Complexes triflates de l’uranium

Uranium triflate complexes. We review here the different preparations of uranium triflates that we have developed in the course of these last years in our laboratory. Protonation of 〚U〛–R and 〚U〛–NR₂ (R = alkyl) bonds with pyridinium triflate constitutes a general and efficient route towards triflate complexes. This method is very suitable for the preparation of organometallic compounds such as U(Cp)₃(OTf), U(Cp)₂(OTf)₂(py), U(Cp*)₂(OTf)₂, and U(Cot)(OTf)₂(py), which have been crystallographically characterised. The homoleptic species U(OTf)_n (n = 3, 4) are easily prepared by heating a mixture of uranium turnings or UH₃ in triflic acid. By adjusting the temperature to 120 or 180 °C, either U(OTf)₃ or U(OTf)₄ is isolated. Treatment of UO₃ with pure or aqueous solution of triflic acid leads to the non-solvated uranyl triflate UO₂(OTf)₂, which is more conveniently obtained by heating a suspension of UO₃ in triflic anhydride. This reactant is an excellent dehydrating agent and enables the preparation of UO₂(OTf)₂ and Ce(OTf)₄ from the hydrated starting materials.     

Étude de l’activité prébiotique potentielle de l’acide borique

Boric acid has always been neglected in prebiotic chemistry, because it is not a major component of biological macromolecules. We argue here for a potential catalytic role of boric acid in peptides and nucleic acid synthesis. Today boron is an important component of seawater (0.4 mM). Its concentration at prebiotic times was probably higher. In water, boron mainly appears in the form of boric acid or its derivatives (esters and anhydrides).     

Caractérisation par émission acoustique de l’adhérence et de l’endommagement d’un revêtement : cas d’un revêtement WC–Co sur acier

This note presents a characterization method of the ‘adhesion’ of a coating by acoustic emission technique. In situ acoustic emission measurements which were investigated in terms of amplitude, absolute energy, position of the event, etc. were performed on WC–Co coated specimens prepared by HVOF (high velocity oxy fuel) during four-point bending tests. The microscopic observations of the specimens show two types of cracks: transversal cracks (regularly spaced on the coating surface) and interfacial cracks. Acoustic emission results show two different types of acoustic events in terms of absolute energy and amplitude, which are representative of two cracking mechanisms.     

Analyse élémentaire organique par diffusion de l'hydrogéne á travers la silice et par carbonisation

Résumé En mettant á profit les découvertes des savants français du début de ce siécle sur la perméabilité de la silice fondue pour l'hydrogéne, l'oxygéne et l'azote, nous avons montré que le dosage á l'échelle ultramicro de l'hydrogéne, de l'oxygéne par voie directe, du carbone et du squelette oxydo-carboné d'une molécule d'un même échantillon de substance contenant carbone, hydrogéne et oxygéne, était possible, avec des moyens extrêmement modestes.

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Summary By taking advantage of the discoveries made at the beginning of this century by French scientists with respect to the permeability of fused silica to hydrogen, oxygen, and nitrogen, we have shown that the determination on the ultramicro scale of hydrogen, oxygen by the direct way, and of carbon and of the oxydo-carbonaceous skeleton of a molecule is possible employing the same sample of a substance containing carbon, hydrogen, and oxygen, and by means of extremely modest equipment.

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Zusammenfassung Die zu Beginn dieses Jahrhunderts von französischen Autoren entdeckte Durchlässigkeit von Quarzglas für Wasserstoff, Sauerstoff und Stickstoff bei hohen Temperaturen bietet die Grundlage zur direkten ultramikroanalytischen Bestimmung von Wasserstoff und Sauerstoff. In der gleichen Einwaage einer aus C, H und O bestehenden Substanz lassen sich mit einfachen Mitteln auch der Kohlenstoffgehalt und der von Wasserstoff befreite Rest des Moleküls bestimmen.

     

Dispersion dans l’environnement routier et urbain de Pt,Pd, et Rh émis par les pots d’échappement catalytiques. Étude de la spéciation des éléments

This study highlights the dispersion into the French urban environment of platinum group elements (PGEs) used in catalytic converters. Differences were observed between Pt and Rh on the one hand, and Pd on the other one. One experiment, consisting in passing the corrosive gas emissions from engines over the metals heated to 1000 °C, showed that Pd was severely corroded by nitrogen oxides. It was concluded that Pd is emitted in nitrate form. Hydrolysis of this nitrate form leads to the formation of soluble species. In situ pH and E_h measurements in the soils concerned confirm this theory when the results are compared with the Pd species predominance diagram.     

Identification de modèles tribologiques par pendule

A model is developed to calculate the period and damping of a rigid pendulum's oscillations when submitted to friction. A theoretical investigation shows that the pendulum is a sensitive accurate tool for assessing friction phenomena of very low intensity. The process of measurement and analysis method is then used to identify a real pendulum.     

Élaboration de ciments apatitiques composites pour la rétention du césium et de l’iode

A composite apatitic cement made of hydroxyapatite loaded with β-tricalcic phosphate, zirconium phosphate and activated carbon has been elaborated and its ability to block cesium and iodine tested. It could be used as a buffer material in order to prevent the rapid release of soluble fission products (cesium and iodine) in aqueous solutions leaching the spent fuel, in the case of direct disposal of spent fuel.     

Méthodologie de synthèse des analogues à chaînes ouvertes de l’huperzine A

An efficient synthesis of two analogues of huperzine A via a one-step synthesis of E alkene.     

Détermination de la constante de vitesse de transfert d’hydrogène par le di-n-butyl 4,7,10-trioxaundécylstannane aux radicaux alkyle primaires

The rate constant of hydrogen transfer by dibutyl 4,7,10-trioxaundecylstannane to primary alkyl radicals was determined at different temperatures by the study of the free radical reduction of 6-bromo-1-hexene by this stannane. This work shows that this compound is an excellent substitute to tri-n-butyltinhydride     

De Réaumur à la Première Guerre Mondiale : les étapes de la maîtrise de l’acier,l’essor des aciers spéciaux

After a brief survey of the history of steel-making we shall concentrate on the period extending from Réaumur's work on that topic (1722) until World War one. Those two centuries saw the transition from empirical steel production processes to industrial steel manufacturing. We shall see how problems linked to the quenching and cementation of steel caused both scientists and steel manufacturers to develop efficient research leading to the establishment of an authentic metal science around 1900. These scientific discoveries then contributed to the prodigious development of special steel. Originally produced empirically to fulfill military requirements, these new types of material were to answer the needs of emerging industries, such as electrical engineering, as well as car manufacturing.     

Dosages en analyse minérale par extraction à l'aide de cations colorés : Dosages par extraction à l'aide de colorants basiques c. dosage de traces de phosphate par la safranine

An extraction method is described for the determination of traces of phosphate in the form of phosphomolybdate, in which safranine is used as complexing agent.     

Greffage de modèles de lignines sur polystyrène par lien imino

The synthesis of oligolignols grafted on polymer (LFP) was engaged in order to further study the properties and the biodegradation mechanisms of the different lignin sub-units in conditions more similar to those involved in the natural polymer. Simplified lignin models were grafted by an imine or amino link to a 1 % amino ethyl polystyrene resin without any protection of the hydroxyl functions. Infrared was used to characterize the supported models. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASsupported lignine / synthesis / lignin models     

Méthode de détermination des épaisseurs de films conducteurs par mesures de résistance de surface

To evaluate the thickness of the layers of a Ti/TiAl₃/Al system, experiments based on surface resistance measurements have been performed. The thickness of each layer is deduced from a comparison among calculated and measured values of resistance. We present here a solution for the simulation of surface resistance and the results obtained with this method on Ti/TiAl₃/Al systems.     

Caractérisation mécanique par flexion,nano- et macro-indentation de revêtements de SiC par PACVD micro-onde sur acier

Various SiC coatings were deposited on a steel substrate in a micro-wave plasma activated CVD device, then tested by a depth-sensing indentation instrument for measurement of their hardness and of their Young's modulus. Tests by three-point bending and by Vickers and Rockwell C indentation yielded consistent results about their rupture and adherence properties. The mechanical origin of the damage patterns induced by indentation is discussed on the basis of a numerical simulation of the testing.