Viscosity-temperature relationships in the system Na2Si2O5-Na4Al2O5

The viscosity-temperature relationships of five melts on the join Na2Si2O2-Na4Al2O5 (5, 10, 20, 30 and 40 mole percent Na4Al2O5) have been measured in air, at 1 atm and 1000–1350°C with a concentric cylinder viscometer. All the melts on this join of constant bulk polymerization behave as Newtonian fluids, in the range of shear rates investigated, and the melts exhibit Arrhenian viscosity-temperature relationships. Isothermal viscosities on this join initially decrease and then increase with increasing mole percent Na4Al2O5. The minimum viscosity occurs near 20 mole percent Na4Al2O5 at 1000°C and moves to higher Na4Al2O5 content with increasing temperature. The observation of a viscosity minimum along the join Na2Si2-O5-Na4Al2O5 is not predicted based on earlier viscosity data for the system Na2O-Al2O3-SiO2 (RlEBLlNG, 1966) or based on calculation methods derived from this and other data (Bottinga and Weill, 1972). This unexpected behavior in melt viscosity-temperature relations emphasizes the need for a more complete data set in simple silicate systems. Previous spectroscopic investigation of melts on the join Na22Si2O5-Na4Al2O5 offer a structural explanation for the observed viscosity data in terms of a disproportionation reaction involving polyanionic units. Macroscopically, the viscosity data may be qualitatively reconciled with the configurational entropy model for viscous flow (Richet, 1984)