Browsing by Author "Robinson, M"

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    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.
  • Thumbnail Image
    Item
    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.
  • No Thumbnail Available
    Item
    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.