Vaporization study on the niobium monoxide and niobium metal systems by mass-spectrometric methods

The vapor pressures over single-phase NbO(s) and two-phase NbO(s) — Nb(s) were measured by the mass-spectrometric method in the temperature range 1948–2301 K. The main vapor species over both systems were observed to be NbO(g) and NbO₂(g). By applying the second and the third law treatments of thermodynamics to the partial vapor pressures of NbO(g) and NbO₂(g), the enthalpies of vaporization were calculated for the following reactions: NbO(inNbO(s)) = NbO(g), NbO₂(inNbO(s)) = NbO₂(g), 2NbO(s) = Nb(s) + NbO₂(g), 2NbO(s) = Nb(s)+NbO(g) + 0(g). The enthalpies of formation and the dissociation energies of NbO(g) and NbO₂(g) obtained from these reactions were in good agreement with the previous results obtained from the vapor pressure measurements on NbO₂(s) by the present authors. The partial pressures of oxygen were calculated as a function of temperature from the partial pressures of NbO₂(g) and NbO(g), from which the partial molar enthalpies and entropies of oxygen in the system of NbO(s) and Nb(s) were determined.