Experimental investigation and thermodynamic description of the BaO-containing oxide system

Abstract

A thermodynamic description of the metallurgical and ceramic system is beneficial when predicting the phase equilibria, thermodynamic properties and other significant properties. The mathematical expression of Gibbs energy has been adopted to realize the thermodynamic description of the systems. Employing the CALPHAD technique, the thermodynamic parameters in the Gibbs energy functions of various phases in the systems can be optimized by fitting with the experimental data available. The thermodynamic assessment of the unary and binary systems plays a fundamental role in the establishment of the thermodynamic database, because the accuracy of the unary and binary data largely determines the reliability of the extrapolation into higher order systems. In the present thesis, experiments were conducted to study the phase equilibria in the BaO-MgO, BaO-Al2O3 and BaO-SiO2 systems. For the BaO-Al2O3 and BaO-SiO2 systems, the liquids was constructed using a high temperature equilibration/quenching/EPMA technique. The phase stability and phase relations (below 1523 K) in the BaO-MgO, BaO-Al2O3 and BaO-SiO2 systems were identified by XRD and SEM/EDS. The literature data combined with the personal experiments were evaluated for thermodynamic modelling. Thermodynamic assessment was performed by means of the CALPHAD technique for the BaO-SrO, BaO-CaO, BaO-MgO, BaO-Al2O3, BaO-SiO2 and SrO-SiO2 systems. For the BaO-SrO, BaO-CaO and BaO-MgO systems, substitutional solution model was adopted to express the molten phase. For the BaO-Al2O3, BaO-SiO2 and SrO-SiO2 systems, associate solution model with one associate was applied to describe the molten phase. Moreover, two associates, Ba2SiO4 and BaSiO3, were tested to model the liquid behavior in the BaO-SiO2 system. The calculated phase diagrams and thermodynamic properties were compared with experimental data. Sets of consistent thermodynamic parameters were presented for the assessed oxide systems. The isothermal section of the BaO-Al2O3-SiO2 ternary system at 1773 K was determined by an equilibration/quenching technique and SEM/EDS. On the basis of the MTOX oxide database, the isothermal sections of this ternary system at 1673 K, 1773 K, 1873 K and 1973 K were calculated.