Niobium electrodeposition

The presence of oxides and the formation of oxofluoro-complexes in molten electrolytes may be sometimes unwanted, but in many cases they are the fundamental features of the system. For instance, the formation of oxide complexes in alkali-alkaline earth chloride melts may be mentioned. The formation of oxofluoride complexes in molten cryolite-alumina melts, used as electrolytes for aluminum production, is typical as well. On the other hand, the presence of oxofluoride complexes in electrolytes used for niobium production was initially regarded as unwanted. Recently, however, it has been proven that their presence in niobium electrolytes plays an important role in the niobium electrodeposition. In the following, some technologically important examples of systems containing halides and oxides will be described. 

Platinum electrodeposition on to tantalum had been carried out as early as 1913 and the use of platinised tantalum as an anode suggested in 1922 , whilst platinum electrodeposition on to niobium was first successfully carried out in 1950 . 

The anionic composition of the cathodic product is not the only parameter that can be controlled through electrolysis conditions. Grinevitch et al. [559] reported on the investigation of the co-deposition of tantalum and niobium during the electrolysis of fluoride - chloride melts. Appropriate electrodeposition conditions were found that enable to obtain either pure niobium or alloys. 

Use of low-temperature molten systems for electrolytic processes related with tantalum and niobium and other rare refractory metals seems to hold a promise for future industrial use, and is currently of great concern to researchers. The electrochemical behavior of tantalum, niobium and titanium in low-temperature carbamide-hilide melts has been investigated by Tumanova et al. [572]. Electrodeposition of tantalum and niobium from room/ambient temperature chloroaluminate molten systems has been studied by Cheek et al. [573],... 

Chong et al. [742] have described a multielement analysis of multicomponent metallic electrode deposits, based on scanning electron microscopy with energy dispersive X-ray fluorescence detection, followed by dissolution and ICP-MS detection. Application of the method is described for determination of trace elements in seawater, including the above elements. These elements are simultaneously electrodeposited onto a niobium-wire working electrode at -1.40 V relative to an Ag/AgCl reference electrode, and subjected to energy dispersive X-ray fluorescence spectroscopy analysis. Internal standardisation... 

The electrodeposition of Al-Nb alloys has been reported in a BPCI-AICI3 ionic liquid containing NbCls or Nb3Clg [60]. A pentavalent niobium species is reduced... 

Titanium can likewise be electrodeposited from nonaqueous organic electrolytes. At circa 18 °C, 1-75 iim thick coatings are deposited from electrolytes containing dimethylsulfoxide (DMSO) [169]. The baths utilized do not exhibit high stability. Zirconium, hafnium, niobium, and aluminum are also expected to afford electroplated coatings from similar composite systems. Electroplating of titanium from electrolytes based on aromatic solvent mixtures has recently been reported [171]. 

Electrodeposition is one of a few techniques used to crystallize high-melting materials at convenient temperatures [e.g., tantalum carbide, TaC (mp 3900°C) and niobium carbide, NbC (mp 3500°C) are crystallized at 750°C]. The preparation involves electrolysis of melts containing Ta20s (or Nb205), Na2B407, Na2C03, NaF, and KF. 

Cheek GT, De Long HC, Trulove PC (2(XX)) Electrodeposition of niobium and tantalum from a room-temperature molten salt system. Proc Electrochem Soc 99-41 527-533... 

Wainer, E. (1959) Transition-metal hahdes for electrodeposition of transition metals. US Patent 2,894,886. Timax Corp. (1960) Electrolytic preparation of pure niobium and tantalum. British Patent 837,722. 

THE ELECTRODEPOSITION OF ALUMINUM-NIOBIUM ALLOYS FROM CHLOROALUMINATE ELECTROLYTES... 

Several electrodeposits were formed at potentials ranging from 0 to -O.IV. EDS examination of the as-deposited surface indicated the presence of aluminum and niobium in all of the electrodeposits examined. Chlorine was not detected in any of the samples indicating that the deposits contained no entrained electrolyte. Figure 7 is a plot of alloy composition as a function of deposition potential. The highest niobium concentration detected was 13.5% (atomic fraction). This was observed at a deposition potential of O.OV. As the deposition potential is made more negative, the niobium concentration is dramatically reduced. This implies that the kinetics for aluminum deposition are much faster than that of niobium, or that the niobium reduction is simply mass transport limited in the potential range examined. The fact that pure niobium deposits are apparently not achievable at potentials more positive of the aluminum deposition potential may be an indication that the codeposition of niobium and aluminum at negative potentials follows an mduced codeposition mechanism i.e., niobium deposition is only possible when aluminum is codeposited. 

Figure 8. X-ray diffraction patterns from electrodeposits formed following the anodic dissolution of niobium into 52 48 AlCl3 NaCl at 190 T. The electrodeposits contained (a) 2.16%, (b) 4.84% and (c) 13.46% atomic fraction niobium.

Chamelot P., Lafage B and Taxil P., Using square wave voltammetry to monitor molten alkali fluoride baths for electrodeposition of niobium, (1997), Electrochemica Acta, 43, 607-616. 

Christensen E, Xindong Wang, Von Barner J.H., Ostvold T. and Bjerrum N.J., the influence of oxide on the electrodeposition of niobium from alkali fluoride melts, (1994), /. Electrochem. Soc., 141, 1212-1220. 

Cohen U., Electrodeposition of Niobium-Germanium alloys from molten fluorides, (1983), J. Electrochem Soc., 130, 1479-85. 

Charging curves were recorded during the deposition of hafnium on niobium at current densities from 4 lO " to 5 10 A/cm. Plots of the dependence of the limiting concentration of hafiiium in the p, a+P and a-phases (Fig.2) were obtained on the basis of these curves. The curves obtained made it possible to establish the variation of the phase composition during the electrodeposition of hafiiium on niobium and to determine the time intervals for the formation and growth of the phases on the surface of the cathode at different current densities and a temperature of 750°C [3]. 

The construction of our electrolytic cell for deposition of hafiiium coatings has been described [7]. Distilled water was used to wash the cathodic deposits. Niobium and steel (St.3) substrates 1 mm thick were used for electrodeposition of coherent hafiiium coatings. These substrates had a mean deviation in the profile of Ra=0,l 1-0,13 pm. 

Cohen, U. (1981). High rate of electrodeposition of niobium from molten fluoride using periodic reversal steps and the effect on grain size, J.Electrochem.Soc. 128, 731-740. 

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