The effect of porosity on the residual thermal stresses generated on cooling from the fabrication temperature in ceramic composites and laminates is calculated. It is suggested that the stresses can be tailored via the porosity effect on Young's modulus only, since porosity does not affect the thermal expansion behaviour of the composite constituents. Using a previously derived Young's modulus-porosity correlation, which takes into account the effects of pore shape and orientation, the thermal stresses for a number of composite systems were calculated. For matrix-type composite materials, it is shown that by controlled variation of the amount and shape of porosity in the matrix, it should be possible to obtain the desired level of internal thermal stresses, while keeping the highest possible value of the composite Young's modulus and strength. For composites with laminate structure, both porosity incorporation in one of the constituents and control of the layer thickness should lead to the optimization of laminate design.

The nucleation of perfect or partial dislocations from a surface-step of a f.c.c. or diamond-like material is frequently observed. In a recent paper, it is shown how the "stress—stacking-fault energy" plane can be divided into three zones where a partial or a complete dislocation will or will not nucleate. In compound semiconductors, dislocations can dissociate into α or β partial dislocations with mobilities appreciably different. In this paper, this effect is taken into account and yields to large modifications in the partial or perfect dislocations nucleation conditions. The case of GaAs is specially examined.

Les propriétés élastiques dans certains systèmes multicouches de faible période présentent des anomalies qui sont généralement associées à des modifications des propriétés structurales de chacune des couches. Ainsi, plusieurs études ont mis en évidence par diffraction des rayons X des déformations de même signe dans le plan et selon la normale au plan des couches déposées. Certains auteurs ont alors proposé dans le cas de systèmes métalliques W/Ni et Nb/Cu l'utilisation d'un coefficient de Poisson négatif. Ce résultat est surprenant car la valeur de ce coefficient pour les métaux est généralement positive. Dans cet article, nous présentons une nouvelle interprétation reposant sur la détermination expérimentale par la méthode des sin2Ψ de la référence (paramètre libre de contrainte) servant de base au calcul des déformations. Nous montrons alors que l'introduction du paramètre du matériau massif comme référence est une hypothèse irréaliste dans le cas des films minces et multicouches (W et Ag/Ni) qui peut conduire à des résultats erronés et en total désaccord avec ceux obtenus par d'autres techniques. Ainsi, l'anomalie élastique au niveau du coefficient de Poisson soulevée par certains auteurs dans la littérature n'est pas imputable à la structure elle-même mais à l'analyse qui est faite des données expérimentales obtenues par diffraction des rayons X.

Admittance spectroscopy has been performed on Al dots deposited on porous Silicon grown on p+ substrates. They exhibit d.c. current-voltage characteristics of a metal-insulator-semiconductor structure containing a large concentration of localized states in the insulator. The admittance is composed of a frequency (ω) independent conductance while the reactance varies as ω s with s = 1. This strongly suggests that conduction through porous Silicon layers is governed by hopping between surface states.

Dc conductivity measurements for Z-cut natural quartz plates, at different temperatures and values of the applied electric field were made. The evolution of I−V characteristics shapes is studied for electrodiffusion times up to 100 h (in interstitial alkali ions transport region). The non-ohmic behavior is discussed and is considered to be due to the space charge produced by the uncompensated Al3+ ions substitutional for Si4+.

Après nos études concernant l'influence du dopage en terres rares sur les propriétés supraconductrices du binaire Mo3 Se4, nous avons étendu ces recherches à d'autres cations, en particulier aux alcalino-terreux et à d'autres éléments tels que Pb, Sn, In, Cu, Ag et U. Nos résultats montrent que la température critique du binaire (T c = 6,45 K) croît avec la concentration en cation, ceci d'autant plus que la taille du cation est importante. Toutefois, il semble exister une valeur limite pour la température critique, celle-ci n'ayant jamais dépassé 7,1 K. Seule une variation de la position du niveau de Fermi pourrait être à l'origine d'une telle augmentation car aucune influence directe du magnétisme du cation n'a été observée.

By scratching the (0001)Si surface of 6H-SiC followed by annealing, dislocations were introduced in the crystal that were subsequently characterized by Transmission Electron Microscopy (TEM). Schottky diodes were then manufactured from the dislocated crystal and their electrical properties were studied by capacitance-voltage (C−V), current-voltage (I−V), and thermally-stimulated capacitance (TSCap) measurements. It was found that the deformation introduces deep traps Maynly located in the upper third of the bandgap promoting a significant increase in the series resistance of the diodes. The as-introduced dislocations were predominantly 30° partials and their core nature was determined to be Si(g) by the technique of Large Angle Convergent Beam Electron Diffraction (LACBED). The compensation effects observed after deformation are presumed to be caused not only by Si(g) dislocations but also by the other defects generated during the deformation step.

L'étude de la signature acoustique permet de mesurer les vitesses de propagation des ondes acoustiques longitudinale et transverse dans les matériaux massifs, vitesses à partir desquelles on détermine leurs propriétés mécaniques. Pour des matériaux en couche mince, ces vitesses ne sont plus mesurées directement Mays calculées à partir des vitesses accessibles expérimentalement : celles des modes de Lamb généralisés. Ces modes ont des vitesses dispersives et leur mesure dépend du liquide servant de couplant, dans lequel la couche est immergée. Dans cette étude, les liquides de couplage (méthanol, eau et solutions aqueuses de iodure de potassium KIaq. 6 M et d'hydroxyde de potassium KOHaq. 12 M) sont choisis en fonction de leurs paramètres acoustiques. La modélisation de la signature acoustique permet de prévoir leur influence respective, comment ils vont favoriser la détection des modes rapides et, d'une façon générale, augmenter la précision sur la mesure de la vitesse de tous les modes accessibles. Les études expérimentales confirment que la caractérisation de chaque mode peut être optimisée par le choix d'une part de la fréquence et d'autre part du liquide de couplage en fonction de sa vitesse, sa densité et son atténuation.

Silver sulphide Ag2 S thin films have been prepared on pyrex glass substrates by the spray pyrolysis technique using silver acetate and thiourea as starting materials. The depositions were carried out in the range of substrate temperatures from 100 to 300 °C. Measurements of X-ray diffraction show that the deposited layers at 250 °C were well crystallized and oriented preferentially in the direction $(\bar{1}03)$ . Films surfaces were analysed by contact Atomic Force Microscopy (AFM) in order to understand the effect of the deposited temperature on the surface structure. On the other hand, from transmission and reflection spectra, the band gap energy determined is about 1.14 eV. In the same way, an iterative method based on the optical reflectance and transmittance of Ag2S thin film prepared at 250 °C were used to obtain the refractive index and the extinction coefficient. Their values vary in the wavelength range 640-2200 nm from 2.38 to 2.81 and from 0.001 to 0.277 respectively.

L'objectif final de cette étude est de réaliser un transformateur impulsionnel supraconducteur aux performances proches de celles calculées pour une transition instantanée du primaire supraconducteur. Nous avons construit un transformateur torique de 500 J, associé à un système de déclenchement destiné à provoquer une transition massive de l'enroulement primaire. Cet article décrit les étapes ayant conduit à la réalisation de ce transformateur et de son système de déclenchement.Lors d'une transition réelle, des points chauds et de fortes tensions entre spires peuvent apparaître au sein de l'enroulement primaire. Pour notre transformateur de 500 J, le calcul de ces tensions a montré qu'elles peuvent dépasser les 3 kV entre spires de couches adjacentes. La connaissance des vitesses de propagation de fronts résistifs a permis de montrer que la température maximale pouvant être atteinte en un point du primaire est de 80 K. Ce point correspond au départ d'une zone résistive supposée être la seule à se développer.Nous avons étudié l'influence de la transition réelle du primaire sur l'impulsion générée dans le secondaire. Pour la charge que nous avons choisie, une transition de 50 % du primaire est suffisante pour obtenir une impulsion de courant très proche de celle calculée dans l'hypothèse d'une transition instantanée du primaire. Nous espérons obtenir une transition complète du primaire grâce à notre système de déclenchement. Ce dernier est constitué de cinq chaufferettes placées sous chacune des couches du priMayre. Alimentées en parallèle par une décharge capacitive très brève (< 40 µs), elles permettent d'envisager un début de transition sur chacune des spires du priMayre 3,5 ms plus tard.

The thermal behaviour of a sample submitted to a sinusoidal optical irradiation or a sinusoidal electronic irradiation is modelled in axisymmetrical two-dimensional geometry. This modelling mixes an integral transform and a one-dimensional thermal harmonic response. It shows the spherical thermal behaviour of the harmonic heat diffusion. The same solving method is applied to the Navier-Stokes equation using one-dimensional acoustic harmonic responses. The latter solution leads to the calculation of the displacement field. Finally, the two heat sources are compared from thermal and thermoelastic pointsof view.

Al2+ (320 keV) ion implantations at room temperature for doses ranging from 3 × 1011 to 3 × 1014 ions cm−2 have been performed in p-type Hg0.78Cd0.22Te. The samples were analyzed by infrared reflectivity providing quantitative results on electrically active defects. A change of type of the free carriers is observed after implantation, for all doses, in a region of thickness greater than the projected range of ions. The dominant phenomena in this change of type seems to be the diffusion of Hg interstitials which takes place during implantation. From 3 × 1011 to 3 × 1013 ions cm−2, the diffusion length of Hg interstitials follows a classical equation with a linear dependence on the squareroot of the implantation time. For doses greater than 3 × 1013 ions cm−2, Hg interstitials diffuse less efficiently. This could be due to the capture of some single interstitials by interstitial dislocation loops as observed by diffuse X-ray scattering.

Généralement, les matériaux métalliques présentent, particulièrement à hautes températures, en complément des propriétés visqueuses, des effets de restauration qui s'opposent à l'écrouissage du matériau. Par hautes températures nous entendons une température telle que T ≥ 0,4 T M et où T M est la température de fusion de l'alliage consideré. Pour l'alliage AlMgSi1 cette limite sera partiellement dépassée dans le domaine de température examiné, soit : 25 < T ≤ 200 °C. Ce document présente les incidences de la viscosité associée à la restauration statique sur le comportement inélastique de l'alliage AlMgSi1. On présente un modèle viscoplastique basé sur un potentiel d'écoulement dit de transition (PET) capable de décrire les observations expérimentales reportées. Le point de départ est le modèle viscoplastique de Chaboche [1]. Finalement, l'ensemble des résultats expérimentaux sont comparés qualitativement et quantitativement aux prévisions issues du modèle.

Multilevel converters appear to be one of the most interesting solutions able to solve problems linked to high voltage applications. Various solutions exist to realise this type of converter. One constraint appearing on these converters is the balance of the voltages across capacitors used as DC voltage sources. This balance can generally be obtained by an appropriate control. This paper proposes a balance control which can easily be added to the usual process. An application to a series-association of voltage source inverters operating as an active filter is developed. The balance control is presented, simulations and experimental results validate its efficiency.

Solid state phase equilibria in the ternary Fe-Ga-As diagram were determined at 600 °C using powder X-ray diffraction as experimental techniques. Very limited solid solubilities were measured in the binary constituent Fe-Ga and Fe-As compounds. In the Fe-rich part of the diagram, a ternary phase was evidenced which corresponds in fact to a solid solution into which Ga and As substitute one another on the same sublattice. This phase, which can be expressed by the general formula Fe3Ga2−x Asx (0.20 ≤ x ≤ 1.125), crystallizes in hexagonal symmetry; it is structurally derived from the NiAs type-structure (B81) and can be considered, owing to its c/a ratio (1.23 ≤ c/a ≤ 1.28) close to $\sqrt{3}/\sqrt{2}$ , as hexagonal-pseudocubic. The original feature of this experimental diagram is the occurrence of a tie-line between this ternary phase and the semiconductor GaAs which differs significantly from the theoretical diagram, estimated from simplified calculations proposed by Schmid-Fetzer. In particular, it is the first time that such a ternary phase Mx (Ga, As)y (M = transition metal) is found to be in thermodynamic equilibrium with GaAs. This fact suggests that the solid state interdiffusions which occur during the annealing of a Fe/GaAs heterostructure could lead to a ferromagnetic, epitaxial and stable Fe3GaAs/GaAs contact.

Nous proposons une approche pour synthétiser automatiquement le modèle à topologie variable des convertisseurs statiques (modèle discret idéal des semi-conducteurs), principalement en vue de leur propre simulation et notamment celle des entraînements électromécaniques.

In this paper we propose a new method of data analysis to interpret results obtained by Thermally Stimulated Depolarisation Current (TSDC) measurements on glassy materials. We give a new law to link the evolution of the relaxation time τ with the temperature T.While the classical law (compensation law) is only convenient in a small range of temperatures (around the compensation temperature T c), a calculation of the depolarisation current, taking into account this new law, leads to a curve I vs. T very close to the real depolarisation current measured on a polymer in the whole range of temperatures.In addition this new law allows the determination of the compensation temperature T c from the complex spectrum which needs single scan of measurements. To our knowledge it is the first time that this determination is proposed from the complex spectrum.

Light emission in the Au/Au junction of a scanning tunneling microscope has been detected in air and the property of the control of emission yield has also been evidenced. Experimental results presented in this paper clearly show that this property can still be observed on mineral oil-covered gold surfaces.

Formation conditions, optical nonlinear properties and applications of F2 − color centers in LiF monocrystals are investigated. The efficiency of F2 − centers generation depending of irradiation conditions and content of OH− impurities of the LiF monocrystal are also presented. Thermal stability of the centers generated by electron irradiation was studied. It was measured the variation of absorption factor of the LiF: F2 − crystal in the −40 °C to 60 °C temperature range. The LiF: F2 − crystals obtained by an efficient method of irradiation with electrons generated by a linear accelerator were employed in Q-switching a Nd: YAG laser cavity with output energies of 18 mJ and fluctuations less than 5% in the −40 °C to 60 °C temperature range.

We study the limitation possibilities of bulk Bi high T c materials. For this we test these materials with a.c. or d.c. currents above their critical currents. We study particularly the evolution of the voltage with time or with current. The material, the value of the current and the time duration play important parts. For sintered Bi samples the voltage depends only on the current even for values much larger than the critical current. With textured samples the V(I) curves shows an hysteretic behaviour due to a warming up. The textured materials are more interesting than sintered ones in terms of required volume for the current limitation. In both cases the superconductors are in a dissipative state but not in the normal state. This state is nevertheless reached if the dissipated energy inside the sample is sufficient. We have tried to apply a magnetic field on the samples in order to trigger a more effective limitation. The voltage increases but with a limited effect for currents much higher (3–4 times) than the critical zero field current. We think that the dissipative state is due mainly to the grain boundaries which become resistive above the critical current.