Glass-forming Melts Research Articles

To understanding of slow and non-monotonic relaxation in Al–Y eutectic melts

We discuss the nature of slow relaxation processes in glass-forming eutectic melts right after melting. For specific, we focus on the binary metallic melt Al–Y, which in addition to the slow relaxation shows unusual non-monotonic dynamics. We argue this slow dynamic the result of non-linearity of diffusion processes in the initially non-homogenous sample, and the nature of slow relaxation processes in eutectic melts after melting is similar to the nature of spinodal decomposition when the reason for the slowdown is the thermodynamic instability. To support this assertion we considered the model with combined Gibbs potential of the Al-Y liquid solution, in which the presence of the stoichiometric phase remains is taken into account. We show that in this system the instability mathematically described by the Cahn–Hilliard type equation can develop, and that fluctuation accounting in the considered model allows qualitatively describe the non-monotonic relaxation observed in the Al-based non-equilibrium melts.

Predictive relation for the α-relaxation time of a coarse-grained polymer melt under steady shear.

We examine the influence of steady shear on structural relaxation in a simulated coarse-grained unentangled polymer melt over a wide range of temperature and shear rates. Shear is found to progressively suppress the α-relaxation process observed in the intermediate scattering function, leading ultimately to a purely inertially dominated β-relaxation at high shear rates, a trend similar to increasing temperature. On the basis of a scaling argument emphasizing dynamic heterogeneity in cooled liquids and its alteration under material deformation, we deduce and validate a parameter-free scaling relation for both the structural relaxation time τα from the intermediate scattering function and the "stretching exponent" β quantifying the extent of dynamic heterogeneity over the entire range of temperatures and shear rates that we can simulate.

In situ characterization of foam morphology during melting of simulated waste glass using x-ray computed tomography

During batch-to-glass conversion, a glass-forming melt connects, creating a foam layer between the batch and the glass melt. Due to its transience and opacity, investigation of this foam layer presents a formidable challenge. In this work, we use in situ x-ray computed tomography to characterize the foam morphology that evolves during batch-to-glass conversion of a simulated nuclear waste glass. Rapid 1-min scans with 38 μm voxels were performed to capture the foam structure during heating. Geometric volume, total porosity, and bubble size distribution are reported. Using evolved gas analysis and combining the temperature-dependent melt viscosity with x-ray data, we describe the evolution of foam structure during the foam growth and subsequent collapse.

Thermodynamic model and Raman spectra of binary barium borate glassforming melts

The set of 34 baseline subtracted and thermally corrected Raman spectra of BaO–B2O3 glassforming melts with the composition xgBaO–(1 − xg)B2O3 (xg = 0.20; 0.25; 0.30; 0.35; 0.40; 045; 0.50; 0.55; 0.60) measured at temperatures ranging from 600 to 1100 °C was analyzed. The thermodynamic model of Shakhmatkin and Vedishcheva was evaluated for each glass melt. Nine following system components (defined as stable crystalline phases of the BaO–B2O3 binary phase diagram) were considered: BaO, B2O3, 2BaO·5B2O3 (Ba2B5), 2BaO·B2O3 (Ba2B), 3BaO·B2O3 (Ba3B), 4BaO·B2O3 (Ba4B), BaO·B2O3 (BaB), BaO·2B2O3 (BaB2), and BaO·4B2O3 (BaB4). The equilibrium molar amounts of system components were used for the evaluation of the molar amounts of basic structural units Tn (trigonal boron with n-bridging oxygen atoms, n = 0, 1, 2, 3) and Q4 (tetragonal boron with 4-bridging oxygen atoms). Only three structural units (Q4, T3, and T2) with the not negligible equilibrium molar amount were found. The significant correlation between equilibrium molar amounts of Q4 and T3 was found. Malfait’s decomposition was performed for the most abundant units, i.e., T3 and T2. Multivariate curve resolution analysis performed for two components resulted in the Raman spectra (so-called loadings) and relative abundances (so-called scores) of each component. The obtained loadings were in good agreement with the partial Raman spectra obtained by the Malfait’s decomposition.

Revisiting the Stokes-Einstein relation for glass-forming melts.

Molecular dynamics simulations of Ni36Zr64, Cu65Zr35 and Ni80Al20 were carried out over a broad range of temperature (900-3000 K) to investigate the Stokes-Einstein (SE) relation for glass-forming melts. Our results reproduce experimental structural and transport properties. Results show that the breakdown temperature of the SE relation (TSE) equals the dynamical crossover temperature (TA) and both are roughly twice the glass-transition temperature (Tg) for the three glass-forming melts (TSE = TA ≈ 2.0Tg). The product of the individual component self-diffusion coefficient and viscosity Dαη can be roughly regarded as a constant at the transition zone (a small temperature range around TSE) in which the temperature behaviors of self-diffusion coefficient and viscosity switch from high-temperature Arrhenius to a low-temperature VFT behavior. Below TSE, the decoupling of component diffusion coefficients was found. In particular, the decoupling of component diffusion coefficients can be ascribed to the decoupling of the partial pair structural correlation of components, which can be clearly reflected by the intersection of the high-temperature and low-temperature behaviors of the ratio between the partial pair correlation entropy of components (Sβ2/Sα2). Furthermore, the ratio between the partial pair correlation entropy of components may be used to predict the validity of the SE relation, in the absence of both transport coefficients and atomic coordinates.

Bell-shaped “dendrite velocity-undercooling” relationship with an abrupt drop of solidification kinetics in glass forming Cu-Zr(-Ni) melts

Abstract We analyze the crystal growth kinetics of rapidly solidifying glass-forming Cu50Zr50 alloy melts. Formulating a dendrite growth model we predict all features of crystallization kinetics in Cu50Zr50 from thermodynamically controlled growth (governed by the change in Gibbs free energy on solidification) to the kinetically limited regime (governed by atomic attachment at the solid/liquid interface). Comparing critical undercoolings observed in the crystallization kinetics with experimental data on melt viscosity, atomistic simulation's data on liquid microstructure and theoretically predicted dendrite growth velocity allows us to conclude that the dendrite growth kinetics strongly depends on the cluster structure changes of the melt.

COOPERATIVE FREE VOLUME RATE MODEL APPLIED TO THE PRESSURE-DEPENDENT SEGMENTAL DYNAMICS OF NATURAL RUBBER AND POLYUREA

ABSTRACT We apply the cooperative free volume (CFV) rate model for pressure-dependent dynamics of glass-forming liquids and polymer melts, focusing on two new applications of the model, to natural rubber and to polyurea. In CFV, segmental relaxation times, τ, are analyzed as a function of temperature (T) and free volume (Vfree), where the latter provides an insightful route to expressing dynamics relative to using the system's overall total volume (V). Vfree is defined as the difference between the total volume and the volume at close packing and is predicted independently of the dynamics for any temperature and pressure using the locally correlated lattice equation-of-state analysis of characteristic thermodynamic data. The new results for natural rubber and polyurea are discussed in the context of results on a set of polymeric and small-molecule glass formers that had previously been modeled with CFV. We also discuss the results in the context of recent connections that we have made with the density-scaling approach.

Long-Range Mass Transport during Structural Transitions in Metallic Glass-Forming Melts

We investigate the structure and dynamics of the bulk metallic glass-forming alloys Zr_{41.2}Ti_{13.8}Cu_{12.5}Ni_{10}Be_{22.5} and Zr_{58.5}Cu_{15.6}Ni_{12.8}Al_{10.3}Nb_{2.8}. Combining insitu synchrotron x-ray diffraction and quasielastic neutron scattering with electrostatic levitation, we directly observe an abrupt change in the temperature dependence of the first structure factor maximum of these melts. We find that the kinetics of this liquid-liquid transition during cooling are on the order of tens of seconds, whereas its onset temperature depends only weakly on the applied cooling rate. Such slow transition kinetics require long-range mass transport, which is incompatible with a transition mechanism involving only local structural changes as in oxides or molecular liquids.

Presolidification Changes in the Structural–Dynamic Characteristics of Glass-Forming Metallic Melts during Deep Cooling, Vitrification, and Hydrogenation

The amorphization and solidification of binary and multicomponent transition-metal-based melts are considered. The presolidification structural changes and the atomic dynamics in metallic melts and other liquid substances that occur during forced quenching and deep supercooling are analyzed and compared as functions of their composition, the atomic size, the character of interatomic bonds, and the presence of eutectics and compounds in phase diagrams. The hydrogenation of a melt changes presolidification processes, affecting the atomic configuration, the modulation of a frequency spectrum, and the fluctuations of band structure (which determines the character of vitrification).

The cooperative free volume rate model for segmental dynamics: Application to glass-forming liquids and connections with the density scaling approach⋆.

In this paper, we apply the cooperative free volume (CFV) rate model for pressure-dependent dynamics of glass-forming liquids and polymer melts. We analyze segmental relaxation times, [Formula: see text] , as a function of temperature (T and free volume ( [Formula: see text] , and make substantive comparisons with the density scaling approach. [Formula: see text] , the difference between the total volume (V and the volume at close-packing, is predicted independently of the dynamics for any temperature and pressure using the locally correlated lattice (LCL) equation-of-state (EOS) analysis of characteristic thermodynamic data. We discuss the underlying physical motivation in the CFV and density scaling models, and show that their key, respective, material parameters are connected, where the CFV b parameter and the density scaling [Formula: see text] parameter each characterize the relative sensitivity of dynamics to changes in T , vs. changes in V . We find [Formula: see text] , where [Formula: see text] is the value predicted by the LCL EOS at the ambient [Formula: see text] . In comparing the predictive power of the two models we highlight the CFV advantage in yielding a universal linear collapse of relaxation data using a minimal set of parameters, compared to the same parameter space yielding a changing functional form in the density scaling approach. Further, we demonstrate that in the low data limit, where there is not enough data to characterize the density scaling model, the CFV model may still be successfully applied, and we even use it to predict the correct [Formula: see text] parameter.

Effect of Ag and γ-ray irradiation on the specific heat of glass transition of Se78Te20Sn2 glassy alloy

The specific heat of glassy materials is a significant parameter that reveals useful information about the local circumstances for the vitrification of a glass-forming melt through its solidification. However, the melt after the formation of glass is known as a highly viscous liquid. In the present work, we are reporting the effect of silver (Ag) on specific heat capacity of glass transition of Se78Te20Sn2 glass alloy in terms of glass and equilibrium specific heats (i.e., Cpg, and Cpe) and then in addition to it, we are reporting the effect of gamma-ray (γ-ray) irradiation (60Co source, 12 kGy dose) on the same glassy alloy. A comparative analysis of Ag addition and γ-ray irradiation on these thermodynamic parameters for the parent glass is discussed in detail.

Structure and dynamics of glass-forming alloy melts investigated by application of levitation techniques

Abstract In this work results of studies on the short-range order and on the atomic dynamics in different stable and undercooled glass-forming metallic melts are reviewed. In order to undercool the melts deeply below the melting temperature and to avoid chemical reactions of the melts with crucible materials, the samples are containerlessly processed utilizing the electromagnetic or the electrostatic levitation technique. The short-range structure of the melts is studied by neutron diffraction, while the atomic dynamics are investigated by quasielastic neutron scattering. The relationship between short-range structure and atomic dynamics is discussed within the mode coupling theory of the glass transition. We will show that taking the time- and space-averaged structural information provided by measured partial structure factors as an input, mode coupling theory is able to explain the experimental results concerning the activation energies for self-diffusion and the coupling/decoupling behavior of the self-diffusion coefficients of the different alloy components.

Analysis of viscosity data in As2Se3, Se and Se95Te5 chalcogenide melts using the pressure assisted melt filling technique

The pressure-assisted melt filling technique (PAMFT) is promising approach for integrating new materials such as glasses or semiconductor into fiber, while only a small amount of starting material (<0.1 mg) is actually required for the implementation of such hybrid optical fibers of length of several centimeters. PAMFT has also the capability to provide viscosity data of glass-forming melts determined during the filling process. The main advantage of this technique – the demand of only small amounts of material (<0.1 mg) is accompanied by the fact that the molten material is encapsulated inside a silica capillary under high pressure. Therefore, PAMFT represents a promising method for measuring highly volatile and corrosive materials, such as chalcogenides. Here we present a detailed analysis on applying PAMFT for viscosity measurement of chosen chalcogenide systems with respect to experimental conditions (applied pressure and capillary diameter) and physical materials properties (surface tension and contact angle towards silica glass). For our study, we chose two well-known chalcogenide materials (Se and As2Se3) as representatives of non-wetting and wetting liquids towards silica glass. The melt viscosities were measured in the range of 0.01–25 Pa·s showing that PAMFT is a suitable method for the determination of viscosity of chalcogenide melts providing reliable data even if in the situation the influence of surface tension and contact angle is neglected. We also present new viscosity data on Se95Te5 chalcogenide melt obtained using PAMFT.

Relaxation aspects of the liquid–glass transition

Relaxation theories of the glass transition and viscous flow of glass-forming melts are presented. The focus is on modern representations of the glass transition equation qτg = δTg that describes the appearance of a glassy state during cooling. Here, q = dT/dt is the temperature change rate during melt cooling and τg is the relaxation time at the glass transition temperature Tg. Various methods for calculating the characteristic temperature band δTg during the liquid–glass transition are considered. The generalized equation for the dependence of Tg on the melt cooling rate is derived. Based on the model of delocalized atoms, a modified kinetic glass transition criterion is discussed. A generalized viscosity equation for glass-forming liquids is derived.

Solute dynamics in the liquid solution with stoichiometric compounds formation

We show that in a binary solution in the presence of liquid phase of non-permanent composition, and a stoichiometric phase, for solute concentration exceeding some critical value, the instability mathematically described by the Cahn–Hilliard type equation can develop. Within the framework of the model system a dispersion relation is constructed. This relation allows to determine the concentration fluctuations dependence on time, and to investigate dynamics of structural factor and of amplification rate. The observed instability can explain the processes of slow non-monotonic relaxation in the glass-forming metal melts upon melting.

Stretched and compressed exponentials in the relaxation dynamics of a metallic glass-forming melt

The dynamics of glass-forming systems shows a multitude of features that are absent in normal liquids, such as non-exponential relaxation and a strong temperature-dependence of the relaxation time. Connecting these dynamic properties to the microscopic structure of the system is challenging because of the presence of the structural disorder. Here we use computer simulations of a metallic glass-former to establish such a connection. By probing the temperature and wave-vector dependence of the intermediate scattering function we find that the relaxation dynamics of the glassy melt is directly related to the local arrangement of icosahedral structures: Isolated icosahedra give rise to a liquid-like stretched exponential relaxation whereas clusters of icosahedra lead to a compressed exponential relaxation that is reminiscent to the one found in a solid. Our results show that in metallic glass-formers these two types of relaxation processes can coexist and give rise to a dynamics that is surprisingly complex.

Redox and volatility of Tc(CO)3+ compounds in waste glass melting

The behavior of non-pertechnetate (Tc(I)) species was studied by preparing three portions of dried low-activity waste-glass simulated feed, combining it with either the [fac-Tc(CO)3(OH)]4 “tetramer,” the [fac-Tc(CO)3(OH)(H2O)2] “monohydroxo,” or the [fac-Tc(CO)3Cl3]2- “trichloro,” and then heating samples to 600, 800, and 1000 °C. For the feeds and the heat-treated samples, Tc concentrations were measured by liquid scintillation counting (LSC) after NaOH and HNO3 fusion, and the results were used to determine the Tc volatility. For unheated samples and samples heated to 600 °C, the speciation of Tc was measured by X-ray absorption near-edge structure (XANES) analysis, while partitioning of Tc in the glass-forming melt was determined by leaching the samples in water and measuring the Tc concentration of the leachate. The current results are discussed in connection with previously reported data for Tc(VII) and Tc(IV) species to rationalize the potential mechanism on how different speciation of Tc affect its retention in glass.

The hidden disintegration of cluster heterogeneity in Fe-based glass-forming alloy melt

The evolution of liquid metal at high temperature is known much less than their solid states. This is partially due to that the message concerning clusters, metastable phase or heterogeneity in liquid is usually too slight to be traced. Here, we shed some light on the nature of structural evolution of Fe-based glass-forming alloy during overheating process by the investigation of high-temperature melt viscometry and first principles simulations. It was found that a structural transition around 1400 ℃ occurred in the melts of initial homogeneous ingot, heterogeneous ingot and amorphous ribbons jointly, and was confirmed by the results from differential scanning calorimeter (DSC), and ab initio molecular dynamics (AIMD). Combining these results with Fe-Si-B ternary phase diagram and the melting characteristics of Fe-B compounds, it is safe to conclude that the disintegration of Fe2B-type clusters to Fe3B-type clusters leads to the observed transition. These results offer a significant reference for the preparation and property control of Fe-based amorphous alloys.

The influence of short-range structures on atomic caging in glass-forming Cu-Zr-Al melts

Atomic caging is an important dynamic process that influences liquid vitrification. In this article, we have studied bulk glass-forming copper-zirconium-aluminum (Cu-Zr-Al) systems using quasielastic neutron scattering and molecular dynamics simulation in order to understand the influence of short-range structures on atomic caging. We found that, in Cu-Zr-Al melts, the long range atomic transport process occurs via a jump diffusion process, and temperature dependence of the atomic diffusion process is non-Arrhenius. Furthermore, the Cu diffusion coefficient at a given temperature decreases with Al addition. Inherent structures obtained from molecular dynamic simulation trajectories show that the dominant short range structures above the melting temperature of these melts are 〈0,3,6,4〉 and 〈0,2,8,2〉 and the percentage of these structure increases with decreasing temperature. The residence time of the atoms in the cage was found to be directly correlated with the number of dominant short-range structures in these melts. Our results show that transient local short-range structures have a strong influence on atomic caging in the glass-forming metallic melts.

Comment on "Glass Transition, Crystallization of Glass-Forming Melts, and Entropy" Entropy 2018, 20, 103.

In a recent article, Schmelzer and Tropin [Entropy 2018, 20, 103] presented a critique of several aspects of modern glass science, including various features of glass transition and relaxation, crystallization, and the definition of glass itself. We argue that these criticisms are at odds with well-accepted knowledge in the field from both theory and experiments. The objective of this short comment is to clarify several of these issues.