Energetics of dissolution of Gd2O3 and HfO2 in sodium alumino-borosilicate glasses

In addition to their industrial and consumer-oriented applications, sodium alumino-borosilicate glasses are leading candidates for encapsulation of reprocessed commercial and defense-related nuclear waste. Quantification of the thermochemical and physical properties of these glasses is necessary for a complete understanding of processes occurring in these systems. In this study, 16 glass samples were studied including the base glass (20Na2O–15B2O3–5Al2O3–60SiO2), six Gd-doped glasses (0.45, 0.92, 1.72, 3.25, 4.74, 7.67, 10.53 mol% Gd2O3) and eight Hf-doped glasses (0.15, 0.38, 0.77, 1.57, 3.27, 5.09, 7.06, 11.53 mol% HfO2). Drop solution calorimetry using lead borate solvent at 976 K indicates that these glasses are energetically stable with respect to their binary crystalline oxides. There is a sharp change in the enthalpy of formation between glasses with more than 1.6 mol% Gd2O3 or HfO2 addition and glasses of lower Gd2O3 or HfO2 content. At higher doping levels, the apparent partial molar enthalpy of solution of HfO2 in the glass is close to zero, consistent with the formation of nanometer-sized heterogeneities. A possible explanation is that glasses with more than 1.6 mol% added oxides consist of regions of Gd2O3 or HfO2-rich glass perhaps containing regions of medium-range order dominated by Gd2O3 or HfO2 and/or their nanocrystals and regions of Gd2O3 or HfO2 poor glass. Glass transition temperatures from DSC experiments indicate similar trends with a change in slope near 1.6 mol% Gd2O3 or HfO2. These data are consistent with structural and spectroscopic studies that suggest the onset of clustering and related changes in structure in this composition range.