Thermal expansion of glasses in the As2Se3–AsI3 system

Thermal expansion coefficients (α) of glasses in the As₂Se₃–AsI₃ system are measured in the glass transition region and temperature dependence of the fictive temperature is calculated on the basis of relaxation model. It is found that the increase of AsI₃ content results in: an increase of α, decrease of the glass transition temperature (T_g), increase of the α change at T_g, an effect of quenching rate on α, and also changes in the structural relaxation times spectrum. The data are discussed within the framework of the assumption that the addition of AsI₃ to As₂Se₃ results in: (1) destruction of the As₂Se₃ glass network, (2) structural inhomogeneity of the glasses increase, (3) the temperature dependence of chemical–structural equilibria occurring in the liquid state increases.     

Relationship between viscous dynamics and the configurational thermal expansion coefficient of glass-forming liquids

We propose a model to describe the relationship between the viscosity of a glass-forming liquid and its configurational contribution to liquid state thermal expansion. The viscosity of the glass-forming liquids is expressed in terms of three standard parameters: the glass transition temperature (T_g), the liquid fragility index (m), and the extrapolated infinite temperature viscosity (η_∞), which are obtained by fitting of the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) expression to measured viscosity data. The model is tested with experimental data for 41 different glass-forming systems. A good correlation is observed between our model viscosity parameter,h(T_g, m, η_∞), and the configurational coefficient of thermal expansion (i.e., the configurational CTE). Within a given class of glass compositions, the model offers the ability to predict trends in configurational CTE with changes in viscosity parameters. Since viscosity is governed by glass network topology, the model also suggests the role of topological constraints in governing changes in configurational CTE.     

Sound velocity in liquid and glassy selenium

The speed of longitudinal sound waves at 7 and 22 MHz has been measured in liquid, supercooled, and amorphous selenium, including the region around the glass transition temperature, T_g, near 35 °C. In amorphous selenium the speed of shear waves at 7 MHz was also measured. The experiments were performed with high purity Se (99.9999%) hermetically sealed in an evacuated silica ampoule. Four temperature regions with strongly different relaxation times can be distinguished between room temperature and the melting point: (1) a glassy state below T_g, which is stable on the time scale of the experiments, (2) a glassy state above T_g, which is metastable on the time scale of the experiments, (3) a region where homogeneous crystal nucleation occurs, and (4) a supercooled liquid, which is stable on the time scale of the experiments. Each region is marked by a change in the slope of the temperature dependence of the sound velocity. Near the glass transition temperature the velocities of longitudinal and transverse sound exhibit hysteresis with a step-like drop on heating and a more continuous rise on cooling. The step-like anomaly in sound velocity may be a general property of the glass transition.     

Kinetics of non-isothermal crystallization and glass transition phenomena in Ga10Se87Pb3 and Ga10Se84Pb6 chalcogenide glasses by DSC

The crystallization process affects solid properties through the crystal structure and morphology established during the transition process. An important aspect of the crystallization process is its kinetics, both from the fundamental point of view of amorphous material as well as the modeling and phase transition. In the present research work, non-isothermal crystallization data in the form of heat flow vs. temperature curves has been studied by using some well known models for amorphous Ga₁₀Se₈₇Pb₃ and Ga₁₀Se₈₄Pb₆ chalcogenide glasses, prepared by the melt quenching technique. The glass transition phenomena and crystallization of these glasses have been studied by using non-isothermal differential scanning calorimetery (DSC) measurements at constant heating rates of 5, 10, 15, 20, 25 and 30 K/min. The glass transition temperature (T_g), crystallization temperature (T_c), and melting temperature (T_m) were determined from DSC thermograms. The dependence of T_g and T_c on the heating rate was used to determine different crystallization parameters such as the order parameter (n), the glass transition energy (ΔE_g) and the crystallization activation energy (ΔE_c). The results of crystallization were discussed on the basis of different models such as Kissinger's approach and the modification for non-isothermal crystallization in addition to Johnson, Mehl, Ozawa and Avrami.     

Temperature dependence of self-diffusion coefficient in several liquid metals

The temperature-dependent self-diffusion coefficient D(T) in liquid metals between the melting temperature T_m and boiling point T_b is modeled in terms of the relationship among D, liquid viscosity η, and liquid–vapor surface tension γ_lv. The model predictions for D(T) correspond to available experimental and molecular dynamics (MD) simulations results for liquid alkali metals Li, Na, Rb and Cs, semi-metal Al, transition and noble metals Ni, Cu, Ag and Au.     

Distinguish bonding characteristic in metallic glasses by correlations

By statistically analyzing 48 kinds of metallic glasses, we report clear correlations between the dimensionless ratio of glass transition temperature/Debye temperature (T_g/Θ_D) and density (ρ), and between Young's modulus or shear modulus and T_g/ρ, for the glasses consisting of only metal elements, while the metallic glasses alloyed with metalloid elements exhibit distinct deviation from the correlations. It is suggested that the alloying of metalloid elements would show covalent-like bonding characteristics in metallic glass, and the found correlations can be used to distinguish different bonding characteristics in metallic glasses.     

Reverse metal-non-metal transition in semiconducting melts

Electrical conductivity, σ(T), and thermopower, α(T), measurements were performed for liquid TlAsSe₂, CuTlSe₂, Cu_0.2(CuAsSe₂)_0.8, Cu_0.4(CuAsSe₂)_0.6, and Cu_0.6(CuAsSe₂)_0.4 alloys in a wide temperature range (700<T<1800 K) under ambient pressures of argon gas (up to 25 MPa). A reverse metal-non-metal high temperature transition is observed when temperature increases to about 1500 K. A mechanism involving the opening of a pseudogap is proposed. The mechanism suggests formation of both a mobility edge responsible for the electrical conductivity behavior, and a density of states edge that is responsible for the thermopower behavior.     

Relaxation of glasses: The Kohlrausch exponent

Relaxation of glasses is analyzed in the framework of the generalized Vogel-Fulcher-Tamann law. In this model the relaxation times of the cooperative and individual motions (α and β) verify the relation log τ_α(T,T′) = 1/n(T,T′) log τ_β(Τ) . In the glass state the equivalent temperature T′ is the temperature of the liquid (at equilibrium) which has the volume of the glass, T′ is then the function of the aging time and of the experiment time. In the liquid state (T = T′), n(T) ~ T − T₀ is the Kohlrausch exponent (T₀ is the Vogel temperature). In the glass state the parameter, n(T,T′), is a function of the aging time and temperature, of the rheological parameters of the liquid (WLF constants C₁ and C₂) and of the expansion coefficients of the liquid and glass states. The Kohlrausch exponents deduced from the properties below Tg (volume, creep, stress relaxation) are deduced from the generalized VFT model.     

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.     

The slow β-relaxation observed in Ce-based bulk metallic glass-forming supercooled liquid

Dynamic mechanical relaxation measurements are performed on a Ce-based metallic supercooled liquid close to its glass transition temperature T_g. An obvious excess wing is observed both in the temperature and frequency dependent loss modulus curves by the calculation the relaxation time of the α-relaxation in supercooled liquid with the fit by the combination of the Kohlrausch–Williams–Watts and Vogel–Fulcher–Tamman equation. The results indicate that the slow β-relaxation process exists in the metallic liquid and arises from the small-scale translational motions of atoms that are linked in its metastable islands.     

An electrospray technique for hyperquenched glass calorimetry studies: Propylene glycol and di-n-butyl phthalate

We describe an electrospray technique for in situ preparation, for differential scanning calorimetry study, of samples of molecular liquids quenched into the glassy state on extremely short time scales (hyperquenched). We study the cases of a hydrogen-bonded liquid, propylene glycol, PG and a Van der Waals liquid, di-n-butyl phthalate DBP. Using a fictive temperature method of obtaining the temperature dependence of enthalpy relaxation, we show that the electrospray method yields quenching rates of ∼10⁵ K/s, while the more common method, dropping a sealed pan of sample into liquid nitrogen, yields only 120 K/s. These hyperquenched samples start to relax, exothermically, far below the glass temperature, at a temperature (0.75T_g) where the thermal energy permits escape from the shallow traps in which the system becomes localized during hyperquenching. This permits estimation of the trap depths, which are then compared with the activation energy estimated from the fictive temperature of the glass and the relaxation time at the fictive temperature. The trap depth in molar energy units is compared with the ‘height of the landscape’ for PG, the quasi-lattice energy of the liquid based on the enthalpy of vaporization, and the single molecule activation energy for diffusion in crystals. The findings are consistent with the mechanism of relaxation invoked in a current model of relaxation in glassforming liquids. In the case of di-n-butyl phthalate we investigate the additional question of sub-T_g annealing effects. We find the ‘shadow’ glass transition, (an annealing prepeak) seen previously only in multicomponent mineral and metallic glasses. The phenomenon is important for understanding microheterogeneities in viscous liquid structures.     

Influence of Nd2O3 on the thermal and dielectric properties of Pb-based lead borosilicate glasses

Neodymium has been employed in various Pb-based glass compositions containing ZnO, MgO, CaO, and BaO, to obtain low melting glasses for use as dielectric materials. Thermal analysis (DTA, TMA) was applied for both qualitative and quantitative thermal properties (T_g, CTE) of the glasses, and impedance analysis of the dielectric properties (ε_r, tanδ) was performed. It was found that with increasing Nd₂O₃, the glass transition temperature (T_g), the dilatometer softening point (T_d) and dielectric constant (ε_r) became higher. Also, a peak shift suggesting neodymium oxide introduced structural changes was found with IR spectra and XPS. These seem to support the conclusion that the addition of increased amounts of Nd₂O₃ can lead to a more rigid glass structure.     

Relaxations in polystyrene below the glass transition studied by viscosity measurement

The shear viscosity of organic glass polystyrene has been determined under pure shear deformation mode from room temperature up to the glass transition temperature. A mechanical model of series connection of anelasticity and viscosity was used to determine the viscosity of the material. Relaxation time for the viscous flow was determined as a function of temperature. The relaxation was composed of two thermal-activation type relaxation processes: the high temperature relaxation (HTR) and the low temperature relaxation (LTR). In both relaxations the relaxation time was represented as τ=τ_o exp(E/k_BT), and the values of τ₀ and E were different in specimens treated differently – aged, loaded, and annealed. The observed τ₀ and E were not independent of each other but a compensation effect, a linear decrease of logτ₀ with increased E, was seen. The results were explained using the idea of cooperative relaxations of relaxing elements. HTR and LTR were considered to correspond to the structural and the slow relaxations, respectively, and the relaxing elements could respectively be a single atom or molecule and a segment in molecular chains.     

Magnetic resonance study of the crystallization behavior of InF3-based glasses doped with Cu2+, Mn2+ and Gd3+

The crystallization of fluoroindate glasses doped with Gd³⁺, Mn²⁺ and Cu²⁺ heat treated at different temperatures, ranging from the glass transition temperature (T_g) to the crystallization temperature (T_c), are investigated by electron paramagnetic resonance (EPR) and ¹⁹F nuclear magnetic resonance (NMR). The EPR spectra indicate that the Cu²⁺ ions in the glass are located in axially distorted octahedral sites. In the crystallized glass, the g-values agreed with those reported for Ba₂ZnF₆, which correspond to Cu²⁺ in a tetragonal compressed F⁻ octahedron and to Cu²⁺ on interstitial sites with a square-planar F⁻ co-ordination. The EPR spectra of the Mn²⁺ doped glasses exhibit a sextet structure due to the Mn²⁺ hyperfine interaction. These spectra suggest a highly ordered environment for the Mn²⁺ ions (close to octahedral symmetry) in the glass. The EPR spectra of the recrystallized sample exhibit resonances at the same position, suggesting that the Mn²⁺ ions are located in sites of highly symmetric crystalline field. The increase of the line intensity of the sextet and the decrease of the background line in the thermal treated samples suggest that the Mn²⁺ ions move to the highly ordered sites which contribute to the sextet structure. The EPR spectra of the Gd³⁺ doped glasses exhibit the typical U-spectrum of a s-state ion in a low symmetry site in disordered systems. The EPR of the crystallized glasses, in contrast, have shown a strong resonance in g ≈ 2.0, suggesting Gd³⁺ ions in environment close to cubic symmetry. The ¹⁹F NMR spin–lattice relaxation rates were also strongly influenced by the crystallization process that takes over in samples annealed above T_c. For the glass samples (doped or undoped) the ¹⁹F magnetization recoveries were found to be adjusted by an exponential function and the spin–lattice relaxation was characterized by a single relaxation time. In contrast, for the samples treated above T_c, the ¹⁹F magnetization-recovery becomes non-exponential. A remarkable feature of our results is that the changes in the Cu²⁺, Mn²⁺, Gd³⁺ EPR spectra and NMR relaxation, are always observed for the samples annealed above T_c.     

On the role of the β process as precursor of the α relaxation in aromatic polyesters

By performing broad band dielectric spectroscopy measurements in a series of amorphous aromatic polyesters we show that different fast modes, contributing to the β relaxation, appear at temperatures below the glass transition temperature, T_g. At high temperatures, (T > T_g) the different β modes tend to merge and the data have to be described by a single β peak. We tentatively assign a molecular origin to each of the different β modes. But also we find a strong connection among the merged β processes and the precursor of the structural α relaxation, implying that this relaxation may posses an important degree of intra-segmental cooperativity.     

Thermodynamic characteristics of Ti-based glass-forming alloys

Based on thermodynamic characteristics of the stable metallic liquid at melting temperature and the supercooled liquid, the present work calculated the mixing enthalpy ΔH^mix, the mixing entropy ΔS^mix and the Gibbs free energy difference between the supercooled liquid and the resulting crystalline phases ΔG of typical Ti-based amorphous alloys. The results show that for the case of larger ΔS^mix, moderate ΔH^mix for the stable liquid and smaller ΔG for the supercooled liquid, Ti-based alloys tend to achieve high glass-forming ability (GFA). A new parameter, β, defined as (T_g ? T_k)/(T_l ? T_g), has been introduced to evaluate the GFA of Ti-based bulk amorphous alloys (wherein T_g, T_l, and T_k represent the glass transition temperature, the liquidus temperature, and the Kauzmann temperature, respectively). Experimental data imply that the larger the β, the better the GFA for Ti-based amorphous alloys.     

Effect of laser irradiation on thermal and optical properties of selenium–tellurium alloy

The crystallization parameters such as glass transition temperature (T_g), onset crystallization temperature (T_c), peak crystallization temperature (T_p) and enthalpy released (ΔH_C) of the bulk Se–Te chalcogenide glass has been studied by using Differential Scanning Calorimeter (DSC), under non-isothermal condition at a heating rate of 20 K/min. The values of T_g, T_c, T_p and ΔH_C with and without laser irradiation for different exposure time have been studied. The optical absorption of pristine and laser irradiated thermally evaporated Se–Te films has been measured. The films shows indirect allowed interband transition that is influenced by the laser irradiation. The optical energy gap has been found to decrease from 1.61 to 1.38 eV with increasing irradiation time from 5 to 20 min. The results have been analyzed on the basis of laser irradiation-induced defects in the film.     

Crossover region of dynamic glass transition: general trends and individual aspects

The dynamic glass transition of many substances is characterized by significant changes and peculiarities in a narrow frequency temperature range, the crossover region. Crossover temperatures and frequencies as obtained from different experimental methods are compiled for 38 glasses of a variety of substance classes. The crossover temperatures obtained from different methods (T_B, T_t−r, T_on, T_c, T_β) are usually similar for a given substance. We attempt to relate the crossover parameters for different substances to typical properties of conventional α relaxation and Johari–Goldstein-process β below the crossover. The parameters are discussed with respect to empirical concepts from the literature. An attempt is presented to quantify the old, intuitive idea that molecular cooperativity starts in the crossover region and increases for lower temperature.     

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.     

Glass transition and thermal expansivity in silica-polystyrene nanocomposites

The glass transition temperature T_g and the thermal expansion coefficient have been measured using the capacitive scanning dilatometry for silica-polystyrene (PS) nanocomposites with various silica volume fraction up to 50 vol.%. The glass transition temperatures for silica-PS nancomposites show a deviation from the bulk T_g together with a large scatter. Thermal expansivity decreases with increasing silica fraction both below and above T_g. A clear relaxation peak can be observed in the expansivity-temperature curve for silica-PS nanocomposites. The intensity of the peak decreases with increasing silica fraction. The decrement is larger than the value expected on the assumption that silica particles do not participate in the glass transition.