Browsing by Author "Lumpkin, GR"

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    Accommodation, accumulation, and migration of defects in Ti3SiC2 and Ti3AlC2 MAX phases
    (John Wiley and Sons, 2013-10-10) Middleburgh, SC; Lumpkin, GR; Riley, D
    We have determined the energetics of defect formation and migration in Mn+1AXn phases with M = Ti, A = Si or Al, X = C, and n = 3 using density functional theory calculations. In the Ti3SiC2 structure, the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular configuration (C–Si = 1.889 Å) and to two apical Ti atoms (C–Ti = 2.057 Å). This carbon–metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti–Si, 17.3 eV for Ti–C, and 6.1 eV for Si–C. Broadly similar behavior was found for Frenkel and antisite defect energies in the Ti3AlC2 structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti3AlC2 is different and the energy is lower compared with the Ti3SiC2 structure. This effect, together with the observation of a stable C interstitial defect coordinated by three silicon species and two titanium species in Ti3SiC2, will have important implications for radiation damage response in these materials. © 2013, Commonwealth of Australia.
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    Atomic origins of radiation-induced defects and interfacial strengthening in additively manufactured titanium aluminide alloy irradiated with Kr-ions at elevated temperature
    (Elsevier, 2019-04-04) Zhu, HL; Qin, MJ; Aughterson, RD; Wei, T; Lumpkin, GR; Ma, Y; Li, HJ
    The irradiation microstructure of the additively manufactured titanium aluminide (TiAl) alloy subjected to in situ transmission electron microscope (TEM) irradiation with 1 MeV Kr ions at the elevated temperature of 873K was investigated. Triangle and large hexagon shaped volume defects were observed within the γ-TiAl phase in the TEM images of the irradiated microstructure. High resolution TEM images and composition analyses revealed the volume defects were vacancy-type stacking fault tetrahedrals (SFTs). Molecular dynamic simulations showed that the increased diffusion coefficient at the elevated temperature promoted the movement and aggregation of vacancies, leading to the formation and growth of SFTs in the irradiated FCC γ phase. The lamellar interfaces in the irradiation microstructure were more effective for acting as strong sinks to absorb the primary point defects and defect clusters at the elevated temperature. The initial defects at the interfaces of the additively manufactured TiAl alloy enhanced the sink strength of the material and greatly refined SFTs near the lamellar interfaces. © 2019, The Authors.
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    Cation antisite disorder in uranium-doped gadolinium zirconate pyrochlores
    (Elsevier, 2014-09-01) Gregg, DJ; Zhang, ZM; Thorogood, GJ; Kennedy, BJ; Kimpton, JA; Griffiths, GJ; Guagliardo, PR; Lumpkin, GR; Vance, ER
    The incorporation of uranium into gadolinium zirconate (Gd2Zr2O7) is investigated by synchrotron X-ray powder diffraction and X-ray absorption near-edge structure (XANES) spectroscopy. The results suggest that the uranium cation is largely located on the pyrochlore B-site instead of the targeted A-site. Cation disorder in Gd2Zr2O7 and U-doped Gd2Zr2O7 is investigated by positron annihilation lifetime spectroscopy (PALS) which demonstrates the absence of cation vacancies in these systems. This work provides direct evidence for cation antisite (A- and B-site mixing) disorder in U-doped and off-stoichiometric Gd2Zr2O7 pyrochlore. © 2014, Elsevier B.V.
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    Ceramic host phases for nuclear waste remediation
    (John Wiley & Sons, Inc, 2018-01-12) Lumpkin, GR
    This chapter gives a summary of the various types of ceramic nuclear waste forms that have been developed, to various levels, either as alternatives to borosilicate glass for high-level waste (HLW) or as special-purpose materials for separated HLW or legacy wastes. These materials have been designed with a number of criteria in mind, such as compatibility with geological environments, high durability in aqueous fluids, and resistance to changes in properties due to radiation damage, and high waste loadings. In nuclear systems, the main sources of radiation damage include neutrons and alpha, beta, and gamma radiation emitted from radioactive elements. A series of molecular dynamics (MD) studies of the fundamental properties of rutile in relation to radiation tolerance have been reported. Although rutile is normally used as an inert component in nuclear waste forms, it has the capacity to serve as a host phase for radionuclides under the appropriate conditions. © 2018 John Wiley & Sons Ltd.
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    Characterisation of natural substrates with regard to application of surface complexation models
    (OECD, 2001) Waite, TD; Fenton, BR; Payne, TE; Lumpkin, GR; Davis, JA; McBeath, M
    While good correspondence between laboratory sorption data and surface complexation modelling results has been obtained for single oxide phase, much poorer correspondence has been obtained for natural substrates. This result arises, at least in part, from the difficulty in ascertaining the identity of sorbing surfaces and in assigning appropriate values for sorbing surface site concentrations. In an attempt to clarify the nature of possible sorbing phases, we have used a variety of techniques to investigate the surfaces of natural solid substrates from the Koongarra weathered zone. Based on insights gained from the surface characterisation studies, we have then proceeded to assess the applicability of various surface complexation modelling approaches as applied to U(VI) uptake. © 2001 OECD
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    Characterization and leaching behavior of plutonium-bearing Synroc-C
    (Cambridge University Press/Springer Nature, 1996-02-05) Smith, KL; Lumpkin, GR; Blackford, MG; Hambley, M; Day, RA; Hart, KP; Jostsons, A
    Synroc-C containing 10wt% simulated PW-4b-D HLW including 0.62 wt% 239Pu was subjected to MCC-1 type leach tests at 70°C in deionised water, silicate and carbonate leachates for 53 d and deionised water for 2472 d. The normalised total (i.e. unfiltered leachate + vessel wall) Pu leach rates in deionised water, silicate and carbonate leachates for periods up to 53 d were found to be of the order of 10-5, 10-4 and 10-4 g m-2 d-1 respectively. After 2472 d, the differential, normalised, Pu leach rate in deionised water dropped to ∼5 × 10-6 (total) and ∼5 × 10-8 (solution - after filtration through a 1000NMW filter) g m-2 d-1. SEM and AEM were used to characterise our starting material and investigate the secondary phases on the surfaces of leached Synroc-C discs. Calculated and measured normalised Pu leach rates are compared and the partitioning of Pu between zirconolite and perovskite is discussed. © Materials Research Society 1997
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    Characterization and leaching behavior of plutonium-bearing Synroc-C
    (Materials Research Society, 1997-04-22) Smith, KL; Lumpkin, GR; Blackford, MH; Hambley, M; Day, RA; Hart, KP; Jostsons, A
    Synroc-C containing 10wt% simulated PW-4b-D HLW including 0.62 wt% 239Pu was subjected to MCC-1 type leach tests at 70°C in deionised water, silicate and carbonate leachates for 53 d and deionised water for 2472 d. The normalised total (i.e. unfiltered leachate + vessel wall) Pu leach rates in deionised water, silicate and carbonate leachates for periods up to 53 d were found to be of the order of 10-5, 10-4 and 10-4 g m-2 d-1 respectively. After 2472 d, the differential, normalised, Pu leach rate in deionised water dropped to ∼5 × 10-6 (total) and ∼5 × 10-8 (solution - after filtration through a 1000NMW filter) g m-2 d-1. SEM and AEM were used to characterise our starting material and investigate the secondary phases on the surfaces of leached Synroc-C discs. Calculated and measured normalised Pu leach rates are compared and the partitioning of Pu between zirconolite and perovskite is discussed. © Materials Research Society 1997
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    Chemistry and radiation effects of davidite
    (GeoScienceWorld, 2013-10-01) Lumpkin, GR; Blackford, MG; Colella, M
    Davidite (A1−xM21O38) samples from five different geological localities contain approximately 0.2 to 9.5 wt% UO2 (0.02 to 0.65 atoms per formula unit) and <0.1 to 1.3 wt% ThO2 (<0.01 to 0.09 atoms per formula unit). Maximum amounts of other notable cations include 3.7 wt% V2O3, 4.1 wt% Cr2O3, 2.5 wt% Y2O3, 5.6 wt% La2O3, 6.0 wt% Ce2O3, 4.0 wt% MnO, 2.4 wt% ZnO, 2.7 wt% SrO, and 4.9 wt% PbO. As a result of the variation in age and Th-U content, the calculated α decay dose ranges from ~0.2 to 44 × 1016 α/mg (~0.06 to 14.5 dpa). For samples with ages of 275–295 Ma, the critical dose for amorphization based on electron diffraction is ~0.8 × 1016 α/mg. Natural davidite is commonly altered to rutile, ilmenite, titanite, and other minor phases. © 2013, Mineralogical Society of America.
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    Chemistry, microstructure, and alpha decay damage of natural brannerite
    (Elsevier, 2012-01-06) Lumpkin, GR; Leung, SHF; Ferenczy, J
    To investigate the long-term stability of brannerite with respect to alpha decay damage and interaction with aqueous fluids, we have undertaken a study of twelve natural samples from a range of geological environments. Our results indicate that seven of the samples exhibit only minor alteration, usually located within veinlets or around the rim of the sample. The remaining five samples consist of variable amounts of unaltered and altered brannerite. Based on a total of 3 metal cations, the Ti and U contents of unaltered brannerite range from 1.8 to 2.1 and 0.4 to 0.9 atoms per formula unit (apfu), respectively. Maximum amounts of the other major cations on the A-site are 0.48 Ca, 0.22 Th, 0.14 Y, and 0.07 Ln (lanthanide = Ce, Nd, Gd, Sm) atoms pfu. Maximum values of other cations on the B-site are 0.15 Fe, 0.14 Si, 0.09 Al, 0.06 Nb, 0.04 Mn, and 0.04 Ni atoms pfu. Altered regions of brannerite contain significant amounts of Si and other elements incorporated from an aqueous fluid phase, and up to 40–90% of the original amount of U has been lost as a result of alteration. SEM-EDX results also provide evidence for TiO2 phases, galena, and a thorite-like phase as alteration products. Electron diffraction patterns of all samples typically consist of two broad, diffuse rings that have equivalent d-spacings of 0.31 nm and 0.19 nm, indicating that brannerite is rendered completely amorphous by alpha decay damage. Many of the grains also exhibit weak diffraction spots due to fine-grained inclusions of a uranium oxide phase and galena. Using the available age data, these samples have accumulated alpha decay doses in the range of 0.7–200 × 1016 α mg−1. Although imprecise, the U–Th–Pb chemical ages determined by microanalysis are generally consistent with the known ages and geological histories of the brannerite host rocks. To a first approximation, it is possible to understand the results of chemical and isotopic dating of brannerite by treating each sample as a complex system composed of nominally unaltered, altered, and recrystallised areas. The dominant effects are U loss from altered areas and Pb loss from unaltered brannerite and, to a lesser extent, altered brannerite, the structure of which approximates to a Ti–Si–O glass-like network wherein Pb is more comfortable as a network modifier. Crown Copyright © 2011 Published by Elsevier
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    Corrigendum to ‘Atomic origins of radiation-induced defects and the role of lamellar Interfaces in radiation damage of titanium aluminide alloy irradiated with Kr-ions at elevated temperature’ [Acta Mater. 172 (2019) 72–83]
    (Elsevier, 2020-09-15) Zhu, HL; Qin, MJ; Aughterson, RD; Wei, T; Lumpkin, GR; Ma, Y; Li, HJ
    The authors regret that the scale bars in Figure 8(c) BF-STEM and 8(d) HAADF-STEM for volume defects near the γ/γ lamellar interfaces in the Kr-ion irradiated microstructure of the TiAl alloy irradiated at 873 K are mislabelled. The authors would like to apologise for any inconvenience caused. © 2020 Acta Materialia Inc. Published by Elsevier Ltd.
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    Crystal chemistry and ion-irradiation resistance of Ln2ZrO5 compounds with Ln = Sm, Eu, Gd, and Tb
    (John Wiley & Sons, Inc, 2021-12-22) Aughterson, RD; Lumpkin, GR; Zhang, ZM; Avdeev, M; Kong, L
    The previously unattained fabrication of single phase Ln2ZrO5 (Ln = Sm, Eu, Gd, and Tb) compounds via relatively low sintering temperature (1400°C) is achieved in this study using a coprecipitation method. The crystal structures have been investigated by neutron, synchrotron X-ray powder diffraction, and electron diffraction techniques. While the general long-range structure may be well described by the defect-fluorite type structure with Fm3m symmetry, electron diffraction has highlighted a complex underlying modulated structure that varies between each compound. These compounds have been tested for ion-irradiation response using in situ 1 MeV krypton ions and transmission electron microscopy characterization. None of the compounds undergo a crystalline to amorphous transition, even holding at 50 K. Both the underlying fluorite and modulated superstructures are little affected by the irradiation. However, some atomic rearrangements are observed in the postirradiated electron diffraction patterns for the Sm2ZrO5 specimen. © 2021 Commonwealth of Australia.
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    Crystal chemistry and phase manipulation in Synroc
    (Trans Tech Publications Ltd, 1991) Vance, ER; Moricca, SA; Thorogood, GJ; Lumpkin, GR
    Synroc is a multi-phase ceramic designed for geological immobilisation of radioactive waste produced by reprocessing nuclear fuel from power reactors [1]. The main crystalline phases are hollandite, perovskite, zirconolite, and reduced titanium oxide. The compositions of these phases and the nuclides they can incorporate in solid solution are shown in Table 1. Table 1. Principal Phases comprising Synroc Phase Nominal Composition Waste nuclides incorporated Estimated wt%* [2] Hollandite Ba1.14(Al, Tr3+)2.28Ti6O16 Cs, Sr, Rb 25 Perovskite CaTio3 Sr, RE, An 20 Zirconolite CaZrTi2O7 RE, An 20 Titanium Oxide TinO2n-1 - 35 *No HLW present RE = rare earths, An = actinides. The main (Synroc-C) formulation is designed for Purex reprocessing waste and the standard composition is wt%: Al2O3(4.3); BaO(4.4); CaO(8.8); ZrO2(5.6); TiO2(57.9); waste oxides (20). The loading of high-level waste (HLW) oxides can be varied if desired, but probably cannot exceed a value of 30-35% [2]. Several variants of this composition have been formulated at the laboratory scale, with Synroc-D, E and F being directed towards Savannah River (U.S.A.) military waste, encapsulation of high-level nuclear reprocessing waste and unreprocessed spent fuel respectively. © 1991 Trans Tech Publications Ltd.
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    Crystal chemistry and structures of uranium-doped gadolinium zirconates
    (Elsevier, 2013-07-01) Gregg, DJ; Zhang, YJ; Zhang, ZM; Karatchevtseva, I; Blackford, MG; Triani, G; Lumpkin, GR
    A series of uranium-containing gadolinium zirconate samples have been fabricated at 1450 °C in oxidizing, inert and reducing atmospheres. X-ray diffraction, Raman spectroscopy and transmission electron microscopy have been utilized to confirm adoption of pyrochlore or defect fluorite structures. X-ray diffraction allowed determination of the bulk averaged structure while Raman spectroscopy and transmission electron microscopy were used to determine ordering at the microdomain scale. Diffuse reflectance, X-ray absorption near edge structure and X-ray photoelectron spectroscopies indicated a predominantly U6+ oxidation state for all the air-sintered samples, even when Ca2+ or A-site vacancies were incorporated to charge balance for U4+, a mixed U5+/U6+ oxidation state was found for samples sintered in argon, while a mixed U4+/U5+ oxidation state occurred for sintering under N2–3.5%H2. This demonstrates a degree of uranium oxidation state control through sintering conditions, and the potential of using gadolinium zirconates as host materials for uranium in nuclear waste applications.© 2013, Elsevier B.V.
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    Crystal chemistry of the orthorhombic Ln2TiO5 compounds with Ln=La, Pr, Nd, Sm, Gd, Tb and Dy
    (Elsevier, 2015-07-01) Aughterson, RD; Lumpkin, GR; Thorogood, GJ; Zhang, ZM; Gault, B; Cairney, JM
    The crystal structures of seven samples of orthorhombic (Pnma) Ln2TiO5 compounds with Ln=La, Pr, Nd, Sm, Gd, Tb and Dy were refined by Rietveld analysis of synchrotron X-ray powder diffraction (S-XRD) data. With increasing size of the lanthanide cation, the lattice parameters increase systematically: c by only ~1.5% whereas both a and b by ~6% from Dy2TiO5 to La2TiO5. The mean Ti–O bond length only increases by ~1% with increasing radius of the Ln cation from Gd to La, primarily due to expansion of the pair of Ti–O3 bonds to opposite corners of the Ti–O5 square based pyramid polyhedra. For Dy2TiO5 and Tb2TiO5, a significant variation in Ti–O1 and Ti–O4 bond lengths results in an increased deformation of the Ti–O5 base. The particular configuration consists of large rhombic shaped tunnels and smaller triangular tunnels along the b axis, which have implications for defect formation and migration caused by radiation damage or the ionic conductivity. © 2017 Elsevier B.V.
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    Crystal chemistry, alpha decay damage, and geochemical alteration of brannerite and davidite
    (Committee of Asia-Pacific Societies of Microscopy, 2012-02-07) Lumpkin, GR; Zhang, YJ; Leung, SHF; Ferenczy, J
    To investigate the long-term stability of the uranium minerals brannerite and davidite, we have undertaken an electron microscopy study of samples from a range of geological environments worldwide. Our results indicate that both minerals are transformed to an amorphous phase after a dose of approximately 10 16 α mg-1, similar to other rare element oxide minerals. Both minerals are susceptible to alteration via interaction with hydrothermal fluids, although by different mechanisms. The U-Th-Pb chemical ages of brannerite are generally consistent with the known ages and geological histories of the host rocks. These ages are affected by U loss from altered areas and Pb loss from unaltered brannerite.
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    Crystal structure influence on ion-irradiation tolerance of Ln2TiO5 compounds
    (Engineers Australia, 2014-11-26) Aughterson, RD; Lumpkin, GR; Gault, B; Whittle, KR; de los Reyes, M; Smith, KL; Cairney, JM
    As a controllable and reproducible technique ion beam irradiation is routinely used as a method for simulating recoil damage caused by alpha-decay in actinide containing materials and neutron damage within fission and fusion reactor systems [1]. The transition from a crystalline to amorphous structure may lead to larger scale effects in the material properties such as an increase in volume (swelling) linked to the generation and agglomeration of defects and decreased thermodynamic and structural stability. Compounds in this study consist of the nominal stoichiometry Ln2TiO5 (where Ln represents the lanthanide series plus yttrium). There are 4 crystal structure symmetries in this series; orthorhombic Pnma, hexagonal P63/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The final structure is dependent upon the lanthanide size and processing conditions used (pressure and temperature regime) [2]. Ln2TiO5 compounds have been proposed for potential nuclear based applications [3]. For example, Dy2TiO5 has been incorporated into inert matrix fuel as a neutron absorber [4] and Gd2TiO5 has also been proposed for similar use [5] and it’s radiation tolerance has been tested for this purpose [6]. In this study a systematic series of samples of the nominal stoichiometry Sm(x)Yb(2-x)TiO5 (where x = 2, 1.4, 1, 0.6, and 0) are used to test and compare the ion-irradiation tolerance of the major structures within the Ln2TiO5 system of compounds. An improved radiation tolerance with the higher symmetry cubic structures was found, which is consistent with previous studies of similar compounds.
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    Crystal structures of orthorhombic, hexagonal, and cubic compounds of the Sm(x)Yb(2−x)TiO5 series
    (Elsevier, 2014-05) Aughterson, RD; Lumpkin, GR; Reyes, MDL; Sharma, N; Ling, CD; Gault, B; Smith, KL; Avdeev, M; Cairney, JM
    A series of single phase compounds with nominal stoichiometry Sm(x)Yb(2−x)TiO5 (x=2, 1.4, 1, 0.6, and 0) have been successfully fabricated to generate a range of crystal structures covering the most common polymorphs previously discovered in the Ln2TiO5 series (Ln=lanthanides and yttrium). Four of the five samples have not been previously fabricated in bulk, single phase form so their crystal structures are refined and detailed using powder synchrotron and single crystal x-ray diffraction, neutron diffraction and transmission electron microscopy. Based on the phase information from diffraction data, there are four crystal structure types in this series; orthorhombic Pnma, hexagonal P63/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The cubic materials show modulated structures with variation between long and short range ordering and the variety of diffraction techniques were used to describe these complex crystal structure types. © 2014, Elsevier Inc.
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    Cyclic voltammetric experiment - simulation comparisons of the complex mechanism associated with electrochemical reduction of Zr4+ in LiCl-KCl eutectic molten salt
    (The Electrochemical Society, 2012-11-29) Fabian, CP; Luca, V; Le, TH; Bond, AM; Chamelot, P; Massot, L; Caravaca, C; Hanley, TL; Lumpkin, GR
    Nuclear energy increasingly represents an important option for generating largely clean CO2-free electricity and zirconium is a fission product that is expected to be present in irradiated fuels. The present investigation addresses the electrochemical reduction of Zr4+ to Zro in LiCl - KCl eutectic molten salt in the temperature range 425–550°C using cyclic voltammetry (CV), square-wave voltammetry (SWV) and bulk electrolysis. Simulations of the CV data indicate that the initial reduction proceeds through surface confined steps: Zr4+* + 2e− ↔ Zr2+* and Zr2+* + 2e− ↔ Zr* processes (* adsorbed species) followed by a peak-shaped complex diffusion controlled step that consists of a combination of closely spaced processes associated with the reactions Zr4+ + 4e− → Zro and Zr4+ + 3e− → Zr+*. Zr+*, probably in the form of ZrCl* is then further reduced to Zro* at even more negative potentials. The simulations provide the first quantitative analysis of the thermodynamics and kinetics of the Zr4+ reduction in the LiCl-KCl eutectic. © 2012, The Electrochemical Society.
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    Defect evolution in a NiMoCrFe alloy subjected to high-dose Kr ion irradiation at elevated temperature
    (Elsevier B.V., 2016-06-01) de los Reyes, M; Voskoboinikov, R; Kirk, MA; Huang, HF; Lumpkin, GR; Bhattacharyya, D
    A candidate NiMoCrFe alloy (GH3535) for application as a structural material in a molten salt nuclear reactor was irradiated with 1 MeV Kr2+ ions (723 K, max dose of 100 dpa) at the IVEM-Tandem facility. The evolution of defects like dislocation loops and vacancy- and self-interstitial clusters was examined in-situ. For obtaining a deeper insight into the true nature of these defects, the irradiated sample was further analysed under a TEM post-facto. The results show that there is a range of different types of defects formed under irradiation. Interaction of radiation defects with each other and with pre-existing defects, e.g., linear dislocations, leads to the formation of complex microstructures. Molecular dynamics simulations used to obtain a greater understanding of these defect transformations showed that the interaction between linear dislocations and radiation induced dislocation loops could form faulted structures that explain the fringed contrast of these defects observed in TEM. © 2016 Elsevier B.V.
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    Density and structural effects in the radiation tolerance of TiO2 polymorphs
    (IOP Publishing, 2013-08-05) Qin, MJ; Kuo, EY; Whittle, KR; Middleburgh, SC; Robinson, M; Marks, NA; Lumpkin, GR
    The radiation response of TiO2 has been studied using molecular dynamics. The simulations are motivated by experimental observations that the three low-pressure polymorphs, rutile, brookite and anatase, exhibit vastly different tolerances to amorphization under ion-beam irradiation. To understand the role of structure we perform large numbers of simulations using the small thermal spike method. We quantify to high statistical accuracy the number of defects created as a function of temperature and structure type, and reproduce all the main trends observed experimentally. To evaluate a hypothesis that volumetric strain relative to the amorphous phase is an important driving force for defect recovery, we perform spike simulations in which the crystalline density is varied over a wide range. Remarkably, the large differences between the polymorphs disappear once the density difference is taken into account. This finding demonstrates that density is an important factor which controls radiation tolerance in TiO2. © 2013, IOP Publishing Ltd.