Diamond, synthesis

Diamonds Sapphires Diamond synthesis Diamomque Diamoni quell Diamomquelll Diamomte Diamontina Diamox Diamox sodium Diampron [3671-72-5]... 

Figure 15 shows the variation of diamond deposition rates by various activated CVD techniques as well as the HP—HT technique (165). It can be seen that the highest growth rate of activated CVD diamond synthesis is stiU an order of magnitude lower than the HP—HT technique. However, CVD has the potential to become an alternative for diamond growth ia view of the significantly lower cost of activated CVD equipmeat and lower miming and maintenance costs. 

In the attempt at diamond synthesis (4), much unsuccesshil effort was devoted to processes that deposited carbon at low, graphite-stable pressures. Many chemical reactions Hberating free carbon were studied at pressures then available. New high pressure apparatus was painstakingly buHt, tested, analy2ed, rebuilt, and sometimes discarded. It was generally beheved that diamond would be more likely to form at thermodynamically stable pressures. 

The refractory nature of carbon indicated that temperatures of about 2000 K and pressures of 5—10 GPa (50—100 kbar) might suffice. Months after operating pressures of ca 7 GPa (70 kbar) had been attained, a reproducible diamond synthesis called the cataly2ed diamond synthesis was finally achieved. 

Comprehensive accounts of the methods and phenomena involved in diamond synthesis can be found in references 14—16. 

Soon after the first successful diamond synthesis by the solvent—catalyst process, a pilot plant for producing synthetic diamond was estabUshed, the efficiency of the operation was increased, production costs declined, and product performance was improved while the uses of diamond were extended. Today the price of synthesized diamond is competitive with that of natural diamonds. 

Recently, diamond synthesis has been successfully performed under high-temperature, high-pressure conditions in a system using kimberlite powder, various carbonates, sulphates or water as the solvent [13], [14]. Higher pressure and temperature conditions are required in a non-metallic solution than in a metallic solution, and the crystals obtained are mainly simple octahedral, differing from those observed in crystals grown from metallic solutions. Crystals synthesized in a non-metallic solution show the same characteristics as natural diamond Tracht. These observations indicate that the solvent components have a definitive effect upon surface reconstruction, and thus on the morphology of the crystals. 

A quite different set of dynamic high-pressure techniques are based on the use of chemical or nuclear explosions to produce transient shock waves of high peak pressure but short duration. With such methods, one can often penetrate the high-T, P regions where kinetic barriers become unimportant and a catalyst is unnecessary. However, the same kinetics that allows facile conversion of graphite to diamonds as the shock front arrives also allows the facile back-conversion as the shock wave passes. As a pioneer of shock-wave diamond synthesis remarked ruefully, We were millionaires for one microsecond [B. J. Alder and C. S. Christian. Phys. Rev. Lett. 7, 367 (1961) B. J. Alder, in W. Paul and D. M. Warschauer (eds). Solids under Pressure (McGraw-Hill, New York, 1963), p. 385]. 

Specific design aspects for specialities and for superpressure facilities e.g. for the diamond synthesis and others are not addressed. 

Figure 15.6 The (graphite + diamond) phase diagram, including the pressure-temperature region for diamond synthesis with ferrous metals and their alloys as solvent catalysts. Reproduced with permission from H. M. Strong, Early Diamond Making at General Electric , Am. J. Phys., 57, 794-802 (1989). Published by the American Association of Physics Teachers.

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