The Shock-Compression Processes

In this chapter solid-state physics and chemistry future directions in shock-compres- 

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The subject of this chapter is the relationship between macroscale observations and the underlying microscale processes in shock compression. Since the greater part of our current experimental knowledge of the shock compression process involves macroscale observations, we try to infer microscale phenomena from these data. A much more satisfactory approach is the direct real-time observation of microscale processes themselves. This is difficult to do in most cases, so we must still rely on a combination of macroscale measurement, microscale theory, and whatever direct observations of microscale processes that can be made. 

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. 

Although it is probably not possible at present to develop a completely satisfactory quantitative model, a conceptual model that identifies the critical processes can be developed. First, it is apparent that before significant chemical reaction can occur substantial modification must be induced by the shock-compression process. With present knowledge, the problem is one of mechanical deformation, rather than one of chemistry. The materials studies... 

If a phase (density) transformation occurs during the shock compression process, the concomitant change in V versus P will be detected. If the transition requires a brief time to be completed, a double wave will form, with the faster wave traveling in the compressed but yet untransformed material. Upon pressure release the transformed material may change back into the low-pressure form, and this process, if slightly delayed, will again produce a separate wave. 

Shock Synthesis. When graphite is strongly compressed and heated by the shock produced by an explosive charge, some (up to 10%) diamond may form (26,27). These crystaUite diamonds are small (on the order of 1 llm) and appear as a black powder. The peak pressures and temperatures, which are maintained for a few microseconds, are estimated to be about 30 GPa (300 kbar) and 1000 K. It is beheved that the diamonds found in certain meteorites were produced by similar shock compression processes that occurred upon impact (5). 

The present book, with contributions from a group of very knowledgable scientists in the field, is an attempt to provide a basis for addressing Bridgman s concerns. The response requires multidisciplinary contributions from solid mechanics, solid-state physics, materials science, and solid-state chemistry. Certainly, advances in theory, experimentation, and numerical simulation are impressive, and many aspects of shock-compressed solids have been studied in detail. At the fundamental level, however, it is certainly appropriate to question how well shock-compression processes are understood. 

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. 

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). 

Figure 8.6. Stress-particle velocity impedance diagram of the shock-compression and wave-interaction process leading to planar spall.

On a different note, after some 50 years of intensive research on high-pressure shock compression, there are still many outstanding problems that cannot be solved. For example, it is not possible to predict ab initio the time scales of the shock-transition process or the thermophysical and mechanical properties of condensed media under shock compression. For the most part, these properties must presently be evaluated experimentally for incorporation into semiempirical theories. To realize the potential of truly predictive capabilities, it will be necessary to develop first-principles theories that have robust predictive capability. This will require critical examination of the fundamental postulates and assumptions used to interpret shock-compression processes. For example, it is usually assumed that a steady state is achieved immediately after the shock-transition process. However, due to the fact that... 

Use of terms such as shock-compression science, or shock-compression processes casts such a broad net that little technical communication is accomplished. Within the overall framework of interest in materials under... 

The study clearly shows that the observed electrical signals are electrochemical in origin, and the first-order description of the process is consistent with that expected from atmospheric pressure behaviors. Nevertheless, the complications introduced by the shock compression do not permit definitive conclusions on values of electrochemical potentials without considerable additional work. 

Prior studies of dynamic compaction of powders to achieve high density compacts have devoted effort to development of models of localization of mechanical energy on the surfaces of powders to explain observations of local melting. Unfortunately, the models that have been developed are too narrowly focused and do not realistically consider basic materials response aspects of shock-compression processes. The models fail to account for the substantial observations that show results demonstrating that melting is not the universal, dominant process. 

The reaction is significantly exothermic with a heat of reaction of about 40 kcalmol . This energy will produce a sufficiently high temperature to melt the product and will allow the influence of thermochemical factors to be investigated. The temperature required to initiate the Ni-Al reaction at atmospheric pressure is about 660 °C. This reaction temperature threshold will be encountered in the shock processing, but it should be recognized that the conventional synthesis process is preceded by melting of the aluminum. At the pressure of the shock compression, the melt temperature of the aluminum will be approximately doubled to a value above the mean-bulk tempera-... 

Shock-compressed solids and shock-compression processes have been described in this book from a perspective of solid state physics and solid state chemistry. This viewpoint has been developed independently from the traditional emphasis on mechanical deformation as determined from measurements of shock and particle velocities, or from time-resolved wave profiles. The physical and chemical studies show that the mechanical descriptions provide an overly restrictive basis for identifying and quantifying shock processes in solids. These equations of state or strength investigations are certainly necessary to the description of shock-compressed matter, and are of great value, but they are not sufficient to develop a fundamental understanding of the processes. 

Shock phenomena, such as shock-induced polarization, have no known counterpart in other environments. In that regard, the distinctive behaviors present the greatest opportunity to determine details of shock-compression processes. Unexplored phenomena, such as electrochemistry [88G02], offer considerable potential for developing improved descriptions of shock-compressed matter. 

In this paper, I shall discuss four problems related to detonation which we have studied by the method of MD (1) Pulsed heating of a lattice. (2) Thermal relaxation in the shock compression of a lattice. (3) Profiles of a detonation wave. (4) Some preliminary results from a new study of the mechanism of heating of vacancy clusters in a lattice under shock compression. From these results I wish to show how the MD method complements the continuum method by providing details of many atomistic processes which would be difficult to investigate by theoretical or experimental means. 

For the reasons given above, a number of authors [23-28] have applied MD to shock wave studies in an effort to obtain details of the various shock compression processes that are not easily available from the conventional continuum method. We have carried out calculations of the shock compression of one-, two- and three-dimensional systems in both solid and liquid phases [29,30], using essentially the same model as in Fig. 1. Here I shall first summarize the general features of the shock profile from our studies, then I shall discuss one representative case, with special reference to the thermal relaxation problem, as an illustration of some of the general results. 

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