Pressure-temperature diagram, diamond synthesis

Figure 9. Pressure-temperature diagram for diamond synthesis showing typical conditions (P, Ti). The supersaturation Ac is a function of P Ty giving rise to different crystal morphology at different conditions.

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.

Another approach to diamond synthesis involves energetic ion or laser beams to produce local areas where, for short duration, carbon atoms are subjected to pressure and temperature conditions that reach into the diamond stable region of the carbon phase diagram. After rapid quenching to ambient temperature and pressure, diamond thus formed remains metastable with respect to graphite. Attempts to deposit diamond by such techniques have been only moderately successful. 

Figure 1. Carbon phase diagram with temperature and pressure ranges corresponding to various diamond synthesis processes, as indicated by the shaded areas. (Reproduced with permission from Ref 3, American Chemical Society, 1989)...

Figure 1. P T diagram of the low-pressure, low-temperature labile equilibriums of carbon solution 1 = graphite-diamond equilibrium line, 2 = glassy carbon-diamond transition line, 3 = range of pneumatolytic hydrothermal processes, 4=oxidative corrosion of diamond, 5 = anticipated area of diamond hydrosynthesis, 6 and 7 = diamond synthesis from glassy carbon precursors, 8 = low-pressure, low-temperature hydrothermal homoepitaxy of diamond. Reproduced from [15] with permission from A. Szymanski.

Figure 5. Pressure and temperature conditions of the diamond synthesis (a) shock wave production of diamond (b) high temperature, high pressure regime for the synthesis of diamond (c) catalytic region for diamond formation (d) chemical vapor deposited diamond and (e) transformation of Cjo into diamond. The most recent review of the P, T phase diagram of carbon can be found elsewhere [151].

Careful experiments on the thermodynamics of graphite and carbon, notably by Rossini and Jessup at the U.S. National Bureau of Standards, led to a prediction by Leipunskii that pressures of 40 to 60 kbar would be required for diamond synthesis at temperatures of 1000 to 1700°C. The Rossini-Jessup-Leipunskii phase diagram turned out to slightly underestimate the pressure required for the stabdity of diamond, and was particularly important as a guide for experimentalists. The phase diagram of Berman and Simon, proposed shortly after diamond synthesis was successful, has been verified by a number of careful experiments. 

In section two of tUs paper we describe the technical aspects of C02-laser heating in a DAC. The third section focuses on the methods for measuring melting temperatures at variable pressures, the fourth section on the determination of high pressure and temperature phase diagrams, and in the fifth section some experiments focusing on the synthesis of diamond and cubic BN from organic precursors will be described. 

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