Physique
Propagation of Lamb waves in an immersed periodically grooved plate: Experimental detection of the scattered converted backward waves
Guided waves propagation in immersed plates with irregular surfaces has potential application to detection and assessment of the extent, depth and pattern of the irregularity. The complexity of the problem, due to the large number of involved parameters, has limited the number of existing studies. The simplest case of irregularities of practical interest is the two-dimensional corrosion profile. Even this case is in general so complex, that one can extract several amplitude dominant periodic surfaces only by using a Fourier spectrum of the surface. Guided waves in plates, with one or both free surfaces having periodic perturbations of different shapes, have been presented in specialized literature. In this paper is studied the propagation of Lamb waves in an aluminum plate with a periodic grooved surface on only one side and immersed in water. The interaction between an incident Lamb wave and the grating gives rise to retro-converted waves. Preliminary numerical simulation by the finite element method is performed in order to obtain key parameters for the experiments. It is shown that retroconverted waves radiating into the water are detectable although their amplitudes are small. The phonon relation is verified for the leaky Lamb modes. The damping coefficients of the leaky Lamb modes in the grooved immersed plate are evaluated. Voir les détails
Mots clés : Liquid–solid interfaces, Rough plate, Periodic grating, Lamb waves, Converted modes
First principles calculations of structural, electronic and opticalproperties of Zn1-xBexSeyTe1-y quaternary alloys
The structural, electronic and optical properties of Zn1-xBexSeyTe1-y quaternary alloys are investigatedusing the full potential-linearized augmented plane wave (FP-LAPW) method within density functionaltheory (DFT). We used both the Wu–Cohen and the Engel–Vosko generalized gradient approximationsof the exchange-correlation energy that are based on the optimization of the total energy and the corre-sponding potential, respectively. Some basic physical properties, such as lattice constant, bulk modulus,electronic band structures, and optical properties (dielectric constant and refractive index) are calculated,nonlinear dependence on the compositions x and y are found. In addition, the energy band gap of zinc-blende Zn1-xBexSeyTe1-y quaternary alloys lattice matched to GaAs substrate is investigated. To ourknowledge this is the first quantitative theoretical investigation on Zn1-xBexSeyTe1-y quaternary alloysand still awaits experimental confirmations Voir les détails
Mots clés : alloys, Ab initio calculations, Electronic structure, optical properties
Effects of sol concentration on structural, morphological and optical waveguiding properties of sol-gel ZnO nanostructured thin films
Nanostructured ZnO thin films with different precursor concentrations (0.5-0.8 M) have been deposited on glass substrates by sol-gel dip coating technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible spectrophotometer, and m-lines spectroscopy have been employed to investigate the effect of solution concentration on structural, morphological, optical and waveguiding properties of ZnO thin films. XRD spectra have shown that all the films are polycristalline and exhibit the wartzite hexagonal structire. SEM micrographs and AFM images have revealed that morphology and surface routhness of the thin films deprnd on sol concentration. The UV-visible transmittanceresults show a high transparency in the visible range and a shift of the maximum transmittance to the higher wavelength with increasing sol concentration. Waveguiding properties such as refractive index, number of propagating modes and attenuation coefficient measered at 632.8 nm wavelength by m-lines spectroscopy indicate that our ZnO slab waveguides are single mode and demonstrate optical losses estimated around 1.5 decibel per cm (dB/cm) for the thin film prepared with a sol concentration of 0.7 M. Voir les détails
Mots clés : ZnO, sol-gel, thin film
Modeling of InGaN / GaAs Photovoltaic Tandem with GaAs/AlAs Bragg Mirror Rear Surface Reflector
In this work, a parametric study of a dual junction tandem based on In0.53Ga0.47N on GaAs has been carried. In order to obtain reflection of unabsorbed photons from the bottom of the device, Bragg reflectors (BR) composed of GaAs/AlAs, with appropriate thicknesses, was placed in the rear surface of the GaAs sub-cell. With this intention, the current-voltage curves are calculated for different front recombination velocities and the influence of the bottom cell thickness on efficiency has been studied. The results of simulation show that the structure’s efficiency can attain 29% under 1-sun AM1.5 illumination, for a front recombination velocity value of 1e3cm/s and 10μm bottom cell thickness. This efficiency will decrease with increasing the operating temperature. Voir les détails
Mots clés : Bragg Reflector, Carrier Lifetimes, Recombination Velocity, Efficiency, Temperature
First-principles prediction of the structural, elastic, thermodynamic, electronic and optical properties of Li4Sr3Ge2N6 quaternary nitride
Structural parameters, elastic constants, thermodynamic properties, electronic structure and optical properties of the monoclinic Li4Sr3Ge2N6 quaternary nitride are investigated theoretically for the first time using the pseudopotential plane-wave based first-principles calculations. The calculated structural parameters are in excellent agreement with the experimental data. This serves as a proof of reliability of the used theoretical method and gives confidence in the predicted results on aforementioned properties of Li4Sr3Ge2N6. The predicted elastic constants Cij reveal that Li4Sr3Ge2N6 is mechanically stable but anisotropic. The elastic anisotropy is further illustrated by the direction-dependent of the linear compressibility and Young’s modulus. Macroscopic elastic parameters, including the bulk and shear moduli, the Young’s modulus, the Poisson ratio, the velocities of elastic waves and the Debye temperature are numerically estimated. The pressure and temperature dependence of the unit cell volume, isothermal bulk modulus, volume expansion coefficient, specific heat and Debye temperature are investigated through the quasiharmonic Debye model. The band structure and the density of states of Li4Sr3Ge2N6 are analyzed, which reveals the semiconducting character of Li4Sr3Ge2N6. The complex dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron energy-loss function are calculation for incident radiation polarized along the crystallographic directions and for energy up to 40 eV. Voir les détails
Mots clés : Quaternary nitride, First-Principles calculations, Elastic constants, Electronic structure, optical properties, Thermal Properties
Structural, elastic, electronic, chemical bonding and optical properties of Cu-based oxides ACuO (A = Li, Na, K and Rb): An ab initio study
Ab initio total energy calculations were performed to study in details the structural, elastic, electronic, chemical bonding and optical properties of Cu-based ternary oxides ACuO (A = Li, Na, K and Rb). Optimized atomic coordinates and lattice constants agree well with the existing experimental and theoretical data. Numerical estimations of the six independent elastic constants Cij and their related properties for monocrystalline ACuO were obtained. A set of elastic moduli for polycrystalline ACuO, namely bulk modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio r, Lamé coefficients k and Debye temperature hD were evaluated. Band structure, total and site-projected l-decomposed densities of states, charge-carrier effective masses, charge transfers and charge density distribution maps were obtained; analyzed and compared with the available theoretical data. Complex dielectric function, refractive index, extinction coefficient, reflectivity and loss function spectra were calculated with an incident radiation polarized parallel to both [100] and [001] crystalline directions. Voir les détails
Mots clés : Cu-based oxides Ab initio calculations Elastic constants Electronic properties Chemical bonding
Trace and essential elements determination in baby formulas milkby INAA and k0-INAA techniques
As a part of the food analysis program executedat Es-Salam research reactor by neutron activation analysislaboratory, the concentration of As, Ba, Br, Ca, Fe, K, La,Rb and Zn in baby formulas milk have been determined byusing INAA and k0-NAA techniques. It was found that theconcentration of all elements obtained by both techniqueswas relatively identical for the three brands of the 0–6 and6–12 months of analyzed samples. In addition, the analyticalresults have been compared with those given byproducers. Voir les détails
Mots clés : Trace and essential elements, NAA method, Milk products, Analysis
Deposition of tin(II) sulfide thin films by ultrasonic spraypyrolysis: Evidence of sulfur exo-diffusion
Tin Sulfide (SnS) thin films were deposited by ultrasonic spray pyrolysis technique, on glass substrate heated at 280 °C, with different deposition times. The used precursor SnCl2 and thiourea are dissolved in methanol. X-ray diffraction (XRD) analysis indicates that films are mainly composed with orthorhombic SnS phase at low deposition time. With increasing deposition time, the hexagonal SnS2 phases become dominant. SnO2 and metallic Sn phases have been detected with increasing deposition time. Scanning electron microscopy (SEM) observations reveal that films surfaces are rough with the presence of bubbles due to S2 gas exo-diffusion from the bulk during film growth. A model of S gas formation is presented. Voir les détails
Mots clés : Tin Sulfide Thin films Spray pyrolysis
ELECTRODEPOSTION OF ZnO NANOWIRES FOR ENERGY HARVESTING
Zinc oxide is a transparent n-type semiconductor with a direct bandgap ranging between 3.3 and 3.5 eV. Polycrystalline ZnO has found numerous interesting applications such as piezoelectric transducers varistors , phosphors, transparent conducting films, transparent windows or nanostructured electrode for solar cells. ZnO nanostructures such as nanowires (NWs) and nanorods (NRs) showed attractive characteristics for certain applications ZnO, is among the most interesting materials in terms of piezoelectric properties that are relatively large compared to other non-ceramic piezoelectric materials . It also has the advantage of being easily burn which is of great interest for microtechnolog.To get, the ZnO thin films and nanowire must present a good cristallinity with preferential orientation (002) direction. Also, films electrodeposited must have good morphological properties, and NW must show good alignment and have direction of growth perpendicular to the substrate. This work is related to the fabrication of ZnO nanowires and nanrcords at room temperature also we showed the influence of the concentration of ZnCl2 on the morphological and structural properties of the deposits made by electrochemical route (NW and thin films). We made these deposits in an aqueous solution at room temperature and without buffer layer. Voir les détails
Mots clés : ZnO, nanowires, thin film, electrodeposition.
Electrochemical synthesis of ZnO thin film and nanowire for piezoelectric applications
ZnO thin films and ZnO nanowire (NW) having piezoelectric qualities can be used in respectively SAW devices (Surface Acoustic Wave) and nanogenerators. In this work, we are interested in optimizing experimental conditions of ZnO electrodeposition to obtain thin films and nanowire with good piezoelectric performances. To achieve this goal, the ZnO thin films and nanowire must firstly present a good cristallinity with preferential orientation along (002) direction. Also, films electrodeposited must have good morphological properties (not porous and with insignificant surface rugosity), and NW must show good alignment and have direction of growth perpendicular to the substrate. The electrodeposition of ZnO was performed in a classical three electrodes electrochemical cell, under potentiostatic mode and at room temperature. The substrate was a molybdenum foil of 0.1 mm thick and with 99.95% purity. The electrolytic bath was initially composed of 5mM ZnCl2, 5mM H2O2 and 100 mM KCl (supporting electrolyte), pH value being about 6.8. The initial conditions which give a well aligned of ZnO-NW (see fig. 1) were progressively modified in order to obtain a ZnO thin film. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Electrochemical impedance spectroscopy (EIS), to obtain information about microstructure, morphology, composition and electronic properties of the films and nanostructures electrodeposited. Voir les détails
Mots clés : ZnO, nanowires, thin film, electrodeposition, piezoelectric.