As reported by Lebedev, the endothermic effect of borosilicate crown glasses is in the temperature range of 550 to 610 °C. The highest heating temperature in Lebedev’s experiments is 700 °C. This is much lower than 1200°C where an endothermic effect of fused silica can be observed. Because of this, Lebedev did not find any endothermic effect of pure silica in his experiments. The large difference in endothermic effect temperature ranges between borosilicate crown glass and fused silica is caused by the difference in their chemical composition. Sodium silicate glasses with different Na2The O concentration can be used to illustrate the influence of chemical concentration on the temperature range of endothermic effects.
Figure 3a is the Na2O-SiO2 Phase diagram showing the change in liquidus temperature of the binary glass as the concentration of sodium oxide varies16. As sodium oxide in the glasses increases from 0 to 11.3% by weight, 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 equivalent to that of pure fused silica, the sodium cations are not involved in the entire process of crystal formation, including the formation of embryonic clusters. At 1470 °C, sodium silicate glasses with a sodium oxide concentration of less than 11.3% by weight have the same critical temperature Tc as pure quartz glass17. Below Tc, the transformation process from disorder to order is expected to take a few hundred degrees of the temperature range to complete. Therefore, the Tg of these glasses should be close to 1200 °C, just like pure quartz 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 decreases from 1470 to 870 °C in the composition range of 11.3–24.5 wt%. The corresponding crystal, through which the sodium silicate glass solidifies, is in this temperature range (upbeta)-Tridymite, don’t (upbeta)-Cristobalite. The critical temperature Tc separating the two different temperature ranges is the inter-crystal polymorphic inversion temperature (upbeta)-Tridymite and (upbeta)-Quartz, that’s 870 °C17. In the glass forming process above 870 °C, the formed clusters (upbeta)-Tridymite embryos, not (upbeta)-Cristobalite embryos. Although the (upbeta)-Tridymite embryo clusters have a different shape and number of facets than those of (upbeta)-cristobalite, they still experience the disorder for order transition in the temperature range below 870 °C. Since the arrangement of SiO4 Tetrahedron in plan of (upbeta)-Tridymite hexagonal structure is the same as that in (111) plan of (upbeta)-Cristobalite face centered cubic structure18Fig. 2a and b can also be used to illustrate the stabilized structure of (upbeta)– Tridymite embryonic clusters formed on the facets. The final temperature of this transition, Tg, is expected to be a few hundred degrees below 870°C and can be determined experimentally. For example, point T, indicated by an arrow in Figure 3a, represents sodium silicate glass with 15 wt% NaO2. The liquidus and critical temperatures of this particular glass are found from Figure 3a to be 1340 and 870°C, respectively. The temperature range 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% NaO2 as a function of temperature is found from references19.20 and is plotted in Figure 3b. There in the Na2O – SiO2 For systems that differ very little in compositions expressed in wt% and in mole%, the Cp data found above represent the thermal properties of glass represented by point T in Figure 3a. The sharp increase in Cp from 480 to 560 °C in Fig. 3b shows that heat absorption increases rapidly in the temperature range from 480 to 560 °C. This temperature range of sodium silicate glass showing the endothermic effect is very close to that observed by Lebedev for borosilicate crown glass as shown in Fig.1.