3,733 research outputs found

    Towards predicting and tailoring properties of energetic materials

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    The field of energetic materials (EMs) involves the study of materials (explosives, propellants, and pyrotechnics) that can release a significant amount of energy when initiated. This property renders EMs particularly useful to a wide array of industries including space travel (rocket propellants), mining (demolition charges), and defence applications. The propensity to release a significant amount of energy upon initiation means these materials are inherently dangerous, as such they are subjected to stringent safety requirements, and must be rigorously characterised prior to use. The safety of an EM is often quantified through the evaluation of the sensitivity (propensity to initiate) with respect to different stimuli such as impact, shock, friction, and electric spark. The focus of this work is the impact sensitivity, a solid-state property which can be influenced through changes in the orientation of molecules in 3D space (polymorphism or co-crystallisation) as well as through changing the structure or bonding environment of the molecules comprising the material. Prediction of this metric has been shown in previous work within the group to be computationally achievable for molecular EMs if the crystal structure of the material is known. This is completed through use of the vibrational up-pumping methodology. Vibrational up-pumping refers to the process by which mechanical impact energy excites delocalised low energy motions in a material and is subsequently channelled upwards into localised molecular vibrations. The vibrational states excited through up-pumping are termed the two-phonon density of states, which represents a measure of how efficiently the initial energy can become trapped on the molecular vibrations. Projection of the twophonon density of states onto the underlying vibrational character yields the up-pumped density which shows a correlation with experimental impact sensitivity. To this date, this method has been applied exclusively to molecular EMs, successfully reproducing experimental sensitivities. While important, focusing on solely molecular materials overlooks those of growing importance such as co-crystals, salts and coordination polymers. Application of the vibrational up-pumping methodology to materials from these areas of growing interest forms the backbone for the work presented in this thesis. Chapter 2 addresses a number of areas within the vibrational up-pumping methodology that could be improved upon, namely, the generation of consistent phonon density of states (g(w)) spectra as well as partial g(w) spectra, the determination of the location of uppermost phonon frequency (Wmax) and the interrogation of vibrational modes within the solid-state vibrations to track the local modes of vibration (bond stretches and angle bends). Three Python scripts have been developed to address these problems and improve the efficiency and applicability of the process by which the impact sensitivity of an EM is predicted via the vibrational up-pumping methodology. Chapter 3 focuses on two unexpected findings that had recently come to light in the EMs group at Edinburgh: a co-crystal of FOX-7 with the non-energetic p-phenylenediamine (PPD) that appeared to be more hazardous to mechanical impact than the pure EM, and a new high-pressure polymorph of 3,4,5-trinitro-1H-pyrazole (TNP) that was markedly more sensitive to initiation than the ambient pressure polymorph. For the former study, strong hydrogen bonding interactions significantly altered the molecular conformation of FOX- 7. For the latter, the molecular conformation remained unchanged in the ambient and high-pressure polymorphs, meaning that crystal packing or pressure-induced vibrational mode hardening must account for the increase in mechanical sensitivity. Taken together both studies present challenges for the up-pumping model, which if successful would allow important structure/property connections to be made. Chapter 4 focuses on salt coordination polymers, all of which present as exceptionally sensitive EMs. The study began with lead azide (LA), which is often used in small quantities as a detonator for a much larger mass of a less sensitive EM. It is well documented that lead has drastic adverse effects to both people and the environment and as such REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) has issued a ban on the use of LA. This has necessitated the development of a number of ‘green’ copper-containing replacements (DBX-1, DBX-2, DBX-3 and Cu(ADNP)) with comparable impact sensitivity and detonation characteristics such that they could potentially be used as drop-in replacements. This type of EM has not been studied before using the vibrational up-pumping procedure; they present a number of unique challenges, exemplified primarily by the need to separate the lattice modes from the molecular modes, which is a key requirement of the vibrational up-pumping model. In this chapter a full discussion on a range of mechanochemical models are investigated, from simple phonon heating, through to up-pumping and consideration of target (i.e. trigger mode) activation. Culminating in the development of a workflow for the treatment of such materials in the future within the vibrational up-pumping methodology. In Chapter 5 the emphasis switches towards applying the up-pumping model in a wider capacity to explore the effects of molecular structure on the impact sensitivity of molecular energetics. Here, the investigation centred on a series of chemically related EMs from three common families, namely pyrazoles, tetrazoles and nitrate esters. A number of these materials only differ by the location or substitution of a single functional group, and yet taken together cover a wide range of impact sensitivity response. Successful predictions of their respective impact sensitivities by the up-pumping model would therefore present a unique opportunity to fully explore structure/property relationships, with molecular flexibility, functional group identity and proximity being key structural features to explore. The data set also allowed further exploration of the trigger mode activation introduced in Chapter 4, where only the weakest bonds in the molecules are vibrationally excited by up-pumping. This approach improves the physical basis for impact sensitivity prediction. Collectively, this thesis explores the application of the vibrational up-pumping methodology to various EMs that present with greater structural complexity than the single-component molecular materials that it was initially designed to model. This work has been aided by the development of supplementary Python scripts which attempt to improve both the efficiency and applicability of the vibrational up-pumping methodology. If successful this work will act to considerably validate vibrational up-pumping, as well as to provide the opportunity to explore in-depth structure/property relationships, to understand the physical basis of impact sensitivity. Such understanding may lead to the development of tailored EMs with desired physical properties in the future

    Raman Spectroscopy Techniques for the Detection and Management of Breast Cancer

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    Breast cancer has recently become the most common cancer worldwide, and with increased incidence, there is increased pressure on health services to diagnose and treat many more patients. Mortality and survival rates for this particular disease are better than other cancer types, and part of this is due to the facilitation of early diagnosis provided by screening programmes, including the National Health Service breast screening programme in the UK. Despite the benefits of the programme, some patients undergo negative experiences in the form of false negative mammograms, overdiagnosis and subsequent overtreatment, and even a small number of cancers are induced by the use of ionising radiation. In addition to this, false positive mammograms cause a large number of unnecessary biopsies, which means significant costs, both financially and in terms of clinicians' time, and discourages patients from attending further screening. Improvement in areas of the treatment pathway is also needed. Surgery is usually the first line of treatment for early breast cancer, with breast conserving surgery being the preferred option compared to mastectomy. This type of operation achieves the same outcome as mastectomy - removal of the tumour - while allowing the patient to retain the majority of their normal breast tissue for improved aesthetic and psychological results. Yet, re-excision operations are often required when clear margins are not achieved, i.e. not all of the tumour is removed. This again has implications on cost and time, and increases the risk to the patient through additional surgery. Currently lacking in both the screening and surgical contexts is the ability to discern specific chemicals present in the breast tissue being assessed/removed. Specifically relevant to mammography is the presence of calcifications, the chemistry of which holds information indicative of pathology that cannot be accessed through x-rays. In addition, the chemical composition of breast tumour tissue has been shown to be different to normal tissue in a variety of ways, with one particular difference being a significant increase in water content. Raman spectroscopy is a rapid, non-ionising, non-destructive technique based on light scattering. It has been proven to discern between chemical types of calcification and subtleties within their spectra that indicate the malignancy status of the surrounding tissue, and differentiate between cancerous and normal breast tissue based on the relative water contents. Furthermore, this thesis presents work aimed at exploring deep Raman techniques to probe breast calcifications at depth within tissue, and using a high wavenumber Raman probe to discriminate tumour from normal tissue predominantly via changes in tissue water content. The ability of transmission Raman spectroscopy to detect different masses and distributions of calcified powder inclusions within tissue phantoms was tested, as well as elucidating a signal profile of a similar inclusion through a tissue phantom of clinically relevant thickness. The technique was then applied to the measurement of clinically active samples of bulk breast tissue from informed and consented patients to try to measure calcifications. Ex vivo specimens were also measured with a high wavenumber Raman probe, which found significant differences between tumour and normal tissue, largely due to water content, resulting in a classification model that achieved 77.1% sensitivity and 90.8% specificity. While calcifications were harder to detect in the ex vivo specimens, promising results were still achieved, potentially indicating a much more widespread influence of calcification in breast tissue, and to obtain useful signal from bulk human tissue is encouraging in itself. Consequently, this work demonstrates the potential value of both deep Raman techniques and high wavenumber Raman for future breast screening and tumour margin assessment methods

    First-principles calculations of anharmonic phonons in diamond and silicon at high temperature and pressure

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    Many ab initio approaches for calculating anharmonic phonon dispersion relations have recently been developed, taking advantage of improvements in computational power. In this thesis, anharmonic phonons in the diamond-type semiconductors silicon and diamond are studied using two of these recently developed ab initio techniques to better understand the role of anharmonicity in these materials at elevated temperatures and pressures. The two techniques are the self-consistent phonon method as implemented in the alamode code and the temperature dependent effective potential approach implemented in the TDEP code. Both these approaches rely on density functional theory calculations to compute anharmonic phonon frequencies from first principles. The renormalisation of the zone-centre optical phonon of silicon is calculated using both methods. The TDEP approach accurately reproduces the experimentally observed temperature dependence of the zone-centre phonon, whereas alamode underestimates the renormalisation. This underestimation is determined to originate from the exclusion of certain phonon–phonon interaction processes in a series expansion central to the self-consistent phonon method. In particular, an interaction process involving three phonons is identified to contribute strongly to the anharmonic phonon renormalisation. An attempt was made to extend alamode to include this interaction, which was, regrettably, unsuccessful. The TDEP approach is then applied to diamond in the same manner as silicon. The zone-centre optical phonon is calculated and a comparison to available experimental data is made. The approach is again found to accurately reproduce the experimental data. Consequently, the TDEP approach is used to investigate the so-called quantum isotope effect in diamond. Deviations from the harmonic frequency ratio of the zone-centre phonons are used to investigate the anharmonic nature of the interatomic potential, as well as to search for an experimentally suggested “inversion” of the quantum isotope effect at high pressure. No such inversion of the quantum isotope effect is observed in the calculations made here. A detailed comparison of the effect of different exchange–correlation functionals and pseudopotentials on the density functional theory calculations is made, ultimately recommending local density approximation as the most accurate predictor of phonon frequencies in diamond. Finally, the Raman frequency of natural diamond is calculated at high temperature and pressure using the highly accurate TDEP method. Improvements are made to the stochastic sampling process, eliminating unwanted scatter from misaligned eigenvectors at degenerate points in the Brillouin zone and increasing the precision of the method. The calculated Raman frequency is used to suggest a calibration of the high-frequency edge of the Raman signal from a diamond anvil, which is used as a pressure marker in very-high-pressure experiments. The suggested calibration extends to pressures up to 1 TPa and temperatures up to 2000 K

    Activating Methane and Other Small Molecules: Computational study of Zeolites and Actinides

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    Exploring the catalytic properties and reactivity of actinide complexes towards activation of small molecules is important as human activities have led to the increased distribution of these species in nature. Toward this end, it is important to have a computational protocol for studying these species, in this thesis we provide details on the performance of multiconfigurational pair-density functional theory (MC-PDFT) in actinide chemistry. MC-PDFT and Kohn-Sham Density Functional Theory (KS-DFT) perform well for these species with indications that the former can be used for species with even greater static electron correlation effect. In addition, we study the activity of organometallic trans-uranium complexes towards the electrocatalytic reduction of water. We conclude that, with a guided choice of ligand, neptunium complexes can provide similar reactivity when compared to organometallic uranium complexes.Conversion of methane to methanol has been a major focus of research interest over the years. This is largely due to the abundance of natural gas, of which methane is the major constituent. Copper-exchanged zeolites have been shown to be able to kinetically trap activated methane as strongly-bound methoxy groups, preventing over-oxidation to CO2, CO and HCOOH. In this stepwise process, there are three cycles; an initial activation step to form the copper oxo active site, methane C-H activation and lastly simultaneous desorption of methanol and re -activation of the active site.. We provide detailed description of the pathway for the formation of over oxidation products. It is observed that to ensure high selectivity to methanol and prevent further hydrogen atom abstraction by extra-framework species, the methyl group must be stabilized from the copper-oxo active sites. There is a temperature gradient between the steps in the methane-to-methanol conversion cycle which is an impediment to industrial adoption of this approach for methane-to-methanol conversion. To mitigate this, we have investigated the impact of heterometallic extra-framework motifs on the temperature gradients of each step. Using periodic DFT, we provide detailed descriptions of the mechanistic pathways for each of the three steps. We were subsequently able to design motif(s) with great methane C-H activities as well as the abilities to be formed and regenerated at nearly the same temperatures. We found [Cu-O-Ag] and [Cu-O-Pd] to be potential candidates for isothermal or near-isothermal operations of the methane-to-methanol conversion cycle. Finally, we provide insights to the changes in optical spectra of activated copper-exchanged zeolites, gaining an understanding of the evolution of these systems on a molecular level will provide opportunities to achieve improved reactivity

    Functional Nanomaterials and Polymer Nanocomposites: Current Uses and Potential Applications

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    This book covers a broad range of subjects, from smart nanoparticles and polymer nanocomposite synthesis and the study of their fundamental properties to the fabrication and characterization of devices and emerging technologies with smart nanoparticles and polymer integration

    Crystal Structures of Metal Complexes

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    This reprint contains 11 papers published in a Special Issue of Molecules entitled "Crystal Structures of Metal Complexes". I will be very happy if readers will be interested in the crystal structures of metal complexes

    Hydrogen-bonding receptors for anion recovery in a capacitive deionisation system

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    Receptors are ubiquitous throughout nature and are found heavily within biological systems. This has led to synthetic supramolecular chemists to modify or develop analogous mimics of these receptors with high affinity and specificity for a range of target compounds, for potential commercial use. One group of particular interest are receptors that function through the formation of hydrogen bonds to the guest species. This class of receptor has been shown to have a range of different structural geometries and binding motifs, that allow for the sequestration of a number of different species. In the context of this work, anionic hydrogen-bonding receptors, specifically for ‘phosphate’- in most cases dihydrogenphosphate- and bicarbonate are of interest. Phosphate is an integral part of the DNA backbone, however a organophosphorus containing compounds also comprise a large group of chemical weapons which can have a devasting impact on the bodies ability to function. Chemical weapon compounds, such as sarin and Novichok, are based on the functionalisation of a central phosphate core which can be biotransformed into a highly potent active species within the body. Phosphate is also an essential component of plant fertilizers and is used on a huge scale in order to maintain global food security. However, phosphate loss as a consequence of agricultural run-off leads to reduced availability of essential minerals as well as large scale eutrophication. One such method that could be utilised for the recovery of phosphate is electrochemical capacitive deionisation. The principle and idea of capacitive deionisation has been around since the late 1960’s to early 1970’s and has been shown to be a suitable method for the desalination of low-to-medium salinity input streams. The purpose of the work within this thesis was to modify and synthesise receptors that could be covalently attached to porous carbon electrodes and impart selectivity to a capacitive deionisation system. In Chapter 1, the importance of ‘phosphate’, biologically and commercially is addressed before an in depth look at some of the phosphate specific hydrogen bonding receptors that have been reported in the literature. The design of a successful hydrogen bonding receptor relies on the correct orientation of the binding motifs and the range of structural scaffolds have been shown to be useable. Following this, the electrochemical principles of capacitive deionisation and its suitability for the recovery of phosphate are detailed, including some examples of capacitive deionisation set-ups and the overall processes involved. Chapter 2 details the theory of the techniques used throughout this thesis, which include, but not limited to, 1H and 13C NMR for the structural elucidation of the synthesised receptors and cyclic voltammetry which was used for the attachment of organic groups to an electrode. The historical and theoretical background established in Chapters 1 and 2 will lead into the work undertaken in Chapters 3-5. Chapter 3 focusses on the first of three hydrogen bonding receptors synthesised. Building upon previous work within the field, two neutral indole-based receptors were modified to include two different potential attachment points for the electrode- a carboxylic acid and an alkyne. Following the successful synthesis of the alkyne-based receptor, 1H NMR titrations were used to confirm the affinity of the new receptor for dihydrogenphosphate. Chapter 4 introduces the second anion of interest, bicarbonate. The underlying principles for hydrogen bonding are the same for bicarbonate, as in phosphate, however a different receptor was synthesised. The carbazole receptor synthesised contained free amine groups that were proposed to act as points of attachment to an already surface bound organic spacer group. 1H NMR titrations are once again used to determine the affinity of the receptor for the bicarbonate anion. Finally, Chapter 5 introduces the second of the dihydrogenphosphate-specific receptors, this time based on the amino acid leucine. UVVis titrations with a number of different anions were used to determine the affinity of the receptor. Within this chapter, methods for the attachment of organic groups are detailed including the electroreduction of 4-nitrobenzene diazonium and the direct oxidation of the alkyne

    Understanding and controlling structural distortions underlying superconductivity in lanthanum cuprates

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    The suppression of superconductivity in layered lanthanum cuprates near x = 1/8 coincides with a structural phase transition from a low-temperature orthorhombic to a low-temperature tetragonal phase. The low-temperature phases are characterised by a static tilt of the CuO6 octahedra away from the layering axis in distinct directions. It remained an open question whether the orthorhombic-to-tetragonal phase transition would only occur in the context of competing electronic orders in the lanthanum cuprates. This thesis proposes a novel approach to studying the orthorhombic-to-tetragonal phase transition using the novel La2MgO4 system. La2MgO4 adopts the layered Ruddlesden-Pepper structure of the lanthanum cuprates but lacks the strong electron correlations and octahedral distortions associated with the Jahn-Teller active Cu site. Combining first-principles simula- tions using density-functional theory with experimental data on the novel La2MgO4 system, the context in which these structural phases can occur is detailed, outlining the key param- eters determining the stability of the phase which suppresses bulk superconductivity. The same sequence of structural phase transitions occurs in La2MgO4 as in La1.875Ba0.125CuO4, and the tetragonal phase is stabilised via steric effects beyond a critical octahedral tilt magnitude. Larger Jahn-Teller distortions favour the orthorhombic phase. The effect of isotropic and anisotropic pressure on La2MgO4 and La2CuO4 is explored. These form the basis for a structural mechanism to understand the experimental trends of the bulk superconducting transition temperature under uniaxial pressure. Finally, the justification for the methodology used throughout this thesis to simulate these systems is provided, highlighting that DFT+U accurately describes their atomic and electronic structure.Open Acces
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