REFINING OF TANTALUM BY SILICON DEOXIDATION

The removal of residual oxygen from tantalum by pyrovacuum treatment u sing silicon deoxidation (as SiO(v)) was studied. The possibility of e liminating oxygen from a Ta-Si-O solid solution by preferential evapor ation of SiO(v) was indicated by calculations based on the experimenta l or estimated values for the activities of silicon and oxygen in the solid solution as well as for the standard free energies of formation of tantalum suboxide (TaO(v)) and silicon monoxide (SiO(v)). The exper imental results obtained by treating Ta-Si-O alloys of known silicon a nd oxygen contents at temperatures ranging from 2073 to 2273 K under 1 x10(-3) Pa pressure for 2 h indicated the occurrence of silicon deoxid ation. In this temperature range, oxygen removal has occurred faster a nd to a lower residual level in Ta-O alloys containing silicon as comp ared to Ta-O alloys containing no silicon. When both silicon and oxyge n concentrations have reached relatively lower levels, viz., approxima te to 1 at% O and 0.3 at% Si, oxygen removal by SiO(v) vapourisation d ecreases with an increasing contribution of sacrificial deoxidation (a s TaO(v)). (C) 1998 Elsevier Science S.A.