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Abstract: X-ray Absorption Fine Structure techniques have been used on Comet Wild2/81P tracks from the Stardust mission. Fe-XANES and EXAFS have been performed on aerogel sections from Tracks 41 and 162 as well as the mid and terminal positions of Track 134. This is the first use of EXAFS in the study of early Solar System materials. With EXAFS, we have measured Fe–O and Fe–S bond lengths and thus, together with complementary XANES measurements, identified Fe-rich phases. In particular, we show that ferric-rich phases in 2 Tracks (41, 162) have Fe–O bond 1st shell bond lengths of 1.99–2.01 Å and Fe K absorption edge and pre edge centroid positions consistent with being hematite-dominated grains. These iron oxides can be clearly distinguished from a magnetite grain, present in Track 134. We also demonstrate the identification of the Mg-rich end member olivine using EXAFS with XANES in Track 162. The terminal grain of Track 134 is pyrrhotite, its first atomic shell has an Fe–S structure, with 4 nearest neighbouring S atoms at a distance of 2.29 ± 0.05 Å. Our XANES results show the presence of Fe3+-bearing grains along the Stardust tracks and suggest either flash-cooling of an Fe–S–SiO–O2 gas during capture or the presence of a Fe–Ni–S–O melt along the cometary tracks during impact capture in the aerogel, rather than the capture process being solely associated with reduction of cometary phases. Accurate determination of Comet Wild2 redox conditions requires the identification of phases, in particular terminal grains, which have not experienced this melting. For instance, the larger hematite-rich grains (>10 μm) are more likely to be cometary in origin. EXAFS provides a valuable new analytical technique to study fine-grained early Solar System materials.

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Abstract: In this thesis two analyses are performed to understand the intrinsic ability of bulk metallic glasses ( BMGs ) to deform plastically, based on their elastic properties. In the first of these, a Blackman diagram is utilised, which plots the ratios of second order elastic constants C 12 / C 11 and C 44 / C 11 . Two physically meaningful conditions are represented on such a p lot : 1) proximity to the Born mechanical instability criterion , and 2) deviation from the zero Cauchy relationship ( C 12 = C 44 ), which indicates whether central or non - central forces govern material behaviour. Those alloys with greatest tendency for plastic s train are found closest to the Born condition, which is also found to correspond to BMGs with a large kinetic glass fragility index, m . Additionally, that plastic alloys exhibit the largest positive deviation from the Cauchy condition suggests that non - dir ectional bonding is a feature of deformable BMGs. In the second anaylsis, three isomechanical groups are found to exist when a representative group of 33 BMGs was studied, by plotting the Young’s ( E ) and shear ( G ) moduli versus k b T g / Ω (Botlzmann constant, glass transition temperature and atomic volume respectively). An analysis of covariance reveals that the variation in the gradient between each group on such plot s are statistically meaningful, indicating fundamental differences in both bonding and structure . These are rationalis ed as resulting from the extent of non - directional bonding and the ability for enhanced structure diversity in the potential energy landscapes (PELs) of fragile glass formers, in com parison to strong glass formers. This abili ty for structure variation in the liquid state can be translated to the solid state via the non - ergodicity parameter, α . This ability for an enhancement in the variety of local minima in the PEL of fragile glass formers is further studied by comparing the ability of high, low and intermediate m BMGs to retain a low modulus and low surface hardness during suction casting in copper dies, observed by performing nanoindentation traverses a cross the cross - section of as - cast rods. The most fragile alloy ( Pd 77.5 S i 16.5 Cu 6 ) is found to show the greatest propensity to retain a soft surface, while the effec t is most limited in the strong composition ( Ce 70 Al 10 Cu 10 Ni 10 ) . Finally, the extended x - ray absorption fine structure (EXAFS) method is used to understand the sourc e of the mech a nical homogeneity observed across the as - cast rods, which is found to potentially not be controlled by nearest neighbour s. Instead, cluster rotation may control the variable mechanical properties (including plastic yield), and so length scale s beyond the first atomic shell appear to be critical.

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Abstract: Recently glasses based on gallate and aluminate networks have aroused interest in laser technology, for example, for the use as the host for .Iaser active ions. As these glasses are not good glass-formers other ingredients such as silica are normally added to improve their glass abilities. In this work I have succeeded in producing these glasses without the need for these additions. The structures of these pure rare-earth gallate and aluminate glasses made by aerodynamic levita- tion and laser heating techniques have been studied including neutron and X-ray diffraction and spectroscopy. The following results have been obtained. The structures of rare-earth gallate glasses, R2Ga6012 and R3Ga5012 where R = Pr and Nd, were studied using neutron diffrac- tion with the isomorphic substitution technique. A good agreement between the structural models from MD simulation and MD-RMC for the difference functions and the full experimental data sets was achieved. The mean Ga-O coordination number was found to be 4.1(1). The results also show a mixture of 6, 7 and 8-fold coordinated sites for the rare-earth ions with an average coordination number of 7.7(1). A more detailed study using a combination of neutron diffraction, Extended X-ray Absorption Fine Structure (EXAFS), MD simulation and MD-RMC refinement was applied to obtain the detail of the local structure of Pr3Ga5012 glasses at Pr and Ga K-edges. The nucleation and phase separation in the (Y203)x(Al203)1-x glassy systems produced by an aerodynamic levitation and laser heating was studied using micro-focus EXAFS. Turbidity was found to occur in the x < 0.25 and x > 0.35 glasses. At x < 0.25, even though no large inclusions (> 1?m) were seen in the turbid glasses, a phase separation into a polycrystalline sample of YAP (x = 0.50) and pure alumina was found. It is concluded that the turbidity in glassy samples at x ? 0.25 is due to the formation of nano meter size crystallites. For x = 0.36 and 0.375 (known as YAG), a nucleation of YAG crystals as spherical inclusions was found in a glass matrix giving rise to the turbidity in these glasses. Finally, a study of the structures of BaTiAl206 glasses was made in order to understand the processes giving rise to their unusual properties. Black and opaque, and clear and transparent BaTiAb06 glasses produced by aerodynamic levitation and laser heating by fast and slow quench rates were studied. Neutron and X-ray diffraction, MD simulation, MD-RMC refinement and X-ray absorption spectroscopy have been used and combined to determine the structure of the glass especially with regard to the coordination structure around the Ti ions. Evidence is found to show that the Ti ions occur in four fold and higher fold oxygen coordinated sites while the Al ions remain to- tally four fold coordinated. Very small differences in the structure of the two glasses are observed confirming that the opacity arises largely due to a small number of optical defects in present in the same overall glass structure.