Valence Electron Theory And Thermodynamics Analysis Of Diamond Growth Mechanism At HPHT

Man-made diamond single crystals not only have the excellent performances of high rigidity, corrosion resistance and high wearing resistance but also the fine properties of optics, acoustics, thermotics and electricity, which make them play important roles in the development of morden science and technology. Undoubtedly, diamond growth mechanism is significant to instructing the commercial production of diamond. However, due to the difficulty of on-line observation at high temperature and high pressure (HPHT) the difficulty of theoretical study is so hard that the academic viewpoints about the growth mechanism especially the key problem about carbon source are still not consistent, and few efforts have been put into the mechanism research in these years. The study on diamond growth mechanism is still a crucial groping subject.Our team has investigated the pattern, composition and structure of catalyst and metal film surrounding as-grown diamond systematically with many kinds of experimental methods in earlier stage. In this paper, according to the former experimental results gained by our team the valence electron structure(VES) of phases existent during the course of diamond growth and the electron density of interfaces were calculated and analyzed with EET and TFDC theory, and the carbon source problem and effect of catalyst were analyzed. Thereby, the diamond crystal growth was investigated in the viewpoint of electron structure. At the meantime, the carbon source problem was analyzed further combined with thermodynamic theory. Accordingly, a new theoretical path was explored to fulfill the study of diamond synthesis mechanism, and a new thought to design the composition of catalyst was put forward.According to the essence of thermal expansion and generalized Hooke's law, the relation between lattice constant of crystal and temperature and pressure were established based on the linear thermal expansion coefficient and elastic constant of crystal. The errors between the lattice constants of graphite at different temperature and pressure calculated with this method and the experimental data are small, which validate the feasibility of the calculational method in this paper. Then the changes of the lattice constants of phases existent during the course of diamond growth with temperature and pressure were calculated to supply a basis for the calculation of VES of crystal at HPHT.According to the valence electron theory, the electron density of carbon source phase/diamond must be continuous, which is the boundary condition for diamond crystal growth. The VES analysis of diamond and graphite shows that the minimum electron density differences between the common planes of diamond and graphite are about 80% at normal temperature and pressure, while they are about 60% at 1600 K and 5.5Gpa. Although their electron densities are approaching with the increace of temperature and pressure, the difference is extraordinarily bigger than 10%, that is, they are not continuous at the first approximation. This discontinuity can't satisfy the boundary condition of diamond crystal growth. Therefore from the viewpoint of electron structure, the cabon source for diamond growth with the method of catalyst at HPHT does not come from graphite directly. The bonds' energy of graphite changes unconspicuously with the change of temperature and pressure. The energy of the strongest bond is about 240 kJ/mol, while the bond energy among the parallel layer planes is very small and the planes are mainly bonded by Van der Waals force. During the diamond growth, parts of the graphite dissolve into the melting catalyst with the manner of C atoms, then form the carbides or solid solutions with metal or alloy catalyst.The former studies about metal catalyst have verified the excellent application prospect and academic value of diamond synthesis with Fe-based catalyst. Our team investigated systematically the Fe-Ni catalyst and metal film after diamond synthesis, and found that there were a lot of Fe₃C andγ-(Fe,Ni) phases on the interface of film/diamond, and considered Fe₃C andγ-(Fe,Ni) as carbon source phase and catalysis phase respectively. Accordingly, in this paper the diamond growth mechanism was investigated with the example of diamond growth from Fe-Ni-C by calculating and analyzing the VES of main phases in metal film and the electron density of interface. The analyses on the VES of Fe₃C and electron density of Fe₃C/diamond interface show that the electron density of the interface is continuous at the first approximation, which can satisfy the boundary condition of diamond growth. Therefore, the carbon source of diamond growth with catalyst at HPHT comes from the carbon atom groups separated from Fe₃C instead of the direct transformation of graphite structure. Moreover, the electron densities of two main crystal planes of Fe₃C are continuous with that of (111) plane of diamond, which can explain perfectly the parallel direction relationship between Fe₃C in the inclusion of diamond crystal and the (111) plane of diamond. The analyses on the VES ofγ-(Fe,Ni) and electron density of Fe₃C/γ-(Fe,Ni) interface show that the electron density of the 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. Thus it can be seen that the results based on valence electron theory analysis meet with the former experimental results gained by our team.In order to analyze the effect of different kinds of catalyst and instruct designing catalyst composition with electron theory, the electron density of the diamond/carbide interfaces which are probably formed during the diamond growth with transition group metals (Fe, Ni, Mn, Co) and their alloys as catalyst and the interfaces formed ofγ-Me solid solutions with different mixture ratio and Me₃C corresponding. The results show that the electron densities across Me₃C/diamond and y-Me/Me₃C interfaces are all continuous at the first approximation, which indicates Me₃C andγ-Me can be considered as carbon source phase and catalysis phase respectively. The electron density continuities of carbide/diamond interfaces are different with carbide, moreover, the better the electron density continuity of interface, the lower the chemical potential getting over to fulfill structure transformation, and the easier the transformation to diamond structure, so the better the diamond quality in the same synthesis condition. The electron density continuity of Fe, Ni, Mn, Co alloying carbide/diamond interfaces are mostly better than that of monometallic carbide/ diamond interfaces; the electron density continuity of the Mn based and Co based carbide/diamond interfaces are better than other carbides; the electron density continuity of the (Fe,Ni)₃C/diamond is the best among Fe based and Ni based carbides. The electron density continuitiy ofγ-Me/Me₃C interfaces are different with the different compositon ofγ-Me. With the increasing of continuity, the growth rate of diamond crystal quickens. The electron density continuity ofγ-(Fe,Ni)/Fe3C interface ebbs with the adding of the Ni content. That is, the growth rate of diamond crystal becomes lower with the increase of the Ni content of Fe-Ni catalyst in the same synthesis condition. The VES analyses of the effect of catalyst mostly meet with the experimental results. Accordingly, a new thought about the design of catalyst composition can be put forward that the Me₃C type carbides must be formed by the effect of catalyst and graphite; the electron density continuity across Me₃C/diamond interface is high; and the electron density continuity acrossγ-Me/Me₃C is appropriate.According to the results of valence electron theory analysis, the diamond growth was also analyzed with thermodynamic theory, and the changes of volume with temperature and pressure were involved in the calculation. The results show that the Fe₃C phases have been formed before diamond nucleation; at the temperature and ressure range of the diamond synthesis method with catalyst, the Gibbs free energies of Fe₃C(?)C(diamond)+3γ-Fe and graphite(?)diamond are all negative, but the former is more little than the latter, which means the former will take place more easily.Therefore, form the viewpoint of thermodynamics the formation of Fe₃C reduces the potential energy of transformation from graphite to diamond, and the diamond crystal growth with Fe based catalyst comes from the decomposition of Fe₃C instead of the direct transformation from graphite structure to diamond structure. Moreover, the P-T (Pressure-Temperature) equilibrium of P_eq(GPa)=1.036+0.002367(K) is gained, which is closer to the equilibrium gained by Bundy. Thereby the feasibility of the thermodynamic calculational method in this paper is verified.The analyses based on the valence electron theory and thermodynamics both hold up the viewpoint that the carbon source of diamond crystal growth with the method of catalyst at HPHT comes from the decomposition of carbide instead of the direct transformation from graphite to diamond. Accordingly, the diamond growth with the method of catalyst at HPHT can be described as following.