The Johari-Goldstein β-relaxation of glass-forming binary mixtures

The present paper shows, by means of broadband dielectric measurements, that the primary α- and the secondary Johari-Goldstein (JG) β-processes in binary mixtures are strongly correlated. This occurs for different polar rigid probes dissolved in apolar glass-forming solvents, over a wide temperature and pressure range.We found that the coupling parameter n = 1 − β_KWW and the ratio between α- and β-relaxation time reduce on increasing the size of the solute solved within the same apolar matrix. Moreover, such a ratio is invariant when calculated at different combinations of P and T maintaining either the primary or the JG relaxation times constant. Dielectric spectra measured at different T-P combinations but with an invariant α-relaxation time are well superposed in both the α- and β-frequency ranges. Experimental results can be rationalized by Coupling Model equation.     

Viscosity of Ba-B-Si-Al-O glass measured by indentation creep test at operating temperature of IT-SOFC

Viscosity of a specific Ba-B-Si-Al-O glass used for intermediate-temperature solid oxide fuel cell was measured using indentation creep tests. Responses of shear strain to corresponding shear stress at the operating temperature of solid oxide fuel cell were analyzed, and the results revealed that the glass system possesses Newtonian flow behavior at 600-630 °C. In addition, the stress exponent and the activation energy for viscous flow at different temperatures and stresses were also determined. Finally, the absolute-rate theory was adopted to describe the viscous flow for the glass. The results were compared with other glass systems.     

Basal and Non-Basal Slip in Anthracene Single Crystals

The role of basal and non-basal slip in the deformation of anthracene single crystals is considered in the light of published evidence and further work to establish their relative importance is described. Compression and shear tests on suitably orientated crystals show that the critical resolved shear stress for non-basal slip is at least ten times greater than that for basal slip. It is concluded that at room temperature slip generally takes place on the (001) [010] and (001) [110] systems and that the non-basal (201) [010], (100) [010], (100) [001], (010) [100] and (010) [001] systems suggested previously are only activated to any other extent under deformation conditions which severely limit basal slip.     

Search for instantaneous radiation near the instant of break momentum of various fissioning nuclear systems at low excitation energies

The main results of studying the properties of “instantaneous” neutrons and γ photons during the fission of ^{233, 235}U(n _th, f) and ²³⁹Pu(n _th, f) nuclei and spontaneous fission of ²⁵²Cf, which were performed on the WWR-M reactor at the St. Petersburg Nuclear Physics Institute, Russian Academy of Sciences, are presented. Along with obtaining the main characteristics of the instantaneous radiation from fission fragments, these studies were also aimed at gaining deeper insight into such exotic processes as the emission of break neutrons and γ photons from a fissioning nucleus near the break point. These investigations were performed on different experimental setups using different analytical methods. This approach allowed us not only to find but also to reduce to minimum possible systematic effects. The yields of break neutrons were found to be about (5–7) × 10⁻² of the total number of neutrons per ^{233, 235}U(n, f) fission event and approximately twice as much for ²³⁹Pu(n, f) and ²⁵²Cf. The coefficient of T-odd asymmetry for γ photons is in agreement with the estimate obtained on the assumption that the observed effect is mainly related to the γ photons emitted by excited fragments with highly oriented angular momenta. This fact gave grounds to conclude that the desired break γ photons cannot be reliably selected (within the obtained experimental accuracy) against the much larger background of γ photons from fission fragments.     

Slowing down of water dynamics in disaccharide aqueous solutions

The dynamics of water in aqueous solutions of three homologous disaccharides, namely trehalose, maltose and sucrose, has been analyzed by means of molecular dynamics simulations in the 0-66 wt.% concentration range. The low-frequency vibrational densities of states (VDOS) of water were compared with the susceptibilities χ″ of 0-40 wt.% solutions of trehalose in D₂O obtained from complementary Raman scattering experiments. Both reveal that sugars significantly stiffen the local environments experienced by water. Accordingly, its translational diffusion coefficient decreases when the sugar concentration increases, as a result of an increase of water-water hydrogen bond lifetimes and of the corresponding activation energies. This induced slowing down of water dynamics, ascribed to the numerous hydrogen bonds that sugars form with water, is strongly amplified at concentrations above 40 wt.% by the percolation of the hydrogen bond network of sugars, and may partially explain their well-known stabilizing effect on proteins in aqueous solutions.     

Interrelation between the parameters of equations of viscous flow and chemical composition of glassforming melts

The number of free parameters of viscosity equations are reduced to two by assuming that the glass transition temperature T_g is the temperature at which viscosity is 10^13.5 [dPa s]. In this way the Avramov and Milchev AM equation is becoming while the VFT equation is transformed to the form: . It is demonstrated that the dimensionless fragility parameter a of the AM equation depends on composition, x, as α = α_ο + 6x, where α_ο = 1 − 2 for silicates and α_ο = 2.75 for borates. We show that the fitting parameters of AM and of VFT equations are correlated. Thus the relationship between the pre-exponential constants is lg η_V ≈ lg η_A − 2. There is also relationship between the fragility parameter a and the divergence temperature T_o of the VFT equation.We develop an alternative approach to consider the vibration frequency of building units of the system. The superposition of the independent vibrations of the atoms constituting the building unit causes a “beat” of the latter.     

Strain-eliminating chemical bonding self-organizations within intermediate phase (IP) windows in chalcogenide, oxide and nitride non-crystalline bulk glasses and deposited thin film binary, ternary and quaternary alloys

Transitions into, and out of intermediate phases (IPs) with minimal strain have been identified to date by Boolchand and co-workers, in bulk glasses, primarily by the extraordinary low values of the change in enthalpy, ΔH_nr, associated with non-reversible heat flow, and by Lucovsky and coworkers in deposited thin films, and at dielectric–semiconductor interfaces by combining spectrographic characterizations, primarily synchrotron X-ray absorption and X-ray photoemission, and electrical measurements. This paper emphasizes chemical bonding self-organizations that minimize macroscopic strain within the IP windows, and identifies for the first time the necessary and sufficient conditions for IP windows to open, and to close, as a function of changes in the alloy composition. Percolation theory, and in particular competitive and synergistic double percolation provide a quantification of IP window first and second transition compositions that account for many of the experimentally determined IP window threshold transitions and IP window widths identified to date.     

Breakdown of the fractional Stokes–Einstein relation in silicate liquids

The fractional Stokes–Einstein relation postulates a direct relationship between conductivity and shear flow. Like viscosity, the electrical resistivity of a glass-forming liquid exhibits a non-Arrhenius scaling with temperature. However, while both viscosity and resistivity are non-Arrhenius, here we show that these two properties follow distinct functional forms. Through analysis of 821 unique silicate liquids, we show that viscosity is best represented using the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) model, whereas the resistivity of the same compositions more closely follows the Avramov–Milchev (AM) equation. Our results point to two fundamentally different mechanisms governing viscous flow and conductivity and therefore cast doubt on the general validity of the fractional Stokes–Einstein relation.     

Nearly Metallic [CH(13)y]x - Importance of Solitons,Crystal Order,Hopping and Band Conduction

Polyacetylene doped in the range of one to five percent has been shown to have low Pauli susceptibility yet high electrical conductivity. Earlier studies of polyacetylene doped with iodine to these nearly metallic levels show that essentially all charges go into soliton-like states. The conductivity (σ) and thermopower (S) of many samples are in quantitative agreement with charge transport via variable range hopping among soliton-like levels. Other doped samples have a lover density of states at the Fermi level with differing σ(T) and S(T). The strong sensitivity of the density of states to disorder leads to the proposal of a parallel conduction mechanism of thermal activation of charge carriers to “high mobility” extended band states. This mechanism may dominate in less disordered samples.     

Anelastic indentation recovery of bioglass at room temperature

Using the microindentation technique, micro-indentations were made on bioglass 45S5 by a spherical diamond indenter for the indentation loads in the range of 50-500 mN at room temperature. Complete anelastic recoveries of the indentation depths and the impression marks were observed for the first time. The recovery rate of the indentation depth decreased with increasing the indentation load. A phenomenological model was proposed to explain the indentation recovery. It found that the recovery rate of the indentation depth was a power-function of the indentation load as supported by the experimental results.     

On the solidification of the Fe50Cr15Mo14C15B6 bulk-amorphized alloy

Investigations of the effect of the initial liquid phase state on the processes of solidification in Fe₅₀Cr₁₅Mo₁₄C₁₅B₆ bulk-amorphized melts have been carried out by differential thermal analysis, X-ray structural analysis, metallography and viscosimetry. The anomalies caused by changing the relation of the atom microgroups of boride and carbide types have been discovered for the first time in polytherms of the melt viscosity in the vicinity of 1653 K and 1793 K. The structural changes observed in the liquid phase result in anomalies in the Fe₅₀Cr₁₅Mo₁₄C₁₅B₆ melt supercooling behaviour and a change in the crystallization mechanism. Structural transformation temperatures, can be considered to be an additional factor in the search for promising alloys with optimized glass forming abilities and enhanced service properties.     

The past, present, and future of the Johari-Goldstein relaxation

A short overview is given of the discovery of the Johari-Goldstein (JG) relaxation. The current perception that it plays an important role in the molecular processes leading to the alpha relaxation is due to K. L. Ngai and associates. Recently it has become apparent that the activation barriers for the JG relaxation are too high to be consistent with an intra-basin process, and the relaxation must involve an inter-basin one, contrary to the original view of Johari and this author.     

Relaxation mechanism of GaP/InGaAlP/GaAs heterostructure

Transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) were carried out to investigate the structural properties of the GaP/In_0.48(Al_0.7Ga_0.3)_0.52P heterostructures grown on GaAs (0 0 1) substrates. The lattice-matched In_0.48(Al_0.7Ga_0.3)_0.52P/GaAs material system could be used as a defect-free substrate because no lattice misfit exists between the In(AlGa)P layer and the GaAs substrate. Both TEM and HREM measurements indicated that there were not only misfit dislocations, but also microtwins at the GaP/In(AlGa)P heterointerface. The mechanism of the microtwins formation is elucidated.     

Dipolar relaxation of propylene glycol molecular cluster confined in carbon nanotubes of different chiralities—computer simulation study

We performed fully atomistic molecular dynamics simulation of propylene glycol molecules confined in single-walled carbon nanotubes of similar diameters but different chiralities to study the effect of the nanotube surface geometry on dipolar relaxation characteristic of the studied system. We show that it follows stretched exponential characteristic and that the chirality of the nanotube considerably affects thermal activation characteristic of the process.     

Defect model for the mixed mobile ion effect revisited: An importance of deformation rates

The progress in understanding the behavior of glassy mixed ionic conductors within the concept of the defect model for the mixed mobile ion effect [V. Belostotsky, J. Non-Cryst. Solids 353 (2007) 1078] is reported. It is shown that in a mixed ionic conductor (e.g., mixed alkali glass) containing two or more types of dissimilar mobile ions of unequal size sufficient local strain arising from the size mismatch of a mobile ion entering a foreign site cannot be, in principle, absorbed by the surrounding network-forming matrix without its damage. Primary site rearrangement occurs immediately, on the time scale close to that of the ion migration process, through the formation of intrinsic defects in the nearest glass network. Neither anelastic relaxation below glass transition temperature, T_g, nor viscoelastic or viscous behavior at or above T_g can be expected being observed in this case because the character of the stress relaxation in a wide temperature range is dictated above all by the deformation rates employed locally to the adjacent network-forming matrix. Since the ion migration occurs on the picosecond time scale, the primary rearrangement of the glass network adjacent to an ionic site occurs at rates orders of magnitude higher than those of the critical minimum values, so the matrix demonstrates brittle-elastic response to the arising strain even at temperatures well above T_g, which explains, among other things, why mixed alkali effect is observable in glass melts.     

The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623 K. Two weak relaxations have been observed around 337 K and around 402 K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W = (0.84 ± 0.03) eV ) and above ( W = (0.82 ± 0.03) eV) the temperature range where the β relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the β relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the β₁ sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the β₂ sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573 K, is high than 8895 s.     

Bias field induced impedance relaxation of TlGaSe2 semiconducting layered crystal at room temperature

Complex Impedance Spectroscopy has been employed for the study of absorption kinetics at TlGaSe₂ layered crystal under Bias voltage at room temperature. Two relaxations mechanisms have been observed. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The complex impedance spectra were fitted by the superposition of two Cole‐Cole types of relaxations. The fast relaxation in the higher frequency region may be due to dipolar relaxation and the slow relaxation in the low frequency region may be due to Maxwell‐Wagner type relaxation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A simple model for cooperative and non-exponential processes in non-crystalline polymers

A quite simple Markov chain model is presented in an attempt to understand several characteristic properties of the structural relaxation in glass-former materials. A second order Markov model is proposed for a small region of the material. Only a few parameters are necessary for the definition of the material in the model equations namely the energy levels accessible to that small region and the transition probabilities between them. The later are considered dependent on the energy levels and the potential barrier heights between them, on temperature, on a reference transition rate parameter and on a cooperativity or memory parameter related to the interaction with the neighbour regions. Isothermal relaxations at different temperatures were simulated with selected values of the model parameters. As a result, non-exponentiality of energy relaxations and its temperature dependence appears directly related to the cooperativity parameter. Markov chain model shows also non-linearity and memory effects, and a transition between two different regimes in the dependence of the relaxation times with temperature.     

Double relaxation processes in the glass system xEu2O3·PbO·2B2O3 studied by broadband dielectric spectroscopy

In this work we study the effect of introducing europium oxide in lead borate glasses, in concentrations up to x = 0.20, using the impedance spectroscopy in a large frequency range (0.1 Hz to 9 GHz). Two relaxation processes can be identified. The low frequency process deviates considerably from the Debye type and is modelled by the Cole-Davidson expression. The high frequency relaxation is also a non-Debye type and is fitted by the Cole-Cole expression. These results imply some distribution of relaxation times, due to the interaction between the dipoles.The behaviour of the dielectric strength is similar in the low and high frequency regions, indicating a cluster formation at a critical concentration of europium oxide (x = 0.03).     

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.