Physique
RELATIONSHIP BETWEEN AE PARAMETERS AND FRACTURE OF COMPOSITE MATERIALS UNDER SOLICITATIONS
Abstract: Acoustic emission (AE) techniques have been extensively studied in concrete engineering. There has been much research in relating AE parameters, such as energy, amplitude, count number, rise time to physical properties such as crack area delaminating, pull out, break down. Our work is performed using of carbon fiber reinforced polymer to study the evolution of AE parameters under solicitations. Meanwhile the waveforms created by the failure were monitored by AE sensors attached to the concrete beam.Examination of the waveforms produced by a range of four sensors with different frequency responses reveals that the fracture depth affects the AE parameters detected during failure. Relationship between fracture area and AE parameters was examined. These results can be used to aid the quantification of damage size based on energy release from concrete structures in the field. Voir les détails
Mots clés : acoustic emission, carbon fiber, sollicitation, quantifi
Optimal Stochastic Excitation for Linear Flaw Detection in a Solid Material.
The field of ultrasonic nondestructive testing has known a great development during the recent years. In order to increase the faw detection sensitivity, many improvements have been made in the equipment and the sensors technology. In the present work, the optimal command which maximizes the flaw detection is investigated experimentally. A parametric optimization consisting of finding the optimal excitation frequency which maximizes the Euclidean distance between a reference medium and a medium with a linear faw has been obtained automatically by using the gradient descent algorithm. Moreover, the waveform excitation optimization has been considered. A set of stochastic signals have been transmitted to the medium. A closed loop optimization process based on a genetic algorithm allowed to find the optimal excitation without a priori knowledge on the shape of the signal. This optimal excitation converged to a sinusoidal pulse with the optimal frequency found by the parametric optimization. Voir les détails
Mots clés : Optimal command, nondestructive testing, Gradient descent algorithm, Genetic algorithm, Ultrasound
Analytical Modeling of Dual-Junction Tandem Solar Cells Based on an InGaP/GaAs Heterojunction Stacked on a Ge Substrate
An analytical model is used to describe the electrical characteristics of a dual?junction tandem solar cell performing with a conversion efficiency of 32.56%under air mass 1.5 global (AM1.5G) spectrum. The tandem structure consistsof a thin heterojunction top cell made of indium gallium phosphide (InGaP) ongallium arsenide (GaAs), mechanically stacked on a relatively thick germa?nium (Ge) substrate, which acts as bottom cell. In order to obtain the bestperformance of such a structure, we simulate for both the upper and lowersub-cell the current density–voltage, power density–voltage, and spectral re?sponse behaviors, taking into account the doping-dependent transportparameters and a wide range of minority carrier surface recombinationvelocities. For the proposed tandem cell, our calculations predict optimalphotovoltaic parameters, namely the short-circuit current density (Jsc), open?circuit voltage (Voc), maximum power density (Pmax), and fill factor (FF) areJsc = 28.25 mA/cm2, Voc = 1.24 V, Pmax = 31.64 mW/cm2, and FF = 89.95%,respectively. The present study could prove useful in supporting the design ofhigh efficiency dual junction structures by investigating the role of differentmaterials and physical parameters. Voir les détails
Mots clés : Analytical modeling, tandem solar cell, Spectral response, conversion efficiency
Optimal pump excitation frequency for improvement of damage detection by nonlinear vibro acoustic modulation method in a multiple scattering sample
We present a method to systematically optimize nonlinear damage detection in multiple scattering media by the nonlinear Vibro-Acoustic Modulation (VAM) technique. The latter consists here of exciting a medium simultaneously with a high frequency ultrasonic sinusoidal burst and with a low frequency continuous sinusoidal wave. Modulation of the high frequency (probe) by the low frequency (pump) is made possible by the presence of nonlinear scatterers, i.e. cracks, defects. A signal processing technique consisting of a closed loop system drives the automatic adaptation of the pumping frequency, yielding to the optimization of the nonlinear modulation (NM) of the output probing coda signal without a priori information on the medium and the scatterers. The correlation coefficient between a reference output probe signal without the pumping wave and an output modulated probe signal with a pumping wave was considered as our cost function. A multiple scattering solid beam where nonlinear scatterers can be controllably added or removed is designed and tested. The first step of this study is an empirical search of the correlation coefficient dependency on the pumping frequency to verify the performances of the proposed method. Then the implemented optimization algorithm based on genetic algorithm (GA) is used to find automatically the optimal pumping frequency. The obtained optimization results show a good agreement with the empirical study. Moreover, the genetic algorithm allowed to find the optimal pump frequency adapted to each configuration of nonlinear scatterers. This relatively fast search of the optimal nonlinear response could be extended to nonlinear scatterer imaging applications using the information on the resonant modes spatial shapes together with the associated optimal response. Voir les détails
Mots clés : Optimal command, Genetic algorithm, Nonlinear- Vibro Acoustic Modulation, Crack detection
Optimal pump excitation frequency for improvement of damage detection by nonlinear vibro acoustic modulation method in a multiple scattering sample.
We present a method to systematically optimize nonlinear damage detection in multiple scattering media by the nonlinear Vibro-Acoustic Modulation (VAM) technique. The latter consists here of exciting a medium simultaneously with a high frequency ultrasonic sinusoidal burst and with a low frequency continuous sinusoidal wave. Modulation of the high frequency (probe) by the low frequency (pump) is made possible by the presence of nonlinear scatterers, i.e. cracks, defects. A signal processing technique consisting of a closed loop system drives the automatic adaptation of the pumping frequency, yielding to the optimization of the nonlinear modulation (NM) of the output probing coda signal without a priori information on the medium and the scatterers. The correlation coefficient between a reference output probe signal without the pumping wave and an output modulated probe signal with a pumping wave was considered as our cost function. A multiple scattering solid beam where nonlinear scatterers can be controllably added or removed is designed and tested. The first step of this study is an empirical search of the correlation coefficient dependency on the pumping frequency to verify the performances of the proposed method. Then the implemented optimization algorithm based on genetic algorithm (GA) is used to find automatically the optimal pumping frequency. The obtained optimization results show a good agreement with the empirical study. Moreover, the genetic algorithm allowed to find the optimal pump frequency adapted to each configuration of nonlinear scatterers. This relatively fast search of the optimal nonlinear response could be extended to nonlinear scatterer imaging applications using the information on the resonant modes spatial shapes together with the associated optimal response. Voir les détails
Mots clés : Optimal command, Genetic algorithm, Nonlinear Vibro Acoustic Modulation, Crack detection
Characterization of the Polylactic acid stretched uniaxial and annealed by Raman spectrometry and Differential scanning calorimetry
In this work, we have been interested in the characterization of the effect of heat treatment and mechanical treatment on the crystallinity of polylactic acid (PLA) film by two techniques, DSC and Raman spectroscopy. The results obtained by the DSC for the stretched film shows the appearance of a broad peak of crystallization around 120 °C, a rise in melting peak in a significant way, which shows that the uniaxial stretching has increased the crystallinity of the PLA, whereas for the annealed film appearance of a double melting peak. The result obtained by Raman spectroscopy shows new peaks appears at 922 cm−1 and 540 cm-1 after stretching and annealed process, indicating the crystallization process. Voir les détails
Mots clés : PLA, glass transition, Raman spectroscopy, DSC
SnS THIN FILMS DEPOSITION BY SPRAY PYROLYSIS: SOLVENT INFLUENCE
Tin monosulfide (SnS) films are a new generation of absorber layers for thin ?lm heterojunction solar cell. The goal of the present study is the investigation of the role of the solvent on SnS thin films properties. Films were synthesized by ultrasonic spray pyrolysis technique. The used solution is a mixture of SnCl2.2H2O and thiourea (SC(NH2)2 precursors prepared with two different solvents: methanol and distilled water. X-ray diffraction (XRD) analysis reveals the SnS orthorhombic polycrystalline phase in different films. Using methanol as solvent leads to Sn2S3 secondary phase formation. While, film prepared with distilled water contains SnS2 as secondary phase. Scanning electron microscopy (SEM) observations reveal that films deposited with the methanol are rough with the presence of craters bubbles on the surface due to gas exo-diffusion during film growth. However, the film deposited with distilled water has a smooth, uniform, homogeneous and pinholes free surface. The electrical measurements reveal that films are p-type semiconductors, the dark conductivity increases from 3.07×10-4 (W.cm)-1 in film prepared with methanol to 5.15×10-3 (W.cm)-1 when using distilled water. We inferred that using distilled water leads to films with better quality than methanol as solvent. Voir les détails
Mots clés : Tin monosulfide, Spray pyrolysis, Thin films, solar cells
Simulation des propriétés électriques d'une jonction à base de nitrure de gallium. Application au détecteur de rayonnement
Le nitrure de gallium (GaN) est l'un des composés semi-conducteurs les plus intéressants, bien adaptés pour fonctionner dans n'importe quelle application dans laquelle la température ou le niveau de radiation pourrait endommager l'électronique conventionnelle. Récemment, la détection du rayonnement ultraviolet (UV) au moyen de dispositifs à base de GaN a suscité un grand intérêt dans de nombreux domaines importants, nécessitant des détecteurs à haute performance, à faible courant d'obscurité et à haute sensibilité. Dans ce travail, nous avons évalué, au moyen d'une étude de simulation en utilisant le simulateur de conception assistée par ordinateur (TCAD) Silvaco-Atlas, les caractéristiques électriques de deux types de photodétecteurs UV: le premier est un photodétecteur à barrière de Schottky (Pt / n-GaN) tandis que le deuxième est un photodétecteur "p+-n-n+" à base du composé ternaire Al0.17Ga0.83N. Les caractéristiques électriques densité de courant - tension et réponse spectrale monochromatique, sous différentes intensités de lumière UV en polarisation inverse et directe, ainsi que les effets de la température et les pièges sont présentées. Les résultats de simulations, ont montré que les densités de courants sous des tensions de polarisation inverse et dans l’obscurité, sont extrêmement faibles. Par contre, en illuminant les détecteurs par des rayons UV avec différentes intensités, nous avons montré que les densités de photo-courants augmentent de plusieurs ordres de grandeur, ce qui confirme l'utilité des structures étudiées pour la détection des rayons UV. D'autre part, on a constaté une réduction significative de la réponse spectrale des détecteurs étudiés, sous l'effet de la présence de certains niveaux de piégeage. Voir les détails
Mots clés : simulation, GaN, Photodétecteur, Ultraviolet, température, Réponse spectrale
Contribution à la caractérisation et synthèse de pigment de peinture à base de calamine. Evaluation de la qualité et analyse des incertitudes sur les propriétés.
The iron oxide pigment consists of 53.18% iron with a siliceous matrix. The calamine itself contains 73.83% iron. The density of the pigment and calamine is respectively ρ=3.0665 g/cm3 and ρ=5.4203 g/cm3. The pigment is of oolitic structure. Calamine has a homogeneous structure of the three iron oxides. The grain size of the compounds is between 0.6 and 40 μm with a D50 of less than 8 μm. Their specific surfaces are 1.6 and 1.5 m2/g. The calamine has an ability to keep its magnetization unlike the pigment which has a low magnetization.Simultaneous Thermal analysis shows a mass increase of 3.602% between 400 and 1000 °C for calamine and exotherm equal to 1.128 W/g. For the iron pigment, this analysis shows an overall mass loss equal to 11.05% accompanied by heat absorption equal to 1.926 W/g. For the iron pigment, this analysis shows an overall mass loss equal to 11.05% accompanied by a heat absorption equal to 1.926 W/g. Concerning the mixtures with 5, 10, 15, 20, 25 and 35% of calamine, we find a decrease of the loss of mass. The flow of heat takes a polynomial pace increasing.The SEM observations show a homogeneous structure of the calamine composed of grains of iron oxides of different sizes ranging from 1 μm to 10 μm and aggregates of small particles. As for the iron oxide pigment, it is composed of aggregates of grains more or less rounded formed of oxides of iron and gangue. The EDS analysis shows a dominance of the iron element for both materials. The optical measurement spectra show that the three compounds do not absorb any visible radiation and absorb significant fluxes in the near UV. They reflect all the incident radiation in the visible but reflect very little near UV radiation. The X-ray diffraction of calamine shows that the crystalline phases of its constitution are mixtures of wustite, magnetite and hematite. As for the iron oxide pigment, it is composed of the following crystalline phases: goethite, hematite, fayalite, silica, phosphorus pentoxide, and hausmannite. The prediction model that we developed deals with the modeling of mass loss and thermal behavior as a function of the mixing rate by a combined model of "neuronal network NN" and the MTCS simulation, which allowed us to calculate this mass loss and the corresponding energy variation. The model based on the neural network approach, by its high precision, offers us a reduced range of uncertainties compared to the linear model. Voir les détails
Mots clés : Pigment iron, scale (calamine), iron oxides, grinding, Simultaneous thermal analysis, X-ray diffraction, spectrophotometry.
Étude des propriétés structurales électroniques et thermodynamiques de l’alliage quaternaireZn1-xMgxSeyTe1-y
First-principles calculations are performed to study the structural, electronic and thermodynamic properties of Zn1-xMgxSeyTe1-yalloys using the full potential-linearized augmented plane wave method (FP-LAPW) within the density functional theory (DFT). In this approach the Perdew-Burke-Ernzerhorf generalized gradient approximation (PBE-GGA) was used for the exchange-correlation potential. Moreover, the modified Becke Johnson approximation (mBJ) was also used for band structure calculations. First for the MgX binary compoundsvarious phases were considered in order to confirm the most stable one and to predict the transition pressure between different phases. The lattice constant for theternary alloys exhibits a small deviation from the Vegard's law. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The bowing of the fundamental gap versus composition predicted by our calculations is in good agreement with available theoretical data. In addition, we have studied the thermal properties of these alloys using the Debye model implemented in Gibbs program. Finally, the energy band gap of Zn1-xMgxSeyTe1-y quaternary alloys lattice matched to InAsandZnTesubstrats was investigated. To our knowledge this is the first quantitative theoretical investigation on Zn1-xMgxSeyTe1-y quaternary alloys and still awaits experimental confirmations. Voir les détails
Mots clés : Zn1-xMgxSeyTe1-y Quaternary alloys, Functional density (DFT), Approximation of the generalized gradient (PBE-GGA), mBJ approximation, InAs and ZnTe substrates