Thermodynamics of Na2O-SiO2 melts

The thermodynamic properties of Na2O-SiO2 solid (942-1285 K) and liquid (1103-1719 K, 19.5-61.8 mol % Na2O) silicates were studied by Knudsen cell mass spectrometry. To determine the activities of the constituent oxides, these were reduced to volatile suboxides directly in effusion cells. Mass spectra of the saturated vapor over Na2O-SiO2 showed the presence of the Na+, Na2O+, NaO+, O2+, TaO+, TaO2+, NbO+, NbO2, MoO+, MoO2+, MoO3+, and NiO+ ions resulting from the ionization of the Na, Na2O, NaO, NaO2,O2, TaO, TaO2, NbO, NbO2, MoO, MoO2, MoO3, and NiO molecules. The activities calculated by two different procedures were found to coincide within the experimental error. The enthalpies and Gibbs energies of formation of sodium silicates were shown to be extremely low. The formation of solid orthoand metasilicates is accompanied by a decrease in entropy, in contrast to the other sodium silicates. Sodium orthosilicate has the lowest enthalpy and Gibbs energy. A thermodynamic model for Na2O-SiO2 melts is proposed which relies on associated solution theory and takes into account silica polymerization. The model describes the composition and temperature dependences of the activities of the constituent oxides in the melt with an accuracy no worse than the experimental error (2-3%). The model, in combination with the thermodynamic functions of formation of all the intermediate solid phases, was used to calculate phase equilibria in the Na2O-SiO2 system. The results agree well with the experimental data obtained by physicochemical methods.