Shock-Wave Processing

The high pressure generated by the shock waves from an explosion can produce the direct cind essentially immediate conversion of graphite into diamondJ l 

A schematic ofthe process is shown in Fig. 12.7. A mixture of graphite and nodular iron is placed inside a 25 cm diameter cavity in a lead block. A flat metal plate, uniformly coated with TNT on the back side, is placed in front of the cavity. The TNT is detonated and the plate impacts the cavity at a peak velocity of 5 km/s. A peak pressure estimated at 300 kbar and a temperature of approximately 1000 K are maintained for a few microseconds. The formation of diamond is assisted by the presence of an iron solvent-catalyst. The diamond crystals are then separated by selective acid digestion and sedimentation. 

Oniy very small polycrystalline diamonds are produced with a maximum particie size of 2tpproximately 60 The technique is commercialized by Dupont in the U.S. Because of its small particle size the material is iimited to appiications such as polishing compounds. 

Hgure 12.7. Schematic of shock-wave processing of diamond. 

0 NATURAL AND HIGH-PRESSURE SYNTHETIC DIAMOND PRODUCTION 

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Q. Johnson, A. Mitchell, and L. Evans, X-Ray Diffraction Evidence for Crystallographic Order and Isotropic Compression During the Shock-Wave Process, Nature 231, 310-311 (1971). 

Nonel. Trade name of a new non-electric device for the firing of expl charges. The basic unit consists of, in place of electrical lines, safety fuses or detonating cord, a plastic hose (3mm diam) the inner wall of which is coated with a thin layer of expl. A shock wave initiated by a special initiator passes thru the hose with a speed of approx 2000m/sec. A spectator observes this shock wave process as a flash in the hose, which is not destroyed by the shock wave In order to initiate a charge, the Nonel line must be combined with a conventional detonator, and branching is possible... 

The shapes of the interatomic potential curves are approximations chosen for mathematical convenience. Such potential functions are generally used in discussions on a variety of properties of molecules and lattices optical absorption and luminescence, laser action, infrared spectroscopy, melting, thermal expansion coefficients, surface chemistry, shock wave processes, compressibility, hardness, physisorption and chemisorption rates, electrostriction, and piezoelectricity. The lattice energies and the vibration frequencies of ionic solids are well accounted for by such potentials. On heating, the atoms acquire a higher vibrational energy and an increasing vibrational amplitude until their amplitude is 10-15% of the interatomic distance, at which point the solid melts. 

Natural and High-Pressure Synthetic Diamond 289 3.4 Shock-Wave Processing... 

Tree J 1975 Shock wave studies of elementary chemical processes Modern Deveiopments in Shock Tube Research ed G Kamimoto (Japan Shock Tube Research Society) pp 29-54... 

A laser beam is capable of putting so much energy into a substance in a very short space of time that the substance rapidly expands and volatilizes. The resulting explosive shock wave travels through the sample, subjecting it to high temperatures and pressures for short times. This process is also known as ablation. 

Other. Because a foam consists of many small, trapped gas bubbles, it can be very effective as a thermal insulator. Usually soHd foams are used for insulation purposes, but there are some instances where Hquid foams also find uses for insulation (see Eoamed plastics Insulation, thermal). Eor example, it is possible to apply and remove the insulation simply by forming or coUapsing the foam, providing additional control of the insulation process. Another novel use that is being explored is the potential of absorbing much of the pressure produced by an explosion. The energy in the shock wave is first partially absorbed by breaking the bubbles into very small droplets, and then further absorbed as the droplets are evaporated (53). 

Shock-compression processes are encountered when material bodies are subjected to rapid impulsive loading, whose time of load application is short compared to the time for the body to respond inertially. The inertial responses are stress pulses propagating through the body to communicate the presence of loads to interior points. In our everyday experience, such loadings are the result of impact or explosion. To the untrained observer, such events evoke an image of utter chaos and confusion. Nevertheless, what is experienced by the human senses are the rigid-body effects the time and pressure resolution are not sufficient to sense the wave phenomena. 

The fact that shock waves continue to steepen until dissipative mechanisms take over means that entropy is generated by the conversion of mechanical energy to heat, so the process is irreversible. By contrast, in a fluid, rarefactions do not usually involve significant energy dissipation, so they can be regarded as reversible, or isentropic, processes. There are circumstances, however, such as in materials with elastic-plastic response, in which plastic deformation during the release process dissipates energy in an irreversible fashion, and the expansion wave is therefore not isentropic. 

As pointed out in Section 2.4, shock waves are such rapid processes that there is no time for heat to flow into the system from the surroundings they are considered to be adiabatic. By the second law of thermodynamics, the quantity (S — Sg) must be positive for any thermodynamic process in an isolated system. According to (2.54), this quantity can only be positive if the P-V isentrope is concave upward. Thus, the thermodynamic stability condition for a shock wave is... 

Grady, D.E. (1977), Processes Occurring on Shock Wave Compression of Rocks and Minerals, in High Pressure Research Applications in Geophysics (edited by Manghnani M.H. and S. Akimoto), Academic Press, New York, pp. 389-438. 

The structure/property relationships in materials subjected to shock-wave deformation is physically very difficult to conduct and complex to interpret due to the dynamic nature of the shock process and the very short time of the test. Due to these imposed constraints, most real-time shock-process measurements are limited to studying the interactions of the transmitted waves arrival at the free surface. To augment these in situ wave-profile measurements, shock-recovery techniques were developed in the late 1950s to assess experimentally the residual effects of shock-wave compression on materials. The object of soft-recovery experiments is to examine the terminal structure/property relationships of a material that has been subjected to a known uniaxial shock history, then returned to an ambient pressure... 

Although the difference in final strength f, integrated through both the actual shock wave and the computational shock wave, will be mitigated by dynamic recovery (saturation) processes, this is still a substantial effect, and one that should not be left to chance. These are very important practical considerations in dealing with path-dependent, micromechanical constitutive models of all kinds. 

We first consider strain localization as discussed in Section 6.1. The material deformation action is assumed to be confined to planes that are thin in comparison to their spacing d. Let the thickness of the deformation region be given by h then the amount of local plastic shear strain in the deformation is approximately Ji djh)y, where y is the macroscale plastic shear strain in the shock process. In a planar shock wave in materials of low strength y e, where e = 1 — Po/P is the volumetric strain. On the micromechanical scale y, is accommodated by the motion of dislocations, or y, bN v(z). The average separation of mobile dislocations is simply L = Every time a disloca-... 

Underlying all continuum and mesoscale descriptions of shock-wave compression of solids is the microscale. Physical processes on the microscale control observed dynamic material behavior in subtle ways sometimes in ways that do not fit nicely with simple preconceived macroscale ideas. The repeated cycle of experiment and theory slowly reveals the micromechanical nature of the shock-compression process. 

D.C. Wallace, Computer Simulation of Non-Equilibrium Processes, in Shock Waves in Condensed Matter (edited by Y.M. Gupta), Plenum, New York, 1986, pp. 37-49. 

R.A. Graham, Shock Compression of Solids as a Physical-Chemical-Mechanical Process, in Shock-Waves in Condensed Matter—1987 (edited by S.C. Schmidt and N.C. Holmes), Elsevier Science, Amsterdam, 1988, pp. 11-18. 

Dremin, A.N. and Breusov, O.N., Processes Occurring in Solids Under the Action of Powerful Shock Waves, Russian Chem. Rev. 37 (5), 392-402 (1968). Gilman, J.J., Dislocation Dynamics and the Response of Materials to Impact, Appl. Meek Rev. 21 (8), 767-783 (1968). 

Graham, R.A., Morosin, B., Venturini, E.L., Carr, M.J., and Beauchamp, E.K., Shock-Compression Processes in Inorganic Powders, in Metallurgical Applications of Shock-Wave and High-Strain-Rate Phenomena (edited by Murr, L.E., Staudhammer, K.P., and Meyers, M.A.), Marcel Dekker, New York, 1986, pp. 1005-1012. 

Employers, at a minimum, must have an emergency action plan that will facilitate the prompt evacuation of employees when there is an unwanted release of a highly hazardous chemical. This means that the employer s plan will be activated by an alarm system to alert employees when to evacuate, and that employees who are physically impaired will have the necessary support and assistance to get them to a safe zone. The intent of these requirements is to alert and move employees quickly to a safe zone. The use of process control centers or buildings as safe areas is discouraged. Recent catastrophes indicate that lives are lost in these structures because of their location and because they are not necessarily designed to withstand over-pressures from shock waves resulting from explosions in the process area. 

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