Liste des documents
Theoretical prediction of the structural, electronic,and thermal properties of Al1-xBxAs ternary alloys
First-principles calculations are performed to study the structural, electronic, and thermalproperties of the AlAs and BAs bulk materials and Al1-xBxAs ternary alloys using the fullpotential-linearized augmented plane wave method within the density functional theory.The structural properties are investigated using the Wu–Cohen generalized gradientapproximation that is based on the optimization of total energy. For band structurecalculations, both Wu–Cohen generalized gradient approximation and modified Becke-Johnson of the exchange-correlation energy and potential, respectively, are used. Thedependence of the lattice constant, bulk modulus, and band gap on the composition x wasanalyzed. The lattice constant for Al1xBxAs alloys exhibits a marginal deviation from theVegard's law. A small deviation of the bulk modulus from linear concentration depen-dence was observed for these alloys. The composition dependence of the energy band gapwas found to be highly nonlinear. Using the approach of Zunger and coworkers, themicroscopic origins of the gap bowing were detailed and explained. The quasi-harmonicDebye model was used to determine the thermal properties of alloys up to 500 K. Voir les détails
Mots clés : Band structures, First principle calculations, Ternary alloys, Thermal Properties
Inverse problem for material analysis by ultrasound
Internal defects detection by ultrasound non destructive testing is widely used in industry. Ultrasonic time signal data are diffi cult to interpret since they require continuous signal analysis for each point of the piece. Inverse problem in materials analysis puts some challenges because the composition variables are both discrete and continuous and because the engineering properties are highly nonlinear functions. In this paper we address the non linear features of back scatted ultrasonic waves from steel plate, for understanding its micro structural behaviour. The experiments show a challenging interface between material properties, calculations and ultrasonic wave propagation modelling Voir les détails
Mots clés : PACS: 43.25.Hg ; 43.35.Zc ; 43.60.Hj ; 43.60, Multiscale analysis • Structural noise • Ultrasonic NDT/NDE • Wavelet
Numerical investigations of ultra wide-band stacked rectangular DRA excited by rectangular patch
In this paper, a numerical study of a new ultra wideband (UWB) dielectric resonator antenna (DRA) is presented. The proposed structure consists of two stacked dielectric resonators excited by rectangular patch and operated from 3 GHz to 11 GHz (an impedance bandwidth of 115%), covering the full UWB spectrum. The analysis is carried out using the Finite Difference Time Domain (FDTD) method and two commercial electromagnetic simulators. The numerical results are given and compared in terms of reflection coefficients, radiation pattern and gain. The computed FDTD results are in good agreement with those of simulations. Voir les détails
Mots clés : numerical analysis, Finite Difference Time Domain (FDTD) method, stacked dielectric resonators
Experimental Investigations of Ultra-Wideband Antenna Integrated with Dielectric Resonator Antenna for Cognitive Radio Applications
In this paper, an ultra-wideband (UWB) patch antenna integrated with a dielectric resonator is proposed for cognitive radio applications. The patch antenna is fed by a coplanar waveguide (CPW) line, and it consists of a rectangular monopole having an elliptical base, and operates from 2.44 to 12 GHz. This UWB antenna is intended to collect the information. Moreover, the proposed structure integrates a narrow-band rectangular dielectric resonator antenna (RDRA) for operation, with very good isolation between the two ports (transmission coefficient S21 less than -20 dB). The RDRA provides a bandwidth from 5.23 GHz to 6.11 GHz. The electromagnetic analysis is carried out using tow commercial software tools. Furthermore, to validate the proposed concept, experimental measurements are also performed. Voir les détails
Mots clés : Integrated UWB/NB antenna, Cognitive radio applications
Prediction of the conversion efficiency of a GaSb thermophotovoltaic converter heated by radioisotope source
In recent years, Gallium Antimonide (GaSb), which has smallest bandgap among III-V semiconductors family, became the subject of extensive investigations in the field of thermophotovoltaic (TPV) converters, because of the recent improvements in optoelectronic technology. In this paper, we investigated the heat to electricity conversion efficiency of a GaSb radioisotope thermophotovoltaic (RTPV) converter, taking account of the photons with energy below the cells bandgap using a comprehensive analytical process. The results show that a conversion efficiency greater than 28% can be obtained for radiator’s temperature of 1600k, at ambiant temperature. This efficiency will decrease as the cell temperature increase. Voir les détails
Mots clés : Radioisotope, Emissivity, Thermophotovoltaic, Efficiency, Temperature
Analysis and optimization of In1-xGaxAsySb1-y thermophotovoltaic cells under low radiator temperatures
In this paper, we investigated the heat to electricity conversion efficiency of In1-xGaxAsySb1-yradioisotope thermophotovoltaic (RTPV) converter with x=0.8 and y=0.18, taking account of the photons with energy below the cells bandgap using a comprehensive analytical process. This was done with a computer program designed for this reason, which allowed the computation of the cell performance under a variety of specified incident radiation spectra as well as a variety of material parameters. The results show that for an emissivity value of 0.78, a cell thickness of about 7µm with low front recombination velocity(700cm/s),a conversion efficiency greater than 29% can be obtained for radiator’s temperature of1300°k at ambient temperature. This efficiency will decrease as the cell temperature increase. Voir les détails
Mots clés : Radioisotope, Emissivity, Recombination Velocity, Efficiency, Temperature
Structural, optical and electrical properties of TiO2 thin films synthesized by sol–gel technique
The influence of annealing temperature on the structure, optical and electrical property of TiO thinfilms with (101) preferential orientation were deposited on glass substrates by sol–gel technique has beenstudied. As-deposited films were amorphous, and the XRD studies showed that the formation of anatase phasewas initiated at annealing temperature close to 400 °C. The grain size of the film annealed at 550 °C was about22 nm. The transmission spectra, recorded in the UV visible range reveal a relatively high transmissioncoefficient (~70%) in the obtained films. The transmittance data analysis indicates that the optical band gap (E)is closely related to the annealing temperature, an indirect band gap ranging from 3.43 eV to 3.04 eV wasdeduced. The electrical resistivity measurement that were carried out in function of the annealing temperature showed a sharp decrease in resistivity was found to be 0.0802 .cm.2g Voir les détails
Mots clés : TiO2Thin films, Annealing, DRX, anatase, Optical property, resistivity
Improvement of photoelectrochemical and opticalcharacteristics of MEH-PPV using titaniumdioxide nanoparticles
The use of bulk heterojunctions can increase the ef?ciency of exciton dissociation inpolymer-based photovoltaics. We prepared and characterized bulk heterojunctions ofpoly[2-methoxy-5-(20-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and titaniumdioxide nanoparticles deposited by spin coating on indium tin oxide substrates. Thesurface morphology of the MEH-PPVþTiO2 composite ?lms revealed that addition ofTiO2 nanoparticles increased the ?lm roughness. The effect of TiO2 nanoparticles on thephotoelectrochemical and optical characteristics of MEH-PPV polymer heterojunctionswas studied. Addition of TiO2 nanoparticles improved the absorbance of MEH-PPVcomposite ?lms. Moreover, the photocurrent of the composite devices increased withthe TiO2 nanoparticle concentration. These observations provide an insight into newapproaches to improve the light collection ef?ciency in photoconductive polymers Voir les détails
Mots clés : Organic–inorganic compounds MEH-PPV Morphology Photoelectrochemical properties
Morphology and Photoelectrochemical Characterization of MEH-PPV/PCBM Composite Film Doped with TiO2 Nanoparticles
Poly[2-methoxy-5-(20-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and titanium dioxide (TiO2) nanoparticles (n-type) were dissolved, mixed and deposited by physical methods (spin-coating) on indium tin-oxide (ITO) substrate. The incorporation of the titanium dioxide nanoparticles changed the morphology and increased the roughness of polymers film (MEH-PPV/PCBM), and the photocurrent density of the composite (MEH-PPV/PCBM +n-TiO2) was higher than that of single MEH-PPV/PCBM film. The study showed that the presence of n-TiO2 particles in the polymeric film improves the photoelectrochemical properties of MEH-PPV/PCBM composite. Voir les détails
Mots clés : Photocurrent density, Electrochemical property, Hybrid coating, Conducting Polymer, titanium dioxide, Organic nanostructures
Correlation between structural and optical properties of SiO2/TiO2multibilayers processed by sol-gel technique and applied toBragg reflectors
SiO2 and TiO2 thin layers processed by sol-gel technique have been deposited,alternatively, on glass substrates and Si (111) wafer. Dip-coatedmultibilayerswere characterized by different experimental techniques:XRD,SEM, FTIRandUV-VIS-NIR. The obtainedX-ray diffraction patterns analysishave shown that our films crystallize in anatase and rutile phases whateveris the number of bilayers and the corresponding grain sizes increasefrom 5.48 nmto 16.11 nm. The SEMmicrograph shows that our layers arehomogeneous without any visual cracks. The FTIR spectra have shownthat the vibration of Si-O-Ti bonds becomes intense by the increase in thenumber of bilayers. This increase, on the on hand, decreases the transmissioncoefficient from 4.58% to 0.55% and increases the width of the stopband shown in UV-VIS-NIR spectra. On the other hand, the band-gap decreasesfrom3.73 eV to 3.59 eV. In addition, a pseudo band-gap is locatedbetween 300 nmand 400 increasing from1.76 eVto 2.29 eV.? 2013 Trade Science Inc. - INDIA Voir les détails
Mots clés : Sol-gel;, Anatase;, Rutile;, Stop band;, Si-O-Ti;, Band-gap.