Effets produits par l'irradiation avec des neutrons et avec des rayons gammas sur la série jaune et la série verte du spectre d'absorption excitonique de la cuprite

The effects of irradiations in a reactor and in a Cobalt 60 source on the green and yellow series of the excitonic absorption spectrum of cuprous oxide are studied. In the reactor irradiation experiments, the effects of fast neutrons are separately studied from those of thermal neutrons and of gamma rays. It is shown that the perturbations of the absorption spectra of Cu₂O by fast neutrons are very similar to those produced by an external electric field. They are also similar to those produced by quenching which creates probably internal strains. Charged defects occur probably in the crystal during bombardment and these charged defects produce an internal electric field and strains. It is not possible to study separately the effects of the electric field and the effects of strains. The electric field can be evaluated by comparison of its effects on the excitonic spectrum with those of an external field. An evaluation of the rate of creation of defects by fast neutrons in the crystal is deduced. It is also shown that the effects of gamma-rays are very different from those of fast neutrons. It is possible with gamma-ray irradiations to decrease residual internal fields which may exist in a crystal.     

Présentation simple de la pression émise par un transducteur focalisant

The numeric computing of the pressure field emitted by a focusing transducer is relatively long. That is why we have developed a simple method to predict the pressure of such transducers. This model requires to compute only one integral and is called 1-dimensional model. It provides a rapid calculation of the radial and axial pressures. These pressures will be compared with the results of the 2-dimensional model and validated by experiment.     

Etude du spectre de l'holmium atomique: II. Interprétation paramétrique des niveaux d'énergie et des structures hyperfines

Energy levels of neutral holmium have been interpreted by means of parametric studies of the groups of configurations (4f¹¹6s² + 4f¹⁰6s²6p + 4f¹¹5d6s) and (4f¹⁰5d6s² + 4f¹¹6s6p), taking into account the mixing effects due to electrostatic interactions. Both real and effective parameters have been fitted from 71 odd and 70 even levels, respectively. The eigen-functions have been used for a determination of the magnetic dipole A and electric quadrupole B hyperfine constants. Values of the parameters obtained from various configurations are compared. Results of parametric studies are also reported for 4f¹⁰(⁵I)5d²6s.     

Sur la remanence de la magnétisation par déplacement des parois de Bloch et la cinétique de sa desaimantation

Sans résumé

---

,

     

Analyse de la conduction de la chaleur aux temps ultra-courts dans un solide par la thermodynamique irréversible étendue et la dynamique moléculaire

Instantaneous heat propagation and thermodynamic local equilibrium cannot be assumed when solving space and time microscale problems. Therefore, we reconsider the thermodynamics basis of the Fourier law in order to obtain the new heat conduction models: the hyperbolic heat equation (EH) and the modified hyperbolic equation (EHM). We have performed molecular dynamics (DM) experiments which are independent of any thermodynamic model, to test the macroscopic approaches. We show that the solutions of the EH and the EHM do not agree with the numerical experiments and that the MD results are strongly dependent on the way from which the macroscopic conditions are simulated in the microscopic point of view.     

Constante de force du vibrateur CH en relation avec la structure électronique de la molécule

Une expression analytique est recherchée pour le calcul des constantes de force k _CH dans le cadre de la méthode CNDO/2. Les dérivées des intégrales de résonance qui apparaissent dans cette expression sont évaluées empiriquement. Un ajustement statistique est effectué sur les valeurs expérimentales, la paramétrisation originale de la méthode CNDO/2 étant conservée. Dans cette optique, deux évaluations des dérivées β_{μ, ν}′ et β_{μ, ν}″ des intégrales de résonance par rapport à la distance CH sont proposées:

1. β_{μ, ν}′ et β_{μ, ν}″ sont supposées constantes.

2. β_{μ, ν}′ et β_{μ, ν}″ sont posées proportionnelles aux dérivées S _{μ, ν}′ et S _{μ, ν}″ des intégrales de recouvrement.

L'une ou l'autre de ces évaluations traduit l'évolution des constantes de force grâce à l'effet conjugué d'une contribution ‘statique’ (fonction des éléments de la matrice densité) et d'une contribution ‘dynamique’ (dépendant de la variation pendant la vibration de ces mêmes éléments).

An analytical expression is sought for k _CH bond force constants, within the frame of the CNDO/2 method. Some integrals appearing in this expression are empirically valued. A statistical adjustment is effected on the experimental data, the original parameters of the CNDO/2 method having been preserved. Two appraisals of β_{μ, ν}′ and β_{μ, ν}″ derivatives of the resonance integrals are compared with each other.

1. β_{μ, ν}′ and β_{μ, ν}″ are constants.

2. β_{μ, ν}′ and β_{μ, ν}″ are in direct ratio to S _{μ, ν}′ and S _{μ, ν}″ derivatives of the overlap integrals.

Either of these valuations explains the evolution of the bond force constants, thanks to the aggregate effect of a ‘static’ contribution (function of elements of the density matrix) and thanks to a ‘dynamic’ contribution (depending on the variation during the vibration of these same elements).     

Extension de l'etude du spectre d'arc du scandium: II. Interprétation théorique

A theoretical investigation of the even and odd configurations of Sc I has been performed to check the experimental labelling of energy levels and to display the effects of configuration interaction. The values of the radial parameters have been fitted by the least-squares method and compared to ab initio integrals.     

Etude de l’effet des dimensions des cristallites sur l’observation des raies du spectre excitonique de CuI

Previous investigations [1] have suggested the possibility of an effect of the sizes of the grains of a thin layer on the intensity of the exciton lines which can be observed at low temperatures. An experimental study of this effect has been carried out with CuI. It is possible to show that for thin layers formed of very small crystals, the exciton lines do not appear at all in the spectrum. The first line (n=1) appears in thin layers formed of crystals which have a size about of 860 Å, but the second line (n=2) appears only when the size of the grains is about 1000 Å. Finally, the third line (n=3) is observed only in single crystals or in thin layers which have crystals about 2000 Å?3000 Å [3]. The theory of this effect will presumably be mathematically very complicated but the physical principles implied in the described observations have been examined qualitatively in a previous paper [4] and are in good agreement with the observations.     

Etude par résonance paramagnétique électronique de la variation en fonction de la température du couplage hyperfin du radical tert-butyle

Le radical tert-butyle obtenu par irradiation γ à 77°k du chlorure de tertiobutyle peut se présenter sous deux structures différentes. Une forme pyramidale, ayant pour origine les contraintes exercées par le réseau cristallin, et pour laquelle le couplage a ¹³C avec le carbone central est une fonction décroissante de la température. Une forme plane pour laquelle a ¹³C croît avec la température. L'étude du radical plan a permis de séparer les contributions relatives des vibrations, a ¹³C _v , et de la polarisation des électrons des couches internes et des liaisons, a ¹³C_pol, à la valeur du couplage a ¹³C. Le résultat obtenu confirme la relative insensibilité de a ¹³C_Pol, par rapport aux détails des substituants du carbone central, prévue par les calculs de Melchior.     

Mesures thermiques et profilométriques 3D pour étudier l'influence de la pression apparente sur la résistance thermique et la microgéométrie des contacts métalliques

Load effects on thermal resistance and on microgeometry of metal contacts : thermal and profilometric study. Thermal contact resistances R_c between pairs of metallic samples have been measured, depending on the pressure P as well as on the maximum pressure P^* reached during the evolution of the stress. Moreover, microgeometry measurement of surfaces in contact have been performed by 3D optical analysis. Metals of different compositions and machined differently were considered. Finally we suggest: lg R_c = K₁ + K₂.lg P + K₃.lg P^* with constants depending on the nature of the metal and the way it has been machined.We find that P^* neither changes too much the roughness parameters nor the Abbott's curve of the variation of the bearing area with the displacement of the plan of contact. On the other hand, the size of contact sports and their surface density are strongly modified as the bearing area changes; their modifications are almost independent of P^*, and are specific to each material.     

Influence des différents scénarios techniques, énergétiques et économiques sur la décision d'installer un nouvel échangeur de chaleur

Influence of different technical, energetic and economic scenarios on the decision of installing a new heat exchanger. This article proposes to examine the decision of installing a new heat exchanger in a working industrial unit in order to improve its energy performances. This examination is based on a new graphical method permitting us to approach the decision while taking into consideration the operating, technological and conjunctural variables.The analysis of the results shows that improvement in energy performances of an industrial unit does not always contribute to improving its economic performances. Factors that govern this contribution are in increasing importance: the choice of heat exchanger's configuration, the energy scenarios, the energy to be saved and the choice of the construction material of the heat exchanger.     

Etude des mouvements moléculaires du furanne en phases plastique et liquide par relaxation magnétique nucléaire

Les temps de relaxation spin-réseau du proton ont été mesurés dans les phases liquide et plastique du furanne pur et en solution dans le furanne perdeutéré, les relaxations nucléaires d'origine intra et inter-moléculaire ont été ainsi séparées. Des temps de corrélation pour le second harmonique sphérique τ₂ ont été déduits de ces mesures et comparés aux temps de corrélation du premier harmonique sphérique τ₁ obtenus par des mesures antérieures d'absorption micro-onde ; les valeurs du coefficient de diffusion translationnelle sont estimées en phase liquide.

Sur la base de ces résultats et en utilisant, pour la phase plastique, les donnés cristallographiques, un modèle est proposé pour dépeindre les mouvements réorientationnels des molécules dans chacune des deux phases condensées.

Dans le liquide, près du point triple, un modèle de type Ivanov s'applique avec des angles moyens de saut voisins de 50° et des durées de piegeage orientationnel de l'ordre de 1,3 ps. En augmentant la température, les temps de résidence tendent à devenir comparables aux durées de saut ou même inférieurs.

Dans la phase plastique, les sauts réorientationnels ont lieu de façon concertée (simultanée) entre les quatre orientations permises. Un modèle analytique décrivant les mouvements d'un ensemble de rotateurs dans un potentiel de symétrie rectangulaire permet d'expliquer les résultats expérimentaux en supposant que les sauts ont lieu de façon privilégiée entre puits de potentiel non adjacents. Un calcul numérique confirme l'existence de quatre puits de potentiel séparés par de faibles barrières et montre que les corrélations d'orientation ont principalement pour origine des intéractions de type Van der Waals entre atomes d'hydrogène de molécules voisines.     

Étude théorique des transferts couplés de chaleur et de masse lors de la désorption du couple zéolithe 13X-eau

A mathematical model based on scale changing has been established, in order to study heat and mass transfers in a zeolithe bed during water desorption. The model has been solved numerically by the finite-volume method, for the case of two-dimensional transfers. The numerical simulation is used to present the time-space evolution of the solid temperature, the pressure and the moisture content in the reactor and to determine the sensitivity to some parameters (heating temperature, outlet temperature, heat coefficients and reactor geometry). The effects of the particle diameter, the kinetic constant of desorption as well as the effective thermal conductivity are discussed.     

Etude des mouvements moléculaires en milieu liquide à partir du profil des bandes de vibration dans l'infrarouge et des fonctions de corrélation

We have studied the fundamental bands of chloroform and bromoform, both pure and in solution in various solvents; also the first harmonics of v ₁, and a few other harmonics and combination bands. The correlation functions of v ₁ and v ₂ and band moments of v ₁ have been calculated. The comparison of our results with those obtained in microwave and far IR and Raman spectra offers an opportunity to discern, in widths and correlation functions, what can be attributed to vibration and what originates in rotational diffusion. Our results are interpreted with the assumption that rotational diffusion is produced by small angles jumps and that the vibrational effect is very important. The v ₁ band shows an additional widening not accounted for in existing theories.     

Conductivité thermique de mousses cellulaires de carbone: Mesures par méthode flash et interprétation

This paper deals with the thermal diffusivity measurement of cellular carbon foams by the flash method. The thermal conductivity is obtained from the thermal diffusivity by using specific heat and density of carbon foam. These materials constitute non-homogeneous porous semitransparent media, hence the classic experimental set-up of the flash method must be adapted. A study is carried out in order to show the influence of the sample thickness, of the temperature, of the enclosed gas pressure and of the gas composition on the effective thermal conductivity. Identification functions are established in the monodimensional and bidimensional cases. Results indicate the dependence of the equivalent thermal conductivity upon the sample thickness. This result, attributed to radiative transfer, is confirmed by modelling. Radiative thermal conductivities are obtained with a Monte-Carlo code. Experimental and theoretical results are compared. It is demonstrated that the concept of effective conductivity is not relevant to these materials.     

Polarisabilités optiques moléculaires de dérivés benzèniques en solution déterminées par des mesures d'effet Kerr statique

Une expression théorique simplifiée de la constante de Kerr permet une comparaison cohérente des mesures d'effet Kerr statique et de diffusion Rayleigh dépolarisée. Cette comparaison ainsi que des mesures de moments dipolaires sont effectuées sur une série de composés benzèniques en solution diluée dans le tétrachlorure de carbone. La méthode d'exploitation des résultats expérimentaux conduit à la détermination du tenseur des polarisabilités optiques moléculaires α de ces composés. Un accord satisfaisant est obtenu en comparant la constante spécifique de Kerr S _k et l'anisotropie optique moléculaire γ², calculées à partir de ces polarisabilités, à leurs valeurs expérimentales.     

Evolution avec la température de la réflectivité, de l'indice complexe et de la sélectivité spectrale du nitrure de zirconium

The reflectance of zirconium nitride has been measured at 20, 500 and 700°C, within the range 0.22 to 5.50 μ m. The optical constants have been determined using Kramers-Kronig analysis. The spectral selectivity of this compound noticeably depends on temperature; the absorptance increases from 0.28 at 20°C to 0.36 at 700°C while the emittance increases from 0.06 to 0.14.     

Amélioration du transfert thermique par un dépôt poreux sur la paroi d'un échangeur tubulaire

A study of flow regime and heat transfer in an annular heat exchanger partially filled with a porous medium is presented in this work. Constant heat flux and constant wall temperature boundary conditions on the inner cylinder are considered, while the outer cylinder is assumed adiabatic. The study is for both the thermal entry region and the thermally fully developed region. The flow in the porous region is modelled either by the Darcy-Brinkman equation for which an exact solution is developed or by the Darcy-Brinkman-Forchheimer equation in order to take into account inertial effects. For this case a numerical solution based on a control volume method is discussed. The results emphasize the effect of the porous layer attached to the inner cylinder on the thermal development length and heat transfer rate. It is shown that the porous substrate reduces the thermal entry length. When the effective thermal conductivity of the saturated porous medium is of the order of the fluid thermal conductivity, the local Nusselt does not vary monotonically with the thickness of the substrate. However, the use of a porous matrix always leads to an increase in the heat transfer rate provided its thermophysical properties and thickness are well chosen.