Heats of Solution, Density, and Refractive Index Changes in Sodium Borate Glasses Subjected to High Pressure

The densification produced in sodium borate glasses by pressures to 40 kbars at 25° and 250°C was measured. At a constant temperature and pressure, the densification decreased with increasing alkali content. For specimens pressed at 40 kbars and 250°C, the densification ranged from 14.2% for B₂O₃ to 6.3% for the 33.3 mole % Na₂O composition. Under the same conditions, the refractive index increase ranged from 4.8 to 1.1%, and the decrease of the molar refraction of the oxygen ions ranged from 2.5 to 1.5%. The heats of solution of the pressed glasses were more negative than those of the corresponding unpressed glasses. The heats of solution of both the pressed and unpressed glasses pass through a minimum near 20 mole % Na₂O. A decrease in density was observed at room temperature for all pressed specimens. Electron micrographs were made of two of the pressed specimens. The results could be explained on the basis of a repacking of structural units.