Journal of Non-Crystalline Solids: X最新文献

Combining thermal and pressure effect represents a novel approach to modify glass properties. However, the microscopic structural origin of these property modifications is complex and far from fully understood, especially in multicomponent glasses with mixed glass formers. In this paper, we have utilized classical molecular dynamics simulations with a set of composition dependent potentials to investigate pressure-quenching effect on sodium borosilicate glasses. Processes including hot compression, cold compression and subsequent annealing on the structures and properties are investigated and compared. It was found that applying pressure up to 10 GPa at the glass transition temperature led to permanent densifications and a dramatic increase of elastic moduli by 90%, while thermal annealing reversed the increase and applying pressure at ambient temperture did not increase the modulus. The main structural change of the hot compressed sample is the increase of four-fold coordinated boron while silicon remains four-fold coordinated. The sodium environment shows an increase of coordination number and a decrease of NaO and NaNa bond distances. Medium range structure is also changed with an increase of 8-membered rings. These results provide atomistic insights of the pressure quench effect on borosilicate glasses.

The viscoelasticity of glass-forming fluids contains sustantial information about space-time rigidity. Viscoelasticity and rheology provide alternative experimental, computational and theoretical ways to asses chemical composition effects in the relaxation of supercooled liquids near the glass transition. In particular, the transverse current correlation and transversal dynamical structure factor contain space-time information allowing to relate the dynamical gap of transversal vibrational modes with floppy modes and relaxation times in the liquid. Here, a short revision is made of the subject, including simulations of Tellurium, a typical chalcogenide glass. Our results are similar to those obtained for typical metallic liquids. To rationalize this result, an statistical mechanics analysis in the strain ensemble is performed by using a model that incorporates flexibility and hard-core potentials. This shows that the entropy is akin to a hard-cord fluid as angular bonds only renormalize the entropy if they are not substantially affected by temperature effects. Finally, a comparison is made with Selenium, where bond breaking effects do not allow such a straight-forward treatment.

Glass-forming liquids are broadly classified as being fragile or strong, depending on the deviation from Arrhenius behavior of their relaxation times. A fragile to strong crossover is observed or inferred in liquids like water and silica, and more recently also in metallic glasses and phase change alloys, leading to the expectation that such a crossover is more widely realised among glass formers. We investigate computationally the well-studied Kob-Andersen model, accessing temperatures well below the mode coupling temperature T_MCT. We find that relaxation times exhibit a crossover in dynamics around T_MCT, and discuss whether it bears characteristics of the fragile to strong crossover. Several aspects of dynamical heterogeneity exhibit behavior mirroring the dynamical crossover, whereas thermodynamic quantities do not. In particular, the Adam-Gibbs relation describing the relation between relaxation times and configurational entropy continues to hold below the dynamical crossover, when anharmonic corrections to the vibrational entropy are included.

C. Austen Angell has been a great inspiration for liquid and glass scientists including myself. He was a close collaborator of mine on glass dynamics and thermodynamics, particularly on the sub-T_g relaxation in glass. Here, in memory of his seminal contributions to science and our collaboration, I revisit our published data and analyze our unpublished data, as well as review some of important results reported recently in the literature, concerning the sub-T_g relaxation in glass, a field on which Austen has profoundly impacted. I highlight new insights into the dynamics, thermodynamics, and structural heterogeneity in glass far from equilibrium, based on the hyperquenching-annealing-calorimetry (HAC) experiments. I describe key extensions and applications of the HAC approach in glass research. Finally, I present the perspectives and challenges regarding the sub-T_g relaxation in glasses with fictive temperatures well above T_g.

The question of whether static stress in fused silica relaxes at room temperature is still under debate. Here, we report experimental data investigating the behavior of fused silica at room temperature when subjected to static tensile stress up to 2 GPa stress levels under different environmental conditions. Our measurements are performed using an innovative combination of methods; a femtosecond laser is used to accurately load a monolithic microscale test beam in a non-contact manner, fabricated using femtosecond laser processing and chemical etching, while the stress is continuously monitored using photoelasticity over an extended period spanning several months, in both dry and normal atmospheric conditions. The results are of practical importance, not only for photonics devices making use of stress-induced birefringence but also for flexures subjected to static loads.

Hypersonic sound velocities have been measured by Brillouin scattering for glasses and liquids of the system CaO-MgO-Al₂O₃-SiO₂ (CMAS). Transverse wave velocities are reported for ten samples from 293 up to temperatures ranging from 1000 to 1600 K, depending on composition, and longitudinal velocities for six samples from 293 to the interval 1890–2360 K. For the 13 CMAS compositions, the temperature dependences of acoustic velocities are constant within three temperature intervals: (i) from 293 K to the standard glass transition temperature T_g, (ii) from T_g, to the temperature T_rx of the onset of structural relaxation at the timescale of Brillouin scattering where transverse waves disappear, (iii) and finally above T_rx where only longitudinal waves thus remain observed. The implications of these results for structural relaxation, shear-wave propagation and shear modulus are then discussed.

We discuss possible extraneous effects entering in the conventional measures of “fragility” at atmospheric pressure that may obscure a characterization of the genuine super-Arrhenius slowdown of relaxation. We first consider the role of density, which increases with decreasing temperature at constant pressure, and then the potential influence of the high-temperature dynamical behavior and of the associated activation energy scale. These two effects involve thermodynamic parameters and the strength of the “bare” activation energy reflecting the specific bonding between neighboring molecules. They vary from system to system with, most likely, little connection with any putative collective behavior associated with glass formation. We show how to scale these effects out by refining the definition of fragility and modifying the celebrated Angell plot. We dedicate this note to our great and so inspiring friend, Austen Angell, and associate in this tribute another dear colleague who died too soon, Daniel Kivelson.

Lithium-ion-conducting oxide glass electrolytes in the multicomponent systems Li₂O–B₂O₃–SiO₂–P₂O₅-LiX (LiX = Li₃N, Li₂SO₄, Li₂CO₃, and LiI) were synthesized using a mechanochemical technique. The crystallization temperature and ionic conductivity of multicomponent glasses with added lithium salts or Li₃N were evaluated. Because the crystallization temperature is a measure of the ability of glasses to resist crystallization at high temperature, glasses with high conductivity and high crystallization temperature are desirable electrolytes. In this study, the lithium-ion conductivity of the glasses was found to be correlated with the crystallization temperature, and it was difficult to increase both the conductivity and crystallization temperature. The addition of lithium salts (Li₂SO₄, Li₂CO₃, and LiI) increased the conductivity but decreased the crystallization temperature. Nitrogen doping by the addition of Li₃N improved both these properties of the oxide glass electrolyte. Therefore, oxynitride glasses are desirable electrolytes owing to their thermal stability and lithium-ion conductivity.