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-15B₂O₃-5Al₂ O₃-60SiO₂), 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 1.6mol% 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 1.6mol% 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.