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ABSTRACT.The electrochemical reduction of B(III) to 8(0) in KBF4-LiF-NaF-KF melts has been studied by voltammetric and chronopotentiometric methods. Glassy carbon, Pt, and Ag were used as working electrode materials. Only in the case of Ag was the reduction not complicated by interaction between boron and the electrode material. On a silver electrode 8(111) was reduced to B(0) in a single irreversible step in the KBF4 concentration range up to 5.7 X 10 mole percent (mb). The cathodic half-wave potential was -1.34 V vs. an Ag/AgC1 reference electrode at 700°C. The diffusion coefficient of BF at 700°C was determined to be 2.06 X i0' cm2 s'. Further increase of the KBF4 concentration above 5.7 x 10-2 mb leads to a change in the reduction process. An ohmic resistance control becomes the limiting factor of the boron electroreduction process. The "apparent surface resistance" changes from 3,0 to 21.6 11 cm2 as the temperature decreases from 700 to 550°C, respectively. Furthermore at KBF4 concentrations higher than 5.7 X 10-2 mb a second reduction peak and a corresponding anodic peak appeared on the voltammograms. These peaks were attributed to formation of alkali metal borides. InfrodudionDue to various unique physical and chemical properties, refractory metal borides have attracted increasing attention by researchers recently."7 Molten salts provide some possibilities for implementing a technology for refractory metal boride deposition either by using chemical reaction with the substrate' or electrolysis. Two approaches of the latter method are available: (1) boron electrochemical deposition on a refractory metal substrate followed by diffusion into the metal phase2 or (ii) a direct electrochemical synthesis. The electrochemical synthesis has the advantage compared to the galvanodiffusion method, that the solid-state diffusion, which is often a rate-determining Electrochemical Society Active Member.step, is avoided. Further, it allows production not only of coatings but also powders. Most investigations3"7 on the electrochemical synthesis of refractory metal borides are concerned with development of coating technologies. Therefore more knowledge about the mechanism of the electrochemical deposition of boron and refractory metals is needed.Unlike the electrochemistry of the refractory metals, the electrochemical behavior of boron is not clear. Investigations concerning the mechanism of the cathodic reduction of boron ions"4'6'8'20 do not give a comprehensive idea of the mechanism and the kinetics of the boron electrodeposition. It was shown"° for platinum electrodes that the reduction process B(III) -B(0) proceeds reversibly in fluoride melts. However this reaction was shown by several authors to be irreversible at glassy carbon elec- Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, inc. 3179 trodes."2° These authors as well as others'8"9 suggested that the process of boron(III) reduction in chloride-fluoride and fluoride melts proceeded in one step without the participation of inte...

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The electrochemical behavior of tantalum in the form of K2TaF7 in an LiF-NaF-KF eutectic melt has been studied by linear voltammetry in the temperature range of 560 to 815~ with and without additions of Na20. An amperometric titration has been performed by measuring the heights of the cathodic and the anodic peaks. It was shown that at a molar ratio Na20/K2TaF7 = 1 the predominating complex in the melt is TaOF~ , whereas with an Na20/K2TaF7 molar ratio of 2 it is TaO2F(= =-I)-, probably in the form of TaO2F~-. Increase in the Na20/K2TaF7 molar ratio in excess of two leads to a decrease of tantalum concentration in the melt, and precipitation of KTaQ occurs. Both the fluoro complex and the monooxefluoro complex were reduced to metal in a single five-electron step. The fluoro complexes, in the temperature range 625 to 815~ with potential scan rates <0.5 V 9 s i, discharge quasi-reversibly, but at potential scan rates >0.5 V 9 s -I they discharge irreversibly. Monooxofluore complexes discharge irreversibly at all temperatures and scan rates studied. The diffusion coefficient of the tantalum fluoro complex depends on the temperature as log D = -2.55 -2044/Twith an activation energy of 39.1 kJ 9 mol -~. For the tantalum monooxofluoro complex the dependence is log D = -2.35 -2293/T with an activation energy of 43.9 kJ 9 mol -I.