Thermodynamics of glass formation and metastable solidification of molten Y 3Al 5O 12

Abstract

Drop solution calorimetry was used to determine the enthalpy of solution in lead borate for three different materials with the chemical composition Y 3Al 5O 12. Crystalline yttrium aluminum garnet (YAG), a crystalline mixture of 3 YAlO 3 (perovskite) plus Al 2O 3 (α-alumina), and a glass, were all synthesized by containerless melting and cooling. The enthalpies of drop-dissolution per mole of Y 3Al 5O 12 were 472.49 ± 4.19, 440.35 ± 3.59, and 196.02 ± 3.39 kJ/mol, respectively, for the garnet, the crystalline mixture, and the glass. The garnet phase is thermodynamically stable with respect to the corresponding mixture of α-alumina and perovskite confirming that the two phase mixture is metastable. The enthalpy of vitrification of YAG is 276.47 ± 5.40 kJ/mol (13.82 ± 0.27 kJ/g atom). This high value for the enthalpy of vitrification is related to the reluctant glass forming ability of the YAG composition. The heat of fusion of Y 3Al 2O 12 is estimated to be 25.8 kJ/g atom or 516 kJ/mol at its melting point, 2240 K.