Graphite/diamond transformation mechanism under the action of an iron-based catalyst

Abstract

Although it has been nearly 70 years since iron-based catalysts were used to synthesize diamond from graphite at high pressure and high temperature (HPHT), the graphite to diamond transformation mechanism in this process is still a mystery. It is found that the formation of a eutectic melt between carbon and an iron-based catalyst plays a critical role in the graphite to diamond transformation at HPHT. In this work, the eutectic temperature of Fe–Ni–C (diamond and graphite) was determined using the in situ high-pressure temperature measurement (HPTM) and the high-pressure differential thermal analysis (HPDTA) method up to 8 GPa in a large-volume cubic press. The experimental results of XRD, Raman, SEM and XPS indicate that the transformation of graphite to diamond occurs in the Fe–Ni–C eutectic melt: the C–C bonds in graphite are broken in the eutectic melt, and the free carbon atoms in the melt form sp³ bond states in the thermodynamically stable region of diamond. In this process, the role of iron-based catalysts is to reduce the environmental conditions, in which the C–C bonds in graphite are destroyed, and to make the carbon atoms that are separated from the sp² bond state form the sp³ bond state in the diamond P–T phase stable region.