The flow of glass at high stress levels. II. The effect of phase separation on viscosity

The effects of phase separation on the viscosity of glass-forming liquids has been investigated using a fiber-elongation technique. The materials studied were a borosilicate glass and a 0.14 Na₂O·0.86 SiO₂ glass. In the latter case, the viscosity has been measured at temperatures of 501 and 555 °C, and found to depend strongly on the extent, scale, and morphology of the phase separation. The faster development of a large-scale interconnected submicrostructure at 555 °C has been associated with the viscosity of samples tested at this temperature being higher than those at 501 °C, for tests up to about one mouth in duration. The borosilicate glass was tested at temperatures of 536, 482, 460, and 448 °C. In contrast with the results obtained previously on a homogeneous rubidium silicate glass, no evidence for non-Newtonian behavior was found on this phase-separated glass, even for tensile stresses as large as 2.3 × 10¹⁰ dyne/cm².