Experimental and numerical study of air-coupled surface waves generated above strips of finite impedance

A series of laboratory experiments are described in which air coupled surface waves are generated from a point source in the frequency range between 800 and 1700 Hz above a surface composed of a lattice of small cavities. Since the sound pressure near the lattice of cavities can be greater than if the surface was rigid, passive amplification is obtained. Moreover, directional receivers can be designed by restricting the lattice of cavities to a strip of finite width. These directional receivers are shown to provide more amplification than would be obtained with a semi-infinite lattice of cavities. Theoretical calculations using a boundary element method predict gains of up to 6 dB, relative to the pressure on a rigid plane when the strip width is of the order of 1 wavelength. The strips are further modified by changing their shape or by adding reflectors to further enhance the amplification. Gains of 12 dB have been measured and predictions show that gains of 20 dB are theoretically possible.     

Fresnel phase matching: a universal phase matching scheme

We present new theoretical concepts for Fresnel phase matching. A guided wave approach is described, which allows us to intrinsically take into account all the physical processes involved. To cite this article: M. Raybaut et al., C. R. Physique 8 (2007).     

A low divergence,N2 UV laser without “kaleidoscopic” effects

A low divergence (0.2 × 0.3 mrad), N₂ UV laser is discribed. A maximum intensity of 5 MW/mrad² has been obtained. The maximum brightness was 2 × 10¹³W/sterad cm². The “equivalent source” of the laser has been determined. These results were reached by using a relatively low voltage transmission line with a curved electrode profile.     

Application des methodes d'analyse thermique a l'etude du frittage des poudres metalliques

The study of nickel and iron ex-carbonyl powders, using various thermal analysis techniques permitted clarification of the initial mechanism of sintering. Solid-gas reactions prior to “bridging” of grains were determined: water desorption, reduction of oxygen and carbon-monoxide surface groups, decomposition of carbides. Initiation of sintering (bridging) can be explained by the prominent part played by impurities and particularly carbon.     

Transient kinetic studies involving exciplex formation. Methods of analysis

In this paper, rate parameters are extracted from fluorescene decay data in the framework of Feedback-Type Kinetics (k₈ < k₄). The exciplex dissociation (k₄) and fluorescene decay rate constants (k₈) have been obtained for the five following systems: For this task we have added to the straight line method of Ware which is valid in certain cases only at high perturber concentrations, a new but similar Taylor series method applicable at low concentrations. Thereafter, a safer and more general method of kinetic analysis based on a quasi-linear χ² minimization procedure has been developed and applies to data in any range of concentration.