Mechanochemical processes in energetic materials: a computational and experimental investigation

Abstract

Energetic materials (explosives, propellants and pyrotechnics; EMs) encompass a broad range of materials. These materials are used across a wide spectrum of applications, including civil and defence. For example, HMX, RDX and TNT are well known EMs with defence applications. Silver fulminate is instead used in house-hold Christmas crackers and ammonium nitrate is used for numerous industrial applications. Common to all EMs is their propensity to rapidly release energy upon external perturbation. The amount and type of energy that is required to initiate an EM can vary across orders of magnitude. Some materials (e.g. triaminotriperoxide, TATP) initiate with < 1 J of impact energy, while others (e.g. triaminotrinitrobenzene, TATB) cannot be initiated without > 100 J of impact energy. Understanding which materials can be handled safely is therefore of critical importance for maintaining the safe use of EMs across all sectors.

Current trends in EM research include a drive to develop new materials with decreased sensitivities. While it is relatively straightforward to selectively modify some properties (e.g. environmental impact), very little is understood about what constitutes a sensitive material. At present, a new EM must be synthesised and its sensitivity tested. However, with no a priori knowledge of the potential sensitivity of a novel EM, synthesis is accompanied by substantial hazard, as well as time and financial costs. It is therefore pressing to develop a fundamental understanding of what dictates a sensitive material, and hence develop a mechanism to predict these properties. A particularly promising model to explore impact sensitivity of EMs is based on vibrational up-pumping, i.e. the up-conversion of vibrational energy. This thesis explores the application of this model to a set of azide, organic molecular and polymorphic materials.

Azide-based EMs share the common N3− explosophore. The electronic structure of this anion was followed as a function of its normal modes of vibration. It was found that excitation of the bending mode is sufficient to induce athermal electronic excitation of the molecule, and spontaneous decomposition. This is valid both in the gas and solid states. It is therefore suggested that this vibrational mode is largely responsible for decomposition of the azide materials. Based on calculations of the complete phonon dispersion curves, the various pathways to vibrational energy up-pumping were explored, namely via overtone and combination pathways. In particular, the relative rates of up-pumping into the N3 − bending mode were investigated.

Remarkable agreement is found between these up-pumping rates and the relative ordering of the impact sensitivity of these azides.

The calculated vibrational structures of organic molecular EMs were first compared with experimental inelastic neutron scattering spectra and found to provide accurate representation of the low temperature vibrational structure of these complex crystals. The decomposition pathways for organic EMs are not known and hence no target frequency could be unambiguously identified. Instead, the up-pumping model was developed for these materials by investigating the total rate of energy conversion into the internal vibrational manifold. A number of qualitative trends were identified, which may provide a mechanism for the rapid classification of EMs from limited vibrational information. The overtone pathways were found to offer a good agreement with experimental impact sensitivities of these compounds. However, the increased complexity of the vibrational structure of the organic EMs as compared to the azides required a more thorough treatment of the up-pumping mechanism to correctly reflect experimental sensitivities. The effects of temperature on up-pumping were also explored.

The sensitivity of organic EMs is known to differ across polymorphic forms. Most notable are the HMX polymorphs. The calculated vibrational structure of two HMX polymorphs was confirmed by inelastic neutron scattering spectroscopy. The up-pumping model developed for molecular organic EMs was therefore extended to a comparison of these two HMX polymorphs. The polymorphic forms of FOX-7 were also investigated under the premise of the up-pumping model. Upon heating, FOX-7 undergoes two polymorphic transformations, which increases the layering of the materials. It therefore offered an opportunity to explore the widely-held hypothesis that layered materials are less sensitive than non-layered materials. The metastable γ-form was successfully recovered, and its experimental impact sensitivity investigated by BAM drop-hammer method. However, upon impact, the γ- polymorph appeared to convert to the α-form and initiate at the same input energy. Hence a considerable deficiency of experimental methods is identified when studying polymorphic materials. FOX-7 was therefore explored within the framework of the up-pumping model. The inelastic neutron scattering spectrum was collected for γ-FOX-7, which confirmed the calculated vibrational structure.

It was shown that within the up-pumping model, the layered γ-polymorph is predicted to be less sensitive than the α-form, and results from a decrease in the maximum phonon-bath frequency. Hence a new mechanism is proposed to describe the insensitivity of layered compounds.

The work presented in this thesis explores the applications of vibrational up-pumping to rationalise and predict the relative impact sensitivities of a range of EMs. Despite the approximations employed in construction of the model, it leads to excellent correlation with experimental results in all cases. This work therefore opens the door to a new fully ab initio approach to designing new EMs based solely on knowledge of the solid-state structure.