Relaxation In Glasses Research Articles

Cl- skeleton modification and K+ charge compensation construct Eu2+ subcrystalline luminescent structure in NaAlSiO4

In this work, the highly homogeneous NaAlSiO4 structure was constructed by glass relaxation crystallization method. The evolution of the subcrystalline luminescence structure was examined by using Eu2+ as a structural probe. The optimization of luminescent properties and phase structure changes of Cl- substitution, Eu2+ occupancy and K+ charge compensation were studied by XRD, EDS, PL spectra, PLE spectra and luminescence thermal stability analysis. The research revealed the AB layer spacing of the crystal stretched and the hexatomix ring opened by Cl- directionally replace O2-(O5), inducing more Eu2+ to selectively occupy the Na+(I) site, resulting in local charge defects in the lattice. The transition emission efficiency of Eu2+ was increased by 50% with introducing K+ for directional charge compensation. In comparison with NaAlSiO4:2%Eu2+, the NaAlSiO3.96Cl0.04:2%Eu2+-8K+ phosphor showed a 20.82% increase in ΔE, indicating improved thermal sensitivity in this phosphor series, which holds promising potential for application in LED devices.

Enthalpy relaxation of sodium aluminosilicate glasses from thermal analysis

AbstractThe sodium aluminosilicate (NAS) glass family is important for many different industrial applications, but glass relaxation has not yet been thoroughly studied in this system. Thermal analysis techniques such as differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC) can provide insight into the enthalpy relaxation of glass by measuring the glass transition temperature (Tg), activation energy, and enthalpy of relaxation. MDSC is mostly used to study nonoxide and low Tg glasses, and there is much debate about whether the nonreversing heat flow analysis method is accurate. To the authors’ knowledge, this is the first paper using MDSC to study these NAS compositions, and one of few papers to report MDSC on high Tg oxide glasses. We report on one set of modulation conditions that obtain a linear response using Lissajous curves, as well as comparing the activation energy calculated from DSC with the enthalpy of relaxation obtained from MDSC. Our results show that the activation energy and enthalpy of relaxation do not give the same compositional minimum in relaxation, and therefore more work is needed to investigate the validity of the nonreversing heat flow approach for high Tg oxide glasses.

Thickness-Dependent Segmental Dynamics in Supported Thin Films: Insights from a Dynamically Correlated Network Model.

A large body of experimental studies shows that the local dynamics in supercooled liquids are significantly altered by spatial nanoconfinement. In a previous study, we proposed a concept of a dynamically correlated network (DCN) model, which assumes that segments in a supercooled liquid undergo cooperative rearrangements within a network-like cluster. We further demonstrated that a model modified for freestanding thin films can predict for the glass transition dynamics in atactic polystyrene (PS) films consistent with experimental results. In this study, we adapted the model to apply it to supported thin films by introducing a layer of virtual vacant segments at the free surface and virtual anchoring segments at the liquid/substrate interface. The latter segments, carrying a finite number of virtual segments, reduce mobility at the interface. We evaluated the cooperative cluster size and distribution with respect to temperature and film thickness, along with the average relaxation time and glass transition temperature Tg for supported thin films of PS. The model predicted that the thickness dependence of Tg for PS becomes stronger with increasing time scale, and this result agreed well with experimental data across different timescales from pseudothermodynamic and dynamic measurements. The results provide insights into the origin of the dynamical decoupling between pseudothermodynamic and dynamic glass transition behaviors.

Inelastic light scattering study of fast relaxation in dibutyl phthalate glass

The organic glass-forming dibutyl phthalate (DBP) has been studied applying low-frequency Raman and Brillouin spectroscopy in the temperature range from 90 K to 200 K, covering glassy and liquid states. Spectra of fast relaxation in the GHz-THz frequency range were determined from the inelastic light scattering spectra. The line width and position of the Brillouin peak were used to determine the Q-1-factor, which is inversely proportional to susceptibility at the Brillouin peak frequency. It was discovered that the temperature evolution of the fast relaxation susceptibility and the Q-1-factor agree well with a mobility parameter from pulse EPR experiment. The findings show that peculiarities in the temperature dependence the EPR data found early for DBP have are accompanied by a redistribution of the relaxation times.

Atomistic origin of structural relaxation in lead metasilicate and lithium disilicate glasses

AbstractA combination of density measurement, differential scanning calorimetry, refractometry, and Raman and 29Si nuclear magnetic resonance (NMR) spectroscopy is employed to characterize the evolution of the structure and physical properties of lithium disilicate (Li2Si2O5) and lead metasilicate (PbSiO3) glasses during physical aging. Both glasses display marked increase in density, enthalpy recovery, and refractive index following a temperature down‐jump near their glass transition regions. The collective analysis of Raman and NMR spectra reveals that the aging‐induced structural relaxation in these silicate networks involves a significant disproportionation of Q‐species as well as their spatial clustering.

Long-term elasto-static compressive loading drives rejuvenation of a metallic glass

The current work focuses on the impact of mechanical protocol which can lead to either rejuvenation or relaxation of metallic glasses (MGs) through elasto-static compressive loading (ECL) treatment. Here we demonstrate that, even after imposing a long-term ECL process, the Zr52.5Cu17.9Ni14.6Al10Ti5 MG subjected to an elasto-static stress of 85% of the yield stress at room temperature shows varying degrees of rejuvenation without any relaxation. The MG undergoes an increasing trend in structural rejuvenation with an increased stored energy and a more disordered structure. Nanoindentation results reveal that the variation of mechanical responses emerges from the complementary effects of the thermal activation process and the structural heterogeneity of glassy phase. The spital heterogeneities can effectively reduce the activation barriers of the shear transformation zones (STZs) and widen their distributions, which will perturb the shear-banding processes from single motion to collective movements, leading to shear band multiplication/branching and an increase in the elastic energy density cut-off. As a result, the plasticity is significantly enhanced and the yield strength is decreased. These findings shed light on that ECL process is an effective way to enhance the structural heterogeneity and the level of rejuvenation of a monolithic MG, which may allow the design of MGs with desirable mechanical properties.

Fast relaxation in metallic glasses studied by measurements of the internal friction at high frequencies

We performed high-frequency (0.4–1.7 MHz) measurements of the internal friction (IF) on 14 bulk metallic glasses (MGs). It is found that 12 of these MGs display relaxation IF peaks at temperatures ≈ 400–500 K, which are weakly affected by heat treatment within the amorphous state. The corresponding relaxation time is about 0.3 μs. This fast relaxation is reported for the first time in the literature. The apparent activation enthalpy for 4 MGs is determined. It is shown that if measured at a low frequency of 1 Hz, these IF peaks would appear near room temperature or by 30–50 K below it. Thus, the IF peaks found in the present investigation are direct analogues of low-temperature/low-frequency β′- and/or γ-relaxations described in the literature. It is argued that the origin of these IF peaks is related to the relaxation of structural defects similar to dumbbell interstitials, which are frozen in solid glass from the liquid state upon melt quenching. These defects can be considered as elastic dipoles and their re-orientation in the applied stress provides anelastic deformation and corresponding IF peak. After correction for temperature dependence of the shear modulus, the true activation enthalpy (0.62–1.08 eV) and activation entropy (4 ≤ΔS∕kB≤15) for defect re-orientation are determined.

Correlating dynamic relaxation and viscoelasticity in metallic glasses

Correlating dynamic relaxation and viscoelasticity in metallic glasses

Unveiling crystallization and relaxation dynamics interplay in a deeply supercooled glass

Crystallization in glasses below their transition range (Tg) is crucial for understanding material stability and performance. However, data on the time evolution of crystallized fractions in this temperature range is scarce. This study investigates the interplay between structural relaxation and crystallization kinetics in a model silicate glass (5BaO·8SiO2) below Tg. Analyzing the number of crystals, their size, and the crystallized volume as a function of the treatment time, we demonstrate that a single parameter accounting for structural relaxation effectively describes these three crystallization processes. Notably, we found that glasses prone to rapid crystallization below Tg can fully crystallize before reaching complete structural relaxation. These findings highlight the complex interplay between relaxation dynamics and crystallization in deeply supercooled glasses, with implications for understanding and controlling these processes in various materials.

Influence of Nucleation on Relaxation, Molecular Cooperativity, and Physical Stability of Celecoxib Glass.

Although nucleation is considered the first step in the crystallization of glass materials, the structure and properties of the nuclei are not understood well. Influence of nucleation on the structure and dynamics of celecoxib glass was evaluated in this study. The nuclei for Form III were induced by annealing the glass at freezing temperature, and their impact on the relaxation behavior was investigated using thermal analysis and broadband dielectric spectroscopy to find accelerated α relaxation and suppressed β relaxation. In addition, observed after nucleation was a decrease in cooperativity of the molecular motion, presumably because of the appearance of void spaces in the glass structure. During long-term isothermal crystallization studies, crystal growth to Form III was accelerated in the presence of the nuclei, whereas this effect was less remarkable when a different crystal form dominated the crystallization behavior. These observations should provide more detailed insights into the nucleation mechanism and impact of nucleation on molecular dynamics including physical stability of pharmaceutical glasses. In addition, discussed is the remarkable acceleration of the crystallization rate of the celecoxib glass just below its Tg, which could be understood by diffusionless crystal growth.

Effect of the nature of the solid substrate on spatially heterogeneous activated dynamics in glass forming supported films.

We extend the force-level elastically collective nonlinear Langevin equation theory to treat the spatial gradients of the alpha relaxation time and glass transition temperature, and the corresponding film-averaged quantities, to the geometrically asymmetric case of finite thickness supported films with variable fluid-substrate coupling. The latter typically nonuniversally slows down motion near the solid-liquid interface as modeled via modification of the surface dynamic free energy caging constraints that are spatially transferred into the film and which compete with the accelerated relaxation gradient induced by the vapor interface. Quantitative applications to the foundational hard sphere fluid and a polymer melt are presented. The strength of the effective fluid-substrate coupling has very large consequences for the dynamical gradients and film-averaged quantities in a film thickness and thermodynamic state dependent manner. The interference of the dynamical gradients of opposite nature emanating from the vapor and solid interfaces is determined, including the conditions for the disappearance of a bulk-like region in the film center. The relative importance of surface-induced modification of local caging vs the generic truncation of the long range collective elastic component of the activation barrier is studied. The conditions for the accuracy and failure of a simple superposition approximation for dynamical gradients in thin films are also determined. The emergence of near substrate dead layers, large gradient effects on film-averaged response functions, and a weak non-monotonic evolution of dynamic gradients in thick and cold films are briefly discussed. The connection of our theoretical results to simulations and experiments is briefly discussed, as is the extension to treat more complex glass-forming systems under nanoconfinement.

Atomic cluster dynamics causes intermittent aging of metallic glasses

In the past two decades, numerous relaxation or physical aging experiments of metallic glasses have revealed signatures of intermittent atomic-scale processes. Revealed via intensity cross-correlations from coherent scattering using X-ray photon correlation spectroscopy (XPCS), the observed abrupt changes in the time-domain of atomic motion does not fit the picture of gradual slowing down of relaxation times and their origin continues to remain unclear. Using a binary Lennard-Jones model glass subjected to microsecond-long isotherms, we show here that temporally and spatially heterogeneous atomic-cluster activity at different length-scales drive the emergence of highly non-monotonous intensity cross-correlations. The simulated XPCS experiments reveal a variety of time-dependent intensity-cross correlations that, depending on both the structural evolution and the q-space sampling, give detailed insights into the possible structural origins of intermittent aging measured with XPCS.

Structural relaxation of Sb2Se98 chalcogenide glass and its effect on following crystallization

This work focuses on a comprehensive study of the structural relaxation of chalcogenide glass and the possible influence of relaxation on subsequent crystallization. Chalcogenide glass of composition Sb2Se98 was studied using differential scanning calorimetry. Non-isothermal cyclic experiments were performed, as well as isothermal annealing at temperatures in the range of 25–35 °C. The Tool-Narayanaswamy-Moynihan model was used to describe the relaxation data, and all methods of estimating some of its parameters were applied. Finally, the parameters were refined by numerical fitting and their values for individual types of experiments are discussed. The reproducibility of individual relaxation datasets is also described in detail. Attention is also paid to subsequent crystallization after the relaxation experiments.

Connection between Mechanical Relaxation and Equilibration Kinetics in a High-Entropy Metallic Glass.

The slow transition from an out-of-equilibrium glass towards a supercooled liquid is a complex relaxation phenomenon. In this Letter, we study the correlation between mechanical relaxation and equilibration kinetics in a Pd_{20}Pt_{20}Cu_{20}Ni_{20}P_{20} high-entropy metallic glass. The evolution of stress relaxation with aging time was obtained with an unprecedented detail, allowing us to pinpoint new interesting features. The long structural relaxation towards equilibrium contains a wide distribution of activation energies, instead of being just associated to the β relaxation as commonly accepted. The stress relaxation time can be correlated with the equilibration rate and we observe a decrease of microstructural heterogeneity which contrasts with an increase of dynamic heterogeneity. These results significantly enhance our insight of the interplay between relaxation dynamics and thermodynamics in metallic glasses.

Evolution of coupling modes between α and β relaxations in metallic glass-forming liquids revealed by nano-calorimetry

The coupling behavior of α and β relaxations in supercooled liquids (SLs) is a key scientific issue that relates to the essence of glass transition and the modulation of glass properties. Here, we observe the evolution of α and β relaxations in Au50Ag7.5Cu17.5Si25 alloys from glass to SLs by nano calorimetric technique. We find that all the β participates in α relaxation in these metallic glasses (MGs) upon heating. Two typical coupling (or decoupling upon cooling) modes of α and β relaxations in Au-based MG SLs have been discovered, dependent on the degree of coupling between α and β relaxations in glasses. The coupling degree increases during the isothermal annealing process as the energy state of the glass decreases. Above a critical degree the decoupling mode changes from “β splits from α” to “α splits from β”. A survey including metallic glasses, polymer glasses and oxide glasses shows the general existence of the critical degree of coupling in glasses. The differences in local structure feature in glasses with different coupling degree is elucidated by analysis on atomic nonaffine displacements using molecular dynamics simulations. Further our work demonstrates that the plasticity of MGs can be effectively modulated by altering the coupling behavior between α and β relaxation.

Kinetics of physical aging of a silicate glass following temperature up- and down-jumps.

In this article, we investigate the structural relaxation of lithium silicate glass during isothermal physical aging by monitoring the temporal evolution of its refractive index and enthalpy following relatively large (10-40 °C) up- and down-jumps in temperature. The Kohlrausch-Williams-Watts function aptly describes the up- and down-jump data when analyzed separately. For temperature down-jumps, the glass exhibits a typical stretched exponential kinetic behavior with the non-exponentiality parameter β < 1, whereas up-jumps show a compressed exponential behavior (β > 1). We analyzed these datasets using the non-exponential and non-linear Tool-Narayanaswamy-Moynihan (TNM) model, aiming to provide a comprehensive description of the primary or α-relaxation of the glass. This model described both up- and down-jump datasets using a single value of β ≤ 1. However, the standard TNM model exhibited a progressively reduced capacity to describe the data for larger temperature jumps, which is likely a manifestation of the temperature dependence of the non-exponentiality or non-linearity of the relaxation process. We hypothesize that the compressed exponential relaxation kinetics observed for temperature up-jumps stems from a nucleation-growth-percolation-based evolution on the dynamically mobile regions within the structure, leading to a self-acceleration of the dynamics. On the other hand, temperature down-jumps result in self-retardation, as the slow-relaxing denser regions percolate in the structure to give rise to a stretched exponential behavior.

Atomistic mechanism of structural and volume relaxation below glass transition temperature in a soda-lime silicate glass revealed by Raman spectroscopy and its DFT calculations.

To elucidate the atomistic origin of volume relaxation in soda-lime silicate glass annealed below the glass transition temperature (Tg), the experimental and calculated Raman spectra were compared. By decomposing the calculated Raman spectra into specific groups of atoms, the Raman peaks at 800, 950, 1050, 1100, and 1150cm-1 were attributed to oxygen and silicon in Si-O-Si, non-bridging oxygen in the Q2 unit, bridging oxygen in low-angle Si-O-Si, non-bridging oxygen in the Q4 unit, and bridging oxygen in high-angle Si-O-Si, respectively. Based on these attributions, we found that by decreasing the fictive temperature by annealing below Tg - 70K, a homogenization reaction Q2 + Q4 → 2Q3 and an increase in average Si-O-Si angle occurred simultaneously. By molecular dynamics simulation, we clarified how the experimentally demonstrated increase in average Si-O-Si angle contributes to volume shrinkage; increasing Si-O-Si angles can expand the space inside the rings, and Na can be inserted into the ring center.

Numerical and atomistic models for predicting structural relaxation in glasses

From optical fiber to high-tech displays, glass relaxation plays a critical role. Due to the non-equilibrium nature of glass, there is a constant evolution associated with relaxation that influences manufacturing and properties. Despite the importance of glass relaxation, it remains a challenge to understand and predict. Models to predict the behavior of glass relaxation have been proposed for more than 80 years and have become increasingly more accurate at the cost of simplicity. These models have also been influenced by the models that have come before them. In this article, we review the history of glass relaxation models and how they connect to modern computational approaches and discuss the remaining challenges of glass relaxation modeling.

Densification of sodium and magnesium aluminosilicate glasses at ambient temperature: structural investigations by solid-state nuclear magnetic resonance and molecular dynamics simulations.

Sodium and magnesium aluminosilicate glasses with compositions 20Na2O-20Al2O3-60SiO2 (NAS) and 20MgO-20Al2O3-60SiO2 (MAS) were subjected to a 12 and 25 GPa compression and decompression at room temperature, resulting in density increases from 3.7% to 5.3% (NAS) and from 8.2 to 8.4% (MAS), respectively. The pressurization at 25 GPa was done on 17O-enriched glasses, to facilitate characterization by 17O NMR. The structural changes associated with this process have been investigated by solid state 29Si, 27Al, 23Na, 25Mg, and 17O magic-angle spinning NMR and compared with the situation in thermally relaxed glasses and/or glasses prepared at ambient pressure. While in the Na aluminosilicate glass only subtle structural changes were observed in a sample densified at 12 GPa, the average coordination number of Al 〈CN(Al)〉 increases moderately from 4.00 to 4.26 by pressurization at 25 GPa. In the Mg-based system, 〈CN(Al)〉 increases from 4.34 to 4.57 to 4.83 in the sequence 10-4 GPa → 12 GPa → 25 GPa. The experimental result at 25 GPa was qualitatively confirmed by molecular dynamics (MD) simulations. Overall, pressurization results in more positive 29Si and 17O chemical shifts, most likely reflecting a reduction in the Si-O-Si and Si-O-Al bonding angles in the pressurized glasses. Furthermore, the results are also consistent with either an increased number of non-bridging O-atoms upon pressurization, or a larger number of Si-O-Al or Al-O-Al linkages. The significantly higher sensitivity of MAS, compared to NAS glass, to an increase in 〈CN(Al)〉 upon pressurization, provides a good structural rationale for its significantly higher crack initiation resistance.

Effects of ionic cross-linking on the non-exponentiality of the [formula omitted]-relaxation in metaphosphate glass melts as measured by dynamic light scattering

Photon correlation spectroscopy was conducted on a series of metaphosphate glass melts to measure the liquid's dynamic structure factor near the glass transition and thereby characterize the degree of non-exponentiality in its time decay. Each metaphosphate consists of polymeric chains of [PO4]−1 monomers interspersed by a variety of modifying cations (Ag+, Ba2, Ca2+, Sr2+) which can potentially form inter-chain crosslinks via ionic bonding. Our results show that the non-exponentiality of the α-relaxation decreases monotonically with increasing field strength of the cation-oxygen bond.