Tungsten fluorides : synthesis and characterization, and electrochemical investigations in the FLINAK molten salt eutectic

Author

Sven Erik Eklund

Date of Award

12-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

James Q. Chambers

Committee Members

Craig E. Barnes

Abstract

An improved method over a previous technique has been developed to determine the ppm oxide concentration of fluoride salts. The oxide is released as oxygen gas by the reaction of the test salt with potassium bromotetrafluoride at 450° C. The molecular oxygen released is then passed through a zirconia oxygen pump which selectively removes the oxygen. The current response is recorded as a chronoamperogram, from which the ppm oxide content of the salt can be obtained. Oxygen recovery from an yttrium oxide standard was better than 99%. The precision of analysis of FLINAK was better than 13% for samples containing 110 to 170 ppm oxide. The LCD was 12 pg of oxygen, and the analytical range was 120 ppm to >20% for a 0.1 g sample.

Mixtures of 1) WF₆ and KF and 2) WF₆, W and MF (where M = Na, K, Rb or Cs) at temperatures near or above the melting point of the alkali salt produced K₂WF₈ and KWF₇ in the first case, and all four MWF₆ products in the second case, where MF was the limiting reactant. Stoichiometric amounts of MWF₆ and the corresponding MF salt produced two new compounds, K₂WF₇ and Rb₂WF₇, and the compounds Na₃WF₈, K₃WF₈ and Rb₃WF₈. Longer reaction times, with excess W and MF, produced a mixture of K₃WF₈ and K₃WF₆, from which crystals of the new compound K₃WF₆ were isolated. Raman spectra were acquired for all of the compounds, and crystal structures were obtained for K₂WF₇ and K₃WF₆. The chemical reduction of WF₆ in FLINAK containing tungsten metal produced equilibrium mixtures of W(V) and W(lll) in the melt, rather than the WF₄ species suggested by previous work. The optimum plating baths reported in the previous studies was in the range where WF₆^3- and WF₈^3- were found to be in approximately equal proportions.

The electrochemistry of tungsten was adversely affected by residual HF in the melt, but could be removed by an extended sparge or by maintaining a vacuum over the melt. The W(V) discharge in FLINAK suggested both quasireversible and irreversible behavior and was dependent upon the bath temperature and the scan rate. This behavior was similar to the tantalum electrochemistry in FLINAK.

Recommended Citation

Eklund, Sven Erik, "Tungsten fluorides : synthesis and characterization, and electrochemical investigations in the FLINAK molten salt eutectic. " PhD diss., University of Tennessee, 1999.
https://trace.tennessee.edu/utk_graddiss/8804