Structural Relaxation Of Glass Research Articles

On the thermally activated structural relaxation in glasses

The asymmetric double-well potential model for thermally activated structural relaxation of Gilroy and Phillips is re-examined. Analytical expressions for the attenuation and the velocity variation of acoustic waves, valid on a large range of temperature and frequency, are found. An application to a-SiO 2 shows that this model using a constant distribution of asymmetries is no longer valid for T ⪆ 150 K . A refinement of the model is obtained by considering a Gaussian distribution of asymmetries. Good agreement is obtained in fitting ultrasonic results.

Ergodic behavior in supercooled liquids and in glasses.

Ergodic behavior in liquids, supercooled liquids, and glasses is examined with a focus on the time scale needed to obtain ergodicity. A measure, d(t), which is based on the time-averaged energies of the individual particles and which is referred to as the ``energy metric,'' is introduced to probe the approach to ergodic behavior. We suggest that d(t) obeys a dynamical scaling law for long but finite times and that it can be used to characterize the degree of stochasticity in measure preserving systems with large numbers of degrees of freedom. Examination of d(t) indicates that the configuration space is explored by a ``diffusive'' process in the space of the dynamical energy variables used in constructing the energy metric. The characteristic diffusion constant associated with this process is argued to be analogous to the well-known maximal Lyapunov exponent which is often used to characterize stochasticity in systems with few degrees of freedom. Based on the long-time behavior of d(t) it is shown that ergodicity is effectively broken in the glassy state. In addition to broken ergodicity, the possibility that a subtle symmetry is broken as the liquid-to-glass transition takes place is examined. It is suggested that a ``discrete'' symmetry, to be referred to as the statistical symmetry, is broken in the glassy phase. This is illustrated by analyzing the distribution of the energy of the particles. Based on this, we expect long-time dynamics and structural relaxation in glasses to be dominated by fluctuations in domains of finite length within which the particles are highly correlated. This is in accord with the ideas of Adams and Gibbs. All of the above arguments are illustrated with the aid of molecular-dynamics simulations of soft-sphere mixtures.

High-frequency structural relaxation in supercooled liquids

High-frequency structural relaxation in glass forming solutions of LiCl in water was studied using complex conductivity measurements in the frequency range 0.5 MHz to 32 GHz for temperatures between 300 and 160 K. The conductivity data show a low-frequency plateau, and a strong frequency dependence above a characteristic frequency. Both the low-frequency conductivity and the characteristic frequency decrease strongly with temperature, scaling approximately as the inverse of the viscosity, suggesting that hydrodynamic processes govern structural relaxation. The ionic conductivity can be related to the mechanical properties using the Stokes–Einstein relation. The frequency dependence cannot be understood in terms of a single viscoelastic relaxation time. A semiphenomenological version of the mode coupling theory of supercooled liquids, using the experimentally determined temperature dependence of the static viscosity as a parameter describes the frequency dependence of the data reasonably well.

Ionic conductivity and glass transition in superionic conducting glasses (Agi) 1 − x(Ag 2MoO 4) x ( x = 0.25, 0.30, 0.35) : I. Experimental results in the liquid and glassy states

The electrical conductivity of the (AgI) 1 − x (Ag 2 MoO 4 ) x system ( x = 0.2, 0.25, 0.3) was measured in both the liquid and the glassy state. The temperature dependence of the observed dc conductivity in the liquid state is well expressed by the Vogel-Tumman-Fulcher (VTF) equation, while in the glassy state it is described by the Arrhenius law. In particular, an abrupt change in the temperature derivative of the conductivity was found at the glass transition temperature T g . the value of which was independently determined by thermal analysis. The frequency dependence of the conductivity in the glassy state was found only at temperatures far below T g , which may be due to the decrease in the transition rate of conducting Ag + cations. The structural relaxation of glasses appears to yield anomalous behavior of the conductivity near T g , such as deviations from the Arrhenius law, annealing and hysteresis effects.

Modeling of viscoelastic and structural relaxation in glass

Contemporary relaxation models, as formulated by means of kinetics, irreversible thermodynamics and the theory of viscoelasticity are reviewed. While they are successful in describing the main relaxations in the vicinity of T g, or in the vicinity of the main maximum of relaxation times distribution, none of them accounts quite satisfactorily for both the main and secondary (sub- T g, β-, short-times-tale-) relaxations. This area along with the response of glass to the high amplitude influences are viewed as most promising for future modeling efforts.

Dielectric Relaxation at the Glass Transition as a Free Volume Process. A Single Relaxation Time Approach

AbstractFree volume theories describing structural relaxation in glasses are applied to the α‐dielectric relaxation occurring around Tg (the glass transition). Only a single relaxation time, τ, which represents the main contribution to the experimentally observed distributions, is considered. The influence of the thermal story is discussed. The unrealistic experimental values of the activation energy and preexponential factor found in the glassy zone are explained in this framework as also are the compensation laws associated with the glass transition. Measurements of τ in the range 10−8 to 104 s are performed in PVC and PAr, showing good agreement with the used model.

GLASS TRANSITION AS A FUNCTION OF COOLING RATE

The method is described for calculating structural relaxation in glasses cooled very rapidly. The major obstacle for the calculation was the assumption in the original model of Arrhenius behavior for the equilibrium viscosity. In this paper the Macedo-Litovitz' hybrid equation is used for the equilibrium viscosity. The glass transition region is shown to shift to higher temperatures and to broaden as the cooling rate increases. Glasses with higher Tg e.g. SiO2 are shown to have broader glass transition region than glasses with lower Tg but similar activation energy.

Model of Structural Relaxation in Glass with Variable Coefficients

The model of structural relaxation by Narayanaswamy deals only with the constant temperature coefficients of the glass property, p. The assumption is seldom valid for real glasses. In this work, equations are obtained for the fictive temperature of glass if the temperature coefficient, dp/dT, is a linear function of T or of 1 /T2. The temperature dependence of p has no significant influence on the calculated fictive temperatures.

Ferromagnetic resonance and spin wave excitations in metallic glasses: The effects of thermal ageing and long-range magnetic ordering

New ferromagnetic resonance experiments under degenerate resonance conditions are reported for FeNiPB metallic glass ribbons in annealed and polished samples. An interpretation of these experimental results in terms of established theories in Ferromagnetic and Spin Wave Resonance Spectroscopy (FSWR) is proposed. The spin-wave analysis is used to interpret the effects of magnetic anisotropy on the lineshapes of metallic glasses before and after annealing. The conclusion is reached that a model of dipolar-coupled regions which assume a common resonance frequency band is appropriate in ferromagnetic metallic glasses. The role of surface pits scattering, microheterogeneities in chemical composition, clustering processes and long-range magnetic ordering in these systems is discussed in relation to the changes of glass properties by annealing. We also conclude that FSWR techniques are more sensitive than differential scanning calorimetry, X-ray scattering and static, magnetic techniques which are currently used to study structural relaxation in glasses.

Prigogine–Defay ratio for systems with more than one order parameter

The Prigogine–Defay ratio, Π≡ΔCp Δκ/TV (Δα)2, has been derived for systems whose states need to be specified in terms of a number of order parameters z by considering the conditions of tangency of the equilibrium free energy surface and the free energy surface for constant values of the order parameters. It is shown that Π?1. The equality applies for the case of a single order parameter or for the case of more than one order parameter which satisfies the constraint (∂V/∂zi)/ (∂S/∂zi) = (∂V/∂zk)/(∂S/∂zk). The inequality applies for the case of more than one order parameter which does not satisfy this constraint. The relevance of the Prigogine–Defay ratio to the kinetics of the structural relaxation in glass is discussed.

Elastic effects of structural relaxation in glasses at low temperatures

A complete theoretical description of the effect of the relaxation of two-state structural defects on the elastic properties of glasses at low temperatures is given. New experimental data on the ultrasonic attenuation in vitreous silica and borosilicate glass in the temperature range between 0.3 and 4 K and at frequencies between 30 and about 500 MHz are presented and explained by this theory. We propose that over the entire temperature range from our lowest temperature of 0.3 to about 100 K the attenuation is due to the relaxation of defects with similar atomic structure.

Analysis of Structural Relaxation in Glass Using Rate Heating Data

A method was developed to determine the kinetic parameters controlling structural relaxation in the glass transition region from data acquired during continuous heating or cooling. The nonexponential character of the relaxation is accounted for by assuming an equilibrium isothermal relaxation function of the form exp [−(t/Toe)ß], where Toe is a relaxation time and 0<β1. The data are linearized using the method of Narayanaswamy, and the continuous temperature variation during heating or cooling is dealt with by invoking the superposition principle. The analysis yields the kinetic parameters A, the relaxation‐time preexponential term; Δh★, the relaxation‐time activation enthalpy; x, a term describing the relative effects of temperature and structure on the rate of relaxation; and β. The method was applied to analysis of the variation of the enthalpy of vitreous B2O3 during rate heating through the transition region following rate cooling through the same region at a variety of rates.

ChemInform Abstract: LIGHT‐SCATTERING MEASUREMENTS OF STRUCTURAL RELAXATION IN GLASS BY DIGITAL CORRELATION SPECTROSCOPY

Chemischer InformationsdienstVolume 6, Issue 24 Physical Inorganic Chemistry ChemInform Abstract: LIGHT-SCATTERING MEASUREMENTS OF STRUCTURAL RELAXATION IN GLASS BY DIGITAL CORRELATION SPECTROSCOPY CHING C. LAI, CHING C. LAISearch for more papers by this authorP. B. MACEDO, P. B. MACEDOSearch for more papers by this authorC. J. MONTROSE, C. J. MONTROSESearch for more papers by this author CHING C. LAI, CHING C. LAISearch for more papers by this authorP. B. MACEDO, P. B. MACEDOSearch for more papers by this authorC. J. MONTROSE, C. J. MONTROSESearch for more papers by this author First published: June 17, 1975 https://doi.org/10.1002/chin.197524029AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume6, Issue24June 17, 1975 RelatedInformation

Light‐Scattering Measurements of Structural Relaxation in Glass by Digital Correlation Spectroscopy

Digital clipped auto correlation spectroscopy was used to investigate the kinetic features of thermodynamic fluctuation processes which relax on time scales from 10−6 to 102 s. The autocorrelation function associated with structural relaxation in Na2O· K2/O·6SiO2, glass was determined in the annealing region. The results indicate a nonexponential correlation function that can be fitted with the decay function exp ‐ (t/τβ)β, where β and τβ are adjustable parameters. The parameter β which characterizes the deviation from a simple exponential, was essentially constant (β= 0.57 ± 0.02) from 468 ° to 550 ° C. The characteristic time constant, τβ ranged from 1.35 × 10−2 to 6.30 s. The computed average relaxation times were compared with shear viscosity data reported for a similar glass; both the temperature dependence and the magnitude agreed well with the predictions of viscoelastic theory.

A Model of Structural Relaxation in Glass

Study of the time dependence of physical properties in the transformation range of glass is complicated by the “memory effect” and the inherent nonlinearity which are characteristic of structural relaxation. A multiparameter model of structural relaxation is presented that differs from earlier models in that it takes account of both these effects. This model fits available experimental data well; these data were obtained for the most part by observing the evolution of properties (such as density or refractive index) following a step change in temperature. The present model also permits prediction of the physical properties of glass subjected to arbitrary and more complex temperature‐time histories. It should, therefore, also be useful in the rational design of heat treating processes such as annealing.