Abstract
Density and viscosity results are presented for ternary Na2O·GeO2·B2O3 melts (∼600° to 1300°C) and glasses containing as much as 35 mole % Na2O. Synthetic partial molar volume models indicate a fairly broad stability region for BO4 tetrahedra in the B2O3‐rich melts. Similar models for GeO2‐rich melts reveal a more limited stability region for GeO6 octahedra. The expansion coefficient contours and viscosity isotherms confirm the volume‐based conclusions for the liquid state. The high‐temperature volume models were used to develop glass volume models that agree to within several percent of experiment. It has been concluded that the melts and glasses possess similar structures. The relatively greater compositional stability of GeO6 octahedra in the presence of B2O3 (compared to Al2O3) can be related to the smaller average number of oxygens around boron (III), at a fixed O/Ge ratio, compared to aluminum (III). Evidence is presented for a slight decrease of the thermal stability of GeO6 octahedra in the GeO2‐rich melts above about 1000°C.
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