Anomalous Enthalpy Relaxation in Vitreous Silica

Abstract

It is a challenge to calorimetrically determine the glass transition temperature (Tg) of vitreous silica. Here, we demonstrate that this challenge mainly arises from the extremely high sensitivity of the Tg to the hydroxyl content, but also from decrease of the Tg with repeating the calorimetric scans. It is known that the liquid fragility (i.e., the speed of the viscous slow-down of a supercooled liquid at its Tg during cooling) has impact on enthalpy relaxation in glass. Here, we find that vitreous silica (as a strong system) exhibits striking anomalies in both glass transition and enthalpy relaxation compared to fragile oxide systems. The anomalous enthalpy relaxation of vitreous silica is discovered by performing the hyperquenching-annealing-calorimetry experiments. We argue that the strong systems like vitreous silica and vitreous Germania relax in a structurally cooperative manner, whereas the fragile ones do in a structurally independent fashion. We discuss the origin of the anomalous enthalpy relaxation in the HQ vitreous silica.

Sub‐Tg enthalpy relaxation in a milling‐derived chalcogenide glass

“…(B) The difference between the C p2 and C p1 curves (Δ C p ), that is, the rate of the energy release for the as‐milled Ag 3 PS 4 sample (Curve B). For comparison, the Δ C p curve (Curve A) of a hyperquenched silicate glass with basaltic composition (mol%): 47.5SiO 2 ·9.1Al 2 O 3 ·6.6FeO·15.9CaO·16.9MgO·2.0Na 2 O·0.7K 2 O . [Color figure can be viewed at wileyonlinelibrary.com]…”

“…The trapped enthalpy can be released by reheating in a controlled manner. Extensive studies have been conducted in recent years with respect to the enthalpy relaxation in hyperquenched glasses . These studies are useful for understanding the enthalpy relaxation in high‐energy ball‐milling‐derived glasses due to the common feature of energetic excitation.…”

“…These studies are useful for understanding the enthalpy relaxation in high‐energy ball‐milling‐derived glasses due to the common feature of energetic excitation. By performing DSC measurements, we observed several fascinating relaxation phenomena of the as‐milled Ag 3 PS 4 glass as shown in Figure , which are in strong contrast to the relaxation behavior in the hyperquenched basalt glass …”

“…Through a high‐energy ball‐milling process, the starting crystalline materials are excited from lower to higher potential energy state, that is, to the glass state. In contrast, through a thermal quenching process, the supercooled liquid is transformed from a high potential energy state to a lower potential energy state, that is, to the glass state . In other words, a potential energy level can be reached either from the low‐energy crystalline state via mechanical milling or from the high‐energy liquid state via thermal quenching.…”

“…Second, we measure the enthalpy relaxation in both the as‐milled and the dynamically/isothermally heated Ag 3 PS 4 glass using a differential scanning calorimeter (DSC). Finally we compare the relaxation pattern in the Ag 3 PS 4 glass with that of a hyperquenched basalt glass . By doing so, we can explore the special relaxation features of the milling‐derived Ag 3 PS 4 glass, reveal its energetic and structural heterogeneities, and also provide insights into the mechanism of the ball‐milling induced amorphization.…”