The correlation between the pressure sensitivity and the fragility/glass transition temperature in bulk metallic glasses

The correlations between the pressure sensitivity and the fragility/glass transition temperature have been addressed in various bulk metallic glasses in the present work. The results demonstrate that the pressure sensitivity of bulk metallic glasses is closely related to both the fragility index (m) and the glass transition temperature (T_g). The physical origin of the correlations has been discussed from their disordered structure, which is determined by the glass transition behavior and the glass transition temperature.     

Calorimetry based on energy absorbed from time-dependent fields

The energy that a supercooled liquid absorbs from a time dependent field is used to determine the time scales involved in the dynamic heat capacities of the slow degrees of freedom. The time resolved changes in fictive temperatures are obtained from dielectric relaxation data derived from the same field that provides the energy input. For a generic molecular glass-former, it is found that dielectric and thermal time scales are locally correlated and agree within 15%. For a monohydroxy alcohol and an ionic liquid, however, such an identity between dielectric and calorimetric time scales is not observed. It is demonstrated that only the field induced change of the fundamental susceptibility, χ(ω), can be exploited for this dynamic calorimetry, whereas χ₃(ω) is not sensitive to these effects.     

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.     

Mössbauer study of structural relaxation at the glass transition

Mössbauer spectra of very dilute solutions of ⁵⁷Fe²⁺ ions in propane-(1,2)diol show distinct anomalies in the quadrupole splitting, the linewidth and the recoilfree fraction near the glass transition temperature. The observed time and temperature dependence of these anomalies can be correlated with structural relaxation processes which occur in a glass upon stabilization into the supercooled liquid phase.     

On the number of amorphous phases in n-butanol: Kinetics of free radicals oxidation by oxygen in frozen n-butanol

We report on the discovery of a new solid, presumably amorphous n-butanol at ambient pressure. According to the literature data the melting point T_m of n-butanol is 183 K and the glass transition temperature T_g is 118 K. If kept isothermally at a fixed temperature between 130 and 160 K, the supercooled liquid n-butanol undergoes remarkable phase transformations to a solid phase. The new phase converts to liquid at a temperature of about 170 K. It is presumably amorphous because foreign substances dissolved in liquid n-butanol keep the same state in this new phase of butanol. The kinetics of free radical oxidation by dissolved oxygen in both solid amorphous phases is investigated in detail.     

Optical Properties of Bi2O3-TeO2-B2O3 Glasses

Glasses of the Bi₂O₃-TeO₂-B₂O₃ ternary system were developed and their linear and nonlinear optical properties were investigated. The absorption edges of these glasses were found to be 367-384 nm with a good transmittance in visible wavelength, although they exhibit the refractive indices as high as 1.98-2.12 at 633 nm. The absorption edges are quite steep and they are analyzed by the Urbach theory. The obtained Urbach energies of these glasses are 73-79 meV which are comparable to silica glasses. The high refractive index and its glass composition dependency are discussed according to the basics of the electronic polarizability and optical basicity. The high third order nonlinear susceptibility χ⁽³⁾ = 2.0 × 10^− 12 esu at 800 nm was also obtained in the 36Bi₂O₃-18TeO₂-46B₂O₃ glass.     

2H NMR studies of supercooled and glassy aspirin

Acetyl salicylic acid, deuterated at the methyl group, was investigated using ²H NMR in its supercooled and glassy states. Just above the glass transition temperature the molecular reorientations, studied using stimulated-echo spectroscopy, demonstrated a large degree of similarity with other glass formers. Deep in the glassy phase the NMR spectra look similar to those reported for the crystal and below 20 K they are indicating that rotational tunneling plays a role. Measurements of the spin–lattice relaxation times for temperatures below 150 K reveal a broad distribution of correlation times in the glass. The dominant energy barrier characterizing the slow-down of the methyl group motion is significantly lower than the well-defined barrier in the crystal.     

Octahedral fluoride glasses: Raman spectra and structure of niobium pentafluoride

Raman spectroscopy is used to characterize the NbF₅ phases in the temperature range 80–500 K. A new clear glass is formed by quenching the melt to liquid nitrogen temperatures having a glass transition at ～206 K and crystallization at ～233 K. For all phases including the melt, the glass, the supercooled liquid, the crystalline solid and the gas, the Raman spectra show a rather common high frequency band at ～760 cm^?1 which is attributed to the Nb–F terminal frequency of partially bridged ‘NbF₆’ octahedra. Based on the systematics of the Raman spectra for all phases and the literature physicochemical data a model is proposed for the glass and the liquid phases where ‘NbF₆’ octahedral bridged in cis and/or trans configurations form a variety of cyclic and/or chain structures which intermix building up the overall structure. At exceptionally low energies (<11 cm^?1) a rather weak in intensity Boson peak is observed in the glass which shifts to even lower energies with increasing temperature. Librational and/or tortional motions of the bridged octahedra participating in the glass structure are possible candidates for the origin of this peak.     

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.     

Electronegativity difference, atomic size parameter and widths of supercooled liquid regions of Sb2S3-As2S3-Sb2Te3 glasses

Electronegativity difference Δ_x, atomic size parameter δ and width of supercooled liquid region (ΔT_x = T_x − T_g, where T_x is the onset crystallization temperature and T_g the glass transition temperature) are analysed for glasses of the ternary system Sb₂S₃-As₂S₃-Sb₂Te₃ as a function of arsenic atomic percentage.Correlation is investigated between the two bonds parameters (Δ_x and δ) and the width of supercooled liquid region ΔT_x (which is generally reliable in estimating the stability against the crystallization of the glasses). It is found that this width of supercooled liquid region of the glasses in Sb₂S₃-As₂S₃-Sb₂Te₃ system depends on electronegativity difference and atomic size parameter.     

‘Intra-cluster rearrangement' model for the α-process in supercooled liquids, as opposed to ‘cooperative rearrangement of whole molecules within a cluster'

A new model for the α-relaxation process in supercooled liquids and glasses is proposed which distinguishes between a structurally correlated region (cluster) of molecules and a unit of molecules for rather independent, correlated rearrangement motion. The essential aspects of the model are that the α-process is due to rearrangement of one or a few molecules within the cluster, while essentially the same motion in the space between the clusters is responsible for the β-process. The model leads to the following expectations: (i) absence of divergent behavior of α-relaxation times at non-zero temperature (e.g., Kauzmann temperature), (ii) close agreement between the glass transition temperatures, T_g, for the α-relaxation in liquid and crystalline phases of the same composition and (iii) possibility of crystal nucleation proceeding much below the T_g, and evidence for the latter two is presented.     

Deformation behavior of a Ni59Zr20Ti16Si2Sn3 metallic glass matrix composite reinforced by copper synthesized by warm extrusion of powders

A metallic glass matrix composite (MGMC) reinforced by copper short fibers has been prepared by warm extrusion of powders, and its deformation behavior at room temperature and in the supercooled liquid region of the metallic glass has been investigated. A mixture of Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ metallic glass powders and copper powders is extruded in the supercooled liquid region of the metallic glass with an extrusion ratio of 5. The volume fraction of the copper phase is 0.2. After extrusion, initially spherical powders are elongated along the extrusion direction; no pores are visible. The MGMC shows a high failure strength of around 1.85 GPa, slightly lower than that of the as-cast Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ metallic glass, under uniaxial compression. However, due to the crack bridging mechanism produced by the randomly distributed copper short fibers, the MGMC does not catastrophically fail by a single shear band propagating across the whole monolithic sample. In the supercooled liquid region of the metallic glass, the MGMC shows large elongation to failure but fails by cavitation due to the preexisting Ni-based crystalline powders.     

Thermal and electrical properties of BaO-B2O3-ZnO glasses

Glasses in the BaO-ZnO-B₂O₃ system were examined as potential replacement for PbO glass frits with low firing temperature (500-600 °C) for the dielectric layer of a plasma display panel (PDP). The glasses were evaluated for glass transition temperature (T_g), thermal expansion coefficient (α) and dielectric constant ε. The electrical and the thermal properties were also compared with theoretical data calculated by a known empirical equation. T_g of the glasses varied between 480 and 560 °C, and α was in the range of 7-9×10⁻⁶ K⁻¹. The dielectric constant ranges from 14 to 19 and the theoretical data showed lower α and ε than the experimental data. The results suggest that BaO-ZnO-B₂O₃ glasses would be suitable as an alternative to Pb-based dielectric layer in PDPs.     

Observing the twinkling fractal nature of the glass transition

A fundamental understanding of the nature and structure of the glass transition in amorphous materials is currently seen as a major unsolved problem in solid-state physics. A new conceptual approach to understanding the glass transition temperature (T_g) of glass-forming liquids called the twinkling fractal theory (TFT) has been proposed in order to solve this problem. The main idea underlying the TFT is the development of dynamic rigid percolating solid fractal structures near T_g, which are said to be in dynamic equilibrium with the surrounding liquid. This idea is coupled with the concept of the Boltzmann population of excited vibrational states in the anharmonic intermolecular potential between atoms in the energy landscape. Solid and liquid clusters interchange or “twinkle” at a cluster size dependent frequency ω_TF, which is controlled by the population of intermolecular oscillators in excited energy levels. The solid-to-liquid cluster transitions are in accord with the Orbach vibrational density of states for a particular fractal cluster g(ω) ~ ω^df − 1, where the fracton dimension d_f = 4/3. To an observer, these clusters would appear to be “twinkling.” In this paper, experimental evidence supporting the TFT is presented. The twinkling fractal characteristics of amorphous, atactic polystyrene have been captured via atomic force microscopy (AFM). Successive two-dimensional height AFM images reveal that the percolated solid fractal clusters exist for longer time scales at lower temperatures and have lifetimes that are cluster size dependent. The computed fractal dimensions, ≈ 1.88, are shown to be in excellent agreement with the theory of the fractal nature of percolating clusters in accord with the TFT. The twinkling dynamics of polystyrene within its glass transition region are captured with time-lapse one-dimensional AFM phase images. The autocorrelation cluster relaxation function was found to behave as C(t) ~ t^− 4/3 and the cluster lifetimes τ versus width R were found to be in excellent agreement with the TFT via τ ~ R^1.42. This paper provides compelling new experimental evidence for the twinkling fractal nature of the glass transition.     

Supercooled liquids and plastically crystalline phases: Indications for a similar manifestation of a crossover in the evolution of the dielectric spectra

We compare the susceptibility spectra (10⁻⁶ Hz–10¹² Hz) of glass forming liquids and plastically crystalline (PC) phases. In both the cases, a similar spectral change is observed while cooling. Whereas at high-temperatures the frequency-temperature superposition (FTS) principle holds for the α-process with a stretching parameter significantly below 1 it fails below a certain temperature T_x. Below T_x, in the case of supercooled liquids, in addition to a broadening of the α-relaxation peak the excess wing appears, and the corresponding power-law exponents β (α-peak) and γ (excess wing) of the distribution of correlation times show a similar dependence on the time constant τ_α, explicitly 1/β and 1/γ both are linear in lg τ_α. In the PC systems studied, an excess wing is missing and the failure of the FTS principle for the α-relaxation peak directly shows up below T_x. Again, the parameter of the Cole–Davidson susceptibility 1/β_CD is linear in lg τ_α below T_x, and constant above, allowing to identify T_x. In PC phases the crossover temperature T_x may be found much closer to the glass transition temperature T_g as compared to supercooled liquids, and thus can be well studied by standard dielectric spectroscopy.     

Dielectric characteristics of Ce3+ doped Sr0.61Ba0.39Nb2O6 with Cole‐Cole plots technique

In this paper, we have investigated two‐relaxator mechanism and dielectric characteristics of Ce³⁺ doped Sr_0.61Ba_0.39Nb₂O₆ with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginer part of the complex dielectric susceptibility in vicinity of ferroelectric‐paraelectric phase transition has been studied in the frequency region 0.1 kHz–10 MHz. The measurements of the dielectric constant of the real and imaginer parts show strongly frequency dependence. The investigations of the dielectric constant revealed a non‐Debye type dielectric relaxation for Ce⁺³ doped SBN61 by using Cole‐Cole plots. It reveals the coexistence of the two dielectric relaxators in vicinity of the phase transition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vibrational entropy near glass transition in a chalcogenide glass and supercooled liquid

Raman spectroscopic measurements have been performed on Ge₂₀Se₈₀ glass and supercooled liquid at temperatures ranging between 298 and 500 K. Temperature dependent softening of vibrational mode frequencies has been used in conjunction with the available vibrational density of states data at ambient temperature to estimate the relative contributions of vibrational and configurational entropies across glass transition. Nearly 20% of the additional entropy above glass transition is estimated to be vibrational. Thermal expansion effect on vibrational mode softening is found to be insufficient to account for the anharmonic component of vibrational entropy implying possible coupling between the vibrational and configurational entropies at temperatures above T_g. These results may have important consequences in shaping our understanding of various aspects of glass transition.     

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.     

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.     

Role of nickel ion coordination on spectroscopic and dielectric properties of ZnF2-As2O3-TeO2:NiO glass system

20ZnF₂-30As₂O₃-(50 − x)TeO₂:xNiO (0 ≤ x ≤ 2.0) glasses were synthesized. The glasses were characterized by X-ray diffraction, scanning electron microscopy, EDS and DSC techniques. A variety of properties, i.e. optical absorption, infrared, magnetic susceptibilities and dielectric properties (constant ?′, loss tan δ, a.c. conductivity σ_ac over a wide range of frequency and temperature) of these glasses have been carried out. The analysis of results of all these studies has indicated that the nickel ions occupy both octahedral and tetrahedral positions and the gradual increase of NiO content in the glass matrix causes growing proportions of Ni²⁺ ions that occupy octahedral positions. The luminescence spectra of these glasses have exhibited a broad emission band in region 1200-1450 nm identified due to ³T₂(3F) → ³A₂(3F) octahedral transition of Ni²⁺ ions. The luminescence efficiency and cross section have been found to be the highest for the glass containing highest concentration of NiO. Finally it is concluded that the higher the concentration of octahedrally positioned Ni²⁺ ions, the higher is the luminescence efficiency.