La luminescence de l'europium divalent dans les fluoroberyllates alcalino-terreux

The M_1?xEu_xBeF₄ (M = Ca, Sr, Ba) phases are efficient UV phosphors. Emission consists of both a broad d-f band and narrow f-f lines, the relative intensities being strongly dependent on host lattice, europium content and temperature     

Nature de l'effet jahn-teller cooperatif dans les fluorures alcalinoterreux ACuF4. Influence sur les proprietes magnetiques

ESR and magnetic susceptibility measurements performed on the ACuF₄ copper 2+ fluorides (A = Ca, Sr, Ba) show that antiferrodistortive ordering of the copper orbitals in CaCuF₄ and SrCuF₄ (of KBrF₄-type structure with square planar coordination of the Cu²⁺ ions) leads to paramagnetism down to 4.2 K. Ferrodistortive ordering in BaCuF₄ (BaMnF₄ type - elongated octahedral coordination with parallel dz² orbitals) gives a 1-D antiferromagnetic behavior.     

Utilisation de la deflection photothermique pour l'etude de l'effet du chlore sur l'absorption par les etats de la bande interdite dans a-SiHCl

Chlorinated and hydrogenated amorphous silicon (a-SiHCl) have been prepared by glow-discharge of a gas mixture (SiCl₄+H₂). Optical absorption and photothermal deflection spectroscopy have been made on samples with different chlorine contents. Due to its higher sensitivity, the photothermal deflection spectroscopy shows a decrease of the gap states absorption at low energies when the chlorine content is increased.     

Interactions magnetiques dans les fluorures de type weberite

In the fluorides of weberite type the various magnetic behaviours have been correlated with structure and nature of the cations.     

Emission f-f de l'europium divalent dans les phases MFCl (M = Ca,Sr, Ba) et BaLiF3

The luminescence spectra of the M_1?xEu_xFCl (M = Ca, Sr, Ba) and Ba_1?xEu_xLiF₃ phases with PbFCl and perovskite structures, show narrow lines cooresponding to transitions inside 4f⁷ configuration.     

Étude en impulsion de l'injection et de la conduction dans les structures Si-SiO2-Al

We have studied the injection mechanisms at the Si-SiO₂ interface and the conduction in Si-SiO₂-Al devices. The use of a pulsed voltage makes it possible to apply fields of the order of 10⁷ V/cm to the thin oxide layer. The experimental I(V) characteristics prove the existence of the following mechanisms:-at weak fields (ε<3.5 x 10⁶ V/cm), Schottky injection of electrons at the temperature of the thermal lattice vibrations;-for 3.5 x 10⁶ V/cm<ε<8 x 10⁶ V/cm, injection of hot electrons (T_m? 6500°K) produced by an avalanche effect;-for ε>8 x 10⁶ V/cm, tunnel injection of electrons of an effective mass m? 0.4m₀.     

Absorbeurs selectifs de l'energie solaire elabores a partir d'acier inoxydable : Influence de la composition chimique et de l'etat metallurgique de l'alliage sur les proprietes optiques

Efficient photothermal converters exhibit high solar absorptance and low thermal emittance. An original method has allowed spectrally selective coatings to be obtained by chemical conversion of stainless steel. These coatings exhibit high selectivity (absorptance higher than 95 % and emittance lower than 20 %) and excellent durability.The dependance of the optical properties of these coatings on the chemical composition and metallurgical state of the substrate is studied. Low emittance appeared to be closely related to the corrosion resistance of the metallic surface in the treatment bath. Owing to this fact, convenient conditions can be found to produce selective coating for each alloy.     

Etude de la segregation d'impuretes telles que Se et Te dans l'antimoniure de gallium

We have study the segregation coefficient k of selenium in gallium antimonide for slightly and heavily doped single crystals. We show that k is a function only of the growing rate. We compare there results to tellurium ones. We deduce the value of k when v is zero: this value is k_o = 0,013. We show also that the variation of the number of free carriers is linear with N_d the number of donors, in n type as well as p type.     

Application de l'etude matricielle de la transformation de burgers a la precipitation des composes NiMo dans les aciers maraging a 18% Ni

This paper concerns a geometrical analysis by coincidence lattice method of aging processes in maraging steels. It is shown an equal formation probability for NiMo and Ni₃Mo precipitates in this martensitic matrix.     

Conduction à l'obscurité dans les couches minces de CdTe,type N

In order to determine the trap levels in the band gap of n-type CdTe thin films, we have studied the dark conduction in high electric fields. Our study enables us to propose values for the density of a uniform distribution of traps located in the band gap, η_t ≈ 5 × 10¹³ eV^-1, and for the position of the Fermi level, E_c - E_F0 ≈ 0,666 eV. We have also determined the width and the position of this distribution in the band gap to be ΔE = E₂ - E₁ ≈ 0,107 eV and E_c - E₁ ≈ 0,5 eV respectively. These values enable us to construct a possible model for the band gap.     

Resonance paramagnetique electronique du manganese divalent dans l'alumine β

Single crystals of β alumina doped with manganese ions have been prepared by high frequency induction melting and slow cooling, followed by Na⁺ - Mn²⁺ exchange in molten MnCl₂. EPR study shows that Mn²⁺ ions occupy two kinds of sites. The first one, with an axial zero field tensor corresponds to the tetrahedral positions in the spinel blocks, close to the sodium planes. This seem to indicate that aluminium vacancies in β alumina are partly located in this site. The other, with an orthorhombic zero field tensor, and high D and E values, belongs to the conduction planes.     

Mesure par diffraction de rayons X de l'anisotropie du pyrocarbone dans les particules combustibles d'un reacteur a haute temperature

An X-ray diffraction technique for measuring the anisotropy factors of pyrocarbon coatings on fuel particles for high temperature reactors has been developed. The measurements yield directly absolute values of BAF averaged over about 100 particles, with an accuracy of ± 0.01. A modified procedure for measuring a single irradiated particle is considered. BAF-values obtained from discs and from particles coated in the same run do not exhibit any simple correlation.     

Détermination expérimentale de l'efficacité des électrons rétrodiffusés dans l'émission électronique secondaire de l'or et de l'aluminium

This paper deals with the experimental determination of the backscattered electron contribution to the secondary electron emission yield of gold and aluminium.We have used Bronshtein's method based on the (δ, η) diagrams, and the measurement range has been extended towards the low energy region in order to complete earlier results.We have established the validity limits of the method, and have achieved the expected accuracy.     

Etude dilatometrique de la transition ferroelastique de l'orthophosphate de plomb monocristallin

Single crystals of lead phosphate Pb₃ (PO₄)₂ have been grown by Czochralski pulling. Thermal expansion has been investigated in this material by standard dilatometry and by X-ray powder diffraction, between room-temperature and 250° C, across the 180° C ferroelastic phase transition. Behaviour of the spontaneous strain, a most important quantity for ferroelastics, is deduced from thermal expansion measurements along the b and c monoclinic axes. The large room-temperature value of the strain component corresponding to a shear in the (100) plane of the crystal, 22 × 10^?3, agrees with the value derived from lattice parameters. A jump of the spontaneous strain is observed at the Curie point which discloses a first order transition. This result, confirmed by differential thermal analysis measurements, is also consistent with recent birefringence data. The estimated transition heat is 0.17 cal/g.     

Etude de la cinetique d'oxydation de magnetites finement divisees. I - Influence de la substitution par l'aluminium

The kinetics of finely divided, (Fe²⁺Fe³⁺_2?xAl³⁺_x)O₄ type (0 ? x ? 2), substitution magnetites oxidation into metastable phase γ(Fe³⁺_1?yAl³⁺_y)₂O₃ (x = 3y) has been accounted for by means of the law of diffusion, under variable working conditions, of the cationic vacancies generated at solid-gas interface with a diffusion coefficient that decreases when the stoichiometric difference is high. The crystallites size and the trivalent substituant percentage increases, in both cases, slow down the reaction rate.     

Anisotropie de la decomposition d'un solide. II-mise en evidence d'un gradient thermique dans la phase solide a l'aide d'un thermographe infra-rouge

Monocrystalline platelets of pentahydrate Copper sulfate are submitted to temperature and pressure conditions such that they cause this salt to be converted into a trihydrated salt. Thermal equilibrium of these platelets is ensured before the reaction starts. Temperature is recorded as moving away from the superficial domain undergoing conversion, by means of an I.R. scanning camera fitted with a cooled detecting device. The occurence of a thermal gradient from the reactive solid-solid interface can, thus, be shown. It seems, therefore necessary to take into account the physical properties of the crystal, which are involved in the occurence of thermal gradients in the solid, to explain the anisotropy of this interface displacement during a conversion where a gas is removed.     

Amelioration des conditions de synthese de l'hydrure de magnesium a l'aide d'adjuvants

Addition of metals or alloys whose hydrides have a high dissociation tension allows a considerable increase of the hydrogenation rate of magnesium. The influence of the nature of the adjuvant and its specific concentration, of temperature and hydrogen pressure on the reaction rate, have been carefully studied. The pressure-composition isotherms have been determined especially when the adjuvant is LaNi₅.     

Influence de la substitution oxygene-fluor dans les materiaux ferroelectriques de structure derivee de la chiolite

All oxide fluorides with ferroelectric properties so far investigated have been obtained by slight substitution of oxygen by fluorine in ferroelectric oxides (less than 10 %) : the Curie temperature decreases very rapidly as the substitution rate rises. A solid solution with formula Na₅(W_3?xNb_x)O_9?xF_5+x (0 ? x ? 2) and a chiolite-related structure has been identified in this work. Three phase transitions have been observed. The corresponding temperatures decrease slowly with increasing x, but the high Curie temperature ($\text{T}{}_{\mathrm{<mtext>C</mtext>}}\text{?800}\text{K}$) observed for a relatively large substitution rate (for $\text{x}\text{= 0 :}\text{F}\text{O+F}\text{= 0.36}$) shows that the material belongs to a family of true ferroelectric oxide fluorides. This conclusion is supported by the fact that neither the pure oxide nor the fluoride chiolites are ferroelectric.