This dissertation describes the single-molecule magnet (SMM) behaviour of f-block cyclooctatetraenyl sandwich complexes. Chapter one introduces the concepts that dictate SMM behavior particularly in f-elements. The emphasis is to understand the origin of magnetic behaviour and the properties that make lanthanide elements particularly interesting to explore. Current strategies used to predict such behaviour are discussed and a literature review on the subject is provided. Chapter Two describes the magnetic properties of eight isostructural lanthanide sandwich complexes utilizing 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion as the ligand, [Li(DME)3][LnIII(COT”)2] (Ln = Ce, Nd, Gd, Tb, Dy, Ho, Er, Tb, COT” = 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion, DME = dimethoxyethane). The complexes display a wide range of magnetic behaviour. The best performing SMM was the erbium complex, which had a blocking temperature of 8 K. Investigating different lanthanide ions with the same ligand enabled us to evaluate our findings in relation to current models used to predict SMM behaviour in lanthanide complexes. Chapter three extends the discussion of lanthanide sandwich complexes to include higher symmetry cyclooctatetraenyl complexes of ErIII and DyIII, [K(18-C-6)][LnIII(COT)2] (18-C-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane, COT = cyclooctatetraene).The change in symmetry evoked by removing the trimethylsyl- (TMS) groups on the ligand greatly influenced the magnetic properties of both complexes. Ab initio calculations revealed that the magnetic relaxation in the ErIII complex occurs via the second excited state which contributes to the very high blocking temperature of 10 K in this complex. Chapter four presents an organometallic building block approach to create triple decker lanthanide COT” complexes of GdIII, DyIII and ErIII with a molecular formula of LnIII2(COT”)3. Synthetically, we couple together the sandwich complexes discussed in Chapter 2 by oxidatively removing one ligand to produce linear complexes where the two metals are bridged by an aromatic COT” ligand. The magnetic properties of all complexes are compared to their respective mononuclear analogs. Most interesting is the unprecedented 4 K increase in blocking temperature of the triple decker ErIII analog compared to the ErIII mononuclear sandwich complex discussed in Chapter 2. This increase is due to a ferromagnetic dipole-dipole interaction between the ErIII ions through the COT” ring. The aromatic bridging ligand provides a GdIII - GdIII interaction of J = -0.448(1) cm-1. Chapter five extends the discussion of magnetic exchange coupling to include linear K2(THF)4[LnIII2(COT)4] (Ln = Gd, Dy, Er, COT = cyclooctatetraenyl dianion, THF = tetrahydrofuran) complexes of GdIII, DyIII and ErIII. Each complex is composed of two LnCOT2 units bridged linearly by a potassium ion. The magnetic interaction between metal ions is much weaker than in the triple decker complexes discussed in Chapter 4, with a GdIII-GdIII interaction of J = − 0.007(4) cm–1. The magnetic properties of the quadruple decker complexes were compared to their mononuclear equivalents (Chapter 3). Surprisingly, the ErIII complex showed an increase in magnetic blocking temperature over its mononuclear analog despite the large ErIII-ErIII separation of 8.819 Å. Ab initio calculations revealed that this increase is due to single ion effects, most likely an increase in symmetry. Chapter six deviates from lanthanide magnetism to study the magnetic properties of uranium sandwich complexes with multiple ligand systems and oxidation states. Prior to this study the SMM behaviour of uranium sandwich complexes was unknown. We report the synthesis, structure and magnetic properties of both uranium-COT” sandwich complexes and uranium-cycloheptatrienyl complexes with oxidation states spanning (III)-(V). None of the complexes showed zero-field SMM behaviour, indicating a sandwichtype ligand is not appropriate for harnessing the SMM character in uranium. We compared the slow magnetic relaxation of isostructural and valence isoelectronic uranium and neodymium complexes. The improved energy barrier in the uranium complex further motivates the use of uranium in SMM design due to its large spin-orbit coupling.

Sub-surface precipitations of UH3 have been modelled with Finite Element methods. The model includes a pre-stressed oxide layer, metal anisotropy, metal plasticity and a hyper elastic coherent hydride. The model was used to investigate UH3 precipitating at a variety of depths from 0 to 2um (spherical shape), with a variety of sizes from 0.08 to 0.8 um diameter (spherical shape) and with a variety of shapes from horizontal ellipsoid to vertical ellipsoid (depth 0.5 um). UH3 precipitation close to the surface was found to be energetically favourable as observed in experiments. Simulations on the shape of the precipitate found that the vertical ellipsoidal precipitates were found to be energetically favoured in contrast to what has been experimentally observed. In all cases the system could not accommodate the phase change by elastic deformation alone but by a combination of elastic and plastic deformation. When anisotropy is introduced into the metal matrix, the precipitate is surrounded by compressive and tensile regions. Tensile regions are found in the x-y plane adjacent to the precipitate and it is suggested that these regions are more likely to transform into further UH3 (through increased hydrogen diffusion, solubility or ease of phase change). Such precipitate development in the lateral direction would result in the experimentally observed horizontal ellipsoids. Multiple sub-surface precipitates were simulated (0.5 um depth) and it is suggested that compressive regions that develop between the precipitates could act as a barrier to coalescence. The surrounding stress regions and energetic factors suggest that there is a barrier to amalgamation for a distance of 1.5 um. Whereas for precipitates closer than 1.5 um there is an energetic benefit to coalescence. The transformation from sub-surface UH3 precipitates into growth centres (exposed UH3 on the surface) was examined by monitoring the oxide stress. This work also suggests that the transformation of sub-surface UH3 to growth centres could be retarded by an increased oxide thickness and the presence of a work hardened layer. To aid confidence in the model nano-indentation experiments were carried out on constrained UH3 surface films in the absence of air. Collected data shows a bulk modulus of 180 +/- GPa which is more in line with DFT calculated results as compared to Diamond Anvil Cell experimental work. The nano-indentation work represents the first time this type of data has been derived for UH3 in this way.

The M-shell x-ray production cross section for thorium and uranium have been determined for protons of energy 0.4 - 4.0 MeV, helium ions of energy 0.4 - 6.0 MeV, carbon ions of energy 4.5 - 11.3 MeV and oxygen ions of energy 4.5 - 13.5 MeV. The total cross sections and the cross sections for individual x-ray peaks in the spectrum, consisting of the following transitions Mz (M4-N2, M5-N3, M4-N3), Ma (M5-N6,7), Mb (M4-N6, M5-O3, M4- O2), and Mg (M4-O3, M5-P3, M3-N4, M3-N5), were compared to the theoretical values determined from the PWBA + OBKN and ECUSAR. The theoretical values for the carbon and oxygen ions were also modified to take into account the effects of multiple ionizations of the target atom by the heavier ions. It is shown that the results of the ECUSAR theory tend to provide better agreement with the experimental data.

The work has been focused on the preconcentration techniques for determination of uranium on the modified sorbent, with the final use of ICP-OES and ICP-MS. First ICP-OES at a wavelength of 385.958 nm was used for determination of uranium. Parameters were optimized and the effect of mineral acids (HCl, HNO3), tensides (Septonex, Ajatin) and organic reagents like (4-(2-pyridylazo)resorcinol, ammonium pyrrolidinedithiocarbamate, 8-hydroxyquinoline-5-sulphonic acid and 1,2-dihydroxyanthraquinone-3-sulphonic acid) was observed. For ICP-MS an isotope 238U was chosen. As an internal standard 209Bi (200 ?g•l-1) was used. Modified silica gel was used (Silicagel tethered with C18, C8 alkyls and phenyl) and two types of Amberlite XAD 4 and XAD 16 for the preconcentration of uranium (VI). Commercially produced Amberlite was modified as follows: Drying at 100 °C (24 hours), milled, minced sorbent was selected and fractionated to particles size ranging from 0.32 to 0.63 ?m and finally activated in methanol. The deionised water and Septonex (5•10-3 mol•l-1) at pH 8 were used for Amberlite conditioning. The optimal sorption of uranium (VI) proceeded at pH 8 in the presence of 4-(2-pyridylazo)resorcinol or ammonium pyrrolidinedithiocarbamate with Amberlite XAD-16 moreover with 1,2-dihydroxyanthraquinone-3-sulphonic acid. All organic reagents have mass concentration five time larger than uranium . For the elution of uranium the mixture of 1 mol•l-1 HNO3 with acetone (ratio of 1:1) was used. Acetone was evaporated and the final analysis performed using ICP-OES. The preconcentration of uranium (VI) using Silicagel-C18 conditioning was performed with ethanol, deionised water and Zephyraminu (5•10-4 mol•l-1) at pH 8. For the Silica-C8 and Silicagel-Phenyl the same conditioning procedure without zephyramin was used. Silica-C18 had the highest sorption efficiency with 1,2-dihydroxyanthraquinone-3-sulphonic acid, Silicagel-C8 with ammonium pyrrolidinedithiocarbamate and Silicagel-Phenyl with 8-hydroxyquinoline-5-sulfonic acid, all organic reagents have mass concentration five time larger than uranium concentration. As an optimal elution mixture acetone and ethanol (ratio of 1:1) in the presence of 1 mol•l-1 HCl was evaluated. Acetone and ethanol was evaporated and the final analysis was performed using ICP-MS. Silicagel-C18 was evaluate as the most effective in the presence of zephyramin and 1,2-dihydroxyanthraquinone-3-sulphonic acid. Sorption was also tested in the presence of 20 ?g•l-1 microelements (Be, V, Co, Ni, Y, Pb, Th, Cd) and in the presence of a defined concentrations of K, Na, Ca, Mg, Al and Fe. Direct analysis of uranium was carried out on samples of water from the river Ploucnice and Turonian aquifers containing uranium, 3,5 and 19,3 ?g•l-1. Different type of water required preconcentration of uranium (VI) on Silica-C18 in the presence of 1,2-dihydroxyanthraquinone-3-sulphonic acid and zephyramin. The analysis results gave statistically satisfactory results, which were confirmed using standard addition of 20 ?g•l-1.

Please read the abstract in the dissertation. / Dissertation (MSc)--University of Pretoria, 1991. / gm2014 / Chemistry / unrestricted

Cílem této práce je studium systému UCo1−xRuxGe nacházejícího se na rozhraní magnetické nestability. Podrobná studie magnetických a transportních vlastností po- lykrystal· s r·zným obsahem ruthenia odhalila komplexní změnu těchto parametr· v rámci celé série. UCoGe je slabý ferromagnet s TC ∼ 3 K a malým magnetickým momentem 0.035 µB/f.u. Pozorovali jsme strmý nár·st těchto hodnot až po hodnotu koncentrace ruthenia xmax ≈ 0.1(TC,max = 9 K a µ0 = 0.11 µB). Podstata tohoto ná- r·stu byla studována na mono krystalu o složení UCo0.88Ru0.12Ge získaném metodou plovoucí zóny. Difrakce polarizovaných neutron· provedená na tomto krystalu se slo- žením UCo0.88Ru0.12Ge vysvětlila posílení magnetismu změnou vzájemné orientace magnetických moment· na iontu uranu a kobaltu. Antiparalelní uspořádání, které bylo dříve publikováno pro UCoGe je změněno na paralelní uspořádání v dopova- ném UCo0.88Ru0.12Ge kdy dojde k reorientaci momentu na kobaltu. Silná anizotropie UCo0.88Ru0.12Ge se promítá do teplotní závislosti odporu a teplotní roztažnosti, které se dramaticky liší pro všechny tři osy. Tento prvotní nár·st TC a magnetického mo- mentu je následován poklesem směřujícím ke koncentraci xcr ≈ 0.31 kde magnetické uspořádání mizí. Podrobná studie kritických exponent· teplotních závislostí elek- trického odporu,...

The basic fuel unit of the High Temperature Reactor (HTR) of the Pebble Bed Modular Reactor (PBMR) is a uranium dioxide kernel coated with a buffer layer, an inner pyrolytic carbon (IPyC) layer, a silicon carbide (SiC) layer and an outer pyrolytic carbon (OPyC) layer and is commonly referred to as a TRISO particle. Thousands of these micro-spheres are embedded in a graphite matrix and pressed to form a fuel sphere. During the manufacture of the TRISO particles and the fuel spheres there is a production of TRISO particles with cracked/broken layers, especially the SiC layer. Before the irradiation of the fresh fuel in the nuclear reactor it is of the utmost importance to quantify the failed fractions in fresh fuel as this information is very useful in the general understanding of fuel behaviour, calculation of risk and safety margins, and prediction of long term fuel behaviour. For this reason the burn-leach method has been applied for the quality control of the fresh fuel. In this work, several aspects of the burn-leach method that affect the efficiency of the method were studied. Aspects that were investigated are: qualitative aspects, layer properties, quantitative aspects, variants of the burn-leach method and lastly statistical information from the burn-leach data. The results obtained were as follows: Studies in this dissertation suggest that partial leaching of uranium in TRISO particles with a defective SiC layer was a phenomenon that exists. Although UO2 kernel equivalents were successfully determined by burn-leach method for particles with fully broken SiC layers, certain particles leached uranium amounts that did not correspond to single UO2 kernel equivalents; Evidence of occurrences of ‘slow leaching’ in an acidic medium were evident for certain particles. There were remnants of uranium dioxide kernels that had been partially leached after the full 16 hours. This behaviour led to inconclusive results on the absolute number of defective particles in a given population; Investigations suggest that there is at least circumstantial evidence that the BL method combined with X-ray tomography provides information about the integrity of the SiC layer, and why one particle leaches and the other does not. Neither the burn-leach nor the leach-burn-leach analysis is sufficient to be used as a stand-alone method to quantify the number of particles with defective SiC layers in a given TRISO particle population. The two tests need to be coupled to other techniques such as high resolution tomography for an extensive quantification of the layer defects; Burn-leach has to be designed to test for the layer integrity on a microscopic level as opposed to testing for the broken shells only, as was done by the normal burn-leach based on the German program. The leach time was not sufficient in its present form; Burn-leach results indicated that oxidation times of 96 hours at 750 °C under atmospheric pressure did not negatively affect the mechanical strength of the silicon carbide layer of freshly-manufactured TRISO particles, as these particles did not have a high failure fraction. / Dissertation (MSc)--University of Pretoria, 2013. / Materials Science and Metallurgical Engineering / Unrestricted

Thesis: M.S., Massachusetts Institute of Technology, Department of Nuclear Engineering, 1979 / Bibliography: leaves 195-198. / M.S. / M.S. Massachusetts Institute of Technology, Department of Nuclear Engineering

The U/Th dating method has been applied to five archeological sites in France. The U/Th method relies upon the co-precipitation of uranium with calcium carbonate in speleothems formed in caves. Because 230Th forms in the calcite from the decay of 234U a radiometric clock is begun in the newly deposited calcite. Dates are derived from measuring the isotopic abundances of the uranium and thorium in the calcite. For many archeological samples, preroasting of the sample before analysis is necessary to improve the yields. Normally, relative dates for archeological sites are derived from the comparison of paleoclimatic interpretations determined from sedimentological, faunal, and palynological studies of the cave sediments with global climatic records. These methods have established that the Mousterian culture and Neanderthals appeared in Europe at the beginning of the Würm, 80 Ka BP. Absolute dates determined for samples from Lachaise, Montgaudier, Pech de l'Aze, Abri Vaufrey, and Grotte 13, where archeological or faunal material is associated stratigraphically with speleothems sampled, have established that there were several regional climatic phenomena experienced in southern France. These events are dated at 80 to 120 Ka BP, interpreted to be the Riss/ Würm interglacial, and at 38 to 50 Ka, interpreted to be the Würm I/II interstadial. Furthermore, archeological materials and human skeletal remains associated with these sites and the speleothems therein, have proven that the Nean~erthals must have evolved prior to 150 Ka BP, but that they did not develop their Mousterian culture until about 125 Ka BP. / Thesis / Master of Science (MSc)

In recent years, a variably high amount of uranium (U) and radon (Rn) were found in the water and airof the Kiirunavaara mine in Kiruna, northern Sweden. The Swedish Agency for Marine and WaterManagement (Havs och vattenmyndigheten) has maximum limits for “special polluting substances” inwater that is released to natural recipients (HVMFS 2019:25), therefore, the mining company LKAB,owner and operator of the Kiirunavaara mine, decided to conduct a study to investigate the issueregarding these findings, triggering this master thesis.From LKAB’s geochemical database, 25 samples were selected, prepared and studied throughpetrographic microscopy. Fourteen samples went through more than 70h of EPMA and BSE imageanalysis and 5 of them went through MLA. All methods were performed either in Uppsala’s Universitylaboratories or in Kiruna, at LKAB´s research facility in Malmberget. The ultimate goal of this projectwas to determine the primary metal source of uranium that could cause future problems in the watertreatment processes and leakages to recipients.In order to aboard this study in Kiirunavaara, where no previous uranium related study was everconducted, the main objectives were divided into identifying the main mineral carrier(s) of uranium thatcould be the source for these U anomalies, the characterization of the rock types and mineralassemblages associated with U-bearing minerals, and last, to try to propose preliminary implications forU solubility and distribution in the rock mass and mine water. This study found uraninite, thorite and anunknown species bearing uranium in some of the samples, both hanging and foot wall, all related tohydrothermal alterations and anhydrite. The water circulation throughout the rock mass of the minecould have been the trigger and transporter of these elements that were causing the anomalies in the firstplace.