Synthetic diamond The quest for a method to synthesize diamonds from
10.4.6.3 Synthetic diamond The quest for a method to synthesize diamonds from
carbonaceous material dates back to the nineteenth century. After World War II, research was stimulated by political uncertainties in Africa that threatened to jeopardize supplies of boart. The first announcement of the successful synthesis of diamonds from graphite was made by the General Electric Company, USA, in 1955. (Later, it transpired that the ASEA, Stockholm, Swe- den had achieved a comparable result two years earlier but had not publicized it.) These remarkable methods simultaneously subjected graphite to extremely high static pressures and high temperatures. 1
The physical conditions necessary for synthesis are customarily discussed in terms of the type of phase diagram shown in Figure 10.14. The key feature is
the ascending Berman-Simon line 2 representing equi-
librium between diamond and graphite. Diamond is stable above this line and graphite is stable below it. Diamond is able, of course, to exist below the line in a metastable condition at ordinary pressures; how- ever, it was found experimentally that heating above
a temperature of 1800 K caused rearrangement of its extremely strong C–C bonds and transformation into graphite. From the diagram it may be deduced that the reverse transition, from graphite to diamond, will take place above the line at similar temperatures if a
1 Much was owed to the pioneering work of the Nobel Prize winner P. W. Bridgman (1882–1961) at Harvard
University, USA, on methods for developing ultra-high pressures.
2 So named in recognition of the theoretical contribution of two Oxford physicists, R. Berman and F. Simon.
Figure 10.14 Pressure versus temperature diagram for carbon (after Bovenkerk et al., 1959, pp. 1094–8) .
pressure of at least 60 kb is applied simultaneously. The GEC method achieved these conditions. It also relies upon an addition of a metal (e.g. Ni, Cr, Mn, Fe, Co) which acts as a molten solvent for carbon and a catalyst for diamond crystallization and growth. The liquidus line for the eutectic mixture of carbon and nickel is superimposed on the diagram in order to define the diamond-growing region (shaded). Early synthetic crystals were usually grown under conditions of temperature and pressure well above the Berman- Simon line and were consequently weak and friable, containing stacking faults and metallic inclusions. As improved methods for measuring process temperature and pressure became available, experience showed that synthesis in the shaded region just above the Berman- Simon line gave slower and more controllable growth.
Later GEC experiments at pressures up to 200 kb transformed well-crystallized graphite into small diamond crystallites, about 0.1 mm in size, with a wurtzite-type structure showing hexagonal symmetry.
Subsequently, natural ‘hexagonal’ diamonds 3 (in association with cubic diamonds) were identified in meteorites; presumably they formed on impact with the earth. Small ‘hexagonal’ diamonds have also been synthesized by explosive shock-loading techniques that are capable of developing pressures as high as 500–1000 kb. These conditions only apply over a period of a few microseconds and thus tend to restrict
a physical transition which is time-dependent.
Synthesis is now practised worldwide and is capa- ble of producing diamonds which meet precise physi- cal and chemical requirements. Mined diamonds are inevitably more variable in quality. The maximum size of synthetic diamonds is in the order of 1 mm. Demand for industrial diamonds far exceeds that for
3 Called lonsdaleite in honour of the eminent crystallographer, Professor Dame Kathleen Lonsdale
Ceramics and glasses 339 gemstones; approximately 85–90% of industrial dia-
monds are synthesized and are mainly consumed in abrasion processes. Synthetic and natural diamond abrasives compete with silicon carbide and alumina. As mentioned elsewhere, diamond machining is a final operation in the production of many of the new engi- neering ceramics.