The kinetics of structural relaxation of bulk and ribbon glassy Pd40Cu30N10P20 monitored by resistance and density measurements

Density and in situ high precision electrical resistance measurements on bulk and ribbon glassy Pd₄₀Cu₃₀Ni₁₀P₂₀ in the initial state, after cold rolling and quenching from the supercooled liquid state are performed. Three relaxation stages resulting in different signs of the electrical resistance relaxation are determined. It is shown that the resistance change upon structural relaxation is predominantly controlled by the resistivity relaxation while the volume change has a minor effect on the resistance. Cold rolling has a complex impact on the electrical relaxation similar to the effect of plastic deformation on the dislocation anelasticity in crystals. Quenching of fully relaxed samples from the supercooled liquid state recovers structural relaxation and the amount of the resistance relaxation can be several times bigger than that in the initial state. It is concluded that relaxation of the electrical resistance as dependent on different conditions (bulk/ribbon samples, thermocycling, rolling and the parameters of quenching from the supercooled liquid state) is rather complex but it is unlikely that the degree of the relaxation is governed by the amount of the free volume. This conclusion agrees with the established fact of equal density of initial glassy and crystallized bulk samples and earlier findings of nearly equal shear viscosity of bulk and ribbon samples different by four orders of magnitude in the production quenching rate.     

Kinetics of self-bleaching in some photodarkened Ge–As–S amorphous thin films

Three amorphous Ge–As–S films with the average coordination numbers of 2.4–2.8 were prepared by thermal evaporation. True relaxation that is self-bleaching of photodarkened state has been studied. Considerable differences in kinetics of relaxation of photodarkened state were observed for Ge₁₂As₁₇S₇₁ (Sa 1) and Ge₁₅As₂₀S₆₅ (Sa 3) amorphous thin films. In both cases, the relaxation (self-bleaching) followed stretched exponential, however, flexible matrix of Sa 1 film relaxed significantly faster than the matrix of Sa 3 film. The amorphous film Ge_25.5As_29.5S₄₅ (Sa 8) was found to be insensitive to illumination. It is suggested that the network rigidity may significantly influence the magnitude of photodarkening and the rate of relaxation of photodarkened state.     

Broadband dielectric spectroscopy and aging of glass formers

We present the results of aging experiments on a variety of glass formers, including xylitol, glycerol, propylene carbonate, and [Ca(NO₃)₂]_0.4[KNO₃]_0.6 (CKN). In addition, broadband dielectric spectra of xylitol and CKN are provided. We demonstrate that, irrespective of which dielectric quantities (real and imaginary part of dielectric permittivity and modulus) are analyzed, and irrespective of the spectral region where they have been measured, the aging-time dependence always is governed by the structural rearrangements constituting the α-relaxation. If the time dependence of the structural relaxation time during aging is taken into account, relaxation times and stretching parameters, fully consistent with equilibrium data, are obtained. In CKN, both the structural relaxation time and the strongly decoupled conductivity relaxation time are deduced from the same dielectric aging experiment.     

In situ high temperature 27Al NMR study of structure and dynamics in a calcium aluminosilicate glass and melt

The temperature dependence (25–1400 °C) of ²⁷Al NMR spectra and spin–lattice relaxation time constants T₁ have been studied for a calcium aluminosilicate (43.1CaO–12.5Al₂O₃–44.4SiO₂) glass and melt using an in situ high temperature probe, and the glass has been characterized by ambient temperature, high field MAS NMR. The peak positions and the line widths show a consistent behavior as motional averaging of the quadrupolar satellites increases with increasing temperature. The rate of decrease with temperature of T₁ drastically increases near the glass transition temperature T_g, which suggests a change in NMR relaxation process from vibrational to translational motions. Above the T₁ minimum (≈1200 °C), NMR correlation times obtained from T₁ are in good agreement with shear relaxation times estimated from viscosity, suggesting that microscopic nuclear spin relaxation is controlled by the same dynamics as macroscopic structural relaxation, and thus that atomic-scale motion is closely related to macroscopic viscous flow.     

Polarization relaxation in (CH3NH3)5Bi2Br11 ferroelectric crystals

The time‐dependent relaxation of retained polarization in unipolar methylammonium bromobismuthate, (CH₃NH₃)₅Bi₂Br₁₁, single crystal was examined by optical observation of the domain structure. The polarization relaxation is attributed to the presence of a built‐in internal bias field caused by structured disorder in the crystal. The internal field accounts for the broad spectrum of potential barriers for the domain walls. The slow polarization decay, in the long‐time regime (t > 1 s), follows a stretched exponential and/or power‐law function. On the assumption that relaxation centers (domains) are noninteracting, a distribution function of relaxation times is reconstructed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of orientation and crystallization on dielectric and mechanical relaxations in uniaxially stretched poly(ethylene naphthalene 2,6 dicarboxylate)(PEN) films

Microstructural analysis (crystallinity, orientation) have been performed on stretched and isothermally crystallized poly(ethylene naphthalene 2,6 dicarboxylate)(PEN) films. The crystallinity ratios are higher at drawing temperatures below T_g than above T_g, and for similar draw ratios, a higher orientation can be obtained at drawing temperature below T_g than above T_g. The molecular mobility study by dielectric relaxation spectroscopy (DRS) and mechanical relaxation spectroscopy (MRS) was carried out as a function of draw ratio (λ) and drawing temperature (T_draw). Drawing in glassy state apparently splits the α-relaxation into two components: an α-low component which exhibits a strongly accelerated dynamics but this relaxation process is masked by microstructural rearrangements occurring while heating followed by an α-high relaxation process. The two sub-glass processes (β and β¹) are influenced by the drawing in the glassy state. This can be observed from the increase of relaxation amplitudes and was related to a high disorder in the amorphous phase that is induced by drawing below glass transition temperature. Drawing at 160 °C induces the opposite trend associated with crystallization and confinements effects. Differences in viscoelasticity behaviour were found by MRS in tensile mode parallel and perpendicular to stretching direction. As a result, when comparing oriented and crystallized samples with the same crystallinity ratios, a strong effect of morphology on the location and amplitude of the three relaxations of PEN can be found.     

Growth behavior of amorphous Ca and La modified PbTiO3 thin film

The growth behavior of amorphous Ca and La modified PbTiO₃ (PLCT) thin film has been studied in terms of the dynamic scaling approach, and the various scaling exponents associated with self-affine film surface have been characterized by small angle x-ray scattering technique. The results indicate that the amorphous relaxation has much influence on the growth behavior of films. For the light amorphous relaxation, the static scaling exponent α is 0.57, which suggests the surface diffusion dominates the surface morphology. As the effect of amorphous relaxation increasing, α is 0.30, suggests the desorption process is controlled mainly by the atomistic relaxation.     

Molecular dynamics of poly (oxybutylene)s confined in pores of Vycor glass

We have investigated the relaxation dynamics of poly (oxybutylene), POB, chains of various lengths, with different end groups (OH and CH₃) in bulk and in confinement using dielectric relaxation spectroscopy. It is known that POB chains exhibit, apart from the segmental (main) relaxation process, the dielectric normal mode process which reflects the global chain motions. The comparative study of OH- and methyl-ended POB chains in bulk reveals the marked effect of association of hydroxyl end groups and indicates that the decrease in the density of the methylated specimens results in the acceleration of both relaxation processes, main and normal mode relaxations. For the confined POB chains the data indicate that the segmental relaxation process becomes faster (the effect being more pronounced for the methylated POB chains). A decrease in T_g of about 8 K for the methyl- and 2 K for OH-ended POB chains is estimated. Regarding the average relaxation rates of the normal mode process our data indicate that global chain mobility is not affected by the confinement.     

Enthalpy relaxation in hyperquenched glasses of different fragility

The enthalpy relaxation in hyperquenched (HQ) glasses with a wide range of fragilities is studied by performing annealing and differential scanning calorimetric (DSC) experiments. In this work, the enthalpy relaxation behavior of annealed HQ glasses is characterized in terms of the excess heat capacity (C_p,exc) given by the difference between the first and the second DSC measurements on the HQ glasses. The shape of the C_p,exc curves depends on the fragility of the glass system, which implies that during annealing the mechanism of the structural relaxation of the HQ strong systems differs from that of the HQ fragile systems. The details of the fragility dependence of the C_p,exc curves have been discussed in terms of the energy landscape and the structure of the liquids.     

On the mechanism of dielectric relaxation in vanadium phosphate glasses

The mechanism of dielectric relaxation in xV₂O₅?(100?x)P₂O₅ glasses (x=50?70) has been investigated using the stimulated dielectric relaxation currents (SDRC) technique. The dielectric relaxation in metal-glass-metal systems is found to be non-linear with regard to voltage bias. Schottky barrier formation at the metal-glass interface, rather than a conventional Debye-type relaxation, is suggested to account for the observed behavior. The density of states N(E_F) as determined from the SDRC is in excellent agreement with the density of paramagnetic states found by the EPR technique.     

Physical ageing and the Johari-Goldstein relaxation in molecular glasses

Based on some of our earlier dielectric relaxation studies during structural relaxation of molecular glasses, we describe certain features specific to the change in the Johari-Goldstein (JG) relaxation and examine their consequences for understanding of the molecular mechanism of the JG process. The parameter for the distribution of relaxation times increases slightly (loss curve becomes narrower) and the relaxation rate either remains constant or increases on ageing. In all cases, contribution to permittivity from the JG relaxation, Δε_JG decreases with time, with a rate constant k according to a relation, Δε_JG (t) = Δε_JG (t → ∞) − [Δε_JG (t = 0) − Δε_JG ( t → ∞)][1− exp[−(kt)]. This reduces to an equation of the type Δε_JG(t) = a + b exp(− kt) where a and b are constants of the glass as well are dependent on the thermal history of quenching of the glassy system. On decreasing the temperature, the relaxation rate for a rigid molecular glass follows the Arrhenius equation in a range that extends from liquid to glass, but deviates from it as a result of structural relaxation of the two alcohols, while the distribution of relaxation times decreases. The variation of Δε_JG with temperature shows an increase in slope on heating through T_g or else a deep and broad minimum before T_g is reached and the slope increases are remarkably similar to the changes observed for volume and thermodynamic properties on heating a glass. These findings need to be considered for a molecular mechanism in the potential energy view of JG relaxation, in analyzing the physical ageing of the α-relaxation process. It is argued that a recent suggestion for considering the JG relaxation within the picture of potential energy landscape may not be inconsistent with its molecular origin and dynamics in localized regions of structurally inhomogeneous glass. But there is still need to determine how the apparent dynamic heterogeneity evident from the broad distribution of the JG relaxation times can be reconciled with the dynamic heterogeneity used to explain less-broad spectra of the α − relaxation process.     

Load relaxation behavior of a Zr41.2Ti13.8Cu12.5Ni10 Be22.5 bulk metallic glass

The load relaxation behavior within the supercooled liquid region of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass has been investigated. To explain the relationship between normalized stress and relaxation time, two different stress relaxation modes such as a Kohlrausch-Williams-Watts (KWW) behavior and a simple power law were applied to the short and long relaxation time regimes, respectively. The apparent activation energy for stress relaxation is 126 ± 10 kJ/mol. Flow curves were obtained by converting load-displacement data into a flow stress-strain rate relation, resulting in three different deformation characteristics through a wide strain rate region interpreted in terms of strain rate sensitivity. A prediction of hot workability has also been attempted by constructing a power dissipation map based on a dynamic materials model.     

Electronic relaxation in zinc iron phosphate glasses

The electrical and dielectric properties of 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses, melted at different temperatures were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe(II)/[Fe(II) + Fe(III)] ratio. With increasing Fe(II) ion content from 17% to 37% in these glasses, the dc conductivity increases. Procedure of scaling conductivity data measured at various temperatures into a single master curve is given. The conductivity of the present glasses is made of conduction and conduction-related polarization of the polaron hopping between Fe(II) and Fe(III), both governed by the same relaxation time, τ. The high frequency dispersion in electrical conductivity arises from the distribution in τ caused by the disordered glass structure. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions.     

Sub-Tg relaxations of dipolar solutes in glass forming solvents

Johari–Goldstein(or JG) relaxation of rigid molecular solute acetone in various glass forming solvents was studied, and compared with the relaxation of acetone molecules occurring in the cages of acetone clathrate hydrate. The activation energy (ΔE_β) of the JG-process for the solute increases with a decrease in the size of the cage of the host (solvent) matrix, and increase in interaction of the solute molecules with the solvent. We have also studied the sub-T_g relaxation(s) due to some flexible molecular solutes, viz. some esters of phthalic acid. These solutes in isopropylbenzene matrix exhibit only one sub-T_g relaxation, whereas in o-terphenyl matrix exhibit an additional sub-T_g process which may be identified with JG type of relaxation. This observation lead us to the conclusion that the β-process observed in the glassy states of these pure solutes is predominantly intramolecular in nature.     

Effect of structural relaxation on helium diffusion and solubility in vitreous B2O3

Helium permeation, diffusion, and solubility in vitreous B₂O₃ were measured as a function of thermal history and as a function of time at constant temperature. Volume relaxation measurements were also made on similar specimens. The correspondence between the effective relaxation times for gas mobility and molar volume suggests that the changes observed in both properties results from the same changes in the glass structure. This relaxation mechanism is described by the expression τ′ = 10^?4 exp(-18 000/RT), where τ′ is in seconds and the activation energy is in cal/mole. It is concluded that helium mobility is a function of the molar volume of the glass.     

Microscopic aspects of Stretched Exponential Relaxation (SER) in homogeneous molecular and network glasses and polymers

The “diffusion to traps” model quantitatively explains “magic” stretching fractions β(Tg) for a wide variety of relaxation experiments (nearly 50 altogether) on microscopically homogeneous electronic and molecular glasses and deeply supercooled liquids by assuming that quasi-particle excitations indexed by Breit–Wigner channels diffuse to randomly distributed sinks. Here the theme of earlier reviews, based on the observation that in the presence of short-range forces only d* = d = 3 is the actual spatial dimensionality, while for mixed short- and long-range forces, d* = fd = d/2, is applied to four new spectacular examples, where it turns out that SER is useful not only for purposes of quality control, but also for defining what is meant by a glass in novel contexts. The examples are three relaxation experiments that used different probes on different materials: luminescence in isoelectronic crystalline Zn(Se,Te) alloys, fibrous relaxation in orthoterphenyl (OTP) and related glasses and supercooled melts up to 1.15T_g, and relaxation of binary chalcogen melts probed by spin-polarized neutrons (T as high as 1.5T_g). The model also explains quantitatively the appearance of SER in a fourth “sociological” example, distributions of 600 million 20th century natural science citations, and the remarkable appearance of the same “magic” values of β = 3/5 and 3/7 seen in glasses.     

Glass transition temperature dependence on heating rate and on ageing for amorphous selenium films

The study, by differential thermal analysis (DTA) of the glass transition of thermally evaporated amorphous selenium, allows the effect of ageing on structural properties to be observed. An enthalpic diagram is used to explain the shift of the glass transition region, with heating rate, and also with ageing. These shifts are directly dependent on structural relaxation in amorphous selenium. The model used, to express the change of the structural time relaxation with temperature, and with the departure of thermodynamic equilibrium leads to a slightly different expression for the dependence of T_g on heating rate than that previously published (by Moynihan et al.).The activation enthalpies of the studied phenomena compared with other results, seem to show that structural relaxation in a-Se principally involves bond breaking and reformation of chains.     

Low frequency relaxation in liquid crystals in relation to structural relaxation in glass-formers

Liquid crystals (LC) are the state of matter intermediate between isotropic liquids and the crystalline state. LC-forming molecules have strongly anisotropic shapes (rod-like in most cases). This leads to an interaction potential that consists of distance-dependent and orientation-dependent parts. Rotational dynamics of LC molecules falls into two frequency regions. Rotations about the short axes are strongly hindered by the potential barrier and thus coupled to fluctuations of the molecular centers of mass. This in turn causes these longitudinal or “flip-flop” motions, characterized by a relatively large relaxation time τ_||, to exhibit considerable temperature, pressure and volume dependences. Experimental relaxation times determined to date for various LC phases (nematic, smectic A, C, and E) for different thermodynamic conditions (isobaric, isothermal and isochoric) are discussed herein, adopting the formulae applied for characterization of the structural relaxation times of glass-formers (GF). This analysis appears fruitful; in particular, the strength parameter characterizing the steepness of the interaction potential can be determined from the relaxation times, and τ_|| is independent of temperature and pressure along the nematic-isotropic transition line, similar to the behavior of the structural relaxation time along certain transitions in GFs.     

Complex nature of the β relaxation and fragility in aromatic polyesters

The following experimental study shows the dielectric relaxation behavior of random copolyesters formed by butylene terephthalate and butylene isophthalate comonomeric units. The experimental data were analyzed using two relaxation modes describing the sub-glass relaxation, characterized by different activation energies. We propose that the high frequency secondary relaxation β₁ is associated to the most flexible part of the repeat unit, whereas the low frequency mode β₂ has been assigned to atoms of the ester groups linked to the aromatic rings. A strong correlation between the dynamic fragility of these polymers and the monomer architecture was also demonstrated.