According to the solution model, diamond nuclei ought to form inside the original volume of molten catalyst. In an experiment performed in 1977, the author observed that the first diamond nuclei were actually precipitated out inside the original volume of graphite where molten metal intruded. Moreover, macroscopic graphite flakes were recrystallized in regions where diamond had not yet nucleated. However, as soon as a diamond nucleus was formed, it served as the carbon sink and sucked up microscopic flakes suspended in molten catalyst. As a result, microscopic flakes could not be accumulated to form macroscopic flakes.
Once a diamond nucleus reaches a critical size, it will continue to grow in the graphite volume. However, there is always a thin film (<0.1 mm) of molten catalyst that separates diamond from graphite. This thin film will wrap around the diamond and it grows continually along with the diamond. Meanwhile, diamond is feeding on microscopic flakes of recrystallized graphite that drifts across the strait of thin film.
It would appear that the sequence of events for the diamond synthesis under high pressure is as follows:
(1) Molten metal invades into graphite in structural weak regions.
(2) Graphite disintegrates to form microscopic flakes.
(3) Catalyst atoms penetrate microscopic flakes by intercalation, and then shuffle the stacking sequence to become rhombo-hedral graphite.
(4) Rhombohedral flakes are puckered under the influence of molten catalyst and they stick together to form a diamond nucleus.
(5) The diamond nucleus is growing by continually feeding on microscopic flakes.
(6) As the diamond grows in size, molten catalyst is pulled in by the capillary effect, so the thin metal envelope can expand continually around the growing diamond.
The above model suggests that diamond is not nucleated from supersaturated solution of carbon atoms, but by puckering suspended graphite flakes (Fig. 4.9). The growth of diamond is also
Figure 4.9. Diamond is wrapped in a metal film that is impregnated with suspended graphite flakes. These flakes are continually disintegrated form the surrounding graphite disk. The immersed flakes are reshuffled in stacking sequence to form rhombohedral graphite. When these recrystal-lized flakes reach the diamond surface, they have been puckered and may attach to the growing diamond in chunks measured in micrometer. The catalyst side of the diamond crystal is growing slowly by the attachment of dissolved carbon atoms. The unequal rate of growth is often reflected in the asymmetrical appearance of the diamond crystal. Note that there is a temperature gradient across the catalyst envelope. Graphite, with electrical resistance 10,000 times higher than that of molten metal, is hotter than the catalyst metal in the corner regions where they are incorporated to form rays of inclusions.
dominated by adding microscopic chunks of flakes instead of discrete solutes. This conclusion is consistent with the observation that diamond facing graphite is growing nearly twice faster than that facing catalyst (growing predominantly by adding atoms). Moreover, when microscopic diamond is dissolved in molten catalyst, diamond may not nucleate at all. This situation is evidenced when gem diamond is grown with a seeded crystal using temperature gradient method. In this case, the growth rate achieved by feeding dissolved diamond atoms are comparable to that of growing saw diamond in the catalyst side. This growth rate is about half of that in graphite side.
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