Kinetics Of Structural Relaxation Research Articles

Enthalpy, volume and structural relaxation in glass-forming silicate melts

Through structural relaxation, the configuration of a viscous liquid changes to allow the Gibbs free energy to be minimum in response to temperature variations. In this review, the practical importance of relaxation in silicate melts is first illustrated by configurational heat capacity and entropy and their connection with viscosity via Adam-Gibbs theory. Relaxation effects on thermal expansion and compressibility are then examined, and the similarity of the kinetics of structural, enthalpy and volume relaxation is pointed out. Turning to microscopic mechanisms, we finally stress the importance of Si-O bond exchange and its decoupling with the motion of network-modifying elements near the glass transition.

The kinetics of irreversible structural relaxation and rheological behavior of metallic glasses under quasi-static loading

It is shown that the directional structural relaxation (DSR) model supplemented by the Maxwell rheological model allows the derivation of a set of simple equations that describe within a common framework a number of mechanical relaxation phenomena induced by irreversible structural relaxation of metallic glasses under quasi-static loading. A comparison of the model predictions with literature experimental data is made. It is concluded that the equations derived are supported by available experimental findings.

Structural relaxation kinetics of antimony borate glasses with covalent bonding character

The structural relaxation kinetics in the glass transition for xSb2O3⋅(100−x)B2O3 glasses with covalent bonding character has been examined from viscosity and heat capacity measurements. These binary glasses have low glass transition temperatures, Tg=250–290 °C, but show a high thermal resistance against crystallization. The degree of fragility m estimated from the activation energy for viscous flow (Eη=290–531 kJ mol−1) is m=29–51. The activation energy for enthalpy relaxation, ΔH=297–602 kJ mol−1, is evaluated from the cooling rate dependence of the limiting fictive temperature. The ΔH values are very close to the Eη values, meaning that the decoupling between enthalpy relaxation and viscous flow is small. The values of Kovacs–Aklonis–Huchinson–Ramos (KAHR) parameter θ estimated from ΔH/RTg2 are 0.13–0.25, where R is the gas constant. The glasses with 30–60 mol % Sb2O3 have very similar Eη, m, ΔH, and θ values. It has been demonstrated that the structural relaxation kinetics of the binary antimony borate glasses is affected by the boron coordination numbers (i.e., BO3 and BO4) and the covalent bonding character of Sb–O bonds, and consequently these glasses are regarded as a highly strong glass-forming system in the fragile/strong classification concept.

Thermodynamic aspects of the glass transition of silicates

The marked increase in second-order thermodynamic properties observed at the glass-transition signals the onset of configurational changes in viscous liquids. Experimental determinations in the glass-transition range will illustrate this point for thermal expansivity and demonstrate that the kinetics of volume, enthalpy and structural relaxation are identical for silicate liquids. Within the framework of the Adam–Gibbs theory, heat capacity and viscosity data may be combined to calculate configurational entropies and gain insights into the potential energy barrier to viscous flow. Finally, the considerable effect of water on the glass transition temperature of geologically relevant silicates is presented.

Kinetics of structural relaxation in glasses: A hierarchical model of relaxation centers

A definition is proposed for the characteristic time of structural relaxation τstr. rel in metallic glasses—the universal parameter describing the kinetics of various processes associated with the structural relaxation. The behavior of τstr. rel in model situations of isothermal annealing of a glass and its heating at a constant rate is considered. In the former case, the time τstr. rel is equal to the age of the sample. In the latter case, after a time, τstr. rel reaches a steady-state value. This is in agreement with the τstr. rel values determined from the creep rate. The kinetics of structural relaxation is analyzed with due regard for the hierarchical structure of the phase space in glasses. The kinetic equation taking into account this feature is proposed, and the results of its numerical solution are given.

Kinetics of Structural Relaxation in Polyester Glasses.

Structural relaxation processes in amorphous poly (ethylene terephthalate) (PET) and in amorphous lightly branched PET of ethylene terephthalate-trimellitic (TMA) copolyesters at temperatures below their glass transition temperatures (Tg) have been monitored by dilatometric volume measurements of the polymer glasses after they were subjected to several different thermal histories. Both of thermal annealing at temperatures above Tg and chain branching due to incorporation of TMA into PET were found to cause appreciable decreases in the effective volume relaxation time. A new molecular kinetics model of structural relaxation was presented to provide a unified interpretation of the experimental results.

The Kinetics of Irreversible Structural Relaxation and Homogeneous Plastic Flow of Metallic Glasses

The Kinetics of Irreversible Structural Relaxation and Homogeneous Plastic Flow of Metallic Glasses

The Kinetics of Irreversible Structural Relaxation and Homogeneous Plastic Flow of Metallic Glasses

The paper is devoted to a comprehensive review of the Directional Structural Relaxation model developed recently. The model treats homogeneous plastic flow of metallic glasses as a result of structural relaxation oriented by the external stress. The analytical expressions for the macroscopic plastic strain rate due to irreversible structural relaxation of metallic glasses are derived. A comparison of the predictions of the model with the experimental data is given. It is concluded that the model provides a good basis for the understanding of homogeneous plastic flow of metallic glasses under various experimental conditions.

Kinetics of structural relaxation and regularities of plastic flow of metallic glasses

On the basis of a calculation of the structural relaxation rate and an experimental acoustical-emission determination of the temperature of the transition from localized to uniform flow it is argued that the type of plastic deformation of metallic glasses is uniquely determined by the kinetic structure of the relaxation. In the case of a kinetically hindered structural relaxation, which is characteristic for tests of initial samples at temperatures T<380–420 K, a localized dislocational deformation is realized. At higher temperatures, “memory” of the thermal prehistory of the samples is lost (aging at room temperature), the structural relaxation rate grows abruptly and plastic flow becomes uniform viscoplastic flow.

Kinetics of relaxation and crystallization of poly(ethylene terephthalate)

We study the kinetics of structural relaxation and crystallization of poly(ethylene terephthalate) (PET). It is demonstrated that the activation energy depends strongly on fictive temperature T f. The dependence of the glass transition temperature T g on the heating rate q + is investigated for samples that were previously cooled down at a rate q −=12.5 K/min. Angell et al. [M. Tatsumisago, B. Halfpap, J. Green, S. Lindsay, A. Angell, Phys. Rev. B 45 (1992) 10091] compare the thermal properties of many glasses to their `fragility'. The dimensionless fragility is a measure of the dependence of the activation energy for spatial rearrangement on changes of structure. We define a parameter α, easily determinable from experimental, which is close to one for `strong' substances and much larger for `fragile' systems. According to the present measurements α is extremely large for polymers (i.e. this is a class of very fragile substances).

Long-term behaviour of PEI and PEI-based composites subjected to physical aging

Glass-forming materials, including thermosetting and thermoplastic resins commonly used in polymer-matrix composites for high-performance applications, undergo structural relaxation when they are cooled through the glass transition region owing to the glassy non-equilibrium state. The process of moving towards equilibrium consists essentially of a densification of the matter and follows complex paths deriving from its inherent non-linear and non-exponential character. The structural relaxation is observable in a laboratory time-scale at temperatures below but close to Tg, and is related to the durability of polymeric materials because it is characterized by changes of structure-sensitive properties until equilibrium is approached. Polymer-based composites suffer the same shortcoming even if the properties are fiber-dominated. In this paper sub-Tg annealing studies have been carried out with both plain PEI and its carbon-fiber composites in order to observe the kinetics of structural relaxation. The experimental technique used is differential scanning calorimetry (DSC), which measures the enthalpy recovery during the structural relaxation. The investigation regarding the plain resin consisted of aging treatments at Tg−10°C, Tg−20°C and Tg−30°C, for different annealing times ranging between 0 and 168 h. The experimental data were used to calculate the parameters of a long-term predictive model for the enthalpy relaxation based on the Narayanaswamy approach. The structural relaxation of the in situ resin (i.e. the resin constrained into the fibers lattice) was also investigated at Tg−20°C taking into account the marked decrease of the glass transition temperature resulting from the overall manufacturing process. The comparison between the plain and the in situ matrix was done on the basis of the same degree of undercooling (i.e. the same distance from Tg). It was found that the composite aged faster than the plain matrix. The macroscopic effects of structural relaxation on PEI based composites were analyzed by fatigue tests in four-point bending geometry at different level of stress ratio R (the ratio of the minimum to the maximum stress). The aged samples showed a higher characteristic strength, which resulted in a correspondingly higher fatigue life compared to the as manufactured materials.

Phase diagram of organic superconductor κ-(ET) 2Cu[N(CN) 2]Br as it is seen from isothermal resistance relaxation

Phase diagram of κ-(ET) 2Cu[N(CN) 2]Br superconductor was studied by resistance measurements of structural relaxation kinetics and properties of nonequiliblium phases after isochronous annealing. We show, that (1) the activation energy of the lattice relaxation time below 100 K corresponds well to the potential barrier heights for ET molecule conformation; (2) the shape of the diagram corresponds to that of the ethylene ordering phenomena with superstructure formation; (3) the known anomalies of the electronic properties of the salt, namely resistance hump at around 100 K and pseudogap behavior at 60 K correspond to upper and lower hysteresis branches of the ethylene transformation, while the main second order phase transition proceeds at 80 K; (4) the state that comes our of the transformation is not an ordered state, but rather contains superstructure interacting with the Fermi surface and causing its partial destruction. We discuss these findings, together with high superconducting T c and negative isotope effect as indications of crucial soft lattice contribution to the superconducting properties in organics.

Nonmetal to metal crossover and ethylene ordering in the organic superconductorκ−(BEDT−TTF)2Cu[N(CN)2]Br

Phase transformations in organic superconductor $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}{\mathrm{Cu}[\mathrm{N}(\mathrm{CN})}_{2}]\mathrm{Br}$ and its deuterated analog have been studied by resistance measurements of structural relaxation kinetics under isothermal conditions. The nonmetallic to metallic resistance temperature dependence crossover observed in the salts and anomaly of spin susceptibility temperature dependence at 60 K are shown to be due to first-order phase transitions caused by ordering of the terminal ethylene groups. The experimentally determined transformation sequence is in good agreement with the microscopic mean field model of ethylene ordering, taking into account two possible configurations of ethylene groups and their interactions with anions.

Kinetics of Reversible Structural Relaxation in Amorphous Alloys

Kinetics of Reversible Structural Relaxation in Amorphous Alloys

Structural relaxation of As 2S 3 glass by length dilatometry

Structural relaxation of As 2S 3 glass was studied by dilatometry. The Tool–Narayanaswamy–Moynihan (TNM) model was successfully applied to the quantitative description of isothermal experiments performed within 40°C below T g. The activation energy of the relaxation process is identical within the experimental error with that of viscous flow, which was found to be 267 kJ/mol. The non-linearity and non-exponentiality parameters were evaluated as x=0.31 and β=0.82, respectively. The temperature dependence of the normalized volume relaxation rate agrees well with the prediction based on the TNM model. This approach can be used for comparison of structural relaxation kinetics in various non-crystalline materials.

Contributions of dynamic heterogeneity to non-exponential electrical relaxation in ionically conducting glasses and melts

The dynamic heterogeneity model for structural relaxation in glassforming liquids assumes that the non-exponential structural relaxation kinetics are due to a distribution of independently relaxing nanoregions which relax at different rates. A simple expression based on this model for the kinetics of the structural relaxation process has been extended to give an estimate of the contribution of microscopic heterogeneity to the non-exponentiality of the electric field relaxation in ionically conducting glasses and melts. Analysis of data near the glass transition temperature T g indicates that for the majority of inorganic glasses, which have very large values of the logarithm of the decoupling index log R τ(T g ) near T g, microscopic heterogeneity makes at best a very minor contribution to the non-exponentiality of the electrical relaxation and, hence, that the source of this non-exponentiality is primarily correlations among the diffusive motions of mobile ions. The opposite result is obtained, however, for glasses such as 0.4Ca(NO 3) 2–0.6KNO 3 with smaller values of log R τ(T g ) . The model also indicates that microscopic heterogeneity should make increasing contributions to non-exponential electrical relaxation in melts as the temperature is increased above T g. This is in accord with the decrease with increasing temperature of the KWW non-exponentiality parameter β σ for electrical relaxation observed for melts above T g.

Relaxation processes and metastability in amorphous hydrogenated silicon investigated with differential scanning calorimetry

In this work we investigated the thermodynamics and kinetics of structural relaxation in a-Si:H films using differential scanning calorimetry (DSC). The relation between structural relaxation and light-induced metastability (Staebler–Wronski effect, SWE) has also been investigated. The a-Si:H films were prepared by RF (13.56 MHz) glow discharge decomposition of the mixtures (10% SiH 4+90% H 2) and (5% SiH 4+95% He), and by 55 kHz plasma-enhanced chemical-vapor deposition (PECVD) of pure silane for high deposition rates. Detailed analysis of the peaks on DSC curves shows that the low-temperature effect (LTEP) is caused by the relaxation of weak Si bonds and that the high-temperature effect is caused by hydrogen effusion with subsequent relaxation of the microstructure. A method for the calculation of kinetic parameters of structural relaxation using DSC data is proposed. We found from the joint investigation of DSC and SWE that both the light-induced defect generation and the LTEP on DSC curves have a common nature and are described by the same reaction between weak Si–Si bonds and dangling bonds. We show that the rates of this reaction in both the forward and the reverse directions are controlled by hydrogen microstructure.

The role of structural relaxation in the plastic flow of metallic glasses

The role of structural relaxation in the plastic flow behavior of metallic glasses is analyzed both theoretically and experimentally. The characteristic time of structural relaxation is calculated as a function of glass thermal prehistory. It is revealed that heating above the room temperature by several tens of Kelvins results in a sharp, by several orders of magnitude, decrease of this time. It is argued that localized “inhomogeneous” dislocation-like flow occurs on loading if the characteristic time of structural relaxation is much greater than the characteristic loading time, while “homogeneous” viscous deformation is observed in the opposite case. Precise measurements of acoustic emission in a Co-based metallic glass being loaded at different temperatures and strain rates are employed for verification of this statement. It is shown that the inhomogeneous → homogeneous flow transition occurs at temperatures somewhat higher than T=400 K, and the transition temperature increases by ≈ 40 K as the strain rate increases by two orders of magnitude. Theoretical estimations show that for the inhomogeneous flow the characteristic time of structural relaxation in the loaded state is indeed much greater than the characteristic loading time. It is concluded that the kinetics of structural relaxation determines the flow mode of metallic glasses in a unique manner. The kinetically “frozen” structural relaxation gives rise to a crystalline-like localized flow under load while intensive structural relaxation facilitates a viscous glass-like behavior.

Raman scattering in glasses at high temperature: The Boson peak and structural relaxation kinetics in glasses

Raman scattering in glasses at high temperature: The Boson peak and structural relaxation kinetics in glasses

Raman scattering in glasses at high temperature: the Boson peak and structural relaxation kinetics in glasses

Inelastic light scattering as a function of temperature was carried out on various halide glasses. In their low frequency Raman spectra, all glasses show a broad feature around 50 cm −1, the so-called Boson peak. This Boson peak is associated with the existence of an intermediate range order in the glass. The Boson peak is due to an increase in the vibrational density of state, over the Debye value, caused by localized excitations. The low frequency Raman scattering containing this dominant spectral line (Boson peak) is interpreted in its relation to the amplitude and extent of the density fluctuations in glasses. The glasses considered in this study are fragile glass formers and the measurements extend from room temperature to the glass transition region. The spectral form of the Boson peak is the same for different glasses and no strong dependence on their respective chemical compositions is observed. The degree and range of disorder in the glasses is obtained in a quantitative sense from the behavior of the Boson peaks with temperature and compared to Rayleigh-Brillouin scattering data on the same glasses.