Breakdown of the fractional Stokes–Einstein relation in silicate liquids

The fractional Stokes–Einstein relation postulates a direct relationship between conductivity and shear flow. Like viscosity, the electrical resistivity of a glass-forming liquid exhibits a non-Arrhenius scaling with temperature. However, while both viscosity and resistivity are non-Arrhenius, here we show that these two properties follow distinct functional forms. Through analysis of 821 unique silicate liquids, we show that viscosity is best represented using the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) model, whereas the resistivity of the same compositions more closely follows the Avramov–Milchev (AM) equation. Our results point to two fundamentally different mechanisms governing viscous flow and conductivity and therefore cast doubt on the general validity of the fractional Stokes–Einstein relation.     

Residual and configurational entropy: Quantitative checks through applications of Adam–Gibbs theory to the viscosity of silicate melts

In this paper the traditional view that glasses possess residual entropy, which can be determined by calorimetric means, is quantitatively supported by applications of Adam and Gibbs configurational entropy theory to the temperature, composition and pressure dependences of the viscosity of silicate melts. This theory is also in harmony with the mechanisms of viscous flow, as understood from NMR experiments, according to which viscosity is controlled by the rate of bond rearrangements between network-forming cations and oxygens. As a matter of fact, Adam and Gibbs basic expression relating structural relaxation times to the reciprocal of the product of temperature and configurational entropy can be derived from a phenomenological analysis of the temperature dependence of the activation energy for viscous flow. Adam–Gibbs theory thus works well for silicate melts because network-modifying cations also play a role in bond rearrangements such that, as a bulk property, configurational entropy is actually relevant to structural relaxation and flow.     

Structural aspect of possible interrelation between fragility (length) of glass forming melts and Poisson’s ratio of glasses

The analysis of the development of the notion of fragility (length) of glass forming melts and the nature of Poisson’s coefficient of glasses is presented for the period from 60s of 20th century, basing on structural relations. It is shown that the most distinct structural correlations have been obtained in the framework of ideas of Zachariasen and Hägg, activation theory of viscous flow, and lattice theory of Mie and Grüneisen. It is shown that the existence of universal correlations between fragility and Poisson’s ratio is physically baseless yet. The reason is that the systems under comparison are in frozen and in metastable states and the physical meanings of the properties under consideration are incompatible. The possibility of particular correlations between fragility of glass forming melts and Poisson’s ratio is discussed. Such correlations possibly exist within special groups of substances which have related structures. The theory of viscous flow for the transition from glass to metastable liquid is refined in the framework of the elastic continuum theory.     

Influence of water on the viscosity and thermal expansion of sodium trisilicate glasses

The transformation range viscosity, activation energy for viscous flow and the dilatometric critical temperature of sodium trisilicate glasses are all decreased by addition of water, whereas the thermal expansion coefficient in the elastic region is not altered to a significant extent. Although these effects are similar to those observed in vitreous silica, they are considerably reduced in magnitude. Since sodium trisilicate glasses are already highly depolymerized by the presence of Na₂O, the additional depolymerization due to small additions of water does not seem adequate to explain these results. It is suggested that water behaves similarly to the alkali oxides and that the results of this study may be due to an extreme case of the mixed-alkali effect.     

Evaluation of activation energy of viscous flow of sol–gel derived phenyl-modified silica glass

The temperature-induced softening behavior in sol–gel derived phenyl-modified low-melting glass (phenyl glass) was investigated in terms of the activation energy for the viscous flow. The temperature dependence of the relative viscous flow was measured from the falling rate of a needle loaded with a constant weight. The activation energy for the viscous flow of phenyl-modified silica glass was found to be irrespective of the time of drying the sample phenyl-modified silica glass, which directly affects the extent of polymerization. Furthermore, the obtained activation energy was in considerably good agreement with that for the viscous flow of potassium alkali glass, and approximately twice larger than that of linear amorphous polymer (polystyrene). This result suggests the common microstructural feature of glassy materials interspaced by additive substances like Na/K or covalently bonded chemical functions such as phenyl groups.     

Viscosity of liquid albite,a network material

The viscosity of liquid albite (NaAlSi₃O₈) has been determined over the temperature range 800–1000°C. The results are combined with data obtained at higher temperatures to obtain the overall log η versus $\text{1}\text{T}$ relation. The relation indicates Arrhenian behavior, with an apparent activation energy of ～95 kcal mol^?1. By comparing the viscous flow behavior of albite with that of the other materials, including SiO₂, GeO₂ and B₂O₃, it is suggested that liquid albite is a random network of SiO₄ and AlO₄ tetrahedra.     

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 (T_g), 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(T_g, 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.     

Dual nature of the orientational effect of ultrasound on liquid crystals

The new model of thresholdless distortion of the orientational structure in a homeotropic layer of nematic liquid crystal with free ends in ultrasonic field has been experimentally substantiated for the first time. The model is constructed within the concepts of nonequilibrium thermodynamics and statistical hydrodynamics of liquid crystals for the frequency range in which the elastic and viscous wavelengths are, respectively, longer and shorter than the layer thickness. The main regularities of the phenomenon, which relate the conditional effect threshold to the ultrasonic frequency and layer thickness, have been established based on the experimental data for (20–150)-μm-thick layers in the frequency range of 0.1–9 MHz. These data are compared with the results of numerical calculations, performed taking into account two mechanisms of liquid crystal structure distortion (convective and nonlinear relaxation ones).     

On the effect that acoustic boundary conditions have on the structural transitions in nematics in the field of ultrasonic compressional waves

The peculiarities of the structural transition in a homeotropic layer of nematic liquid crystal (NLC) affected by compressional waves in a cell with acoustically soft or hard boundary conditions at its ends have been studied for the frequency range where the wavelength of viscous wave is smaller than the layer thickness, while the wavelength of the elastic wave in NLC exceeds the layer thickness. The data obtained are analyzed within the known theoretical models developed for these two geometries based on the unified hypothesis, which postulates the flow mechanism of the orientational effect of ultrasound on NLC.     

Strongly correlating liquids and their isomorphs

This paper summarizes the properties of strongly correlating liquids, i.e., liquids for which the virial/potential energy correlation coefficient is above 0.9 for equilibrium fluctuations in the NVT ensemble. The definition and properties of strongly correlating liquids' isomorphs are given, and various isomorph invariants are discussed. The cause of strong virial/potential energy correlations is also discussed, and it is argued that strongly correlating liquids are not merely to be thought of as approximate inverse power-law liquids. The experimental predictions for strongly correlating glass-forming liquids include: i) density scaling; ii) isochronal superposition; iii) that there is a single function of frequency from which all frequency-dependent viscoelastic response functions may be calculated; iv) that strongly correlating liquids are approximately single-parameter liquids with close to unity Prigogine-Defay ratio; v) that the fictive temperature initially decreases for an isobaric temperature up jump. The paper also briefly discusses the “isomorph filter”, which provides a necessary condition for universality of theories for the non-Arrhenius temperature dependence of the relaxation time.     

Viscosity Measurement of La3Ga5SiO14 Melt

The viscosity of La₃Ga₅SiO₁₄ melt was measured by a fixed-crucible rotor technique in the range between 1520 and 1596 °C. The melt was found to be Newtonian. The viscosity data ranged between 73 and 97 mPa. s, decreasing with increasing temperature. The activation energy of the viscous flow and the volume of a viscous flow unit estimated from the measured data were 90 kJ/mol and 3 × 10⁻³ nm³, respectively.     

Genuine Johari–Goldstein β-relaxations in glass-forming binary mixtures

Broadband dielectric spectroscopy measurements were performed on glass-forming binary mixtures, composed of rigid polar molecules dissolved at low concentration in apolar viscous solvent (tristyrene). Dielectric spectra were dominated by the polar molecule contribution, so enabling the study of its dynamic behavior. A well resolved secondary relaxation, not attributable to internal degrees of freedom, was visible in both the liquid and glassy states: for its intermolecular nature, it can be called a ‘genuine’ Johari–Goldstein (JG) relaxation, in the sense that it is a local and non-cooperative process but entailing the motion of the molecule as a whole. Among our results, the following ones are noteworthy: (a) polar systems in the neat state showed an excess wing that became a well resolved JG-peak on mixing with the apolar solvent; (b) the time-scale distance between structural and JG loss peak increased with the apolar solvent fraction; (c) broader the structural loss peak was, larger was the separation in the frequency scale between structural and JG peak; (d) the JG relaxation time showed a non-Arrhenius temperature behavior above T_g, paralleling that of the structural relaxation time. All the results can be rationalized in the framework of Coupling Model.     

Interrelation between shear modulus and the molecular parameters of viscous flow for glass forming liquids

The relationship between the free energy of viscous flow activation, the instantaneous modulus and the molar volume of kinetic units overcoming the barrier was derived by the author in the simplest form, ΔG^≠ = F_∞V¹, thirty-eight years ago. It was the result of the common solution for most general equations for the viscosity coefficient, the rate constant for shear relaxation in accordance with Maxwell’s relation. Here it is shown that this equation may be derived in the framework of the theory of elasticity and/or hydrodynamics. In this equation, V¹ = 8(r₀)³N_A, where V¹/N_A in the theory corresponds to the cube volume containing an inscribed molecule (atom) of radius r₀. The experimental proof of the equation shows that the atomic radii found from the viscous and elastic parameters match those obtained from direct structural investigations (X-ray and neutron scattering) with an average accuracy not worse than ∼5% (oxide, fluoride and chalcogenide glass melts). The theory needs development for molecular liquids with more complex structure. It is obvious that Maxwell’s relation is valid for modeling of viscous flow at the molecular level in supercooled liquids.     

Structural transitions of nematics upon interaction of coherent acoustic waves

The features of the structural transition of homeotropically oriented nematic liquid crystal (NLC) upon the interaction of coherent longitudinal and shear waves in a cell with acoustically soft boundary conditions at the ends for the frequency range, where the viscous wavelength is smaller and the elastic wavelength in NLC is larger than the mesophase layer thickness, have been experimentally investigated. The data obtained are analyzed within a model developed for this problem geometry based on a hypothesis postulating the flow mechanism of the orientational effect of ultrasound on NLC.     

Application of modified Lennard–Jones potentials to structural and dynamical studies for liquid Al

Based on the dense gas-like model of viscosity and Born–Green viscosity formula, the modification of Lennard–Jones (LJ) potential has been done to make it suitable to describe the properties of liquid Al. Experimental data involving viscosity and pair correlation function (PCF) of liquid Al are used to derive the potential parameters. The structural properties and dynamical heterogeneity of liquid Al in cooling processes have been studied via molecular dynamics simulations to test the accuracy of new pair potential. Calculated results are comparable with those derived by Mei’s embedded-atom method (EAM), exhibiting correct trends as a function of temperature.     

Viscosity of germanium sulfide melts

Viscosities of Ge_xS_1−x melts (0.30⩽x⩽0.44) have been measured by penetration viscometry from 10⁷ to 10¹³ Pa s. The temperature dependence of equilibrium viscosities in this range can be expressed approximately by a simple Arrhenius equation. Both the heat capacity change at the glass transition and the activation energy of viscous flow monotonously increase with germanium content as predicted by the Adam–Gibbs theory. Therefore, the connectivity of the germanium–sulfur network is reduced due to decreasing concentration of sulfur which causes increasing fragility of the undercooled liquid. The glass transition temperature exhibits a maximum near the GeS₂ composition where heteropolar bonds are predominantly formed.     

An electrospray technique for hyperquenched glass calorimetry studies: Propylene glycol and di-n-butyl phthalate

We describe an electrospray technique for in situ preparation, for differential scanning calorimetry study, of samples of molecular liquids quenched into the glassy state on extremely short time scales (hyperquenched). We study the cases of a hydrogen-bonded liquid, propylene glycol, PG and a Van der Waals liquid, di-n-butyl phthalate DBP. Using a fictive temperature method of obtaining the temperature dependence of enthalpy relaxation, we show that the electrospray method yields quenching rates of ∼10⁵ K/s, while the more common method, dropping a sealed pan of sample into liquid nitrogen, yields only 120 K/s. These hyperquenched samples start to relax, exothermically, far below the glass temperature, at a temperature (0.75T_g) where the thermal energy permits escape from the shallow traps in which the system becomes localized during hyperquenching. This permits estimation of the trap depths, which are then compared with the activation energy estimated from the fictive temperature of the glass and the relaxation time at the fictive temperature. The trap depth in molar energy units is compared with the ‘height of the landscape’ for PG, the quasi-lattice energy of the liquid based on the enthalpy of vaporization, and the single molecule activation energy for diffusion in crystals. The findings are consistent with the mechanism of relaxation invoked in a current model of relaxation in glassforming liquids. In the case of di-n-butyl phthalate we investigate the additional question of sub-T_g annealing effects. We find the ‘shadow’ glass transition, (an annealing prepeak) seen previously only in multicomponent mineral and metallic glasses. The phenomenon is important for understanding microheterogeneities in viscous liquid structures.     

The vitreous phase of porcelain stoneware: Composition,evolution during sintering and physical properties

High performance ceramic tiles (ISO 13006 Group BIa, water absorption < 0.5%) are composed of porcelain stoneware: a compact and light-colored material containing a large amount of vitreous phase, which governs sintering behavior and affects geometrical, mechanical and functional properties of finished products. Ninety-three porcelain stoneware tiles were analyzed for bulk chemistry (XRF) and quantitative phase composition (XRD-Rietveld) in order to calculate both chemical composition and physical properties of the vitreous phase; their evolution during the sintering process was followed by lab simulation of industrial firing and quenching in the 1100–1200 °C range. Porcelain stoneware tiles contain 40% to 75% wt. of a vitreous phase having a quartz-feldspathic composition with an alumina excess coming from clay minerals breakdown. Vitreous phase formation by feldspars melting is a fast phenomenon, starting from ~ 1050 °C, that is mostly accomplished before viscous flow begins densification, which goes on involving a slow-rate quartz dissolution. Sintering kinetics is expected to be controlled by viscosity and surface tension of the liquid phase, which appear to depend essentially on the alumina content (hence on the mullite stability) along with the Na/K and Na/Ca ratios. At any rate, a microstructural control on sintering is claimed as the rheological behavior of the viscous phase (i.e. the matrix containing both liquid phase and fine-grained crystals of quartz and mullite) is substantially different from that of the liquid phase only.     

Molecular dynamics simulation of viscosity in supercooled liquid and glassy AgCu alloy

The viscosity of the model AgCu alloy is simulated by several methods using (i) correlation functions through the Green–Kubo formalism, (ii) a non-equilibrium molecular dynamics approach and (iii) creep tests under constant stress. Temperature dependences of the shear viscosity and the diffusion coefficient show the breakdown of the Stokes–Einstein relation well above the glass-transition temperature T_g. This observation is interpreted as a manifestation of the development of heterogeneities in the supercooled liquid approaching T_g. Based on a generalized Einstein formula for the viscosity of liquid, a temperature dependence of heterogeneity degree of supercooled liquid is estimated. Using the dependence of the deformation rate on external stress, the activation volume is evaluated to be four atomic volumes in liquid state. However, below the mode-coupling temperature T_c the activation volume increases by several times.