Diamond, Synthetic

Abstract

In 1955, a team of research workers at General Electric developed the necessary high pressure equipment and discovered solvent–catalytic processes by which ordinary forms of carbon could be changed into diamond. In the catalyzed synthesis process, a mixture of carbon (eg, graphite) and catalyst metal is heated high enough to be melted, while the system is at a pressure high enough for diamond to be stable. Graphite is then dissolved by the metal and diamond is produced from it. Effective catalysts are Cr, Mn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, and Ta, and their alloys and compounds. Many kinds of apparatus have been devised for simultaneously producing the high pressures and temperatures necessary for diamond synthesis. An early successful design is the belt apparatus. A belt apparatus is capable of holding pressures of 7 GPa (70 kbar) and temperatures of up to 3300 K for periods of hours. Size, shape, color, and impurities are dependent on the conditions of synthesis. Lower temperatures favor dark‐colored, less pure crystals; high temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures produce octahedral faces. If diamond seed crystals are placed in the active diamond growing zone of a typical graphite–catalyst metal apparatus, new diamond usually forms on the seed crystals. Excellent growth can be obtained if pressure and composition are held relatively constant. In the direct graphite‐to‐diamond process, diamond forms from graphite without a catalyst. The refractory nature of carbon demands a fairly high temperature (2500–3000 K) for sufficient atomic mobility for the transformation. When graphite is strongly compressed and heated by the shock produced by an explosive charge, some (up to 10%) diamond may form. The annual production of diamond by this process is only a small fraction of total industrial diamond consumption. Diamond can form directly from graphite at pressures of about 13 GPa (130 kbar) and higher at temperatures of about 3300–4300 K. No catalyst is needed. The transformation is carried out in a static high pressure apparatus. Diamond forms in a few milliseconds. Metastable growth of diamond takes place from gases rich in carbon and hydrogen at low pressures where diamond would appear to be thermodynamically unstable. In a typical use of this method, a mixture of hydrogen and methane is fed into a reaction chamber at a pressure of about 1.33 kPa (10 torr). The substrate upon which diamond forms is at about 950°C and lies about 1 cm away from a tungsten wire at 2200°C. This method has been actively studied since 1993. As of this writing (1993) the price of synthesized diamond is competitive with that of natural diamonds. The bulk of synthetic industrial diamond production consists of the smaller crystal sizes up to 0.7‐mm particle size (25 mesh). This size range has wide utility in industry, and a significant fraction of the world's need for diamond abrasive grit is now met by synthetic production yielding thousands of kilograms per year. Semiconducting diamonds are prepared by adding small amounts of boron, beryllium, or aluminum to the growing mixture. The production of synthetic sintered diamond masses having excellent mechanical properties has only been achieved recently. About 90% of industrial diamond is synthesized at high pressures because its price is relatively low, and it can be tailor‐made for efficiency in each application.