Jianhong Gong

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.

Compositions, mechanical properties and microstructures of cBN-based composites sintered with Al or TiC

ABSTRACT Cubic boron nitride (cBN)-based composites were prepared from cBN–Al and cBN–TiC mixtures, respectively, under high pressure and high temperature (HPHT). During sintering, Al reacted with cBN and produced AlN and AlB2, while TiC did not react with cBN. With increasing Al from 5–20 wt-%, the strength of the cBN-based composites decreased first, then increased. AlN and AlB2 could strengthen the cBN-based composites to a certain extent. In the preparation from cBN–TiC, 10 wt-% addition of TiC was the optimal formulation to obtain the densest microstructure. Hardness of the two kinds of composites displayed different tendencies. On the other hand, cBN-based composites were treated at 1200°C for 1200 s. It was found that the isothermal oxidation process could be described by parabolic laws with slight oxidation scales formed on samples’ surfaces in different morphologies.

Preferential etching by flowing oxygen on the {100} surfaces of HPHT single-crystal diamond

Application of diamond is determined by its oxidation behaviour in some measure. Oxidation process of single-crystal diamond prepared under high pressure and high temperature (HPHT) has been studied by the thermal analysis, scanning electron microscope (SEM) and Raman spectrometer. The result of a simultaneous thermal analysis indicates that single-crystal diamond is oxidized at ~ 818°C at a heating rate of 5°C/min in the flowing oxygen. Based on the data of the thermal analysis at different heating rates, the activation energy is calculated by the Kissinger method. A weight loss rate increases with the rising heat treatment temperature from 600 to 800°C. After the oxidation at 800°C, etch pits emerge on the {100} surfaces of single-crystal diamond, while the {111} surfaces are smooth. Shapes of the etch pits on the {100} surfaces are inverted pyramidal hollows, with edges direction parallel to the <110> direction.

Microstructure of the (100) and (111) facets of boron-doped diamonds under HPHT

Abstract Boron-doped diamond single crystal was synthesised from Fe–Ni–C–B system at 1773 K and 5.6 GPa for 10 minutes. The crystal shape, structure and morphology of boron-doped diamonds were studied using X-ray diffraction, atomic force microscopy and field emission electron microscopy. Results showed that the {111} faces of boron-doped diamond are prevailing. Field emission electron microscopy and atomic force microscopy analysis indicate the terrace-like and zigzag growth steps both present on (100) and (111) facets with the terrace-like steps are dominant on (100) crystal plane, and the height of the steps is in the range from 3.10 to 5.02 nm. The steps growth is proposed to result from the boron atom incorporation.