Caractérisation par RMN du solide des matrices de stockage solide de l'hydrogène

On peut distinguer trois moyens de stockage de l'hydrogène à bord d'un véhicule : sous forme liquide (à 20K sous 10 bars), sous forme comprimée (350 bars), ou dans des matériaux solides sous forme physisorbée ou chimisorbée. Nous nous sommes plus particulièrement intéressés à l'étude de ces matériaux solides. Parmi ceux cités dans la littérature, nous avons choisi d'étudier deux systèmes qui se prêtent bien à une étude RMN : les alanates et les clathrates. L'objectif de notre étude est le développement et la validation d'outils de caractérisation par spectroscopie et imagerie de résonance magnétique nucléaire (RMN) de la structure de matériaux pour le stockage de l'hydrogène, et de leur évolution au cours des cycles de charge/décharge. Les deux principaux volets de cette étude sont d'une part l'analyse de la structure locale des matériaux et la compréhension de ses éventuelles modifications, et d'autre part, l'analyse in-situ de la distribution et de la diffusion de l'hydrogène au sein du matériau de stockag

Computational simulations of solid state NMR spectra: a new era in structure determination of oxide glasses

The application of the MD-GIPAW approach to the calculation of NMR parameters, line widths and shapes of the spectra of oxide glasses is reviewed. Emphasis is given to the decisive role of this approach both as an interpretative tool for a deeper understanding of the spectral behavior of complex systems and as a predictive instrument to map NMR data in a distribution of structural parameters and vice versa (structural inversion method). After a brief overview of the basic features of oxide glasses and the experimental techniques routinely employed to investigate their structure, a general description of the computational methods usually adopted to generate sound structural models of amorphous materials is offered. The computational recipe used to compute the solid state NMR spectra of oxide glasses and to establish quantitative structural-NMR property relationships is then described. Finally, these concepts are applied to 'simple' network former glasses and more complex silicates, aluminosilicate, phosphosilicate and borosilicate glasses of scientific relevance. The final section is dedicated to the future developments that will hopefully improve the computational approach described overcoming some of the current limitations

Citation

Second-order dipolar order in magic-angle spinning nuclear magnetic resonance J. Chem. Phys. 135, 154507 (2011) Single crystal nuclear magnetic resonance in spinning powders J. Chem. Phys. 135, 144201 (2011) Resistive detection of optically pumped nuclear polarization with spin phase transition peak at Landau level filling factor 2/3 Appl. Phys. Lett. 99, 112106 (2011) High-resolution 13C nuclear magnetic resonance evidence of phase transition of Rb,Cs-intercalated singlewalled nanotubes J. Appl. Phys. 110, 054306 (2011) Distribution of non-uniform demagnetization fields in paramagnetic bulk solids J. Appl. Phys. 110, 013902 (2011) Additional information on J. Chem. Phys. In this article, we present an alternative expansion scheme called Floquet-Magnus expansion (FME) used to solve a time-dependent linear differential equation which is a central problem in quantum physics in general and solid-state nuclear magnetic resonance (NMR) in particular. The commonly used methods to treat theoretical problems in solid-state NMR are the average Hamiltonian theory (AHT) and the Floquet theory (FT), which have been successful for designing sophisticated pulse sequences and understanding of different experiments. To the best of our knowledge, this is the first report of the FME scheme in the context of solid state NMR and we compare this approach with other series expansions. We present a modified FME scheme highlighting the importance of the (timeperiodic) boundary conditions. This modified scheme greatly simplifies the calculation of higher order terms and shown to be equivalent to the Floquet theory (single or multimode time-dependence) but allows one to derive the effective Hamiltonian in the Hilbert space. Basic applications of the FME scheme are described and compared to previous treatments based on AHT, FT, and static perturbation theory. We discuss also the convergence aspects of the three schemes (AHT, FT, and FME) and present the relevant references

On the microscopic fluctuations driving the NMR relaxation of quadrupolar ions in water

International audienceNuclear Magnetic Resonance (NMR) relaxation is sensitive to the local structure and dynamics around the probed nuclei. The Electric Field Gradient (EFG) is the key microscopic quantity to understand the NMR relaxation of quadrupolar ions, such as 7 Li + , 23 Na + , 25 Mg 2+ , 35 Cl − , 39 K + , or 133 Cs +. Using molecular dynamics simulations, we investigate the statistical and dynamical properties of the EFG experienced by alkaline, alkaline Earth, and chloride ions at infinite dilution in water. Specifically, we analyze the effect of the ionic charge and size on the distribution of the EFG tensor and on the multi-step decay of its auto-correlation function. The main contribution to the NMR relaxation time arises from the slowest mode, with a characteristic time on the picosecond time scale. The first solvation shell of the ion plays a dominant role in the fluctuations of the EFG, all the more that the ion radius is small and its charge is large. We propose an analysis based on a simplified charge distribution around the ion, which demonstrates that the auto-correlation of the EFG, hence the NMR relaxation time, reflects primarily the collective translational motion of water molecules in the first solvation shell of the cations. Our findings provide a microscopic route to the quantitative interpretation of NMR relaxation measurements and open the way to the design of improved analytical theories for NMR relaxation for small ionic solutes, which should focus on water density fluctuations around the ion

Extended Czjzek model applied to NMR parameter distributions in sodium metaphosphate glass

The Extended Czjzek Model (ECM) is applied to the distribution of NMR parameters of a simple glass model (sodium metaphosphate, $\mathrm{NaPO_3}$) obtained by Molecular Dynamics (MD) simulations. Accurate NMR tensors, Electric Field Gradient (EFG) and Chemical Shift Anisotropy (CSA), are calculated from Density Functional Theory (DFT) within the well-established PAW/GIPAW framework. Theoretical results are compared to experimental high-resolution solid-state NMR data and are used to validate the considered structural model. The distributions of the calculated coupling constant $C_Q\propto |V_{zz}|$ and of the asymmetry parameter $\eta_Q$ that characterize the quadrupolar interaction are discussed in terms of structural considerations with the help of a simple point charge model. Finally, the ECM analysis is shown to be relevant for studying the distribution of CSA tensor parameters and gives new insight into the structural characterization of disordered systems by solid-state NMR.Comment: 17 pages, 12 figures to be published in J. Phys.: Condens. Matte

Effect of changing the rare earth cation type on the structure and crystallization behavior of an aluminoborosilicate glass

4 pagesAn aluminoborosilicate glass, containing high amount of rare earth (RE) accordingly to the following composition 50.68 SiO2 - 4.25 Al2O3 - 8.50 B2O3 - 12.19 Na2O - 4.84 CaO - 3.19 ZrO2 - 16.35 RE2O3 (wt.%), is currently under study for the immobilization of nuclear waste solutions. In this work, we wanted to investigate the effect of changing the RE cation type on the glass structure and on its crystallization behavior. For this purpose, a glass series was elaborated in which the nature of the RE is varying from lanthanum to lutetium. In this glass series, only little effect was observed on the glass structure. On the contrary, a strong impact was put in evidence on the crystallization behavior through different heat treatments. A slow cooling of the melt at 1°C/min, revealed significant crystallization of apatite Ca2RE8(SiO4)6O2 in sample containing rare earths with ionic radii close to that of calcium. Another heat treatment consisting of successive nucleation and growth stages, performed to force the crystallization in the bulk and reduce any surface crystallization effect, put in evidence the existence of a strongly heterogeneous second rare earth rich silicate phase for samples containing RE with low ionic radius (from Y to Lu)

Effect of the nature of alkali and alkaline-earth oxides on the structure and crystallization of an aluminoborosilicate glass developed to immobilize highly concentrated nuclear waste solutions

A complex rare-earth rich aluminoborosilicate glass has been proved to be a good candidate for the immobilization of new high level radioactive wastes. A simplified seven-oxides composition of this glass was selected for this study. In this system, sodium and calcium cations were supposed in other works to simulate respectively all the other alkali (R+=Li+, Rb+, Cs+) and alkaline-earth (R'2+=Sr2+, Ba2+) cations present in the complex glass composition. Moreover, neodymium or lanthanum are used here to simulate all the rare-earths and actinides occurring in waste solutions. In order to study the impact of the nature of R+ and R'2+ cations on both glass structure and melt crystallization tendency during cooling, two glass series were prepared by replacing either Na+ or Ca2+ cations in the simplified glass by respectively (Li+, K+, Rb+, Cs+) or (Mg2+, Sr2+, Ba2+) cations. From these substitutions, it was established that alkali ions are preferentially involved in the charge compensation of (AlO4)- entities in the glass network comparatively to alkaline-earth ions. The glass compositions containing calcium give way to the crystallization of an apatite silicate phase bearing calcium and rare-earth ions. The melt crystallization tendency during cooling strongly varies with the nature of the alkaline-earth.Comment: nombre de pages:

Modifications structurales et défauts ponctuels paramagnétiques induits par irradiation électronique externe de la hollandite Ba1.16Al2.32Ti5.68O16

Des matrices BaxCsy (M,Ti)8O16 (x+y<2, M cation trivalent) de type hollandite, sont envisagées pour confiner spécifiquement le césium radioactif. Afin de simuler l'effet des rayonnements b, les modifications structurales et les défauts ponctuels paramagnétiques produits par irradiation électronique externe à température proche de l'ambiante d'une hollandite de composition simplifiée sans césium Ba1,16Al2,32Ti5,68O16 ont été étudiés par RPE et RMN. Des modifications ont été observées au niveau de l'environnement des cations Al3+ et Ti4+, résultant de la formation de lacunes d'oxygène et d'une augmentation du désordre dans les tunnels associée à des déplacements d'ions baryum. Des centres à électrons (Ti3+) et à trous électroniques (O2-) ont été observés. Ceux-ci sont relativement stables à température ambiante mais des recuits (traitements isochrones entre 50 et 800°C, traitements isothermes à 300°C) engendrent la formation d'autres défauts issus des défauts précédents correspondant à des ions Ti3+ de surface de type titanyl et des agrégats d'oxygène