Etude de la formation d'un complexe de surface graphite-eau relation avec la reaction d'oxydation

Using a gas flow apparatus under atmospheric pressure with infrared CO and CO₂ detection and an apparatus operating under low pressure with a mass spectrometer for analysis, we have found the following: The chemisorption of water becomes appreciable above 200°C. The water chemisorbed on graphite previously degassed at 1000°C forms a surface complex which on raising the temperature decomposes with simultaneous evolution of CO and H₂, thus showing that hydrogen enters into the composition of the complex. The programmed thermal decomposition of the complex shows two peaks. The first peak corresponds to the sites which disappear with the degassing of the CO and H₂; these sites are formed by the ‘labile’ carbon atoms created by the prior degassing. The second peak corresponds to the sites which are constantly renewed after degassing the CO molecules, and which are therefore formed by graphite atoms normally linked to the graphite lattice. The reaction rate of the oxidation of graphite by water shows two transient rates before attaining a stationary value. Correlations between the CO thermo-desorption and these reaction rates are found which indicate that the formation of a surface complex is a necessary step in graphite-H₂O reaction.     

Etude de l'effet retardateur du zinc sur l'hydratation de la pate de ciment Portland

The retarding effect of zinc on the hydration of C₃S and C₃A, the two principal Portland cement components, has been investigated by X - ray diffraction. The results show that the C₃S retardation is more important than that of C₃A. This retardation is due to the precipitation of an amorphous layer of zinc hydroxide around the anhydrous grains. The effect of this coating depends on its permeability. The hydration reaction starts again through the transformation of the zinc hydroxide into the crystalline calcium zinc hydroxide Ca Zn₂ (OH)₆, 2H₂ O.     

Cinetique de formation d'une couche de passivation - II. Etude theorique de l'oxydation intensiostatique d'un alliage binaire

In an earlier paper we have presented a theoretical scheme accounting for the electrochemical behavior of metals under constant anodic current and with formation of compact oxide film. The kinetics of growth of this resulting oxide film, which insulates the metal from the electrolyte, was supposed to be governed both by the electron exchange reaction and by the diffusion of point defects (vacancy, interstitial) across the oxide film. In this paper, this theoretical scheme is used to describe the electrochemical behavior of binary alloys under similar conditions and with possible formation of a composite oxide. The variation of interfacial concentration of defects versus time is derived and we have drawn the chronopotentiometric curves corresponding to different types of point defects in the oxide and to different concentrations of the elements in the alloy.     

L'electrolyse de l'alumine dans la cryolithe fondue. Etude du mecanisme de decharge cathodique a l'aide de l'electrode a disque tournant

The use of a rotating disk platinum electrode, on which aluminium has been previously deposited, has allowed us to determine the mechanism of the cathodic reaction during the electrolysis of cryolite—alumina melts. This mechanism requires three stages within the Nernst's diffusion layer. The first of these is the dissociation of the ion AlF^3?₆, at δ′ from the electrode, forming the ions Al³⁺ and F^?. The second one is the discharge of ions Al³⁺ diffusing towards the cathode and the third one, at δ″ from the electrode, is the reaction of the ions F^? with the ions AlF^?₄ to give again the ions AlF^3?₆. The ions Na⁺, which transport most of the current serve to maintain electro-neutrality.     

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.     

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.     

Transferts de matiere a la paroi d'une cuve mecaniquement agitee

Mass transfer of the wall of a stirred tank was studied using the electrochemical method. As local values were first investigated the influence on the mass transfer coefficient of the ‘entrance effect’ was pointed out. Overall coefficient was then obtained by integration of the modified local values. The influence of the dimension and of the position of the agitator was deduced and also the importance of the presence of baffles.     

Possibilites de la diffusion centrale dans l'examen d'etats alteres du ciment

After a review of the idea of specific surface in relation to x-ray diffuse scattering, we examine in a comparative way the data gathered on normal cements and on cements having been subjected to different types of modifications. We show that these modifications are very evident on pure kinds (of cement), but are much more uncertain on concrete.     

Transferts de chaleur a la paroi d'une cuve mecaniquement agitee

Local heat transfer coefficient between the agitated liquid contained in a tank and the wall of the tank has been determined for baffled and unbaffled tanks. The diameter of the six blade-turbine used was one third of the tank diameter. The overall coefficient has been calculated by integration of the local values. Results show the influence of the position of the turbine and of the baffles.     

Application de la thermoluminescence a l'etude de l'hydratation du silicate bicalcique

Hydraulic properties differences between β and γ dicalcium silicate varieties are well known. In this work, a water vapour chemisorption on the surface of both solid phases is shown by thermoluminescence and by quadrupole mass spectrometry.The similar behaviour of β and γ dicalcium silicate varieties in those experiments means that the fundamental difference between their hydraulic reactivities is not due to this hydration step.     

Etude des cinetiques successives d'insertion et de desorption du brome dans un pyrocarbone

A single sample of pyrocarbon was submitted to 15 successive sorptiondesorption cycles in a bromine vapor atmosphere, at various temperatures and pressures. The sample mass and diamagnetic anisotropy were simultaneously measured in situ as a function of time. It is concluded that the initial and final states of the evolution of a system may be varied in a rather broad range without impairing the validity of Pacault's rule. Apparent activation energies of 1–2 kcal/mole (sorption) and 15–25 kcal/mole (desorption) are derived from the results. A quantitative interpretation of the successive sorptions kinetics may be achieved with a simple diffusion process along the basal planes of the graphitic crystallites. Some empirical correlations between bromine content and diamagnetism during the sorption or desorption processes are found but their physical meaning is not yet clear.     

Influence du passage d'une bulle sur le coefficient instantané de transfert de chaleur à une paroi immergée dans un lit fluidisé

This paper deals with the study of the vigorous particle mixing around an artificial bubble injected in a fluidized bed using a hot film anemometric technique.The fluctuations of the heat transfer coefficient have been studied as a function of the bubble characteristics and position with respect to the probe.In the bubble injection axis the heat transer coefficient depends very much on the bubbling frequency but only slightly on the bubble diameter. On the other hand, the distance x between the transfer surface and the bubble track was found very important. In this study we also determined the critical values of the bubble diameter and frequency which characterize the beginning of interaction between two successive bubbles.We propose a velocity potential model so as to take into account the form of the agitation zone of particles around the bubbles.     

Etude experimentale de la fatigue du beton en flexion 3 points

The results of fatigue tests under three point bending carried out on concrete prismatic specimens showed a significant scatter in the material life time. This scatter did not allow any prevision of the fatigue life time. The concrete damage through the fatigue history seems to be dependent on the development of deformations that grow at constant rate during a large part of the test.

Résumé

Les résultats des essais de fatigue en flexion 3points menés sur des éprouvettes prismatiques en béton se car actérisent par une dispersion dans la durée de vie de ce matériau. Cette dispersion empêche toute prévision de durée de vie. L'endommagement du béton au cours de l'histoire de fatigue semble être lié à l'évolution des déformations. Celles-ci évoluent à vitesse const ante pendant une bonne partie de l'essai.     

Cinetique du couple redox ferro-ferricyanure a des electrodes d'oxyde et de graphite. Effets de la nature de l'electrode et du cation alcalin

At an electrode made of graphite or of the mixed oxide La_0.8Sr_0.2CoO₃, it is shown with the rotating disk method that the limited current of oxidation of ferrocyanide is governed by the diffusion of a participating species only with K ⁺ and Cs ⁺. Conversely, with Na ⁺ and Li ⁺, it is governed by the diffusion and by a heterogenous reaction which takes place before the charge-transfer. This step is apparently catalysed by the surface of the oxide. By scanning potential cyclic voltametry, it is shown that, with Na ⁺ and Li ⁺, the oxidation reaction is semifast, though it is an order of magnitude faster with K ⁺. By conductibility measurements, we have verified that the number of ion pairs formed is larger with K ⁺ than with Na ⁺ and Li ⁺. A mechanism in term of “contact” and “non contact” ion pairs is proposed.     

Etude electrochimique de l'oxydation simultanee de la galene (PbS) et de l'ethylxanthate—condition de formation du xanthate de plomb en surface de la galene

The electrochemical oxidation of galena (PbS) has been investigated in the pH range 9–10, by controlled potential electrolysis using a special bed electrode. In the absence of xanthate, the oxidation of PbS leads to the formation of elemental sulphur, thiosulphate and lead species (Pb(OH)₂). Simultaneous oxidation of galena and xanthate has also been studied and the products characterized by their uv-visible spectrum. The oxidation produces lead xanthate and dixanthogen. It has been established that lead xanthate comes from an ion exchange reaction between excess xanthate in solution and lead species resulting from the lead sulphide oxidation.     

Etude du mecanisme et de la cinetique de la reduction des ions uranyles dans des solutions d'acide phosphorique

The electrochemical reduction of uranyl ions in 0,1-9 M phosphoric acid has been investigated by polarography, cyclic voltammetry, chronopotentiometriy and potentiostatic coulometry.In concentrated phosphoric acid solutions (H₃PO₄ < 1 M), an adsorption pre-wave appears in addition to the main wave of diffusion. The electrochemical reaction involves a single electron. Kinetic usually corresponds to a quasi-reversible system, but it evolves according to the variation of U(VI) and H₃PO₄ concentrations. So, when the concentration of U(VI) increases and/or that of H₃PO₄ reduces, the system becomes reversible.     

Etude d'une cuve agitee munie de chicanes: transfert de matiere a une sphere fixe

The authors have studied the hydrodynamics of a liquid in a cylindrical vessel stirred by a standard disk-turbine.To do so, they have measured the mass transfer to a spherical probe through an electrochemical technique.The experimental results have permitted to draw a “chart” of the stirring induced by a turbine with a D/T ratio of $\text{1}\text{3}$. It has been established that the velocity of the liquid is proportional to the peripheral speed of the stirrer, varies with the distance from the turbine, and depends on the diameter of the turbine. The authors have determined the pumping capacity of the stirrer.     

Sur l'electrosorption d'ethylxanthate au cours de la polarisation anodique d'electrodes de platine et de galene

The main results concerning the galena flottation by xanthates which are relevant from an electrochemical point of view are presented. The usual kinetic theories were unable to explain the results obtained in previous electrochemical studies on xanthate oxidation at various electrodes. A modified treatment is reported. It reconciles data indicating the same phenomenology as that taking place when a monolayer is chemisorbed as well as quantitative results suggesting the presence of a thick layer. The result previously obtained with platinum and galena electrodes can thus be quantitatively interpreted. It is concluded that the complexity of the chronoamperometric data derives more from the adsorbed layer structure than from the reaction scheme. The latter is composed of the xanthate physisorption followed by chemisorption. The adsorbed phase with tens of monolayers is assumed to be analogous to a lamellar phase of a liquid crystal. It is electrically conducting and its thickness is not dependent on the substrate. However. the nature of the latter controls the oxidation rate and the possible occurrence of passivation. The electrochemical results and those obtained by other technics on dispersed systems are well can be interpreted by the proposed model.     

Etude par thermoluminescence de l'hydration de l'aliminate tricalcique

Thermoluminescence shows that the hydration of tricalcium aluminate is deeply influenced by structural defects of the solid. Hydration reaction excites centers of the solid detected by thermoluminescence; this energy storage is related to trapped electrons. Hydration of tricalcium aluminate in the presence of gypsum is not influenced by traps but thermoluminescence allows to corroborate the well-known mechanism of this reaction.