Thermodynamic properties and phase equilibria in the MnO–SiO2 system
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. ΔfG[½(MnO · SiO2)]/J mol–1=–(10090 ± 616)–(0.81 ± 0.42)(T/K) and ΔfG[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.