Masses volumiques et conductibilités molaires des mélanges (Ag-M)Br fondus

Nous avons mesuré, par la méthode de la poussée hydrostatique, les masses volumiques des mélanges (Ag-M)Br avec M = Na, K, Rb, Cs. Nous donnons les variations de la masse volumique et du volume molaire en fonction de la température et de la composition. Les volumes molaires sont trouvés pratiquement additifs. A partir de résultats précédemment obtenus sur la conductibilité spécifique nous pouvons calculer les conductibilités molaires. Les conductibilités d'excès sont importantes et nous tentons de les relier à la structure des mélanges

Electrocatalyse sur oxydes métalliques

The electrocatalysis on metallic oxide for oxygen réactions (ORR, oxygen reduction reaction, and OER, oxygen evolution reaction) is reviewed. The emphasize is put on the spinel and perovskite families. The properties of the oxides depend on the preparation procedure. Some methods of preparation of powders and films of mixed oxides are briefly presented : thermal decomposition of salts, freeze drying, spray pyrolysis, sol-gel, sputtering and electrochemical. Characterization of the products by X-ray diffractometry is not sufficient. Surface optical methods like, for instance, near grazing X-ray diffraction are useful with very thin films, together with UV-vis-NIR reflectance spectroscopy for their identification. Transmission and/or reflexion IRFT spectroscopy is also useful, as the thickness detection lower limit can be as low as 5 nm in the best case with this technique. Optical spectra of Co3O4 and NiCo2O4 are presented. The knowledge of the cationic distribution in the mixed oxide is essential to a better understanding of the electrocatalytic properties. Cyclic voltammetry provides an electrochemical mean of characterization of the oxide/solution interface. A double electrode flow through cell has been designed to investigate the ORR on films and to determine the ratio of the direct reduction rate constant (k1) to the indirect (peroxide) reduction rate constant (k2). This will be exemplified with the copper manganite spinel Cu1+xMn2-xO4 on which k1/ k2 increases from 2 to 9 when x increases from 0.2 to 0.4

Propriétés de transport des sels fondus : nouvelles possibilités de la thermoélectricité

L’approximation de Wagner-Pitzer pour le calcul de l'entropie partielle molaire d’un ion est justifiée à partir de plusieurs mesures comprenant le pouvoir thermoélectrique initial des mélanges d’halogénures d’argent et d’halogénures alcalins, et le pouvoir thermoélectrique des sels purs.Une estimation de la chaleur réduite de transfert est alors possible. Cette estimation permet de mettre en évidence les différences de comportement des séries chlorures, bromures, et nitrates pour le phénomène de diffusion thermique

Surface characterization of LaNiO3/Ni-PVC composite

The perovskite LaNiO3 was prepared by the sol-gel method. Electrodes of LaNiO3/Ni-PVC were obtained by painting a conductive Ni-PVC composite substrate with a suspension of the LaNiO3 powder in an ethanol-tetrahydrofuran mixture. They were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and energy dispersive spectroscopy (EDS). The study of the electrochemical response of this composite was performed in alkaline solutions by cyclic voltammetry, open circuit potential and by voltammetric charges. The state of the surface and the contribution of the Ni-PVC substrate in the electrochemical behavior of LaNiO3 were studied for the same electrode after prolonged repetitive potential cycling and oxygen evolution. (C) 1999 Elsevier Science Ltd. All rights reserved

Initial stages of cobalt oxidation by FTIR spectroscopy

The behaviour of specular reflectance of cobalt oxides on metal has been studied theoretically (CoO/Co system) and experimentally (Co3O4/Co system; the thin films are obtained by chemical spray pyrolysis) in the infrared region. CoO may be identified by a LO mode at 585 cm-1 and by a TO mode at 345 cm-1. Co3O4 is characterized by five bands respectively at 690 (LO), 660 (TO), 605 (LO), 560 (TO) and 390 cm-1. These experimental results allow the characterization of CoO and Co3O4 grown on metal during the initial stages of thermal oxidation (400–750 °C). For film thicknesses in the range 10-1000 nm, the amounts of CoO and Co3O4 in the duplex structure layers may be estimated by FTIR reflectance spectroscopy. FTIR and XRD studies reveal a cyclic behaviour in the growth of the Co3O4/CoO/Co system