Ab initio calculations study of the electronic, optical and thermodynamic properties of NaMgH3, for hydrogen storage

The structural stability, electronic structure, optical and thermodynamic properties of NaMgH₃ have been investigated using the density functional theory. Good agreement is obtained for the bulk crystal structure using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy. It is found from the electronic density of states (DOS) that the valence band is dominated by the hydrogen atoms while the conduction band is dominated by Na and Mg empty states. Also, the DOS reveals that NaMgH₃ is a large gap insulator with direct band gap 3.4 eV. We have investigated the optical response of NaMgH₃ in partial band to band contributions and the theoretical optical spectrum is presented and discussed in this study. Optical response calculation suggests that the imaginary part of dielectric function spectra is assigned to be the interband transition. The formation energy for NaMgH₃ is investigated along different reaction pathways. We compare and discuss our result with the measured and calculated enthalpies of formation found in the literature.     

A large ultrasonic bounded acoustic pulse transducer for acoustic transmission goniometry: modeling and calibration

A large, flat ultrasonic transmitter and a small receiver are developed for studies of material properties in acoustic transmission goniometry. While the character of the wave field produced by the transmitter can be considered as a plane wave as observed by the receiver, diffraction effects are noticeable near critical angles and result in the appearance of weak but detectable arrivals. Transmitted ultrasonic waveforms are acquired in one elastic silicate glass and two visco-elastic acrylic glass sample plates as a function of the angle of incidence. Phase velocities are determined from modeling of the shape of curves of the observed arrival times versus angle of incidence. The waveform observations are modeled using a phase propagation technique that incorporates full wave behavior including attenuation. Subtle diffraction effects are captured in addition to the main bounded pulse propagation. The full propagation modeling allows for various arrivals to be unambiguously interpreted. The results of the plane wave solution are close to the full wave propagation modeling without any corrections to the observed wave field. This is an advantage as it places confidence that later analyses can use simpler plane wave solutions without the need for additional diffraction corrections. A further advantage is that the uniform bounded acoustic pulse allows for the detection of weak arrivals such as a low energy edge diffraction observed in our experiments.     

Synthesis,characterization and kinetic computations of fullerene (C60)–CuO on the mechanism decomposition of ammonium perchlorate

CuO, C₆₀–CuO, and Al/C₆₀–CuO nanostructures were synthesized and characterized by scanning electron microscope (SEM)/energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and fourier transform infrared spectroscopy (FTIR). differential scanning calorimetry (DSC)/thermogravimetric analysis (TGA) measurements were performed to study the influence of these additives on ammonium percolate (AP) thermal decomposition. From the comparison of DSC and TGA plots, the catalytic effect of CuO and C₆₀–CuO has been clearly noticed in which the lower temperature decomposition of AP was decreased from 331 °C to 315 °C, 310 °C, and 303 °C (in the presence of CuO, C₆₀–CuO, and Al/C₆₀–CuO, respectively) and the HTD was dropped from 430 °C (pure AP) to 352 °C, 335 °C, and 317 °C (for the compounds AP/CuO, AP/C₆₀–CuO, and AP/Al/C₆₀–CuO, respectively). The kinetics of the samples were investigated by isoconversional models and compared with an iterative procedure. The results of pure AP indicated a complex decomposition process involving three decomposition steps with specific reaction mechanism. The nanocatalysts incorporated in the AP have clearly affected its decomposition process in which the reaction mechanism and the number of stages were changed.     

Contribution to the study of structural and elastic properties of wüstite under pressure up to 140 GPa by pseudopotential calculations

Using first-principles calculations based on the density functional theory and the generalized gradient approximations, we have studied the effect of high pressures up to 140 GPa on the structural and elastic properties of wüstite. Our results indicate that FeO undergoes a structural phase transition from NaCl-type (B1) to NiAs-type (B8) almost at the pressure of 77 GPa. The density increases across this transition by about 5%, which is a higher value than that obtained in other researches. We can clearly present the wüstite elastic properties and isotropic wave velocities which are not already studied in this range of pressure, and we could compare these results with the available experiment data, especially with that of PREM model.     

Existence d'une sélection continue pour les applications multivoques à images non nécessairement convexes

In this Note, we prove a continuous selection existence result for the multivalud maps with not necessarily convex values, but continuously contractible, a notion that we define below. To cite this article: Y. Askoura, C. R. Acad. Sci. Paris, Ser. I 337 (2003).     

Ab initio study of the structural,electronic, elastic and magnetic properties of Cu2GdIn,Ag2GdIn and Au2GdIn

We preformed first-principle calculations for the structural, electronic, elastic and magnetic properties of Cu₂GdIn, Ag₂GdIn and Au₂GdIn using the full-potential linearized augmented plane wave (FP-LAPW) scheme within the generalized gradient approximation by Wu and Cohen (GGA-WC), GGA+U, the local spin density approximation (LSDA) and LSDA+U. The lattice parameters, the bulk modulus and its pressure derivative and the elastic constants were determined. Also, we present the band structures and the densities of states. The electronic structures of the ferromagnetic configuration for Heusler compounds (X₂GdIn) have a metallic character. The magnetic moments were mostly contributed by the rare-earth Gd 4f ion.     

Global weak solutions of a three-dimensional flow model for a multi-species mixture in porous media

We consider an initial boundary value problem for a non-linear differential system consisting of one equation of parabolic type coupled with a n × n semi-linear hyperbolic system of first order. This system of equations describes the compressible miscible displacement of n + 1 chemical species in a porous medium, in the absence of diffusion and dispersion. We assume the viscosity of the fluid mixture to be constant. We prove, in three space dimensions, the existence of a global weak solution with non-smooth initial data for the concentration. The proof is based on the artificial viscosity method together with a compensated compactness argument. © 1998 B. G. Teubner Stuttgart—John Wiley & Sons, Ltd.     

Asymptotic approximation of the solution of Stokes equations in a domain with highly oscillating boundary

We consider a viscous incompressible flow in an infinite horizontal domain bounded at the bottom by a smooth wall and at the top by a rough wall. The latter is assumed to consist in a plane wall covered with periodically distributed asperities which size depends on a small parameter, and with a fixed height. We assume that the flow is governed by the stationary Stokes equations. Using a boundary layer corrector we derive and analyze a first order asymptotic approximation of the flow.      

Spectral boundary homogenization in domains with oscillating boundaries

We study the asymptotic behavior of the solutions of a spectral problem for the Laplacian in a domain with a rapidly oscillating boundary. We consider both cases where the eigenvalues of the limit problem are simple and multiple. We construct the leading terms of the asymptotic expansions for the eigenelements and verify the asymptotics.