Abdeslam Barhoun

Synthesis and Structural Characterization of Carbon Powder by Electrolytic Reduction of Molten Li2 CO 3 ­ Na2 CO 3 ­ K 2 CO 3

A new method was proposed for the preparation of carbon powders based on the electrochemical reduction of a fused eutectic mixture of lithium-sodium-potassium carbonates at 450°C. Transmission electron microscopy observations have revealed the presence of three forms of carbon in the powder samples depending on the deposition potential values: amorphous carbon, graphite, and fibers. The presence of graphite and amorphous carbon was confirmed by X-ray diffraction measurements. The d 002 values were rather constant whatever the sample and close to 0.34 nm. Moreover, surface analyses by X-ray photoelectron spectroscopy revealed the presence of nearly metallic and ionic lithium belonging to lithium oxides. The specific areas were measured by the Brunaucr-Emmett-Teller (BET) method. Whatever the deposit potential values, the total BET specific surface area is much higher than that corresponding to external one because of the pore walls contribution. It has been shown that, for carbon powder dried at 400°C under vacuum, the specific surface area decreased as the deposition potential became more cathodic: from about 850 m 2 g -1 at -2.4 V vs. CO 2 -O 2 to about 500 m 2 g 1 at -6.0 V vs. CO 2 -O 2 . With increasing negative values of the potential, the nucleation and the growth of carbon competes with the formation of nearly metallic lithium and/or lithium oxides. The presence of the latter induces the closure of some nanopores.

Study of the Electrochemical Reduction of Zr4 + Ions in Molten Alkali Fluorides

The electrochemical behavior of Zr 4+ in potassium-free fused alkali fluorides has been studied in the temperature range of 773-1123 K. With an inert tungsten or molybdenum electrode, a well-defined oxydo-reduction wave was observed during cyclic voltammetry experiments. The reduction of Zr 4+ is a single four-electron reversible step, and the process is diffusion controlled. Chronopotentiometry measurements give rise to the diffusion coefficient values of Zr 4+ ions. For instance, a value 2.92 X 10 -5 cm 2 s -1 was found at 1010 K. The reduction of Zr 4+ ions was also studied using a nickel electrode for which Ni-Zr alloys formation was pointed out as revealed by cyclic voltammetry. The analysis of these deposits by scanning electron microscopy (SEM) and energy dispersive X-ray showed the presence of a multilayered Ni-Zr alloy with different Zr contents. On graphite electrodes, zirconium carbides have been evidenced on cyclic voltammograms and the formation of Zr metal was observed by SEM.

Electrochemical and thermodynamic properties of titanium chloride solutions in various alkali chloride mixtures

The stability ranges of the various oxidation states of titanium ions in fused alkali chlorides were studied by electrochemical techniques. The thermodynamic properties of solutions were calculated and the Gibbs energy of formation of titanium solutions was determined. The presence of strong donors of chloride ions such as cesium chloride enhances the stability of the high oxidation states. The results are interpreted in terms of formation of chlorocomplexes.

Electrodeposition of Tantalum in NaCl ‐ KCl ‐ K 2TaF7 Melts

Electroreduction of dissolved in at 720°C was investigated by transient electrochemical techniques. The primary reaction involves a single reversible five electron step . In addition to the metal deposition, other insoluble compounds were generated during the reduction step; their influence increases as the potential becomes more negative. The formation of these compounds was progressively inhibited by addition of sodium fluoride; simultaneously, a shift of the deposition potential towards more negative values was observed. This result was interpreted as a consequence of the formation of an increasing amount of fluoride rich complexes such as or . When oxide ions were present in the bath, perturbations appeared, which indicates that the reduction of electroactive species containing oxygen occurs at a potential more negative than for oxygen free complexes. The diffusion coefficient of the complex ion containing Ta(V), measured by chronopotentiometry and cyclic voltammetry, is about . The characterization of the tantalum deposits obtained at different overpotentials was determined by x‐ray diffraction and by scanning electron micrography.

Electrochemical Behavior of Solutions of Niobium Chlorides in Fused Alkali Chlorides

The electrochemical properties of solutions of niobium chlorides in the fused eutectic LiCl-KCl were studied by transient electrochemical techniques. The fundamental role of temperature was determined. At temperatures ranging from 380-600 o C, two reversible redox reactions were detected: Nb(V)+e↔Nb(IV), and Nb(IV)+e↔Nb(III). Reduction of Nb(III) led to metallic niobium, however the deposition was perturbed by the formation of insoluble non-stoichiometric niobium subhalides (mean oxidation state between 2 and 3) at the electrode surface

Influence of Oxide and Alloy Formation on the Electrochemistry of Ti Deposition from the NaCl ­ KCl ­ NaF ­ K 2TiF6 Melt Reduced by Metallic Ti

The redox reactions in KCl-NaCl-NaF-K 2 TiF 6 melts reduced by titanium metal have been studied by cyclic voltammetry and chronopotentiommetry. At platinum and nickel electrodes waves due to alloy formation were seen preceding the Ti(III) → Ti metal deposition wave. The presence of oxide species could be detected as waves on voltammograms performed with platinum electrodes. Deposits on nickel plates in general consisted of multiphase Ni-Ti layers. By varying the deposition potential outer layers with compositions ranging from pure titanium over NiTi to nickel-rich alloys such as Ni 3 Ti could be obtained. Pulse plating produced the best titanium coating. With constant potential electrolyses it was also possible to deposit titanium diboride coatings of high quality from the mixed chloride-fluoride melts.