Browsing by Author "Harvey, EJ"

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    Ion irradiation of ternary pyrochlore oxides
    (American Chemical Society, 2009-07-14) Lumpkin, GR; Smith, KL; Blackford, MG; Whittle, KR; Harvey, EJ; Redfern, SAT; Zaluzec, NJ
    Polycrystalline synthetic samples of Y2Ti2−xSnxO7 with x = 0.4, 0.8, 1.2, and 1.6, together with Nd2Zr2O7, Nd2Zr1.2Ti0.8O7, and La1.6Y0.4Hf2O7, were irradiated in situ in the intermediate voltage electron microscope (IVEM)-Tandem Facility at Argonne National Laboratory using 1.0 MeV Kr ions at temperatures of 50 to 650 K. Determination of the critical amorphization fluence (Fc) as a function of temperature has revealed a dramatic increase in radiation tolerance with increasing Sn content on the pyrochlore B site. Nonlinear least-squares analysis of the fluence-temperature curves gave critical temperatures (Tc) of 666 ± 4, 335 ± 12, and 251 ± 51 K for the Y2Ti2−xSnxO7 samples with x = 0.4, 0.8, and 1.2, respectively. The sample with x = 1.6 appears to disorder to a defect fluorite structure at a fluence below 1.25 × 1015 ions cm−2 and remains crystalline to 5 × 1015 ions cm−2 at 50 K. Additionally, the critical fluence-temperature response curves were determined for Nd2Zr1.2Ti0.8O7 and La1.6Y0.4Hf2O7, and we obtained Tc values of 685 ± 53 K and 473 ± 52 K, respectively, for these pyrochlores. Nd2Zr2O7 did not become amorphous after a fluence of 2.5 × 1015 ions cm−2 at 50 K, but there is evidence that it may amorphize at a higher fluence, with an estimated Tc of 135 K. The observed Tc results are discussed with respect to the predicted Tc values based upon a previously published empirical model (Lumpkin, G. R.; Pruneda, M.; Rios, S.; Smith, K. L.; Trachenko, K.; Whittle, K. R.; Zaluzec, N. J. J. Solid State Chem. 2007, 180, 1512). In the Y2Ti2−xSnxO7 pyrochlores, Tc appears to be linear with respect to composition, and is linear with respect to rA/rB and x(48f) for all samples investigated herein. © 2009, American Chemical Society
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    Ion irradiation of ternary pyrochlores
    (Materials Research Society, 2008-12-01) Whittle, KR; Smith, KL; Blackford, MG; Redfern, SAT; Harvey, EJ; Zaluzec, NJ; Lumpkin, GR
    Synthetic pyrochlore samples Y2Ti2-xSnxO7 (x=0.4, 0.8, 1.2, 1.6), Nd2Zr2O7, Nd2Zr1.2Ti0.8O7, and La1.6Y0.4Hf2O7, were irradiated in-situ using the IVEM-TANDEM microscope facility at the Argonne National Laboratory. The critical temperatures for amorphisation have revealed a dramatic increase in tolerance with increasing Sn content for the Y2Ti2-xSnxO7 series. This change has also found to be linear with increasing Sn content. Nd2Zr1.2Ti0.8O7 and La1.6Y0.4Hf2O7 were both found to amorphise, while Nd2Zr2O7 was found to be stable to high doses (2.5x10^15 ions cm-2). The observed results are presented with respect to previously published results for irradiation stability predictions and structural disorder. © Materials Research Society 2009
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    Radiation tolerance of A2Ti2O7 compounds at the cubic-monoclinic boundary
    (Australian Nuclear Association, 2006-10-15) Lumpkin, GR; Harvey, EJ; Smith, KL; Blackford, MG; Zaluzec, NJ
    Ceramic waste forms provide attractive alternatives to the direct disposal of spent fuel or the immobilisation of high-level radioactive waste in borosilicate glass. They are particularly suited for the disposal of actinide wastes (e.g., from partitioning strategies, or for excess Pu from defence purposes) and furthermore they exhibit very low dissolution rates in aqueous fluids, making them attractive candidates for certain repository scenarios (e.g., deep disposal). For general background information on these materials, including studies of the crystal chemistry, aqueous durability, and the behaviour of natural analogues in geological systems, readers are referred to references [1-4]. Over the design lifetime of ceramic waste forms, the actinide elements will undergo alpha decay, resulting in damage to the crystalline structure primarily due to alpha recoil collision cascades. In certain materials, this will lead to a crystalline-amorphous transformation accompanied by volume expansion and reduced chemical durability. The performance in aqueous fluids may be compromised by cracking, increased surface area, and decreased thermodynamic stability of the amorphous phase. Consequently, the radiation damage effects have been of particular interest in ceramic waste forms. Detailed reviews of radiation damage effects can be found in references [5-7]. Some aspects of the alpha decay process have been simulated by irradiation with heavy ions under controlled experimental conditions. In this study, we conducted in situ ion irradiation experiments using the IVEM-Tandem Facility at Argonne National Laboratory to determine the radiation response of Gd2Ti2O7 pyrochlore and two monoclinic, layered perovskite-type phases, Nd2Ti2O7 and La2Ti2O7. For each compound, the critical amorphization dose Dc was determined as a function of temperature and used to establish Tc , the critical temperature, above which the compound remains crystalline. Together with previous data for the A2Ti2O7 compounds, our results show a clear reversal in the trend of Tc versus the cation-anion radius ratio rM/rX. Our experimental results are discussed in the general context of the potential factors that control the susceptibility of a given compound to amorphisation, which include aspects of the structure, bonding, and disorder energy. For the A2Ti2O7 compounds we also show that the critical temperature correlates with the electronic structure (e.g., 4f occupancy) of lanthanide cations. This appears to be a unique result in the world of ion irradiation studies, but the story is complicated by the phase transition from pyrochlore to the layered perovskite structure in this system. Our ultimate goal here is to illustrate the need for a detailed understanding of the physical processes of radiation damage and the potential for predictive modelling of waste form performance.
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    Radiation tolerance of A2Ti2O7 materials - a question of bonding?
    (Cambridge University Press/Springer Nature, 2007) Whittle, KL; Lumpkin, GR; Smith, KL; Blackford, MG; Harvey, EJ; Zaluzec, NJ
    The resistance of Ln2Ti2O7 (Ln=lanthanide) compounds to radiation damage is an important topic in the understanding and development of new materials by which radioactive nuclear waste can safely be immobilised. A model has been developed, from previously published density functional theory and molecular orbital theory simulations of the band structure for Ln2Ti2O7 materials. This model provides a chemical interpretation of radiation stability. © 2007 Materials Research Society