Li Hesheng1, 2, Xu Bin2,3, Gong Jianhong1,2, Li Li1,2, Tian Bin1,2, Li Musen1,2
(1. School of Materials Science and Engineering, Shandong University, 250061
2. Shandong Engineering Research Centre for Superhard Materials, 273500
3. School of Materials Science and Engineering, Shandong Jianzhu University, 250101)
Abstract In this paper, the diamond formation mechanism in Fe-Ni-C system at high-pressure and high-temperature (HPHT) is summarized,which is established on base of the experimental and theoretic results: 1) characterization of microstructure of metallic film surrounding diamond; 2) calculation and analysis of valence electron structure of related phase (including diamond, graphite, Fe3C((Fe, Ni)3C), γ-(Fe, Ni) and so on) involved in the process of diamond synthesis, based on the empirical electron theory of solid and molecules (EET) and the improved Thomas-Fermi-Dirac theory by Cheng (TFDC). The results indicates that there is no diamond (like-diamond structure), but are large number of metallic carbides and transition phase just like γ-(Fe, Ni) in the metallic film. In addition, some crystal planes of γ-(Fe, Ni) have the crystalline orient relationship with those of some metallic carbides. The results of calculation show that the valence electron densities of common planes in graphite were not continuous with those of planes in diamond at the first order of approximation. On the contrary, the continuous relationship of valence electron densities appears on the interface of Fe3C and diamond as well as that of Fe3C and γ-(Fe, Ni). It means that the boundary condition of diamond growth can be satisfied on the interface of Fe3C and diamond. All the results suggest that the direct carbon source of diamond growth is not graphite. The carbon atoms groups possess SP3 structure, which decompose from metallic carbides, deposit on the diamond nucleus surface and make it grow, which agrees with the solvent-catalyst theory.
Keywords synthetic diamond; synthesizing mechanism; Fe-Ni-C catalytic system; EET; TFDC; high-pressure and high-temperature (HPHT)