Sciences des matériaux

Nombre total de résultats : 417
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Improving the sensitivity of Raman signal of ZnO thin films deposited on silicon substrate

A. Hammouda, A. Canizarès, P. Simon, A. Boughalout, M. Kechouane  (2012)
Publication

Raman spectroscopy results of DC reactive sputtering of ZnO thin films deposited on silicon substrates are reported. Monocrystalline silicon wafers are frequently used as substrates for thin film deposition, and when working on thin films materials of rather low Raman cross-sections such as ZnO, it is difficult and even sometimes hopeless to extract the Raman information relevant to the film from the silicon signal.In this paper, we show that using a 457 nm wavelength of the laser excitation combined with the effect of silicon Raman selection rules through convenient orientation of the silicon substrate, the 520 cm-1 Si signal can be sufficiently attenuated to give access to the ZnO Raman information, whatever the important difference of intensity between them. Voir les détails

Mots clés : Raman spectroscopy, Thin films, Silicon, Zinc oxide, DC reactive sputtering deposition

CHARACTERIZATION OF THE CRITICALLY REFRACTED LONGITUDINAL (LCR) WAVES AND THEIR USE IN DEFECT DETECTION

Wahiba Djerir, Mohamed Ourak et Tarek Boutkedjirt  (2014)
Publication

The critically refracted longitudinal (LCR) waves are used in various domains of nondestructive evaluation, especially for residual stress measurements. However, few works have characterized the associated ultrasonic beam. In this article, the characterization of the LCR ultrasonic beam, both numerically and experimentally, is first clarified in order to provide some answers to questions that arise about its behavior in elastic solids. The aim of the second part of this work is to investigate the use of the LCR waves for the detection of surface defects of different sizes in aluminum. For that, the effect of defects at fixed depth (5 mm) with various diameters (2, 4, 6, and 8 mm) and fixed diameter (8 mm) with various depths (5, 7, and 10 mm) in an aluminum sample have been investigated at frequencies around 1 MHz. It has been experimentally found that the amplitude of the (LCR) wave decreases and its frequency spectrum changes with a given defect. The rate of the decrease of the amplitude and the change in the spectrum is related to the increase of the defect depth. The study shows that the effect on the propagating of the LCR waves is larger as the defect depth increases.The critically refracted longitudinal (LCR) waves are used in various domains of nondestructive evaluation, especially for residual stress measurements. However, few works have characterized the associated ultrasonic beam. In this article, the characterization of the LCR ultrasonic beam, both numerically and experimentally, is first clarified in order to provide some answers to questions that arise about its behavior in elastic solids. The aim of the second part of this work is to investigate the use of the LCR waves for the detection of surface defects of different sizes in aluminum. For that, the effect of defects at fixed depth (5 mm) with various diameters (2, 4, 6, and 8 mm) and fixed diameter (8 mm) with various depths (5, 7, and 10 mm) in an aluminum sample have been investigated at frequencies around 1 MHz. It has been experimentally found that the amplitude of the (LCR) wave decreases and its frequency spectrum changes with a given defect. The rate of the decrease of the amplitude and the change in the spectrum is related to the increase of the defect depth. The study shows that the effect on the propagating of the LCR waves is larger as the defect depth increases. Voir les détails

Mots clés : critical angle, critically refracted longitudinal (LCR) wave, main lobe, NDE, NDT, surface defect

Investigation properties of Au – Porous a-Si0.60C0.40as Humidity sensor

Assia Boukezzata, Aissa Keffous, Ghania Nezzal, Noureddine Gabouze, Mohamed Kechouane, Kahina Zaafane, Amirouche Hammouda, Patrick Simon, Hamid Menari  (2013)
Publication

In this letter, we report the study of a humidity sensor based on porous amorphous silicon carbide (PASiC) as a function of exposure time and relative humidity (RH). Resistive humidity sensors Au–PASiC/Si(p) were fabricated through evaporating coplanar interdigital gold electrodes. The PASiC was formed by anodization of a-Si0.70C0.30 in HF/ethylene glycol (ETG) solution at current density of 50 mA/cm2 for 30 s. The structural properties of a-Si0.70C0.30 thin films deposited by DC magnetron co-sputtering using single silicon crystal, 6H-SiC targets and the PASiC films were investigated by FT-IR and Raman spectroscopy. Hygroscopicity was studied by measuring the resistance of the PASiC under various RH. The results show that the measured resistance highly depends on the applied bias voltage. Moreover, the response signal against RH is found linear for an applied voltage of 2 V. Finally, the response and recovery times were determined around 13 s and 20 min, respectively. Voir les détails

Mots clés : Porous SiC, Electrochemical etching, RH sensors, Au electrodes

Aluminium-induced crystallization ofamorphous silicon films deposited byDC magnetron sputtering on glasses

F. Kezzoula, A. Hammouda, M. Kechouane, P. Simon, S.E.H.Abaidia, A. Keffous, R. Cherfi, H. Menari, A. Manseri  (2011)
Publication

Amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) films were deposited by DC magnetron sputtering technique with argon and hydrogen plasma mixture on Al deposited by ther- mal evaporation on glass substrates. The a-Si/Al and a-Si:H/Al thin films were annealed at different temperatures ranging from 250 to 550 ◦C during 4 h in vacuum-sealed bulb. The effects of annealing temperature on optical, structural and morphological properties of as-grown as well as the vacuum- annealed a-Si/Al and a-Si:H/Al thin films are presented in this contribution. The averaged transmittance of a-Si:H/Al film increases upon increasing the annealing temperature. XRD measurements clearly evi-dence that crystallization is initiated at 450°C. The number and intensity of diffraction peaks appearing in the diffraction patterns are more important in a-Si:H/Al than that in a-Si/Al layers. Results show that a-Si:H films deposited on Al/glass crystallize above 450 ◦C and present better crystallization than the a-Si layers. The presence of hydrogen induces an improvement of structural properties of poly-Si prepared by aluminium-induced crystallization (AIC). Voir les détails

Mots clés : Crystallization, Thin films, Hydrogenated amorphous silicon, AIC, Raman, XRD

Application of Inverse Methods for Spatial Deconvolution of Pulsed Ultrasound Fields Radiated in Solids

W. Djerir, T. Boutkedjirt, A.B. Bouda and A. Satour  (2010)
Publication

When measuring the ultrasound field, the signal provided by the receiving transducer is affected by its spatial properties. Particularly, the displacement normal to its surface is spatially averaged because of the receiver finite size. In this study, we show using a numerical simulation, the effectiveness of the spatial deconvolution of these effects for a rectangular transducer. For that, three methods allowing the inversion of the aperture effect are tested 1) Wiener’s method; 2) the power spectral equalization (PSE) method, and 3) the maximum a posteriori (MAP) method. The obtained results show that the three methods are able to reconstruct the ultrasound field from the spatially averaged values and the quality of the reconstruction depends strongly upon the signal to noise ratio (SNR) and the spatial frequencies of the ultrasound field investigated. Voir les détails

Mots clés : Deconvolution, Spatial filter, Wiener filter, Power spectral equalization method, Maximum a posteriori method.

Comparison of Simulation Methods of Pulsed Ultrasonic Fields Radiated in Isotropic Solids

W. Djerir, T. Boutkedjirt and A. Badidi Bouda  (2009)
Publication

The modelling of pulsed ultrasonic fields radiated by transducers in materials of various types is of particular interest in industrial applications of non destructive testing. Knowledge of the ultrasonic field allows defining and optimizing the control conditions. For the calculation of harmonic and pulsed ultrasonic fields, various methods have been used such as exact, semi-analytical, asymptotic and purely numerical methods. In this work, these methods are reviewed for the case of a planar and circular ultrasonic source coupled to a semi-infinite isotropic solid. In the second part, the solution of a semi-analytical method, which uses particular approximations, is compared to that of an exact one. The numerical calculations achieved by using the exact method are very tedious but allow the interpretation of physical phenomena involved. In contrast to that, the semi-analytic one permits the calculations to be undertaken in a more reasonable time. However, Rayleigh waves and head waves are not revealed by this method. Voir les détails

Mots clés : Ultrasound, NDT, Pulsed field, Isotropic Solid, Transducer.

Characterization of Grains Size by Ultrasounds

A. Badidi Bouda, M. S. Aljohani, A. Mebtouche, R. Halimi, W. Djerir  (2009)
Publication

A mechanical or thermal treatment of a material can change, among other things, the average grains size. It depends on temperature, holding time, cooling condition or rolling stress. The average grain size, as well as its influence on the propagation velocity and attenuation coefficient of ultrasonic waves, also affects the wave frequency propagating through the material. Grain size is an indicator of material fatigue. It can therefore be used in monitoring or fatigue damage prevention. In this paper, we study the effect of various heat treatments hence different steel average grain sizes on the ultrasonic wave frequency after crossing the material. We have performed the same experimental study on aluminum samples. The different grain sizes are obtained by rolling. The frequency shift measurement of longitudinal waves is achieved by immersion with two probes of different frequencies 2.25 and 5 MHz. The experimental results are shown as curves giving the frequencies depending on the grain size. Heat treatments on steel and aluminum rolling performed on the samples have yielded a grain sizes gradient. Our results are consistent with the theory because of the important path in the sample and in this case the down shift frequency is paramount. They show a direct relationship between the frequency shift and the average grain size. It is therefore possible to trace quantitatively to an average grains size from the frequency of an ultrasonic wave that has passed through this material and hence its thermal or mechanical fatigue state. Voir les détails

Mots clés : Attenuation, Frequency Spectrum, Grain Size, Scattering, Ultrasound (US)

Acoustic emission signal denoising to improve damage analysis in glass fibre reinforced composites

Abida Satour, Silvio Montrésor, Mourad Bentahar, Rachid Elguerjouma and Fouad Boubenider  (2014)
Publication

In this contribution, continuous wavelet denoising technique is developed to enhancethe signal to noise ratio of acoustic emission (AE) signals. The time– frequency characteristics of the recorded noise are first determined by calculating the noisewavelet coefficients. Then, an algorithm able to proceed denoising by extracting noise features from those corresponding to AE signals is developed. The denoising procedure is applied to get meaningful signals when the detection threshold is decreased from 30 dB to 25 dB. Furthermore, the spectral flatness measure was applied to eliminate signals originating from noise. Experiments were conducted on cross-ply composite specimens [±45°]6S to examine the efficacy of the proposed technique. The denoised signals were analysed using the continuous wavelet transform and were also analysed in terms of the dominant frequency band which was processed by a fast Fouriertransform. Results show that denoised low-amplitude AE hits are well correlated withhigh-amplitude AE hits (A≥30 dB) in terms of damage characterisation. Voir les détails

Mots clés : acoustic emission, polymer-based composite, Denoising, Signal processing, damage characterisation

Use of Guided Waves for Inspection of Composite Skin-Honeycomb Core

A. Satour, F. Boubenider, A. Badidibouda, R.Halimi  (2010)
Publication

Sandwich plates, made of two aluminum layers and a metal honey comb core are used inthe aircraft industry. The purpose of this study is to show the ultrasonic guided waves sensibility to discover delamination in skin-honeycomb sandwich structures used in aeronautics. Separation between the skin and the core can appear during the manufacture or after use. In this work, Lamb's waves are used to control this kind of plane structure. Indeed, these waves have the advantage to putin vibration the totality of the plate that we want to control and they propagate on long distances without too much attenuation. The revealing, by the guided waves, of the unsticking which canmeet on such sandwiches, between the plate and the honeycomb core is studied and commented. Voir les détails

Mots clés : Lamb waves, Sandwich structure, Dispersion curves, Displacements.

Effect of the heat treatment on the microstructural evolution of the Nickel based superlloy

Mohamed Retima, Bouyegh Saida, Hacène Chadli  (2011)
Publication

The effect of heat treatment on the microstructure of cobalt-rich nickel based superalloy was studied applying scanning electron microscopy (SEM) and transmission electron microscopy (TEM).  The aim of the present work was to investigate the formation and evolution of different phases during the heat treatment of superalloy similar to Udimet 500. The presence of a relatively high volume fraction of γ’ particles in the γ matrix suggests on inefficient cooling rate during oil quenching from 1150°C. Carbides such as MC primary carbides of the type TiC and MoC as well as secondary carbides M23C6 (Cr23C6) were found in grains and at grain boundaries. Voir les détails

Mots clés : superalloy; heat treatment;, SEM and TEM; γ' particles; carbides .