Properties Of Supercooled Liquids Research Articles

The role of the dipole moment orientations in the crystallization tendency of the van der Waals liquids \u2013 molecular dynamics simulations

Computer simulations of model systems play a remarkable role in the contemporary studies of structural, dynamic and thermodynamic properties of supercooled liquids. However, the commonly employed model systems, i.e., simple-liquids, do not reflect the internal features of the real molecules, e.g., structural anisotropy and spatial distribution of charges, which might be crucial for the behavior of real materials. In this paper, we use the new model molecules of simple but anisotropic structure, to studies the effect of dipole moment orientation on the crystallization tendency. Our results indicate that proper orientation of the dipole moment could totally change the stability behavior of the system. Consequently, the exchange of a single atom within the molecule causing the change of dipole moment orientation might be crucial for controlling the crystallization tendency. Moreover, employing the classical nucleation theory, we explain the reason for this behavior.

Continuous time random walk concepts applied to extended mode coupling theory: a study of the Stokes–Einstein breakdown

In an attempt to extend the mode coupling theory (MCT) to lower temperatures, some years back an Unified theory was proposed which within the MCT framework incorporated the activated dynamics via the random first order transition theory (RFOT). The theory successfully showed that there is hopping induced diffusive dynamics and the modified MCT coupled to the activated motion continues till low temperatures. Here we show that the theory although successful in describing other properties of supercooled liquids is unable to capture the Stokes–Einstein breakdown. We then show using continuous time random work (CTRW) formalism that the Unified theory is equivalent to a CTRW dynamics in presence of two waiting time distributions. It is known from earlier work on CTRW that in such cases the total dynamics is dominated by the fast motion. This explains the failure of the Unified theory in predicting the SE breakdown as both the structural relaxation and the diffusion process are described by the comparatively fast MCT like dynamics. The study also predicts that other forms of extended MCT with Markovian hopping kernel will face a similar issue. We next modify the Unified theory by applying the concept of renewal theory, usually used in CTRW models where the distribution has a long tail. According to this theory the first jump given by the persistent time is slower than the subsequent jumps given by the exchange time. We first show that for systems with two waiting time distributions even when both the distributions are exponential the persistent time is larger than the exchange time. We also identify the persistent time with the slower activated process. The extended Unified theory can now explain the SE breakdown. In this extended theory at low temperatures the structural relaxation is described by the activated dynamics whereas the diffusion is primarily determined by the MCT like dynamics leading to a decoupling between them. We also calculate a dynamic lengthscale from the wavenumber dependence of the relaxation time. We find that this dynamic length scale grows faster than the static length scale.

Avoided criticality and slow relaxation in frustrated two-dimensional models

Frustration and the associated phenomenon of "avoided criticality" have been proposed as an explanation for the dramatic relaxation slowdown in glass-forming liquids. To test this, we have undertaken a Monte-Carlo study of possibly the simplest such problem, the 2-dimensional XY model with frustration corresponding to a small flux, $f$, per plaquette. At $f=0$, there is a Berezinskii-Kosterlitz-Thouless transition at $T^*$, but at any small but non-zero $f$, this transition is avoided, and replaced (presumably) by a vortex-ordering transition at much lower temperatures. We thus have studied the evolution of the dynamics for small and moderate $f$ as the system is cooled from above $T^*$ to below. While we do find strongly temperature dependent slowing of the dynamics as $T$ crosses $T^*$, and that simultaneously the dynamics becomes more complex, neither effect is anywhere nearly as dramatic as the corresponding phenomena in glass-forming liquids. At the very least, this implies that the properties of supercooled liquids must depend on more than frustration and the existence of an avoided transition.

Simulation Study of Dynamic Features during the Isothermal Relaxation Processes in Supercooled Liquid and Glassy Pb

The molecular dynamics simulations on the isothermal relaxation properties of supercooled liquid and glassy Pb have been performed. The calculated diffusion constant of Pb is well agreed with the experimental result. Results indicate that the relaxations of supercooled and glassy Pb present more and more pronounced heterogeneity with decreasing temperature. The appearance of the second plateau of mean square displacement (MSD) and the non-zero plateau of Non-Gaussian parameter (NGP) is corresponding to the rest of structure rearrangement.

Nucleation and Thermophysical Properties of Glass-Forming Liquids

The intimate links among structure, nucleation, and thermophysical properties are demonstrated from a few select experiments on high-melting-temperature metallic alloy liquids, made possible by containerless processing techniques. To complement the ground-based studies, future experiments on the International Space Station using a new electromagnetic levitation facility developed by the European Space Agency are discussed. The knowledge obtained from these studies is expected to deepen the understanding of supercooled liquid properties, elucidate their role in nucleation and glass formation, and test and develop more advanced theories of nucleation. All of these are expected to improve liquid phase processing techniques for crystalline and amorphous materials so that better functional properties may be achieved.

Simulation study on the dynamic mechanisms of nucleation during the initial stage of supercooled liquid potassium

The solidification process of liquid metal potassium is simulated by using the molecular dynamics method. According to the evolution properties of average atomic energy in system, bonding type and clustering type among atoms, and the dynamic parameters of mean-square displacement and non-Gaussian parameter, the dynamic mechanisms in initial nucleation of supercooled liquid potassium are deeply studied. It is found that the supercooled liquid region can be divided into two different stages according to the evolutions of thermodynamic, dynamic and structural properties of supercooled liquid. And the potential crystallization nuclei appear in the lower temperature region of supercooled liquid. It is also found that the potential crystallization nuclei are formed with the disaggregations of icosahedron clusters during the -relaxation regime, and the critical size of nucleus is about 300 atoms.

The effect of cluster dynamics on the α-relaxation in supercooled liquids

The dielectric relaxation in a supercooled liquid is analyzed in a model in which the liquid is composed of slowly moving molecules within clusters and fast ones between them. If the dielectric α-relaxation of molecules inside a cluster is described by a stretched exponential function then the cluster dynamics, with molecules leaving and entering the clusters, can lead to the relaxation of the system’s polarization being described by a stretched exponential function with a larger stretching exponent, that depends on the rate at which the molecules leave the clusters. The implications of this for time–temperature superposition and other properties of supercooled liquids are discussed.

Model Energy Landscapes

The multidimensional potential-energy “landscape” formalism offers useful insights into the properties of supercooled liquids and glasses. However, its mathematical fundamentals present formidable subtlety and complexity. In the interests of developing a useful approximation for the statistical mechanics of landscapes, we have developed a simple family of models describing the energy-depth distribution of landscape basins. Our analysis begins with the “Gaussian” model that has been advocated in the recent literature, a physically appealing and thermodynamically rather accurate description that straightforwardly predicts a positive-temperature ideal glass transition. Careful enumeration of low-lying basins reveals however that the Gaussian model requires modification in the form of a logarithmic correction. Consequently, we have carried out algebraic and numerical analyses of a logarithmically modified Gaussian model, including depth dependence of the mean intrabasin vibrational free energy. The logarithmic modification has the effect of eliminating the positive-temperature ideal glass transition of the precursor pure-Gaussian model. Nevertheless, it is sufficiently similar to that unmodified model at and above any kinetic glass transition temperature to be able to represent measurable calorimetric data with reasonable accuracy.

Comment on “Quasisaddles as relevant points of the potential energy surface in the dynamics of supercooled liquids” [J. Chem. Phys. 116, 10297 (2002)

Recently, the properties of supercooled liquids have been studied by mapping instantaneous configurations onto minima of the gradient squared. It was originally suggested that this mapping would probe higher-order saddle points of the potential energy surface. However, it was subsequently shown that the majority of the minima of this function are only local minima and so do not correspond to saddles. In this comment, we provide a critique of the suggestion made by Angelani et al. [J. Chem. Phys. 116, 10297 (2002)] that although these minima are not true saddles, they are almost saddles (hence the term quasisaddles). This issue has important implications for the interpretation of the results obtained by this approach.

Fragility of Metallic Glass Forming Liquids

ABSTRACTBased on the available kinetic and thermodynamic data, we compare the kinetic fragility and thermodynamic fragility of different metallic glass forming liquids. The results indicate a correlation between the kinetic and thermodynamic fragility in metallic glass forming liquids, consistent with the energy landscape model which predicts a connection between the kinetic and thermodynamic properties of supercooled liquids. The metal - metalloid glass forming alloys such as PdNiCuP are found to exhibit a distinctively different correlation as compared to early – late transition metal – type metallic glass forming alloys such as ZrTiCuNiBe. For the same thermodynamic fragility the former exhibit a much larger kinetic fragility indicating that the two classes of alloys have a different liquid structure. In addition, the relationship between the kinetic fragility and the Gibbs free energy difference between the undercooled liquid and the crystalline phases has been discussed in both metal metalloid glass forming alloys and early - late transition metal – type metallic glass forming alloys.

Observed anomalies in supercooled liquids described by frustration-limited domain theory

The properties of supercooled liquids are particularly interesting because they cannot be understood simply as an extension of liquid properties at temperatures above the melting point, and in this sense they are anomalous. These anomalies include apparent divergences and thermodynamic contradictions. We give a general description of a select set of anomalies and discuss how they can be understood within the theory of frustration-limited domains.

Supercooled Liquids and Polyamorphism

We have discovered a solid, apparently amorphous phase of triphenyl phosphite to which the supercooled liquid converts, a phase distinct from both the glass and the crystal. To date, this is the clearest and best identified case of a first-order transition from a liquid to another apparently amorphous condensed phase. We discuss this phenomenon in terms of a recently formulated theory of supercooled liquids that predicts and naturally incorporates the existence of such low-temperature phases, thereby suggesting that its existence is a general phenomenon intimately connected with the existence and properties of supercooled liquids. In accord with the theory, we also suggest that although the X-ray scans do not indicate any lattice structure, these apparently amorphous phases may in fact be defect-ordered structures with large unit cells.

A dynamic measure of order in structural glasses

Although some patterns of physical behavior are common in the glass transition and in the properties of supercooled liquids and glasses (characteristic viscoelasticity, temperature dependence of viscosity and relaxation times, property evolution through “physical aging”, difficulties in performing equilibrium measurements or simulations, etc.), it is difficult to arrive at a definition of the glass transition which distinguishes it from other phenomena exhibiting similar features. The present paper addresses this problem by defining a dynamical measure of order involving the average “shape” of particle trajectories in supercooled liquids. This dynamic order parameter should provide a measure of “closeness” to the glass transition and some indirect insights into the physical nature of supercooled liquids and glasses. Arguments are given that the proposed dynamic measure of order [“generalized capacity”, C( T)] is related to the temperature-dependent “effective hydrodynamic radius” R H ( T) measured in supercooled liquids and model numerical calculations are given to support this view. Some consequences of the intermittent particle motion at low temperatures for stress relaxation are also discussed.

Glass Transition and Viscoelastic Behaviors of La<SUB>55</SUB>Al<SUB>25</SUB>Ni<SUB>20</SUB> and La<SUB>55</SUB>Al<SUB>25</SUB>Cu<SUB>20</SUB> Amorphous Alloys

It has been found that amorphous La55Al25Ni20 and La55Al25Cu20 alloys exhibit a distinct glass transition and the temperature span between Tg and Tx, ΔTx(=Tx−Tg) exceeds 50 K. This is the first evidence for the formation of an amorphous phase with ΔTx above 50 K in a metal-metal type system. With the aim of clarifying the glass transition behavior and the properties of supercooled liquid in the metal-metal type amorphous alloys, the changes in fundamental properties of specific heat (Cp), viscosity (η), tensile yield strength (σy), tensile elongation including elastic elongation (εf) and viscoelasticity in the glass transition region were examined for both amorphous alloys. The transition brought about significant changes in the above-described properties, i.e., for the La55Al25Ni20 alloy, an increase of Cp by 12 J/mol·K and εf from 2.43 to 188% and a dramatic decrease of η to below 1012 Pa·s and σy from 710 to 3.47 MPa. In addition, the complex modulus decreased from 33.5 to 1.79 GPa and the loss tangent increased rapidly from 9×10−3 to 3.32. Similar significant changes in the fundamental properties were also observed for another La55Al25Cu20 amorphous alloy and hence these amorphous alloys were concluded to be typical glassy materials.

La<SUB>55</SUB>Al<SUB>25</SUB>Ni<SUB>20</SUB>およびLa<SUB>55</SUB>Al<SUB>25</SUB>Cu<SUB>20</SUB>アモルファス合金のガラス遷移と粘弾性挙動

It was found that amorphous La55Al25Ni20 and La55Al25Cu20 alloys exhibit a distinct glass transition and the temperature span between Tg and Tx, ΔTx(=Tx−Tg) exceeds 50 K, as evidence of the formation of an amorphous phase with ΔTx above 50 K in a metal-metal type system. With the aim of obtaining further information on the glass transition behavior and the properties of supercooled liquid, the changes in fundamental properties of specific heat (Cp), viscosity (η), yield tensile strength (σy), tensile elongation including elastic elongation (εf) and viscoelasticity by glass-supercooled liquid transition were examined for both amorphous alloys. The transition brought about significant changes in the above-described properties, i.e., for the La55Al25Ni20 alloy, an increase of Cp by 12 J/mol·K and εf from 2.43 to 188% and a dramatic decrease of η to below 1012 Pa·s and σy from 710 to 3.47 MPa. In addition, the complex modulus decreased from 33.5 to 1.79 GPa and the loss tangent increased rapidly from 9×10−3 to 3.32. Similar significant changes in the fundamental properties were also observed for another La55Al25Cu20 amorphous alloy and hence these amorphous alloys were concluded to be a typical glassy material.

Structure Relaxation in Super-Cooled Liquids

Dynamical properties of super-cooled liquids are studied in the intermediate wavenumber region. A polarization function is shown to describe the relaxation of the structure sustaining effective potentials. Effects of its slow relaxation are discussed on the dynamical structure factor as well as the transverse current correlation function for liquid argon.

Thermodynamic properties of supercooled liquids

Thermodynamic properties of supercooled liquids

Discontinuities in the physical properties of supercooled liquids

The viscosity, electrical conductivity and density of several liquids have been measured over a range of temperatures both above and below the melting-point. The results confirm that the activation energy of viscous flow is greater in the supercooled state than in the normal liquid, the slope of the log η , 1/ T graphs increasing by about 10% on passing into the supercooled region. In the series of liquids formed by co-ordinating boron trifluoride to acetic acid and its methyl, ethyl, n -propyl and n -butyl esters the effect increases with the chain length of the alkyl radical. A similar discontinuity in the log k , 1/ T curves indicates that the activation energy for ionic migration is also greater in the supercooled than in the normal state. Although this discontinuity in electrical conductivity has not been noted previously, it is shown to be implied in the numerical results of earlier authors. In contradistinction to viscosity and electrical conductivity, the density varies linearly with temperature over the whole range. These results are discussed in relation to the published data on the other physical properties of supercooled liquids.