Interaction between U/UO2 bilayers and hydrogen studied by in-situ X-ray diffraction

This paper reports experiments investigating the reaction of H$_{2}$ with uranium metal-oxide bilayers. The bilayers consist of $\leq$ 100 nm of epitaxial $\alpha$-U (grown on a Nb buffer deposited on sapphire) with a UO$_{2}$ overlayer of thicknesses of between 20 and 80 nm. The oxides were made either by depositing via reactive magnetron sputtering, or allowing the uranium metal to oxidise in air at room temperature. The bilayers were exposed to hydrogen, with sample temperatures between 80 and 200 C, and monitored via in-situ x-ray diffraction and complimentary experiments conducted using Scanning Transmission Electron Microscopy - Electron Energy Loss Spectroscopy (STEM-EELS). Small partial pressures of H$_{2}$ caused rapid consumption of the U metal and lead to changes in the intensity and position of the diffraction peaks from both the UO$_{2}$ overlayers and the U metal. There is an orientational dependence in the rate of U consumption. From changes in the lattice parameter we deduce that hydrogen enters both the oxide and metal layers, contracting the oxide and expanding the metal. The air-grown oxide overlayers appear to hinder the H$_{2}$-reaction up to a threshold dose, but then on heating from 80 to 140 C the consumption is more rapid than for the as-deposited overlayers. STEM-EELS establishes that the U-hydride layer lies at the oxide-metal interface, and that the initial formation is at defects or grain boundaries, and involves the formation of amorphous and/or nanocrystalline UH$_{3}$. This explains why no diffraction peaks from UH$_{3}$ are observed. {\textcopyright British Crown Owned Copyright 2017/AWE}Comment: Submitted for peer revie

5d-5f Electric-multipole Transitions in Uranium Dioxide Probed by Non-resonant Inelastic X-ray Scattering

Non-resonant inelastic x ray scattering (NIXS) experiments have been performed to probe the 5d-5f electronic transitions at the uranium O(4,5) absorption edges in uranium dioxide. For small values of the scattering vector q, the spectra are dominated by dipole-allowed transitions encapsulated within the giant resonance, whereas for higher values of q the multipolar transitions of rank 3 and 5 give rise to strong and well-defined multiplet structure in the pre-edge region. The origin of the observed non-dipole multiplet structures is explained on the basis of many-electron atomic spectral calculations. The results obtained demonstrate the high potential of NIXS as a bulk-sensitive technique for the characterization of the electronic properties of actinide materials.Comment: Submitted to Physical Review Letters on 31 December 200

Temperature-dependent Hall scattering factor and drift mobility in remotely doped Si:B/SiGe/Si heterostructures

Hall-and-Strip measurements on modulation-doped SiGe heterostructures and combined Hall and capacitance–voltage measurements on metal-oxide-semiconductor (MOS)-gated enhancement mode structures have been used to deduce Hall scattering factors, rH, in the Si1 – xGex two-dimensional hole gas. At 300 K, rH was found to be equal to 0.4 for x = 0.2 and x = 0.3. Knowing rH, it is possible to calculate the 300 K drift mobilities in the modulation-doped structures which are found to be 400 cm2 V – 1 s – 1 at a carrier density of 3.3 × 1011 cm – 2 for x = 0.2 and 300 cm2 V – 1 s – 1 at 6.3 × 1011 cm – 2 for x = 0.3, factors of between 1.5 and 2.0 greater than a Si pMOS control

Magnetic Excitations in NpCoGa5

We report the results of inelastic neutron scattering experiments on NpCoGa$_{5}$, an isostructural analogue of the PuCoGa$_{5}$ superconductor. Two energy scales characterize the magnetic response in the antiferromagnetic phase. One is related to a non-dispersive excitation between two crystal field levels. The other at lower energies corresponds to dispersive fluctuations emanating from the magnetic zone center. The fluctuations persist in the paramagnetic phase also, although weaker in intensity. This supports the possibility that magnetic fluctuations are present in PuCoGa$_{5}$, where unconventional d-wave superconductivity is achieved in the absence of magnetic order.Comment: 4 pages, 5 figure

Understanding the complex phase diagram of uranium: the role of electron-phonon coupling

We report an experimental determination of the dispersion of the soft phonon mode along [1,0,0] in uranium as a function of pressure. The energies of these phonons increase rapidly, with conventional behavior found by 20 GPa, as predicted by recent theory. New calculations demonstrate the strong pressure (and momentum) dependence of the electron-phonon coupling, whereas the Fermi-surface nesting is surprisingly independent of pressure. This allows a full understanding of the complex phase diagram of uranium, and the interplay between the charge-density wave and superconductivity

Spin Correlations In Actinide Materials: A Neutron Study Of USb

Measurements have been made of the short-range spin correlations in USb, a metallic compound that orders antiferromagnetically. The system has no transverse fluctuations, and the longitudinal spin correlations are anisotropic, showing stronger interactions within the ferromagnetic sheets than between them. The results of this and other experiments can be understood with simple concepts involving the bonding of 5f electrons. © 1978 The American Physical Society

Reservoir quality and burial model evaluation by kinetic quartz and illite cementation modeling : Case study of Rotliegendes, north Germany

Silicate reaction kinetics provide a complementary means to other established paleothermal indicators such as organic maturation for evaluating thermal reconstructions. In this study we combine the use of an organic maturation model with kinetic models for quartz and illite cementation to evaluate burial history scenarios for five subsalt wells in lithologically and structurally complex Rotliegendes reservoirs. Models for organic maturation are most sensitive to maximum temperature and provide no direct evidence for the time of peak temperature or the overall duration of high temperatures. By contrast, the kinetics of quartz cementation are much more strongly influenced by the duration of exposure to high temperatures compared with organic indicators. Kinetic models for fibrous illite formation similarly are sensitive to time and temperature and provide predictions for the time of illite formation that can be compared with radiometric dates. Used collectively, these organic and inorganic paleothermal indicators provide improved constraints on thermal evolution compared with conventional approaches. In this study we use these indicators to evaluate two alternative burial history scenarios. Scenario 1 incorporates a hypothesized Jurassic heat flow peak together with significant Late Jurassic deposition and subsequent erosion. Scenario 2 omits the Jurassic heat flow peak and omits the deposition and erosion of the Upper Jurassic. Although both of these scenarios are consistent with organic maturation data, scenario 2 leads to a far better match with quartz cement volumes and fibrous illite K-Ar dates