Glass-forming Systems Research Articles

Many-Body Nature of Relaxation Processes in Glass-Forming Systems.

Most glass-forming systems are composed of basic units interacting with each other with a nontrivial anharmonic potential. Naturally, relaxation and diffusion in glass formers is a many-body problem. Results from recent experimental studies are presented to show the effects of many-body relaxation and diffusion manifested on the dynamic properties of glass formers. Considering that the effects are general and critical, the problem of glass transition will not be solved until the many-body nature of the relaxation process has been incorporated fundamentally into any theory.

Communication: Correlation of the instantaneous and the intermediate-time elasticity with the structural relaxation in glassforming systems

The elastic models of the glass transition relate the increasing solidity of the glassforming systems with the huge slowing down of the structural relaxation and the viscous flow. The solidity is quantified in terms of the instantaneous shear modulus G(∞), i.e., the immediate response to a step change in the strain. By molecular-dynamics simulations of a model polymer system, one shows the virtual absence of correlations between the instantaneous elasticity and the structural relaxation. Instead, a well-defined scaling is evidenced by considering the elastic response observed at intermediate times after the initial fast stress relaxation. The scaling regime ranges from sluggish states with virtually pure elastic response on the picosecond time scale up to high-mobility states where fast restructuring events are more apparent.

Correlating the supercooled liquid region width with the fragility parameter in bulk metallic glasses

A linear correlation of fragility parameter D* with supercooled liquid region width ΔTx for Ca-based bulk metallic glasses (BMGs) was revealed. This relationship is found in La- and Zr-based BMGs as well and extended to several glass-forming systems. The origin of this phenomenon lies in the close relation between crystallization and temperature dependence of viscosity. This relationship can be formulated by ΔTx0.33=6.8×10-3×(D*)(Tg0.33)+2(K), indicating that the unique variation of the viscosity with the temperature correlates with the location and width of the supercooled liquid region. Moreover, an approximation of fragility parameter D* for BMGs can be evaluated by the formula.

Application of Phase Transition Theory to a Glass-Forming System

We have investigated the crystallization of a monatomic simple liquid in equilibrium, where the constituents interact through the Lennard-Jones-Gauss (LJG) potential. By incorporating a perturbation expansion into a density functional approach, we obtain a phase diagram covering a wide range of the parameter space. The phase diagram agrees qualitatively with that obtained by molecular dynamics (MD) simulations. The MD simulations show that the system cannot be crystallized, even if the temperature is sufficiently low, in a certain region of the parameter space. In this parameter region, we find that the coexisting density of the fcc crystal is unexpectedly high owing to a decrease in the free energy and show that the third-nearest-neighbor site of the fcc crystal coincides with the second minimum in the LJG potential.

Brillouin Neutron Spectroscopy as a Probe to Investigate Collective Density Fluctuations in Biomolecules Hydration Water

The role of water in the behaviour of biomolecules is well recognized. The coupling of motions between water and biomolecules has been studied in a wide time scale for theselfpart whilecollectivedynamics is still quite unexplored.Self-dynamics provides information about the diffusion processes of water molecules and relaxation processes of the protein structure.Collectivedensity fluctuations might provide important insight on the transmission of information possibly correlated to biological functionality. The idea that hydration water layers surrounding a biological molecule show aself-dynamical signature that differs appreciably from that of bulk water, in analogy with glass-former systems, is quite accepted. In the same picture Brillouin terahertz spectroscopy has been used to directly probecollectivedynamics of hydration water molecules around biosystems, showing a weaker coupling and a more bulklike behaviour. We will discuss results of collective modes of hydration water, arising from neutron Brillouin spectroscopy, in the context of biomolecules-solvent interaction.

Distribution of tellurite polymorphs in the xM2O–(1 − x)TeO2 (M = Li, Na, K, Cs, and Rb) binary glasses using Raman spectroscopy

Raman spectra of xM2O–(1−x)TeO2 (M=alkali metal atom) binary glass-forming oxide systems were measured over a wide composition range and for all alkali-types in an effort to extend the structural aspects of previous works on this field and reveal the composition- and modifier type-induced structural changes in a quantitative manner for the first time. Analysis of the Raman spectroscopic data after a reduction procedure and focusing on spectral effects caused by structural changes allowed the “quantitative” monitoring of the well-known transformation of the TeO4 (predominating in the low modifier content glass) into TeO3 trigonal pyramids at increased alkali content. The main finding pertains to the estimation of the relative populations of the basic building blocks for a wide compositional range and for different alkali-types directly from the Raman spectroscopic data, thereby permitting the calculation of the average number around Te atoms versus modifier content and alkali-type. The variation of the coordination number implies that the structure of all M2O–TeO2 tellurite glasses with the same composition is comparable and almost independent of alkali-type.Spectroscopic evidence suggests that the presence of neutral trigonal pyramid TeO2/2(O) units is questionable, while the increase of M2O content leads to a gradual transformation of TeO4/2 initially to TeO3/2O− and then to TeO1/2O−(O) trigonal pyramid units depending on the modifier content. The structural model based on Raman spectroscopic results is in agreement with the findings of NMR work on binary alkali–tellurite glasses.

Relationship between viscous dynamics and the configurational thermal expansion coefficient of glass-forming liquids

We propose a model to describe the relationship between the viscosity of a glass-forming liquid and its configurational contribution to liquid state thermal expansion. The viscosity of the glass-forming liquids is expressed in terms of three standard parameters: the glass transition temperature (Tg), the liquid fragility index (m), and the extrapolated infinite temperature viscosity (η∞), which are obtained by fitting of the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) expression to measured viscosity data. The model is tested with experimental data for 41 different glass-forming systems. A good correlation is observed between our model viscosity parameter,h(Tg, m, η∞), and the configurational coefficient of thermal expansion (i.e., the configurational CTE). Within a given class of glass compositions, the model offers the ability to predict trends in configurational CTE with changes in viscosity parameters. Since viscosity is governed by glass network topology, the model also suggests the role of topological constraints in governing changes in configurational CTE.

The non-Gaussian dynamics of glycerol

We have combined incoherent quasielastic neutron scattering experiments and atomistic molecular simulations to investigate the microscopic dynamics of glycerol moving away from the hydrodynamic limit. We relate changes in the momentum transfer (Q) dependence of the relaxation time to distinct changes of the single-particle dynamics. Going from small to large values of Q, a first crossover at about 0.5 Å−1 is related to the coupling of the translational diffusion dynamics to the non-Debye structural relaxation, while the second crossover at a Q-value near the main diffraction peak is associated with the Gaussian to non-Gaussian crossover of the short-time molecular dynamics, related to the decaging processes. We offer an unprecedented extension of previous studies on polymeric systems towards the case of the typical low-molecular-weight glass-forming system glycerol.

Vibrational Properties of Bioprotectant Mixtures of Trehalose and Glycerol

In this work vibrational spectra of mixtures of two glass-forming bioprotectant systems, i.e., trehalose and glycerol, are collected at very low temperature by using the indirect geometry time-of-flight (t.o.f.) TOSCA spectrometer at the ISIS Pulse Neutron Facility (Rutherford Appleton Laboratory, Oxford, U.K.). The main aim of this work is to investigate, through inelastic neutron scattering (INS), the vibrational behavior of trehalose and its mixtures with glycerol at different concentration values in order to characterize the changes induced by glycerol on the trehalose hydrogen bonded network. The obtained experimental findings, which are discussed and interpreted in the framework of previous INS, quasi elastic neutron scattering (QENS) and molecular simulation data obtained on trehalose/glycerol mixtures at different concentration and temperature values, will be linked to the different mixtures bioprotectant effectiveness.

Large Raman Gain in a Stable Nanocomposite Based on Niobiosilicate Glass

The optical properties of composite materials can be adjusted by controlling the constituents and morphology of the composite structure. The optical nanocomposite approach offers opportunities to produce high-performance and relatively low-cost optoelectronic media suitable for many applications. In this paper, a nanocomposite material obtained by a high niobium content glass belonging to the K2O–Nb2O5–SiO2 (KNS) glass-forming system is prepared and characterized. We focus on the investigation of spontaneous and stimulated Raman scattering. Strong changes of Raman spectra in nanocomposite material with respect to bulk sample are proved and discussed, while a significant enhancement of Raman gain (up to 25 times higher) and of its bandwidth with respect to SiO2 glass is reported. This family of glasses is a promising candidate for next generation Raman gain applications for use in the format of integrated optics as well.

Universality in Self-Diffusion of Atoms among Distinctly Different Glass-Forming Liquids

The long-time self-diffusion coefficients D(S)(L) in distinctly different glass-forming liquids are analyzed from a unified point of view recently proposed by the present author. It is shown that as long as the systems are in equilibrium, they are all described by the following two types of master curves, depending on whether the control parameter is intensive or extensive: D(S)(L)(x) = d(0)x(-1)(1 - x)(2+η) exp[cx(3+η)(1 - x)(2+η)] for a reduced intensive control parameter x, such as a reduced inverse temperature, and D(S)(L)(x) = d(0)x(-1)(1 - x)(2) for a reduced extensive control parameter x, such as a reduced volume fraction, where d(0) and c are constant. Here, the exponent η (≠0) results from many-body correlations in a supercooled liquid state. The constants η and c depend on the systems and are given by (η,c) = (4/3,62) for fragile liquids, (5/3, 62) for strong liquids, and (0,0) for other glass-forming systems in which the peak heights of their non-Gaussian parameters are always much less than 1.0. It is also shown that all of the data for the diffusion coefficient start to deviate from the master curves at lower temperatures (or higher volume fraction), where the systems become out of equilibrium, leading to a glass state.

High mixing entropy bulk metallic glasses

Bulk metallic glasses (BMGs) are usually based on a single principal element such as Zr, Cu, Mg and Fe. In this work, we report the formation of a series of high mixing entropy BMGs based on multiple major elements, which have unique characteristics of excellent glass-forming ability and mechanical properties compared with conventional BMGs. The high mixing entropy BMGs based on multiple major elements might be of significance in scientific studies, potential applications, and providing a novel approach in search for new metallic glass-forming systems.

Atomic packing symmetry in the metallic liquid and glass states

Recently, we have revealed that the atomic packing in both metallic liquids and supercooled liquids can be described globally by the spherical-periodic order (SPO), while the global feature of the glassy solids can be characterized by the local translational symmetry (LTS) imposed on the SPO (Liu XJ, et al. Phys Rev Lett 2010;105:155501). In this study, we have conducted a systematic study of the effects of chemical compositions and radiation resources on the evolution of pair distribution function (PDF) profiles in the model Zr–Cu and Zr–Cu–Al glass-forming systems, by theoretical atomistic simulations coupled with synchrotron X-ray scattering experiments. Our results indicate that the global symmetry feature is held very well even in some complex cases, as long as their short-range orders can be carefully identified.

New insights from variable-temperature and variable-pressure studies into coupling and decoupling processes for ion transport in polymer electrolytes and glasses

A fresh analysis of literature data shows how the influences of temperature and pressure on ion transport and structural relaxation in glass-forming systems may be combined within the framework of ‘master plots’ based on the equation E A = M · V A , to reveal new insights into coupling and decoupling effects in a wide range of systems. E A,σ and V A,σ are, respectively, instantaneous activation energies and volumes for ionic conductivity and the parameter, M σ , is a corresponding ‘process modulus’. For structural relaxations occurring at the glass transition, the appropriate modulus is given by M s = T g · d P/d T g . We can now identify typical behaviour patterns for fragile liquids on the one hand, and typical inorganic glasses on the other. Thus, the parameters, M σ and M s , for fragile systems such as molten Ca(NO 3) 2:KNO 3 (CKN) or a typical polymer electrolyte such as a complex of LiCF 3SO 3 in PPG, are found to remain constant over a wide range of temperatures down to T g , despite changes in the temperature (and pressure) dependences of the ionic conductivities, as indicated for example by a return to Arrhenius behaviour in the case of CKN, or by so-called Stickel plots and changes in the VTF parameters for the polymer electrolytes. If E* and V* are activation energies and volumes assigned to elementary steps, when again E* = M · V*, we can go further and identify the microscopic processes driving forward structural relaxation. In the case of inorganic glasses, where usually we find the decoupling index R τ ≈ 10 12, we identify two distinct decoupling paradigms represented by strong and fragile systems respectively, where in both cases the activation volumes for ion transport are very similar to the corresponding ionic volumes. In the former case (typified by the strongly cross-linked silicate and aluminosilicate systems), the negative activation volumes for structural relaxation (negative values of d T g /d P) are clearly indicative of a ‘water-like’ behaviour attributable to the collapse of the network under pressure. On the other hand, for the more fragile fast-ion conducting silver iodomolybdate glass, the experimental results show that M s (at T g ) ≈ M σ (in glass), implying some recoupling of structural relaxation to ion transport. Arguments based on the dynamic structure model lead us to predict that a similar close link should exist between M s (at T g ) and M σ in the relatively fragile lithium and sodium borate glasses, thus highlighting the need for more information concerning the effects of pressure on the glass transition temperatures of common inorganic glasses.

Achieving long time scale simulations of glass-forming systems

Glass-forming systems have posed an especial challenge for atomistic simualtions given their complicated non-crystalline structure and the long time scales involved with glass transition and relaxation phenomena. In this article, we review two recent techniques for extending the time scales of these simulations. First, we describe the enthalpy landscape approach, which uses inherent structure and transition point mapping to develop a set of coarse-grained master equations for computing long time dynamics. Accounting for the broken ergodic nature of glass, these master equations can be solved on any arbitrary time scale. Second, we discuss the Kinetic Monte Carlo method and its application to glassy systems. Kinetic Monte Carlo provides an effective means of sampling rare events without losing the detailed atomistic description of the glass structure.

Structural origin of enhanced slow dynamics near a wall in glass-forming systems

Spatial confinement is known to induce a drastic change in the viscosity, relaxation times, and flow profile of liquids near the glass (or jamming) transition point. The essential underlying question is how a wall affects the dynamics of densely packed systems. Here we study this fundamental problem, using experiments on a driven granular hard-sphere liquid and numerical simulations of polydisperse and bidisperse colloidal liquids. The nearly hard-core nature of the particle-wall interaction provides an ideal opportunity to study purely geometrical confinement effects. We reveal that the slower dynamics near a wall is induced by wall-induced enhancement of 'glassy structural order', which is a manifestation of strong interparticle correlations. By generalizing the structure-dynamics relation for bulk systems, we find a quantitative relation between the structural relaxation time at a certain distance from a wall and the correlation length of glassy structural order there. Our finding suggests that glassy structural ordering may be the origin of the slow glassy dynamics of a supercooled liquid.

Structural signature of slow dynamics and dynamic heterogeneity in two-dimensionalcolloidal liquids: glassy structural order

Glassy states are formed if crystallization is avoided upon cooling or increasing density.However, the physical factors controlling the ease of vitrification and the nature of glasstransition remain elusive. Among various glass-forming systems, colloidal liquids are one ofthe most ideal glass-forming systems because of the simplicity and controllability of theinteractions. We use numerical simulations of two-dimensional polydisperse andbinary hard discs to tackle both of these longstanding questions. For polydispersesystems, we systematically control the polydispersity, which can be regarded asthe strength of frustration effects on crystallization. We reveal that crystal-likehexatic order grows in size and lifetime with an increase in the colloid volumefraction or with a decrease in polydispersity (or frustration). We stress that hexaticordering in hard disc systems is a direct consequence of dense packing and amanifestation of low configurational entropy. Our study suggests an intriguingscenario that the strength of frustration controls both the ease of vitrificationand the nature of the glass transition. Vitrification may be a process of hiddencrystal-like ordering under frustration for this system. This may provide not only aphysical basis for glass formation, but also an answer to another longstandingquestion on the structure of amorphous materials: ‘order in disorder’ may be anintrinsic feature of a glassy state of many materials. For binary mixtures, onthe other hand, the relevant structural feature linked to slow dynamics is nothexatic order, but an amorphous structure of low structural entropy. These resultssuggest that slow dynamics is associated with bond orientational order linked tothe crystal for a weakly frustrated system, whereas to amorphous structures oflow configurational entropy for a strongly frustrated system. This suggests anintrinsic link between structure and dynamics in glass-forming materials: slowdynamics is linked to structuring (‘glassy ordering’) towards low configurationalentropy. We discuss the nature of ‘glassy order’ responsible for slow dynamics.

A new criterion for predicting glass forming ability of bulk metallic glasses and some critical discussions

A modified criterion, β', defined as Tg/Tx–Tg/(Tl·η) (Tg, Tx, Tl) and η denote the glass transition temperature, the onset of crystallization temperature, the liquidus temperature and constant coefficient, respectively), has been proposed for assessing the glass-forming ability (GFA) of bulk metallic glasses (BMGs). A survey of the readily available experimental data associated with the thermal analysis of various BMG alloys demonstrates that the new β' criterion exhibits much better correlation with the GFA than other currently used criteria. But, it is more significant from statistical results to note that, all criteria including the currently proposed criterion β', which are all combined by Tg, Tx and Tl, almost have equal ability in measuring the glass forming ability in any glass-forming system.

Verification and rectification of the physical analogy of simulated annealing for the solution of the traveling salesman problem

The aim of the present study is to elucidate how simulated annealing (SA) works in its finite-time implementation by starting from the verification of its conventional optimization scenario based on equilibrium statistical mechanics. Two and one supplementary experiments, the design of which is inspired by concepts and methods developed for studies on liquid and glass, are performed on two types of random traveling salesman problems. In the first experiment, a newly parameterized temperature schedule is introduced to simulate a quasistatic process along the scenario and a parametric study is conducted to investigate the optimization characteristics of this adaptive cooling. In the second experiment, the search trajectory of the Metropolis algorithm (constant-temperature SA) is analyzed in the landscape paradigm in the hope of drawing a precise physical analogy by comparison with the corresponding dynamics of glass-forming molecular systems. These two experiments indicate that the effectiveness of finite-time SA comes not from equilibrium sampling at low temperature but from downward interbasin dynamics occurring before equilibrium. These dynamics work most effectively at an intermediate temperature varying with the total search time and thus this effective temperature is identified using the Deborah number. To test directly the role of these relaxation dynamics in the process of cooling, a supplementary experiment is performed using another parameterized temperature schedule with a piecewise variable cooling rate and the effect of this biased cooling is examined systematically. The results show that the optimization performance is not only dependent on but also sensitive to cooling in the vicinity of the above effec-tive temperature and that this feature is interpreted as a consequence of the presence or absence of the workable interbasin dynamics. It is confirmed for the present instances that the effectiveness of finite-time SA derives from the glassy relaxation dynamics occurring in the "landscape-influenced" temperature regime and that its naive optimization scenario should be rectified by considering the analogy with vitrification phenomena. A comprehensive guideline for the design of finite-time SA and SA-related algorithms is discussed on the basis of this rectified analogy.

Evaluation of the Dyre shoving model using dynamic data near the glass temperature

The temperature dependence of the dynamics of glass-forming systems remains an important fundamental problem in glass physics. Here we use literature data [S. A. Hutcheson and G. B. McKenna, J. Chem. Phys. 129, 074502 (2008)] reanalyzed with the Baumgärtel-Schausberger-Winter (BSW) [M. Baumgärtel, A. Schausberger, and H. H. Winter, Rheol. Acta 29, 400 (1990); M. Baumgärtel and H. H. Winter, ibid. 28, 511 (1989); M. Baumgärtel and H. H. Winter, J. Non-Newtonian Fluid Mech. 44, 15 (1992)] model of complex fluid dynamics to evaluate the Dyre shoving model [J. C. Dyre, N. B. Olsen, and T. Christensen, Phys. Rev. B 53, 2171 (1996); J. C. Dyre, Rev. Mod. Phys. 78, 953 (2006)] that relates the temperature dependence of viscosity to the infinite-frequency shear modulus and its temperature dependence. In Dyre's model, the free-energy barrier for a "flow event" is attributed to the work done in shoving aside the surrounding molecules; the free-energy barrier is proportional to the glassy modulus, which increases as the temperature decreases. In the present work, the glassy modulus was obtained by the extrapolation to zero time or infinite frequency of the Kohlrausch-Williams-Watts (KWW) [G. Williams and D. C. Watts, Trans. Faraday Soc. 66, 80 (1970); F. Kolrausch, Pogg. Ann. Phys. 12, 393 (1847)] and BSW [M. Baumgärtel, A. Schausberger, and H. H. Winter, Rheol. Acta 29, 400 (1990); M. Baumgärtel and H. H. Winter, ibid. 28, 511 (1989); M. Baumgärtel and H. H. Winter, J. Non-Newtonian Fluid Mech. 44, 15 (1992)] functions to experimental data for m-toluidine and sucrose benzoate. It was found that the glassy modulus obtained from the KWW function for m-toluidine and sucrose benzoate [S. A. Hutcheson and G. B. McKenna, J. Chem. Phys. 129, 074502 (2008)] provides a consistent picture of the temperature-dependent dynamics within the framework of the shoving model. A similar analysis using a BSW description of the dynamics provides consistency for the sucrose benzoate but not for the m-toluidine.