Sodium-iron fluorophosphate glasses: Part 2: EPR and mössbauer resonance study

EPR and Mössbauer measurements have been performed on sodium-iron fluorophosphate glasses. Mössbauer spectra show the presence of Fe³⁺ and Fe²⁺ both in octahedral coordination sites. However, for low iron concentration, EPR suggests a small number of Fe³⁺ tetrahedra coexisting with Fe³⁺ octahedra. On the basis of these results, Fe³⁺ tetrahedrally and/or octahedrally coordinated, connected with P(O, F)₄ tetrahedra would behave as a network-former cation, whereas Fe²⁺ would substitute for Na⁺ as a netweork-modifier. Long range magnetic order previously observed by magnetic measurements has been confirmed for iron-rich compositions.     

The effective dielectric constant of plasmas — A mean field theory built from the electromagnetic ionic T-matrix

This work aims to obtain the effective dielectric constant tensor of a warm plasma in the spirit of the derivation of a mixing law. The medium is made of non point-like ions immersed in an electron gas with usual conditions relating the various lengths which define the problem. In this paper the ion dielectric constants are taken from their RPA responses as developed in a previous paper [1]. Furthermore the treatment of the screening effects is made through a mathematical redefinition of the initial problem as proposed in Ref. [1]. Here the complete calculation of the T-matrix describing the scattering of an electromagnetic wave on an isolated ion immersed in an “effective medium” is given. It is used for building , in the spirit of a mixing law, a self-consistent effective medium theory for the plasma dielectric tensor. We then extend the results obtained in Ref. [1] to higher orders in ion or dielectric inclusion densities. The techniques presented are generic and can be used in areas such as elasticity, thermoelasticity, and piezoelectricity.     

Cascaded optical parametric oscillation with a dual-grating periodically poled lithium niobate crystal

We demonstrate continuous-wave cascaded optical parametric oscillation in which the signal field of the primary parametric oscillator internally pumps the secondary parametric oscillator. Wavelength tuning is achieved with temperature tuning and a fan-out grating structure of a dual-grating periodically poled lithium niobate crystal. Above the secondary threshold the primary signal power is clamped, and all the other output powers increase linearly with the input pump power, in accordance with theory. Cascaded parametric oscillation offers a convenient and efficient way to generate multiple tunable outputs.     

From the dilute solution to the pure compound: Extraction strategy based on a multi-stage process of phase separation

It is very rare that a one-step process of extraction leads to the pure compound with a high degree of purity specified by an industrial application. The various stages of a synthesis process and possible secondary reactions may lead to the synthesis of more or less complex and highly diluted solutions. In this work, the rationale and strategy for extraction and purification of a high added value compound are discussed. All the thinking is based on the knowledge and the exploitation of phase diagrams and then developed for different unit operations of the process. The most significant research tools are the experimental data and the modelling of phase equilibrium to estimate the yield of each step of extraction. The significant example chosen involves all the basic methods of phase separation, starting with liquid-vapour equilibrium: stripping of high volatility components and then more or less complex distillation are classically employed. The theoretical plateau number can be deduced from the equilibrium equation curves. The second step is based on the study of the liquid-liquid equilibrium and is an intermediate step for enrichment of the solution when distillation is not possible. A final step based on solid-liquid equilibrium consists of the selective crystallization of the pure product at low temperature, in order to satisfy the requirements of purity and safety imposed by industrial use. The conclusion includes all isolation operations in the form of a general extraction and purification scheme.

     

On The Curvature of the Real Milnor Fiber

Let C be a germ at O R² of a real analytic plane curve, andC^C its complexification; let C_t B be a fiber of a real smoothdeformation of C in the ball B = B(O,). The following inequalityis proved between the integrals of real curvature k of C_t andthose of Gaussian curvature K of : The sharpness of this inequality is proved in the case whereC is a real irreducible germ. Similar results are proved foran affine algebraic curve C R² of degree d. 2000 MathematicsSubject Classification 14H20, 14H50, 53A04.     

Décomposition du percarbonate de O,O-t-butyle et O-isopropényle en solution: Acétonylation des esters,acides et nitriles

The Free-Radical Decomposition of O,O-t-Butyl and O-Isopropenyl Peroxycarbonate in Solution: the Acetonylation of Esters, Acides and Nitriles The free-radical decomposition of O, O-t-butyl and O-isopropenyl peroxycarbonate in substratres possessing mobile H-atoms (S? H) consists mainly in an induced chain process leading to acetonylated derivatives of the solvent. Fairly good yields are obtained but teh acetonylation of functional substrates often gives mixtures of isomers. In the case of methyl acetate, the acetonylation occurs on the C-atoms adjacent to the carbonyl (acylox moiety) and to the O-atom (alkox moiety). However, the relative amounts of the isomeric products depend on the concentration of the peroxycarbonate solutions; at lowest concentration, methyl 4-oxopentanoate (acyloxy moiety) is obtained selectively. It is assumed that the free radicals issued from the solvent are able to abstract H-atoms of other molecules of solvent before adding to the double bond of the peroxycarbonate; the more the peroxycarbonate solution is diluted the more the transfers from the C-atom adjacent to the carbonyl to the radicals adjacent to the O-atom are favoured. In the case of methyl alkanoates, H-transfers from the α-C-atoms to β-radicals of teh acyloxy moiety may account for the orientation of the process. Owing to similar H-transfer processes, the acetonylation of functional esters, of acids and nitriles is selective in most cases.     

Cobalt carbonyl catalyzed sRN1 carbonylation of aryl and vinyl halides by phase transfer catalysis

The phase transfer catalyzed, cobalt carbonyi catalyzed carbonylation (1 atm. P.) of aryl and vinyi bromides under photostimulation (350 nm) affords the corresponding unsaturated acids in high yield.     

High-yield solution-phase synthesis of di- and trinucleotide blocks assisted by polymer-supported reagents

A new solution-phase phosphoramidite approach is reported for oligonucleotide synthesis employing recyclable solid-supported reagents. It uses polyvinyl pyridinium tosylate as the activator of a nucleoside-3'-O-phosphoramidite in the coupling step with a 5'-OH nucleoside or dinucleotide. The resulting phosphite triester was either sulfurized or oxidized using polystyrene-bound trimethylammonium tetrathionate or periodiate. This method avoids complicated purification steps, as excess reagents are easily removed by filtration. [reaction: see text]