Thermodynamic properties and phase equilibria in the MnO–SiO2 system

Abstract

The thermodynamic properties of the MnO–SiO₂ system have been examined in the temperature range 1369–1817 K and concentration range 40–80 mol% MnO. Equilibria of the reactions between MnO–SiO₂ mixtures and molybdenum or niobium were studied by the Knudsen effusion technique coupled with mass-spectrometric analysis of the evaporated products. Mn⁺, SiO⁺, NbO⁺, NbO₂⁺, MoO⁺, MoO₂⁺ and MoO₃⁺ were detected in the mass spectra of the saturated vapour and originated from Mn, SiO, NbO, NbO₂, MoO, MoO₂ and MoO₃ produced by reduction of manganese and silicon oxides with Mo or Nb. The Mn⁺, SiO⁺, NbO₂⁺ and MoO₂⁺ peaks were the most intense. The intensities of Mn⁺ and SiO⁺ ion currents were measured for all the samples. The activities of the components were determined by various methods and similar results were obtained. Δ_fG[½(MnO · SiO₂)]/J mol^–1=–(10090 ± 616)–(0.81 ± 0.42)(T/K) and Δ_fG[⅓(2MnO · SiO₂)]/J mol^–1=–(11560 ± 588)–(1.30 ± 0.38)(T/K). The Gibbs energies of formation of MnO · SiO₂ and 2MnO · SiO₂ were approximated by the equations. The thermodynamic functions of the liquid mixtures were described by the associated solution model under the assumption that complexes MnO · SiO₂, 2MnO · SiO₂ and polymeric SiO₂ exist in the melts. The thermodynamic functions of the MnO–SiO₂ system have been applied for computation of the phase diagram and the results are in good agreement with the available data.