Low frequency in situ metrology of absorption and dispersion of sound absorbing porous materials based on high power ultrasonic non-linearly demodulated waves

The present work is related to acoustic in situ free-field measurements of sound absorption in porous materials, such as cellular plastic foams, glass-wool or recycled felt materials. The emphasis is given towards fine metrology of absorption in view of potential industrial applications. A powerful ultrasonic array working at 40 kHz is used. It enables to measure absorption acoustical data down to 100 Hz due to the exploitation of the non-linear ultrasonic demodulation phenomenon in air. Fine measurements of acoustic absorption are compared to numerical predictions based on the “equivalent-fluid” model (when the squeleton frame is motionless), and to some measurements performed on a Brüel and Kjaer impedance tube. Dispersion curves are also measured and compared to the numerical predictions for some automotive felt materials which are compressed at various ratios. Data obtained with a dedicated portable instrument are also discussed for the same type of materials and configurations.     

Photoluminescence processes in LaOCl:Bi and YOCl:Bi

The luminescence of rare earth oxychlorides LnOCL:Bi (with Ln = La and Y) is studied in detail. The main excitation bands, responsible for the blue and UV fluorescences, are attributed to the bismuth activator substituting for Ln³⁺ in the rare earth tetragonal site. The weak shoulder of these bands and the complex decay times are assumed due to a parasitical phase, La₂O₃ or Y₂O₃ doped by bismuth, an X-ray analysis confirming their presence in the samples. Finally, the existence of these phases does not seem to have an important effect upon the interpretation of the luminescence processes of the main center, as shown by the configuration curve model.     

LEED-Auger characterization of GaAs during activation to negative electron affinity by the adsorption of Cs and O

A combination of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) has been used to study the formation of the negative electron affinity (NEA) condition on surfaces of p-type, degenerate, (100) and (111) GaAs. Activation to NEA is achieved by adsorbing Cs and O onto atomically clean GaAs in repetitive cycles of first Cs and then O. Before activation, the clean GaAs surfaces exhibit their characteristic LEED patterns. However, once obtained, there is no significant correlation between the quality of these LEED patterns and the final activation. The adsorption of both Cs and O during activation to NEA is amorphous. Auger measurements have shown that the first photoemission maximum occurs after the adsorption of about a half monolayer of Cs. The initial O adsorption occurs on the GaAs surface between the Cs atoms. The adsorbed O interacts strongly with Cs at any stage during the activation. Peak photosensitivities, after completion of the Cs and O adsorptions, were in the range 400 to 1100 $\text{μA}\text{lumen}$. The final activation does not correlate with the quantity of Cs and O on the surface. The temperature dependence of the photosensitivity of NEA GaAs (100) activated at ?170°C has a broad maximum at about ?50°C and a subsidiary maximum at about 160°C. In addition, the photoemission at ?170°C can be either increased or decreased by having heated the sample up to 200°C, even though no Cs or O desorption has taken place. These results can be traced to changes in work function rather than to changes in bulk properties. While the LEED patterns from clean GaAs show no structural changes with temperature, such changes are observed when Cs is on the surface. It is suggested that changes both in photoemission and in LEED patterns are due to the temperature-induced mobility of Cs on GaAs. An atomic model for the NEA surface is discussed in terms of a layer of Cs and O atoms about 10 Å thick on the GaAs.     

Etude en fonction du temps de l'oxydation des magnetites substituees a l'aluminium et au chrome au moyen de la conductivite electrique

The oxidation of substituted magnetites (Fe²⁺Fe_2?x³⁺M_x³⁺O₄^2?(O < x < 2) into the lacunar phase γ(Fe_1?z³⁺M_z³⁺)O₃^{2 ?} (x = 3z) was followed over time by electrical conductivity over the temperature r Curves σ = f(t) are different according to whether we deal with an n or p-type spinel. However, for intermediate substitution rates (1,1 < x < 1,8), n-type ferrichromites are oxidized after an initial period in the same way as p-type semi-conductors.For the initial period, the chemisorption kinetics of oxygen upon these n-type samples, reduced under vacuum, was found to follow Elovich's law with an activation energy depending on the degree of coverage.     

Les Sels de Bismuth (III) Comme Catalyseurs Dans la Reaction D'ouverture Des Epoxydes Par Les Amines

BiCl 3 and Bi (OTf) 3 catalyze the opening of epoxides ( 1-7 ) by amines ( 8-12 ). High regioselectivities are observed. BiCl 3 et Bi (OTf) 3 catalysent la réaction d'ouverture des époxydes ( 1-7 ) par les amines ( 8-12 ). La réaction est fortement régiosélective.     

Complexes of Tin(IV) Tetrachloride with 3-Formylpyridine Semicarbazone and Thiosemicarbazones

The reaction of tin(IV) tetrachloride with 3-formylpyridine semicarbazone and different 3-formylpyridine thiosemicarbazones produces [Sn(HL)CL 3 ][SnCl 5 ] where HL stands for the neutral ligand. The tin(IV) complexes were characterized using a variety of spectroscopic techniques. Coordination through the pyridine nitrogen occurs in all cases. Solvation studies in DMSO indicated that dissociation of the ligands and their complete replacement by solvent molecules occurs.     

Le Triflate de Bismuth(III) Comme Catalyseur Dans la Reaction de Carbonyl-Diels-Alder

In presence of bismuth(III) triflate, a carbonylated electrophile (ethyl mesoxalate or glyoxylate) and usual diene led selectively (60-100) the hetero carbonyl-Diels-Alder reaction with the ene reaction product. Bi(OTf) 3 exhibits strong catalytic activity and reacts under mild conditions. En présence de triflate de bismuth(III) le mésoxalate et le glyoxylate d'éthyle réagissent avec des diènes usuels pour donner un mélange de carbonyl-Diels-Alder et du produit de la ène réaction. Bi(OTf) 3 présente une forte activité catalytique et réagit dans des conditions douces.     

The Rate Problem of a Thermally Regenerable Ion-Exchange System

Earlier equilibrium studies have established the thermal dependence of the equilibrium between salt solution and a mixed bed of weakly basic and weakly acidic ion-exchange resins. High resin utilization can be achieved if the resin properties and equilibrium conditions are optimized; the equilibrium characteristics of polyacrylic acid and polyvinylbenzyldiethylamine resins are quite suited for the practical desalination of brackish waters.

However, the adsorption rates exhibited by normal-sized resin beads of this type are much too slow for satisfactory operation of the process because of the low concentration of protons available for transfer between the resins. It is shown that increasing the porosity of the resins improves amine resin kinetics 10-fold and carboxylic acid resin kinetics 6-fold. Nevertheless such improvements are still inadequate for practical purposes, and it is concluded that for satisfactory rates to be achieved systems having much shorter diffusion paths are necessary.

Two further approaches to the rate problem are discussed, both involving the synthesis of novel resin systems. A mixed bed of microbeads (10-20 p) reacts at acceptable rates but presents mechanical problems; the magnetic flocculation of finely divided magnetic resins is reported as one possible solution to this problem. Another avenue is the synthesis of normal-sized beads of the amphoteric and snake-cage variety. Resins of this type that exchange at suitable rates are described.     

Liquid-crystalline comb copolymers with lipoamino acid side chains Synthesis and structure study

In order to obtain liquid-crystalline polymers without using classical mesogenic groups, comb copolymers consisting of a polyacrylamide main chain and lipoamino acid side chains have been synthesized. These copolymers were obtained by the polymerization of lipoamino acid macromonomers. These macromonomers were obtained from, α,ω-aliphatic amino acids by linking a polymerizable group at the amino end and an α-amino acid at the carboxyl end. The macromonomers were then transformed into comb copolymers by free-radical polymerization. These comb copolymers exhibit mesophases both in aqueous solution and in the anhydrous state. The range of stability and the structures of the mesophases were determined by X-ray diffraction. Two types of structures were found, corresponding to the lyotropic lamellar and hexagonal mesophases. The influences of the nature of the amino acid and the water concentration on the domain of stability and the geometrical parameters of the mesomorphic structures were investigated.