Glass-forming Systems Research Articles

On the dependence of the properties of glasses on cooling and heating rates: I. Entropy, entropy production, and glass transition temperature

The glass transition is theoretically described in terms of a generic non-equilibrium thermodynamics approach employing De Donder's structural order parameter method, appropriate expressions for the relaxation behavior of glass-forming systems and a simplified but qualitative correct model of glass-forming melts with one order parameter related to the free volume of the system. Employing this approach the behavior of a variety of thermodynamic quantities describing glass-forming systems in vitrification and devitrification processes is interpreted theoretically. The present paper is devoted to the computation of the entropy, the entropy production and the glass transition temperature in dependence on the cooling and heating rates, varying latter parameter in a broad interval. A comparison with experimental results is given and some further consequences and possible extensions are discussed briefly.

Development of static and dynamic structures in glass-forming metallic alloys

Glass-forming metallic systems are either simple and show a marginal glass-formation tendency with high transformation rates towards the crystalline state or the systems offer a high stability of the undercooled liquid state against crystallization, but are chemically complex. However, metallic systems can be utilized as model systems if the variations in kinetic stability are used as a tool to select appropriate alloy systems for a limited range of problems to be investigated. Here, marginal glass-forming Al-based metallic glasses were used to study the early stages of crystal formation. A bulk glass-forming Pd-based system provided sufficient kinetic stability against crystallization to allow analyzing the relaxation spectrum near the glass transition by modulation calorimetry. These measurements were analyzed in the framework of a fluctuation model of the glass transition. Finally, possible interrelations between the length scale of dynamic heterogeneities and structural heterogeneities in the deeply undercooled liquid are discussed.

Measurements of fractal dimensionality in 0.1Cs 2O–0.9TeO 2 glass-forming tellurite system by Raman spectroscopy

Low-frequency Raman spectroscopy was utilized in order to estimate the fractal dimensionality for tellurite 0.1Cs 2O–0.9TeO 2 binary glass-forming system in a temperature range, which includes the glassy and supercooled liquid state. A variation in fractal dimensionality was observed and calculated directly from the Raman spectroscopic data. This variation can be interpreted as an indication of structural alterations caused by temperature variation and correlated with existing structural models concerning tellurite network. Low-frequency Raman spectroscopy has proved to be a valuable tool for studying the fractal dimensionality and structural transformation in amorphous phases. The estimation of the localization degree of lattice vibration in disordered materials is important and discussed in the context of current phenomenological status of the field.

Heterogeneous dynamics in columnar liquid crystals of parallel hard rods

In the wake of previous studies on the rattling-and-jumping diffusion in smectic liquid crystal phases of colloidal rods, we analyze here for the first time the heterogeneous dynamics in columnar phases. More specifically, we perform computer simulations to investigate the relaxation dynamics of a binary mixture of perfectly aligned hard spherocylinders. We detect that the columnar arrangement of the system produces free-energy barriers that the particles should overcome to jump from one column to another, thus determining a hopping-type diffusion. This phenomenon accounts for the non-Gaussian intercolumn diffusion and shows a two-step structural relaxation that is remarkably analogous to that of out-of-equilibrium glass-forming systems and gels. Surprisingly enough, slight deviations from the behavior of simple liquids due to transient cages is also observed in the direction perpendicular to this plane, where the system is usually referred to as liquidlike.

Fractional Exponent of the Modified Stokes-Einstein Relation in the Metallic Glass-Forming Melt Pd<sub>43</sub>Cu<sub>27</sub>Ni<sub>10</sub>P<sub>20</sub>

The diffusion coefficient in the metallic glass-forming systems such as Pd-Cu-Ni-P exhibits a marked deviation from the Stokes-Einstein (SE) relation in the proximity of the glass transition temperature. Such a deviation is characterized by the fractional exponent p of the modified SE expression. For the material Pd43Cu27Ni10P20, it has been reported that it takes the value p = 0.75. In this work, it is shown that the value of p is highly correlated with the ratio ED / ENB, where ED and ENB are the activation energies for diffusion coefficient D and cooperativity NB defined by the Bond Strength-Coordination Number Fluctuation (BSCNF) model. The present paper reports that for the metallic glass-forming melt Pd43Cu27Ni10P20, the fractional exponent p can be calculated accurately within the framework of the BSCNF model.

Prepeak and First Sharp Diffraction Peak in the Structure Factor of (Cs2O)0.14(B2O3)0.86 Glass: Influence of Temperature

Neutron diffraction measurements on (Cs(2)O)(0.14)(B(2)O(3))(0.86) glass were performed at varying temperature over an extended range from room temperature to 800 K. It was found that, in the same Q range where the first sharp diffraction peak (FSDP) is observed in the static structure factor of almost all glass-forming systems, cesium borate glass shows two peaks. The intensities of these peaks increase with temperature, and their positions shift to lower Q values, in agreement with the peculiarities of the FSDP of network glasses. A description of this anomalous temperature dependence in terms of thermal relaxations of strained bonding arrangements of boron oxide units lying on the boundaries of cages present in the boron skeleton matrix is suggested. By comparing the diffraction patterns of a (Cs(2)O)(0.14)(B(2)O(3))(0.86) sample before and after a high-temperature thermal treatment with the spectra of cesium crystals, a correspondence between the medium-range structure in the glass and the related crystalline phases has been inferred.

Evidence for critical-like behavior in ultraslowing glass-forming systems

Glass transition constitutes one of main problems of condensed matter physics and material engineering that remains unsolved. The common acceptance of the Vogel-Fulcher-Tammann (VFT) equation for portraying the primary relaxation time or shear viscosity indicated a possible phase transition, hidden below the glass transition temperature (T(g)). Recently Hecksher [Nat. Phys. 4, 737 (2008)] delivered strong empirical arguments that VFT description lacks a direct experimental basis and thus theories not predicting a dynamic divergence should be focused on. We present clear evidence for a superiority of critical-like divergent equation τ(T)=τ(0)(T-T(C))(-ϕ) and T(C)<T(g) for liquid crystalline (LC) glass formers and orientationally disordered crystals (ODIC). Such dependence was already known for spin-glasslike systems and the dynamical scaling model, although the latter was hardly explored so far. The pressure-related behavior is also discussed. Results obtained support arguments for the suggested direct link between critical phenomena and vitrification [Tanaka, Nat. Mater. 9, 324 (2010)]. LCs and ODICs may be considered as simple experimental model systems for the structural glass formers group.

Communication: Resolving the vibrational and configurational contributions to thermal expansion in isobaric glass-forming systems

A fundamental understanding of isobaric thermal expansion behavior is critical in all areas of glass science and technology. Current models of glass transition and relaxation behavior implicitly assume that the thermal expansion coefficient of glass-forming systems can be expressed as a sum of vibrational and configurational contributions. However, this assumption is made without rigorous theoretical or experimental justification. Here we present a detailed statistical mechanical analysis resolving the vibrational and configurational contributions to isobaric thermal expansion and show experimental proof of the separability of thermal expansion into vibrational and configurational components for Corning Jade glass.

Universal scaling between structural relaxation and caged dynamics in glass-forming systems: Free volume and time scales

It is shown that the Debye–Waller factor (DW), a measure of the cage dynamics, is contributed by free-volume in o-terphenyl (OTP) and glycerol. An elementary ansatz provides an alternative way to get the reduced DW from Positron Annihilation Lifetime Spectroscopy (PALS). The ansatz supports the scaling of the slow relaxation with the fast caged dynamics over about ten decades in relaxation times in OTP and glycerol. Both PALS and neutron scattering experiments show that, in order to evidence the scaling, the observation times must be shorter than the time scales of the relaxation processes.

Heterogeneous slow dynamics and the interaction potential of glass-forming liquids

The role of the intermolecular interaction potential on the dynamic and thermodynamic properties of model glass-forming mixtures is investigated through molecular dynamics simulations. Variations of the repulsive exponent m in the well-studied Lennard-Jones Kob-Andersen mixture are shown to have a negligible effect on the fragility and dynamic correlation volumes when quenches are performed at constant pressure. The number of dynamically correlated particles, estimated from the temperature derivative of a two-point dynamic correlation function, is approximately invariant to m at any fixed relaxation time. Further, the density scaling property of a model tetrahedral network glass-former, based on inverse power law and Lennard-Jones potentials, is investigated. The optimal scaling exponent γ is close to zero and does not superpose the data well. The breakdown of density scaling is consistent with the absence of correlation between fluctuations of the virial and the potential energy. These results emphasize the crucial role of structural many-body correlations in glass-forming systems and show the need of investigations of more complex and realistic model liquids.

Dynamics of glass-forming bioprotectant systems

In this work the dynamics of glass-forming mixtures showing a bioprotective function towards organisms under stress conditions is investigated. The behaviour of mixtures of two known glass-forming systems, such as α- d-glucopyranosyl α- d-glucopyranoside (trehalose) and 1,2,3-propanetriol (glycerol), are studied as a function of temperature and concentration. Both the fast local dynamics and the diffusive dynamics are determined by Inelastic Neutron Scattering and Quasi Elastic Neutron Scattering, respectively; more specifically the Boson peak behaviour, the mean square displacements and the translational linewidth as a function of temperature, wave-vector Q and concentration are characterized. The findings highlight an anomalous dynamic behaviour when a small amount of the alcohol is added to the disaccharide. This feature could be relevant in understanding the bioprotection mechanisms.

Generalization of the Cole–Davidson and Kohlrausch functions to describe the primaryresponse of glass-forming systems

We introduce a three-parameter step-response function which is based on a generalizationof the Cole–Davidson (CD) and Kohlrausch (K) functions, and which provides ahighly flexible susceptibility description for viscous liquids. A second parameterα characterizing the overall width, in addition to a parameterβ determining the high-frequency behavior of the susceptibility, allows for acontinuous change of the spectral shape from the CD-type to the K-type.We prove that the function fulfills mathematical conditions required for astep-response function. When applying the function to interpolate dielectric spectra ofneat (pure) glass formers, it is possible to keep the high-frequency parameterβ temperature-independent while varying the parameterα to account for the change of the overall width. This analysis might suggest that thefailure of frequency–temperature superposition in glass formers is reflected bya broadening in the low-frequency region instead of in the high-frequency one.

Entropy of the hard-core model

The entropy of the hard-core model is analyzed subject to dimension, system size and particle density. Inspired by the exactly solvable one-dimensional case and the results from a virial expansion, we suggest a scaling relation for the two-dimensional and three-dimensional case. These relations are compared with numerical results, obtained by a modified Widom's particle insertion method. The notion of finite size effects, relevant for atomic glass-forming systems, is discussed in the context of the present results.

Neutron Scattering and Polymer Dynamics

Nowadays polymers are ubiquitous in our daily life because they are durable, cheap to produce, easy to process and exhibit very versatile and favorable mechanical properties. For example, depending on temperature or time the same polymer may be viscous, rubber elastic, very tough with high impact strength or even brittle. Polymers are composed of macromolecules which are built up by a large number monomer units linked together by covalent bonds. Due to this connectivity, the relevant processes driving the dynamics in polymers depend on the length scale of observation. While at typical inter- and intra-macromolecular length scales (≈ 10 Å and below) polymer dynamics display the universal features of glass-forming systems, at larger distances the macromolecular character of the polymer chains prevails –there, entropy and topological constraints (‘entanglements’) play the major role. Carbon and Hydrogen are in most cases the main constituents of polymers. Therefore, neutron scattering combined with isotopic substitution (H/D labeling) is an extremely well-suited technique to study the dynamical processes of polymeric materials at a molecular level. Motions of particular molecular groups or given polymer chains, e. g. in a polymer blend, can be selectively investigated by neutron scattering. However, the restricted dynamic window offered by this technique prevents a complete characterization of the complex polymer dynamics, including several multi-scale processes the characteristic times of which span over many orders of magnitude. The combination with other experimental techniques is, in most cases, essential to fully characterize the processes; in addition, the use of complementary MD-simulations has proved to be crucial for the interpretation of the neutron-scattering results.

Infrared signatures of the peptide dynamical transition: A molecular dynamics simulation study

Recent two-dimensional infrared (2D-IR) experiments on a short peptide 3(10)-helix in chloroform solvent [E. H. G. Backus et al., J. Phys. Chem. B 113, 13405 (2009)] revealed an intriguing temperature dependence of the homogeneous line width, which was interpreted in terms of a dynamical transition of the peptide. To explain these findings, extensive molecular dynamics simulations at various temperatures were performed in order to construct the free energy landscape of the system. The study recovers the familiar picture of a glass-forming system, which below the glass transition temperature T(g) is trapped in various energy basins, while it diffuses freely between these basins above T(g). In fact, one finds at T(g) approximately 270 K a sharp rise of the fluctuations of the backbone dihedral angles, which reflects conformational transitions of the peptide. The corresponding C=O frequency fluctuations are found to be a sensitive probe of the peptide conformational dynamics from femtosecond to nanosecond time scales and lead to 2D-IR spectra that qualitatively match the experiment. The calculated homogeneous line width, however, does not show the biphasic temperature dependence observed in experiment.

Energetics of local clusters in Cu64.5Zr35.5 metallic liquid and glass

Correlation between the cluster energy and its population and dynamics can provide a better understanding of the complicated energy landscape of disordered metallic systems. We propose a method to analyze the cluster energy distribution for different kinds of short-range order (local clusters) in liquid and glass systems. By applying this analysis to an interesting and important glass forming system—Cu64.5Zr35.5 we observe a direct correlation between the energy and dynamics of the cluster in this realistic glass-forming system. This study suggests that dynamic arrest originates from the environment-dependent energetics of local clusters.

Glass formation, thermal properties, and elastic constants of La–Al–Co alloys

The compositional dependence of glass formation and thermal and elastic properties was clarified for the ternary La–Al–Co bulk glass-forming system. The existing linear correlation between La concentration and characteristic temperatures, i.e., the glass transition temperature Tg and the onset temperature of crystallization Tx, as well as the elastic moduli in this system can give a useful guideline for the chemical design of desirable bulk metallic glasses (BMGs) with tunable physical properties in advance. The relationship between Tg and elastic constants for the La–Al–Co BMGs can be quantitatively described using a microscopic model proposed by T. Egami.

Prediction of Glass-Forming Compositions in Metallic Systems: Copper-Based Bulk Metallic Glasses in the Cu-Mg-Ca System

A novel methodology for predicting specific compositions for glass-forming alloys based on efficiently packed atomic cluster selection, liquidus lines, and ab initio calculations is presented. This model has shown applicable adaptation to many known metallic and ceramic oxide glass-forming systems and has led to the discovery of soon to be reported Ag- and Zn-based bulk metallic glasses (BMGs). As a model system, glass formation in the Cu-Mg-Ca ternary system has been assessed using this alloy design methodology, which has led to the discovery of a number of Cu-based BMGs with compositions ranging from Cu-33 to 55 at. pct, Mg-18 to 45 at. pct, and Ca-18 to 36 at. pct. Included in this work are the calculated values of associated cluster binding energies and correlations between physical and thermal properties of these glassy compositions, which show significant physical evidence to support the likely existence of such clusters.

Dynamic Arrest in Multicomponent Glass-Forming Alloys

We report radiotracer diffusivities in a Pd43Cu27Ni10P20 melt, presenting for the first time a complete set of data for all components over the whole relevant temperature range. While a vast decoupling of more than 4 orders of magnitude is observed between the diffusivity of Pd and of the smaller components, at the glass transition temperature Tg, the diffusivities of all components merge close to the critical temperature Tc of mode coupling theory. For Pd, the Stokes-Einstein relation holds in the whole range investigated encompassing more than 14 orders of magnitude suggesting the formation of a slow subsystem as a key to glass formation in systems with dynamic asymmetry.

Study of the relaxational and vibrational dynamics of bioprotectant glass-forming mixtures by neutron scattering and molecular dynamics simulation

In this work inelastic neutron scattering (INS) and quasielastic neutron scattering (QENS) data, collected at different temperature values by the OSIRIS and IRIS spectrometers at the ISIS Facility (Rutherford Appleton Laboratory, Oxford, UK) on mixtures of two glass-forming bioprotectant systems, i.e., trehalose and glycerol, as a function of concentration are presented. The data analyses show that the fast local dynamics, measured by INS, as well as the diffusive dynamics, measured by QENS, exhibit in the investigated mixtures a switching-off maximum in the same concentration range corresponding to a very low glycerol content. This effect can be accounted for by a not-ideal mixing process of the pure constituents due to an increased hydrogen bonding network strength. The experimental studies are completed by molecular dynamics simulation findings.