Obtention et etudes des verres refractaires a base d'alumine et de sesquioxyde de neodyme

The amorphous phase obtained by rapid quenching with an hammer-anvil device of solar furnace melted binary mixture (1?x)Al₂O₃-xNd₂O₃ was investigated by thermal analysis and optical methods. By this way the domain of this amorphous phase obtention is determined. It is concluded that the amorphous phase is indeed a glass exhibiting no short range order which recristallizes circa 900°C in the two equilibrium phases βAl₂O₃ and NdAlO₃.     

Etude comparee des proprietes electriques des hexaborures d'europium et d'ytterbium

Electrical conductivity and Seebeck coefficient of semiconducting EuB₆ and YbB₆ have been investigated and compared. The results are discussed in terms of a band model in which the conduction band is the 5d or 6s band of the rare-earth sublattice.     

Etude par conductivite electrique et par spectrometrie d'absorption infra-rouge de la precipitation d'hematite a partir de ferrites lacunaires

The hematite precipitation from zinc or cobalt-substituted defective spinels of type $\text{(M}{}_{\mathrm{x}}{}^{\mathrm{2+}}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>(8?2x)</mtext><mtext>3</mtext>}}\text{■}{}_{\mathrm{<mtext>(1?x)</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ has been investigated using electrical conductivity and infrared spectrometry. The experimental data show that the temperature and kinetic of hematite precipitation depend of substitution extent x and might be closely associated with the cation vacancy in the spinel lattice. Stabilization of spinel solid solution by the cation is more pronounced for zinc than for cobalt and the effect of the cation is interpreted in terms of cation distribution in the spinel lattice.     

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.     

Determination par fluorescence x du phosphore incorpore dans les films anodiques d'aluminium et de niobium formes en milieu phosphorique

The incorporation of phosphorus impurities from phosphoric acid baths into the dielectric films formed on aluminium and niobium by anodic polarisation have been studied by X-ray fluorescence analysis. Results are discussed in relation to the metallic substrate and electrochemical parameters such as acid concentration, apparent current density and limiting voltage of film formation.     

Etude par diffraction des rayons X et par spectroscopies d'absorption infra-rouge et de diffusion raman des tantalates de strontium et de baryum de structure de type “blocs 1×2”

The structural characteristics from X ray diffraction powder patterns and indexed according to the Pnma space group are given for SrTa₂O₆ isomorphous with SrNb₂O₆ and for a modification of BaTa₂O₆ isomorphous with CaTa₂O₆. Cell parameters are refined for each compound. The force field of different niobates and tantalates of “1×2 block” type structure calculated from a vibrational analysis is then compared and the relative stability of the two modifications of BaTa₂O₆ is shown.     

Etude cristallochimique d'une nouvelle serie d'oxydes doubles de rhenium et de terres rares

The preparation of a new series of heptavalent rhenium compounds of Ln₃ReO₈ type (Ln = rare earths) is described. The crystals are orthorombic for Ln = La (space groups Ccmm, Ccm2₁ or Cc2m) and Ln = Pr, …, Gd (space group B22₁2), face centered cubic of fluorite type for Ln = Tb, …, Lu and Y.     

Caracterisation et localisation de la molecule de para-xylene dans une zeolithe pentasil : Cas des complexes formes par deux boralites avec le para-xylene

Precise X-ray powder diffraction investigations on pentasil type zeolites having respectively the H_1.4B_1.36Al_0.02Si_94.6O₁₉₂ and H_3.2B_3.2Al_0.02Si_92.8O₁₉₂ compositions reveal that the visual modifications observed on the step-scan profiles can be interpreted by localizing the centroid of the p-xylene molecule on the mirror plane of the 4c site in the Pnma space-group. Among the three possible orientations of the p-xylene molecule, the best fit is obtained for the solution where the line joining the methyl groups is parallel to the straight channel. Several trial and error calculations show that the organic molecule might present a statistical disorder with two simultaneous orientations in the channels.     

Etude par spectroscopie infra-rouge de l'eau adsorbee sur un xerogel d'oxyde de vanadium

The infrared study of water adsorbed on vanadium oxide xerogel (V₂O₅ 1,6 H₂O; V₂O₅ 0,5 H₂O) suggests two models for the structure of adsorbed water molecules. Water molecules of V₂O₅ 1,6 H₂O are involved in hydrogen bonds with the V = O groups. This arrangement would give an interlamellar spacing of 1,155 nm, as is found by X - ray studies. In the case of V₂O₅ 0,5 H₂O water molecules are not involved in hydrogen bonds and are this localized in the holes of the structure of vanadium oxide units.     

Interfaces semi-conducteur-electrolyte: Correlations entre le potentiel de bande plate et les echelles d'electronegativite

We have tried to correlate the flat band potential of a semi-conductor with the electronegativity scales. From this correlation the value of the flat band potential for new compounds could be deduced.     

Oxydation a haute temperature d'alliages Fe-20Cr-xAl

The oxidation of Fe-20Cr-xAl (x=2, 4, 6) and Fe-20Cr-6Al-0.025 Ce alloys was studied in pure oxygen (1 bar), the temperature ranging between 800 and 1100°C. The parabolic law is obeyed when alloys containing 4 or 6 % Al are oxidised. An α-Al₂O₃ scale is formed. The oxidation rate does not depend on the composition. The reaction is more complex with the Fe-20Cr-2Al alloy. An iron and chromium oxide layer is formed at the beginning. Alumina precipitates in the metal. After a few hours a continuous α-Al₂O₃ barrier is formed. The lower oxidation rate (when compared with other alloys) is then justified and the reason why the parabolic law is not obeyed can be explained.     

Etude des arseniures de cuivre precipites par l'hydrogene arsenie

The action of AsH₃ on cupric ion solutions permits to precipitate the phases Cu_5?xAs₂ and β-Cu_3?xAs, and also annother phase of composition near Cu₃As. It is shown that the variation of the composition of these arsenides results from an oxydo-reduction reaction, dependent on the pH of the solution, with trivalent arsenic as second member of the couple.     

Etude par spectrometrie d'absorption infra-rouge des facteurs influencant la transformation ordre-desordre de ferrites γ lacunaires derivant de l'oxydation de spinelles ferreux

The order-disorder phenomenon in defect γ-spinels of type $\text{(Fe}{}_{\mathrm{<mtext>(8?8y)</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{M}{}_{\mathrm{<mtext>8y</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{□}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ and $\text{Fe}{}_{\mathrm{<mtext>(8?2x)</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{M}{}_{\mathrm{x}}{}^{\mathrm{2+}}\text{□}{}_{\mathrm{<mtext>(1?x)</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ with M³⁺ = A1³⁺, Cr³⁺ and M²⁺ = Zn²⁺, Co²⁺, Mn²⁺ obtained from ferrous spinels was investigated using IR spectrometry. These compounds possess a vacancy ordering on octahedral sites for substitution extents of less than 0.30 and no ordering has been observed for substitution extent > 0.40. The ordering process is also influenced by the nature and position of the trivalent or divalent cation, particle size, heating temperature and oxidation time.     

Etude cristallographique et par resonance magnetique nucleaire de l'oxyfluorure ScOF

The structure and especially ordering between oxygen and fluorine has been determined by X-ray diffraction and NMR. ScOF has the baddeleyite type structure, where scandium is linked to three fluorines and to four oxygens. The cationic coordination of fluorine and oxygen is respectively triangular and tetrahedral. An ordered anionic distribution has been determined without any ambiguity by refining the structure for the various oxygen-fluorine configuration possibilities and by comparing calculated and observed second moments.     

Cristallochimie des sulfures mixtes de chrome et d'erbium

Inventory of a family of mixed chromium-erbiumsulfides (belonging to orthorhombic or monoclinic systems). Two of three parameters keep constant values, only the third (b) changes. The erbium atoms occupy octahedral regular and prismatic seven-coordinated sites. Chromium atoms are at the center of octahedral sites sometimes deformed by Jahn-Teller effect. We show how these structures are built from same basal pattern. An agreement between geometric pattern and chemical formula is laid down, it allows to confirm value of b parameter.     

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.     

Verres fluores a base de metaux lourds. Etude de la corrosion par la vapeur d'eau atmospherique

Chemical durability of $\text{ThF}{}_4\text{BaF}{}_2$ glasses has been investigated in atmospheric moisture conditions. Corrosion of the melt by the vapor contaminates all the vitreous bulk. The corrosion of OH free glass until T_g is only a surface phenomenon depending on a diffusion process.     

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.     

Rendement d'émission électronique secondarie déduit d'un modéle analytique: Comparaison avec les valeurs expérimentales pour l'or et l'aluminium

Conditions for comparison between experimental data and the theoretical results obtained from Amelio's analytical model of secondary electron emission are defined. We show that the quantity numerically comparable is the yield δ₀ due to the contribution to secondary electron formation by the primary electrons during their penetration into the target. The results show that the model presented leads to a choice of the mean free path which is unrealistic and inconsistent with the theory in terms of the Boltzmann transport equation.