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High pressure catalytic synthesis of diamond crystals of graphite and metal catalyst is always at the junction of nucleation and plenty of graphite carbon source towards the direction of growth. Regardless of the iron-based, nickel-based catalyst or flakes, powdered catalyst, the main characteristics of the diamond synthesis are: After nucleation of diamond is always 30-50μm thick layer of a metal film wrapped, by this layer of film, graphite diffusion transfer carbon to the surface of diamond and diamond structure eventually catalysis. Thus, this layer film is a carbon source which diamond growth. High pressure line detection is extremely difficult, but can be synthesized by rapid cooling block so that under normal temperature and pressure of the metal coating can maximize the retention of its many high pressure and temperature information. Because the crystal growth and disappearance occurred in diamond / metal envelope interface, it is more detailed and microscopic study of diamond / envelope interface will be very helpful to reveal the diamond growth mechanism. This paper were prepared by powder metallurgy Fe-Ni-C iron-based catalyst tablets, graphite as the initial carbon source, high temperature and pressure conditions (5.5GPa, 1623K) synthetic diamond crystals under experiment. Scanning electron microscopy (SEM), field emission scanning electron microscope (FESEM), spectrum analyzer (EDS), X-ray diffraction (XRD), high resolution (HRTEM), field emission transmission electron microscope (FETEM) characterization methods such as, System characterization and analysis of diamond / iron-coated side of the interface are coated microscopic morphology, atomic composition, phase structure, identified the diamond growth under high temperature and pressure have an effect of iron-based metal coating phase. In order to characterize the microstructure elements contained in phase (carbon, iron, etc.) information about the electronic structure, find diamond catalytic mechanism of the growth process, the use of electron energy loss spectroscopy (EELS), the interface to the interface from the envelope depths of carbon atom sp2 to sp3-bonds change key status changes of iron 3d electrons as well as with the outer electrons of carbon atoms complexed situation. In addition, the combination of thermodynamic theory to explain the catalyst phase in which the process of change, to explore its role in diamond synthesis and mechanism. Of iron-based metal coating (100) interface has been FESEM observation, and found the interface there are less than 1OOnm aspect tetragonal columnar protrusions structure, combined SUPRA55 and SU-70-type field emission scanning electron microscope configured EDS spectrometer respectively Film Interface Quartet convex structure, pyramid-shaped protrusion structure composed of atomic enclave point analysis. The results showed that: tetragonal pyramid-shaped convex protrusion structure and the structure agglomerations contain iron, nickel and carbon. By XRD of the diamond / Film Interface done a full spectrum analysis, we found the interface consists of three phases y-(Fe, Ni), Fe3C and (Fe, Ni) 23C6 components. Shows XRD peak intensity from the interface there are lots of y-(Fe, Ni) phase, and the interface of the carbon-rich phase is Fe3C, there is no structure of graphite and diamond. In order to diamond growth mechanism for further analysis and reasoning, the choice of the energy resolution of less than 2nm of FETEM observe the envelope (envelope from the envelope to the depths of the interface extends) the phase changes, the observation position is: Film Interface and coated inner (respectively, from the Film Interface 2μm, 4μm and 6μmm depth) of four dimensions. The results showed that the coating at the interface and from the interface 2μm The phase of γ-(Fe, Ni), Fe3C, 4μm and 6μm from the interface at the γ-(Fe, Ni), Fe3C, and graphite. Since diamond crystal coated surface and direct contact, the interface on the diamond growth phase is the most direct part of an effect. Thus, it can be concluded that: the interface a deep (eg 4μm and 6μm Office), graphite and diamond can not be directly into the catalytic structure, but the diamond / envelope interface, carbon atoms form by precipitation into a Fe3C diamond structure. Therefore, the envelope of the interface Fe3C found illustrates the loss of graphite growth of diamond rather than graphite carbon phase is Fe3C. Based on the above, the choice of the energy resolution of less than 2nm of FETEM, combined with field emission EELS attached to carbon and iron atoms in the electronic structure were characterized. The results showed that: the interface to the envelope from the inner envelope, C-sp3 content increased from 78.15% to 87.33%, EELS spectrum peak and peak intensity can be clearly seen at different depths in the envelope of the electronic structure of carbon atoms have been changed significantly; envelope from the interface to the inner envelope, iron 3d electron share decreased to 4.54 from 5.64 electron / atom. According to valence bond theory, Fe3C formation should make the iron atom 3d electron share increases, while the actual opposite. This is because there are a lot of γ-(Fe, Ni) of the 3d and Fe3C unpaired electron interaction, the carbon atoms in the Fe3C class Sp3 into the interface structure and the separate envelope, the diamond growth, and ultimately at the interface resulting γ-(Fe, Ni) of the 3d electron share decline. Therefore reasonable to assume that iron-based metal catalyst HPHT synthetic diamond catalytic mechanism is: catalytic phase γ-(Fe, Ni) using the outer unpaired 3d electron catalytic or alter the electronic structure of carbon atoms, making the carbon Fe3C atom from sp2 to sp3 transition state gradually, eventually forming a diamond at the interface. Consider HPHT diamond synthesis conditions on the physical opposite of diamond metal catalyst thermodynamic processes of structural change were calculated. Found Fe3C, Mn3C, Ni3C and graphite to diamond transition free energy changes are negative, can form diamond. However, graphite and diamond energy barrier between the presence, resulting in a diamond synthesis temperature and pressure range of graphite is difficult to directly into diamond and carbide to diamond Me3C free energy change of graphite to diamond phase change than the free energy change is more negative. Therefore, from a thermodynamic point of view can be drawn Fe3C, Mn3C, Ni3C carbon in the carbon than graphite into diamond easier, that the growth of diamond under high temperature and pressure can only be Me3C carbide carbon source, rather than graphite. Description Fe3C easier to change!
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