Thermodynamic properties and phase equilibria in the Na2O–SiO2 system

Abstract

Knudsen effusion mass spectrometry and the method based on generation in the effusion cell of reduction reactions producing volatile suboxides were used to study the thermodynamic properties of the Na₂O–SiO₂ system in solid and liquid states. Nb, Ta, Mo or Ni served as reductants. Na⁺, Na₂O⁺, NaO⁺, O₂⁺, MO⁺, MO₂⁺ (M=Nb, Ta, Mo), MoO₃⁺ and NiO⁺ were detected in the mass spectra of the saturated vapour. Na, TaO₂, NbO₂, MoO₂, NiO and NaO₂ were found to be the main vapour species and their partial vapour pressures were measured and used for calculation of a(Na₂O). The activity of SiO₂ was calculated using the Gibbs–Duhem equation. The Gibbs energies of formation of solid sodium silicates from β-Na₂O and quartz were approximated using the following equations (in J mol^-1): 1/111/31/21/51/3