Effects of zinc additive on the HPHT synthesis of diamond in Fe–Ni–C and Fe–C systems

In the series of experiments at 5.0–7.0 GPa and 1300–1800 °C with the duration from 15 min to 2 h, the diamond crystals were synthesized in the Fe–Ni–C and Fe–C systems with zinc additive and the capability of zinc for converting graphite to diamond were also investigated in this work. Compared with the diamond synthesis using conventional catalysts, much higher temperatures are required for the nucleation and growth of diamond in Fe–Ni–Zn–C and Fe–Zn–C systems. The morphology of synthesized diamond crystals changes regularly from cub-octahedron to octahedron in the Fe–Ni–C system with increasing zinc additive, while only octahedral diamonds form in the Fe–Zn–C system. The diamond growth is accelerated by appropriate addition of zinc in conventional catalysts while the excessive zinc additive may have a suppressive effect on the diamond nucleation. We also estabish the essential dependence of diamond nucleation and morphology on the composition of crystallization medium in the Fe–Ni–Zn–C and Fe–Zn–C systems. Based on our analysis of the diamond surface configuration, we suggest that the formation of the different defects on the {100} and {111} faces is most likely due to the two simultaneous growth process on the {111} faces.

Research Highlights
► We investigate the capability of zinc for converting graphite to diamond.
► Much higher pressure and temperature are required for diamond nucleation and growth.
► The diamond growth is accelerated by appropriate zinc addition.
► The excessive zinc additive has a suppressive effect on diamond nucleation.
► We suggest that there are two simultaneous growth processes on the {111} faces.