Fine Structures And Phase Transition Mechanism Of Diamond/Fe-Based Metal Film Interface

Diamond crystal synthesized at high temperature and high pressure always forms on the diamond/film interface and grows towards rich-carbon graphite direction. Whether it is iron-based catalyst, nickel-based catalyst or a flake catalyst, powder catalyst, one of the most important characteristics and basic phenomena during diamond growth is that there always exists a 30μm-50μm thick metal film covering on a growing diamond. through which graphite carbons diffuse constantly to the diamond surface and transfer to diamond structure eventually, Therefore, this film is where the growth of diamond, because the crystal growth and disappearance occurred in the diamond/film interface. Despite of the difficulty of line detection at high temperature and high pressure, much information of diamond growth at HPHT can be remained in the film at room temperature and ambient pressure when the cell assembly is cooling rapidly after finishing the synthetic process. Therefore, meticulously, more microscopic study of diamond/film interface will be very helpful to reveal the mechanism of diamond synthesis and growth.Diamond single crystals were synthesized in a cubic anvil apparatus under a high temperature of approximately 1623K and a high pressure of 5.5GPa using graphite as the carbon source and Fe-Ni alloy manufactured by powder metallurgical technology as the molten catalyst. The microstructure, composition, phase structure of iron based metal film interface were characterized by means of Scanning Electron Microscopy (SEM), Field Emission Scanning Electron Microscopy (FESEM), X-ray Diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Field Emission Transmission Electron Microscopy (FETEM) to determine the metal film phase composition about diamond growth under high pressure and high temperature. In order to characterize the electronic structure information of carbon, iron, nickel atoms, search catalytic mechanism in the process of diamond growth, we research the change of sp2 to sp3 status, the 3d electron change of iron and nickel atom from the interface to the inner layer interface by mean of EELS technique. In addition, combined with thermodynamic theory to explain the catalyst phase in the process of change and discuss its role in diamond synthesis and mechanism.SUPRA55 and SU70-type FESEM was used to investigate (100) film, it found that a length and width of less than lOOnm respectively convex quadrilateral columnar structure. EDS data showed that there were iron, nickel and carbon elements in the convex quadrilateral columnar structure and pyramid columnar convex accumulation area.XRD of the diamond/film interface to do a full-spectrum analysis that the interface consists of three phasesγ-(Fe,Ni), Fe3C and (Fe,Ni)23C6. From XRD peak intensity it can be seen there are a lot of y-(Fe,Ni) phase, and a carbon-rich phase is Fe3C. There is no graphite and diamond structure.In order to further analysis the growth mechanism of diamond, we also use the energy resolution of less than 2nm FETEM to observe the phase change in the diamond/film interface and the inner layer of the film. Observation position is the film interface and the inner layer of film (2μm,4μm and 6μm depth from the diamond/film interface). It was found that there existed y-(Fe,Ni), Fe3C at the position of 0μm and 2μm from the diamond/film interface corresponding to y-(Fe,Ni), Fe3C and graphite at the 4μm,6μm. As the interface contacting with the diamond crystal directly, the phase on diamond/film interface play the role of diamond growth directly. Therefore, we can conclude that:a certain depth away from the diamond/film interface (for example,4μm and 6μm), graphite can not be directly catalyzed into diamond structure, but in the diamond/film interface, graphite carbon atoms were gradually turned into diamond structure from Fe3C. Therefore, Fe3C discovery and the disappearance of graphite on the diamond/film interface, which fully illustrates the carbon source of diamond growth is Fe3C instead of graphite.On above basis, the diamond/film was investigated by FETEM combined with EELS and found that C-sp3 content increased from 78.15%to 87.33% corresponding to the 3d-occupancy of Fe decreased from 5.64 to 4.54 electron/atom from diamond/film interface to the inner layer of film. EELS peak and intensity can be clearly seen that the electronic structure of carbon atoms had changed dramatically in the different layers of the film. According to valence bond theory, Fe3C the formation should increase the 3d electron occupancy contrast to the actual experiment. Analysis result that there are a lot ofγ-(Fe,Ni) in the 3d unpaired electron interaction with the Fe3C. Well then, there must be a catalytic phaseγ-(Fe,Ni) using 3d unpaired electrons affect carbon atoms in Fe3C lattice and make them transform to sp3-like state in the film/diamond interface, then carbon atoms with the sp3-like structure are separated from the Fe3C and stack on the growing diamond crystal, resulting in 3d-state occupancy for Fe reduction on the interface eventually. Therefore, it is believed that the catalytic mechanism of diamond synthesis at high temperature and high pressure:catalytic phaseγ-(Fe,Ni) using 3d unpaired electrons change the electronic structure of carbon atoms, make the carbon atoms of Fe3C transformed from sp2 to sp3 gradually and the formation of diamond on the interface ultimately.Consider the diamond synthesis conditions of high temperature and high pressure, the Gibbs free energy of the decomposition from Me3C-type (Me:Fe, Ni, Mn) phases to diamond were calculated with the determinant method ofΔG<0 in thermodynamics theory. It was found that the Gibbs free energies of Me3C→C (diamond)+3Me is more negative than graphite→diamond, which means the former will take place more easily at the range of 1500K～1700K,5GPa～6GPa. Therefore, from the viewpoint of thermodynamics the formation of Me3C explained that the diamond crystal growth with catalysts comes from the decomposition of Me3C instead of the direct transformation from graphite structure to diamond structure, which suggested Fe3C was more easily transformed to diamond.