Diamond Shock Wave

When a supersonic flow emerges from a rocket nozzle, several oblique shock waves and expansion waves are formed along the nozzle flow. These waves are formed repeatedly and form a brilliant diamond-Uke array, as shown in Fig. C-5. When an under-expanded flow, i. e., having pressure higher than the ambient pressure is formed at the nozzle exit, an expansion wave is formed to decrease the pressure. This expansion wave is reflected at the interface between the flow stream and the ambient air and a shock wave is formed. This process is repeated several times to form a diamond array, as shown in Fig. C-6 (a). 

1 Shapiro, A. H., The Dynamics and Thermodynamics of Compressible Fluid Flow, The Ronald Press Company, New York, 1953. 

2 Liepman, H. W., and Roshko, A., Elements of Gas Dynamics, ohn Wiley Sons, New York, 1957. 

3 Kuethe, A. M., and Schetzer, J. D., Foundations of Aerodynamics, John Wiley Sons, New York, 1967. 

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Figure C-5. Diamond shock wave array formed downstream of a rocket nozzle.

A quite different set of dynamic high-pressure techniques are based on the use of chemical or nuclear explosions to produce transient shock waves of high peak pressure but short duration. With such methods, one can often penetrate the high-T, P regions where kinetic barriers become unimportant and a catalyst is unnecessary. However, the same kinetics that allows facile conversion of graphite to diamonds as the shock front arrives also allows the facile back-conversion as the shock wave passes. As a pioneer of shock-wave diamond synthesis remarked ruefully, We were millionaires for one microsecond [B. J. Alder and C. S. Christian. Phys. Rev. Lett. 7, 367 (1961) B. J. Alder, in W. Paul and D. M. Warschauer (eds). Solids under Pressure (McGraw-Hill, New York, 1963), p. 385]. 

The use of shock waves was imitated from nature, more specifically from meteorite craters. Among other things, unique forms of diamonds were found in there, i.e. hexagonal diamonds formed out of graphite. Furthermore the mineral stishovite was also found there. 

The cubic y-modification has been recently observed under a pressure of 15 GPa and temperatures above 2000 K by the laser heating technique in a diamond cell [23] and in shock-wave compression experiments with pressures >33 GPa at 1800 K and >50 GPa at 2400 K [29]. This modification is often designated as the c-modification in the literature in analogy to the cubic boron nitride (c-BN). It has a spinel-type structure in which two silicon atoms are octahedrally coordinated by six nitrogen atoms, one silicon atom is coordinated tetrahedrally by four nitrogen atoms (Fig. 3c). The atomic coordinates for the cubic modification are given in Table 2. From calculations it is shown that this structure should have a high hardness similar to that of diamond and c-BN [23]. 

Shock-wave phenomena are important in meteorite impacts where high-pressure minerals are often formed. Small diamonds useful for lapping and polishing are made commercially by shocking graphite mixed with iron and copper. The metals cool the diamonds before they can transform back to graphite on pressure release. 

It is evident from the phase diagram that diamond may be obtained in a very wide pressure-temperature range, thus allowing several synthesizing methods to work in various regions. Those mainly applied are conversion of graphite to diamond by a flux method, direct conversion by shock wave compression, and direct conversion by static compression. Synthetic diamond is mostly produced by the flux method, which will be outlined below. 

Jeanioz, R., T. J. Ahrens, H. K. Mao, and P. M. Bell (1979). BI-B2 transition in calcium oxide from shock-wave and diamond-cell experiments. Science 206, 829-30. 

Explosives, as a special kind of material, can produce a special state of materials—detonation. In a long historical period of time, explosives liave been used for tlie purposes of destruction, elimination, or decomposition, and tliese purposes still account for tlie main utilization of explosives. In recent years it has been found that explosives can be used for the opposite purposes of construction, creation, and synthesis. Using the liigh pressure, high temperature and high mass speed produced by explosive detonation, new substances or substances with unique features can be produced. Two examples are given in this chapter shock-wave-induced chemical reactions for material synthesis and ultrafme diamond syntliesized by explosive detonation. Much research work l as been done in these fields in recent years, but the quantity and the deptli of these studies are far from sulficient. There remain many unsolved problems and unexplored fields. 

Shock Wave Chemistry and Ultrafine Diamond from... 

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