A Pathway to the Athermal Impact Initiation of Energetic Azides

Abstract

Energetic materials (explosives, propellants, and pyrotechnics) are used in a broad range of public and private sector applications. The design of novel, safe materials is therefore of critical importance. At present, physical mechanisms able to rationalize the impact sensitivity properties of energetic materials remain limited. Investigation therefore has required lengthy synthesis and experimental testing. On the basis of knowledge of the effects of mechanical impact, an ab initio model is developed to rationalize and describe the impact sensitivity of a series of crystalline energetic azide materials. It is found that electronic excitation of the azido anion is sufficient to permit bond rupture and therefore offers a plausible mechanism for initiation of these materials. The athermal excitation can be achieved through consideration of nonadiabatic vibronic processes. Across the series of azides studied here, the electronic structure of the azido anion is found to remain largely constant. By considering only the relative rates of vibrational energy transfer within the crystalline materials, it is found that a direct correlation exists between the relative impact sensitivity and the rate of energy up-conversion. Thus, the present contribution demonstrates a fully ab initio method to describe the athermal initiation of ideal, crystalline energetic materials and predict their relative sensitivity. Without the need for any experimental input beyond a crystal structure, this method therefore offers a means to selectively design novel materials for targeted application.

Predicting the reactivity of energetic materials: an ab initio multi-phonon approach

“…This has been demonstrated previously for inorganic systems. 56 As has been noted for a set of energetic azide materials, 37 there is no noticeable correlation between band gap values and the reported impact sensitivity of the molecular EMs. This also holds for the materials tested here, see Table 2.…”

“…a single vibrational normal coordinate that is responsible for initiating N À 3 . 37 In such cases, a direct up-pumping model can be discussed. 25 Such a mechanism is possible if either (1) the target mode is sufficiently isolated such that the multi-phonon density of states is negligible in its vicinity, or (2) the target mode has sufficiently strong coupling to the phonon bath to permit rapid, direct transfer.…”

“…32 Despite the promise offered by a model rooted in multi-phonon up-pumping to rationalize impact sensitivity of EMs, models based on experimental data alone do not offer predictive value. Correspondingly, the rst results of vibrational up-pumping methods that rely on an ab initio input based on gas phase 35 and periodic structure models 34,36,37 have now appeared. Using zone-centre vibrational frequencies for molecular crystals, Bernstein 36 was able to rank the relative impact sensitivity of six chemically similar organic EMs.…”

“…Using zone-centre vibrational frequencies for molecular crystals, Bernstein 36 was able to rank the relative impact sensitivity of six chemically similar organic EMs. In our previous work, 37 we have investigated the vibrational uppumping in nine azide-based crystals by calculating the complete vibrational dispersion curves. A correct ranking of these materials was achieved when both overtone and combination pathways were taken into account in the up-pumping process.…”

Bond Strain and Rotation Behaviors of Anharmonic Thermal Carriers in ‐RDX

“…Some studies indicate that phonon modes that lie between 1000 and 2000 cm À 1 (between 30 THz and 60 THz) are responsible for scisson of key NÀ N bonds [4,5,6]while others suggest that low frequency lattice phonons include deformation of NÀ N bonds [7,8]. Moreover, the pathway to scission may not necessarily entail only the strain of bonds [9]. Knowing how atom and bond motions, including both strain and rotation of bonds, participate in phonon modes will help understand possible mode coupling and anharmonic effects involved in initiation.…”