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.

含能材料（炸药、推进剂和烟火剂）在军事与民用领域均发挥着重要作用。其对冲击、震动、摩擦及火花引发的敏感性不仅关乎性能表现，更是制造、加工、储存、运输及使用过程中安全性的核心要素。我们近期基于振动上泵浦理论（vibrational up-pumping theory）开发了一种敏感性预测模型，并将其应用于一系列结构多样、敏感性各异的含能材料。该模型解释了晶体振动模式将机械冲击能量通过低能声子浴模式（low-energy phonon bath modes）传递至分子振动的难易程度。近期一项引人注目的进展是发现，通过喷雾干燥等技术制备的非晶态含能材料对机械引发的敏感性显著低于其晶态形式，这为制备对意外引发敏感性更低、因而本质上更安全的含能配方开辟了新途径。然而，这种非晶态特性阻碍了声子态密度（phonon densities of states）的直接计算，因此我们旨在通过非弹性中子散射光谱（INS spectra）的实验测量直接获取该数据，以评估振动能量传递并对敏感性进行预测，进而将预测结果与实验测量数据相关联。