Unmixing in sodium-silicate melts: influence on viscosity and heat capacity

The influence of unmixing on the physical properties of sodium silicate glasses and liquids has been investigated by calorimetry and viscometry. Whereas the miscibility gap extends up to about 20 mol% Na2O near the glass transition range, unmixing did not affect the measurements made up to 850 K on samples with 17.5 and 20 mol% Na2O. For a Na2O content of 15 mol%, in contrast, the viscosity was time-dependent and non-Newtonian as a result of the increase of the volume fraction of silica-rich inclusions embedded in a sodium silicate matrix. In differential scanning calorimetry (DSC) experiments made up to 1100 K, the influence of phase separation was also seen up to 17.5 mol% Na2O as a negative heat capacity anomaly due to the enthalpy release associated with the unmixing process.