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

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

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

Mobilité atomique dans les composés intermétalliques TiAl, Ni3Al et Ti2AlNb

Les composés intermétalliques ont des potentialités importantes comme matériaux de structure aux hautes températures. Un certain nombre de leurs caractéristiques (stabilité structurale, résistance au fluage) sont gouvernées par un transport de matière. Afin de prévoir l'évolution à long terme de ces matériaux, il est donc important de caractériser leur mobilité atomique et les coefficients de diffusion. Des résultats concernant les composés intermétalliques TiAl, Ni3Al et Ti2AlNb, qui sont les plus prometteurs pour de futures applications, sont présentés ici

DYVACS (DYnamic VACuum Simulation) code: gas density profiles for dynamic conditions in particle accelerators - simulations for the LHC and the FCCee

International audienceThe computation of residual gas density profiles in particle accelerators is an essential task to optimize beam pipes and vacuum system design. In a hadron collider such as the LHC, the beam induces dynamic effects due to ion, electron and photon-stimulated gas desorption. The well-known VASCO* code developed at CERN in 2004 was already used to estimate pressure profiles in steady state conditions. Nevertheless, some phenomena are not taken into account such as the ionization of residual gas by the electron clouds and the evolution of the electronic density related to the electron cloud build up. Therefore, we proposed an upgrade of this code by introducing electron cloud maps** to estimate the electron density and the ionization of gas by electrons to calculate pressure evolution in dynamic conditions. Results obtained with DYVACS reproduces with a good accuracy the experimental dynamic pressure recorded in the VPS beam pipes sector*** of the LHC from the proton beam injection to the stable beam period, for several materials of vacuum chamber. Additionally, DYVACS was used as a predictive tool to compute the pressure evolution in the beam pipes for the Future Circular Collider e- e+

Swift heavy ion irradiation of pyrochlore oxides: Electronic energy loss threshold for latent track formation

International audiencePyrochlore pellets with the Gd(2)(Ti(2-x)Zr(x))O(7) stoichiometry (x = 0, 1 and 2) were irradiated with swift heavy ions in order to investigate the effects of electronic excitation and to determine the electronic stopping power threshold for track formation. XRD results showed that the electronic excitation induced by 870 MeV Xe and 780 MeV Kr ions leads to: (i) a crystalline-amorphous transition for Gd(2)Ti(2)O(7) and Gd(2)TiZrO(7), (ii) a phase transition towards an anion-deficient fluorite structure (order-disorder transition) for Gd(2)Zr(2)O(7). Thus, zirconate pyrochlores present a better radiation resistance under swift heavy ion irradiation than titanate pyrochlores. Moreover results underline the existence of an electronic stopping power threshold around 13-14 keV/nm, below which phase transformations do not occur. (C) 2010 Elsevier B.V. All rights reserved

Radiation tolerance of fluorite-structured oxides subjected to swift heavy ion irradiation

Expérience GANIL : Both fluorite-type crystals were irradiated at room temperature with 944-MeV Pb53+ ionsInternational audienceFluorite-structured materials are known to exhibit an excellent structural stability under irradiation. The radiation stability of urania and yttria-stabilised cubic zirconia single crystals submitted to intense electronic excitations induced by 944-MeV Pb53+ ions was investigated. Various analytical tools (TEM, AFM, RBS/C, XRD) were employed to examine the modifications induced at the surface and in the crystal bulk. At low fluence irradiation leads to the formation of localised ion tracks whose centre is hollowed in the surface region over a depth of 100 nm and to the formation of nanometer-sized hillocks. Both features are interpreted as resulting from an ejection of matter in the wake of the projectile. Track overlapping at high fluence results in the formation of micrometer-sized domains ( 50 nm) in the crystal bulk characterised by a slight disorientation ( 0.2 ) with respect to the main crystallographic orientation of the crystal

Dynamic pressure evolution during the LHC operation

International audienceFor the accelerator community and the vacuum scientists, the understanding of the beam interactions with a vacuum chamber is fundamental to provide solutions to mitigate pressure rises induced by electron, photon, and ion molecular desorption. Moreover, beam instabilities induced by ion and electron clouds must be investigated in order to find solutions to reduce them. This study presents in situ measurements of pressure evolutions and electrical currents performed during the LHC RUN II (2018). The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. These charged particles are accelerated away from the beam and reach the vacuum chamber wall, inducing, among other phenomena, stimulated desorption and secondary electron emission. Moreover, protons emit synchrotron radiations that also induce photodesorption and photoelectron production. Experimental measurements of the electrical signals recorded on copper electrodes were compared to calculations considering both the secondary electron yield of copper and the electron energy distribution. All measurements performed with the Vacuum Pilot Sector in the LHC ring show the importance of taking into account a large variety of phenomena in order to understand the pressure evolution in the LHC. Results show that the multipacting threshold, corresponding to an increase in the electron cloud density, strongly depends on the number of protons per bunch. Finally, the ion current was measured with a biased electrode lower than -500 V. It was much higher than expected, pointing its origin not only from simple beam-gas ionization but also from the ionization of the residual gas by the electron cloud

Radiation stability of fluorite-type nuclear oxides

International audienceOxides with the fluorite-type structure are radiation tolerant materials. They are widely used or envisaged in hostile nuclear environments, such as nuclear fuels or inert transmutation matrices for actinide burning. Study of the radiation stability of this class of solids in various radiative fields is of major importance. Two issues which may affect the stability of materials are considered in this work: the production of radiation damage (ballistic contribution); the modification of the matrix composition by doping (chemical contribution). Both contributions may drastically affect the solid stability. Urania and zirconia single crystals were chosen as fluorite-type canonical systems. They were implanted with low-energy inert gases (He or Xe). The damage in-growth, due to both ballistic and chemical contributions, was investigated by in situ RBS/C experiments in the channelling mode and TEM. Two main steps in the disordering kinetics were observed for both inert gases. Relevant key parameters were found to be: the number of displaced lattice atoms created by the slowing-down of energetic ions during the implantation process,, the concentration of noble gas atoms in the solid which cause the formation of large stress fields surrounding gas aggregates. (C) 2008 Elsevier B.V. All rights reserved

Phase transformations induced by heavy ion irradiation in Gd2_2O3_3 : Comparison between ballistic and electronic excitation regimes

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