Origins of the Energy and Safety of Energetic Materials and of the Energy & Safety Contradiction

Abstract

Energy & Safety (E&S) are the two primary concerns among the properties and performance of energetic materials (EMs): energy governs the effectiveness of application, and safety affects the use of EMs, even though their definitions are still subject to discussion. Energy may refer to reaction heat (heat of detonation or combustion), detonation properties, power, or working ability; while, safety is usually evaluated by various sensitivities, which are the degrees of an EM in response to various external stimuli: higher sensitivity represents lower safety. Energy and safety are thought to be in contradiction with each other, i. e., there exists a so-called E&S contradiction for EMs 1. This contradiction is often regarded as mainly responsible for the slow evolution of EMs. The practical utility of an EM is often determined by its energy content, in particular its useful energy content. This useful energy content is strongly related to the ingredients and density, the loading density and the anticipated release mechanism. In general, the mechanism is extraordinarily complex and thus makes it difficult to detail it. Detonation can produce high pressure (several tens of GP) and high temperature (several millions of K) conditions, and these conditions can almost take the reaction to completion. It also suggests the feasibility of use of thermodynamic cycle theory in these cases. Even though relatively little is known about material equations of state (EOS) under detonation conditions, shock experimentation on a wide range of materials has generated sufficient information to allow reasonably reliable thermodynamic modeling to proceed. That is, the energy can reliably be described by thermodynamic modeling, for which, only composition, density and heat of formation are needed. Or simply, considering chemical energy release, it can be seen as the heat of reaction, which is a state function and is only determined by the primary and final states and is unaffected by the detailed intermediate steps. In this case, the energy is in fact the variation of bond energy, i. e., the result of the bond energy of reactants minus that of products. Because safety is strongly related to the detailed pathway from initial to final state of an EM in response to an external stimulus, it is more kinetic in nature. Reasonably, the E&S contradiction is a thermodynamic-kinetic contradiction. This can be simply illustrated by Figure 1. EMs are kinetically stable while thermodynamically unstable substances. Their chemical energy release (ΔE1) are the heats of detonation reactions, which are the difference in potential between the energetic reactant and final products. These products are usually stable small molecules: a larger difference suggests a higher ΔE1 (or a higher energy (E)). On the other hand, the energy barrier (ΔE2) for decaying of an EM should be higher if it is required to be more stable. Obviously, ΔE2 is strongly responsible for safety (S). Therefore, the so-called E&S contradiction can simply be regarded as the ΔE1 & ΔE2 contradiction. Plot showing the energy and safety of EMs. Is the ΔE1 & ΔE2 contradiction unavoidable? Or, does it mean that higher ΔE1 always goes with lower ΔE2? This issue may well be addressed at different levels. First, on the level of a molecule, we can discuss the ΔE1 & ΔE2 contradiction from the heat of molecular decomposition and molecular stability, which represent ΔE1 and ΔE2, respectively. In principle, ΔE1 is determined by the difference in bond energy summation between the reactant and products. For given products, the higher ΔE1 requires the smaller bond energy summation of the reactant. The smaller bond energy summation of the reactant may imply a less stable reactant. On the other hand, the higher ΔE2, indicative of the higher molecular stability, requires a more stable reactant. Thus, the ΔE1 & ΔE2 contradiction does indeed exist at molecular level: firstly, the higher ΔE1 requires the smaller bond energy of the reactant; secondly, in contrast, the higher ΔE2 requires larger bond energy. However, such a contradiction remarkably appears only on the molecular level. After all, with respect to the structures of applied EMs, the molecular structures are basic but not the only factor. In general, applied EMs are multi-scale and hierarchical. At the level of the molecule, as pointed out above, the E&S contradiction appears to be inherent. At the crystal level, it has already been verified that molecular packing can influence mechanical sensitivity. In recent studies, we found that the face-to-face π-π stacking is preferred for most efficiently buffering against external mechanical stimuli, facilitating low mechanical sensitivity 2-4. Additionally, strengthening the intermolecular interactions by energetic co-crystallization is also thought to favor low mechanical sensitivity 5. Even though the enhanced intermolecular interactions, or the elevated lattice energy, can decrease ΔE1 a little, the impact sensitivity can be improved significantly. This is a part of crystal engineering of EMs. It seems that energetic ionization is also a kind of crystal engineering of EMs, by which both the molecular stability and the intermolecular interactions are enhanced to increase energy performances while retaining safety 6. Therefore a mixture of effects is the final form of EMs. Numerous efforts have been used tapply various additives to improve the sensitivities of EMs, as well as the other properties and performances. These are merely the usual formulation techniques in practice. Even though these formulation techniques are varied, they can only prevent hot spot formation by reducing the efficiency of external stimulation energy initiating molecular decomposition. Thereby, the E&S contradiction can be alleviated. Chaoyang Zhang Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang, Sichuan, China We greatly appreciate the financial support from theNational Natural Science Foundation of China (U1530262 and 21673210) and Scientific Challenge Project of China.