Experimental and ab initio infrared study of χ-, κ- and α-aluminas formed from gibbsite

χ-, κ- and α-alumina phases formed by dehydration of micro-grained gibbsite between 773 and 1573 K are studied using infrared spectroscopy (IR). The structural transitions evidenced by X-ray diffraction (XRD) were interpreted by comparing IR measurements with ab initio simulations (except for the χ form whose complexity does not allow a reliable simulation). For each phase, IR spectrum presents specific bands corresponding to transverse optical (TO) modes of Al-O stretching and bending under 900 cm-1. The very complex χ phase, obtained at 773 K, provides a distinctive XRD pattern in contrast with the IR absorbance appearing as a broad structure extending between 200 and 900 cm-1 resembling the equivalent spectra for γ-alumina phase. κ-alumina is forming at 1173 K and its rich IR spectrum is in good qualitative agreement with ab initio simulations. This complexity reflects the large number of atoms in the κ-alumina unit cell and the wide range of internuclear distances as well as the various coordinances of both Al and O atoms. Ab initio simulations suggest that this form of transition alumina demonstrates a strong departure from the simple pattern observed for other transition alumina. At 1573 K, the stable α-Αl2Ο3 develops. Its IR spectra extends in a narrower energy range as compared to transition alumina and presents characteristics features similar to model α-Αl2Ο3{dot operator} Ab initio calculations show again a very good general agreement with the observed IR spectra for this phase. In addition, for both κ- and α-Αl2Ο3, extra modes, measured at high energy (above 790 cm-1 for κ and above 650 cm-1 for α), can originate from either remnant χ-alumina or from surface mode

Order parameter and connectivity topology analysis of crystalline ceramics for nuclear waste immobilization

We apply bond order and topological methods to the problem of analysing the results of radiation damage cascade simulations in ceramics. Both modified Steinhardt local order and connectivity topology analysis techniques provide results that are both translationally and rotationally invariant and which do not rely on a particular choice of a reference structure. We illustrate the methods with new analyses of molecular dynamics simulations of single cascades in the pyrochlores Gd2Ti2O7 and Gd2Zr2O7 similar to those reported previously (Todorov et al 2006 J. Phys.: Condens. Matter 18 2217). Results from the Steinhardt and topology analyses are consistent, while often providing complementary information, since the Steinhardt parameters are sensitive to changes in angular arrangement even when the overall topological connectivity is fixed. During the highly non-equilibrium conditions at the start of the cascade, both techniques reveal significant localized transient structural changes and variation in the cation connectivity. After a few picoseconds, the connectivity is largely fixed, while the order parameters continue to change. In the zirconate there is a shift to the anion disordered system while in the titanate there is substantial reversion and healing back to the parent pyrochlore structure

Fundamental Phenomena and Applications of Swift Heavy Ion Irradiations

Peer reviewe

Phase transformation induced by ion implantation in cubic stabilized zirconia

Cubic yttria-stabilized zirconia possesses a high stability against radiation. No amorphization of this material has been observed, even at high ion fluences leading to the production of a large amount of defects. Nevertheless irradiation with energetic particles may induce microstructural evolutions and phase transformations. In the present paper we demonstrate that a cubic-to-rhombohedral phase transformation occurs in yttria-stabilized zirconia implanted with He ions. This transformation consists in a rhombohedral deformation of the cubic cell along the directions due to residual stresses induced by implantation

PARTENARIAT QUALITE (APPLICATION A UN FOURNISSEUR D'ETUIS EN CARTON)

DIJON-BU Médecine Pharmacie (212312103) / SudocSudocFranceF

DYVACS (DYnamic VACuum Simulation) code: Calculation of gas density profiles in presence of electron cloud

International audienceThe computation of residual gas density evolution in the LHC in the presence of proton beams was performed with a new simulation code called DYVACS (DYnamic VACuum Simulation). It is a modification of the VASCO code in order to take into account dynamic effects such as the electron cloud phenomenon leading to an increase of both the electron- and the ion-induced gas desorption. Results obtained with the DYVACS code is compared to pressure measurements recorded during a typical physics fill in the Vacuum Pilot Sector of the LHC. First results show a good agreement between the calculated pressure and the experimental values

Xenon versus helium behavior in UO2 single crystals: A TEM investigation

The behavior of He and Xe implanted into UO2 single crystals is studied by in situ TEM experiments before and after annealing up to 700 °C. TEM micrographs show that annealing induces the formation of noble-gas bubbles in both cases. However, the size (25 nm for He and 3–5 nm for Xe) and the nucleation temperature (600 °C for He and 400 °C for Xe) of bubbles depend on implanted species. These results are explained by the radiation damage produced by ion implantation (different by a factor of 100 for the two elements) and the diffusion mechanisms involved in each case