Hexagonal diamond synthesis

Diamonds also occur in meteorites, probably as a result of high pressures produced dynamically by impact (10,11). The shock or explosive mode of synthesis is a viable process for fine diamond powders of both the cubic and hexagonal (lonsdaleite) polymorphs (12) naturally or otherwise. Some diamonds in space appear to have formed by processes more closely related to the low pressure chemical vapor deposition processes described later (see... 

Cubic BC2N. Hetero-diamond B C—N compounds have recently received a great interest because of their possible applications as mechanical and optical devices. The similar properties and structures of carbon and boron nitrides (graphite and hexagonal BN, diamond, and cubic BN) suggested the possible synthesis of dense compounds with all the three elements. Such new materials are expected to combine the best properties of diamond (hardness) and of c-BN (thermal stability and chemical inertness). Several low-density hexagonal phases of B,C, and N have been synthesized [534] while with respect to the high-density phases, different authors report contradictory data [535-538], but the final products are probably solid mixtures of c-BN and dispersed diamonds [539]. 

Cubic boron(lll) nitride is manufactured from hexagonal BN in a high pressure synthesis at 50 to 90 kbar and 1500 to 2200°C in the presence of alkali or alkaline earth metals as catalysts. Cubic boron(III) nitride is, after diamond, the hardest known material. It is utilized in the grinding agent sector instead of diamond, due to its better chemical resistance at high temperatures. 

Because fonnation of cubic boron carbonitride is of great fundamental interest with respect to superhard materials much additional effort is needed to succeed in the preferential synthesis of the cubic B-C-N phase. As follows from the above results the most promising way would be synthesis under nonequilibrium conditions such as flash-heating at static pressures or shock-wave compression [140]. Successful synthesis of cubic BC2.5N solid solution in 18% yield by shock-compression of hexagonal BC2.5N has been reported [143]. The material obtained was a single cubic BC2.5N phase with a diamond-like structure and crystals between 5 and 20 nm in size. 

Feynman s vision of miniaturization and the Drexler-versus-Smalley debate on feasibility of mechanosynthetic reactions using molecular assemblers were discussed. Fullerenes are the third allotropic form of carbon. Soccer-ball-structured Cgo with a surface filled with hexagons and pentagons satisfies Euler s law. Howard patented the first generation combustion synthesis method for fullerene production. The projected price of the fullerenes has decreased from 165,000 per kg to 200 per kg in the second-generation process. Fullerenes can also be synthesized using chemical methods, a supercritical extraction method, and the electric arc process. Applications of fullerenes include high temperature superconductors, bucky onion catalysts, advanced composites and electromechanical systems, synthetic diamonds. 

Figure 2.3. The phase diagram for carbon. Solid lines represent equilibrium phase boundaries Position A commercial synthesis of diamond from graphite by catalysts B P/T threshold of very fast (< 1 ms) transformation of diamond to graphite C P/T threshold of very fast transformation of graphite to diamond and D single-crystal hexagonal graphite transforms to retrievable hexagonal-type diamond (Bundy, 1996).

During the next 15 years there were a number of additional innovations in the application of HPHT technology to the synthesis of industrial superabrasives, virtually all of which were pioneered by GE—the Superabrasives business working in close collaboration with the Corporate Research and Development laboratory. In 1956, Wentorf synthesized cubic boron nitride (cBN). Boron nitride does not exist in nature, but the hexagonal form of BN, which has a structure very similar to that of graphite, was a well known synthetic material. The second-hardest material known, cBN has much better abrasion resistance with ferrous alloys and oxidation resistance than diamond. The best catalyst-solvents for cBN synthesis are alkali- and alkaline-earth nitrides and related inorganic salts. GE introduced cBN commercially in 1969 under the tradename Borazon cBN. 

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. 

Another important aspect of adding homopolymer to a block copolymer is the ability to change morphology (without synthesis of additional polymers). Furthermore, morphologies that are absent for neat diblocks such as bicontin-uous cubic double diamond or hexagonal-perforated layer phases have been predicted in blends with homopolymers [183], although not yet observed. Transitions in morphology induced by addition of homopolymer are reviewed elsewhere [1], where a list of experimental studies on these systems can also be found. 

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