Zhao-Yin Hao

Non-equilibrium microstructure of hyper-eutectic Al-Si alloy solidified under superhigh pressure

The non-equilibrium microstructures of hyper-eutectic Al-26.6wt%Si solidified under superhigh pressure (5.5 GPa) have been investigated. The results show that there exists a great deal of primary α phase in hyper-eutectic Al-Si alloy. The non-equilibrium microstructure for hyper-eutectic Al-Si alloy is composed of primary α phase, β phase and (α + β) eutectic phase. The solid solubility of Si in α phase and the solid solubility of Al in β phase increase significantly. The effects of high pressure on the solidification structures of Al-Si alloy are discussed.

Planar defects and dislocations in HPHT as-grown diamond crystals

In the present paper, diamond crystals approximately 0.8 mm in dimension were synthesized at high temperature and high pressure (HPHT) in the presence of FeNi catalyst in the diamond stable region. An argon beam-milling machine thinned the HPHT as-grown diamonds until they were suitable for examination by cross-sectional transmission electron microscopy (TEM). It was shown that there are a number of twins, stacking faults and dislocation networks on (111) planes in the HPHT-grown diamond crystals. During the diamond growth at HPHT, the growing diamond inevitably traps impurities. Dislocation networks near the zone relieve the concentration stresses caused by the impurity-enriched zones. Twins may be formed mainly due to the carbon atoms falling by mistake into positions where a twin crystal can form during diamond growth. Another possibility for twin formation in the HPHT as-grown diamonds is that twins are initially formed during the nucleation process as in CVD diamonds. Moire images reveal that the density of stacking faults is high. The stacking faults may be formed mainly due to rapid growth of the diamond at HPHT. Another possibility for stacking faults formation is related to the condensation of supersaturated vacancies in the HPHT as-grown diamonds on the (111) plane during rapid quenching after diamond synthesis. The terminating of stacking faults on intersecting twins by moire image suggests that the bordering partial has propagated by glide up to the twin interface during diamond growth, this may be described by the reaction of Shockley partial dislocation with a twin on the (111) plane.

Impurities identification in a synthetic diamond by transmission electron microscopy

Three types of impurities, which were trapped in diamond single crystal during process of the diamond crystal growth under high temperature and high pressure in the presence of iron–nickel solvent catalyst, have been successfully and directly investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Both the chemical composition and the crystal structure of the impurities were identified and determined. The impurities may be derived from the starting materials and the medium (pyrophillite) for transmitting the pressure, they consisted of amorphous graphite, f.c.c. (FeNi)23C6 and orthorhombic FeSi2.

Characteristics of some inclusions contained in synthetic diamond single crystals

Abstract Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been successfully used to investigate microstructures of synthetic diamond single crystals grown from the Fe–Ni–C system under high temperature and high pressure. Several types of inclusions incorporated into the diamond during the process of diamond growth were identified. Both the chemical composition and structure of the inclusions in diamond were successfully determined. It was found that the inclusions trapped in the diamond consisted of f.c.c. (FeNi) 23 C 6 , orthorhombic FeSi 2 , f.c.c. silicon carbide and amorphous graphite.

Some inclusions and defects in a synthetic diamond single crystal

Abstract Transmission electron microscopy (TEM) has been used to study inclusions and defects in a synthetic diamond single crystal grown from Fe–Ni–C system under high temperature–high pressure. The (FeNi)23C6 and SiC inclusions trapped in the diamond were identified. The stacking fault, twin and prismatic dislocation in the diamond were directly observed. The formation mechanisms of the inclusions and defects were analyzed.

Some aspects of diamond crystal growth at high temperature and high pressure by TEM and SEM

Abstract Diamond crystal growth process at high temperature–high pressure (HPHT) from Fe–Ni–C system was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Parallel growth cellular interfaces in the growing diamond crystals were directly observed by transmission electron microscopy (TEM) for the first time. The presence of the cellular growth interface indicates that the diamond crystal grows from solution of graphite in molten catalyst and there exists a narrow constitutional supercooling zone in front of the growth interface during the diamond crystal growth. The formation of the cellular interface may be related to the solubility difference between diamond and graphite in molten catalyst. The successive parallel arrays of layers with cellular interface by TEM suggest that the diamond grows from solution layer by layer, which can be further confirmed by morphologies on the surface of growing diamonds and as-grown diamonds obtained by SEM. Some particle clusters were found on the growing parallel layers and diamond surface by TEM and SEM, which may be the diamond atom clusters transmitted to the growing diamond through diffusion. This study provides direct evidence that the diamond is formed at HPHT through graphite continuous dissolution in the molten catalyst to form a colloidal solution, transition of graphite to diamond under the action of the catalyst, diffusion of the diamond atom clusters to the growing diamond, and collection or unification of the diamond subcritical atom clusters on the growing diamond crystal.

Nanosized crystalline foreign particles contained in cubic boron nitride single crystals

Cubic boron nitride (c-BN) crystals with a dimension of 0.3–0.4 mm, which have been synthesized using lithium nitride (Li3N) as a catalyst under high temperature–high pressure (HPHT), were examined by transmission electron microscopy (TEM). It was shown by TEM that there exits some nanostructrued areas containing several types of nanosized foreign particles within the as-grown c-BN crystals. The results indicate that the nanometer inclusions are hexagonal BN (h-BN) with a lattice constant of a=2.054 A, c=6.661 A, tetragonal B25N with a lattice constant of a=8.79 A, c=5.08 A, tetragonal B53N with a lattice constant of a=8.79 A, c=5.08 A, and hexagonal Li3N with a lattice constant of a=3.648 A, c=3.875 A.

Analysis of nanometer inclusions in high pressure synthesized diamond single crystals

Abstract It was shown by transmission electron microscopy (TEM) that there contain nanoscale crystalline inclusions about several nanometers in dimension in diamond single crystals synthesized in the presence of Fe70Ni30 alloy catalyst under high pressure and high temperature (HPHT). The inclusions distribute homogeneously within the diamond matrix. It was indicated by selected area electron diffraction (SAD) patterns combined with EDS analyses that the inclusions consist of three main types: hexagonal graphite with a lattice constant of a=2.463 A , c=6.714 A , f.c.c. (FeNi)23C6 with a lattice constant of a=10.89 A , and f.c.c. γ-(FeNi) with a lattice constant of a=7.146 A .

Defect formation in diamond single crystals grown from the Fe-Ni-C system at high temperature and high pressure

Some defects were formed in diamond single crystals grown from an Fe-Ni-C system at high temperature high pressure (HPHT). These defects were successfully examined by transmission electron microscopy (TEM) and a kind of indirect lattice image called moire fringe. These defects are mainly composed of vacancy-type prismatic dislocation loops, stacking-fault tetrahedra, an array of parallel dislocation lines, and dislocation networks. The formation process of these defects was analyzed briefly. It was suggested that these defects in the diamond crystal were derived from vacancies and inclusions, which were contained in the diamond single crystal during the diamond synthesis at high temperature and high pressure.

Inclusions related to catalyst and medium for transmitting pressure in diamond single crystals grown at high temperature and high pressure from the Fe-C system

Inclusion entrapment in a crystal is one of the most important characteristics for the crystal growth technique from solution. Diamond single crystals grown from the Fe-C system at high temperature-high pressure usually contain inclusions related to the molten catalyst and the medium (pyrophyllite) for transmitting pressure. During the growth of the diamond, the inclusions are trapped by the growth front or are formed through reaction between the contaminants trapped in the diamond. In the present article, the inclusions related to the catalyst and pyrophyllite were systemically examined by transmission electron microscopy. The chemical composition and crystal structure of the inclusions were, for the first time, determined by selected area electron diffraction pattern combined with energy dispersive x-ray spectrometry. It was shown that the inclusions are mainly composed of orthorhombic Fe3C, orthorhombic FeSi2, hexagonal SiO2 and face-centred cubic SiC.