Facteurs de structures partiels et grandeurs thermodynamiques dans les alliages liquides

From the calculation of partial structure factors in liquid alloys developed by Bhatia and Thornton, the authors establish a generalized relationship between these structure factors and the thermodynamic parameters in dilute solutions by introducing Wagner's interaction coefficients. The validity of using models describing the interactions in solution to establish such relationship is discussed.     

Modelisation de la propagation dans les fibres optiques

This paper presents a new method for investigating the effects of mode conversion due to microbends on the transmission characteristics (attenuation, frequency response ...) of multimode optical fibres. The method applies to both step-index and graded-index fibres and takes into account any dependence of the mode coupling coefficient upon propagation angle. The general theory of propagation with mode coupling is developed and we introduce the concept of modes. The propagation problem is then simplified by using hypotheses based on exerpimental results and the solution is obtained with the help of a computer. Practical results concerning the increase of attenuation due to microbends are presented, from which the precautions to be taken when cabling fibres are deduced. It is also shown that in every practical case encountered, the prediction of the total attenuation and of the frequency response of a given fibre, when the launching conditions are specified is obtained from analytical expressions which approximate well with the exact results obtained from the computer. All these results make the method powerful and very useful for system designers when evaluating the effect of introducing an optical fibre into a transmission system.     

Examen de la loi d'arrhenius de la diffusion dans les solides

Validity of the Arrhenius law has been investigated by measuring the influence of temperature on the transport processes in very pure single crystals of sodium iodide. For the first time, due to the high experimental accuracy and the precision of the temperature control, diffusion coefficients could be measured in a solid in steps of a few degrees up to a few tenths of a degree of the temperature of fusion. We have studied: (a) ionic conductivity, (b) self-diffusion of the cation Na⁺ and (c) self-diffusion of the anion I^?. The three transport processes follow the same temperature dependence: (a) strict verification of the Arrhenius law over several orders of magnitude up to about thirty degrees below the melting point and (b) in the last thirty degrees a positive departure from the simple exponential law is observed which increases rapidly to reach 50% at fusion. The different possible reasons of this departure are discussed. The phenomenon seems to be connected with the lattice dynamics before melting.     

Largeur de raie de resonance antiferromagnetique dans GdAlO3 en fonction de la temperature

Calculations of the photoionization cross sections of the positive ions C³⁺, O³⁺, Ne³⁺ and Mg³⁺ are reported.     

Propagation et modulation de la lumiere a 1,064 μ dans les grenats magnetiques

We report for the first time propagation, switching and modulation of light at 1.064 μm in iron garnet films epitaxially grown on a GGG substrate. Absorption was measured at this wavelength.     

Correlations entre la conductivite ionique et la concentration en ions alcalins dans les borates de lithium vitreux

The electrical transport properties of the B₂O₃ - xLi₂O glasses have been investigated. An electrochemical approach allows to give an interpretation of the conductivity based on the carrier concentration in relation with Li₂O content. The mobility of the Li⁺ ions does not seem to depend on the composition.     

Effets d'irradiation sur les proprietes magnetiques des alliages de fer-nickel dans la region invar

We have undertaken the irradiation of iron-nickel alloys of 30, 32, 35 and 40% nickel content to study its influence on the magnetic properties of invar. The initial results obtained from measurements of the magnetization and from the Mössbauer effect, show that electron irradiation could lead to the appearance of an ordered phase.     

Les effets causes par les dimensions de la source dans les systemes optiques eclaires en lumiere spatialement incoherente

It has been shown recently that in order for an incoherent optical system to be linear and space invariant, the image of the source at the filter plane must be larger than the filter aperture. We show in this paper that the source image must be larger than the filter by an amount equal to twice the highest spatial frequency of the object, the frequency being measured in the filter plane.     

Commentaires sur les mesures de flux dans les fibres optiques

Sans résumé     

Fluorescence A 4.3 μm en fonction de la pression dans un laser CO2 supraatmospherique

We have observed the 4.3 μm fluorescence signal produced by a superatmospheric double-discharge CO₂ laser, up to a pressure of 5 atm. The evolution of the signal amplitude with pressure reveals an unusual behaviour which we associate to the hot bands. Moreover, we have deduced the V-V relaxation times of CO₂ in mixtures CO₂:N₂:He of (10:5:85) and CO₂:He of (9:91).     

La méthode de la fonction génératrice et les développements en quasibosons

An isomorphism between the oH' subspace of the physical bound states and a normalized oF functional space is established. This is done by introducing a generating function for these oH' states. By choosing for the oH-space the Bargmann's space, an isomorphism is obtained between an oH_F space of the fermion states and an oH_B space of the boson states. An expansion of any quasiboson operator of oH_B is thus feasible. This is particularly interesting for the Hamiltonian operator. In this case the expansion obtained, similar to the Beliaev-ZeIevinski and Marumori expansions, appear to be simple and more convergent.     

Comportement anelastique de la glace ih dopee avec HF

Modifications of mechanical damping of HF doped ice crystal are studied in a lower frequency range. Three effects are observed: (1) The relaxation peak appeared in a lower temperature range than that of pure ice. (2) High temperature internal friction is increased. (3) High temperature internal friction becomes more amplitude dependant. These effects are interpreted in terms of increase of the content of Bjerrum defects according to the assumption about the substitutional position of fluorine atome in the ice lattice.     

Existence d'orbites quasi-periodiques dans les attracteurs de Birkhoff

For each number between the lower and the upper rotation number of the Birkhoff attractor of a dissipative monotone twist map, there is a periodic or quasi-periodic orbit with rotation number .     

Vitesse des ultrasons dans les gaz rares sous haute pression a la temperature de 298.15 K

By a Pulse Echoes Overlap Method, the speed of ultra sound has been worked out at 15 MHz for the complete noble gas series at temperature 298.15 K in a very wide density range (which corresponds at pressures up to 10 kbar for light noble gases). When the molar density and the isothermal compressibility coefficient are known, the ratio of specific heats can be deduced as a function of density. In the case of argon the evolution of both c_p and c_v has been obtained separately and a value of 3R is determined before the fluid-solid transition. These thermodynamical quantities have been compared with those derived from the numerical equation of state of Hansen for a Lennard-Jones fluid. One notices good agreement between predictions and experimental results for hot gases.     

Methode de mesure des coefficients de relaxation dans les collisions atomiques

A simple model for the study of the anisotropy is proposed. It is found that the analysis of the anisotropy can be made by a direct examination of the alignment signal. This led to a simple way of determining the relaxation coefficients. In the case of isotropy the results obtained are identical to the classical results already known. Different cases are examined according to the direction of the external applied fieldH and the direction of the detected signal.     

Calcul de la fonction correlation de phonon pour la diffraction d'une particule par un solide

A method of calculating expressions of the type $\text{<T exp[?i}\text{k}{}_1\text{u}{}_{\ln }\text{(t}{}_1\text{)] ... exp [?i}\text{k}{}_{\mathrm{n}}\text{u}{}_{\ln }\text{(t}{}_{\mathrm{n}}\text{>}$ which is simpler than that based on Wick's theorem is proposed. Expressions of the above type are widely used in studying the effects of temperature in electron and neutron diffraction.