Li Musen

Carbide Identification in Different Regions of a Thin Metal Film Covering on an HPHT As-Grown Diamond Single Crystal from Ni–Mn–C System

Diamond single crystals were synthesized in the presence of Ni–Mn catalyst under high temperature and high pressure (HPHT). A thin metal film covering on as-grown diamond formed during diamond growth was examined using transmission electron microscopy. It was shown that phase compositions of the region near the as-grown diamond are different from those of other regions in the film. We found γ-(Ni,Mn) solid solution, diamond, Ni3C and Mn23C6 in the region near the as-grown diamond, while graphite, Mn7C3 and γ-(Ni,Mn) could be found in other regions of the film. The relationship between the diamond growth and the carbides in the film was analysed briefly. It is suggested that the carbon source for diamond growth should be closely related to the decomposition of carbides in the region near the diamond single crystal at HPHT, not being directly from that of the graphite structure.

carbon Nitride Compounds Synthesized by Thermal Annealing Amorphous Nanostructured Graphite under the Flow of NH3 Gas

Graphitic-C3N4 (g-C3N4) and pseudocubic-C3N4 (p-C3N4) have been synthesized by thermally annealing high-energy ball milled amorphous nanostructured graphite powders under NH3 atmosphere. The experimental results by x-ray, transmission-electron microscopy, selected electron area diffraction and parallel electron energy loss spectroscopy indicated that g-C3N4 grew from the milled graphite powders in the presence of NH3 gas at a temperature of 1050°C. After treatment at a temperature of 1350°C, the pseudocubic-C3N4 phase forms. It was believed that the high-energy ball milling generates nanosized amorphous graphite structures, under subsequent isothermal annealing in a flow of NH3 gas, the carbon nitride compound can easily form through reaction of nanostructured carbon with nitrogen of NH3.

Twin diamond crystals grown at high temperature and high pressure from the Fe-Ni-C system

Twin diamond crystals grown at high temperature and high pressure (HPHT) in the presence of FeNi catalyst have been examined by transmission electron microscopy (TEM). Direct observation by TEM shows that there are a large amount of twins which lie on the {111} planes in the HPHT-grown diamonds. The twins in the diamond may be formed and may extend into the inner crystal from the twin nucleus formed in the nucleation process. The twins can be formed due to the carbon atoms falling mistakenly into positions where a twin crystal can form during diamond growth, or condensation of supersaturated vacancies on the {111} plane. Some hexagonal dislocation loops related to supersaturated vacancies are found on the twins. The Moire fringe image reveals that stacking faults terminate on the intersecting twin boundary. This suggests that, at the temperature that the HPHT diamond is grown, the bordering partial has propagated by gliding up to the twin interface, which can be described by the reaction of a Shockley partial dislocation with a twin on the {111} plane.

Effect of metallic film in diamond growth from an Fe-Ni-C system at high temperature and high pressure

The metallic film surrounding a diamond single crystal, which plays an important role in the diamond growth from an Fe-Ni-C system, has been successfully investigated by using transmission-electron microscopy (TEM), Raman spectroscopy and x-ray photo-electron spectroscopy (XPS). Diamond and graphite were not found in surface layer (near diamond) of the film by TEM and Raman spectroscopy, but a parallel relationship exists between the (1) plane of γ-(Fe,Ni) and the (100) plane of (Fe,Ni)3C in this region. Compared with that of solvent metal (catalyst) near diamond, the binding energy in the valence bands of iron, nickel and carbon atoms of the film has an increase of 0.9 eV. According to the microstructures on the film obtained by the TEM, Raman spectra, and XPS, the catalytic mechanism of the film may be assumed as follows. In the surface layer of the film, iron and nickel atoms in the γ-(Fe,Ni) lattice can absorb carbon atoms in the (Fe,Ni)3C lattice and make them transform to an sp3-like state. Then carbon atoms with the sp3-like structure are separated from the (Fe,Ni)3C and stack on the growing diamond crystal. This study provides a direct evidence for the diamond growth from a metallic catalyst-graphite system under high temperature and high pressure.

Influence of Dysprosium Distribution on Properties of Sintered and Aged Dy-doped NdFeB Permanent Magnets

Abstract Sintered and aged Dy-doped NdFeB magnets were investigated. The magnetic properties, the microstructures and the compositions were characterized by hysteresis loop instrument, thermal field emission scanning electron microscopy (TFESEM) and energy disperse spectroscopy (EDS), respectively. The results indicate that Dy element is mainly distributed in the Nd-rich phase, Nd-Dy oxides and Dy-rich particles located at the grain boundaries of the sintered magnets, in addition to the main crystal phase Nd2Fe14B. The optimized aging process is beneficial to promote a reasonable diffusion and distribution of Dy element. The Dy contents of Nd-riched phases, Nd-Dy oxide, and Dy-riched particle decrease successively in the sintered, the high temperature aged and the optimally two-stage aged Dy-doped NdFeB permanent magnets. The measurements demonstrate that the enhancement of the coercivity of the aged Dy-doped NdFeB Magnet is caused mainly by the Dy element reasonable distribution.

Interface Instability of Diamond Crystals at High Temperature and High Pressure

Diamond growth instability at high temperature and high pressure (HPHT) has been elucidated by observing the cellular interface in diamond crystals. The HPHT diamond crystals grow layer by layer from solution of carbon in the molten catalyst. In the growth of any other crystals from solution, the growth interface is not stable and should be of the greatest significance to understand further the diamond growth mechanism. During the diamond growth, the carbon atoms are delivered to the growing diamond crystal by diffusion through a diamond crystal-solution boundary layer. In front of the boundary layer, there is a narrow constitutional supercooling zone related to the solubility difference between diamond and graphite in the molten catalyst. The diamond growth stability is broken, and the flat or planar growth interface transforms into a cellular interface due to the light supercooling. The phenomenon of solute trails in the diamonds was observed, the formation of solute trails was closely associated with the cellular interface.

Characterization of Growth Hillocks on the Surface of High-Pressure Synthetic Diamond

Diamond crystals, with dimensions of about 0.5-0.6 mm, were synthesized in the presence of Fe-Ni and Fe-Ni-Si catalyst solvents under high-pressure-high-temperature (HPHT) conditions. The as-known dendritic pattern was clearly seen on the (111) or (100) planes of diamond single crystals grown using Fe-Ni as a catalyst solvent. However, the conventional dendritic pattern was not observed in diamonds grown in the presence of Fe-Ni-Si alloy catalyst. Trigonal-type, pyramid-type, polygonal-type and rectangular-type growth hillocks were clearly observed on the (111) and (100) surfaces of diamonds grown from the Fe-Ni-Si-C system, and the density of the hillocks is very high at some positions. Clear successive growth layers can also be found on the (111) planes of the high-pressure diamond single crystals grown in the presence of Fe-Ni-Si alloy catalyst. The growth hillocks distributed on the (111) and (100) planes of the diamonds generally occur on or near growth steps, and some of the hillocks terminate at certain solid inclusions and voids. Growth hillocks on the (111) and (100) surfaces directly indicate the spiral growth mechanism under HPHT. A possible formation process for growth hillocks is proposed.