Study Of New Iron-based Catalytic Materials For Diamond Single Crystal Synthesis Under High Pressure And High Temperature

In recent years, with the development of high temperature and high pressuretechnique，more and more attention have been taken on the study of new catalyticmaterials. A variety of non-transition metal materials and non-metallic elements haveshow catalytic effects under much higher synthesis conditions. Based on thedevelopment and application of new catalysts materials, diamonds with specificexcellent performances gradually have been developed. of iron-based catalystdeveloped special diamond can be synthesized with excellent performance with thenew iron-based catalyst material. Our works are divided into the following sections:Firstly, we investigate the diamond crystallization in the Fe-Ni-C with aluminumadditive and the capability of aluminum for converting graphite to diamond in theseries of experiment at4.9to5.5GPa and1240to1500℃. Our experimental resultsshow that the growth habits of diamond crystal have been significantly influenced bythe addition of aluminum in catalyst. The crystals color changes from yellow to nearlycolorless. The morphology of synthesized diamond crystals gradually changes fromcub-octahedron to octahedron in the Fe-Ni-C systems with increasing aluminumadditive. The lowest synthesis conditions fell first and then rose with the increase ofaluminum. We found the suitable addition of aluminum is very effective in loweringthe synthesis conditions while the excessive aluminum additive may have asuppressive effect on the diamond nucleation.Second, we explore the diamond synthesis from Fe-Ni catalyst with P3N5additive in a series of experiment at temperature of1250-1550℃and pressure5.0-6.3GPa. We also investigate the influence of nitrogen on diamond crystallization duringthe diamond crystallization. Our results show that the synthesis conditions(temperature and pressure) increase with the increasing amount of P3N5additive. Nitrogen impurity can significantly influence the diamond morphology. The stablegrowth forms of diamond are strip and lamellar shapes in the nitrogen-richenvironment. FTIR spectrum shows the nitrogen concentration increase rapidly withincreasing content of P3N5additive. By spectrum analysis we found that with theincrease of nitrogen concentration, the Ib nitrogen atoms can aggregate to form Acenters form. The highest nitrogen concentration of A-center is approximate840ppm.Third, we report on the influence of hydrogen impurity in metal melts on thegrowth process, morphology, and defect-and-impurity structure of diamond crystals.In a series of experiments, the hydrogen impurity in the growth system was comefrom decomposition of TiH2in the HPHT chamber. It has been found that withincreasing hydrogen concentration in the synthesis system from0.05wt.%to0.5wt.%,the minimum conditions for diamond synthesis increased rapidly. The effects of thehydrogen on the properties of the crystallization medium have been extensivelystudied by XRD and Raman spectra. The results show that hydrogen can change thecharacteristics of catalyst and graphite. The FTIR spectra of the synthesized diamondindicate that the hydrogen atoms can enter to diamond lattice and substitute thecarbon atoms by the form of sp3-CH2-. The diamond growth and morphology is alsosignificantly influenced by the hydrogen impurity. When hydrogen concentration insynthesis system higher than a certain critical value, the growth of diamond issuppressed and we only can obtain some tiny diamond nuclei. We believe that thecurrent research will help us in-depth understanding of the formation and growthprocess of natural diamond.