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.

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+.

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.

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.

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.

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.

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.

Le but de cette étude est de mettre en évidence le comportement non linéaire des générateurs électrochimiques secondaires soumis à des sollicitations impulsionnelles de charge ou de décharge de grande amplitude. L'influence de la création de composés gazeux sur l'impédance de la batterie est examinée. En second lieu, cette non-linéarité au niveau de la réponse à l'impulsion est exploitée pour détecter l'état de charge de la batterie. Les informations issues de la réponse à diverses sollicitations impulsionnelles en courant (différentes durées, différentes amplitudes, en charge ou en décharge) sont fusionnées, afin de détecter l'état de charge de l'accumulateur. Une méthodologie pour prendre en compte le régime de décharge est exposée dans l'article (cas des accumulateurs plomb-acide). Une réalisation expérimentale permettant la détection de l'état de charge d'un accumulateur au plomb est présentée.

The characteristic features of a dc discharge generated between parallel plate electrodes and especially the discharge stabilization by the GaAs semiconducting cathode in such a system are studied. The cathode was irradiated on the back-side with IR light in a particular wavelength range that was used to control the photoconductivity of the material. The semiconductor material was found to stabilize the discharge. The current-voltage and radiation-voltage characteristics of the gas discharge cell with a semiconducting cathode were obtained experimentally. An investigation of the effect of the voltage amplitude on the dynamics of transient processes in the plane semiconductor-discharge gap structure was made for explanation of the light intensity and current decay. Expressions are obtained for the photoelectric gain.

The current-voltage characteristics of a gas discharge system in parallel-plane geometry are studied. The gas discharge system with a photosensitive semiconductor cathode have been studied in a wide range of the gas pressures p (21.3−1013 hPa), inter-electrode distances d (10μm−5mm), and conductivities of the semiconductor. Gallium arsenide $(10^7 -10^8\, \Omegarm\ {\rm cm})$ has been used as the semiconducting cathode. The cathode was irradiated on the back-side with light in a particular wavelength range that was used to control the photoconductivity of the material. The semiconductor material was found to "stabilize" the discharge. When the current is increased above the stable limit, breakdown or small current oscillations begin. The filamentation was primary due to the formation of a space charge of positive ions in the discharge gap which changed the discharge from the Townsend to the glow type.

We compare high resolution roughness measurements on fractured granite, basalt and sandstone samples. For granite and basalt, the fractures display over the full experimental length scale range a self-affine geometry with a characteristic Hurst exponent consistent with the value 0.8 suggested to be universal by several authors. For sandstone, the rugosity spectrum reMayns independent on the fracturation velocity but displays self-affine characteristics only for characteristic length scales roughly larger than the grain size: furthermore, the corresponding Hurst exponent takes a value 0.47 ± 0.05 distinctly smaller than the previous one. At characteristic length scales below the grain size, a clear cutoff is observed in the spectrum. This cut-off is shown through numerical simulations to be largely explainable by the faceted shape of the sand grains and to a minor degree by the finite size of the profilometer sensor tip. None of these factors seems to influence the low frequency part of the spectrum from which Hurst's exponent is determined. The differences between the values of Hurst's exponent may reflect the intergranular nature of fracture in the sandstone samples while no effect of structural heterogeneities on fracture propagation is observed in granite.

This paper presents a contribution to the numerical simulation of the left ventricle taking into account simultaneously the fluid flow inside the cavity and the motion of the cardiac wall. One describes mathematical tools which are useful for such a problem of fluid-structure interaction. After having written the continuous mathematical problem and noted the difficulties which exist to solve such a problem, one makes two important simplifying assumptions: on the one hand, one builds a new problem which, on the mathematical viewpoint, can be distinguished from the continuous problem but which, on the physical and physiological viewpoints, can be considered as acceptable. One builds then a software which has to be considered just as a numerical investigation tool. It can be noted that it is necessary to impose, for the shape of the cardiac wall, a time-evolution. Such a bi-dimensional framework is sufficient to study the numerical aspect of this interaction, but is not sufficient to investigate the effects of major parameters governing the behavior of the left ventricle.