High-temperature heat capacity and density of simulated high-level waste glass

The heat capacities of two simulated high-level waste borosilicate glasses and glass melts have been determined from differential scanning calorimetry and drop-calorimetry measurements between 345 K and 1673 K. The densities of the glass melts have been measured by double-bob Archimedean method between 1273 K and 1573 K. The volume expansivity between glass transition temperature and 1573 K has been determined by combining the measured densities and density of supercooled melts below 1273 K reported previously. Both of the heat capacity and the volume expansivity of the simulated high-level waste glass melts are characterized by a large increase at the glass transition temperature and the rapid decrease with increasing temperature. The configurational contributions to the heat capacity and the volume expansivity at the glass transition temperature are 35% and 88%, respectively. The significant change of the heat capacity is probably attributed to temperature dependence of chemical mixing of boron, aluminum and silicon in the tetrahedral sites, while the change of the volume expansivity is caused by temperature-induced coordination change of boron.