Thermodynamics of glass formation and metastable solidification of molten Y3Al5O12

Drop solution calorimetry was used to determine the enthalpy of solution in lead berate for three different materials with the chemical composition Y3 Al5O12. Crystalline yttrium aluminum garnet (YAG), a crystalline mixture of 3 YAlO3 (perovskite) plus Al2O3 (alpha-alumina), and a glass, were all syn thesized by containerless melting and cooling. The enthalpies of drop-disso lution per mole of Y3Al5O12 were 472.19 +/- 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 alpha-alumina and perovskite confirming that the two phase mixture is metastable. The enthalpy of vitrification of YAG i s 276.47 +/- 5.40 kJ/mol (13.82 +/- 0.27 kJ/g atom). This high value for th e enthalpy of vitrification is related to the reluctant glass forming abili ty of the YAG composition. The heat of fusion of Y3Al2O12 is estimated to b e 25.8 kJ/g atom or 516 kJ/mol at its melting point, 2240 K. (C) 1999 Elsev ier Science B.V. All rights reserved.