Instead of breaking large diamond grains to form micron-sized particles, shock wave may be applied to collapse graphite. Due to the instantaneous rises of pressure and temperature diamond crystallites are formed. However, as the high pressure is maintained for only a few microseconds, such crystallites are virus sized (e.g. 50 nm). But due to the slow cooling of the charge nano particles are sticking together to form sintered polygrits up to 50 microns (Fig. 5.19).
The direct transformation of graphite to diamond by shock wave compaction was commercialized by Du Pont in 1970s. The
process requires the packing of dynamite around a steel tube. Inside the steel tube are intimately mixed graphite powder and copper grains (e.g. 92wt.%, about 1 mm in size). The charge was first compressed to increase the density (e.g. by cold isostatic pressing). The presence of high porosity in the charge can dampen the shock wave so the desired pressure cannot be attained. Moreover, the collapse of large amount of pores may also increase the temperature that can back convert diamond formed.
The charge could be round bars of about 20 cm in diameter by about 5 m long. They were then sealed in steel tubes with a hallow wall that contained vacuum. Many of such steel tubes were packed inside a cylindrical culvert along with several tons of dynamite. The culvert was located in a deep shaft of an abandoned mine.
As the explosive detonated, it progressively collapsed the outer (driver) tube onto the inner (product) wall, subjecting the graphite to enormous pressure, from 8GPa up to as high as 40GPa. The propagation of the shock wave would compress graphite from one end of the steel tube to the other. As a result, microscopic poly-crystalline diamond may form. Such diamonds may have 98% of the theoretical density of diamond (3.52 gm/cc). Because the pressure was excessive, nucleation rate of diamond was extremely fast
with the result of forming crystallites of up to 60 nm. However unlike the dynamite transformed nanodiamond, the shock waves allowed time for millions of crystallites to sinter into micron-sized aggregates.
During the shocking process, temperature might reach 2000°C. Such a high temperature could convert diamond just formed back to graphite. The charge contained copper particles as heat sink that could quench the hot diamond as soon as it was formed. As a result, several kilograms of diamond may be preserved in each explosion (Fig. 5.20).
Shock wave compressed micron diamonds are embedded in unconverted graphite along with metal powder. The latter could be dissolved in acids. Graphite might be oxidized by roasting the acid treated residue with PbO (e.g. at 400). Another method was to exfoliate graphite by soaking the charge in warm concentrate sulfuric acid. The intercalated graphite and subsequently vibrate the puffed graphite away in water with agitation.
Shock wave compressed diamond is formed by direct transition from graphite without using catalyst. Moreover, the transition time is so short (microseconds) that long-range diffusion of carbon atoms are prohibited. Consequently, atoms in graphite must collapse directly into diamond structure.
Graphite contains hexagonal planes that are stacked in sequence. Each graphene plane may be offset to assume one of the three positions relative to the adjacent plane. In the case of 1 H polytype (hexagonal), the position of all graphene planes is matched so the sequence becomes AAA____Alternatively, graphene planes may stack up with offset in alternation as in the case of 2 H polytype (hexagonal), so the sequence now reads ABA Yet, there is one more polytype of graphite that is designated as 3C (rhombohedral). This time the sequence has a triple period in the form of ABC
The common graphite contains most (e.g. 85%) 2H polytype that may mixed in with some (e.g. 15%) graphene sequence in 3C. The 1 H graphite is unstable because of the repulsion of dangling electrons that is lined up in the same position, so it does not exist in normal graphite. It turns out that the carbon atoms in both 1 H
and 3C graphite has matching atoms in adjacent layer so they can readily transform into diamond under extreme pressure, as in the case of shock wave compression. In the case of 1H graphite, it can form hexagonal diamond (lonsdaleite) that exists rarely (e.g. in meteorite or impact crater). On the other hand, 3C graphite can transform directly into a cubic diamond, the common form of diamond of both natural and synthetic origins (Fig. 5.21).
It would be interesting to note that half of carbon atoms in 2H graphite do not have matching atoms in adjacent planes, so the common 2H graphite cannot form diamond directly. In the case of diamond synthesis that utilizes molten metal catalyst, 2H
graphite, with the intercalation of liquid catalyst, has ample time to shuffle into 3C sequence before it transforms into cubic diamond. However, under the shock wave compression, no such time is allowed for shuffling graphene planes, hence, 2H graphite under compression will bounce right back upon decompression. This was why Bridgman failed in making diamond and hence his lament "graphite is nature's best spring".
The inability of 2H graphite to transform directly into diamond has also limited the yield of diamond formed by shock wave compression. As the graphite used in shock wave compression since the time Du Pont invented this method contains only about 15% rhombohedral sequence, this has become the yield of polycrystals formed each time the dynamite exploded. Had the process use rhombohedral graphite as the carbon source, the diamond yield would have been increased dramatically (Sung, US Application No. 2006/0251567). One way to shuffle hexagonal graphite to rhombohedral graphite is to mill graphite with high shear force (e.g. by attritor). The shearing of graphite can slide graphene planes.
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