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.     

Sur la determination de la permittivite complexe des materiaux a tres fortes pertes

The characterization of materials having a high dielectric constant and a large loss angle presents great difficulties, notably due to the very slight phase shift to be determined. From the analysis of the propogation for dielectric materials (in rectangular wave guides and coaxial lines) a new experimental technique is proposed for coaxial lines using the properties due to the impedance shifts which allows the worthwhile increase in the accuracy of the measurements.     

Sur la determination des constantes optiques du calcium a partir de couches epaisses

We investigate how it is possible to determine the best the optical constants of calcium from measurements of the transmittance T and reflectance at the interfaces metal-vacuum R and metal substrate R′ on thin films, using a method of successive approximations. We compare the results obtained using the experimental values of (R, R′, T) or (R, T) or (R′, T), for two films of different thicknesses. We show that a satisfactory result is obtained with the values of (R′, T). We determine, in this manner, the optical constants of calcium in the energy range 2 –5.6 eV, and we compare the results obtained with those determined by other authors.     

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.     

Sur la restitution du signe de la phase en interferometrie

Phase variations are transformed into intensity variations by means of interferometry. It is then possible to form a visible image of a pure phase object. But then information on the sign of the phase or the direction of surface relief is lost. Principles of a method for extracting only the imaginary part of the complex amplitude of the object, which is an odd function of the phase, are studied. From this it is possible to obtain the sign of the phase.     

Theorie semimicroscopique des solutions solides Cr1−xVxN

The solid solutions Cr_1?xV_xN show at low temperatures a progressive establishment of an orthorhombic distorsion and an antiferromagnetic order (of the fourth kind) for $\text{x <}\text{1}\text{3}$ accompagnied by a (cubic) fraction which remains disordered at the lowest temperatures : No ordering occurs for $\text{x >}\text{1}\text{3}$. Assuming a Gaussian distribution of local concentrations in otherwise homogeneous cells (cellular model), the mean magnetic moment, measured by neutron diffraction can be related to a constant moment of Cr provided the local vanadium concentration is smaller than 0.26. The cell size contains 20 metal atoms.     

Glissement de gouttes de liquide sur des solides mous

Viscoelastic braking due to the motion of the wetting ridge during spreading of a liquid drop on a soft, viscoelastic solid is now established in axisymmetric conditions where only capillary forces cause motion of the triple wetting (or dewetting) line. We consider here preliminary results where this same phenomenon of slowing down prevents rapid flow of the liquid on an inclined elastomeric plane.     

Calcul semi-empirique de la temperature de transition des superconducteurs et de l'effet de la pression sur la transition

A method is developed to take into account the contribution of the Umklapprozesse to the electron-phonon interaction in polyvalent metals. This method is used to compute—from first principles—the high-temperature resistivity of non-transition metals as well as the transition temperature T_c of non-transition superconductors along the lines of , and 's theory of superconductivity (Phys. Rev. 108, 1175 (1957)). The effect of pressure on T_c is also computed and its dependence upon the band structure is stressed.     

Sur la contribution des differents porteurs à la zone de charge d'espace d'un semiconducteur multivallée

We study, in the case of the parabolic approximation, the space charge of a two-valley conduction band semiconductor; the electrons associated to these valleys are supposed to have different effective masses. By using a non-degenerate distribution we analyse the contribution of the different carriers to the space charge and the effect of surface band bending, temperature, effective masses, dope and type of material. We point out that the space charge can be governed by either type of carrier or both. By comparing the results with those given by a single valley model, we show the incidence of the initial hypothesis on the value and evolution of the space charge with temperature and surface band bending. As an exemple, we study briefly n-type GaSb.     

Etude par diffraction neutronique de la forme haute pression de FeOOH

A powder sample of iron oxyhydroxide (FeOOH_HP) synthesized under high pressure-high temperature conditions, has been studied by neutron diffraction. The magnetic structure has been determined. The magnetic and chemical unit cells are the same and the antiferromagnetic arrangement is collinear with the spins parallel to the c-axis. The crystallographic structure affinement by the Rietveld method has confirmed that FeOOH_HP is isostructural with InOOH (space group P2₁nm) and has established the positional atomic parameters. The magnetic moment of iron (4.7μ_B at 4.2 K) and the O-H distance in the hydrogen bond (1.11 Å) are discussed with regard to the values obtained for the other forms of iron oxyhydroxide.     

Sur l'evolution des proprietes physiques de la perovskite GdCoO3 entre 77 et 1200 K

GdCoO₃, which has the GdFeO₃ structure, has been studied between 77 and 1200 K by D.T.A., X-ray diffraction, magnetic susceptibility, electric conductivity and thermoelectric power. All properties observed, although different from those of LaCoO₃, fit with the corresponding Goodenough localized electron model. With rising temperature cobalt ions pass progressively from a low-spin Co^III(t⁶_2ge_g⁰) state to a Co³⁺(t⁴_2ge_g²) high-spin state.     

Influence de la temperature sur les intensites des raies Raman de resonance

Using an analytical expression for the resonance Raman scattering tensor, the influence of the temperature on the scattered intensities is studied.The variation of the intensities as a function of the temperature is different from the result given by the polarisability theory. However, it is shown that the usual ratio $\text{I}{}_{\mathrm{<mtext>Stokes</mtext>}}\text{I}{}_{\mathrm{<mtext>antiStokes</mtext>}}$ is still valid with a good approximation.     

Influence des impuretes et de l'inhomogeneite d'un plasma sur la mesure de sa temperature electronique

The effect of impurities on soft X-ray continuum radiation emitted from a high-temperature deuterium plasma has been studied. The results of these calculations are useful in the determination of the electronic temperature of a laser-created plasma. Such a plasma is inhomogenous and contains impurities.     

Application d''un interferometre a ondes multiples a la mesure des pouvoirs reflecteurs

We describe a device which can be used to measure mirror reflectances with a good precision. The method is convenient for plane or spherical mirrors of optical cavities and can be applied over a large spectral range.     

Sur la diffraction d'une onde plane par un reseau metallique de conductivite finie

A numerical-integral equation approach is used to determine the efficiencies of finite conductivity gratings.