ELECTRODEPOSITION OF TANTALUM IN NACL-KCL-K2TAF7 MELTS

Electroreduction of K2TaF7 dissolved in KCl-NaCl at 720-degrees-C was investigated by transient electrochemical techniques. The primary reaction involves a single reversible five electron step Ta(V) + 5e reversible Ta. 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 TaF72- or TaF83-. 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 1.5 x 10-5 cm2 s-1. The characterization of the tantalum deposits obtained at different overpotentials was determined by x-ray diffraction and by scanning electron micrography.