Fundamental Study on the Fabrication of Niobium by the Carbothermic Reduction-Electron Beam Melting Combination Method

Abstract

Niobium and niobium alloys have received considerable attention as engineering materials for use in heat engine applications. In the past several years, the carbothermic reduction-electron beam melting combination method has made it possible to prepare the pure niobium metal. In the present investigation, the possibilities and conditions for winning pure niobium by this process have been considered with regard to the following items: (1) Working conditions and analytical results on the Nb-C-O solid solutions obtained by the carbothermic reduction of Nb₂O₅ in vacuum. (2) Vacuum degassing in the solid state. (3) Refining of niobium by electron beam melting. (4) Evaporation deoxidation via niobium suboxides.
The mechanism of refining of the niobium metal by the present method can be characterized by the next steps, carbon is removed by degassing as CO and the elimination of oxygen takes place as CO as well as volatile suboxides, for instance, NbO and NbO₂. Both phenomena seem to occur during the refining step in the solid state in vacuum at 2200 K and the subsequent electron beam melting. Accordingly, the carbon elimination is possible only when an excess amount of oxygen is present at the beginning of electron beam melting. On the other hand, even after carbon is lowered, the remaining oxygen can be removed by the suboxide evaporation.
With this process the high purity niobium in which both carbon and oxygen contents are reduced to a sufficient degree could be obtained by favorable conditions of reduction and melting.