Influence de la vitesse de graphitisation de produits carbones sur les variations dimensionnelles causees par l'irradiation neutronique

Two types of baked carbon were graphitised by heat treatments at rates varying from 50°C hr⁻¹ to 3300°C hr⁻¹. It was found that for both types of carbon, an increase in the heating rate leads to considerable changes in the texture of the graphitised product, particularly in the apparent density, helium density and in microporosity. In view of these textural differences in graphitised products, different dimensional effects of neutron irradiation were predictable. A method of simulation by bromination showed this to be the case. An irradiation experiment at 550°C to doses of about 3.5 × 10²¹ n cm⁻² EDN confirmed the results obtained by bromination.     

Etude physicochimique et electrochimique de la protection d'un acier au carbone par les monofluorophosphates. II. Influence de l'application d'une couche de peinture sur les surfaces pre-traitees

Résumé A l'aide de méthodes électrochimiques stationnaires (relevé des courbes courant-tension, mesure de la résistance de polarisation) et transitoires (mesure de l'impédance électrochimique) nous avons étudié le comportement en milieu NaCl 3% d'un acier au carbone (XC 38) ayant subi divers pré-traitements avant l'application d'une couche de peinture glycérophtalique.Les mesures d'impédance électrochimique ont permis la détermination de l'évolution au cours du temps des paramètres influant sur les performances de tenue à la corrosion des divers revêtements.On a ainsi mis en évidence — pour de longs temps d'immersion dans le milieu aggressif (>50 h) — l'effet bénéfique des traitements avant peinture et en particulier celui des post-traitements à base de monofluorophosphates, des surfaces ayant subi une phosphatation cristalline.

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XC 38 carbon steel samples pretreated in various ways have been coated with a glycerylphthalate-type paint. The behaviour of the painted samples in 3% NaCl solutions has been investigated by both steady-state (cathodic polarization curves and polarization resistance measurements) and transient (impedance measurements) electrochemical methods.Impedance measurements allow us to follow the evolution, as a function of immersion time, of the parameters influencing the corrosion performance of the different coatings.The favourable effects of prepaint surface treatments have been exhibited for long immersion times in aggressive media, particularly of those based on monofluorophosphate post-treatments applied on zinc phosphated carbon steel.

     

Etude par chronopotentiometrie des reactions anodiques sur electrodes de carbone dans les bains cryolithe—alumine

The anodic behaviour of carbon electrodes in cryolithe-alumina melts has been investigated by use of galvanostatic pulses in a wide range of intensity i (2.3–30 A/cm²). The corresponding transition times τ vary from 0.07 to 10 ms. The classical law describing the galvanostatic process (Sand law) is not obeyed; the experimental quantity i τ ^{$\text{1}\text{2}$}/C^o is always much smaller than the theoretical previsions. In addition, by operating in air-tight apparatus it is shown that a lowering of the pressure leads to a decrease of the transition time. These observations are interpreted in term of variation of the active surface of the electrode. A large part of these variations are attributed to the presence of gaseous compounds which remain on the electrode during the pulse of current. The consequences of this hypothesis on the derivation of the transition time are calculated. A semi-empirical treatment is proposed to take account of this process.     

Etude du mecanisme d'oxydation des noirs de carbone par l'ozone en milieu aqueux

The kinetic study of the oxidation of carbon blacks in an aqueous medium shows two distinct and simultaneous reactions: formation of degradation products which are partially oxidized to carbon dioxide, and a direct oxidation into carbon dioxide. The ratio of oxidized carbon in both reactions is constant and characteristic for a given carbon black. It is shown that degradation products arise from less organized areas of the particles. Determination of crystallinity of the carbon blacks by X-ray diffraction indicates a degree of crystalline disorganization directly related to the interplanatary d spacing. The relation established between the fraction degraded by oxidation and the disorganization factor leads to a classification of the carbon blacks according to the degree of perfection of their interplanar d spacing distribution pattern.     

Role de la temperature de cuisson sur les proprietes de l''anhydrite

The paper briefly reviews the effects of the calcination temperature on some anhydrite properties. The different treatments modify: specific area, granulometry, exoemissivity and dissolution kinetics. Some of these evolutions can be interpreted by a variation of the defects due to the grain sintering.

Résumé

Dans cet article, les auteurs examinent l'effet de la température de cuisson de l'anhydrite sur certaines de ses propriétés. Ces différents traitements modifient notamment: la surface spécifique, la granulométrie, le pouvoir exoemissif et la cinétique de dissolution. Certaines de ces évolutions peuvent s'interpréter par une variation des défauts dûe au frittage des grains.     

Etude de l'action des acides de lewis sur les polymères. Application au greffage de macromolécules sur les surfaces solides

The interaction between a solid surface (metal oxides, carbon blacks…) and macromolecules carrying a carbenium ion formed by a Lewis acid attack, results in a significant insolubilization of the polymer on the surface. Physical modifications of hydrocarbon polymers under the action of Lewis acids have been followed. Saturated (polyethylene, polypropylene, polyisobutylene), unsaturated (polybutadiene, polyisoprene, styrene-butadiene and isoprene-isobutylene copolymers) and styrene polymers have been investigated. Lewis acids generally extract a hydrid ion or add a proton to the double bond of a polymer. In both cases, a polymeric cation is formed: its evolution depends essentially on its stability and on the vicinity of chemical groups on the backbone of the polymer. The carbenium ion reacts with the surface hydroxyl groups. The amount of polymer irreversibly fixed on the solid surface exceeds the one required for the formation of the monolayer. It depends both on the concentration of polymeric cations and on the accessibility of the surface groups to the Lewis acid.     

Influence de l'adsorption du depolarisant ou d'un produit de la reaction electrochimique sur les courants polarographiques—XVIII. Etude de la dimerisation de radicaux mercuroorganiques par voltammetrie a variation lineaire de tension

Compounds with the general formula ArHgX, adsorbed on a mercury electrode are reduced to free radicals, which dimerize on the surface of the electrode. A study by linear potential sweep voltammetry shows that the dimerization involves two free radicals and establishes the validity of the theory elaborated previously for that type of reaction.     

Greffage de molécules à la surface du silicium par voie électrochimique

The hydrogentated surface of silicon exhibits remarkable properties, but poor resistance to oxidation. To improve its stability, surface hydrogen has been replaced by several organic groups. Such grafting can be carried out chemically by a multi-step reaction scheme. However, an electrochemical approach allows direct reaction with the hydrogenated surface. The porous-silicon surface has been partially methoxylated by a controlled anodic dissolution. If has also been methylated using a non-destructive anodic process, with a yield of 80%, limited only by steric hindrance. The methylated surface of porous silicon exhibits a stability against oxidation increased by an order of magnitude.     

Synthese asymetrique par voie electrochimique. Etude de l''influence des differents parametres sur le rendement optique

The influence of different factors which could affect the optical yield of an asymmetric reduction, made by an electrochemical method in the presence of inducing agents, has been investigated in the case of the electrolysis of phenylglyoxylic acid at a mercury cathode. Maximum optical yield is obtained with tiny amounts of a suitable alkaloid; the ability of such compounds to induce optical activity depends both on its adsorption at the mercury and the nature of the substrate. Electrolysis must be done with relatively low current density and with efficient stirring of the mercury pool in order to regenerate usable inducing agent. Subject to the previous determination of the optimum pH, the optical yield of the reaction studied may exceed 20 per cent. The rate determining step of the induction would be the formation of an adsorbed complex between the carbanion and the inducing agent, the protonation of which occurring with at least partial retention of the carbanion configuration.     

Equilibres acide-base donnes par les 1–4 benzodiazepines en solution hydro-alcoolique; incidence sur leur comportement a l'electrode de mercure

The electrochemical behaviour at the mecury electrode of 1, 4-benzodiazepines and 1, 4-benzodiazépines-2-one was investigated. Medazepam, diazepam and desmethyldiazepam were reducibles according to a 2 F.mol^?1 process. The plot of half-wave potentials vs pH is in agreement with the results of the spectrophotometric uv analysis: the more basic atom of the diazepine ring was that of the imine bond (C₅ = N₄).The introduction of an OH substituent on the carbon 3 resulted in 4 F.mol^?1 process (oxazepam and lorazepam). The 2 F.mol^?1 reduced intermediate was isolated. It underwent an acid-catalysed dehydration reaction and yielded in slightly basic solution a dimeric insoluble product. For oxazepam, the dimerisation initiated a disproportionation reaction.     

Etude de la reduction de l'oxygene sur les phtalocyanines monomeres et polymeres—III. Phtalocyanines monomeres de fer en couche mince sur l'or

Two kinds of iron phthalocyanine electrodes are prepared by vacuum deposition on gold substrates. The show remarkable differences of activity concerning O₂ reduction, and are investigated by X-ray, optical spectroscopy and X-ray photoelectron spectroscopy (ESCA) techniques. As predicted in a previous paper [3], the iron ion in the active form, is presenting a high spin d⁵ configuration.     

Etude physicochimique et electrochimique de la protection d'un acier au carbone par les monofluorophosphates. I. Influence d'un traitement de conversion chimique

Résumé A l'aide de méthodes électrochimiques stationnaires (relevé des courbes courant-tension, mesure de la résistance de polarisation) nous avons étudié le comportement en milieu NaCl 3% d'un acier au carbone (XC 38) ayant subi une phosphatation cristalline ou amorphe puis un post-traitement par les monofluorophosphates de zinc ou de potassium. Nous avons montré que tous les traitements rédusaient la corrosion du substrat métallique mais à des degrés différents selon le type de traitement.L'analyse physico-chimique des couches issues des différents traitements a été conduite par analyse chimique, microscopie électronique à balayage et spectroscopie de photoélectrons (ESCA). Dans le cas de la phosphatation cristalline nous avons montré qu'il y a formation d'une solution solide Fe_xZn_3–x(PO₄)₂-4H₂O(1x3); un post-traitement par le monofluorophosphate de potassium entraine la formation d'un composé de formule moyenne Zn_0.5K_1.1PO_3.35F_0.4.

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Different chemical conversion treatments have been applied to a XC 38 carbon steel. Using electro-chemical methods (steady-state cathodic polarization curves, polarization resistance measurements) it has been shown that all the treatments reduce the corrosion rate of the metallic substrate; nevertheless, great differences in the degree of protection provided were found depending upon the treatment used. A physico-chemical analysis of the layers resulting from different treatments has been carried out using chemical analysis, scanning electron microscopy and photoelectron spectroscopy.It has been shown that, in the case of zinc phosphatation, there is formation of a solid solution Fe_xZn_3–x(PO₄)₂-4H₂O(1x3); in the case of zinc phosphatation with a potassium monofluorophosphate post-treatment there is formation of a compound with average stoichiometric formula Zn_0.5K_1.1PO_3.35F_0.4.

     

Determination du courant limite d'electrodialyse par conductivite pour les faibles nombres de reynolds. Cas des solutions de tartrate acide de potassium

In the case of an electrodialysis operation, it is possible to obtain the limiting current by measuring the conductivity of the solution undergoing dilution. Starting from the equation of the constant flux of matter, when the concentration at the membrane falls to zero, (i/σ_d)_crit = δ relates the current density i and the conductivity of the diluate σ_d. The critical value of (i/σ_d) is easily obtained from the drawing of σ _dvs the potential applied to the apparatus. Experiments carried on aqueous solutions of KHC₄H₄O₆ at 8 g/l show that (i/σ_d)_crit) is well related to the Reynold's number by the relation αRe^β. It is then possible to calculate the thickness δ of the diffusion layer as a function of the Reynold's number.     

Etude de la reduction de l'oxygene sur les phtalocyanines monomeres et polymeres—II: Polyphtalocyanines de fer impregnees sur noir d'acetylene Y

The electrocatalytic activity of different iron polyphthalocyanines directly impregnated on carbon blacks during their synthesis is observed as a function of the synthesis parameters. The electrodes are formed by electrophoresis. A reaction mechanism function of the electrolytic medium and of the former treatménts of the electrode is proposed.     

Utilisation des techniques dielectriques pour l'etude in situ de l'activation de la sphalerite par les ions cu2+

Dielectric methods permit to follow in situ the activation by Cu²⁺ ions of a ZnS pulp in ionic solution. The conductibility of the solid is strongly increased. The influence of size, ionic strength and copper concentration is examined.     

Greffages mecanochimiques sur le polychlorure de vinyle. I. Polyméarisations amorcéaes par le malaxage

The grafting of methyl methacrylate and of methacrylic esters on poly(vinyl chloride) by mechanicochemical synthesis can be performed in the Brabender Plastograph. These monomers possess sufficient intermolecular plasticizing power to bring the poly(vinyl chloride)-monomer system into the viscoelastic state required for the formation of free radicals by homolytic scission of polymeric chains. Nevertheless several other monomers (among them the styrene) have interstructural plasticizing ability and can be grafted by mechanicochemical synthesis by addition of an intermolecular plasticizer or a monomer of the first class (methyl methacrylate). Also, except for basic monomers, the monomers stabilize poly(Vinyl chloride) during the mastication, probably because they act as free-radical scavengers.     

Etude du mecanisme de l'inhibition du zinc en milieu HCl n par les sels de phosphonium

The different steps of mechanism for zinc corrosion inhibition in a 1N HCl solution with phosphonium salts Ph_{4 ? y} P⁺nBu_y, X^? (0 ? y ? 4) are clarified according to the nature of the substituting groups.The secondary type of inhibition, involving the reduction products of these salts, ie phosphine does not appear when y is greater than 1. In this case, the electrostatic adsorption and chemisorption are the predominant steps.     

Influence de la pulsation liquide sur le comportement d'un réacteur électrochimique constitué par un lit de particules conductrices. I. Transfert de matière

Sans résumé