Les étoiles variables à longue période

Long-period pulsating stars represent the last stage in the evolution of stars with a mass in the approximate range 1 to 9 solar masses. These pulsationally-unstable stars are variable stars with large visual amplitude. These pulsations trigger shock waves propagating through the very extended atmosphere, which in the end cause a strong mass loss. Their strong mass loss, coupled with their high frequency in the Galaxy, make these stars major contributors to the interstellar matter. Because these stars are also the site of a rich nucleosynthesis, they play a key role in the chemical evolution of the Galaxy.     

Caractérisation des couches minces de Cd1–xZnxS préparées par dépôt chimique

Cd_1–xZn_xS is an important material that can be used to make n-type window-layers for thin film heterojunction solar cells. After a theoretical study of the solution chemistry of cadmium and zinc sulphates, ammonia and thiourea corresponding to sulphurs precipitation, we have been interested in the study of the effect of some parameters (temperature and zinc concentration) on the deposition process of Cd_1–xZn_xS on a glass substrate. X-ray diffraction studies showed that this latter compound crystallises in a hexagonal wurtzite crystalline structure. The AFM analysis showed that a homogenous deposited film was obtained for x = 0.1. The different values of gap energy are in the range 2,42–2,50 eV for x values between 0.08 and 0.28.     

Semimicrodosage de l'acide glycolique libéré au cours des oxydations périodiques

Résumé L'acide glycolique chauffé avec l'acide chromotropique en milieu sulfurique développe une teinte violette permettant un dosage colorimétrique régulier. Nous décrivons un protocole opératoire applicable aux milieux résultant d'une oxydation périodique.

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Summary The violet color, which develops when glycolic acid is warmed with chromotropic acid in the presence of sulfuric acid can be used for the colorimetric determination of glycolic acid. A procedure is given for applying this reaction to solutions of glycolic acid obtained on oxydation by means of periodic acid.

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Zusammenfassung Glykolsäure gibt beim Erwärmen mit Chromotropsäure in Gegenwart von Schwefelsäure eine Violettfärbung, mit deren Hilfe ihre kolorimetrische Bestimmung möglich ist. Es wird eine Arbeitsvorschrift mitgeteilt, die diese Reaktion für Glykolsäure-Lösungen anwendbar macht, die durch Oxydation mit Perjodsäure erhalten werden.

     

Interférences de deux modes propagatifs aux basses fréquences dans des conduits couplés par des perforations périodiques

A discrete model for low frequency acoustic propagation in a lattice made of two waveguides periodically coupled by perforations is presented. Firstly, it is shown that when at least one lattice mode is strongly evanescent, at any frequency, the lattice is equivalent to a simple waveguide with a locally reacting treatment on its walls (equivalent to an impedance); secondly, the resonant maxima of the insertion loss coefficient of a perforated tube muffler is due to the interference of the two modes of the lattice, which can both propagate even at low frequencies if the perforation ratio is not too large.     

Les différentes voies de modélisation macroscopique du procédé de dépôt de SiC par voie gazeuse

The different macroscopic modelling routes and chemical databases are reviewed for the growth of silicon carbide from the vapour phase in the Si-C-H-Ar system. Theses databases have been built up by experts over many years through the critical assessment of primary experimental data and ab-initio calculations. The thermodynamic modelling route addresses several important issues with respect to vapour deposition techniques. This approach is a useful tool in understanding the complex chemistry involved during the growth, but should be used with careful attention to the assumptions underlying the application. The transport modelling approach extends the previous analysis to dynamical systems. It is based on the conservation equations for momentum and heat transfer combined with mass transfer including thermodiffusion and chemical reactions based on thermodynamic and kinetic data. In addition to empirical, lumped chemical kinetic models, we propose a new modelling route linking transfers models with local thermochemical equilibrium (LTCE) computations. The modelling results have been validated with the help of the SiC sublimation technique for the transfer-LTCE concept, and of the chemical vapour deposition technique for lumped chemistry models. The simulated results allow the quantification of the different modelling proposals.     

Possibilités d'obtention de dépôts dans le système céramique ti-v-c-n à partir de sources moléculaires

Monometallic and heterobtmetallic titanium and vanadium compounds were prepared and studied as precursors to the chemical vapor deposition (CVD) of carbide and nitride ceramic thin films. Their thermal properties are discussed according to the chemical environment of the metal atom and their CVD behavior is studied. Two of them, CpTiCl₂N(SiMe₃)₂ and Cp₂VMe₂, are applied to the deposition of thin films within the Ti-V-C-N quaternary system.     

Problèmes spécifiques liés à la vaporisation des précurseurs organométalliques solides utilisés pour les dépôts dans le système V-C-N

The pyrolysis behaviours of titanocene dichloride and vanadocene used as a precursor for the chemical vapor deposition of titanium carbide and vanadium carbide, respectively, are studied. The influence of the gas-phase transport of these compounds on the properties of the deposits is also discussed according to the vaporization procedure.     

Mouvements capillaires durant le séchage d'une pâte granulaire

During the first stage of the slow drying of a liquid within a bead pack, capillary liquid transfers dominate and are governed by the pore size distribution. Deep layers made of coarse grains can be invaded first by air, and the drying rate is controlled by the characteristics of the layers close to the free surface. © 1999 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS     

Dépôt de TiC sur fibres de carbone par CVD réactive: Influence des conditions expérimentales sur les caractéristiques des fibres obtenues

Uniform coating of the filament in a carbon yarn by titanium carbide occurs by direct reaction of titanium tetrachloride vapour and hydrogen with the surface of the fibre at temperatures exceeding 1200 K. Continuous fibre treatment is achieved under atmospheric pressure. Mechanical properties derived from tensile tests on single filaments permit the experimental CVD (chemical vapour deposition) conditions to be optimized. Temperature is the decisive factor: at 1240 K and for 0.8 min treatment, high quality C(TiC) fibres are obtained (σ_R = 2965MPa). At higher temperature the tensile strength decreases. In order to explain this phenomenon, variations in size of the carbon and the titanium carbide crystallites are studied with respect to experimental conditions.     

Modification de la tenue mécanique d'un dépôt type Si — C sur acier par incorporation d'une intercouche métallique

SiC based coatings were deposited on a steel substrate by PACVD from TMS-Ar mixtures. The influence of a metallic interlayer between deposit and substrate on the mechanical stability has been studied through interface analysis and several mechanical tests (scratch, indentation, microbending in a SEM). The deposit adhesion is enhanced by a tantalum interlayer and is a function of its thickness. Micro-bending tests allowed to characterize the damage evolution and to obtain experimental parameters linked with the intrinsic properties of the coatings.     

Linéarisation et homogénéisation en viscoélasticité

Constitutive equations for non-linear viscoelastic materials are expressed first using a causal operator which relates a response function to any loading history. The corresponding tangent linear equations are then defined through the Fréchet derivative of this operator. This definition is used to propose a step-by-step treatment in view of the derivation of the overall behaviour of non-linear non-ageing viscoelastic heterogeneous materials.     

Détermination du module d'young,de la dureté et de la contrainte d'écoulement plastique de dépôts par nano-indentation

Variable residual stresses are present in the metallic coatings of tungsten and chromium manufactured by PVD and modify the quantities measured by depth-sensing indentation instruments: depth of penetration and contact stiffness for a given load. By considering the results of experiments and numerical simulations we discuss the corrections required for improving the determination of the mechanical characteristics of such coatings.     

Précurseurs métallorganiques pour le dépôt chimique à partir dune phase gazeuse de revêtements dans le système Ti-V-C-N

The various kinds of metallorganic precursors which can be used for the chemical vapor deposition of ceramic materials in the Ti-V-C-N system are reviewed. Selection criteria, and the use of one or several precursors for a selected ceramic coating are discussed. Several examples are presented to illustrate the benefits and problems resulting from such a process.     

Géométrie de l'intermittence en turbulence développée

A geometrical interpretation of intermittency in fully developed turbulence is realized through an hierarchy of fractal structures Ω_p of dimensions Δ_p linked each other by the relations Ω_{p + 1} − Ω_p (i.e. Δ_{p + 1} < Δ_p) and γ = (Δ_{p + 1} − Δ_∞)/(Δ_p − Δ_∞) with γ = ((1 + 3/√8)^1/3 + (1 − 3/√8)^1/3)³ and Δ_∞ = 1 and Δ_∞ = 1. This is obtained by the introduction of an entropy jump, defined at the scale r, ΔS_p(r) = (Δ_{p + 1} − Δ_p) ln (r/r₀) characterizing the order level of each sub-structure Ω_p and verifying a linear relation ΔS_p(r) = γ ΔS_{p − 1}(r).     

Étude des propriétés des mélanges pebd régénéré / pebd vierge

This study is a contribution to the valorization of recycled low-density polyéthyléne (LDPE). First the characterization of five recycled LDPE produced from wastes coming from different sources was performed. The physical properties (density, melt flow index), chemical structure (Fourier transform infra-red spectroscopy) and mechanical properties (tensile strength and hardness) were investigated. The effect of the ratio of virgin LDPE on these physical and mechanical properties was studied in the case of the blends of recycled LDPE / virgin LDPE.     

La Rhéoturbidimétrie

Résumé L'état d'agrégation des particules de certaines solutions colloïdales dépend de la valeur du gradient de vitesse qui règne dans ces solutions et de la durée de l'écoulement. On décrit un appareil qui permet de mesurer les variations correspondantes de la turbidité. Les solutions examinées sont placées entre deux cylindres coaxiaux dont l'entrefer constitute la chambre de mesure proprement dite. Le gradient de vitesse est imposé par la rotation du cylindre intérieur. La partie optique et électrique du rhéoturbidimétre est contenue dans un chariot mobile afin de faciliter le nettoyage et le remplissage de la chambre de mesure. Une série d'exemples relatifs à des solutions aqueuses de virus de la mosaïque du tabac et de sérum albumine de buf native ou dénaturée montre les principaux cas susceptibles de se produire selon que l'agrégation provoquée par l'écoulement est ou non réversible en fonction du gradient de vitesse.

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Zusammenfassung Der Aggregationszustand der Teilchen gewisser kolloidaler Lösungen hängt vom Geschwindigkeitsgradienten ab, der während der Dauer des Fließens in der Lösung herrscht. Ein Gerät wird beschrieben, das die entsprechenden Veränderungen in der Trübung zu messen gestattet. Die Lösungen befinden sich zwischen zwei koaxialen Zylindern, deren Zwischenraum die Meßkammer darstellt. Der innere Zylinder rotiert. Die optische und elektrische Anordnung des Rheoturbidimeters ist beweglich angeordnet, um Reinigung und Füllung der Meßkammer zu erleichtern. Eine Reihe von Relativmessungen an vorhandenen Lösungen von Tabakmosaikvirus und Rinderserumalbumin, natives und denaturiertes, zeigen die wesentlichen Fälle, die auftreten und bei denen Aggregation durch Fließen hervorgerufen wird, die reversibel oder nicht reversibel als Funktion des Geschwindigkeitsgradienten ist.

     

Etude polarographique du méthyl-phényl-dodécyl-triméthylammonium (désogène)

Rèsumè Le méthyl-phényl-dodécyl-triméthylammonium (désogène), donne une vague polarographique caractéristique, en utilisant comme électrolyte de support l'hydroxyde de tétraméthylammonium, le CILi ou le ClK.Les propriétés de cette vague ont été étudiées en fonction de la concentration du saponium. La loi d'Ilkovi est vérifiée.Deux électrons interviennent dans la réduction du saponium.

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Summary The cationic detergent désogène (methyl-phenyl-dodecyl-trimethylammonium) gives a characteristic polarographic wave in LiCl or in tetramethylammonium solution. The wave seems to correspond to a 2 electronprocess, following Ilkovi's law at low concentrations. A typical adsorption maximum, proceeded by a prewave wich does not follow the Ilkovi' law is found at higher concentrations. At high ionic strength this maximum disappears.

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Zusammenfassung Das kationische Detergens Désogène (Methylphenyl-Dodeeyl-Trimethylammonium) gibt in Lithiumchlorid- oder in Tetramethylammoniumhydroxydlösung eine charakteristische polarographische Welle. Diese scheint einem 2-Elektronen-Prozeß zu entsprechen und folgt bei niederen Konzentrationen dem Gesetz von Ilkovi. Ein typisches Adsorptionsmaximum, dem eine dem Ilkovi-Gesetz nicht folgende Welle vorangeht, findet sich bei höheren Konzentrationen. Dieses Maximum verschwindet bei hoher Ionenkonzentration.