Enthalpy of mixing of mixed alkali glasses

The enthalpy of mixing was determined for various alkali silicate glasses where alkali species were Li, Na, and K and the total alkali oxide contents were 25 mol% and 33 mol%, using an ion-exchange equilibrium method. It was found that the enthalpy of mixing is negative for all the systems investigated and the magnitude of the enthalpy of mixing was found to increase with alkali content for the Na2O–K2O–SiO2 glass system and also with the size difference of two alkali species for a given alkali concentration. Furthermore, for all the systems investigated the magnitude of enthalpy of mixing was found to be proportional to the difference of molar volumes of the corresponding single alkali glasses. The results were interpreted as a probable cause of the large activation energy of DC electric conductivity, the large mechanical relaxation loss peak, and the large thermometer effect observed for mixed alkali glasses.