180 research outputs found

    Hands-on Science. Celebrating Science and Science Education

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    The book herein aims to contribute to the improvement of Science Education in our schools and to an effective implementation of a sound widespread scientific literacy at all levels of society

    Experimental and Computational Studies of Oxide Ion Conductors

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    The work in this thesis focuses on the study of oxide ion dynamics with the aim to develop improved oxide ion conductors. As the main techniques used to achieve this were ab initio molecular dynamics (AIMD) and quasielastic neutron scattering (QENS), this combined approach is also the focus of the literature review in Chapter 1. Chapter 2 introduces the methods used for synthesis, characterisation, and further study of the materials studied. Chapter 3 investigates the effect of the dopant on the oxide ion dynamics in two doped δ\delta-Bi2_2O3_3 oxide ion conductors: Bi0.852_{0.852}V0.148_{0.148}O1.648_{1.648} and Bi0.852_{0.852}P0.148_{0.148}O1.648_{1.648}. QENS allowed observation of nanosecond dynamics, corresponding to the diffusion of the oxide ions in the Bi-O sublattice via vacancy-hopping, and picosecond dynamics, corresponding to localised motion within the dopant sublattices. AIMD gave further insight into the different oxide ion dynamics in Bi0.852_{0.852}V0.148_{0.148}O1.648_{1.648} and Bi0.852_{0.852}P0.148_{0.148}O1.648_{1.648}, showing that the flexibility of the V coordination environment plays an important role, creating additional vacancies in the Bi-O sublattice, consistent with the superior conductivity of the vanadate. Chapter 4 describes the systematic study of conductivity of the complex scheelite-type materials: Bi3_{3}(BO4_{4})(B'O4_{4})2_{2} (B = Fe, Ga, Fe0.9_{0.9}Ti0.1_{0.1}; B' = Mo) as well as Bi3_3(B2_{2}O8_{8})1/2_{1/2}(B'O4_{4})2_{2} (B = Sc, In; B' = Mo). Impedance measurements indicate that interstitial oxide ions are responsible for conductivity in these materials, and the conductivity of Bi3_{3}(Fe0.9_{0.9}Ti0.1_{0.1}O4.05_{4.05})(MoO4_{4})2_{2} was found to be 1.5 ×\times 103^{-3} S cm1^{-1} at 800 ^\circC, which is comparable to the scheelite-type oxide ion conductor LaNb0.92_{0.92}W0.08_{0.08}O4.04_{4.04} Chapter 5 discusses the study of two hexagonal perovskites: Ba3_3NbMoO8.5_{8.5} and Ba7_7Nb4_4MoO20_{20}. Using variable temperature powder X-ray diffraction, the reversibility of the phase transition in Ba3_3NbMoO8.5_{8.5} was demonstrated for the first time. QENS showed that oxide ion dynamics in both compounds are too slow to be observed on a nanosecond timescale. In Ba7_7Nb4_4MoO20_{20}, AIMD revealed a continuous oxide ion migration pathway in the abab plane, and moreover showed an important out-of-plane contribution to the long-range diffusion. This allowed suggestion of a new doping strategy to further enhance oxide ion conductivity. Chapter 6 discusses results obtained from the first AIMD simulations on a Dion-Jacobson phase oxide ion conductor, CsBi2_{2}Ti2_{2}NbO10δ_{10-\delta}, revealing an important contribution of the O2 site to the long-range diffusion. This suggests that oxide ion migration occurs predominantly via an O1-O2-O1 pathway, demonstrating the importance of rotationally flexible octahedra for high ionic conductivity in this new family of oxide ion conductors

    Synchrotron X-ray operando studies of atomic structure evolution of multi-component Al alloys in liquid state

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    This research has studied one of the challenging scientific issues in materials science, i.e., in real time, understanding quantitatively the 3D atomic structures of multiple component alloys in the liquid state and how the atomic structures evolve with temperatures until the onset of crystal nucleation. Four Al-based alloys were used in the research: (1) Al-0.4Sc, (2) Al-1.5Fe, (3) Al-5Cu-1.5Fe and (4) Al-5Cu-1.5Fe-1Si alloy (all in weight percentage). All alloys were heated up to the liquid state and then cooled down with predefined cooling rates using a dedicated solidification apparatus. During cooling, synchrotron X-ray was used to illuminate onto the samples and the total scattering data were collected at the target temperatures. Based on the total scattering data, the empirical potential structure refinement (EPSR) method was used to model and reconstruct the 3D atomic structures in the liquid state at the selected temperatures for each alloy. The research has demonstrated that the EPSR is a computationally efficient tool for searching and finding the solutions of 3D atomic structures according to the measured total scattering data. For the studied alloys, the research reveals fully the temperature-dependent structure heterogeneity and their evolutions with temperature. The key findings of the research are: (1) For the Al-0.4Sc alloy, at the short-range scale in the liquid state, Sc-centred Al polyhedrons form icosahedral type structures with the Al coordination number in the range of 10–12. As the melt is cooled down, the Sc-centred polyhedrons become more compacted, and the connections between adjacent polyhedrons change from more vertex connection to more edge and then more face-sharing connection. At the medium-range scale, the Sccentred clusters with face-sharing are proved to be the “precursors” for the L12 Al3Sc primary phase in the liquid-solid coexisting region. (2) For the three Fe-containing alloys, atomic structural heterogeneities were found to exist in the 1st atomic shell and beyond. The degree of structural heterogeneities is related with the difference in atom radius, atomic bond length and the chemical preference between different atoms in each alloy. The competition resulted in that the Al-centred clusters expand, i.e., with larger bond length, while the solute atom-centred clusters contract, so with the reduced bond length. (3) At the short-range scale, the structural heterogeneities were characterised by the co-existence and growth of the icosahedra-like (ICO-like) and crystal-like structures. During cooling, the Fe atoms show a higher degree of crystallinity than other atoms in the liquids. At the onset of crystal nucleation, relative percentage of the Fe-centred ICO-like and crystal-like Voronoi polyhedrons (VPs) reaches 8-10%, and the others in the range of 5.8-8.5%. (4) The Fe-centred short-range orders (SROs) tend to connect together via five different modes to form larger Fe-centred medium-range orders (MROs). The percentage of the face-sharing increases almost linearly as the temperature is cooled down, approximately 18-20% at the onset of nucleation in the 3 melts. The Fe-centred MROs gradually approach to the structures of the Al13Fe4 primary phase (monoclinic structure) and are proved to be the nucleation precursors for the Al13Fe4 phases. (5) For the quaternary Al-Cu-Fe-Si alloy melt, the research found that the liquid first transfers into a quasicrystal-like, metastable monoclinic Al13Fe4 phase. Such primary phase was confirmed to have a higher degree of five-fold and crystalline symmetry than the liquid. Upon cooling, the Fe-centred five-fold and crystalline symmetry both get enhanced in liquid, leading to a smaller Al13Fe4-liquid configuration entropy difference and interfacial free energy

    Synthesis and characterisation of cerium based nanocomposites

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    This thesis presents the novel, solution based, single step methodology of reactive infiltration (RI) as a way of synthesising cerium based nanoparticles (CeNP) inside metal organic framework (MOF) structures. Through the complementary precursor-host pairing, the highly basic CeN''3 precursor reacts spontaneous within the protic MOF808 host structure, yielding the composite: CeNP@MOF808. The CeNP@MOF808 was fully characterised using a number of spectroscopic techniques, demonstrating that the structural integrity of the MOF808 host remains unchanged following the RI process, specifically the crystalline arrange and high surface area of MOF808 are retained. A combination of XAS, XPS and EELS spectroscopies demonstrated that the CeNP@MOF808 composite consists of both Ce3+ and Ce4+ oxidation states, in a 3:1 respectively, with a surface Ce ions all existing in the 3+ state. Post-synthetic modifications, thermal and oxidative, of CeNP@MOF808 are presented, along with spectroscopic characterisation of the physio-electronic properties of the modified composites. A notable link between the microstructure of the composite and electronic Ce3+:Ce4+ structure is observed whereby thermal and oxidative post-synthetic modifications that resulted in the loss of MOF808 crystalline arrangement exhibited a concurrent reduction in the Ce3+ proportions. Given the redox active nature of the Ce3+:Ce4+ couple, the CeNP@MOF808 composite was screened as a possible catalyst for the decomposition of the nerve agent simulant dimethyl p-nitrophenylphosphate (DMNP). Through an standard optimised methodology it was demonstrated that the CeNP@MOF808 composite exhibited improve catalytic activity towards the hydrolysis of DMNP, relative to the control MOF808 host structure. The synthesis and characterisation of lanthanide β-ketoiminate complexes Y{OC(Me)CHC(Me)Ni Pr}3 (1) and Ce2{OC(Me)CHC(Me)Ni Pr}6 (2) was reported. Compound 2 is investigated as a possible broad spectrum precursor in the synthesis of CeO2 based nanomaterials through thermal decomposition and RI with MOF808

    Density Functional Theory

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    Density Functional Theory (DFT) is a powerful technique for calculating and comprehending the molecular and electrical structure of atoms, molecules, clusters, and solids. Its use is based not only on the capacity to calculate the molecular characteristics of the species of interest but also on the provision of interesting concepts that aid in a better understanding of the chemical reactivity of the systems under study. This book presents examples of recent advances, new perspectives, and applications of DFT for the understanding of chemical reactivity through descriptors forming the basis of Conceptual DFT as well as the application of the theory and its related computational procedures in the determination of the molecular properties of different systems of academic, social, and industrial interest

    Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter

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    The book is dedicated to the work and achievements of Howard Flack. It combines articles which describe his own work and the advances he made in the field of crystallography, with original research articles which focus on aspects related to Howard Flack's interests

    Understanding the structure and radiation behaviour of complex ceramic oxides Ln2TiO5 (Ln = lanthanide) for actinide immobilisation

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    The generation of electricity from nuclear power sources, whilst proving a low carbon emission alternative to fossil fuel based sources, produces large amounts of actinide containing radioactive wastes as a by-product. As these radioactive wastes pose a significant potential risk to both the environment and public health, a means of safely disposing of them is required. The UK government, and indeed many others across the globe, consider disposal of these wastes in geological repository, using a multi-barrier approach, as the safest method. A principal component of the multi-barrier approach is to either vitrify waste products in glass mediums, such as borosilicate, or immobilise them in ceramic systems. Ceramics are of particular interest due to their high aqueous durability, favourable physical qualities, tolerance of radiation damage and high potential waste loading. The ability of a ceramic wasteform to resist and have predictable responses to radiation damage is key to their implementation. The development of novel Ln2TiO5 ceramics as nuclear wasteforms has recently been undertaken due, in large part, to their reportedly high radiation tolerance. This study aims to characterise the local atomic arrangements of structures associated with Ln2TiO5 series of ceramics and their response to radiation damage by utilising multiple X-ray absorption techniques. The Ln2TiO5 series of ceramics form the end member of the Ln2(Ti2-xLnx)O7-x/2 (x = 0.667) pyrochlore solid solution and as such are often referred to as stuffed pyrochlores due to the additional lanthanide cations ‘stuffed’ onto the Ti-site. Dependent on ionic radius of the lanthanide used, pressure and temperature, different structure types can be obtained. The structures that Ln2TiO5 stuffed pyrochlores have been commonly observed to form include orthorhombic Pnma, hexagonal P63/mmc and cubic Fd-3m and Fm-3m symmetries. The radiation tolerance of these materials is highly dependent upon the crystal structure used. Whilst the long-range order of these materials is relatively well described there is a significant lack in understanding of the short-range order. Various studies have reported conflicting conclusions about the short-range structure of cubic Ln2TiO5. The long-range structures have been observed to be defectfluorite Fm-3m and the short-range order to be possibly consist of a series of Fd-3m pyrochlore nanodomains or to have orthorhombic Pnma symmetry. Through a combination of X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) analyses, the local structure is shown to be well represented by a model based off of a Fd-3m structure where the Ti-O coordination is 5-fold (reduced from an initial 6-fold coordination of the pyrochlore structure). Examination of Ln2TiO5 stuffed pyrochlores, that adopt either an orthorhombic or hexagonal symmetries through X-ray diffraction (XRD), using XAS techniques concludes that they are well represented by models based upon the same structure assigned to their long-range ordering. Furthermore, it is shown that the TiO5 polyhedra associated with both orthorhombic and hexagonal structures are found to have different degrees of centro-symmetry. Understanding the local structure of these materials is vital to predicting how they will behave under extreme environmental conditions, such as in high intensity radiation fields. The successful design of XAS models allowed for further study of the local structural environments of Ln2TiO5 stuffed III pyrochlores through the application of glancing angle X-ray absorption spectroscopy (GAXAS). This technique was used to probe the damaged surface region of ion beam irradiated (Au2+ and Kr+ ) bulk monoliths of orthorhombic Gd2TiO5, hexagonal Dy2TiO5 and cubic Yb2TiO5. The results of this study gave key insights into the structural responses of these materials to radiation damage and allowed for the development of an understanding of mechanisms that drive their responses. The comprehensive study of the structures and radiation response of Ln2TiO5 stuffed pyrochlores provided in this PhD study will facilitate future tailoring of their applications for roles within the nuclear industry and beyond
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