As Lebedev reported, the endothermic effect of borosilicate crown glasses is in the temperature region from 550 to 610 °C. The highest heating temperature in the Lebedev’s experiments is 700 °C. This is much lower than 1200 °C, where endothermic effect of silica glass can be observed. This is why Lebedev did not find the endothermic effect of pure silica in his experiments. The large difference in the temperature regions of endothermic effect between borosilicate crown glass and silica glass is caused by the difference in their chemical compositional. Sodium silicate glasses with various Na2O concentration can be used to illustrate the influence of chemical concentration on the temperature region of the endothermic effects.
Figure 3a is the Na2O-SiO2 phase diagram, which shows the variation of the liquidus temperature of the binary glass when the concentration of sodium oxide varies16. As sodium oxide in the glasses increases from 0 to 11.3 wt.%, the liquidus temperature of the glasses decreases from 1713 to 1470 °C. Because the corresponding solidification crystal of these glasses is \(\upbeta \)-cristobalite, which is the same as that of pure silica glass, the sodium cations are not involved in the entire process of crystal formation, including embryonic clusters formation. Sodium silicate glasses with less than 11.3 wt.% sodium oxide concentration have the same critical temperature Tc as pure silica glass at 1470 °C17. It is expected that the disorder to order transformation process below Tc takes a few hundred degrees of the temperature range to be completed. Thus, Tg of these glasses should be close to 1200 °C, same as pure silica glass; and the endothermic effect of all these glasses should be observed near 1200 °C.
Figure 3a also shows that the liquidus temperature for sodium silicate glasses in the composition range of 11.3–24.5 wt.% decreases from 1470 to 870 °C. The corresponding crystal in this temperature range through which the sodium silicate glass solidifies is \(\upbeta \)-tridymite, not \(\upbeta \)-cristobalite. The critical temperature Tc that separates the two different temperature regions is the polymorphic inversion temperature between crystal \(\upbeta \)-tridymite and \(\upbeta \)-quartz, which is 870 °C17. In the glass forming process above 870 °C, the formed clusters are \(\upbeta \)-tridymite embryos, not \(\upbeta \)-cristobalite embryos. Although the \(\upbeta \)-tridymite embryonic clusters have a different shape and number of facets than that of \(\upbeta \)-cristobalite, they still experience the disorder to order transformation in the temperature region lower than 870 °C. Because the arrangement of SiO4 tetrahedra in the base plan of \(\upbeta \)-tridymite hexagonal structure is the same as that in (111) plan of \(\upbeta \)-cristobalite face-centered cubic structure18, Figs. 2a and b can also be used to illustrate the stabilized structure of \(\upbeta \)-tridymite embryonic clusters formed on the facets. The ending temperature of this transition, Tg, is expected to be lower than 870 °C by a couple of hundred degrees and can be experimentally determined. For example, point T, indicated by an arrow in Fig. 3a, represents sodium silicate glass with 15 wt.% of NaO2. This particular glass’s liquidus temperature and critical temperature are found from Fig. 3a to be 1340 and 870 °C, respectively. The temperature region of the endothermic effect of this glass can be determined from the experimental heat capacity data. The heat capacity Cp of sodium silicate glass with 15 mol.% of NaO2 as a function of temperature is found from references19,20 and is plotted in Fig. 3b. Since in the Na2O – SiO2 system, the compositions expressed in wt.% and in mol.% differ very little, the above found Cp data represents the thermal property of glass presented by point T in Fig. 3a. The sharp rising of Cp from 480 to 560 °C in Fig. 3b indicates that heat absorption increases rapidlyly within the temperature region from 480 to 560 °C. This temperature range of the sodium silicate glass, showing the endothermic effect, are very close to that observed for borosilicate crown glass by Lebedev, as shown in Fig. 1.
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