Volume and structural relaxation in compressed sodium borate glass

Abstract

The structure and properties of glass can be modified through compression near the glass transition temperature (T_g), and such modified structure and properties can be maintained at ambient temperature and pressure. However, once the compressed glass undergoes annealing near T_g at ambient pressure, the modified structure and properties will relax. The challenging question is how the property relaxation is correlated with both the local and the medium-range structural relaxation. In this paper, we answer this question by studying the volume (density) and structural relaxation of a sodium borate glass that has first been pressure-quenched from its T_g at 1 GPa, and then annealed at ambient pressure under different temperature–time conditions. Using ¹¹B MAS NMR and Raman spectroscopy, we find that the pressure-induced densification of the glass is accompanied by a conversion of six-membered rings into non-ring trigonal boron (B^III) units, i.e. a structural change in medium-range order, and an increase in the fraction of tetrahedral boron (B^IV), i.e. a structural change in short-range order. These pressure-induced structural conversions are reversible during ambient pressure annealing near T_g, but exhibit a dependence on the annealing temperature, e.g. the ring/non-ring B^III ratio stabilizes at different values depending on the applied annealing temperature. We find that conversions between structural units cannot account for the pressure-induced densification, and instead we suggest the packing of structural units as the main densification mechanism.