Fireworks, explosives, and even rockets. We know they go bang, but we don’t know why. Despite being used for thousands of years, very little is known about the mechanisms of explosive materials at the atomic scale. Pearl, ISIS’ high pressure neutron diffractometer, is ideal for investigating these materials, known as energetic materials.
Energetic materials work (explode) by releasing energy when triggered by stimuli such as heat, impact and sparks, but their performance can vary under different temperatures and pressures. At Pearl, these conditions can be controlled, and the material looked at in detail at the atomic scale to try to understand why. Researchers from İzmir Institute of Technology and the universities of Edinburgh, Birmingham and Lincoln visited Pearl for this reason.
Their research focused on the energetic material 3,4,5-TNP, which is being studied as a safer and more stable alternative to commonly used explosives and rocket propellants. 3,4,5-TNP is a secondary explosive, meaning the material is classed as being relatively safe to handle. The researchers planned to do a two-hour neutron diffraction experiment under pressures of ~4.4 GPa and subsequently ~5.3 GPa. However, the first experiment was cut short when the sample unexpectedly underwent violent decomposition (in other words, went ‘bang’) in the Pearl beamline.
The team’s initial observations from the neutron experiments were that, no notable structural changes were seen before the explosion. In the second experiment, the sample was compressed to a higher pressure range, up to 5.3 GPa (and between significantly more robust anvils). This time, the team saw that the sample actually underwent a phase transition, where its structure was physically altered (akin to how chocolate isn’t as nice after being melted and cooled). This new form of 3,4,5-TNP was denoted Form II, and after being held at 5.3 GPa for several hours it was this form that exploded spontaneously, which put an end to data collection. The researchers concluded that this new form could be more sensitive to pressure, but more data was needed to support this.
The team expanded their study to include X-ray measurements, performed on single crystals of 3,4,5-TNP. When compared to the neutron diffraction data, both sets showed a new phase forming at 5.3 GPa. When the sample was decompressed, the X-ray diffraction data also showed that the phase change was reversible; Form II reverted to Form I when pressure was reduced. Analysis of the molecular and crystal arrangement of the sample in both forms indicated that difference in explosive sensitivity is not molecular-based, but rather caused by pressure-induced changes in molecular motion within the crystal structure. This was backed up by computational modelling.
Overall, these experiments identified a phase transition of 3,4,5-TNP under pressure, to Form II, which is more sensitive to decomposition. Inevitably, these findings and the unexpectedness of the decomposition will impact safe handling of the material, to avoid unintended pressure loading. The identification of a new form is expected to help understand how appropriate 3,4,5-TNP is for future real-world uses, and provide insight that helps the development of new energetic materials.
