Tailoring structure and properties of silica glass aided by computer simulation

Abstract

By using a combination of experiments and molecular dynamics simulations, our studies show that the elastic response of silica glass to initial compression gradually changes from abnormal to normal with increasing quench pressure, helium content or alkali modifier added in the glass matrix. We uncovered the structural origin of the elastic anomaly in silica glass as localized structural transitions between motifs of different stiffness that are similar to those found in its crystalline counterparts. Pressure-quenching, helium-stuffing, or alkali-modifying plays a different role in changing the structure of silica glass, but all of the resulting structures reduce the propensity for such local structural transitions to take place, thus the degree of elastic anomaly. Our studies demonstrate that by processing in ways that gradually eliminates the elastic anomaly, the degree of silica glass to undergo irreversible densification can be eventually eradicated. This provides a solid foundation for the bottom-up design of new glasses with tunable structure and properties.

Understanding the structural origin of intermediate glasses

“…From the temperature response of Raman bands, we have identified two structural changes that occur in silicate glass networks with temperature. One is bond weakening with temperature and the other is the network stiffening aided by conformation changes in flexible large rings such as the predominant 6‐membered rings in silica glass . These two structural effects come together to control the temperature response of elastic properties which will be discussed next.…”

“…The universal positive temperature dependence of the frequency of the Raman main band in all of the glasses analyzed here suggests that flexible MRO rings always get stiffer with temperature, with the stiffening effect depending on the number of in Si–O–Si bonds in these rings. It is important to note that such conformation changes of flexible rings in glasses are localized in contrast to the extended character of similar transformation in crystals …”

“…In the Na 2 O–SiO 2 system, a 15Na 2 O–85SiO 2 composition and in the Na 2 O–TiO 2 –SiO 2 system, a 15Na 2 O–4TiO 2 –81SiO 2 composition were found to exhibit intermediate elastic responses as a function of temperature. Mechanically modified silica glass by a systematic pressure quenching was found to exhibit intermediate behavior without chemical modification . Brillouin longitudinal frequency shift of albite glass, a glass derived from the naturally occurring sodium feldspar, was found to remain constant with temperature .…”

“…Normal glasses, like window glass, show a negative temperature dependence and a positive pressure dependence of elastic moduli . Anomalous glasses, like silica, on the other hand, show a positive temperature dependence and a negative pressure dependence of elastic moduli . It is natural to imagine that in between there exist intermediate glasses with elastic responses independent of temperature and/or pressure.…”

“…Such glasses have attracted much attention in recent literature with a great potential for developing a‐thermal optical fibers for enhanced telecommunication, sensors, and high‐energy laser systems . These glasses would also prove useful in applications where a broad range of thermal and mechanical stimulation is expected …”