Browsing by Author "Qin, MJ"

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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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    Construction and implementation of a TDCR system at ANSTO
    (Elsevier, 2007-09-03) Qin, MJ; Mo, L; Alexiev, D; Cassette, P
    A triple-to-double coincidence ratio (TDCR) liquid scintillation counting system has been recently constructed at Australian Nuclear Science and Technology Organization (ANSTO). A description of the system and measured activities for sources such as 3H, 14C, and 241Am are presented. © 2008, Elsevier Ltd.
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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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    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.
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    Molecular 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, SC
    The 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.
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    Raman spectroscopic study of natural and synthetic brannerite
    (Elsevier, 2013-06-23) Zhang, YJ; Karatchevtseva, I; Qin, MJ; Middleburgh, SC; Lumpkin, GR
    Raman spectra of a well-characterized natural brannerite and some synthetic brannerite samples have been collected and analysed. For the first time, all 12 theoretical Raman polyhedraof CeTi2O6 with brannerite structure have been positively identified by using quantum chemical calculations based on density functional theory (DFT) and assigned to the spectra. For synthetic samples with Y or Ca substitution on the U site and Fe substitution on the Ti site, Y–O, Fe–O and U–O vibrational modes are tentatively assigned. The U–O bond lengths for the uranyl (UO2)2+ groups, have been calculated by using the measured wavenumbers of ν1 (UO2)2+ symmetric stretching vibrations and compared with the published U–O bond lengths of a natural brannerite. © 2013 Elsevier B.V.
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    Technetium and ruthenium incorporation into rutile TiO2
    (Elsevier, 2013-10-01) Kuo, EY; Qin, MJ; Thorogood, GJ; Whittle, KR; Lumpkin, GR; Middleburgh, SC
    The incorporation of TcO2 into the rutile phase of TiO2 has been theoretically examined to understand the potential of the system for use as a wasteform. The effects of defect clustering and the transmutation of Tc into Ru were considered. Results suggest that as a single defective species, Tc has a temperature-dependent, moderate solubility into rutile TiO2, which increases when clustering is considered. The {2TcTi} cluster that preferentially forms was found to have a Tc–Tc bond length similar to the cation distance in TcO2. The transmutation of Tc to Ru has two effects: first, the preferred binary cluster morphology changes from that of first nearest neighbour for {2TcTi} to that of second nearest neighbour for {2RuTi}. Second, the defect solubility is lower for single RuTi defects and Ru-containing defect clusters. This will likely result in the formation of a secondary phase as transmutation proceeds, if Tc is added to rutile TiO2 to its solution limit. The varying solubility of RuO2 in TiO2 with temperature was compared with previously published experimental data with encouraging results. The use of two different DFT codes (one plane-wave and one local-orbital) was used to find the correct magnetic ordering in the defective lattices, which increased the confidence in the methods and therefore the results. © 2013, Elsevier B.V.
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    Theoretical and experimental Raman spectroscopic studies of synthetic thorutite (ThTi2O6)
    (Elsevier, 2014-03-01) Zhang, YJ; Čejka, J; Karatchevtseva, I; Qin, MJ; Kong, L; Short, KT; Lumpkin, GR; Middleburgh, SC
    Raman spectra of synthetic non-substituted and U/Np/Pu- substituted thorutite (ThTi2O6) samples were collected. Factor group analysis was used to analyse the possible vibration modes. Theoretical simulations based on density functional theory (DFT) were performed on both CeTi2O6 and ThTi2O6. All possible vibrational modes for thorutite have been identified and assigned to the measured spectra. The presence and the effect of higher valence of U in thorutite has been discussed and the U–O bond lengths have been calculated by using the measured wavenumbers of ν1 (UO2)2+ symmetric stretching vibrations. The presence of U6+ in uranyl form can be inferred from the wavenumbers of U–O vibrations. Corresponding Np–O and Pu–O vibrations were not observed in the Raman spectra. Thus the presence of Np6+ and Pu6+ in the studied samples was not established. © 2016 Elsevier B.V.
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    Thermal conductivity and energetic recoils in UO2 using a many-body potential model
    (IOP Science, 2014-11-14) Qin, MJ; Cooper, MWD; Kuo, EY; Rushton, MJD; Grimes, RW; Lumpkin, GR; Middleburgh, SC
    Classical molecular dynamics simulations have been performed on uranium dioxide (UO2) employing a recently developed many-body potential model. Thermal conductivities are computed for a defect free UO2 lattice and a radiation-damaged, defect containing lattice at 300 K, 1000K and 1500 K. Defects significantly degrade the thermal conductivity of UO2 as does the presence of amorphous UO2, which has a largely temperature independent thermal conductivity of ∼1.4Wm−1 K−1. The model yields a pre-melting superionic transition temperature at 2600 K, very close to the experimental value and the mechanical melting temperature of 3600 K, slightly lower than those generated with other empirical potentials. The average threshold displacement energy was calculated to be 37 eV. Although the spatial extent of a 1 keV U cascade is very similar to those generated with other empirical potentials and the number of Frenkel pairs generated is close to that from the Basak potential, the vacancy and interstitial cluster distribution is different. © 2014, IOP Publishing Ltd.
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    Thermal conductivity variation in uranium dioxide with gadolinia additions
    (Elsevier, 2020-11) Qin, MJ; Middleburgh, SC; Cooper, MWD; Rushton, MJD; Puide, M; Kuo, EY; Grimes, RW; Lumpkin, GR
    By combining experimental observations on Gd doped fuel with a theoretical understanding, the variation in thermal conductivity with Gd concentration and accommodation mechanism has been modelled. Four types of Gd accommodation mechanisms have been studied. In UO2−x, isolated substitutional Gd3+ ions are compensated by oxygen vacancies and {2Gd'u:V"o}x defect clusters. In UO2, isolated substitutional Gd3+ ions are compensated by U5+ ions and {Gd'u:U'u}x defect clusters. The results indicate that defect clusters can be considered as less effective phonon scatterers and therefore result in less thermal conductivity degradation. The thermal conductivity predicted for UO2 with {Gd/u:U'u}x defect clusters is in good agreement with experimental data for UO2 with 5 wt% Gd2O3. This supports the previous theoretical results that Gd is accommodated through defect clusters {Gd'u:U'u}x in UO2 in the presence of excess oxygen. © 2020 Elsevier B.V.
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    Transmutation of ABO4 compounds incorporating technetium-99 and caesium-137
    (IOP Publishing, 2017-01-13) Kuo, EY; Qin, MJ; Thorogood, GJ; Huai, P; Ren, CL; Lumpkin, GR; Middleburgh, SC
    The stability of a series of ABO4ABO4 minerals incorporating radioactive 99Tc99Tc during the latter’s β-decay to 99Ru99Ru was investigated theoretically using density functional theory (DFT) computations. The compounds investigated were KTcO4KTcO4, RbTcO4RbTcO4 and CsTcO4CsTcO4. The stability of the latter, CsTcO4CsTcO4, during transmutation, when the caesium consists of the radioactive isotope 137Cs137Cs, was also investigated. For each of the compounds, two similar possible crystal structure types—scheelite and pseudoscheelite—were considered. As the 99Tc99Tc decays, or the 137Cs137Cs decays to 137Ba137Ba, reaction enthalpies were calculated for possible decompositions or precipitations of the transmuting compounds. All the possible decompositions or precipitations investigated had positive reaction enthalpies, suggesting that the transmuting compounds are all chemically stable. Volume and lattice parameter changes, however, suggest that KTcO4KTcO4 would also be structurally stable during transmutation to KRuO4KRuO4, but that CsTcO4CsTcO4 would not be structurally stable during its transmutation to BaRuO4BaRuO4. © 2017 IOP Publishing Ltd.