Synthesis polycrystalline diamond

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

Limited supply, increasing demand, and high cost have led to an intense search for an alternative, dependable source of diamond. This search led to the high pressure (ca 5 GPa (0.5 x 106 psi)), high temperature (ca 1500°C) (HP—HT) synthesis of diamond from graphite in the mid-1950s (153—155) in the presence of a catalyst—solvent material, eg, Ni or Fe, and the subsequent development of polycrystalline sintered diamond tools in the late 1960s (156). 

In the diamond stmcture, carbon atoms are present in sp hybridization, with a tetrahedral stereochemistry and a face-centered cubic stmcture that is shown in Fig. 2.1. Besides natural diamond, synthetic diamond has been produced since General Electric first announced its successful high-pressure synthesis in 1955. Sintered polycrystalline diamond, different types of diamond films, and diamondlike carbon are other types of diamond-related synthetic materials, some of which are noncrystalline [13, 19] these solids have their own terminology [10, 20]. Unhke other carbonaceous solids, diamond has a rather limited and specific relevance to adsorption. Indeed, ever since the publication of a pioneering work... 

To address the incorporation of non-cryslalline phases in polycrystalline diamond films and the morphological instabilities at high growth rates, Ravil conducted an experimental investigation of the combustion synthesis of diamond and proposed a model for the development of morphological instabilities in diamond films, as schematically depicted... 

Carbon in the structural form of diamond is the only element used industrially as a hard material. Each year about ten tons of natural diamond and about twenty tons of synthetic diamond (produced via high temperature high pressure synthesis) are marketed as hard materials. While pure diamond is transparent, a yellow tint results from the replacement of some carbon atoms by nitrogen, and a blue, yellow, or even green tint through substitution of carbon by boron atoms. Polycrystalline diamond with impurities, used as an abrasive, is often black. 

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. 

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. 

The development of low-pressure synthesis methods for diamond, such as the chemical vapor deposition (CVD) technique, has generated enormous and increasing interest and has extended the scope of diamond applications. Highly efficient methods have been developed for the economical growth of polycrystalline diamond films on non diamond substrates. Moreover, these methods allow the controlled incorporation of an impurity such as boron into diamond, which in this case forms a ptype semiconductor. By doping the diamond with a high concentration of boron (B/C = O.Ol), conductivity can be increased, and semi-metallic behavior can be obtained, resulting in a new type of electrode material with all of the unique properties of diamond, such as hardness, optical transparency, thermal conductivity and chemical inertness [1,2]. 

Static Pressure Synthesis. Diamond can form direcdy from graphite at pressures of about 13 GPa (130 kbar) and higher at temperatures of about 3300—4300 K (7). No catalyst is needed. The transformation is carried out in a static high pressure apparatus in which the sample is heated by the discharge current from a capacitor. Diamond forms in a few milliseconds and is recovered in the form of polycrystalline lumps. From this work, and studies of graphite vaporization/melting, the triple point of diamond, graphite, and molten carbon is estimated to He at 13 GPa and 5000 K (Fig. 1)... 

By shock synthesis A carbon material is converted into diamond by the action of a shock wave generated, for example, by a detonation or a projectile. This procedure is employed, for instance, to prepare polycrystalline microdiamond with primary particles measuring in the range of nanometers. 

Synthesis experiments for the formation of a diamond-cBN junction are listed in Table 11. Regarding formation of the junction at low pressures, diamond was used as a substrate for cBN deposition in order to improve adhesion of the cBN film to the substrate (291,294,313). A low-pressure diamond film was also deposited on a substrate of a cBN single crystal (or polycrystal) that was made at high pressures (275,329,332-335) or on a polycrystalline cBN film made at low pressures (336-338). The diamond film could grow epitaxially on some kinds of cBN surfaces (35,332,339). The (lll)B surface of a cBN crystal was believed to be more useful as a substrate for diamond growth than the (111)N surface (332). High-resolution electron microscopy confirmed the diamond-cBN parallel epitaxy (335,339), showing almost no misfit... 

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