. The precipitation mechanisms of micro crystal graphite powder implied that the added Ti (Cu) and related reaction-products in metal-carbon system could change solvent property of the catalyst, which affected solubility curve of graphite.Download high-res image (104KB)Download full-size image