Probing Amorphous Energetic Materials Using Inelastic Neutron Scattering

Energetic materials (explosives, propellants and pyrotechnics) play important roles in both military and civilian applications. Their sensitivities to initiation by impact, shock, friction and spark are crucial not only for their performance, but also for safety in manufacture, processing, storage, transport, and operational use. We have recently developed a sensitivity-prediction model based on a vibrational up-pumping theory and this has been applied to a range of structurally diverse energetic materials with variable sensitivities. The model accounts for the ease with which the vibrational modes of the crystal can channel the energy from a mechanical impact through the low-energy phonon bath modes to reach the molecular vibrations. A fascinating recent development has been the discovery that amorphous energetic materials (prepared through techniques such as spray drying) are substantially less sensitive to mechanical initiation than the crystalline form. This has the potential to open up new avenues for the preparation of energetic formulations that are less sensitive to accidental initiation and are thus intrinsically safer. This lack of crystallinity precludes the direct calculation of phonon densities of states and so we are seeking to obtain these directly through experimental measurements of INS spectra. This will allow us to to assess vibrational energy transfer and hence make predictions about sensitivity that can be correlated with experimental measurements.