Browsing by Author "Middleburgh, SC"
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- ItemAccommodation of excess oxygen in fluorite dioxides(Elsevier, 2013-12-15) Middleburgh, SC; Lagerlof, KPD; Grimes, RWAccommodation of excess oxygen in CeO2, ThO2 and UO2 has been investigated using ab-initio modelling. Calculations indicate that hyperstoichiometry is preferentially accommodated by the formation of peroxide species in CeO2 and ThO2 but not in UO2, where oxygen interstitial defects are dominant. Migration of the excess oxygen defects was also studied; the peroxide ion in CeO2 and ThO2 is transported via a different mechanism, due to the formation of peroxide molecules, to the oxygen interstitial in UO2. Frenkel pair defects were investigated to understand if the interstitial component could assume a peroxide like configuration in the vicinity of the vacancy. While it was already expected that this would not be the case for UO2 since peroxide was not stable, it was also not found to be the case for CeO2 and ThO2 with the peroxide disassociating into a lattice species and a separate interstitial ion. © 2013, Elsevier B.V.
- ItemAccommodation of excess oxygen in group II monoxides(John Wiley and Sons, 2013-01-07) Middleburgh, SC; Lagerlof, KPD; Grimes, RWAtomic scale simulations are used to predict how excess oxygen is accommodated across the group II monoxides. In all cases, the preference is to form a peroxide ion centered at an oxygen site, rather than a single oxygen species, although the peroxide ionic orientation changes from <100> to <110> to <111> with increasing host cation radius. The enthalpy for accommodation of excess oxygen in BaO is strongly negative, whereas in SrO it is only slightly negative and in CaO and MgO the energy is positive. Interestingly, the increase in material volume due to the accommodation of oxygen (the defect volume) does not vary greatly as a function of cation radius. The vibrational frequency of peroxide ions in the group II monoxides is predicted with the aim to provide test data for future experimental observations of oxygen uptake. Finally, calculations of the dioxide structures have also been carried out. For these materials the oxygen vacancy formation energy is always positive (1.0–1.5 eV per oxygen removed) indicating that they exhibit only small oxygen defect concentrations. © 2012, The American Ceramic Society.
- ItemAccommodation, accumulation, and migration of defects in Ti3SiC2 and Ti3AlC2 MAX phases(John Wiley and Sons, 2013-10-10) Middleburgh, SC; Lumpkin, GR; Riley, DWe 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.
- ItemAnisotropy 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, SCIn-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
- ItemAtomic scale modelling for real nuclear engineering problems and applications(Australian Institute of Physics, 2013-02-06) Middleburgh, SCNot available
- ItemCrystal structure, thermodynamics, magnetics and disorder properties of Be–Fe–Al intermetallics(Elsevier B.V., 2015-08-05) Burr, PA; Middleburgh, SC; Grimes, RWThe elastic and magnetic properties, thermodynamical stability, deviation from stoichiometry and order/disorder transformations of phases that are relevant to Be alloys were investigated using density functional theory simulations coupled with phonon density of states calculations to capture temperature effects. A novel structure and composition were identified for the Be–Fe binary ε phase. In absence of Al, FeBe5 is predicted to form at equilibrium above ∼1100 K, while the ε phase is stable only below ∼1500 K, and FeBe2 is stable at all temperatures below melting. Small additions of Al are found to stabilise FeBe5 over FeBe2 and ε, while at high Al content, AlFeBe4 is predicted to form. Deviations from stoichiometric compositions are also considered and found to be important in the case of FeBe5 and ε. The propensity for disordered vs ordered structures is also important for AlFeBe4 (which exhibits complete Al–Fe disordered at all temperatures) and FeBe5 (which exhibits an order–disorder transition at ∼950 K). © 2015 Elsevier B.V.
- ItemDensity 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, GRThe 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.
- ItemFormation and structure of V–Zr amorphous alloy thin films(Elsevier B.V., 2015-01-15) King, DJM; Middleburgh, SC; Liu, ACY; Tahini, HA; Lumpkin, GR; Cortie, MBAlthough the equilibrium phase diagram predicts that alloys in the central part of the V–Zr system should consist of V2Zr Laves phase with partial segregation of one element, it is known that under non-equilibrium conditions these materials can form amorphous structures. Here we examine the structures and stabilities of thin film V–Zr alloys deposited at room temperature by magnetron sputtering. The films were characterized by X-ray diffraction, transmission electron microscopy and computational methods. Atomic-scale modelling was used to investigate the enthalpies of formation of the various competing structures. The calculations confirmed that an amorphous solid solution would be significantly more stable than a random body-centred solid solution of the elements, in agreement with the experimental results. In addition, the modelling effort provided insight into the probable atomic configurations of the amorphous structures allowing predictions of the average distance to the first and second nearest neighbours in the system. © 2014 Elsevier Ltd.
- ItemFormation of (Cr,Al)UO4 from doped UO2 and its influence on partition of soluble fission products(Elsevier, 2013-11-01) Cooper, MWD; Gregg, DJ; Zhang, YJ; Thorogood, GJ; Lumpkin, GR; Grimes, RW; Middleburgh, SCCrUO4 and (Cr, Al)UO4 have been fabricated by a sol–gel method, studied using diffraction techniques and modelled using empirical pair potentials. Cr2O3 was predicted to preferentially form CrUO4 over entering solution into hyper-stoichiometric UO2+x by atomic scale simulation. Further, it was predicted that the formation of CrUO4 can proceed by removing excess oxygen from the UO2 lattice. Attempts to synthesise AlUO4 failed, instead forming U3O8 and Al2O3. X-ray diffraction confirmed the structure of CrUO4 and identifies the existence of a (Cr, Al)UO4 phase for the first time (with a maximum Al to Cr mole ratio of 1:3). Simulation was subsequently used to predict the partition energies for the removal of fission products or fuel additives from hyper-stoichiometric UO2+x and their incorporation into the secondary phase. The partition energies are consistent only with smaller cations (e.g. Zr4+, Mo4+ and Fe3+) residing in CrUO4, while all divalent cations are predicted to remain in UO2+x. Additions of Al had little effect on partition behaviour. The reduction of UO2+x due to the formation of CrUO4 has important implications for the solution limits of other fission products as many species are less soluble in UO2 than UO2+x. © 2013, Elsevier B.V.
- ItemGradual structural evolution from pyrochlore to defect-fluorite in Y2Sn2-xZrxO7: average vs local structure(ACS Publications, 2013-11-25) Zhang, ZM; Middleburgh, SC; de los Reyes, M; Lumpkin, GR; Kennedy, BJ; Blanchard, PER; Reynolds, EM; Jang, LYWe have studied the long-range average and local structures in Y2Sn2−xZrxO7 (x = 0−2.0) using synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, respectively, and by theoretical methods. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defectfluorite at x ∼ 1.0−1.2, X-ray absorption near-edge structure (XANES) results at the Zr L3- and Y L2-edges reveal a gradual structural evolution across the whole compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary, as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defectfluorite region. The Zr and Y L-edge spectra are very sensitive to changes in the local structure; such sensitivity enables us to reveal the progressive nature of the phase transition. Experimental results are supported by ab initio atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures are predicted, and the trends agree well with the experimental XANES results. The calculations predict that the coordination of cations in the Y2Zr2O7 defect-fluorite (normally considered to be 7 for all cations) varies depending on the species with the average coordination of Y and Zr being 7.2 and 6.8, respectively. © 2013, American Chemical Society.
- ItemHydrogen accommodation in α-iron and nickel(Elsevier, 2014-02-25) Fullarton, ML; Voskoboinikov, RE; Middleburgh, SCAb initio calculations have been used to study the effects of hydrogen on vacancy concentrations in αα-Fe and Ni. The presence of H interstitials aided vacancy formation in both metals but via two different mechanisms. In αα-Fe, trapping of H by a vacancy is favourable. However binding of further hydrogen atoms was not predicted to proceed. The thermal equilibrium concentration of H interstitials in comparison to vacancies in αα-Fe is many orders of magnitude higher over a wide temperature range. Excessive H interstitials in solid solution facilitate vacancy formation, lowering the required energy by 0.79 eV (down to 1.41 eV). In Ni, a single H interstitial is not expected to have an impact on the vacancy population, increasing the vacancy formation energy by 0.32 eV. Two bound H interstitials however are predicted to decrease the vacancy formation energy by 0.52 eV with expected arrangement along a 〈〈1 1 1〉〉 direction around the vacancy. The calculations show reasonable agreement with experimental data when comparing crystal lattice contractions of the pure metals and predicted melting temperature of the Me–H alloys. © 2013, Elsevier B.V.
- ItemHydrogen induced vacancy formation in tungsten(Elsevier, 2014-05) Middleburgh, SC; Voskoboinikov, RE; Guenette, MC; Riley, DPAtomic scale modelling methods have been used to study the change in vacancy population when H is introduced into the W bulk matrix. Vacancy defects are predicted to dominate in pure W, and the vacancy concentration is expected to be very small. A mechanism whereby H solutes facilitate vacancy formation has been outlined and a single H interstitial is predicted to reduce the vacancy formation energy from 2.95 eV to 2.23 eV. Clustering of H interstitials in W is predicted to be unfavourable without a W vacancy. H has also been shown to affect the vacancy binding characteristics in W, changing the behaviour from a repelling interaction in pure W, retarding the formation of small vacancy clusters, to an attractive interaction when vacancy-H clusters are considered. The changes in defect behaviour predicted, will have observable implications to operational properties and the application of W in fusion reactor components. © 2014, Elsevier B.V.
- ItemThe incorporation of plutonium in lanthanum zirconate pyrochlore(Elsevier, 2013-11-01) Gregg, DJ; Zhang, YJ; Middleburgh, SC; Conradson, SD; Triani, G; Lumpkin, GR; Vance, ERThe incorporation of plutonium (Pu) within lanthanum zirconate pyrochlore was investigated using air, argon, and N2–3.5%H2 sintering atmospheres together with Ca2+ and Sr2+ incorporation for charge compensation. The samples have been characterised in the first instance by X-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS). The results show Pu can be exchanged for La3+ on the A-site with and without charge compensation and for Zr4+ on the B-site. DRS measurements were made over the wavenumber range of 4000–19,000 cm−1 and the Pu in all air- and argon-sintered samples was found to be present as Pu4+ while that in samples sintered in N2–3.5%H2 was present as Pu3+. The Pu valence was confirmed for three of the samples using X-ray near-edge absorption spectroscopy (XANES). Pu valences >4+ were not observed in any of the samples. © 2013, Elsevier B.V.
- ItemInfluence of plasma impurities on the effective performance of fusion relevant materials(Australian Institute of Physics, 2014-02-04) Riley, RP; Guenette, MC; Deslandes, A; Middleburgh, SC; Lumpkin, GR; Thomsen, L; Corr, CSThe development of a sustainable source of power derived from fusion energy is presently constrained by the limited number of materials capable of operating under such extreme conditions. Plasma facing components within magnetically confined fusion reactors must withstand extremes of temperature and loads, while maintaining a high tolerance to radiation damage from energetic particles or neutrons. More specifically, factors of sputtering yield, thermal conduction, electrical conduction and retention of fuel can all degrade the performance of the reactor and hence detrimentally lower the efficiency. In aiming to improve our understanding of materials capable of operating within the fusion environment, it is essential to establish how present generation materials become degraded. Use of ion beam accelerators and linear plasma devices simulate the respective impact of energetic neutron damage (14.1 MeV) and plasma erosion (H+, D+, He+) within a magnetically confined fusion environment. Methods of characterising changes in the local structure and chemistry of surface and near surface regions of fusion relevant materials quantify material degradation resulting from the uptake of plasma impurities. While complementary density functional theory (DFT) simulations have identified possible mechanisms for degradation of material performance. An overview of material evaluation methods will also be presented.
- ItemIon beam irradiation of ABO4 compounds with the fergusonite, monazite, scheelite, and zircon structures(https://doi.org/10.1111/jace.17288, 2020-06-04) de los Reyes, M; Aughterson, RD; Gregg, DJ; Middleburgh, SC; Zaluzec, NJ; Huai, P; Ren, CL; Lumpkin, GRThe effects of irradiation on CaWO4, SrWO4, BaWO4, YVO4, LaVO4, YNbO4, and LaNbO4 were investigated on thin crystals using 1.0 MeV Kr ions at 50‐1000 K. All of the ABO4 compounds can be amorphized with calculated damage cross sections (σa = 1/Fc0) in the range of ~0.30‐1.09 × 10‐14 cm2 ion−1 at zero Kelvin. The analysis of fluence‐temperature data returned critical temperatures for amorphization (Tc) of 311 ± 1, 358 ± 90, 325 ± 19, 415 ± 17, 541 ± 6, 636 ± 26, and 1012 ± 1 K, respectively, for the compounds listed above. Compared with previous in situ irradiation of ABO4 orthophosphate samples using 0.8 MeV Kr ions, the Tc values of LaVO4 and YVO4 are higher than those of LaPO4 and YPO4 by 82 K and 124 K, respectively. The Tc values of the three scheelite structures, CaWO4, SrWO4, and BaWO4, indicate that they are the most radiation tolerant compounds under these conditions. The A‐B cation anti‐site energies, EfAB, determined by DFT range from 2.48 to 10.58 eV and are highly correlated with the A‐B cation ionic radius ratio, rA/rB, but are not correlated with Tc across the different structure types, suggesting that the formation and migration energies of Frenkel defects play a more important role in damage recovery in these compounds. We also discuss the role of cation and anion charge/iconicity as determined by DFT. ABO4 compounds with the zircon structure and B = P or V have a distinct advantage over those with B = Si as the damaged regions do not appear to be significantly affected by polymerization of (PO4)3− or (VO4)3− groups which might stabilize the amorphous fraction and ultimately lead to phase separation as observed in zircon (ZrSiO4). © 1999-2020 John Wiley & Sons, Inc.
- ItemMolecular dynamics simulations of thermal conductivity of UO2, PuCrO3 and PuAlO3(Australian Institute of Physics, 2014-02-04) Qin, MJ; Kuo, EY; Robinson, M; Marks, NA; Lumpkin, GR; Middleburgh, SCThe thermal conductivities of the PuCrO3 and PuAlO3 precipitates in UO2 fuel have been calculated using non-equilibrium molecular dynamics simulations. The PuCrO3 phase showed a markedly lower thermal conductivity than UO2, which will impact the microstructure, fission product distribution and gas release properties of UO2-based fuels. The PuAlO3, in both its orthorhombic and rhombohedral structures, showed greater thermal conductivity in comparison to PuCrO3, lower than UO2 at low temperatures but higher at elevated temperatures. Additions of Al with Cr to doped fuels is therefore likely to have a beneficial impact on the thermal conductivity of the fuel as opposed to solely doping with Cr.
- ItemNovel chemical synthesis and characterization of CeTi2O6 brannerite(ACS Publications, 2014-06-13) Kong, L; Gregg, DJ; Karatchevtseva, I; Zhang, ZM; Blackford, MG; Middleburgh, SC; Lumpkin, GR; Triani, GCerium titanate CeTi2O6 was prepared by a new soft chemistry route in aqueous solution. A suite of characterization techniques, including X-ray diffraction, thermal analysis, vibrational spectroscopy, and scanning and transmission electron spectroscopy, were employed to investigate the brannerite structure formation and its bulk properties. The synthesized powder formed the brannerite crystal structure upon calcination at temperatures as low as 800 °C. Samples sintered at 1350 °C possess a high level of crystallinity. X-ray absorption near-edge structure results indicate the presence of six-coordinated Ce4+ in the brannerite samples. © 2014, American Chemical Society.
- ItemPartition of soluble fission products between the grey phase, ZrO2 and uranium dioxide(Elsevier, 2013-07-01) Cooper, MWD; Middleburgh, SC; Grimes, RWThe energies to remove fission products from UO2 or UO2+x and incorporate them into BaZrO3, SrZrO3 (grey phase constituent phases) and ZrO2 have been calculated using atomistic scale simulation. These energies provide the thermodynamic drive for partition of soluble fission products between UO2 or UO2+x and these secondary oxide constituents of the fuel system. Tetravalent cation partition into BaZrO3, SrZrO3 and ZrO2 was only preferable for species with smaller radii than Zr4+, regardless of uranium dioxide stoichiometry. Under stoichiometric conditions both the larger and the smaller trivalent cations were found to segregate to BaZrO3 but only the smaller fuel additive elements Cr3+ and Fe3+ segregate to SrZrO3. Partition from UO2+x was always unfavourable for trivalent cations. Additions of excess Cr3+ (as a fuel additive) are predicted make the partition into BaZrO3 and SrZrO3 more favourable from UO2 for the larger trivalent cations. Trivalent fission products with radii smaller than or equal to that of Sm3+ were identified to segregate into ZrO2 only from UO2. No segregation to SrO or BaO is predicted. © 2013, Elsevier B.V.
- ItemPeroxide defect formation in zirconate perovskites(Royal Society of Chemistry, 2014) Middleburgh, SC; Karatchevtseva, I; Kennedy, BJ; Burr, PA; Zhang, ZM; Reynolds, EM; Grimes, RW; Lumpkin, GRAtomic scale modelling suggests that excess oxygen can be accommodated in the group II perovskite zirconates by the formation of peroxide ion defects. This is unprecedented given the lack of charge compensating defects required for standard excess oxygen accommodation. The solution energy of O2 was predicted to be close to zero for BaZrO3, accommodating the peroxide ion defect more easily than in SrZrO3 or CaZrO3. This was experimentally examined by exposing SrZrO3 and BaZrO3 to hydrogen peroxide solution and then carrying out Raman spectroscopy measurements to look for a peak indicative of peroxide ions. A peak was observed at ∼1000 cm−1 in both compositions, suggesting the theoretically predicted peroxide ion is present. © 2014, The Royal Society of Chemistry.
- ItemPreferential formation of al self-interstitial defects in gamma-TiAl under irradiation(Elsevier, 2013-01-01) Voskoboinikov, RE; Lumpkin, GR; Middleburgh, SCEmpirical dynamic calculations were used to observe a distinct increase in aluminium interstitial defects compared to titanium interstitial species remaining after a displacement cascade (even though Frenkel formation energies for both species were found to be similar). Thermodynamic data from static ab-initio models support this interesting result. Calculations were then used to determine whether these interstitials are free to move and therefore have the possibility of migrating to a surface where a passive Al2O3 layer can be formed. © 2012, Elsevier Ltd.