FLINAK

Molten lithium fluoride is used in salt mixtures for an electrolyte in high temperature batteries (qv) (FLINAK) (20), and as a carrier in breeder reactors (FLIBE) (21) (see Nuclear reactors). 

Kolosov, Matychenko and Novichkov reported [566] on the investigation of the electrolysis of pure niobium from K2NbF7 dissolved in a LiF - NaF - KF eutectic melt (called a FLINAK melt). Conditions for obtaining highly uniform niobium coatings thicker than 5 pm were determined. 

White [7] gives a general review that includes information about the preparation and purification of a variety of alkali and alkaline earth halide melts. Information about fluorides can also be found in an earlier review article by Bamberger [8]. Many different halide melts have been used as solvents for electrochemistry, and a complete discussion of all of these melts is outside the scope of this chapter. However, two systems that have generated continuous interest over the years are the LiCl-KC1 eutectic (58.8-41.2 mol%, mp = 352°C) [9] and the LiF-NaF-KF (46.5-11.5-42.0 mol%, mp = 454°C) also known as FLINAK [10]. The former is an electrolyte commonly used in thermal batteries, whereas the latter molten salt is of interest for refractory metal plating. 

FLINAK is purified by treatment with the HF released by ammonium bifluoride (NH4HF2) the HF converts oxide impurities in the melt to H20 [7]. In this purification procedure, the fluoride salt mixture is combined with 15 wt% NH4HF2 and heated to about 500°C in a graphite crucible. The molten mixture is poured into a platinum container and heated to 750°C. Hydrogen is passed through the molten mixture for approximately 2 days. Further purification can be achieved by con-trolled-potential electrolysis at an applied potential of about 3 V between a tungsten cathode and glassy carbon anode. 

At a platinum electrode, highly purified FLINAK has a voltammetric window extending from about +1.5 to -2.0 V vs. the nickel reference electrode [7]. The positive limits of the alkali halide melts discussed herein arise from the oxidation of halide ions, whereas the negative limits are due to reduction of the alkali metal ions. Because chloride ion is substantially easier to oxidize than fluoride ion, the potential window of the LiCl-KCl melt is approximately 1.5 V smaller than that for FLINAK. 

Plambeck [16] summarizes the important physical properties of the molten LiCl-KCl eutectic such as density, viscosity, and electrical conductivity and gives references to some other sources of this data. Tabulated physical property data for FLINAK are scarce but are available in experimental articles. 

Fukada, S., A. Morisaki (2006a), Hydrogen Permeability Through a Mixed Molten Salt of LiF, NaF and KF (Flinak) as a Heat-transfer Fluid , Journal of Nuclear Materials, 358, 235-242. 

Fukada, S., et a1. (2008), Hydrogen Diffusion and Hydrogen Isotopic Exchange on Molten Salts of Mixed Fluorides, Flibe (LiF+BeF2) or Flinak (LiF+KF+NaF) , Proceedings of 2008 Joint Symposium on Molten Salts, Kobe, Japan, 19-23 October, 875-880. 

D. Elwell and G. M. Rao, Mechanism of electrodeposition of silicon from K2S1F6-Flinak, Electrochim. Acta 27(6), 673, 1982. 

The presence of oxide in the melt causes the formation of various niobium oxofluoro-complexes. It was shown that relatively pure Nb coatings can be obtained from FLINAK melts if no/wNb is less than 1. However, it has been shown by Konstantinov et al. (1981) and Khalidi et al. (1991) that, even a small amount of 0 ions can entirely change the mechanism of Nb deposition depending on the types of the niobium oxofluorides formed in the melt. 

Metals in groups 4-6 of the periodic table are galvanically deposited from FLINAK (a eutectic mixture composed of LiF, NaF, and KF with a of 450°C). With the exception of chromium, these metals cannot be precipitated at room temperature from either aqueous solutions or organic solvents. 

Carbides such as W2C and M02C are also galvanically deposited from a FLINAK solution. 

FLINAK is also an appropriate solvent to deposit TaS2 and TiS2 electro-lytically from a solution of K2SiFg and metal sulfides. 

The optimum conditions for the electrodeposition of tantalum in molten fluoride were first established by Senderoff and Mellors using the ternary eutectic mixture LiF-NaF-KF (i.e., FLiNaK) with 15 to 40 wt.% K TaF as solute, in an inert atmosphere. 

Steel bars, coated with tantalum by the FLINAK process, have been borided under low pressure and the properties evaluated. The process (Fig. 1) is conducted by consecutive coating procedures. To define the boride processing data for the diffusion treatment of refractory metals preliminary investigations have been carried out, such i.e. 

FLINAK" eutectic mixture of LiF, NaF, and KF used as melt solvent, DTH, Denmark "EKABOR" Elektroschmelzwerk Kemp ten, Wacker Chemie, Germany 3) This procedure is an intermediate between gas and plasma processing... 

Wendt, H., Reuhl, K., and Schwarz, V. (1992) Cathodic deposition of refractory intermetallic compounds from FLiNaK melts, J. of Appl. Electrochem. 22, 161-165. 

STUDY OF ELECTRODE PROCESSES IN FLINAK-KiTaFT - KBF4 MELT... 

Tantalum in FLINAK-K2TaF7 melt was earlier shown to reduce at V<0.5 V- s in one quasireversible stage [9]. According to [10], the B(III) reduction in a fluoride melt is a more complicated process and depends on the KBF4 concentration. When the latter is lower than 5,710 m/o, the cathodic process B(III) + 3e-> B(0) is irreversible. At... 

Figure 1. Change in the shape of voltammograms of the FLINAK-KBF4 (0.594 m/o) melt with change in K2TaF7 concentration. T =710 C, scan rate 0.25 V s AAg=0.24 cm CK TaF, 2 -0.075 3 - 0.298 ...

K2TaF7to the FLINAK - KBF4 melt resulted in the appearance of new peaks Ri, R2and R3 (Fig. 1, curve 2) and disappearance of peak R5. The peak Ri potential corresponded to that of tantalum discharge in the fluoride melt. Further additions of K2TaF7 resulted in decrease of the number of cathodic peaks. Only Ri and R2 were seen when the molar ratio B/Ta was equal to 1 (Fig. 1, curve 4). 

Thermodynamically obtaining the alloy is easier than each component separately, due to the negative free energy of the formation of the alloy. Wendt et al have verified this point in the case of titanium or zirconium diboride electrosynthesis in Flinak [26] the overall reduction Ti + + 9e TiB2 can be separated into three steps (1) Ti ... 

Wendt H., Reuhl K. and Shwarz V., Cathodic deposition of refractory intermetallic compounds from Flinak melts- I- Voltammetric investigation of Ti, Zr, B, TiB2 and ZrB2, (1992), Electrochemica Acta, 37, 237-244. 

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