Borosilicate glasses display a rich complexity of chemical behavior depending on the details of their composition and thermal history. Noted for their high chemical durability and thermal shock resistance, borosilicate glasses have found a variety of important uses from common household and laboratory glassware to high-tech applications such as liquid crystal displays. In this paper, we investigate the topological principles of borosilicate glass chemistry covering the extremes from pure borate to pure silicate end members. Based on NMR measurements, we present a two-state statistical mechanical model of boron speciation in which addition of network modifiers leads to a competition between the formation of nonbridging oxygen and the conversion of boron from trigonal to tetrahedral configuration. Using this model, we derive a detailed topological representation of alkali-alkaline earth-borosilicate glasses that enables the accurate prediction of properties such as glass transition temperature, liquid fragility, and hardness. The modeling approach enables an understanding of the microscopic mechanisms governing macroscopic properties. The implications of the glass topology are discussed in terms of both the temperature and thermal history dependence of the atomic bond constraints and the influence on relaxation behavior. We also observe a nonlinear evolution of the jump in isobaric heat capacity at the glass transition when substituting SiO(2) for B(2)O(3), which can be accurately predicted using a combined topological and thermodynamic modeling approach.

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The problem of glass relaxation under ambient conditions has intrigued scientists and the general public for centuries, most notably in the legend of flowing cathedral glass windows. Here we report quantitative measurement of glass relaxation at room temperature. We find that Corning® Gorilla® Glass shows measurable and reproducible relaxation at room temperature. Remarkably, this relaxation follows a stretched exponential decay rather than simple exponential relaxation, and the value of the stretching exponent (β=3/7) follows a theoretical prediction made by Phillips for homogeneous glasses.

It is known that the coordination number (CN) of atoms or ions in many materials increases through application of sufficiently high pressure. This also applies to glassy materials. In boron-containing glasses, trigonal BO3 units can be transformed into tetrahedral BO4 under pressure. However, one of the key questions is whether the pressure-quenched CN change in glass is reversible upon annealing below the ambient glass transition temperature (Tg). Here we address this issue by performing 11B NMR measurements on a soda lime borate glass that has been pressure-quenched at ~0.6 GPa near Tg. The results show a remarkable phenomenon, i.e., upon annealing at 0.9Tg the pressure-induced change in CN remains unchanged, while the pressurised values of macroscopic properties such as density, refractive index, and hardness are relaxing. This suggests that the pressure-induced changes in macroscopic properties of soda lime borate glasses compressed up to ~0.6 GPa are not attributed to changes in the short-range order in the glass, but rather to changes in overall atomic packing density and medium-range structures.

We investigate the high-temperature limit of liquid viscosity by analyzing measured viscosity curves for 946 silicate liquids and 31 other liquids including metallic, molecular, and ionic systems. Our results show no systematic dependence of the high-temperature viscosity limit on chemical composition for the studied liquids. Based on the Mauro-Yue-Ellison-Gupta-Allan (MYEGA) model of liquid viscosity, the high-temperature viscosity limit of silicate liquids is 10 −2.93 Pa·s. Having established this value, there are only two independent parameters governing the viscosity-temperature relation, namely, the glass transition temperature and fragility index. It is also critical for understanding the relaxation characteristics of liquids, as in the well-known Angell plot 1 where the logarithm of viscosity, log 10 η, is plotted as a function of the T g -scaled inverse temperature, T g /T. Here, T g is the glass transition temperature, defined as the temperature at which the liquid viscosity equals 10 12 Pa·s, and T is absolute temperature. The slope of the Angell curve at T g defines the fragility index m,Fragility is a common measure of the slowing down of liquid dynamics upon cooling through the glass transition. According to Angell, 1 liquids can be classified as either "strong" or "fragile" depending on whether they exhibit an Arrhenius or super-Arrhenius scaling of viscosity with temperature, respectively. The degree of non-Arrhenius scaling varies greatly among different glass-forming liquids and reflects the second derivative of the viscosity curve with respect to inverse temperature. With the assumption of a universal hightemperature limit of viscosity, η ∞ , Angell proposed that this non-Arrhenius character is directly connected to the fragility index, m, a first-derivative property of the viscosity curve at T g . 2 However, the assumption of a universal high-temperature limit of viscosity, which enables this direct connection between first-and second-derivative properties, has not yet been validated by a systematic analysis of experimental data.In this Brief Report, we analyze viscosity-temperature curves of 946 silicate liquids and 31 other liquids, including water and silica, as well as borate, metallic, molecular, and ionic liquids. Our results show that there is no systematic dependence of η ∞ on composition and point to a narrow spread around η ∞ = 10 −2.93 Pa·s for silicate liquids. This result implies the existence of a universal high-temperature limit of viscosity, indicating that the fragility index m does have a direct relationship to the non-Arrhenius scaling of liquid viscosity (a measure of curvature), at least for silicate liquids. Our results indicate that there are only two independent parameters governing the viscosity of silicate liquids: T g and m.This simplifies the process for modeling the composition dependence of liquid viscosity and is an indication of the universal dynamics of silicate liquids at the high-temperature limit.To evaluate η ∞ , we analyze experimental viscosity data using ...

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The physical origin of stretched exponential relaxation is considered by many as one of the oldest unsolved problems in science. The functional form for stretched exponential relaxation can be deduced from the axiomatic diffusion-trap model of Phillips. The model predicts a topological origin for the dimensionless stretching exponent, with two "magic" values emerging: β = 3/5 arising from short-range molecular relaxation pathways and β = 3/7 for relaxation dominated by longer-range interactions. In this paper, we report experimental confirmation of these values using microscopically homogeneous silicate glass specimens. Our results reveal a bifurcation of the stretching exponent, with β = 3/5 for stress relaxation and β = 3/7 for structural relaxation, both on macroscopic length scales. These results point to two fundamentally different mechanisms governing stress relaxation versus structural relaxation, corresponding to different effective dimensionalities in configuration space during the relaxation process.