Sensitivity Correlations

Peter Politzer; Jane S. Murray    Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA

1 INTRODUCTION

A continuing major objective in the area of energetic materials is to achieve diminished sensitivity, i.e. to reduce vulnerability to detonation initiated by unintentional external stimuli; these can include, for example, impact, shock, heat, friction and electrostatic charge [1,2]. The importance attached to this issue is seen in the establishment, by the North Atlantic Treaty Organization, of the Munitions Safety Information and Analysis Center (MSIAC, formerly NIMIC) [http://hq.nato.int/related/nimic]. This Center promotes efforts to decrease sensitivity and disseminates relevant information.

In general, vulnerabilities to the various types of stimuli follow roughly similar trends, although there are certainly deviations. Thus, Storm et al obtained satisfactory correlations, for diverse groups of compounds, between measured shock and impact sensitivities and between shock sensitivity and the critical temperature at which thermal decomposition becomes self-sustaining [3]. Zeman et al found somewhat more equivocal relationships involving impact and electrostatic spark sensitivity [4].

Energetic compounds are metastable. The introduction of external energy can lead to rapid decomposition, the first phase of which is endothermic but which subsequently becomes highly exothermic, releasing a great deal of energy and gaseous products at high temperatures, and giving rise to a large pressure gradient (shock front) that propagates through the compound at supersonic velocity, producing continuing self-sustaining exothermal decomposition (detonation) [1,2,5,6].

How readily this sequence of events will occur for any given compound (i.e. its sensitivity) depends of course upon its molecular structure and composition, as well as its crystal properties and physical form. Achieving greater insight into molecular factors offers a route to designing new less-sensitive energetic materials; understanding crystal and designing new less-sensitive energetic materials; understanding crystal and physical effects may permit improvement of existing ones, e.g. by modification of crystallization techniques and conditions. For instance, the β polymorph of HMX (1) is more stable toward impact than is the δ [7], and both are made somewhat less sensitive by grinding the crystals. Appropriate alterations of crystal shapes have diminished the shock sensitivities of several explosives, including RDX (2), HMX and CL-20 (3) [8,9].

Over a period of years, considerable effort has gone into developing relationships that correlate (and hence can predict) sensitivity in terms of some molecular or crystal property, usually the former. The first problem is obtaining reliable and reproducible experimental data. These correlations most often focus upon impact sensitivity, which is normally determined by measuring the height from which a given mass, dropped on the material, has a 50% likelihood of causing detonation [1–3,10,11]. The greater is this height, designated h50, the lower is the sensitivity. The results of this “drop-weight” test are very dependent upon the exact procedure that is followed and the condition of the sample, and are frequently difficult to reproduce.

A more serious problem, however, is that the initiation of detonation, as already mentioned and to be further discussed, depends upon a complex interplay of various molecular, crystal and physical factors. It can therefore be viewed as remarkable that relatively good correlations have been established between impact sensitivity and a number of different properties, although they are generally limited to a particular class of compounds, e.g. nitroaromatics. The existence of these relationships certainly does not mean that all of these properties (or perhaps any of them) play important roles in detonation initiation, as was pointed out by Brill and James [12,13]. Many of them may be symptoms of some more fundamental factor; others may happen to correlate with a more relevant property.

In this chapter, we shall present an overview of some of these sensitivity correlations. We shall try, as much as possible, to relate them to a conceptual framework.

2 BACKGROUND

A key concept, with respect to detonation initiation, is that of hot spots, proposed by Bowden and Yoffe [14,15]. These are small regions in the crystal lattice in which is localized some portion of the energy introduced by, for example, impact or shock. If this energy is sufficiently channeled into appropriate molecular vibrational modes, it may result in the endothermic bond-breaking or other step that leads to exothermic decomposition and detonation. For this to happen, hot spots must be large enough and hot enough that they are not prematurely dissipated by thermal diffusion. Bowden and Yoffe estimated their dimensions to be of the order of 1(10− 6 m, temperatures > 700 K and lifetimes of 10− 5 to 10− 3 s [14,15]. Later work, cited by Tarver et al [16], indicated that this description roughly fits shock-induced hot spots but that those due to impact are larger and longer-lived, by factors of about 103. More recently, however, atomic force microscopy studies of RDX have shown that the latter can be as small as ~ 10− 8 m [17]. Using a combination of techniques (scanning electron microscopy, x-ray photoelectron spectroscopy and chemical-ionization mass spectrometry), it has been possible to identify, associated with hot spots, some of the decomposition products of TATB (4) and TNT (5) [18]. Tarver et al have modeled hot spots and the ignition of exothermic decomposition in HMX and TATB [16]. They found that as the hot spot becomes larger, the temperature required for ignition decreases but the time increases.

The formation of hot spots is generally attributed to the presence of lattice defects [11,17,19–23], which could include vacancies, voids, dislocations, misalignments, cracks, impurities, etc. One explanation is that defects induce strain in the lattice, which is relieved, via structural relaxation, by the externally-introduced energy; this results in a disproportionate localization of energy in the neighborhood of the defect, a portion of it being in lattice vibrations [21,22]. The thermal energy of hot spots must be efficiently transferred to appropriate molecular vibrational modes, a process called “up-pumping” [24–27], if bond-breaking and subsequent exothermic decomposition and detonation are to be achieved. The term “trigger linkage” has been applied to the key bond or bonds that are initially ruptured [28]. Any dissipation of hot spot energy, for instance by diffusion, will lessen the likelihood of these processes. Thus Kamlet suggested that free rotation around the trigger linkage can have a desensitizing effect, since it uses energy that could otherwise go into bond-breaking vibrational modes [28,29].

The fact that solids composed entirely of very small particles are less sensitive [7,30] can now be explained on the grounds that these will have smaller hot spots and thus, as shown by Tarver et al [16], require higher temperatures for ignition. Once this has been achieved, however, these higher temperatures will result in more rapid reaction, as has been observed [31].

It should be noted that initiation of detonation can occur even in a homogeneous (defect-free) solid [6,24,32–36] (although this is of less practical significance since energetic compounds typically do contain lattice defects of various sorts). This can occur, for example, if there is efficient anharmonic coupling to channel energy from lattice into the critical molecular vibrations [24,25,36].

3 SENSITIVITY CORRELATIONS

The concepts that were outlined in the last section have suggested several approaches to developing sensitivity correlations. A popular one has been to focus upon the trigger linkage. Kamlet viewed C–NO2 and N–NO2 bonds as playing this role in H/C/N/O explosives, and scission of these bonds, which tend to be the weakest in the molecule [13,37–39], has indeed been shown experimentally to be the first step in the thermal decompositions of many of them [12,13,37,40–42]. Accordingly there have been a number of attempts to correlate sensitivity with properties of C–NO2 and N–NO2 bonds, generally some measure of their stabilities [39,43–46]. On the whole, these have been quite successful, although they are usually limited to a given class of compounds, e.g. nitramines, nitroaromatics, etc. However Owens was able to correlate with impact sensitivity the computed C–NO2, N–NO2 and O–NO2 bond energies of 11 molecules of different types: nitroaromatics, nitramines and nitrates [39].

An. interesting variation of this emphasis upon the trigger linkage was offered by Kohno et al...