Low-Temperature Production of Ti-Al Alloys Using Ionic Liquid Electrolytes: Effect of Process Variables on Current Densi
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DIFFERENT processing methods were reported by many researchers for producing Ti-Al alloys. The electrodeposition of titanium from high-temperature molten chloride/fluoride melt was also extensively investigated.[1–7] However, the electrodeposition of Ti-Al alloy at room temperature or very low temperature was reported recently.[8–10] In recent years, Ti-Al alloy deposition from room-temperature-ionic-liquid (RTIL) electrolytes has gained interest because of the unique chemical and physical properties of RTIL electrolytes such as wide temperature range for the liquid phase, high thermal stability, negligible vapor pressure, low melting point, wide electrochemical window, low energy consumption, and low pollution emission. The electrochemical production of Ti and Ti-Al alloy is complicated due to the variable oxidation states of titanium Ti+2, Ti+3, and Ti+4. The electrochemical behavior of titanium in Lewis acidic chloroaluminate melt was investigated.[10–15] The electrochemistry of TiCl4 in strongly Lewis acid molten salt (AlCl3EtMelmCl) at room temperature was reported by Carlin et al.[13] It was found that the reduction of tetravalent D. PRADHAN, Graduate Student, R.G. REDDY, ACIPCO Professor and Head, and A. LAHIRI, Postdoctoral Fellow, are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487. Contact e-mail: [email protected] Manuscript submitted June 11, 2008. Article published online January 16, 2009. 114—VOLUME 40B, FEBRUARY 2009
titanium Ti (IV) occurs in two one-electron steps. Ti (IV) first reduced to Ti (III) and then subsequently reduced to Ti (II). However, other studies[9,10,12] showed that, when the concentration of Ti (III), i.e., b-TiCl3, exceeded the solubility limit, then Ti (III) was passivated on the electrode surface and was also precipitated. This passive film blocks the electrodes and prevents oxidation and reduction. It was reported that, at higher applied voltage, this film breaks down and produces Ti (IV). Mukhopadhyay et al.[16] reported the formation titanium nanowire on the step edges of highly-orientedpyrolytic-graphite (HOPG) from 1-butyl-3-methyl imidazolium bis ((trifluoromethyl) sulfonyl) amide containing 0.24 (M) TiCl4 at room temperature. The Ti (IV) was reduced to Ti (II), which was subsequently reduced to metallic Ti. The first nanowire was formed at the 1.0 V vs [Fe]+/[Fe] redox couple at the edge of HOPG, and then subsequent straight and highly aligned nanowires were grown parallel to the first. Investigations have been undertaken for the direct electrochemical reduction of TiO2 to titanium metal to make the process more economical. The electrochemical production of aluminum has been reported by various researchers.[17–21] Recently, Reddy et al.[22–25] investigated the electrorefining of aluminum alloy in 1-butyl-3-methyl imidazolium chloride (BmimCl) and the production of high-purity aluminum deposit from AlCl3-BmimCl melt at 100 C ± 3 C. Karpinski et al.[26] studied the finding that, when BmimCl is present in molar e
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