Borosilicate glass viscosity

Densities required for the calculation of viscosity were obtained in three ways. The densities of the salt solutions were measured by Archimedean displacement (9) of a borosilicate glass bob weighed in air, water, and the solvents and solutions. The results were expressed as linear functions of the molar concentration of salt, and the slopes obtained are recorded in Table II. Densities of the solvent mixtures were taken from published tables (10,11). Densities of the stock acetone and methanol were measured by a conventional pycnometer to greater precision than the salt solutions, and they were compared with the literature values for analysis and with the values obtained by displacement as a check. The two methods agreed within 0.0005 g/ml. 

The viscosity of a borosilicate glass is drawn in Figure 6.31(a). Estimate the activation energy of the viscosity and comment on the form of the Arrhenius plot. 

Figure 6.31 Plot of viscosity against temperature for (a) a borosilicate glass, for Question 6.33, and (b) a high-siUca glass, for Question 6.34...

This result makes clear the important influence of configuration size, as represented by a, on the relaxation process. For example, suppose that a = 1cm, Ei and hi = hy = 100 nm. The borosilicate glass at 800°C might have a viscosity as low as r]v = 10 N-s/m, in which case the characteristic time is roughly 10 s or about 10 days. A small metastable material structure held at elevated temperature for that length of time would lose its defining... 

Fig. 2 Viscosity versus temperature characteristics for various glass compositions [7]. 1, Fused silica 2, 96% silica 3, soda lime (plate glass) 4, lead silicate (electrical) 5, high-lead 6, borosilicate (low expansion) 10. aluminosilicate...

Pressing needs a mold that will be gray cast iron (to 1000°C), stainless steel (can be used for borosilicates at 1185°C and glass ceramics at 1480°C), or even bronze. Usually, though, the mold is cooled. The process uses a viscosity of 4kP and has been applied to objects weighing from 5g to 15 kg. The finished object can be fire-polished. This process, shown in Figure 23.19, is quite... 

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