Proprietes interfaciales et reactions anodiques sur electrodes de carbone dans les bains cryolithe—alumine

The influence of gaseous compounds adsorbed on a carbon anode in cryolite—alumina melts without aluminium metal is studied by chronopotentiometry. It shows that the active surface of the electrode depends on different factors such as the nature, the density and the porosity of carbon, the acidity or the basicity of the melt, the nature of alumina (α alumina, γ alumina, hydrated alumina).A schematic representation of these mechanisms is proposed taking into account the wetting properties and the interfacial tensions of the used materials. It is possible to obtain a more coherent representation of the experimental observations and this concept appears as an original way in studying the reactivity of carbons.This research had lead us to improve the experimental technique by using an ultrasonic desorption of the gases or a hollow cathode with vacuum inside. A rapid determination of the alumina content can be obtained in definite experimental conditions; however this method requires the exact knowledge of the active surface of the working electrode.     

Evolution de diverses classes de carbones par irradiation neutronique

We investigated in previous studies the electronic properties of three graphites and one pyrocarbon damaged by neutrons [1,2]. We have now extended this work to various other carbons and we present in this paper a unified presentation of the results and a quantitative evaluation of the damage in terms of positive hole concentration in the valence band vs irradiation dose.The materials tested and the electronic properties which were measured are listed in Table 1. Irradiations were performed at 35°C in conditions identical to those of previous studies [1,2] at the “Melusine” reactor of the Grenoble Nuclear Center. Doses D ranging from 10¹⁷ to 4 × 10²⁰neutrons/cm² were recorded.The variation of the electrical resistivity at room temperature is shown in Figs. 1 and 2; of the Hall coefficient in Table 2 and of the magnetic mean susceptibility in Fig. 5. Figure 4 presents the relative transverse magnetoresistance (i irradiated sample; o non irradiated).The paramagnetic susceptibility χ_p was measured directly by EPR and its variation at 298°K with the neutron dose as shown in Fig. 6. Using the data on variation of χ_pwith temperature, the paramagnetic susceptibility is split[6] into contributions of the “localized paramagnetic centers” (Fig. 7) and of the conduction carriers (Fig. 8). The latter, χ_deloc (Fig. 8) increases more slowly than the dose, while χ,_loc (Fig. 7) increases first very rapidly, then much more slowly at high irradiation dose, when some recombination of point defects takes place.At higher doses when the Hall coefficient begins to decrease with increase of the dose, the data may be used to calculate the number N of positive holes created by irradiation: Figure 9 shows that approx. 4 holes/cm²are produced by 1 neutron/.cm². Figure 10 shows that χ_deloc of carriers is roughly proportional to √N, as expected for the band structure of graphite. It is clear from Fig. 11 that N_loc increases proportionally to N at low irradiation doses, but slower than N at high doses (recombination of vacancies).Although the radiation-induced changes in electronic properties differ widely for different carbons, it is possible to find some regularities in these results. We find that, in a given “class of carbons,” each unirradiated sample j may be characterized by an “equivalent neutron dose” Δ_j^p, Δ_j^p would be the dose needed to alter a property P of the most perfect material of the class from its value to the value equal to the unirradiated sample j. Then all the variations of P with (D + Δ_j^p) should be represented by a single curve for all the carbons of the same “class.” Such a parameter Δ_j^p can be useful only if Δ_j^p is independent from the nature of the property P: Δ_j^p = Δ_j.Figure 12 shows the variation of the diamagnetic mean susceptibility ?χ as a function of the “total dose” D + Δ_j^p (D = real dose). There is indeed a single curve and the values of Δ_j^p. (arbitrarily set at zero for pyrocarbon or for PGCCL) are listed in Table 3. In the same way Fig. 13 shows the “single curve” for the Hall coefficient A and Table 3 lists the corresponding values of Δ_j^A. It may be seen that Δ_j, is independent from the property P only for carbons belonging to the “class of graphites” or for those of the “class of pyrocarbons.” Papyex cannot be placed in any one of these classes, and there are not enough data for all the other carbons.For carbons in the same class, the value of Δ_j, can be interpreted in terms of defect concentration prior to irradiation.Finally, from the “single curves” of Figs. 12 and 13, an “irradiation path” for each class of carbons can be drawn by plotting ?χ vs A. Figure 14 shows that the irradiation and annealing paths of graphites are the same.     

Etude de la reaction du gaz carbonique avec le carbone aux temperatures elevees et sous de basses pressions

In order to complete and to determine more precisely previous data a new investigation on the kinetics of the C + CO₂ reaction has been carried out over a broader range of temperature (800–2000°C) and pressure (between 10^?4 and 10^?1Torr) with an improved apparatus and using more varied carbon samples. All types of carbons exhibit the same peculiarities in kinetics related to changes in the intrinsic reactivity of the surface. The general features of the kinetics are more similar than hitherto believed to those of the C + O₂ reaction. Indeed, the two reactions differ only in their absolute rates. This difference is discussed on the basis of the mechanism previously proposed by Duval.     

Decomposition de l'acetylene,de l'ethylene et du benzene sur le carbone aux tres hautes temperatures et sous de basses pressions

At high temperatures (1000–2000°C) and low pressures (10^?5?10^?2 Torr) ethylene, acetylene and benzene decompose helerogeneously on pyrolytic carbon giving mainly hydrogen and deposited carbon, with collision yields of the order of 10^?4. The kinetics of these carbon deposition reactions show some striking similarities with carbon removal reactions by oxygen or oxygenated compounds.The true reaction order of these decomposition reactions is one above 1400°C, but becomes smaller at lower temperatures. This behaviour, common in gas-solid reactions, is generally interpreted as an inhibition due to chemisorption of some intermediate or reaction product. Evidence is also obtained that decomposition of the hydrocarbon molecules only occurs on peculiar sites of the carbon surface, i.e. the decomposition is not a purely thermal process, but involves a specific chemical interaction with the surface.Moreover, the behaviour of the pyrocarbon surface in carbon deposition reactions is similar to that observed in gasification reactions, i.e. the reactivity of the surface accommodates itself to the temperature and pressure conditions, as revealed by the observation of “transitory” and “stationary rates”. Transitory rates show that the surface deactivates with increasing temperatures (Figs. 4 and 5) [from which a maximum in the stationary rate results (Figs. 1–3)] and decreasing pressures (Figs. 7 and 8). The interpretation assumes that reaction sites are continuously created as an effect of carbon atoms deposition, but also deactivated by a thermal healing process.A main difference between carbon deposition reactions from hydrocarbons and carbon gasification reactions concerns the temperature range where reactivity is temperature dependent: in carbon deposition reactions, deactivation of the pyrocarbon surface is still effective up to much higher temperatures (Fig. 12).     

Decomposition du methane sur le carbone aux tres hautes temperatures et sous basses pressions

The decomposition of methane on pyrocarbon as deposited in the course of the reaction has been investigated at high temperatures (1000–2000°C) and low pressures ( 10⁻² Torr). Experiments have been carried out with electrically heated filaments made of amorphous, pyrolytic or graphitic carbon. Rates of decomposition are calculated from rates of molecular hydrogen formation. Results are very different from those reported previously about the decompositions of ethylene, acetylene, propadiene and benzene on the same substrates.

1. (1) Electron microscope observations never showed any evidence of pyrocarbon deposition (Fig. 4). However some indications do not exclude this possibility in the deep pores of the Acheson graphite.

2. (2) Electrical resistivity measurements confirm the smallness—or even the complete absence—of any pyrocarbon deposition (Fig. 5).

3. (3) Even after many series of ten hours reaction time, the kinetics of methane decomposition are still strongly dependent upon the type of sample. This fact supports also the conclusion that pyrocarbon deposition is absent (in contrast, the pyrocarbon deposited in similar experiments using unsaturated hydrocarbons makes all types of samples tend progressively to a standard chemical behavior).

4. (4) On smooth pyrocarbon surfaces, methane decomposition is hardly detectable; collision yields are of the order 10⁻⁷ (as compared to 10⁻⁴ for unsaturated hydrocarbons, Fig. 6). But with porous samples—especially Acheson graphite—appreciable decomposition occurs; the kinetics show then many similarities with those reported previously for unsaturated hydrocarbons decompositions: especially the reactivity of the carbon surface is temperature and pressure dependent (Figs. 1–3).

The peculiar behavior of methane is explained in terms of chemical reactivity: unlike unsaturated hydrocarbons, methane does not undergo weakly activated chemisorption on the active sites of the carbon surface, and decomposition may then only occur after thermal accommodation of the gas molecules.

Résumé

On étudie la décomposition du méthane sous basses pressions ( 10⁻² Torr) et à hautes températures (1000–2000°C) sur le carbone qui se forme durant la réaction. Les résultats obtenus sont très différents de ceux acquis antérieurement dans des expériences similaires menées avec l'éthylène, l'acétylène, l'aliène et le benzène.

1. (1) Les observations à l'oeil nu et au microscope électronique n'ont jamais permis de déceler un changement de la surface du filament de carbone de départ qui puisse être imputé à un dépôt de carbone pyrolytique. Cependant certains indices n'excluent pas la possibilité d'un tel dépôt dans les anfractuosités profondes des graphites Acheson.

2. (2) Les mesures de résistivité électrique de l'échantillon de carbone ont confirmé l'insignifiance—ou l'inexistence—d'un dépôt de carbone.

3. (3) Même après une dizaine d'heures de réaction la cinétique de la décomposition du méthane varie beaucoup selon le type de filament de départ, preuve supplémentaire que le dépôt de carbone pyrolytique est quasi inexistant (en opérant pareillement avec les hydrocarbures insaturés, le dépôt de carbone pyrolytique uniformise rapidement tous les échantillons et unifie leur comportement chimique).

4. (4) Avec les échantillons à texture fermée et à surface lisse une décomposition du méthane est à peine décelable (rendement de choc 10⁻⁷ contre 10⁻⁴ pour les hydrocarbures insaturés). Avec les échantillons poreux—notamment les graphites Acheson—une décomposition mesurable a lieu: la cinétique s'apparente alors à celles observées avec les hydrocarbures saturés sur des surfaces de carbone lisses, c'est-à-dire qu'elle est révélatrice d'une réactivité de la surface du carbone variable avec la température et la pression.

On interprète le comportement singulier du méthane en terme de réactivité chimique: le méthane, contrairement aux hydrocarbures insaturés ne donne pas lieu à une adsorption très peu activée à la surface du carbone. Sa chimisorption, et par conséquent sa décomposition nécessitent une accommo- dation thermique beaucoup plus complète que dans le cas des hydrocarbures insaturés.     

Les proprietes electriques des fibres de carbone et leur interpretation sur la base d'un modele structurel

The electrical conductivity and thermoelectric power of rayon-based carbon fibers have been measured from liquid helium temperature to 1000°K on samples with various orientations and HTT. The measurements of the conductivities of interstitial compounds of these fibers with potassium show a large increase in density of free carriers due to the intercalation of metal atoms without significant modification of the mobility. The results are explained by using a band model which includes localized states and a mobility gap related to the microstructure of the fibers. It is shown that the density of free carriers is dependent on the number of local defects acting as acceptor centers. The mobility, practically independent of electron-phonon interactions, is limited essentially by scattering on structural defects. This concept is used to explain the conduction properties of carbon fibers saturated with potassium and to evaluate the mobility of the electrons in these compounds.     

Etude de la reaction du carbone avec le sulfure d'hydrogene H2S a hautes temperatures et sous basses pressions

Reaction of carbon with hydrogen sulfide at high temperature (1000–2000°C) and low pressure (10^?4–10^?2 Torr) exhibits the following features:—carbon disulfide CS₂ is the only carbonaceous reaction product. There is no indication that CS₂ would originate from a secondary reaction of the unstable carbon monosulfide CS. Some decomposition of H₂S into its elements is also observed (Figs. 1–3).—as already observed in other high temperature carbon gasification reactions, the intrinsic reactivity of the sample surface is temperature and pressure dependent. Consequently, transitory or stationary rates are observed, depending respectively upon a changing or a stationary surface state of the carbon sample (Fig. 4). The changes in the surface state are more marked for amorphous than for graphitized samples (Fig. 5).—below 1700°C, the true reaction order is smaller than one, as a consequence of the high stability of the carbon-sulfur surface complexes.—for ungraphitized samples, the carbon surface loses slowly and irreversibly its ability to change with changes in pressure and temperature.All these features were previously observed in the reaction of carbon with sulfur vapor: consequently hydrogen sulfide appears to behave simply as a gaseous sulfur carrier. The kinetic behaviour is in agreement with former interpretations assuming presence of peculiar reactive sites, which originate from chemical attack of the solid but disappear due to a surface thermal heating process.Confirming also previous assumptions, sulfur chemisorption strongly affects the reaction kinetics, as shown by the influence of H₂S traces on the kinetics of the C-O₂ reaction (Fig. 10):—at lower temperatures (< 1300°C) there is an important inhibiting effect on the CO production: H₂S is adsorbed strongly on the reactive sites which become inaccessible to O₂ molecules.—in the intermediate temperature range an enhancing effect is observed which is attributed to a hindrance of thermal healing (caused by chemisorption still present).Finally, the kinetic features as a whole are tentatively summarized in a comprehensive diagram where the consistency of experimental results appears clearly.     

Activation de la tourbe de sphaigne par le dioxyde de carbone et agglomération du charbon actif produit

The physical activation of peat moss using CO₂ has been investigated with respect to the operational variables and the characteristics of the active carbon produced. Peat coke is produced during carbonisation with a 30% yield. The activated carbon accounts for 25% of the initial air dried peat moss. Analysis of the active carbon has revealed a surface area up to 600 m²/g and a chemical reactivity similar to commercial active carbon as measured by the iodine, phenazone and phenol indexes. The active carbon thus produced is a powdered material, with a weak abrasive resistance. Agglomeration of the carbon and subsequent pelletization has been done with ammonium ligno-sulfonate, water, and the application of pressure. Resistant pellets can be obtained using 15-20% lignosulfonate and 3000 psi.     

Activation de la tourbe de sphaigne par le dioxyde de carbone et agglomération du charbon actif produit

The physical activation of peat moss using CO₂ has been investigated with respect to the operational variables and the characteristics of the active carbon produced. Peat coke is produced during carbonisation with a 30% yield. The activated carbon accounts for 25% of the initial air dried peat moss. Analysis of the active carbon has revealed a surface area up to 600 m²/g and a chemical reactivity similar to commercial active carbon as measured by the iodine, phenazone and phenol indexes. The active carbon thus produced is a powdered material, with a weak abrasive resistance. Agglomeration of the carbon and subsequent pelletization has been done with ammonium ligno-sulfonate, water, and the application of pressure. Resistant pellets can be obtained using 15–20% lignosulfonate and 3000 psi.     

Texture poreuse et reactivite d'un carbone vitreux et d'un coke de saccharose—I. Examen de l'oxyreactivite et de la carboxyreactivite

Vitreous carbon (in shape of plates and balls) and a saccharose coke, both heat-treated at 1000°C were examined. The variations of the oxyreactivity and of carboxy-reactivity were determined against burn-off for various granulometric portions at different temperatures. Though the average reactivity of the carbons is comparable, differences in structure were detected. In particular, for vitreous carbon balls, the carbon reactivity is greater for the core of the particles than for their periphery. This is most likely due to the treatment undergone by this material at the very last stage of its preparation to mold it to the desired shape. Experimental data are presented which suggest that texture of vitreous carbon balls and of saccharose coke present some similarities.     

Electrode a pate de carbone a liant electrolytique et composes electroactifs peu solubles incorpores: aspects quantitatifs

The behaviour of carbon paste electrodes containing insoluble electroactive compounds and an electrolytic pasting liquid has been studied. An investigation of some parameters which affect the characteristics of voltamperometric and chronoamperometric curves is presented. It is shown that such electrodes can be used for qualitative and quantitative analyses of solid compounds.     

Les monofluorophosphates de zinc et de potassium en tant qúinhibiteurs de la corrosion d'un acier au carbone en solution de NaCl a 3%

Résumé Nous avons effectué l'étude des monofluorophosphates de zinc et de potassium en tant qu'inhibiteurs de la corrosion d'une électrode à disque tournant d'acier au carbone (XC 38) en contact avec une solution à 3% de chlorure de sodium. Les efficacités inhibitrices ont été déterminées à la fois par des méthodes de type électrochimique (relevé des courbes courant-tension stationnaires) et de type gravimétrique (dosage du fer passé en solution). Le monofluorophosphate de zinc se révèle plus efficace que le monofluorophosphate de potassium dans un large domaine de concentration, les maxima d'efficacité étant de l'ordre de 98% et 88% respectivement.L'étude, menée dans un but comparatif, sur des solutions de chlorure de zinc et sulfate de potassium a permis de mettre en évidence que l'ion Po₃F^2– jouait un rôle dans le processus global d'inhibition.

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The study of zinc and potassium monofluorophosphates as inhibitors of the corrosion of a XC 38 carbon steel, using a rotating disc electrode in contact with a 3% NaCl solution, has been carried out.Inhibitive efficiencies have been determined by both indirect electrochemical methods (cathodic polarization curves and polarization resistance measurements) and by a direct method based on titration of dissolved iron.Zinc monofluorophosphate appeared to be a more efficient inhibitor than potassium monofluorophosphate over a wide concentration range, the maximum efficiency being about 98% and 88% respectively. The study of zinc chloride and potassium sulphate for comparative purposes, has suggested the role played by the monofluorophosphate anion PO₃F^2– in the overall inhibition process.

     

Degagement anodique du chlore a une électrode de carbone vitreux dans un mélange fondu de NaCl et FeCl3

Résumé On étudie la surtension de dégagement du chlore à une électrode de carbone vitreux dans un mélange fondu de NaCl et de FeCl₃. On enregistre les courbes tension-courant à très faible vitesse de variation de la tension. La pente de Tafel estRT/F à 178° C dans un intervalle de surtension 3.9–4.2 V. Ce résultat est compatible avec des mécanismes simples à adsorption activée de type Temkin. Il est plus difficile d'expliquer cette pente de Tafel avec une adsorption de type Langmuir.

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The overpotential of anodic chlorine evolution on vitreous carbon, from a molten mixture of NaCl and FeCl₃, is studied. The current-potential curves are recorded at a very slow rate of change of potential with time. The Tafel slope isRT/F at 178°C in the overpotential region 3.9–4.2 V. This result is consistent with simple mechanisms involving Temkin-type activated adsorption. It is more difficult to explain this Tafel slope with a Langmuir-type adsorption.

     

Evolution de l'arrangement et des dimensions des couches d'un carbone graphitable entre 1700 et 2500°C

The use of radial distribution functions of interatomic distances, P(r), in case of a pitch coke treated at temperatures between 1700° and 2500°C allows to follow tridimensional organisation and the growth of the carbon layers as a function of temperature. These results are obtained by comparing the experimental radial distribution functions of a sample treated at temperatures of 1700, 2050, 2100 and 250°C with synthetic functions calculated for a graphitic model with C-C distances less than 10 Å; the agreement of the two types of curves allows essentially:On one hand to evaluate the importance of the distorsions between the layers and to follow their evolution with temperature.On the other hand to determine the size of the diffusing domains, the measurements being confirmed by another method based on the analysis of diffusion band profiles. We have found a sudden enlargement of the average diameter of layers at a temperature of about 2000°C, on the average they change from 80 Å at 1900°C to 300 Å at 2000°C. We have shown[1] that it is in this range of temperature that the interstitial carbon atoms rearrange and are eliminated from between the layers and that the defects affecting the hexagonal rings disappear.The results obtained show that the organisation and growth of graphitic layers are related directly to the evolution of the interstitial carbon atoms: the rearrangement and the elimination of these interstitial atoms at 2000°C sweeps away defects which, up to there, hinder the tridimensional growth of the pitch coke.     

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.

     

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.

     

Les proprietes electrochimiques des diphenylamines et de leurs produits d''oxydation en milieu organique—II. Proprietes des diaryl-5,10 dihydro-5,10 phenazines et des tetraarylhydrazines

This paper is the second in a series devoted to the electrochemical oxidation of diphenylamines and of coupling products obtained from them. Anodic processes with 5,10-diaryl 5,10-dihydrophenazines and tetraarylhydrazines in acetonitrile are studied by voltammetry and coulometry. Absorption and epr spectra of the corresponding cation radicals are described. This study gives the opportunity to discuss an anodic substitution involving a di-cation as the reactive intermediate. Aged solutions of electrogenerated hydrazinium radicals in neutral media and the acidic degradation of tetraarylhydrazines give a similar mixture the composition of which depends upon the nature of the para substituant on the aromatic ring.