Journal of Non-Crystalline Solids: X最新文献

Glasses in the systems Me₂O-ZnO-B₂O₃ with Me = Li, Na, K, Rb (MeZB), Na₂O-ZnO-CuO-B₂O₃ (NZCuB), CaO-ZnO-B₂O₃ (CaZB), and Li₂O-PbO-B₂O₃ (LPbB) as a reference, were studied by differential thermal analysis, dilatometry, rotational viscometry, and heating microscopy. A decrease of viscosity and sintering range was found with decreasing number of fourfold coordinated boron. The viscosity of the alkali zinc borate glasses varies only slightly. LPbB and CaZB stand out by their reduced and increased viscosities, respectively. Sodium, potassium, and calcium zinc borate glasses possess a fragility above 76. All glasses were sintered to full density before crystallization. Mostly binary zinc borate phases govern crystallization. A ternary crystalline phase was detected only in the potassium containing sample. The Weinberg glass stability parameter ranges between 0.07 and 0.12. This is caused by the presence of several crystalline phases and varying melting points of even the same crystalline phase in different glass matrices.

Glass transition temperature and related dynamics play an essential role in amorphous materials research since many of their properties and functionalities depend on molecular mobility. However, the temperature dependence of the structural relaxation time for a given glass former is only experimentally accessible after synthesizing it, implying a time-consuming and costly process. In this work, we propose combining artificial neural networks and disordered systems theory to estimate the glass transition temperature and the temperature dependence of the main relaxation time based on the knowledge of the molecule's chemical structure. This approach provides a way to assess the dynamics of molecular glass formers, with reasonable accuracy, even before synthesizing them. We expect this methodology to boost industrial development, save time and resources, and accelerate the scientific understanding of structure-properties relationships.

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.

Inorganic glasses have been the one of the most important families of inorganic materials used by human society. The ability to fabricate glasses with customized shapes is of high significance for a diverse range of applications. We review the recent advances in the development of techniques for fabricating glass items with pre-designed geometries. Most of these methods are based on the high-temperature densification of the green parts, which are formed by the preshaping of the composites containing inorganic feedstock (e.g., SiO₂) by molding, nanoimprinting and 3D printing, with resolutions down to the sub-micron level. These methods have also enabled the fabrication of multicomponent glass systems and the incorporation of traces of metal ions or nanoparticles as dopants with optical functionalities. In this review, these low-temperature routes are compared with the direct 3D printing route for the fabrication of glass by selective laser melting and fused deposition molding, which rely on high-temperature melting/sintering of glass powers or filaments. Finally, the benefits of different methods for fabricating glass in a customizable manner are discussed and potential future directions are highlighted.

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.