On the electrodeposition and characterization of niobium from fused fluoride electrolytes
The electrodeposition of niobium from a binary electrolyte consisting of KF and NaF was characterized and compared with that from the ternary electrolyte of LiF, NaF and KF. The deposition experiments were conducted at current densities between 5 and 35 mA cm−2 and electrolyte temperatures between 725 and 800°C. DTA measurements indicated the melting points to be 450 and 710°C for the ternary and binary electrolytes with added K2NbF7; however, it was necessary to operate both electrolytes above 725°C to obtain dense coherent deposits. Coating morphology was described by optical and scanning electron microscopy studies, while coating structure and properties were characterized by X-ray diffraction analysis. In particular, a series of diffraction measurements were reported to describe the changes in the microstructure of the deposited material as a function of the preparation conditions.
Oxidation–reduction processes in halide and oxohalide niobium containing melts. Part I: Interaction of fluoride–chloride and oxofluoride–chloride Nb(V) melts with niobium oxides
The interaction between the K2NbF7–KCl–NaCl melt widely used for niobium electrolysis and niobium oxides of higher (Nb2O5) and lower (NbO2 and NbO) oxidation states was studied by linear sweep voltammetry and analysis of the dissolution time dependences. Three monooxofluoride complexes, NbOF4−, NbOF2−5 and NbOF63−, may form at the mole ratio of Nb2O5:K2NbF7=1:3. No significant difference was found in the electrochemical behaviour of the complexes, but their discharge potentials become more negative with increasing number of fluorine anions in the complex. Due to the interaction of K2NbF7 with NbO2 and NbO, oxofluoride Nb(v) and fluoride Nb(iv) complexes form simultaneously in the KCl–NaCl melt. Thus, the content of the latter increase until the molar ratios in the melt become NbO2dissolv.:K2NbF7init.=1:2 and NbOdissolv.:K2NbF7init.=1:3 and decrease with further dissolution of oxides in the melt. Dissolution of lower niobium oxides in melts containing oxofluoride Nb(v) complexes was also studied.
Infrared‐Reflection Spectra of Molten Fluoride Solutions: Tantalum (V) in Alkali Fluorides
Infrared‐absorption spectra derived from reflectivity measurements over the range 4000 to 200 cm−1 have been obtained for molten fluoride solutions of electrochemical interest. The results for alkali fluoride melts containing tantalum (V) over the composition range 21.3 KF:21.3 LiF:TaF5 to 2 KF:2 LiF:TaF5 show two strong bands which increase in intensity with increasing tantalum concentration. One band is found at 535 cm−1 and the other is a split (17‐cm−1) band centered at 300 cm−1. The band positions are independent of concentration and temperature from the melting point to 800°C. Comparison of these spectra with those for well‐characterized solid compounds such as K2TaF7 and CsTaF6 demonstrate that the TaF72— complex ion is the predominant species in these melts.A study of the influence of cations on the spectra of these melts show that when KF is present TaF72— is the only species observed. In the NaF:LiF solvent, TaF6− also appears.The spectra of vapors condensed from the various melts studied show i...