Thermodynamics Of The Interaction Of Niobium And Tantalum with Oxygen And Nitrogen At Temperatures Near The Melting Point

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Report Number: WADD TR 60-655
Author(s): Pemsler, J. Paul
Corporate Author(s): Nuclear Metals, Inc.
Laboratory: Materials Central
Date of Publication: 1961-03
Pages: 46
Contract: AF 33(616)-6627
DoD Project: 7364
DoD Task: 73640
Identifier: AD0262060

Abstract:
The concentration of nitrogen and oxygen present in niobium and tantalum in equilibrium with the pure gas has been determined as a function of pressure at each of three temperatures near the melting point, and in the liquid phase at the melting point. The solubility of nitrogen in niobium is directly proportional to the square root of the pressure (Sievert's Law) up to the solubility limit, where the saturated solution is in equilibrium with Nb2N. The solubility of nitrogen in tantalum shows a negative deviation from Sievert's Law at nitrogen concentrations above 5 atomic percent; beyond the solubility limit saturated solution is converted to Ta2N. The niobium-oxygen and tantalum-oxygen systems obey Sievert's Law up to the solidus point. The temperatures investigated were above the melting points of the metal oxides and no oxide scales were obtained. A curious solubility inversion was noted in the tantalum-oxygen system where the oxygen concentration in equilibrium with a specified pressure of oxygen is lower at 2850 than at 2960°C. Data were used to calculate the partial molar and integral values of the free energy, enthalpy, and entropy of dissociation of solutions of nitrogen in niobium and tantalum and partial molar free energy and enthalpy of dissociation of solutions of oxygen in niobium and tantalum. It was shown that the concentration of hydrogen in equilibrium with metal at high temperature is very low.

Provenance: IIT

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