Preparation of Aerosol Ni-Mn-Co Alloy Powder Catalyst and Its Synthetic Diamond Characteristics
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2 Test method Rapid solidification gas atomization technology The principle of preparing alloy powder is to inject molten metal into the bottom drain, and a gas atomizing nozzle is placed under the bottom drain, when the molten metal flows through the nozzle through the nozzle. The high velocity gas emitted by the nozzle is atomized into small droplets, and then the alloy droplets rapidly solidify and cool into powder when flying in the atomizing gas. Compared with traditional casting technology, rapid solidification gas atomization technology has obvious advantages:
1) The organization is small and the composition is small;
2) It can enlarge the solid solubility of the element and can produce some metastable phase or new phase;
3) It has the advantage of large-scale commercial production of directly preparing powder, low oxygen content, low pollution and simple process. Therefore, a vacuum/inert gas atomization process can be used to prepare a high-quality catalyst alloy powder having a low oxygen content, uniform composition, and fine structure. Using Ni, Mn, Co raw materials,
According to w(Ni)=70%, w(Mn)=25%, w(Co)=5% ratio, considering the burning loss of Mn, vacuum induction melting and casting into ingot, the alloy ingot is remelted and then Ar gas Atomized into powder. The atomization process parameters are: atomization pressure 15 5 MPa, superheat degree 250 ° C, and inner diameter of the draft nozzle 3.5 mm. After the alloy powder was sieved through 30 μm and 63 μm, it was used for test analysis and synthetic diamond test. The phase structure of the powder catalyst was analyzed by X-ray apparatus, the microstructure of the catalyst was observed by a metallographic microscope, the morphology of the powder catalyst and the synthesized diamond were analyzed by scanning electron microscopy, and the compressive strength, room temperature impact strength and high temperature impact strength of the synthesized diamond were examined. 900 ° C, 1100 ° C) and other properties.
3 Results and analysis 3 1 NiMn25Co5 powder catalyst characteristics and microstructure The Ni Mn Co alloy powder prepared by gas atomization method has a spherical shape. Since the smelting under vacuum is carried out by inert gas atomization, the Ni MnCo alloy powder prepared has a low oxygen content of the catalyst, and the oxygen content of the 30-63 μm powder is 160×10-6, and the purpose of the less than 30 μm is 220×10-6. The oxygen content is one of the important indicators to distinguish the quality of the powder catalyst, because the oxygen in the catalyst powder will adversely affect the synthesis of diamond. The reasons for this adverse effect may come from two aspects: First, since the oxygen in the Ni Mn Co alloy is mainly in the form of manganese oxide, the formation of manganese oxide will reduce the surface activity of the powdered catalyst and reduce the powdery catalyst and graphite. The contact area reduces the wettability of graphite, affects the dissolution and diffusion of carbon in the powdered catalyst, and is not conducive to diamond nucleation. Second, the oxidation of the catalyst will cause its lattice constant to become large, thus destroying the catalytic structure. The corresponding relationship loses the role of the catalyst. The powder catalyst has a rich specific surface, which is beneficial to increase the contact area between the catalyst and the graphite, increase the diamond nucleation rate and the synthetic yield, and reduce the production cost.
The metallographic microstructure of the alloy powder is fine dendrites. Since the solidification speed of the alloy powder is high during the gas atomization process, the microstructure is much finer than that of the Ni Mn Co alloy produced by the conventional casting and rolling process, which greatly reduces the composition segregation and improves the uniformity of the structure. Sexuality, there is no rolling texture problem of sheet-like catalyst, which is conducive to the uniform nucleation and growth of diamond and the improvement of crystal integrity.
X-ray diffraction analysis of gas atomized Ni70Mn25Co5 catalyst alloy powder showed that the alloy phase structure was a single face-centered cubic solid solution similar to Ni.
3 2 Ni MnCo powder catalyst synthesis diamond process characteristics Φ23mm × 16mm reaction chamber, respectively, powder catalyst and sheet catalyst synthetic diamond test, the situation is shown in Table 2. It can be seen that the diamond synthesis rate of the diamond catalyst is high, the synthetic yield is increased, the amount of coarse-grained diamond is increased, and the compressive strength is greatly improved. In addition, the powder catalyst is reduced by 20% compared with the sheet-like catalyst, the current density of the top of the hardened carbide is reduced, and the life of the top hammer is improved. The average hammer loss is reduced to 2 to 3 kg per 2000 g of diamond. The consumption ratio of catalyst and graphite is also greatly changed, and about 30% of the catalyst alloy can be saved.
In addition, high-yield abrasive grade diamonds can be synthesized using powder catalysts. For the high-yield abrasive grade diamonds synthesized in the Φ23mm reaction chamber, the average yield is 32g, and the diamond grade is 90% above MBD4 and MBD6. It can be seen that the promotion and application of powder catalyst will effectively promote the technological innovation and progress of China's diamond industry, which will inevitably produce huge economic and social benefits. Diamond morphology synthesized by powder catalyst The diamond single crystal is mainly composed of six-octahedron, and the crystal form integrity rate is high. The total amount of pure inclusions in the crystal is small, and the distribution is fine and small. After testing, the synthesized diamond has good room temperature and high temperature impact strength (TI, TTI), and the test results are shown in Table 3. The powder catalyst and graphite are spatially three-dimensionally distributed, which can produce a relatively uniform temperature field and pressure field, which creates favorable conditions for diamond growth. Therefore, the diamond crystal form can grow symmetrically and the crystal form is relatively complete; and the supply of carbon source is The “powder process†is slow and uniform, and the “sheet process†is rapid and unbalanced, so the diamond crystal synthesized by the powder catalyst is pure, the total amount of inclusions is small, and the inclusions and bubbles are less. Therefore, the powdered catalyst has high diamond symmetry, complete crystal form and pure crystal, so that it has high compressive strength and good thermal shock strength.
4 Conclusions 1) The NiMn25Co5 catalyst alloy powder prepared by gas atomization method has uniform composition, spherical particles, fine and isotropic grains, and has a single face-centered cubic solid solution structure similar to Ni. The oxygen content of 30μm powder is <200×10. -6;
2) The diamond crystal synthesized by the powder catalyst has a high integrity rate, pure crystals, a small amount of inclusions, less inclusions and bubbles, high compressive strength and good thermal shock strength;
3) The use of powder catalyst to synthesize diamond can improve the synthesis yield, save raw and auxiliary materials, greatly reduce the production cost of diamond, and not only can synthesize high-yield abrasive grade diamond, but also greatly improve the proportion of high-quality coarse particles and high-grade diamond.