Comptes Rendus de l'Académie des Sciences - Series IIB - Mechanics-Physics-Chemistry-Astronomy最新文献

In the course of our research concerning the reactivity of tantalocene hydrides towards elemental sulfur, the synthesis of monosulphurated complexes by a sulfur insertion reaction into the Ta-H bond is reported. Some information on the mechanistic pathway for the formation of the disulfide complexes and the synthesis of Cp′₂Ta(CO)(SH) is given.

This note is concerned with junctions of thin plates. The junction is made by a slender rod made of material with low rigidity with respect to that of the plates, allowing a variation of the angle between them. A macroscopic study is not sufficient to obtain all transmission conditions. A local tridimensional study is necessary. We prove then that not only the variation of the angle between the two plates, but also the sliding of one plate with respect to the other must be taken into account to the same order. Thus the behavior of the junction is completely determined.

This paper introduces a new approach to studying the effects of lateral stress on surface elasticity. Recent results in the force modulation with an atomic force microscope (AFM) on the (010) gypsum face are presented and discussed. At the atomic level, the images in the contact mode differ from those mapping the elasticity. At the microscopic level, the images of elasticity vary according to the stress applied to the sample. Without stress, the surface stiffness is uniform. However, a lateral stress induces slipping and shearing processes.

The original structure of 1,3,5-trimethyl-2,4,6-trinitrobenzene (TNM) is composed of separate planar layers with hexagonal pseudo-symmetry, like a prismatic single crystal. These layers are parallel to (20) and the interlayer distance is greater than the standard spacing for two benzene rings. NO₂ group planes and the tallying benzene ring are nearly normal. This and the methyl group resonance minimize the interlayer oxygen-hydrogen distances of two symmetrical molecules in separate layers. Crystal data: triclinic, space group P, a = 8.268(1) Å, b = 8.7890(5) Å, c = 9.932(1) Å, α = 108.12(1)°, β= 113.25(1)°, γ = 95.19(1)°, V= 549.7(2) ų, Z = 2, R(F)= 5.1 %, 1 822 observed reflections ¦F₀² > 3 σ(F₀²)

The annular region between two confocal ellipses is a new mixing geometry defined and constructed in Nancy. When the two walls are displaced following their circumference in opposite directions at constant angular velocity, the steady state streamlines show the appearance of two saddle points joined by two different streamlines, leading thus to heteroclinic trajectories. By an adequate choice of the time-periodic velocity history of the two walls, chaotic trajectories can be obtained in this Stokes flow and only a limited number of periods are necessary to mix a small amount of tracer over the entire domain. The protocol that gives the most rapid mixing is also the one that gives the highest heat transfer rate.

The Replica Fourier Transform introduced previously is related to the standard definition of Fourier transforms over a group. Its use is illustrated by block-diagonalizing the eigenvalue equation of a four-replica Parisi matrix.

An investigation of the generation of Tollmien-Schlichting waves has been carried out with the direct numerical simulation PEGASE code developed at ONERA. The latter was allowed to visualise these waves in a two-dimensional incompressible flow around a roughness element placed on a flat plate, the incoming flow having the form: U_∞ = U₀(1ɛ cos ωt). Quantitative results on the influence of ɛ, and of the dimensions h and l of the rectangular hump on these waves are obtained from a parametric study. These results show good agreement with previous experimental and theoretical studies.

For sensitivity reasons, absorption spectroscopy was not possible at millimeter wavelengths until recently. While emission measurements need collisional excitation of the molecular levels above the cosmic background temperature, absorption measurements are able to detect molecules in low-density regions. However, large collecting area instruments are required. In diffuse clouds, the HCO⁺ ion is frequently seen, more easily than the CO molecule. At the photodissociation transition of CO, around N_H2 ~ 10²¹ cm⁻², the CO column density varies at a much higher rate than the HCO⁺ column density, for values of N_HCO⁺ ~ 10¹² cm⁻². The HCO⁺ to OH abundance ratio is remarkably constant (0.03 to 0.05). Other species such as H₂CO, HCN, HNC, CN, C₂H, C₂H₂, CS, SO, H₂S, can now also be studied in diffuse clouds.

Vacuum prehensile systems using a set of suction cups offer many advantages. However, an important problem appears when there is no matter in front of some suction cups. Indeed the flow rate sucked up in vain induces a considerable waste of energy and the vacuum level may become insufficient for gripping. Therefore it is very useful to disconnect all the suction cups that are not being used for gripping. In this paper, we present a new solution using the sucked up flow rate to detect and to drive the closing of unused suction cups.

We demonstrate a non-intrusive measurement technique, that allows the simultaneous determination of two velocity components and the concentration of a passive contaminant. This technique has been applied to the characterization of the diffusion properties of a turbulent jet.