Une formule asymptotique pour\sum\limits_{n \leqq x} {a_z (n)d(n + 1)}

Sans résumé     

Sur une voie d'acces potentiellement generale aux phosphines fonctionnelles a phosphore optiquement actif

Optically active methyl-phenyl-cyclohexylphosphine sulfide is metallated by BuLi and reacted with ethyl carbonate to yield carbethoxymethyl-phenyl-cyclohexylphosphine sulfide with retention of the optical activity at phosphorus ; this sulfide is converted into the corresponding optically active functional phosphine through a nickelocene reduction-complexation.     

Magnetoelectric effect and magnetization process in Fe5LiO8

After showing the existence of second order M.E. effect β, in Fe₅LiO₈ [1], we try to verify Lee and Rado's assumptions. These authors suppose that the existence of β is connected with the rotation of magnetic domains. We compare the magnetoelectric β and the magnetization σ curves vs the magnetic field H₀. The latter curve reveals the different magnetic modes, and therefore illustrates the behavior of the magnetic domains. This comparison has been made for several directions of the applied field in order to illustrate all possible cases of rotation of the domains and allows us to confirm the basic assumption.     

Stereochimie de l'addition des derives zinciques des ethers propargyliques sur les composes carbonyles

The stereochemistry of the addition of the zinc derivative of ether PhCCCH₂OMe, to aldehydes and ketones has been studied. In the case of saturated carbonyl compounds, the results can be explained by a mechanism which takes into account the steric effects of the substituents. However, with those which contain phenyl, vinyl or alkynyl groups in the α position, conformational and electronic factors must be considered.     

Modelling of dynamic ductile fracture and application to the simulation of plate impact tests on tantalum

A model of dynamic damage by void nucleation and growth is proposed for elastic-viscoplastic materials sustaining intense loading. The model is dedicated to ductile materials for which fracture is caused by microvoiding. The material contains potential nucleation sites where microvoids are generated when the local pressure overcomes the nucleation pressure. A probability density function is adopted to describe the fluctuation of the nucleation pressure within the material. The void growth is described by using a hollow sphere model where micro-inertia effects are accounted for. The matrix weakening due to void growth is also included.The model has been first tested under uniaxial deformation. When the strain rate is assumed constant, the pressure inside the material has nearly a linear response up to a maximum. An analytical expression for the maximum pressure is proposed.Finite element simulations of plate impact tests have been carried out and compared to experiments on tantalum. From simulations based on the proposed model, an increase of the spall strength is observed with higher shock intensities. Therefore, the relationship between the velocity pullback and spall strength usually assumed in the literature (based on the acoustic approach) seems to be inadequate. Velocity profiles are simulated for different flyer thicknesses and different impact velocities with close agreement with experiments.     

Water vapor sorption in naphthalenic sulfonated polyimide membranes

The water vapor uptake of sulfonated polyimides (SP) was investigated by electronic microbalance (IGA, Hiden) from 15 to 55°C. The sigmoïdal isotherms obtained (BET II type) are considered as dual sorption (concave part) plus clustering (convex part) and are fitted with good agreement by Park’s equation. Zimm–Lundberg’s method is used to study the clustering process of water molecules: limit clustering activity, a^*, and the number of molecules per cluster are estimated.

To obtain a better understanding of polymer structure and isotherm analysis, H⁺ (counter-ions of sulfonic groups) were replaced by ions with a smaller hydration shell (Cs⁺ and EDAH⁺). Comparison of the three isotherms shows no significant difference in the water affinity of the cations. This is attributed to a partial control of the sorption by microcavities existing in the membrane.     

Positive electrode materials for lithium batteries based on VOPO4

A detailed electrochemical study of Li insertion in the -form of VOPO₄ and the optimization of the cycling performance are presented. The redox process occurs in one step close to 3.76 V, along with a phase transition. In order to improve the intercalation kinetics, the electronic conductivity was optimized by introducing a mixed valency, and the ionic conductivity was favored by introducing ‘pillaring’ molecules or ions in the interlayer space. In this way, the electrochemical behaviors of -VOPO₄·2H₂O, -Na_xVOPO₄, -K_xVOPO₄ and -Mg_xVOPO₄ (0≤x≤1) have been studied: the hydrate compound shows a good specific capacity (100 mA h/g at a C/5 regime), but a poor cyclability was mainly because of water oxidation. The inserted large alkaline ions improved the cyclability up to 80 cycles (Na⁺) or over 100 cycles (K⁺). Enhancements of the VOPO₄ specific capacity have been obtained as well by mechanical ball-millings.     

Homogenization of elastic–viscoplastic heterogeneous materials: Self-consistent and Mori-Tanaka schemes

This paper deals with the prediction of the macroscopic behavior of a multiphase elastic–viscoplastic material. The proposed homogenization schemes are based on an interaction law postulated by Molinari et al. [Molinari, A., Ahzi, S., Kouddane, R. 1997. On the self-consistent modelling of elastic–plastic behavior of polycrystals. Mech. Mater., 26, 43–62]. Self-consistent schemes are developed to describe the behavior of disordered aggregates. The Mori-Tanaka approach is used to capture the behavior of composite materials, where one phase can be clearly identified as the matrix. The proposed schemes are developed within a general framework where compressible elasticity and anisotropy of the materials are taken into account. Inclusions can have various shapes and orientations. Illustrations of the homogenization procedure are given for a two-phase composite materials. Comparisons between results of the literature and predictions based on the interaction law are performed and have demonstrated the efficiency of the proposed homogenization schemes.