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Pressure-induced structural changes in a sodium borate glass have been studied up to ~ 9 GPa by in situ micro-Raman spectroscopy in a diamond anvil cell at ambient temperature. The results show a decrease in the fraction of boroxol rings and an increase in the fraction of non-ring structures with increasing pressure. These findings are compared with Raman spectra of the same glass composition compressed at 1 GPa at its glass transition temperature (406 °C). We show similarities in the medium-range structural changes of the borate glass compressed at high-pressure/low-temperature (9 GPa, 25 °C) and intermediate-pressure/high-temperature (1 GPa, ~ 400 °C) conditions. The structural changes are elastic during cold compression but inelastic as a result of the hot compression. We discuss this difference in relation to densification and Raman inactive structural changes.

The elastic moduli of glasses are important for numerous applications, but predicting them based on their chemical composition and forming history remains a great challenge. In this study, we investigate the relationship between densification and changes in elastic moduli as a result of isostatic compression up to 1 GPa of various oxide compositions at elevated temperature (so-called hot compression). An approximately linear relationship is observed between the relative changes in density and elastic moduli across a variety of glass families, although these glasses exhibit a diverse range of structural responses during compression owing to their dramatically different chemistries.

Density and hardness of glasses are known to increase upon both compression at the glass transition temperature (Tg) and ambient pressure sub-Tg annealing. However, a serial combination of the two methods does not result in higher density and hardness, since the effect of compression is countered by subsequent annealing and vice versa. In this study, we circumvent this by introducing a novel treatment protocol that enables the preparation of high-density, high-hardness bulk aluminosilicate glasses. This is done by first compressing a sodium-magnesium aluminosilicate glass at 1 GPa at Tg, followed by sub-Tg annealing in-situ at 1 GPa. Through density, hardness, and heat capacity measurements, we demonstrate that the effects of hot compression and sub-Tg annealing can be combined to access a “forbidden glass” regime that is inaccessible through thermal history or pressure history variation alone. We also study the relaxation behavior of the densified samples during subsequent ambient pressure sub-Tg annealing. Density and hardness are found to relax and approach their ambient condition values upon annealing, but the difference in relaxation time of density and hardness, which is usually observed for hot compressed glasses, vanishes for samples previously subjected to high-pressure sub-Tg annealing. This confirms the unique configurational state of these glasses.

The structure and properties of glass can be modified through compression near the glass transition temperature (Tg), and such modified structure and properties can be maintained at ambient temperature and pressure. However, once the compressed glass undergoes annealing near Tg 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 Tg 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 (BIII) units, i.e. a structural change in medium-range order, and an increase in the fraction of tetrahedral boron (BIV), i.e. a structural change in short-range order. These pressure-induced structural conversions are reversible during ambient pressure annealing near Tg, but exhibit a dependence on the annealing temperature, e.g. the ring/non-ring BIII 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.

The structure-property relations of permanently densified sodium metaphosphate oxynitride glasses (NaPO 3 − 3x/2 N x ) with varying N/P ratio are investigated. Densification of the bulk samples is achieved through 1 GPa isostatic compression at the glass transition temperature. The pressure-induced structural transformations are characterized by Raman spectroscopy and solid state 31 P and 23 Na NMR spectroscopy, whereas the glass hardness and brittleness are quantified through Vickers indentation. Nitridation of the ambient pressure sodium metaphosphate glass (NaPO 3 ) results in a step-wise conversion of PO 4 units into PO 3 N and PO 2 N 2 oxynitride species. Upon hot compression, the glasses become permanently denser and harder, but also more brittle. This pressure-induced densification is not accompanied by changes in the types or fractions of the structural units for the NaPO 3 glass. In contrast, densification of the oxynitride glasses (N/P > 0) is accompanied by pressure-induced structural transformations between the oxynitride species. Based on the 31 P NMR results, a structural mechanism for these transformations is proposed.

Glasses can be chemically strengthened through the ion exchange process, wherein smaller ions in the glass (e.g., Na+) are replaced by larger ions from a salt bath (e.g., K+). This develops a compressive stress (CS) on the glass surface, which, in turn, improves the damage resistance of the glass. The magnitude and depth of the generated CS depends on the thermal and pressure histories of the glass prior to ion exchange. In this study, we investigate the ion exchange-related properties (mutual diffusivity, CS, and hardness) of a sodium aluminosilicate glass, which has been densified through annealing below the initial fictive temperature of the glass or through pressure-quenching from the glass transition temperature at 1 GPa prior to ion exchange. We show that the rate of alkali interdiffusivity depends only on the density of the glass, rather than on the applied densification method. However, we also demonstrate that for a given density, the increase in CS and increase in hardness induced by ion exchange strongly depends on the densification method. Specifically, at constant density, the CS and hardness values achieved through thermal annealing are larger than those achieved through pressure-quenching. These results are discussed in relation to the structural changes in the environment of the network-modifier and the overall network densification.

Oxide glasses can be permanently densified through application of high pressure at room or elevated temperature. Such treatment allows for modification of macroscopic glass properties. However, the structural origins of the pressure-induced property changes are not yet fully understood. In this study, we investigate the ability of a glass network to resist densification under pressure at both ambient and elevated temperatures. We study the detailed deformation mechanisms (densification and shear flow) that occur during indentation of series of as-prepared and isostatically compressed mixed Na/K aluminosilicate glasses, which exhibit a pronounced nonlinear scaling in glass properties due to the mixed alkali effect. Following pressure treatment at elevated temperature, an increase in Vickers hardness is observed due to a significant decrease in densification under the indenter. In contrast, the volume of glass developed in the pileup regions due to shear flow is unaffected by the pressure treatment. This change in the relative contributions of these plastic deformation mechanisms can explain the decrease in crack resistance of the glasses induced by the isostatic compression treatment.

Densified glasses recovered from a high-pressure state are of potential technological interest due to their modified physical and chemical properties. Here we investigate the temperature-dependent densification behavior of a sodium borosilicate glass in a gas pressure chamber at 1 GPa. The temperature is varied for a 30 min treatment between 0.6Tg and 1.15Tg, where Tg is the glass transition temperature, and the treatment duration is varied between 10 and 10 000 min for compression at 0.9Tg. Permanent densification occurs for temperatures above 0.7Tg and the degree of densification increases with increasing compression temperature and time, until attaining an approximately constant value for temperatures above Tg. The same temperature and time dependence is also found for the glass mechanical properties (hardness and brittleness) and the network structure, i.e., fraction of three-fold versus four-fold coordinated boron atoms and ring versus non-ring trigonal boron atoms, and the extent of mixing of Si and B. The results provide insights into the temperature-dependence of the network densification and the relative roles of viscous flow and more localized rearrangements.

Glass is ubiquitous, yet our understanding of its structure-function relationships remains far from complete and limits technology. For example, while compression is an important tool in the synthesis of crystalline materials, comparable breakthroughs in preparing bulk glasses are still largely lacking. This work reveals the striking linear dependence of the plastic compressibility of borosilicate glasses on both initial trigonal boron content and relative change in hardness with pressure, with important implications for manufacturing tailored damage-resistant glassy materials.

Glass exhibits a significant change in properties when subjected to high pressure because the short- and intermediate-range atomic structures of glass are tunable through compression. Understanding the link between the atomic structure and macroscopic properties of glass under high pressure is an important scientific problem because the glass structures obtained via quenching from elevated pressure may give rise to properties unattainable under standard ambient pressure conditions. In particular, the chemical strengthening of glass through K(+)-for-Na(+) ion exchange is currently receiving significant interest due to the increasing demand for stronger and more damage-resistant glass. However, the interplay among isostatic compression, pressure-induced changes in alkali diffusivity, compressive stress generated through ion exchange, and the resulting mechanical properties are poorly understood. In this work, we employ a specially designed gas pressure chamber to compress bulk glass samples isostatically up to 1 GPa at elevated temperature before or after the ion exchange treatment of a commercial sodium-magnesium aluminosilicate glass. Compression of the samples prior to ion exchange leads to a decreased Na(+)-K(+) interdiffusivity, increased compressive stress, and slightly increased hardness. Compression after the ion exchange treatment changes the shape of the potassium-sodium diffusion profiles and significantly increases glass hardness. We discuss these results in terms of the underlying structural changes in network-modifier environments and overall network densification.