Co-crystallisation of energetic materials – a step-change in the control of properties and performance of munitions

Abstract

The research described in this thesis seeks to explore a concept that has the potential to make a step-change for the control of the properties of energetic materials (sensitivity, long-term storage, processability, performance, etc.), resulting in safer munitions with enhanced performance. This concept is co-crystallisation and involves crystallisation of the energetic material with one or more molecular components in order to modify the properties of the composition. The concept has been demonstrated in the pharmaceutical sector as a successful means of altering the physical properties of active pharmaceutical ingredients, e.g. solubility, bioavailability, stability to humidity. This project therefore aims to exploit the concepts of crystal engineering and co-crystallisation as applied to selected energetic materials in order to achieve the following objectives: (i) develop an enhanced understanding of how structure influences key properties such as sensitivity, (ii) control the sensitivity of existing, approved energetic materials, and (iii) identify new energetic materials with enhanced properties, e.g. reduced sensitivity, higher performance, and increased thermal stability. The compound 3,5-nitrotriazolone (NTO) was crystallised with a selection of co-formers to produce salts and co-crystals. The structure properties of these materials were explored using single-crystal and powder X-ray diffraction, and structural features were correlated with properties such as crystal density, difference in pKa of co-formers, thermal properties, and sensitivity to impact. Detonation velocities of the co-crystals were calculated based on densities, chemical composition, and heats of formation. Co-former molecules included a series of substituted anilines, substituted pyridines (including 4,4’-bipyridine, 2-pyridone), and substituted triazoles. A co-crystal was formed between NTO and 4,4’-bipyridine on crystallisation from ethanol, whilst a salt was formed when crystallised from water. Upon heating the salt to 50ºC, the co-crystal was formed. Structural differences between the salts formed by NTO with 3,5-DAT and 3,4- DAT were correlated with structural features. 3,5-DAT.NTO is substantially less impact sensitive than 3,4-DAT.NTO, and this is attributed to the layered structure of 3,5-DAT.NTO. An investigation into triazole-based NTO salts under high pressure was conducted. A new polymorph of 3,5-DAT.NTO was discovered upon increasing the pressure to 2.89 GPa. The high-pressure phase appears to retain the layered structure and remains in this phase up to 5.33 GPa, although it was not recoverable upon decompression to atmospheric pressure. The compression behaviour of the unit cell volume for phase I of 3,5-DAT.NTO has been fitted to a 3rd-order Birch- Murnaghan equation of state (EoS) with V0 = 957.7 Å3, B0 = 8.2 GPa and B’0 = 14.7. The unit cell was found to be most compressible in the a and c directions. Under high pressure 3,4-DAT.NTO does not give any indication of a phase change occurring up to 6.08 GPa. The coefficients of the 3rd-order Birch-Murnaghan EoS have been determined to be V0 = 915.9 Å3, B0 = 12.6 GPa and B’0 = 6.5.