Morrison, Carole

Pulham, Colin

Christopher, Imogen Louise

2024-06-28T09:36:30Z

2024-06-28T09:36:30Z

2024-06-28

Energetic materials, encompassing explosives, pyrotechnics and propellants, are of vital importance to the world. They have a range of applications including military, aerospace, industrial and even domestic applications. Their use is ubiquitous throughout the world but historically there has not been a systematic development process for new materials. Because of their ability to release vast amounts of energy upon initiation energetic materials are inherently dangerous to work with. Therefore, it is important to be able to understand not only how dangerous a material can be, but also why these materials behave the way they do. Computational methods provide a valuable tool for screening new energetic materials without having to handle potentially dangerous samples. This thesis is focused on the development of a fully computational screening programme for new energetic materials, wherein the properties of hypothesised materials can be calculated, and the performance of the material assessed in a quick and safe manner. Chapter 3 details a study to determine the best method to calculate the heat of formation of solid EMs. The heat of formation of energetic materials is vital to know, as it is routinely used in software such as EXPLO-5 to calculate the detonation parameters of EMs. While heat of formation can be measured experimentally, several methods have been proposed to calculate heat of formation without having to carry out any experimental measurements. Methods reported for calculating gas phase heats of formation include group equivalence methods and isodesmic equations, as well as a simple semi-empirical quantum mechanical method, known as PM7. Within this thesis these three methods have been compared using a chemically broad series of 20 CHNO containing energetic molecules, while PM7 was tested with a further 31 inorganic compounds. PM7 performed the best (R2 = 0.995). Methods for calculating sublimation or lattice enthalpies, used to calculate solid-state heats of formation, are also evaluated here. The work here allows for heats of formation of a range of systems, single-component molecular solids, salts, and co-crystals to be calculated. For the latter it is noted that an estimate of the lattice energy can be obtained from simply summing the energy terms for the corresponding co-former species. Chapters 4 and 5 are focused on utilising a recently developed computational model to predict the impact sensitivity from a crystal structure alone. This method models the vibrational up-pumping of initial impact energy through a crystal structure, and has thus far accurately predicted the relative sensitivity of well-known EMs, is used extensively to explore structure-property relationships of energetic co-crystals and polymorphic series. Chapter 4 shows how this method can be used to predict the sensitivity of a co-crystal consisting of energetic nitrotriazolone (NTO) and non-energetic 4,4’-bipyridine (BIPY). This chapter also highlights the power of computational methods to understand the structural changes of systems when experimental methods fall short. High pressure crystallography showed unusual behaviour with the crystal undergoing a colour change from colourless to yellow above 4 GPa. This, combined with the knowledge of other bipyridine containing co-crystals led to the assumption that the system undergoes a phase transition from co-crystal to salt at high pressure. However, computational modelling of the proton transfer system through potential energy surfaces along the N—H···N vector along with fitting of 1D Schrodinger equations confirmed that the resulting co-crystal of NTO.BIPY is a proton migration system, with the colour change being due to system compression. Prediction of the impact sensitivity of the co-crystal suggests that the sensitivity is increased in relation to the parent energetic, NTO. This is due to the introduction of BIPY which increases the number of vibrational pathways to initiation. However, the introduction of the non-energetic BIPY simultaneously dilutes the energetic power of this system. Finally in Chapter 5 the effect that polymorphism can have on impact sensitivity is explored. Analysis has been carried out on experimentally determined polymorphs of RDX and DNAN, which both exhibit high pressure polymorphs. For both RDX and DNAN the most sensitive structures are those which exist at high pressures. And for RDX this offers more detailed understanding of the detonation pathway of α-RDX, suggesting that the less sensitive α-RDX undergoes a phase change or molecular conformation change to a more sensitive form during impact sensitivity testing. The results for DNAN polymorphs suggest interesting results as the high pressure form, DNAN-VI is predicted to be highly sensitive, even though molecules pack in parallel layers. This crystal packing is usually indicative of insensitivity to impact, which is contradicted here. It is postulated that the change in the vibrational modes of the methoxy group, which are largely controlled by crystal packing, leads to the increase in the predicted sensitivity. The latter half of Chapter 5 will focus on how the impact sensitivity model will function in a fully computational screening programme. The impact sensitivity of structures of EMs obtained through crystal structure prediction are calculated, and show that impact sensitivity is expected to change significantly across polymorphic forms, therefore when devising a computational screening programme, the best estimation that may be possible is to classify new energetic materials as either primary or secondary explosives. The work presented in this thesis aims to provide pathways for developing a fully computational screening programme for new energetic materials, covering methods used to predict detonation properties through the molecular structure and implementing an ab initio method to predict the sensitivities of solid structures. The latter has shown that crystal packing plays a large role in the sensitivity of a material and has also provided insight into structure-property relationships which can be used for the design of new and safe energetic materials.

https://hdl.handle.net/1842/41928

http://dx.doi.org/10.7488/era/4651

en

The University of Edinburgh

J. E. Arnold, I. L. Christopher, C. A. Morrison and G. M. Day, Structure Prediction and Energy-Structure-Function Maps of Energetic Materials, Unpubl. Work

energetic materials

explosives

pyrotechnics

propellants

fully computational screening programme for new energetic materials

structure-property relationships

Towards a computational screening programme for energetic materials

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy