Grains polycrystalline diamond

Gaudin et al. [91] have suggested that conductivity in polycrystalline diamond films is due to migration of adsorbates such as water, hydrocarbons, N02 and NH3 through the film grain boundaries. These are believed to be capable of influencing the properties of the near surface of the film. 

The soultrananocrystalline diamond (UNCD) is a special form of polycrystalline diamond films consisting of nanoscopically small individual crystallites with diameters of 3-10 nm (Figure 6.4). Owing to this fine structure, the portion of carbon atoms being part of grain boundaries is in its turn markedly increased as compared to microcrystalline diamond films, which again influences properties like the surface conductivity. 

O. Shenderova and D. W. Brenner, Mater. Res. Soc. Symp. Proc., 442, 693 (1997). Coexistence of Two Carbon Phases at Grain Boundaries in Polycrystalline Diamond. 

Successful hydrothermal diamond synthesis was carried out in autoclaves filled with a specially prepared carbon enriched water solution , the composition of which was not disclosed [15,45,46]. The carbon precursor should be fine-grained diamond, vitreous carbon or emulsion of crude oil and water [29]. The presence of free radical catalysts was mentioned and paragenetic crystallization of quartz needles and diamond indicate the presence of silicon [45]. The synthesis was described as a sol/gel colloidal process working in the range 200-600°C and 100-200 MPa. Healing and joining of diamond crystals was reported. After 21 days at 400°C and 170 MPa, thin colorless films of polycrystalline diamond were obtained on (111) surfaces of seed crystals (Fig. 3c). With a reported size of 15-40 pm, these are the largest diamond crystals from hydrothermal experiments. 

Polycrystalline diamond or PCD is produced by sintering micron diamond powders under ultrahigh pressure (>5 GPa) in the presence of a metal catalyst such as cobalt. These materials are commercially available in a range of grain sizes and typically are... 

Diamond abrasive was suspended in a water-based carrier and supplied by a peristaltic pump at a flow rate of 0.75 mL/min. The slurry was based on either monocrystalline or polycrystalline diamond grains with 0.25 pm grit size. 

During the experiments, only the machining time was varied 2, 7,17, 47, and 107 min, for each grain size, for both monocrystalline and polycrystalline diamond slurry. 

Grain size distribution for 25 p.m polycrystalline diamond (a) before lapping (b) after lapping. 

Recently, some works have been done on the HR-EELS spectrnm of single-crystal diamond surfaces and polycrystalline diamond snrface consisting of diamond grains in the micron to nano-size range deposited by CVD (see, e.g.. Refs. 81-85) and of DND powder. ... 

Diamond and cBN powders produced by milling are essentially monocrystalline and dominate the market. However, polycrystalline diamond powder can also be produced by shock synthesis. Under suitable conditions, shock waves produced by explosively driven projectiles can produce HPHT conditions in confined volumes for a sufficient duration to achieve partial conversion of graphite into nanometer-sized diamond grains which can also sinter into micrometer-sized, polycrystalline partieles." This process was commercialized by DuPont to produce a polycrystalline DMP (trade name Mypolex ) that is more friable than monocrystalline DMP and is well suited to fine polishing applications. Hexagonal (graphite-hke) BN will also react under shock-synthesis conditions, but the dense, nanometersized particles that are produced are of the wurtzite phase (wBN) rather than the cubic phase. So far, nano-wBN has not achieved much commercial importance. 

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