Showing papers by "University of Bordeaux published in 1984"

SiC filament/titanium matrix composites regarded as model composites

Abstract: It is generally accepted that silicon carbide CVD-filaments are more suitable than the related boron filaments for reinforcing titanium alloys at medium temperatures Silicon carbide has a higher refractoriness and chemical inertia than boron Moreover, the SiC CVD-filaments retain a much higher fraction of their room temperature mechanical performances up to about 800 to 1000° C and could be produced in the future at a lower price Despite the fact that SiC seems to be less reactive than boron towards most metals, it nevertheless reacts with titanium and its alloys at rather low temperatures (700 to 800° C) Furthermore, silicon carbide when deposited from a gas phase (eg methyl chlorosilane and hydrogen mixtures) often contains small amounts of more reactive species such as elemental silicon or carbon In the same manner, coatings containing elemental carbon have been applied to SiC CVD-filaments in order to reduce their sensitivity to surface abrasion effects Therefore, SiC filament-titanium matrix composites must be regarded as non-equilibrium systems when they are heated at medium or high temperatures The occurrence of chemical interactions between SiC and titanium in the solid state, controlled by diffusion, has already been established either from experiments performed on diffusion couples with a plane interface [1, 2] or on fibrous composite samples [3–12] However, the nature of the phases and the growth mechanism of the filament-matrix (FM) reaction zone remains a subject of controversy The aim of the present contribution is to analyse, for various well characterized SiC-based CVD-filaments (stoichiometric SiC, SiC with a pyrocarbon coating, SiC with a SiC + C coating) and titanium matrices (unalloyed titanium or Ti-6Al-4V): (a) the FM interaction zone composition, (b) its kinetics of growth in the 700 to 1100° C temperature range, and (c) the growth mechanism The filaments which have been used here have been carefully analysed previously on a chemical, microstructural and mechanical point of view in Part 1 In the same manner, the results of the present study on FM chemical interaction will be later correlated with those of different mechanical characterizations [13, 14]

Contourite drift molded by deep Mediterranean outflow

Abstract: Since its inception in early Pliocene time, the deep Mediterranean outflow has eroded and sculptured the southern Iberian margin. The 50-km-long, 300-m-thick Faro Drift, constructed by this powerful bottom current, has been the subject of detailed study. Sandy, silty, and muddy contourite facies can be clearly characterized, and a distinctive vertical arrangement or “sequence” of these facies can be related to hydrodynamic fluctuation. These results are important for the identification of ancient contourites, the reconstruction of paleocirculation patterns, and documentation of the effects of Quaternary climates on the Mediterranean outflow.

Brillouin and raman scattering study of borate glasses

Abstract: We report light scattering results obtained from Brillouin and Raman scattering experiments in a series of B 2 O 3 − x M 2 O glasses (M  Li, Na, K, Rb, Cs and Tl). Raman lines related to the internal vibrations of structural units give information on the composition and concentration dependence of the short range arrangement in the glasses. The “Boson peak” is analysed on the basis of the Martin-Brenig model. The short correlation range is lower than 10 A and increases with the cation radius. From these experiments the very low frequency light scattering (Light Scattering Excess) is extracted. The temperature dependence of the LSE intensity in pure B 2 O 3 glass as well as its variation with composition at room temperature are studied. The Theodorakopoulos-Jackle theory relates this intensity to the acoustic attenuation deduced here from Brillouin linewidth measurements. While a theoretical description of the relation between the temperature variation of both quantities agrees well with experimental results, disagreement is apparent in their composition dependence. Finally, the sound velocity plotted versus concentration gives evidence of the contradictory influence of the hardening due to the change of the boron atom coordination number and the softening related to the distortion of the network by large cations.

Wine phenolics—analysis of low molecular weight components by high performance liquid chromatography

Abstract: Low molecular weight phenolic compounds were separated by high performance liquid chromatography on hydrocarbon-bonded reverse phase packings, with a water-methanol elution gradient. Phenolic compounds were extracted from wine with ethyl acetate. A first extraction at pH 7 enabled isolation of neutral molecules (catechins, procyanidins, flavonols, aromatic alcohols); a second extraction at pH 2 was performed to extract phenolic acids. Successive injections of these two extracts in the chromatograph gave the distribution of the different phenolic compounds in the wine under analysis. Analytical data are given for low molecular weight phenolic compounds present in young red Bordeaux wines from different vine cultivars.

Structural and electrical properties of (TMTTF)2MF6 salts (M = P, As, Sb). Rôle of the anions

Abstract: The structural and electrical properties of different (TMTTF) 2 MF 6 compounds are described and the role of the anion size is investigated through the behaviour of the SbF 6 salt for which a critical increase of the resistivity occurs below 154 K. The effects of alloying (TMTTF) 2 SbF 6 with AsF 6 or PF 6 is also studied and we show that this electrical anomaly still exists for small amount of doping. Finally the existence of structural instabilities involving centrosymmetrical anions is briefly discussed Etude des proprietes structurales et electriques de ces sels ou l'on etudie le role de la taille de l'anion, a partir du comportement du sel de SbF 6 pour lequel un accroissement critique de resistivite est observe a 154 K. Effet du dopage de (TMTTF) 2 SbF 6 par AsF 6 ou PF 6 : l'anomalie de resistivite persiste aux faibles taux de dopage. Discussion de l'existence d'instabilites structurales impliquant des anions centrosymetriques

The distribution of the projection from the parataenial nucleus of the thalamus to the nucleus accumbens in the rat: An autoradiographic study

Abstract: In this study the intrastriatal distribution of afferents arising from the parataenial nucleus of the thalamus was investigated. Tritiated leucine and proline injected into the parataenial nucleus was found to densely label the entire anterior-posterior extent of the medial nucleus accumbens. The projection was for the most part limited to this striatal subregion, although some moderate labelling was found along the medial wall of the anterior caudateputamen. The terminal labelling within accumbens was characterized by a distinct patchiness. Other efferent connections of the parataenial nucleus observed in this study include the thalamic reticular nucleus, the basolateral and central nuclei of the amygdala, the septum, the medial frontal cortex, the entorhinal cortex and subiculum.

Maintained contractions of rat uterine smooth muscle incubated in a Ca2+‐free solution

Abstract: 1 The effects of acetylcholine (10−4m), prostaglandin E2 (10−6m), vanadate (5 × 10−4m) and fluoride (10−2m) have been studied on the mechanical and electrical activities of rat myometrial strips perfused in Ca2+-free EGTA-containing solutions. 2 All four substances produced maintained contractions which could be initiated repeatedly after exposure to Ca2+-free solution for more than 1 h, without a significant decrease. The largest contractions were obtained with vanadate and the smallest ones with acetylcholine. The tension was usually 7–30% of the control contraction triggered by an action potential in Ca2+ containing solution. 3 Maintained contractions induced by fluoride were unaffected by isoprenaline while those induced by acetylcholine, prostaglandin E2 and vanadate were completely relaxed. 4 Prostaglandin E2 -and vanadate-induced contractions were slightly reduced by Na+ removal or by adding Ca2+ antagonists. In contrast, contractions induced by acetylcholine were suppressed in Na+-free solution and largely inhibited in the presence of Ca2+ antagonists. 5 The depolarization induced by acetylcholine in Ca2+-free solution was strongly dependent on the external Na+ concentration. The relationship between the size of the acetylcholine-induced depolarization and the membrane potential (shifted by constant currents) was linear, giving an apparent reversal potential for acetylcholine close to zero potential. 6 In Ca-free solutions and in the presence of atropine, Na+ action potentials of long duration can be evoked which produced contractions of the same order of magnitude as those initiated by acetylcholine-induced depolarizations. 7 These results are consistent with the hypothesis that the maintained contractions in Ca2+-free solutions induced by several stimulants could be related to Ca2+-independent mechanisms (fluoride) or Ca2+ release from an intracellular store. This latter mechanism would include both pharmacomechanical (prostaglandin E2, vanadate) and electromechanical (acetylcholine) coupling.

The deconvolution problem: Fast algorithms including the preconditioned conjugate-gradient to compute a MAP estimator

Abstract: Different aspects of the deconvolution problem are discussed and a solution by the MAP estimator using the a priori knowledge of smoothness and nonnegativity of the unknown signal is proposed. Several fast algorithms are examined and the conjugate-gradient (CG) method is show to have several advantages: it allows computation of a constrained solution and when used with a preconditioning technique it can be faster than other algorithms for general near-to-Toeplitz systems, in particular, for Toeplitz-block-Toeplitz systems.

Fast inversion of triangular Toeplitz matrices

Abstract: An algorithm requiring 0(n \log n) operations is proposed for the inversion of triangular Toeplitz matrices. This algorithm is faster than existing techniques, even for relatively small values of n , and can easily be written in Fortran.

Immunohistochemical localization of dopamine in the brain of the insect Locusta migratoria migratorioides in comparison with the catecholamine distribution determined by the histofluorescence technique.

Abstract: As part of a follow-up study to our previous investigation of the catecholaminergic neurosecretory cells in the brain of adult female locusts (Locusta migratoria migratorioides) by means of the formaldehyde-induced fluorescence method, we have attempted to specify the identity of the amines present in these cells by an immunohistological technique. Using a recently developed anti-dopamine serum, we have demonstrated that the majority of the cate cholaminergic median neurosecretory cells contain dopamine. Moreover, dopamine is present in some cell bodies of other zones of the brain, i.e. the median subocellar neurosecretory cells, perikarya in external areas of the protocerebrum, below the calyces, around the pedunculus, in the optic lobes (between the lobula and the medulla, between the medulla and the lamina), and in external zones of the tritocerebrum. Among the structured neuropils, which were particularly fluorescent in the formaldehyde-induced fluorescence method, only the pedunculus, the posterior part of the central body, the external zones of the α- and β lobes and the proximal part of the lamina contain little dopamine.

Composite materials made from a porous 2D-carbon-carbon preform densified with boron nitride by chemical vapour infiltration

Abstract: 2D-C-C/BN composites (with a BN volume fraction (V BN) up to 0.60) have been obtained by chemical vapour infiltration of hexagonal-BN from a BF3-NH3 mixture, within the pores of a 2D-C-C preform made of a stacking of carbon fabric layers which has been weakly consolidated with pyrocarbon. They were tested in compression on samples with the carbon fabric layers directed either parallel or perpendicular to the load axis. In the first case (p-direction), the compression behaviour is mainly elastic (at least for high enoughV BN). Bothσ ∥ R andE ∥ increase regularly with risingV BN, as the BN-layer deposited within the pores of the preform (mainly located between adjacent carbon layers) becomes thicker,e ∥ R is weak and progressively decreases with increasingV BN. In the second case (o-direction), the stress-strain curve exhibits both elastic and pseudoplastic domains whose respective extension depends onV BN (or onV p). The pseudoplastic behaviour is related to an irreversible microstructural damage of the interlayer C/BN filling.σ ⊥ R ,σ ⊥ E andE ⊥ increase with increasingV BN (or decreasingV p) according to parabolic laws,e ⊥ R is much higher thane ∥ R . The materials remain anisotropic even at highV BN. Oxidation tests in air (or oxygen/argon mixtures) have shown that 2D-C-C/BN, incompletely densified (V p ∼ 0.10) by BN, exhibit a weight loss (oxidation of the carbon skeleton) at low temperatures and a weight increase (oxidation of BN) above 900° C. Oxidation resistance is enhanced by increasingV BN (which results in aV p decrease) and decreasing oxygen partial pressure. It is predicted that a good oxidation protection of the carbon skeleton requires a full densification by BN (V p ∼ 0) and will be effective at medium temperatures. The results of the present study could be easily extended to 2D-BN-BN composites.

Sunflower Seed Proteins: Characterization and Subunit Composition of the Globulin Fraction

Abstract: Salt soluble proteins from sunflower (Helianthus annuus) seeds were fractionated by isoelectric precipitation and analysed by electrophoresis. Three molecular species were detected by gradient polyacrylamide gel electrophoresis of the globulin fraction. Multi-dimensional gel electrophoresis analysis indicates that all these species contained similar intermediary subunits of60 000,54 000,48 000 and 40000 molecular weight, the two former being predominant. As shown by ion-exchange chromatography under dissociating and reducing conditions, the intermediary subunits are composed of disulphide linked pairs of large 'acidic' and small 'basic' subunits. Heterogeneity in molecular weight of these subunits was shown by electrophoretic studies. These results suggest that a major reserve protein in sunflower seeds is similar to 'legumin' of plants of the family Leguminosae.