Calcium-deoxidation of niobium and titanium in Ca-saturated CaCl2 molten salt

Niobium was deoxidized under Ca(liquid)-CaO(solid) equilibrium at 1273 K to a level below the lower Limit of quantitative chemical analysis, 50 mass ppm oxygen. Because Ca does not react with Nb, it is easy to remove the excess Ca and the byproduct CaO, and no reaction layer remains such as for Ti external gettering. Due to direct contact with Ca liquid, however, Nb was contaminated by carbon acid nitrogen. When Ca was supplied as vapor into a CaO-saturated CaCl2 melt, Ca could saturate in the CaCl2 melt, and the carbon and nitrogen impurities in Nb were minimized. The oxygen concentration in Ti was the same as that under Ca-CaO equilibrium, and that in Nb was below 50 ppm. By Ca-halide flux deoxidation using the Ca-saturated but CaO-unsaturated CaCl2, melt, the residual resistivity ratio, RRR, of Nb exceeded that of the starting commercial Nb wire by 500. The oxygen content in Nb was estimated to be smaller than a single ppm. The activity of CaO in the salt was also evaluated at 1273 K using the equilibrium oxygen concentration in Ti. (C) 1999 Elsevier Science S.A. All rights reserved.