Kinetics Of Structural Relaxation Research Articles

Creep kinetics of linearly heated metallic glasses

A quantitative theory of creep in linearly heated metallic glasses is developed in terms of new ideas on the kinetics of irreversible structural relaxation under external mechanical stress. The validity of the resulting flow equation has been confirmed by a specially devised experiment. It is shown that the temperature dependence of Newtonian viscosity is determined by the rate of heating and the energy spectrum of irreversible structural relaxation.

Metastability and relaxation processes in hydrogenated amorphous silicon

The kinetics of structural relaxation in hydrogenated amorphous silicon (a-Si:H) deposited by various methods is investigated by differential scanning calorimetry. The experimental results are used to analyze the nature of the metastable states in a-Si:H and to investigate the relationship between structural relaxation and light-induced metastability (the Staebler-Wronski effect).

Non-exponential structural relaxation, anomalous light scattering and nanoscale inhomogeneities in glasses

Light scattering from glasses in the glass transition region exhibits an anomalous Rayleigh scattering. We observe a maximum in the scattering intensity vs. temperature during heating. It is shown that this behavior is consistent with the presence of nanometer scale inhomogeneities (density fluctuation) which relax at differing rates. Recent observations of anomalous light scattering were carried out on strong glass formers and fragile glass formers. In all cases a hysteresis effect is seen in the light scattering between heating and cooling each sample from room temperature through the glass transition region. From these measurements we suggest that this could be the source of non-exponential structural relaxation kinetics. A model based upon the modified Tool-Narayanaswamy model is found to be in agreement with the calculations based on the experimental results.

The relation between the bulk and ribbon Zr 55Ni 25Al 20 metallic glasses

The planar-flow-casting technique and quenching in flowing water were used to produce the glassy Zr 55Ni 25Al 20 ribbon and bulk samples. The thermodynamic states of these two types of samples were compared using differential scanning calorimetry and analysis of the structural relaxation kinetics. The relaxation enthalpy is 3 times larger and the glass transition temperature is higher by 67.5 K in the case of the ribbon sample. These differences were theoretically confirmed by a kinetic model and were caused by the different quenching rates for the ribbon and bulk samples, namely 10 5 and 10 2 K s −1, respectively. X-ray analysis has shown, that the main difference between the studied glassy structures consists in the different degree of the short-range order in the samples.

Viscosity, configurational entropy and relaxation kinetics of borosilicate melts

The viscosities of one silicate and four borosilicate liquids have been measured from about 1 up to more than 10 14 Pa s when liquid unmixing did not take place. Equilibrium values at the highest viscosities do not show evidence for a change of rheology in the glass transition range. Using calorimetrically determined heat capacities, we account quantitatively for the non-Arrhenian variations of the viscosities over the whole viscosity interval with the Adam-Gibbs theory. The configurational entropies determined from the liquid viscosities depend markedly on composition and are the highest for Na and Ca-rich compositions. The temperatures, T 1, of the empirical Tamman-Vogel-Fulcher viscosity equation are consistent with the temperatures at which configurational entropies vanish. Comparisons between a borosilicate and a window glass having similar viscosity-temperature relationships indicate a slight difference in the kinetics of structural relaxation. This difference agrees with a correlation between increasing fragility and shorter relaxation times and also suggests that the shear modulus at infinite frequency of borosilicates can vary by a factor of 100 as a function of composition.

Surface and bulk structural relaxation kinetics of silica glass

The structural relaxation kinetics of a silica glass were measured by following the IR structural band positions, which are directly correlated with the average Si–O–Si bond angle as well as with the fictive temperature of the glass, as a function of heat-treatment time, temperature and the water vapor pressure. Both surface relaxation and bulk relaxation kinetics were determined by measuring the IR reflection and absorption band positions, respectively. The surface relaxation was much faster than the bulk relaxation and had a smaller activation energy. Also, both relaxation kinetics were faster in the presence of water vapor. The apparent bulk relaxation time determined from the IR absorption band shift was a composite relaxation time consisting of both the relaxation time of the water-catalyzed near surface layer and the true bulk relaxation time of the glass interior which is unaffected by water vapor. The true bulk relaxation time was evaluated and found to have an activation energy consistent with that of the viscous flow.

Kinetics of Structural Relaxation in Physical Aging of Amorphous Poly (Ethylene Terephthalate)

Structural relaxation in amorphous poly (ethylene terephthalate) (PET) quenched from various temperatures above and below the glass transition temperature Tg (ca 68°C) has been studied in terms of time dependence of the specific volume. Experimental results were analyzed using the phenomenological multiparameter model of Moynihan and co-workers to give numerical values of kinetic parameters characterizing the structural relaxation. The kinetics of PET struc-tural relaxation could be divided into two different processes. One was in a temperature range below Tg-20°C, where the kinetic parameters remained constant while the time (ta) and tempera-ture (Ta) of aging varied. The relaxation time of this process was strongly dependent on the instan-taneous state of the PET glass and showed a broad distribution. The other process was in a temperature range from Tg-20°C to Tg, where the kinetic parameters changed with Ta, and the re-laxation time showed an Arrhenius-like temperature dependence and a less broad distribution. Moreover, it was found that thermal pretreatment in the vicinity of Tg was likely to stabilize the glassy PET structure and lead to some extreme retardation in long-term segmental rearrangements and cooperative motions.

Kinetics of structural relaxation of glass-forming melts

This is a review of current knowledge on the kinetics of relaxation of glass-forming melts. The kinetics of the relaxation process is of prime importance for the properties of glasses. A number of approaches describing the structural relaxation of glasses are discussed. The aim is to predict the form of the relaxation function and the temperature dependence of the characteristic time for structural relaxation τ r. Important information on the relaxation behaviour of glasses can be obtained by following the dependence of the glass transition temperature T g on the heating rate and the dependence of T f on the cooling rate.

Silicate melt structural relaxation: rheology, kinetics, and Adam-Gibbs theory

Several applications of the Adam-Gibbs theory of relaxation processes in viscous liquids to the theology of silicate melts are reviewed. This theory reproduces very well the isostructural viscosity observations, it explains the occurrence of shear thinning and offers a rationale for the pressure dependence of silicate melt viscosities. The key feature of the Adam-Gibbs theory is the calculation of the structural relaxation time. This aspect makes it also possible to use it for studying the kinetics of structural relaxation. Kinetic data obtained with static experiments are shown to be in good agreement with this model.

Kinetics of structural relaxation in a constrained dynamics system

The relaxation kinetics of complex systems is discussed in terms of two models. The first one describes the process as a series of parallel reactions, while the second one considers it as a chain of sequential reactions. It is demonstrated that the first model predicts, according to the experimental observations, that the time dependence of the thermodynamic functions follows a stretched exponential law. A discussion is given on whether the effective time, τ eff , that controls the rate of the relaxation process is to be determined: through the average of the jump frequency, 1 〈ν〉 , or by the average of the jump time, 〈τ〉.

Vitrification and structural relaxation of a water-swollen protein, wheat gluten and the thermodynamics of its water–protein ↔ ice equilibrium

To understand the chemical physics of molecular motions in a structurally simple, vegetable native protein containing absorbed water, in which both intermolecular and intramolecular interactions occur, the nature of the glass-transition and structural relaxation of vitrified 52.8 wt.% water swollen wheat gluten have been studied by differential scanning calorimetry (DSC), as has the water ↔ ice phase transformation. Experiments performed during both cooling and heating and with samples of different thermal histories show a broad endothermic feature beginning at ca. 160 K which is barely interrupted by a partial-crystallization exotherm at ca. 250 K. The endothermic features resembled those observed for several simpler hydrated proteins (Biophys. J., 1994, 66, 249; J. Phys. Chem., 1994, 98, 13780), a hydrated crosslinked polymer (J. Phys. Chem., 1990, 94, 2689) and a dry interpenetrating network polymer blend (J. Polym. Sci. B, 1992, 32, 683). Their broadness is a result of the closely spaced multiplicity of small but sharp mini-endotherms and has its origin in the onset of different configurational substrates that become available to the protein's structure as the temperature is increased. The remarkable similarity of these features amongst a broad class of materials is a reflection of the predominant role of the intermolecular energy barriers in determining the structural relaxation kinetics. This has been described in terms of a time-dependent potential-energy surface, which represent a hierarchy of molecular and segmental motions. Ice and freeze-concentrated solution coexist at a thermodynamic equilibrium at T < 273 K. Their respective amounts have been measured between 260 and 273 K and formalism based on equilibrium thermodynamics has been developed, and the DSC scans for cooling simulated. This formalism agrees with the data. The temperature variation of the equilibrium constant for the protein-water ↔ protein-ice coexistence does not agree with that given by the Gibbs–Helmholtz equation, which is a reflection of strong interactions between the water molecules and H-bonding protein segments. The interpretation in terms of the role of protein dynamics in the crystallization of water, and the formalism developed, are general and useful for studies on other complex biomaterials.

The kinetics of stress-oriented structural relaxation in metallic glasses

A theoretical model of the homogeneous flow of as-cast metallic glasses is proposed. Plastic flow within the framework of the model is viewed as a totality of local irreversible shear rearrangement with distributed activation energies. The equations for the macroscopic quantities measured in the experiment are derived.

Structural relaxation and the photoplastic effect in amorphous semiconductors

The kinetics of photoinduced structural relaxation in amorphous As-S films have been investigated by monitoring the changes that occur in their intrinsic and extrinsic stresses during the process of illuminating with bandgap light. It was found that, on illumination, photoinduced stress relaxation is observed and the level of the stress decreases to zero. This fact can be explained in terms of the photoplastic effect, i.e., the considerable decrease in a non-crystalline material's ability to resist plastic deformation, which is caused by illumination. This ability is restored when the illumination is turned off. A macroscopic model is proposed, which explains this effect in terms of viscous flow under irradiation with a viscosity near 1013 Poise.

Anomalous relaxation in amorphous Fe70Cr10.5P11.5C6.5Mn1.5

The structural relaxation kinetics for isothermal annealed Fe70Cr10.5P11.5C6.5Mn1.5 are examined via the Curie temperature, Tc, and Mössbauer spectroscopy. Anomalous kinetics were detected below 280°C, and a phenomenologic description is given of the data obtained.

Kinetics of Structural Relaxation in Metal Glasses

ABSTRACTRelaxation processes in different amorphous transition metal-metalloid (TM-M)-type alloys have been studied using thermopower measurements. The migration of two types of atomic complexes has been considered. For Ti-based amorphous alloys the migration of TM atoms and TM-M complexes diffusion are at least two processes that define the structural relaxation. For Ni and Fe-based alloys the relaxation processes are determined first of all by isolated TM atom migration.

An extended review of structural relaxation models with the mutual correlation of their parameters

Both the metallic and chalcogenide glasses possess the exothermal, endothermal and λ-shaped effects of the apparent specific heat c p , a( T) in the differential scanning calorimeter. The kinetics of structural relaxation of Kissinger, Moynihan and distribution of non-linear relaxation models are used to interpret these three phenomena. Because of the consistency of the models, their parameters are mutually related. The activation enthalpies ΔH Kiss ∗ (T a , t a ) and ΔH M ∗(T a , t a ) were predicted from the material parameters ΔH DNLR ∗, ΔS DNLR ∗ and K DNLR and the heat treatment history of the sample.

Light scattering in halide glasses at high temperatures: anomalous scattering and nanoscale inhomogeneities

Abstract The intrinsic light scattering behavior of several halide glasses has been studied as a function of temperature from room temperature to beyond the glass transition temperature, Tg. The Landau-Placzek ratios and the Brillouin shifts and widths were obtained as a function of temperature. Anomalous scattering results were found in the glass transition region both for the Landau-Placzek ratios and the Brillouin shifts. These results are interpreted as evidence of the physical origin of the non-exponential character of structural relaxation kinetics and their relationship to thermally induced nanoscale inhomogeneities.

Non-exponential structural relaxation, anomalous light scattering and nanoscale inhomogeneities in glass-forming liquids

Light scattering from glass-forming liquids exhibits an anomalous time dependence in the glass transition region, e.g. maxima in the scattering intensity versus temperature curves during heating. It is shown that this behavior is consistent with the presence of nanoscale inhomogeneities (density fluctuations) which relax at different rates. It is suggested that this could be the source of non-exponential structural relaxation kinetics. An expression relating the size of these regions to structural relaxation kinetic parameters has been developed and predicts sizes in excellent agreement with those determined by other methods.

Structural relaxation and nonexponential kinetics of CO-binding to horse myoglobin. Multiple flash photolysis experiments

The geminate recombination kinetics of CO-myoglobin strongly deviates from single exponential behavior in contrast to what is expected for unimolecular reactions (1). At low temperatures, this result was attributed to slowly exchanging conformational states which differ substantially in barrier height for ligand binding. Above 160 K the kinetics apparently slow down with temperature increase. Agmon and Hopfield (2) explain this result in terms of structural relaxation perpendicular to the reaction coordinate, which enhances the activation energy. In their model, structural relaxation homogenizes the kinetic response. Recently, Steinbach et al. (3) proposed a relaxation model which conserves the kinetic inhomogeneity. Below we test these conjectures by single and multiple excitation experiments. This method allows for discrimination between parallel (inhomogeneous) and sequential (homogeneous) kinetic schemes. The kinetic anomaly above 160 K is shown to result from a homogeneous, structurally relaxed intermediate. However a second anomaly is found above 210 K concerning the inhomogeneous phase which may indicate either a shift in activation energy or entropy.

On the Kinetics of Structural Relaxation in Metallic Glasses

On the Kinetics of Structural Relaxation in Metallic Glasses