Research On Mechanism Of Synthesizing Diamond By Fe-based Powder Catalyst

Synthetic Diamond, as a substitute for natural diamond, is a versatile functional material and applied in many fields including the industrial, scientific and defense fields. As a result, it is of great strategic significance to enhance research on synthetic technology and synthesis of raw materials, optimize and improve existing powder catalyst formulations, develop low-cost catalysts, and prepare special-purpose synthetic diamond abrasives.In this paper, based on the research on iron-based catalyst, the rule of influence for synthetic diamond by the different composition of catalyst and different ratio between catalyst and graphite has been studied deeply, and also the mechanism of catalyst in the process of synthesizing diamond has been discussed. Based on above the research, new low-cost catalyst formula suitable for synthesizing granular structure diamond (CSD) has been designed out and the followed study has been carried out as a result:1) Carrying out Synthesis experiments using powdered catalyst with different ratio of Fe and Ni. The results show that, As the Fe content in catalyst is increasing, the lowest and the best synthesizing conditions are improved somewhat, and diamond V-shaped growth area moves to the upper right; diamond particle's crest value is thicker, the mixed yield per unit, static strength and impact toughness all decrease; the tests on the different ratios of catalyst and graphite have shown:increasing the usage of catalyst can enhance yield per unit. Considering the elements such as the cost of raw materials, diamond production and quality, the best effect can be gained by selecting the content of 70% Fe and 30% Ni and with the 3:7 ratio of catalyst and graphite, and the basic principle for designing catalyst composition has also been established.2) Studying deeply the effect on the synthesizing diamond by adding rare earth elements in FeNi30. It shows As followed:rare earth elements can increase effectively oxygen bonding content in catalyst, enhance the purity of powdered catalyst, and exert varying effect on increasing the mixed yield per unit, the proportion of coarse particles, TI and TTI value of single grain, static strength increases, and the magnetic susceptibility; optimized tests show that the best addition of rare earth is 0.4%.3) According to the basic principle for designing catalyst composition, FeMn-based catalysts with different composition have been designed, and by synthesizing diamond tests, it is studied on the rules which different adding elements and contents effect diamond synthesis, and the result shows that:by using FeMn-based catalyst only, diamond can be synthesized on 5.4～5.6GPa and 1450～1600℃, the synthetic conditions is more strictly than that in FeNi-based catalysts, and diamond particle size is less fine, and its crystal form is poorer; when 5% Ni or Co is added into the FeMn catalyst, the synthesizing pressure and temperature can be reduced more or less; adding rare earth elements contributes to the improvement of diamond crystal; when Ni content of 15% remains invariable, and Mn content varies from 15%,20% to 25%, diamond nucleation increases gradually, the particle size becomes thinner, the diamond color varies from gray-green to black, and crystal shape becomes irregular, and when Mn content is 25% the aggregate structure of diamond can been produced; when Ni and Mn contents keep invariable and Co content increases, the particle size becomes coarse, grain shape becomes better, mixed yield increases, black diamond decreases, and crystal transparency increases.4) Aimed at the increasing needs of market for CSD diamond, by advantage of cost of FeMn-based catalyst, the FeMn25Ni15 catalyst suitable for CSD diamond has been developed, and synthetic technics have also been studied deeply, the result shows that:the performance evaluation of CSD diamond synthesized by FeMn-based catalyst can meet the useful requirements and the cost is significantly lower than that of Ni70Mn30 catalyst which is currently used.5) EDS energy spectrum analysis is used to study the effect of metal envelope in the process of synthesizing diamond, and it is found that metal envelope exerts the effect of transporting carbon source and eliminating impurity to outside in the progress of diamond's growing, and after metal envelope breaks down, diamond will stop growing; the analysis for the phase of metal envelope further shows that the metal envelope's phase mainly consists of Fe3C, (Fe,Ni)23C6, andγ-(Fe,Ni), and the graphite content is very small. Therefore, it can be considered that catalyst grain agglomerating exists in the process of the diamond nucleation.6) Discussing initiatively the problem of source of carbon needed during the progress of diamond's growing; and the analysis on current thermodynamic data shows that, the decomposition of metal carbide(Fe3C) in metal envelope reduces the energy barrier during the progress of transforming graphite into diamond, and it benefits more the diamond's formation; however, through analyzing such elements as the formation conditions of Fe3C in Fe-C Phase diagram and the quantity of diamonds transformed from carbon in Fe3C, the result shows that it has little possibility for Fe3C to exist under high temperature and high pressure, and lacks base in theory and in real application for Fe3C to be used as carbon source during the process of synthesizing diamond. Due to limited test's conditions, it needs further study on carbon sources of diamond.