Browsing by Author "Marks, NA"

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    Atomistic simulation of cation ordering and radiation damage in Sr1-3x/2LaxTiO3 defect perovskites
    (Australian Institute of Physics, 2005-01-31) Thomas, BS; Marks, NA; Begg, BD; Corrales, LR; Devanathan, RL
    Sr-1.3x/2LaxTiO3 perovskites are known to contain charge-compensating cation vacancies, which display one-dimensional ordering at high La concentrations. Recently, the radiation resistance of these perovskites has been measured, revealing an anomalously high radiation resistance at around x = 0.2. We use atomistic computer simulation techniques to study short-range cation and vacancy ordering as a function of La concentration and thermal history. Long-range electrostatic effects dominate the interactions, and ordering in one- and two-dimensions is observed. We also give preliminary results on the effects of La concentration and ordering on radiation resistance, including both primary damage creation and defect annealing. © 2005 Australian Institute of Physics
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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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    Mechanisms of radiation damage and properties of nuclear materials
    (Materials Research Society, 2009-11-30) Lumpkin, GR; Smith, KL; Whittle, KR; Thomas, BS; Marks, NA
    A wide range of materials are currently under consideration for use in advanced nuclear fuel cycle applications. The effects of radiation on these materials by exposure to external neutron irradiation and internal alpha and beta decay processes may have significant effects on the physical and chemical properties. This is especially true for materials that are subject to hundreds of displacements per atom during their service life. In this paper, we explore some of the radiation damage mechanisms prevalent in oxide based materials, including mathematical models and other concepts of amorphization (e.g., percolation), the role of defects on picosecond time scales, and longer term effects such as diffusion and recrystallization. As radiation "tolerance" or the ability of a material to maintain crystallinity under intense irradiation is a key issue for many fuel cycle applications, we will briefly review and comment on some of the underlying factors that have been identified as important in driving the short-term damage recovery. These include aspects of the structure (e.g., connectivity, polyhedral distortion), bonding, energetics of defect formation and migration, and melting point and similar criteria. The primary materials of interest here are those under development as special purpose nuclear waste forms, novel materials for separations, inert matrix fuels, and transmutation targets. In this context, we will illustrate the behavior of simple oxides and several more complex oxides such as perovskite, multicomponent fluorite systems, and related derivative structures (e.g., pyrochlore and zirconolite). The damage mechanisms in these materials are briefly compared with those of intermetallic and metallic materials.
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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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    Radiation damage response of ceramics in extreme environments
    (Sociedad Nuclear Mexicana (Mexican Nuclear Society), 2010-10-24) Lumpkin, GR; Smith, KL; Whittle, KR; Thomas, BS; Marks, NA
    Oxide-based and inter-metallic compounds have great potential as new materials for clean and renewable energy production. Many of these materials, especially those designed for operation in Generation IV fission reactors or in fusion reactors, must exhibit robust performance under extreme conditions of temperature, irradiation, and chemical attack. Others, such as nuclear waste forms, may be required to retain radioactive elements for long periods of time in geological repositories. The mechanisms of radiation damage production and recovery in these materials may vary considerably as a function of the damage source, e.g., energetic neutrons in reactor systems versus alpha decay in nuclear waste forms. Furthermore, the kinetics of damage recovery are complicated by multiply activated processes and in certain cases, longer-term diffusion may modify the structural state left by irradiation in the short term. Here, we review some basic concepts regarding the mechanisms of radiation damage in selected ceramic materials, including mathematical models, fluence-temperature relationships, and predictive methodologies. A major consideration for materials performance is the ability of a given compound to resist amorphization. Historically, there are a number of general criteria for radiation resistance, including those involving melting point (thermodynamics), structural freedom, bonding, and energetics of defect formation. These are discussed using specific examples.
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    Structural dependence of threshold displacement energies in rutile, anatase and brookite TiO2
    (Elsevier, 2014-09-15) Robinson, M; Marks, NA; Lumpkin, GR
    Systematic molecular dynamics simulations of low energy cascades have been performed to examine how threshold displacement events are effected by changes in crystal structure. Exploiting the structural proximity of the rutile, anatase and brookite polymorphs of TiO2, a quantitative examination of defect production has been carried out including detailed defect analysis and the determination of values of the threshold displacement energy (Ed). Across all polymorphs comparable values of Ed are reported for oxygen at around 20 eV, with the value for Ti in rutile (73 ± 2 eV) significantly higher than that in brookite (34 ± 1 eV) and anatase (39 ± 1 eV). Quantifying defect formation probability as a function of Primary Knock-on Atom (PKA) energy, simulations in rutile indicate a consistent reduction in defect formation at energies higher than Ed relative to anatase and brookite. Defect cluster analysis reveals a significant proportion of di-Frenkel pairs in anatase at Ti PKA energies around Ed. These clusters, which are stabilised by the localisation of two Frenkel pairs, are associated with a recombination barrier of approximately 0.19 eV. As such, annihilation is likely under typical experimental conditions which suggests an expected increase in the measured Ti value of Ed. Identical O defect populations produced at the threshold by the O PKA in both rutile and anatase explain the comparable values of Ed. At higher O PKA energies, the commencement of defect production on both sublattices in anatase is observed in contrast to the confinement of defects to the O sublattice in rutile. The overall trends reported are consistent with in-situ irradiation experiments and thermal spike simulations, suggesting the contrasting radiation response of the polymorphs of TiO2 is apparent during the initial stages of defect production. © 2014, Elsevier B.V.