Etude de la pérénnité des gels d'altération de verres nucléaires

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Surfactant-assisted lamellar structuration of tunable Co-based hybrid nanosheets

The controlled synthesis of metal organic nanosheets (MONs) still remains a challenge for scientists and is of primary interest in many research areas. In this study, we have developed a simple and reproducible one-pot synthesis method leading to tunable cobalt-based hybrid materials consisting of nanosheets with a well-defined morphology. The synthesis is performed via a solvothermal process assisted by primary amines acting as surfactants. The obtained materials are composed of ultrathin nanosheets with a rectangular morphology (width of about 50 nm and length greater than 100 nm). The different analysis techniques used show that the nanosheets are constituted by the alternation, in their width, of inorganic fringes (cobalt hydroxide) and organic ones (dicarboxylic acid) and that amine molecules stabilize their surface. 3D organisation of the nanosheets can be achieved at a synthesis temperature above 190 °C due to the interaction of alkyl chains of the stabilising amines. A multiparametric study based on SAXS experiments and TEM and PXRD analyses has highlighted the control of structural parameters by variations of some synthesis conditions. The inter-fringe distances are tunable from 1.0 nm to 1.5 nm through the length of the dicarboxylic linker and the distances between nanosheets can be modulated by the length of the alkyl chain of the primary amines. This efficient one-pot process seems to be very promising for the synthesis of tunable multiscale nanostructured MONs and can be implemented in many research areas, opening the way for various applications

Structural and Dynamical Properties of Water Confined in Highly Ordered Mesoporous Silica in the Presence of Electrolytes

International audienc

Stable uranium sols as precursors for the elaboration of nanostructured nc-UO 2 materials

International audienc

Long-term alteration of basaltic glass: Mechanisms and rates

International audienc

Corrosion products formed on MgZr alloy embedded in geopolymer used as conditioning matrix for Nuclear waste—A proposition of interconnected processes

International audienceGeopolymer has been selected as a hydraulic mineral binder for the immobilization of MgZr fuel cladding coming from the dismantling of French Uranium Natural Graphite Gas reactor dedicated to a geological disposal. In this context, the corrosion processes and the nature of the corrosion products formed on MgZr alloy in a geopolymer matrix with and without the corrosion inhibitor NaF have been determined using a multiscale approach combining in situ Grazing Incidence hard X-ray Diffraction, Raman microspectroscopy, Scanning and Transmission Electron Microscopies coupled to Energy Dispersive X-ray Spectroscopy. The composition, the morphology, and the porous texture of the corrosion products were characterized, and the effect of the corrosion inhibitor NaF was evidenced. The results highlighted the formation of Mg(OH)$_{2−x}$F$_x$. In addition, in presence of NaF, NaMgF$_3$ forms leading to a decrease of the thickness and the porosity of the corrosion products layer. Moreover, a precipitation of magnesium silicates within the porosity of the geopolymer was evidenced. Finally, we propose a detailed set of interconnected processes occurring during the MgZr corrosion in the geopolymer

How ion pair formation drives adsorption in the electrical double layer: Molecular dynamics of charged silica-water interfaces in the presence of divalent alkaline earth ions

International audienceThe study by numerical methods of ionic distributions in charged solid-liquid interfaces allows the interpretation of many concepts and phenomena such as the $\zeta$ potential or ion adsorption. Molecular dynamics (MD) can reveal detailed information about electric double layer (EDL), especially the Stern layer, by including electrostatic, van der Waals, and molecular forces. Here, we aim at analyzing the chemical species and the correlations between the ions and the surface including three-body e!ects, one particle belonging to the surface and two to the solution. Correlations are specified on the basis of interionic distance screening. An extensive description is provided from simulations of three alkaline earth metal chlorides (Mg$^{2+}$,Ca$^{2+}$, and Ba$^{2+}$) aqueous solutions at 0.6 mol/L$^{-1}$ in the centers of negatively charged silica nanochannels. The resulting McMillan-Mayer potentials of mean force (PMF) exhibit a decreasing affinity of deprotonated silanol along with the series Mg$^{2+}$ > Ca$^{2+}$ > Ba$^{2+}$, while the formation of bulk M$^{2+}$-Cl$^-$ pairs is in reverse order. A similar trend is obtained for the association constants and the residence times. Over 40% of surface-bound Ba$^{2+}$ ions are correlated with surface-bound Cl$^-$, while the other two cations do not show such trend. When the surface-bound and surface-correlated ions are taken apart, the remaining free ion distributions fit the Poisson-Boltzmann equation well, i.e., the Gouy-Chapman model. This work demonstrates the necessity to account for three-body associations on oxide surface at least for divalent ions

Effect of natural and synthetic iron corrosion products on silicate glass alteration processes

International audienceGlass long term alteration in the context of high-level radioactive waste (HLW) storage is influenced by near-field materials and environmental context. As previous studies have shown, the extent of glass alteration is strongly related to the presence of iron in the system, mainly provided by the steel overpack around surrounding the HLW glass package. A key to understanding what will happen to the glass-borne elements in the geological disposal lies in the relationship between the iron-bearing phases and the glass alteration products formed. In this study, we focus on the influence of the formation conditions (synthetized or in-situ) and the age of different iron corrosion products on SON68 glass alteration. Corrosion products obtained from archaeological iron artifacts are considered here to be true analogues of the corrosion products in a waste disposal system due to the similarities in formation conditions and physical properties. These representative corrosion products (RCP) are used in the experiment along with synthetized iron anoxic corrosion products and pristine metallic iron. The model-cracks of SON68 glass were altered in cell reactors, with one of the different iron-sources inserted in the crack each time. The study was successful in reproducing most of the processes observed in the long term archaeological system. Between the different systems, alteration variations were noted both in nature and intensity, confirming the influence of the iron-source on glass alteration. Results seem to point to a lesser effect of long term iron corrosion products (RCP) on the glass alteration than that of the more recent products (SCP), both in terms of general glass alteration and of iron transport

Mechanical properties of a plasma-modified porous low-k material

International audienceFor 45 nm and beyond microelectronics technology nodes, the integration of porous low dielectric constant (low-k) materials is now required to reach integrated dielectric constant values lower than 2.7. However, porous low-k materials have lower mechanical strength in comparison with traditional dense materials and are also affected by chemical diffusion through the interconnected porosity during the various integration processes. Different types of plasma post-treatments which lead to surface modification of the porous low-k material with possible formation of a top surface layer, change of surface structure and “pore sealing” effect were applied. Highly sensitive instruments for mechanical investigation of thin layers, such as the Ultra Nano Hardness Tester (UNHT) and Nano Scratch Tester (NST) were applied for characterization of the effect of the plasma post-treatments on the mechanical behavior of a porous low-k material. Preliminary results are presented and discussed in this paper

Biosourced adsorbent prepared with rice husk part 1: A complete understanding of the structure of materials, the major role of mineral impurities for metal extraction

Rice husk is a global agricultural co-product and is already the subject of several studies, notably for wastewater treatment. Rice husk is composed of 3 types of components: amorphous silica, organic biopolymers and others (salts, oxides…). Depending on the treatment, rice husk becomes either a carbon-free material or a mixed carbon/silica biochar. In this paper, a thorough characterisation of rice husk was carried out by SEM, TEM, TGA, XRF, XRD, XPS, FTIR, SAXS, SANS, NMR and N2 adsorption-desorption. The results show that silica can be present as dense silica or silica nanoparticles and that native organic matter can be converted to turbostratic carbon. This carbon “drowns” the silica nanoparticles and prevents them from sintering. Particular attention is paid to impurities which play a crucial role in several properties. They are present in different forms, such as CaCO3, KCl or Al2O3 or in the silica lattice. They can be removed, but if retained, they induce sintering and crystallisation of the silica nanoparticles, resulting in a decrease in specific surface area from 330 m2.g−1 to 15 m2.g−1. Moreover, the sorption efficiency of the materials is strongly dependent on the presence of impurities since the extraction rate drops drastically from 99% to <0.5% when the impurities are removed. The maximum capacity reached for nickel is 11.7 mg.g−1