Desorption kinetics of carbon and oxygen in liquid niobium
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I.
INTRODUCTION
THE conventional production of high-purity Nb consists of four stages: (1) the extraction of Nb2O5 from niobium ores through leaching or chlorination/distillation, (2) carbothermic or aluminothermic reduction of Nb2O5 to form metallic sponge or powder, (3) a consolidation process to form electrodes through sintering or remelting, and, finally, (4) refining through electron-beam remelting in vacuum (EBM).[1,2,3] Within these four stages, purification of Nb is mostly achieved during the EBM stage through volatilization of metallic and nonmetallic impurities. In some cases, an additional purification stage, such as electrolytic refining in molten salt, is incorporated before EBM in order to eliminate the less-volatile elements like Ta and W. It should be noted that Niobium can also be purified in the solid state and in ultra-high vacuum at temperatures above 2000 7C, but the purification kinetics is much slower than that in the liquid state, because transport from the interior to the surface in the solid state takes place by diffusion only. For the liquid-state purification, the transport is enhanced by the hydrodynamic flow in liquid metal. For example, it is reported by Schultz et al.[1,4] that the annealing time required for a niobium rod of 10 mm in diameter to reach the necessary parts-per-million level is longer than 1 week at 2400 K under the nitrogen partial pressure of 5z10211 mbar. Thus, EBM has been used widely in industry since the 1950s as a major purification step in the production route of high-purity niobium. For EBM, the electron beam, accelerated in a high potential, is focused on the Nb ingot, causing it to melt and drip down into a water-cooled mold. The solidified ingot is then withdrawn downward at a rate proportional to the melt rate. Since this process is performed in high vacuum, nonmetallic impurities such as H, N, O, and C, and volatile metallic impurities like Al are removed to a great extent via the degassing and vaporization process. Table I shows the typical impurity concentrations before and after EBM.
The final refining, achieved using single or multiple remelts, is affected by the vacuum level, the effective surface area exposed to the ambient furnace atmosphere, the melt superheat, and the fluid flow pattern within the melt pool. The first fundamental results concerning Nb and gas reactions at high temperature and low pressures were reported around 1965.[5] Since then, other research has been done to understand the interaction behavior of interstitials in Nb. However, these studies are mostly limited to the solid state, since the high melting point of Nb (2468 7C) precludes the use of conventional experimental techniques to the liquids. As a result, there is a lack of fundamental understanding of the interaction processes occurring between the gas phase and the liquid Nb. Such an understanding is essential for the efficient refining and control of interstitials in the melt. In a previous article,[6] the kinetics and mechanisms of adsorption of CO in liquid niobium we
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