Graphitization of diamond

Ni surface [171], Fullerene (Ceo) powder of 0-25 pm and graphite powder of 10-15 pm were also used [172]. In Refs. [173, 174], diamond powder of 0.5 pm in size, which was suspended in acetone, was apphed on the Ni surface. For heteroepitaxial growth of diamond on Ni, the seeding step is very important to make oriented diamond nuclei and suppress the graphitization of diamond simultaneously. 

The graphitization of diamond may involve thermally-allowed six re-electron concerted processes an electrocyclic reaction converts a six-membered ring in the diamond lattice into three isolated double bonds as shown in Fig. 12. Subsequently, each of the three cyclohexene rings can undergo a retro-Diels-Alder reaction, multiplying the number of double bonds in a triple cascade reaction 23]. 

At even higher temperatures, bulk graphitization of diamond grains starts to take place and the hardness and wear properties of pcD produced (binderless) start to... 

Boron nitride is chemically unreactive, and can be melted at 3000 K by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of giant molecules as in graphite or diamond (p. 163). The bond B—N is isoelectronic with C—C. 

Carbon is found free in nature in three allotropic forms amorphous, graphite, and diamond. A fourth form, known as "white" carbon, is now thought to exist. Ceraphite is one of the softest known materials while diamond is one of the hardest. 

Synthetic Diamond. In 1955 the General Electric Company announced the successful production of diamonds (see Carbon, diamond, synthetic) from graphite under very high pressure and temperature ia the presence of a metal catalyst. It was later reported that a Swedish company, Allmana Svenska Electriska AB (ASEA), had succeeded ia ptoduciag diamond ia 1953 (35). 

Interest in the synthesis of diamond [7782-40-3] was first stimulated by Lavoisier s discovery that diamond was simply carbon it was also observed that diamond, when heated at 1500—2000°C, converted into graphite [7782-42-5]. In 1880, the British scientist Haimay reported (1) that he made diamond from hydrocarbons, bone oil, and lithium, but no one has been able to repeat this feat (2). About the same time, Moissan beheved (3) that he made diamond from hot molten mixtures of iron and carbon, but his experiments could not be repeated (4,5). 

In this process, diamond forms from graphite without a catalyst. The refractory nature of carbon demands a fairly high temperature (2500—3000 K) for sufficient atomic mobiUty for the transformation, and the high temperature in turn demands a high pressure (above 12 GPa 120 kbar) for diamond stabihty. The combination of high temperature and pressure may be achieved statically or dynamically. During the course of experimentation on this process a new form of diamond with a hexagonal (wurtzitic) stmcture was discovered (25). 

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)... 

The sequence of sheets in graphite is also ABAB however, an examination of the atomic positions shows that they are not simply related to those in either kind of diamond. Thus the simple compression of graphite should not be expected to yield diamond. However, we11-crysta11i2ed graphite, in which the ABAB sequence extends for at least hundreds of layers, tends to form wurt2itic carbon. The rare rhombohedral form of graphite has an ABCABC sequence of sheets, but its scarcity has hindered its study as a source for diamond. 

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