Vaporization study on the niobium-oxygen solid solution by mass-spectrometric method

Abstract

The vapor pressures over the niobium-oxygen solid solution were measured by mass-spectrometric method in the temperature range 2091–2379 K. The main vapor species were observed to be NbO(g) and NbO 2(g). The solidus line of the niobium-oxygen solid solution and the solubility limit of oxygen in niobium metal at high temperatures were determined from the vaporization behavior. The partial pressure of oxygen was calculated as a function of temperature and O/Nb composition from the partial pressures of NbO(g) and NbO 2(g), from which the partial molar enthalpy and entropy of oxygen in the niobium-oxygen solid solution were determined. The compositional dependence of the partial pressure of oxygen was observed to obey the Henry's law at low oxygen concentrations in niobium metal less than about 5 a/o. The partial molar enthalpy in the oxygen concentration range between 1 and 5 a/o was observed to be nearly independent of composition, suggesting the presence of very weak interaction between interstitial oxygens. The compositional dependence of the partial molar entropy of oxygen in this low oxygen concentration range was explained by assuming the occupation of the octahedral sites in b.c.c. niobium lattice by the interstitial oxygens. The partial excess entropy of oxygen was compared with the value derived from the sum of the contributions from the volume expansion, vibrational and electronic heat capacity terms.