Study of the kinetics of free volume in Zr45.0Cu39.3Al7.0Ag8.7 bulk metallic glasses during isothermal relaxation by enthalpy relaxation experiments

Enthalpy relaxation experiments were conducted to study the kinetics of free volume in Zr_45.0Cu_39.3Al_7.0Ag_8.7 bulk metallic glasses (BMGs) during isothermal relaxation using differential scanning calorimetry (DSC) at the temperature within the range from 648 to 684 K. Stretched exponential relaxation functions and the ?esták–Berggren SB (m, n) model were employed to analyze the kinetics of free volume. It is found that the relaxation time decreases from 2643.5 to 242.8 s while the Kohlrausch exponents increase from 0.717 to 0.892 in the investigated temperature range. The activation energy fluctuates slightly in the course of the relaxation process and its mean value is determined to be 239.7 kJ/mol. By fitting the first derivative of the conversion degree as a function of annealing time using the ?esták–Berggren SB (m, n) model, it is found that the values of m at all annealing temperatures are very small and can be approximated as zero. The values of the pre-exponential factors Z and the kinetic parameter n are found to be slightly varying with the annealing temperature, indicating the complexity of the kinetics of the isothermal relaxation. Finally, an approximate rate equation describing the kinetics of free volume during isothermal relaxation is proposed.     

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.     

Study of Isothermal Crystallization Kinetics of 5CB with Differential Scanning Calorimetry and Broadband Dielectric Spectroscopy

The isothermal crystallization kinetics of 4-n-pentyl-4–cyanobiphenyl (5CB) has been studied with Differential Scanning Calorimetry (DSC) and Dielectric Relaxation Spectroscopy (DRS). 5CB is a well characterized material which makes it ideal for a dielectric and thermal comparative study. The effect that isothermal crystallization exerts on the behavior of relaxation processes α and δ by cooling the isotropic liquid below the nematic phase or through the process of cold crystallization by heating it from the supercooled state is explored by comparing the DRS and DSC. Furthermore, by employing the Avrami equation, we compare the ability to probe the crystallization with each technique.     

Effect of temperature and pressure on the structural (α-) and the true Johari–Goldstein (β-) relaxation in binary mixtures

We investigated the microscopic origin of the excess wing through isothermal and isobaric dielectric relaxation measurements for the Quinaldine/tristyrene mixture. Our results show that the excess wing, characteristic of the high frequency side of the structural loss peak in neat Quinaldine, becomes a well resolved Johari–Goldstein secondary relaxation on mixing with the apolar tristyrene. Analyzing the temperature and pressure behavior of the two processes, a clear correlation has been found between the structural relaxation time, the Johari–Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). These results support the idea that the Johari–Goldstein relaxation acts as a precursor of the structural relaxation and therefore of the glass transition phenomenon.     

Dielectric relaxation in vitreous metaphosphates at medium temperatures

Dielectric relaxation experiments were carried out on mixed alkali metaphosphate glasses between room temperature and about 300°C (f = 10³ s^?1). The observed relaxations appeared to be connected with the movement of the alkali ions from site to site. By using stainless steel electrodes it was possible to reveal a relaxation peak free from conduction losses and due only to the migration from site to site of the cations. When two cations of dissimilar sizes are simultaneously present a mixed alkali effect was observed, i.e. an unproportionally large reduction in the alkali ion mobilities. It is shown that this effect can be at least partly explained by considering the changes in the potential energy states upon mixing dissimilar cations due to polarizing forces and coulombic interactions.     

Chemical modification of the electrical behavior of a-Se and its alloys-xerographic studies

The extent to which the photoelectronic properties of amorphous chalcogenides can be varied by chemical modification remains a key issue in the study of this technologically important class of semiconductors. a-Se is a prototypical system whose elemental nature and ambipolarity make it particularly attractive for such a study. In the following, the effect of the equilibrium doping of a-Se with halogens and alkalis will be summarized. Doping effects on thermal generation and deep trapping have been examined using xerographic techniques. The effect of dopants on magnetic properties has been measured by electron spin resonance (esr). The experimental results are compared to predictions of recently proposed defect models.     

Isothermal changes of volume and Young's modulus of amorphous CoP

The isothermal shrinkage and the isothermal increase of Young's modulus E of electrodeposited CoP samples were studied in the amorphous state and during crystallization. The lower limit of the activation spectrum of the elementary processes responsible for these changes is shifted from 0.8 to 1.6 eV by annealing at temperatures ascending from 100°C to 220°C. From the Arrhenius equation a frequency constant of 10^9.5±2.5 was derived.On account of the different behavior of the two properties we conclude that the changes of E in connection with the relaxation of the amorphous structure is mainly a consequence of the homogenization of the material and the shrinkage is mainly a consequence of the annihilation of free volume. Based on the observations a model is proposed concerning the nature of the elementary processes involved in the different steps of structural relaxation. During crystallization the correlation between the increases in Young's modulus and in density is totally removed. These results show that the observation of temporal and thermal changes of only one physical property does not yield full information about the kinetics of structural relaxation and crystallization.     

Uncertainties in crystallization of hen‐egg white lysozyme: reproducibility issue

The reproducibility of biomacromolecular crystallization (tetragonal and orthorhombic lysozyme crystals) was studied by monitoring the evolution of protein concentration during the crystallization process using Mach‐Zehnder interferometer. It was found that formation of both tetragonal and orthorhombic crystals exhibited poor reproducibility. When the crystallization occurred under isothermal conditions, the protein concentration in the solution varied differently in different experiments under identical conditions (for both types of crystals). Moreover, in the case of orthorhombic lysozyme crystallization (under either isothermal or thermal gradient conditions), it is clear that the crystals could not be always readily formed. When formation of tetragonal lysozyme crystals was conducted at a temperature gradient condition, however, the evolution of concentration was reproducible. The phenomena found in this study revealed that biomacromolecular crystallization can be uncertain, which is probably caused by the process of nucleation. Such uncertainties will be harmful for the efforts of screening crystallization conditions for biomacromolecules. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enthalpic structural relaxation in Te-Se glassy system

Differential scanning calorimetry (DSC) measurements were used to study enthalpic structural relaxation in the Te_ySe_(1−y) glassy system. The examined compositions were y = 0.1, 0.2 and 0.3. Single set of Tool-Narayanaswamy-Moynihan parameters was obtained from the curve-fitting procedure for each studied glass. A comparison with our previous measurements on a-Se was made and the development of particular TNM parameters with increasing Te content was discussed in terms of changes in molecular structure of the material. The curve-fitting results were further verified by several independent non-fitting methods.     

Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni₆₀Nb₂₀Zr₂₀ amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, E_x, were determined to be 499.5 kJ/mol and 488.6 kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E_x = 419.5 kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Volume and enthalpy relaxation of a-Se in the glass transition region

Volume and enthalpy relaxation studies of amorphous Se have been performed in the glass transition region by mercury dilatometry and differential scanning calorimetry. For simple temperature jump experiments, as well as for more complex thermal history the volume and enthalpy relaxation data can be described by a single set of kinetic parameters for Tool-Naraynaswamy-Moynihan (TNM) model [Δh¹/R = 42.8 kK, ln(A_TNM/s) = ?133]. Slightly different non-linearity and non-exponentiality parameter were found for volume [x = 0.42, β = 0.58] and enthalpy [x = 0.52, β = 0.65] relaxation data. Similar results were obtained also for Adam-Gibbs-Scherer (AGS) model. The activation energy of viscous flow in the glass transition range is identical with the effective activation energy for relaxation process. A self-consistent data evaluation shows that at moderate departure from equilibrium, volume and enthalpy in amorphous selenium relax in the same way as expressed by TNM and AGS models. Both volume and enthalpy change can be interpreted within the same fictive temperature concept.     

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.     

Charge transport in chlorine doped amorphous Se:Te xerographic photoreceptor films

Time-of-flight drift mobility experiments were carried out on a-Se_1?xTe_x (x = 0-0.1) with chlorine as an additive up to 70 atomic parts per million to investigate the charge transport mechanism in these xerographically important photoreceptor films. Hole drift mobility-temperature data indicate that hole transport in a-Se:Te alloys is controlled by a relatively discrete set of Te-introduced shallow traps (probably Te₁^? centres) at ～ 0.43 eV above E_v whose concentration increases nearly linearly with Te addition. Chlorine doping generates an additional set of traps (probably Cl₀^? centres) around the same energy as Te-induced traps in a similar fashion to the effect of Cl addition to a-Se. Electron drift mobility-temperature data for a-Se:Te alloys containing no Cl can also be interpreted by assuming Te-introduced electron traps at ～ 0.49 below E_c. There was no electron transport observable in Cl-doped a-Se:Te alloys.     

Relation between the dispersion of α-relaxation and the time scale of β-relaxation at the glass transition

Dielectric spectra of several typical molecular glass-formers, showing one or more secondary processes resolved in the glassy state, have been measured at different temperatures. We found that the genuine Johari–Goldstein β-relaxation is connected to the structural relaxation. In fact, a clear correlation was found between the structural relaxation time, the Johari–Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). Moreover, for a group of epoxide oligomers the steepness index is correlated to the broadness of the structural peak and to the time separation between the structural and the Johari–Goldstein relaxation. These results support the idea that the Johari–Goldstein relaxation acts as a precursor of the structural relaxation. A rationale for our results is provided by the coupling model.     

The effect of boron addition on texture and structure of NiMo/Al2O3 catalysts

The effect of support (Al₂O₃) modification with borate ions and the effect of their concentration on the texture and structure of nickel-molybdena catalysts were studied. The surface area and pore volume of the samples were determined using the BET method, whereas pores distribution – besides the BET method also by mercury porosimetry. FT-IR, derivatographic, and diffractometric studies were also performed. Modification with borate ions does not bring about any change in the type of the support pores, nor does it change its crystallic structure. However, a considerable decrease in the surface area of the support was observed. It was shown that modification with borate ions leads to a decrease in thermal stability of catalysts, yet increase in concentration of borate ions raises the thermal stability of the samples. It was found that on the catalyst surface borate ions form a well-dispersed monolayer structure.     

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.     

Thermally Stimulated Discharge Current Studies in Thin Films of Polyacrylamide

TSDC of pure polyacrylamide films of thickness 22 μ grown by isothermal solution growth technique has been studied as a function of polarizing field and polarizing temperature at a constant heating rate of 0.09 K s¹ In these studies only one TSDC peak was observed and the temperature corresponding to this peak was found to be independent of polarizing field strength but dependent on polarizing temperature. The activation energies, relaxation parameters and charge associated with the peak were evaluated. The origin of TSDC is discussed.     

Hole burning experiments with proteins: Relaxations,fluctuations and glass-like features

We present a series of different spectral hole burning experiments, such as aging and waiting time experiments and thermal cycling experiments to gain information on features of the EL of proteins and the associated physics. We focus on heme proteins. Spectral diffusion dynamics in proteins follow a power law in waiting time. If the protein is unfolded the dynamics change to a logarithmic time law as widely observed in glasses. Thermal cycling experiments with proteins show, in addition to random features, also discrete features. There are severe solvent effects demonstrating that the EL is strongly influenced by the solvent. We demonstrate that the hole recovery reaction may strongly depend on protein relaxation, and finally, we link features of the EL to characteristic properties of the folding–unfolding transition.     

Structural relaxations in disordered solids below Tg: Study by thermal-cycling single-molecule spectroscopy

Structural relaxation processes in disordered solids on the length scale of nanometers have been studied in a broad temperature range, from 4.5 K up to the glass transition, with single-molecule spectroscopy and thermal cycling experiments. Irreversible changes of single-molecule spectra after thermal cycles were observed and attributed to relaxation processes in the local environment of the probe molecules of tetra-tert-butylterrylene in polyisobutylene (PIB) and terrylene in solid ortho-dichlorobenzene. The effects of the relaxation processes on the individual parameters of low-energy matrix excitations (tunneling two-level systems and low-frequency vibrational modes) were analyzed, as well as the temperature dependence of the efficiency of relaxation processes in PIB. The results prove that the local matrix relaxations are characterized by a spatially varying distribution of activation energies.     

Crystallization of amorphous silicates far from equilibrium part II: Experimental insight into the key role of decoupled cation mobilities

The crystallization of amorphous and porous Ca–Mg-silicates prepared by a sol–gel method is studied in the glass transition range during isothermal and continuous heating experiments. A starting material of diopside (CaMgSi₂O₆) composition was primarily studied because it is a reference system to study crystal nucleation and growth. The annealed products were characterized by X-ray diffraction and transmission electron microscopy. In the glass transition range, the crystallization is not congruent. It follows a systematic sequence in which the most Ca-rich silicates present in the phase diagram crystallize first. This trend does not obey equilibrium thermodynamics predictions. Instead, this sequence is the result of the decoupled mobilities of network-modifying and network-forming cations. The high surface/volume ratio of gels likely exacerbates this effect compared to compositionally comparable glasses. Altogether, the study shows that local dynamics controls kinetic phase transitions in the glass transition range.