| Industrial diamond single crystals are mostly synthesized from artificial graphite in the present of a 3d-transition metal as a solvent-catalyst under static high pressure and high temperature (HPHT) at present. The nucleation and growth of diamond is carried out in the hermetic cavity, so the in-situ detection to the transition from graphite to diamond is difficult. And the transition mechanism under HPHT is still in dispute.The growth and disappearance of crystal occur at the interface. The carbon atoms or carbon atom groups from catalyst melt transform to diamond just at the diamond/solvent metal interface. The interface for quick-cooling retains much direct information about diamond growth under HPHT. In view of this, in this paper, the morphologies, compositions, phases of the diamond/solvent metal interfaces were investigated by means of field emission scanning electron microscopy (FESEM), energy dispersive spectrum (EDS), transmission electron microscope (TEM). In consequence the main phases which played important roles in the diamond synthesis were determined.When the electron density of the biphase interface is continuous at the first approximation, the phase transition occurring at this interface is feasible in the valence electron structure level. Also there are some phase transitions in diamond growth. Therefore, in this paper, on the basis that the main phases had been determined, the HPHT lattice constants of main phases existed in the course of diamond growth from Fe-Ni-C were calculated based on the linear thermal expansion coefficient and elastic constant of crystal according to the essence of thermal expansion and generalized Hooke's law. Then the valence electron structure (VES) of phases 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). Thus the effect mechanism of main phases existed in the course of diamond synthesis were analyzed from the perspective of VES.Based on the following premise "there exists short range ordered solid structure in the melt" and "the solvent metal at room temperature after synthetic block for rapid cooling retains a lot of information under HPHT", in this paper, optics microscopy (OM), scanning electron microscopy (SEM) were used to study the correlation between diamond synthetic effects and the solvent metal microstructure, which was also to verify the calculation of the valence electron theory. Accordingly, the process of diamond growth at HPHT with Fe-based catalyst was summarized, and the new thought to design the composition of catalyst was put forward.By FESEM, the nanoscale pyramid cone shapes are found on the solvent metal interface corresponding to the diamond (100) crystal surface, and serrate steps are found on the interface corresponding to the diamond (111) surface. According to EDS, the elements (Fe, Ni, C) are found on the solvent metal interface.γ-(Fe,Ni) and Fe3C exist on the solvent metal interface by TEM. To sum up, the main phases that have important effects on diamond growth containγ-(Fe,Ni), Fe3C style metallic carbide and graphite (original carbon source).The VES analyses show that the electron density of diamond/graphite interface is not continuous at the first approximation, while the electron density of Fe3C/diamond interface is continuous at the first approximation, which can satisfy the boundary condition of diamond growth. Therefore, the carbon source of diamond growth with Fe-based catalyst at HPHT comes from the carbon atom groups separated from Fe3C instead of the direct transformation of graphite structure. The electron density of Fe3C/γ-(Fe,Ni) interface is continuous at the first approximation, which illuminates thatγ-(Fe,Ni) plays a role of catalysis phase, that is,γ-(Fe,Ni) improves the decomposition of Fe3C.The interrelated studies between diamond synthetic effects and the solvent metal microstructure show that the synthesis time and the catalyst composition commonly affect the solvent metal microstructure, especially the quantity and shape of cementite. Under different crystallization conditions, the quantity and the shape of Fe3C are different, corresponding to different synthetic effects. There is some close community between diamond growth and the decomposition of Fe3C style carbide. Thus it can be seen that the experimental results agree well with the analytical results based on valence electron theory. The catalyzer with best diamond synthetic effect was also selected.And a new thought about the design of catalyst composition can be offered that the Fe3C type carbides must be formed by the effect of catalyst and graphite; the electron density continuity across Fe3C/diamond interface is high; and the electron density continuity acrossγ-solid solution/Fe3C is appropriate, that is, decomposition velocity of Fe3C is appropriate.As for the Fe-based catalyst, the results of this paper support "solution-catalysis theory". The diamond/solvent metal interface characterization, the analyses based on the valence electron theory and the experimental results of solvent metal microstruture both hold up the viewpoint that the carbon source of diamond crystal growth with the method of Fe-baed catalyst at HPHT comes from the decomposition of carbide. Accordingly, the diamond growth with the method of Fe-based catalyst at HPHT can be described as following.
|
PDF Full Text Request