Study On Effect Mechanism Of Iron-based Metallic Catalyst Phases In Diamond Synthesis At HPHT

The static pressure via which the graphites discs are placed alternatively with catalyst discs in a cell assembly is widely used in industrial production. The main feature of diamond synthesis at HPHT is that diamond nucleates on graphite/catalyst interface, and then grows towards graphite discs. Simultaneously a thin molten metal film forms and covers on the growing diamond and isolates the diamond from the graphite. The metal film, which is the reaction product of metallic catalyst with graphite, is the most direct part of the effect which metallic catalyst have on the transformation from graphite to diamond. Carbons from graphite, which merged into the film, are transmitted to growing diamond crystal and then transformed into diamond structure. Therefore, the study on effect mechanism of phases in catalyst should be focused on the mechanism of phases in the film.In the paper, diamond was synthesized at HPHT by using iron-based metallic catalyst produced by powder metallurgy. The morphology of diamond and its corresponding metallic film interfaces, elemental composition and phase components of metallic film interfaces were studied by means of scanning electron microscope (SEM), field emission scanning electron microscopy (FESEM), energy dispersive spectrum (EDS), electron backscatter diffraction (EBSD). The phases of iron-based metallic catalyst (PIC), which had effect on diamond synthesis, were determined on the results obtained by the methods mentioned above. On this basis, the valence electron structure (VES) of PIC and covalent electron density of interfaces at HPHT were calculated with empirical electron theory in solid and molecule (EET) and thomas-fermi-drac-cheng theory (TFDC), the transformation Gibbs free energy changes (ΔG) for the reactions from PIC to diamond at HPHT were calculated with thermodynamic theory. The effect mechanism of PIC during the course of diamond synthesis was analyzed from the perspective of VES and thermodynamics. Meanwhile, the VES of graphite in the condition of explosive detonation synthesis was calculated with EET, the mechanism of diamond synthesis by explosive detonation was discussed.SEM and FESEM study indicates that the nanoscale pyramid cone shapes and tetragonal shapes exist on the metallic film interface (001), while serrate steps are found on the metallic film interface (111). The diamond growth units have different accumulation mode on different diamond interfaces, which leads to different morphologies of diamond corresponding metallic film interfaces.According to EDS and EBSD analysis, there exist Fe, Ni, C in the film and tetragonal shapes on the metallic film interface, f.c.cγ-(Fe,Ni) is determined in tetragonal shapes.By TEM, f.c.cγ-(Fe,Ni), Fe3C and rhombohedral graphite are observed in the metallic film,γ-(Fe,Ni) and orthorhombic Fe3C exist in the metallic film interface. Ni3C which existed during the course of diamond synthesis at HPHT, decompose and turn intoγ-(Fe,Ni) in the shock chilling process. The phases of iron-based metallic catalyst which have effect on diamond synthesis at HPHT consist ofγ-(Fe,Ni), graphite, Fe3C, Ni3C.Based on the linear thermal expansion coefficient, the relation between lattice parameter of crystal and temperature is established. And the relation between lattice parameter of crystal and pressure is established according to elastic constant in generalized Hook's law. The lattice parameters of phases in iron-based metallic catalyst at HPHT are calculated to supply a basis for the calculation of EET.According to the analysis on the VES of graphite and covalent electron density of graphite/diamond interface, the covalent electron density of graphite/diamond interface is discontinuous at the first order approximation. From the perspective of VES, the carbon source for diamond growth with iron-based metallic catalyst at HPHT can't come from graphite directly.According to the analysis on the VES of Fe3C and covalent electron density of Fe3C/diamond interface, the covalent electron density of Fe3C/diamond interface is continuous at the first order approximation, which can satisfy the boundary condition of diamond growth. The 3d layer electrons of Fe atomic in Fe3C absorb the outermost layer electrons of carbon, which rearranges the electron orbit of carbon to supply electron structure basis for diamond transformation. The carbon atomics, which have rearranged electron orbit, separate from the Fe3C/diamond interfaces and make a transformation to diamond structure. Compared with graphite, the carbon source for diamond growth with iron-based metallic catalyst at HPHT comes from the carbon atomics separated from Fe3C.According to the analysis on the VES of y-(Fe,Ni) and covalent electron density ofγ-(Fe,Ni)/Fe3C interface, the covalent electron density ofγ-(Fe,Ni)/Fe3C interface is continuous at the first order approximation. From the perspective of VES,γ-(Fe,Ni) plays a role of catalysis phase.According to the analysis on the VESs of Fe3C, Ni3C and covalent electron density of Fe3C/diamond, Ni3C/diamond interface, the covalent electron density of the interfaces are continuous at the first order approximation. The carbon source for diamond growth with iron-based metallic catalyst at HPHT can come from the carbon atomics separated from Fe3C, Ni3C. At the same conditions, the continuity of Fe3C/diamond interface is better than the continuity of Ni3C/diamond interface. The result shows that the synthetic effect of iron-based metallic catalyst is better than that of nickel-based metallic catalyst, which is in accordance with the fact that the iron-based metallic catalyst is widely used in diamond synthesis.According to the analysis on the VES of graphite in the condition of explosive detonation synthesis, the VES of graphite can't exist in the condition of explosive detonation synthesis. From the perspective of VES, graphite may decompose during explosive detonation synthesis.According to the analysis on the transformation Gibbs free energy changes (AG) for the reactions of phases in iron-based metallic catalyst to diamond at HPHT, theΔG for the transformation from Fe3C, Ni3C or graphite to diamond are all negative, but theΔG value for the transformation from Fe3C or Ni3C to diamond are much smaller than that for the transformation from graphite to diamond, which means the transformation from Fe3C or Ni3C to diamond take place more easily. From the perspective of thermodynamics, the formation of metallic carbides reduces the potential energy of transformation from graphite to diamond, and the carbon source for diamond growth with iron-based metallic catalyst at HPHT can come from the carbon atomics separated from metallic carbides instead of the direct transformation from graphite. To Ni3C and Fe3C, theΔG value for the transformation from Ni3C to diamond is smaller than that for the transformation from Fe3C to diamond, but theΔG value for these two transformation is on the same level. So it is feasible in thermodynamics for the substitution of nickel-based catalyst by iron-based catalyst. Small amount of Ni powder can be mixed in the iron-based catalyst to prepare iron-nickel alloy catalyst which has optimal cost and is widely used in the world.