Browsing by Author "Lopes, DA"

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    Anisotropy in the thermal expansion of uranium silicide measured by neutron diffraction
    (Elsevier, 2018-09) Obbard, EG; Johnson, KD; Burr, PA; Lopes, DA; Liss, KD; Griffiths, GJ; Scales, N; Middleburgh, SC
    In-situ neutron diffraction patterns were collected for a sample of as-cast U3Si2 during heating to 1600 °C. Anomalous changes were observed above 1000 °C, including the formation of a new diffraction peak not belonging to P4/mbm U3Si2, unequal changes in the peak intensities and onset of anisotropic lattice expansion. The large data-set enabled derivation of a function-fitted isotropic thermal expansion coefficient to high precision, in close agreement with previous dilatometry results but reducing linearly with temperature over the studied interval. Anisotropy in the instantaneous lattice thermal expansion corresponded to anomalies reported by White et al. (2015) at a similar temperature. © Elsevier B.V
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    Crystallographic characterization of U2CrN3: a neutron diffraction and transmission electron microscopy approach
    (Elsevier, 2023-06) Patnaik, S; Mishchenko, Y; Stansby, JH; Fazi, A; Peterson, VK; Jädernäs, D; Thuvander, M; Johnson, KD; Obbard, EG; Lopes, DA
    In this study, neutron diffraction and transmission electron microscopy (TEM) have been implemented to study the crystallographic structure of the ternary phase U2CrN3 from pellet to nano scale respectively. Recently microstructural evaluation of this ternary phase has been performed for the first time in pellet condition, overcoming the Cr evaporation issue during the conventional sintering process. In this work for the first time, the crystallographic structure of the ordered ternary U2CrN3 phase, stabilized in pellet condition, has been obtained by implementing neutron diffraction. For this study, pellets of the composite material UN with 20 vol% CrN were fabricated by powder metallurgy by mixing UN and CrN powders followed by Spark Plasma Sintering (SPS). TEM was used to investigate the nanoscale structure with a thin lamella of the order of 100–140 nm produced by focused ion beam (FIB). The neutron data revealed the phase composition of the pellet to be primarily 54(8) wt.% U2CrN3, in good agreement with the stoichiometry of starting reagents (UN and CrN powder) and metallographic analysis. Neutron data analysis confirms that all the crystallographic sites in U2CrN3 phase are fully occupied reinforcing the fully stoichiometric composition of this phase, however, the position of the N at the 4i site was found to be closer to the Cr than previously thought. TEM and selected area electron diffraction rendered nano-level information and revealed the presence of nano domains along grain boundaries of UN and U2CrN3, indicating a formation mechanism of the ternary phase, where the phase likely nucleates as nano domains in UN grains from migration of Cr. © 2023 The Author(s). Published by Elsevier Ltd. Open access article under the CC BY licence.
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    In-situ neutron characterization of advanced nuclear ruels - the road to a new neutron irradiation testing capability
    (The Minerals, Metals & Materials Society, 2020-02-23) Obbard, EG; Gasparrini, C; Burr, PA; Johnson, KD; Lopes, DA; Anghel, C; Middleburgh, SC; Gregg, DJ; Liss, KD; Griffiths, GJ; Scales, N; Thorogood, GJ; Lumprin, GR
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    Thermal expansion and steam oxidation of uranium mononitride analysed via in situ neutron diffraction
    (Elsevier B. V., 2023-03) Liu, J; Gasparrini, C; White, JT; Johnson, KD; Lopes, DA; Peterson, VK; Studer, AJ; Griffiths, GJ; Lumpkin, GR; Wenman, MR; Burr, PA; Sooby, ES; Obbard, EG
    In situ neutron powder diffraction experiments are applied to physical, kinetic, and microstructural characterization of uranium mononitride as a promising light water reactor fuel material. The temperature-variable coefficient of thermal expansion and isotropic Debye Waller factors are obtained by sequential Rietveld refinement over 499–1873 K. Oxidation of a UN pellet (95.2% density) under flow of 11 mg/min D2O is observed to initiate above 623 K and the rate increases by a factor of approximately 10 from 673 to 773 K, with activation energy 50.6 ± 1.3 kJ/mol; uranium oxide is the only solid corrosion product. Crown Copyright © 2022 Published by Elsevier B.V.