Electrodeposition processes of tantalum(V) species in molten fluorides containing oxide ions
Résumé
The influence of the molar ratio O/Ta noted Ro on the electrodeposition of tantalum in molten fluorides at 800°C is examined. After additions of both K2TaF7 and Na2O , we observed: (i) tantalum deposit is obtained only by reduction of TaF72-; (ii) the oxide ions react with TaF72- to give oxifluoride species, identified first as TaOF52- and for higher oxide content, as TaO2F43-; (iii) the electroreduction of TaOF52- leads to the formation of an unstable bivalent tantalum species (TaO); (iv) the conversion of TaF72- to oxifluoride is complete when Ro is one. Titration of Ta by ICP and oxygen by a carbothermal-reduction technique show that for Ro >1, insoluble compounds Na3TaO4 and NaTaO3 are formed. For Ro > 3, these species seem to be solubilised. Electrodeposition runs on copper cathodes show that pure and coherent Ta coatings are obtained for a lower oxide content (Ro<0.5). For higher Ro, the faradic efficiencies are low. The coatings are powdery and include the presence of Na3TaO4, NaTaO3 and traces of Ta metal, arising from the decomposition of TaO.