High-pressure studies of energetic co-crystals

Abstract

Co-crystals have been widely explored in the field of energetic materials for more than a decade. Their ability to tune the physical, chemical and energetic properties of a material without the need to synthesise new molecules from scratch; along with their ability to count as new intellectual property; has made them attractive candidates for both civil and military applications. However, until now, no-one in published literature has explored the behaviour of these materials under high-pressure conditions – the very conditions that are generated during an initiation event. This thesis tackles that question of how the structures of these novel materials change with increasing pressure. Specif ically, this work focuses on co-crystal systems of two energetic materials – hexaaza-hexanitro-isowurtzitane (CL-20) and nitroguanidine (NQ) – with an array of co-formers, both energetic and non-energetic. All systems have been studied using Paris-Edinburgh pressure cells and neutron powder diffraction at the ISIS Neutron Source in Harwell, Oxfordshire. Each co-crystal has had a Birch-Murnaghan equation of state fitted to its structural data over the respective pressure range. The CL-20 systems used 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), trinitrotoluene (TNT), and hy drogen peroxide (HP) as co-formers. 2(CL-20):HMX was studied up to 3.5 GPa, and found to compress in an unusually isotropic manner, with no phase separations or polymorphic transitions occurring over this pressure range. The bulk modulus (B0) was determined to be 14.1(8) GPa, with pressure derivative B0 = 9.1(9). 2(CL-20):HP was studied up to 5.7 GPa, and also found to compress isotropically with no chemical changes or unusual structural behaviour over that pressure range. B0 and B0 were 13.4(2) GPa and 11.2(3) respectively. In CL-20:TNT, the system was studied up to 3.5 GPa, initially with a similar lack of unusual behaviour. However, an initiation event is believed to have occurred after reaching the next pressure point (4.1 GPa). B0 and B0 up to 3.5 GPa are 10.1(5) GPa, and 11.6(7). The NQ systems used two nitropyridone molecules as co-formers – 2-hydroxy-3,5-dinitropyridone (DNP), and 2-hydroxy-5-nitropyridone (NP). NQ:DNP underwent a phase transition at 0.86 GPa (B0 and B0 of the ambient-pressure phase being 14.4(2) GPa and 4.0), whereas the NQ:NP system showed no phase transitions or separations up to 3.5 GPa, in contrast to earlier X-ray diffraction data of the same material. B0 for NQ:NP was 8.7(9) GPa, with B0 = 9.7(9). A mechanochemical process used to prepare some of the aforementioned materials, Resonant Acoustic Mixing (RAM), was also studied in situ using neutron powder diffraction – the first time this RAM process has ever been monitored using neutron-based methods. Two non-energetic systems, urea:oxalic acid and glycine:oxalic acid, were used in this proof-of-concept study and their co-crystallisation processes were successfully followed by neutron diffraction over a time range of one hour in collection blocks of five minutes. This proves neutron diffraction can be used to successfully follow mechanochemical reactions in situ over shorter timescales than initially thought possible using this technique.

2021-06-25