Heat Capacity and Structural Relaxation of Mixed‐Alkali Glasses

Heat capacities of a series of mixed-alkali glasses of composition (in mol%) 24.4(Na2O + K2O)-75.6SiO2 were measured in the transition region by differential scanning calorimetry. The glass heat capacities at 700 K and the equilibrium liquid heat capacities are the same for all glasses on a per-g atom basis and equal, respectively, to 5.6 ± 0.1 and 6.8 ± 0.1 cal/g atom K. The glass transition temperatures exhibited negative deviations from additivity, but the heat capacity curves in the transition region of all the glasses for identical heating rates and thermal histories could be superimposed on the same reduced plot. This behavior indicates that the shapes of the structural relaxation functions are the same for all the glasses. These results support Shelby's conclusion that there is no unique “mixed-alkali effect” on thermodynamic or structural relaxation properties and that the term should be reserved for properties relating to ionic transport.