Study On Synthesis Technology And Mechanism Of Boron-doped Diamond Single Crystal In Fe-Ni-C-B System Under HPHT

It has been found that the properties of diamond can be significantly enhanced by doping some elements into diamond lattice. Boron atoms, whose atomic radius is close to that of carbon, can enter into diamond lattice relatively easily and nowadays, boron-doped diamond has become a hot spot due to the related advantages in practice. For instance, boron-doped diamond is semi-conductive or even super-conductive and the thermal stability and chemical inertia of boron-doped diamond are much better than those of the normal one. In addition, boron-doped diamond can be manufactured by many kinds of methods. The manufacture and application of special diamond, which is represented by the boron-doped one, will be the important innovative orientation in 21st century. The study on manufacture of boron-doped diamond is significant for the technical progress of the whole industry in China for it will enrich the species of synthetic diamond, upgrade its properties and extend its application fields.However, at present, most studies focus on boron-doped aggregate crystals and poly- crystalline films and only a few on single crystals. Furthermore, almost all the methods for synthetic boron-doped diamond single crystal require strict conditions and involve high cost; therefore, it is hard to obtain high quality boron-doped diamond single crystal under existing industrial conditions.In this light, it is very urgent to obtain synthetic high-quality boron-doped diamond single crystal with cheaper raw materials and simpler method.This paper mainly focuses on synthesis of boron-doped diamond single crystal in Fe-Ni-C-B system under high-pressure and high-temperature (HPHT). The Fe-Ni-C-B reactive system is composed of boron-doped iron-based catalyst alloy and graphite. The boron-doped iron-based catalyst alloy was made by powder metallurgy method by adding boron source into the raw materials. The synthesis technology of boron-doped diamond was promoted by redesigning the composition of the catalyst, improving synthesis process and developing new purification technique. At the same time, the synthesis mechanism of boron-doped diamond under HPHT, consisting of the carbon source and mechanism for diamond growth as well as the formation route, was also studied systemically. This paper is organized based on the whole preparation process of boron-doped diamond including catalyst manufacture, HPHT synthesis and purification, characterization of structure and properties of diamond as well as the synthesis mechanism.As the first step of diamond synthesis, the composition of iron-based catalyst was enhanced by modifying the ratio of pure iron powder to nickel and introducing graphite powder. The chemical composition of raw materials was strictly controlled, the oxygen content, especially. A new kind of formation method for catalyst preparation was developed from powder rolling to sintering, then to punching. This method is helpful to increase the product yield and utilization efficiency of nickel.The composition of boron-doped catalyst was designed by optimizing the boron source and its addition level. The multi-component alloying of catalyst by adding some elements such as copper was discussed as one part of composition design. On the basis of the experimental results, hexagonal boron nitride (hBN) is considered as a feasible boron source. In addition, the addition quantity of hBN should be controlled in the range of a-2a because excessive addition will damage the quality of diamond crystal. Moreover, the added copper is helpful to control the crystal growth rate and enhance the quality of diamond.G4D type graphite disc is admitted as the suitable carbon source for boron-doped diamond synthesis. The structure of assembly bulk was improved by adopting the means of powder catalyst mode to enhance the stability of temperature and pressure in synthesis cell. A new pressure and temperature dynamic match technology was proposed aiming to control the pressure and temperature within the range for high-quality diamond growth. Then the new synthesis technology is realized with the modified high-pressure apparatus. In addition, the real pressure in synthesis cell was calibrated and the temperature in synthesis cell was measured by thermocouple. Due to the experimental work mentioned above, the growth region for high-quality boron-doped diamond single crystal in Fe-Ni-C-B system was definite: P=5.5-5.7GPa,T=1400-1500℃.A new diamond purification process was created by fully utilizing the ferromagnetism and high brittleness of iron-based catalyst and metallic film. The diamond single crystals can be purified completely with mechanical separation methods. Furthermore, the new process is more valuable in practice with no pollution, lower cost and simpler operation.The shape of diamond crystals is cube-octahedral with rough surface and developed {111} faces due to the presence of boron. The shift of Raman scattering peak suggests boron atoms entering into diamond lattice. The B-C covalent bond in diamond is directly found in IR absorption spectra. The boron atoms are inclined to substitute the carbon atoms in diamond lattice on basis of the calculation results with the first principle. In addition, due to the substitution of boron to carbon on the crystal surface, the thermal stability of boron-doped diamond is more excellent than that of the normal one. The surface initial oxidation temperature of the boron-doped diamond is advanced by 170℃, and the apparent activation energy of oxidation process of boron-doped diamond is 3.5 times as much as that of the normal one.The experimental results of this paper further prove that the direct carbon source for diamond growth in Fe-Ni-C-B system under HPHT is metallic carbides, not graphite. The yield and quality of synthetic diamond crystals are determined by the full formation of metallic carbide in catalyst melt, i.e. the solution degree of graphite in it.The diffusion form and route of boron are the same to those of carbon in the process of diamond formation. The boron-doped metallic carbides diffuse through the metallic film surrounding diamond and arrive on the film/diamond interface, where active boron and carbon atoms will precipitate from the carbides and enter into diamond. In general, the formation of boron-doped diamond depends on the decomposition of carbides on the film/diamond interface.Plenty of trace of diamond layer growth was found via modern materials testing technique. The origin of growth steps for diamond layer growth is two-dimensional nucleation at the initial stage and more growth steps originate from the dislocations that result from the mismatch between boron and carbon atom. The layer boron-carbon atom groups, namely the growth unit, which arrive on the surface growing diamond, will be absorbed on the side of the steps and become one part of diamond crystal.