## Physique

#### First principal investigation of structural, morphological, optoelectronic and magnetic characteristics of sprayed Zn: Fe2O3 thin films

Undoped and Zn-doped Fe2O3 thin films were grown through spray pyrolysis. Zinc doping effect on the physical properties was investigated in detail. X-ray diffraction analysis confirms that all the Fe2O3 thin films showed a rhombohedral structure. The surface morphological study shows an interesting dendrite structure. The estimated band gaps energies were increased from 2.13 to 2.21 eV for indirect transition and from 1.80 to 1.85 eV for direct transition as function of doping ratio which was increased from 2 to 8 at. % Zn. The resistivity value (ρ) of un-doped Fe2O3 thin film is 6.06 × 104 Ω. cm and as adding Zn ions, ρ consequently decreased to 52 Ω. cm for 6 at. % Zn-doped Fe2O3 thin films. Vibrating sample magnetometer (VSM) measurements showed an increase of the saturation magnetization with the Zn2+ insertion. Further, a ferromagnetic behavior was observed. Voir les détails

**Mots clés : **Ferromagnetic, semiconductor, Fe2O3, Zinc doping, Low resistivity

#### Unraveling the effect of Bi2S3 on the optical, electrical and magnetic properties of γ-MnS-based composite thin films

(Bi2S3)x (γ-MnS)1-x composite thin films have been deposited onto glass substrates using spray pyrolysis method. The structural and compositional investigations confirmed the co-existence of Bi2S3 and γ-MnS binary compounds in the thin films. The surface morphology indicated that the increase in Bi2S3 concentration influences both the shape and the size of γ-MnS crystallites. The optical analysis via transmittance and reflectance measurements revealed that the band gap energy Eg decreased from 3.29 eV to 1.5 eV in terms of Bi2S3 content. The electrical parameters such as resistivity ρ, mobility μ, carrier concentrations and Hall coefficient have been obtained by Hall Effect measurements. It is found than incorporation of Bi2S3 enhances the conductivity, and p-type conduction of γ-MnS could be converted to n-type at x = 0.5. The vibrating sample magnetometer measurement has revealed that (Bi2S3)x (γ-MnS)1-x composite thin films have a ferromagnetic behavior at room temperature. Voir les détails

**Mots clés : **γ-MnS, Bi2S3, Spray pyrolysis, Magnetic Properties

#### Heat Transfer Mode and Effect of Fluid Flow on the Morphology of the Weld Pool

In this work, the heat transfer by conduction and convection mode, effect of the fluid flow on the morphology of the weld pool and the welding properties are investigated during Tungsten Inert Gas (TIG) process. In the first part, a computation code under Fortran was elaborated to solve the equations resulting from the finite difference discretization of the heat equation, taking into account the liquid-solid phase change with the associated boundary conditions. In order to calculate the velocity field during welding, the Navier-Stokes equations in the melt zone were simplified and solved considering their stream-vorticity formulation. A mathematical model was developed to study the effect of the melted liquid movement on the weld pool. The evolution of the fraction volume of the liquid and the thermal fields promoted the determination of the molten zone (MZ) and the Heat Affected Zone (HAT) dimensions, which seems to be in good agreement with literature. Voir les détails

**Mots clés : **Thermal and flow modeling, TIG welding, Heat affected zone, molten zone

#### Slug length and frequency upstream a sudden expansion in gas– liquid intermittent flow

The purpose of this work is to analyze the influence of a sudden expansion on the upstream behavior of the horizontal gas–liquid slug flow. Measurements were made on a 40 mm ID pipe with and without a sudden enlargement of aspect ratio σ = 0.444. The experiments were carried out with two-phase air–water mixture. The slug lengths and frequencies were measured using a non-intrusive video technique. Upstream the sudden enlargement, it was observed that the mixture velocity has no influence on slug length. The variation of slug frequency is found proportional to the liquid superficial velocity for the two cases within this study. It was also observed that the behavior of the slug length and frequency was affected by the presence of the sudden enlargement. The comparison of the results obtained with various empirical correlations available in the literature showed that the latter are not worthwhile in the case where singularity is installed. Voir les détails

**Mots clés : **slug flow, sudden expansion, slug length, slug frequency

#### Thermal andlfuid fow modeling oflthelmolten pool behavior duringlTIG welding bylstream vorticity method

The present paper deals with the numerical simulation of weld pool development in Tungsten Inert Gas (TIG) process. A mathematical model is developed in order to solve the Navier–Stokes equations expressed in the stream–vorticity formulation coupled with heat equation taking into account the liquid solid phase change. Using the stream–vorticity formulation in incompressible fuid fow, the same problem is solved with reducing the number of transport equations. Therefore, only one transport equation (vorticity) and one Poisson equation (stream) are considered in this model. The FORTRAN programming and the numerical simulation are then achieved using appropriate discretization that ensures the convergence of the numerical methods to solve a large and sparse linear algebraic systems. Furthermore, to solve the radiation phenomena during welding described by the Stefen law, another method is proposed. Thefobtained numerical results are discussed and validate with experimental. Voir les détails

**Mots clés : **Thermal and fuid modeling, TIG welding, Stream vorticity, 304L steel, Numerical simulation

#### Calcul des corrections d’absorption des Neutrons par La Méthode de Monte Carlo sur des Echantillons de Poudres Cristallines

We have studied the angular behavior of the absorption coefficient in diffraction experiments of thermal neutrons incident on highly absorbing cylindrical samples. We have thus developed a code based on the Monte Carlo method to carry out the transport of such thermal neutrons and the evaluation of the finite-geometry induced effects, in addition to the study ofthe role played by multiple scattering, the absorption and the shape of the scattering sample as well. Thus, we were able to determine that the angular behavior of the transmission coefficient of the full-cylinder geometry is different from that of an annular-cylinder geometry containing the same number of scattering material. An enhancement of the diffracted intensity by a factor of 10, for small Bragg angles, is observed whenever the annular-cylinder shape is used instead of the full-cylinder one. Our code is further validated by the agreement of its results with those obtained previously par other workers using semi analytic methods. Voir les détails

**Mots clés : **Absorption coefficient, thermal neutrons, highly absorbing cylindrical samples, Monte Carlo Method

#### Modelling and performance analysis of a GaN-based n/p junction betavoltaic cell

In this work, we optimized the performance of a gallium nitride (GaN)-based n/p junction betavoltaic cell irradiated by the radioisotope nickel-63 (Ni63). In particular, we developed a lab-made software starting from an analytical model that takes into account a set of fundamental physical parameters for the cell structure. The simulations reveal that, by using a Ni63 radioisotope source with a 25 mCi/cm² activity density emitting a flux of beta-particles with an average energy of 17.1 KeV, the cell performs a conversion efficiency (η) in excess of 26%, thus approaching the theoretical limit for a GaN-based device. The other electrical parameters of the cell, namely the short-circuit current density (Jsc), open-circuit voltage (Voc), and maximum electrical power density (Pmax) are 240 nA/cm², 2.87 V, and 660 nW/cm², respectively. The presented analysis can turn useful for understanding the theoretical background needed to better face GaN-based betavoltaic cell design problems. Voir les détails

**Mots clés : **analytical modelling, Gallium nitride, betavoltaic cell, nickel-63 radioisotope, radioactivity density

#### Simulation analysis of a high efficiency GaInP/Si multijunction solar cell

The solar power conversion efficiency of a gallium indium phosphide (GaInP)/silicon (Si) tandem solar cell has been investigated by means of a physical device simulator considering both mechanically stacked and monolithic structures. In particular, to interconnect the bottom and top sub-cells of the monolithic tandem, a gallium arsenide (GaAs)-based tunnel-junction, i.e. GaAs(n+)/GaAs(p+), which assures a low electrical resistance and an optically low-loss connection, has been considered. The J–V characteristics of the single junction cells, monolithic tandem, and mechanically stacked structure have been calculated extracting the main photovoltaic parameters. An analysis of the tunnel-junction behaviour has been also developed. The mechanically stacked cell achieves an efficiency of 24.27% whereas the monolithic tandem reaches an efficiency of 31.11% under AM1.5 spectral conditions. External quantum efficiency simulations have evaluated the useful wavelength range. The results and discussion could be helpful in designing high efficiency monolithic multijunction GaInP/Si solar cells involving a thin GaAs(n+)/GaAs(p+) tunnel junction. Voir les détails

**Mots clés : **GaInP/Si, tandem solar cells, power efficiency, numerical simulations

#### Effect of the c-axis tilting angle in piezoelectric ZnO crystal on the performances of electroacoustic SAW sensors

This paper aims to study the effect of c-axis tilting angle of piezoelectric ZnO/Si on the performances of electroacoustic SAW sensors, the dispersion curves of phase velocity, the electromechanical coupling factor K² and sensitivity to mass loading of Rayleigh and Sezawa modes are studied for different hZnO/λ and for different c-tilting angles (0, θ, 90°). The effect of the tilting angle θ on the performances of electroacoustic devices, is studied by finite element analysis. Based on the obtained results, SAW device is fabricated onto a ZnO/SiO2/Si multilayered structure. The obtained results show best performances and high sensitivity to gas and will contribute in enhancing the sensitivity and performances of SAW electroacoustic devices. Voir les détails

**Mots clés : **surface acoustic waves, electroacoustic devices, Finite Element Analysis, Piezoelectric materials, c-tilted ZnO

#### Effect of calcium phosphate synthesis conditions on its physico-chemical properties and evaluation of its antibacterial activity

The antibacterial activity of non-stoichiometric calcium phosphate particles prepared by precipitationunder controlled experimental conditions at pH ?∼ ?9 and sintered at high temperature was studiedagainst Staphylococcus aureus bacteria. The effects of operating parameters developed according to anexperimental design of Plackett-Burman type on the physicochemical characteristics and the capacityto inhibit bacterial growth were identi fi ed using a thermal analysis ( TGA-DTA-DSC) , x-rayDiffraction( XR) , Raman Spectroscopy, Scanning Electron Microscope(SEM ) and the Kirby BauerMethod. The XRD spectrum shows that the synthetic crystalline nanoparticles powders consist ofmultiphasic calcium phosphateβ -TCP/β -CPP/OCP / HA and that the average particle size is between56 and 123 nm calculated by the Debay-Shearer equation. The Raman spectrum of sintered powdershows the main absorption bands that are assigned to the asymmetric / symmetric P-O stretchingvibrations in PO4− 3and the symmetric O-H stretching mode of the hydroxyl group in addition ofCa-PO4and Ca-OH modes. The samples were found to possess different morphologies consisting ofnano-rods of different lengths, semi / spherical structures and fine granules, in addition to irregularclusters. The antibacterial tests results showed that the high concentration calcium phosphate powderexhibited better antibacterial activity against Staphylococcus aureus bacteria with inhibition zonesranging from 0.2 –0.7 cm. Voir les détails

**Mots clés : **Biomaterials, calcium phosphate, Antibacterial Activity, nanoparticles