Purification of tantalum by means of hydrogen plasma arc melting

Abstract

Further purification of commercially pure Ta metal ( > 99.9%) by hydrogen plasma arc melting (HPAM) has been examined under a pressure of 101.3 kPa using Ar-30 vol% H 2 plasma gas. Most impurities, except for Nb, Mo and W, are removed down to a few mass ppb levels. Concentrations of the elements which deteriorate the large scale integrated circuit performance, such as Fe, Th and U, are also decreased by two or more orders of magnitude. The final concentrations of many impurities in Ta purified by HPAM are lower than those achieved by electron beam melting (EBM) of Ta. The use of Ta, instead of W, as a cathode inside the plasma torch, makes it possible to avoid contamination of the Ta melt with W and Th from the thoriated W cathode commonly used. Possibility for the removal of Th, U and W from the Ta melt during HPAM was also discussed on the basis of thermodynamic calculations.