Phase equilibria and thermodynamics in the Y2O3-Al2O3-SO2-system

The enthalpy of formation of YAM (Y4Al2O9) and Y2Si2O7 (gamma) has been der ived using high-temperature drop solution calorimetry at 1073 K in molten a lkali borate. The enthalpy of formation from oxides at 298 K is -6.24 +/- 6 .21 and -67.08 +/- 5.98 kJ/mol for YAM and disilicate, respectively. The th ermodynamic functions in the Y2O3-Al2O3 and Y2O3-SiO2 systems are assessed from phase equilibrium data and calorimetric measurements for YAG (Y3Al5O12 ), YAP (YAlO3), YAM (Y4Al2O9), Y2SiO5 and Y2Si2O7 phases. The calculated ph ase diagrams of the Al2O3-Y2O3 and Y2O3-SiO2 systems are in reasonable agre ement with the experimental data. Suggestions for further experimental meas urements on solid phases are given. Using limited phase equilibrium data fo r the ternary system Y2O3-Al2O3-SiO2, the thermodynamic functions are asses sed. The liquidus surface, an isothermal section at 1673 K and isopleths in the systems Y3Al5O12-SiO2, YAlO3-SiO2, Y4Al2O9-SiO2, Y2SiO5-Al6Si2O13 and Y2Si2O7-Al6Si2O13 are presented.