Glass-liquid Transition Temperature Research Articles

Crystallization Kinetics of Phosphonium Ionic Liquids: Effect of Cation Alkyl Chain Length and Thermal History.

The effects of alkyl chain length on the crystallization kinetics and ion mobility of tetraalkylphosphonium, [P666,n][TFSI], (n = 2, 6, 8, and 12) ionic liquids were studied by differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) over a wide temperature range. The liquid-glass transition temperature (Tg) and ion dynamics examined over a broad T range were almost insensitive to structural modifications of the phosphonium cation. In contrast, the crystallization kinetics were strongly affected by the length of the fourth alkyl chain. Furthermore, the thermal history of the sample (cold vs melt crystallization) significantly impacted the crystallization rate. It has been found that the nature of crystallization phenomena is the same across the homologous series, while the kinetic aspect differs. Finally, electric conductivity in supercooled liquid and crystalline solid phases was measured for all samples, revealing significant ionic conductivity, largely independent of the cation structure.

Decrease of heat capacity on aging, long and linear extrapolations for fictive temperature, and the case of 20-million-year-old fossil-amber

A glass and its melt have the same value of a physical property at fictive temperature, Tf, which is estimated by matching the area between two linearly extrapolated plots of the heat capacity, Cp, against temperature T, one from Cp of glass and the other from that of the melt. According to the DSC heating scans, the Cp values of 20-million-year-old fossil-amber and its melt-cooled fresh-amber are the same at T < 273 K, and Tf of fossil amber is 0.89 Tg→l, where Tg→l is the glass-liquid transition temperature of melt-cooled amber [Nat. Commun. 4 (2013) 1783]. A comparison of Tf with the variation of stress relaxation time with T led to the conclusion that relaxation dynamics of amber deviates from super-Arrhenius variation. Here we show that use of the same Cps of the two ambers at T > 0 K violates basic thermodynamics, and argue that Cp of (denser) fossil-amber would be less than that of fresh-amber at T < 273 K. The Cp,confof the melt, taken as the difference between the extrapolated Cps of fossil-amber and of its melt, increases by ∼13% on cooling from T= Tg→l to T= 0.72 Tg→l, and this increase and the corresponding increase of (Cp,conf/T)= (∂Sconf/∂T)p are too small to indicate an approach towards a second-order transition of the entropy theory. Hence we conclude that deviation from super-Arrhenius variation of relaxation time of amber is an artefact of a long and linear extrapolation of its melt's Cp used for determining Tf.

Magnitude of Dynamically Correlated Molecules as an Indicator for a Dynamical Crossover in Ionic Liquids.

In this work, we show how the structure and intermolecular interactions affect the dynamic heterogeneity of aprotic ionic liquids. Using calorimetric data for 30 ionic samples, we examine the influence of the strength of van der Waals and Coulombic interactions on dynamic heterogeneity. We show that the dynamic length scale of spatially heterogeneous dynamics decreases significantly with decreasing intermolecular distances. Additionally, we assume that the magnitude of the number of dynamically correlated molecules at the liquid–glass transition temperature can be treated as an indicator for a dynamical crossover.

Effects of Zn doping on the pinning potential and the glass-liquid transition temperature of YBa2Cu3O6+δ films

A pristine YBa2Cu3O6+δ film as well as two Zn-doped YBa2(Cu1−x Zn x )3O6+δ films with doping levels x of 0.2 % and 0.4 %, respectively, grown epitaxially on SrTiO3 single crystalline substrates by pulsed laser deposition, were investigated regarding the effect of the doping on the glass-liquid transition line and the pinning potential distribution. Zn doping did not show any effect on the shape of the transition line but caused a suppression of the transition temperature T GL of about 16 K%−1. Although the doping did not lead to an additional pinning effect, the increase of the critical current density j c towards lower temperatures is mainly driven by the variation of the threshold critical current density, j cm. The width of the critical current density distribution j 0 saturates at low temperatures and only drops above ~ 0.9T GL.

Impact of the Interfacial Energy and Density Fluctuations on the Shift of the Glass-Transition Temperature of Liquids Confined in Pores

The behavior of the low- and high-molecular weight glass formers confined in nanoporous templates remains an unsolved puzzle despite the intensive long-term studies in this matter. Special effort is taken to understand the enhancement of segmental or structural dynamics and the depression of the glass-transition temperatures, Tgs in materials infiltrated in pores of the nanometric size. In this paper, we have analyzed dielectric, calorimetric, and contact angle data collected for various systems to determine which factors are responsible for these effects. It turned out that with increasing interfacial energy, molecules attached to the pore walls (interfacial layer) vitrify at higher temperatures. Moreover, the dynamics of core molecules starts to deviate from bulk-like behavior. Therefore, a greater depression of the glass-transition temperature, Tg, of this fraction of molecules is noted. Also, it was found that the sensitivity of structural dynamics to the density fluctuations, quantified by the pressu...

Angular behavior of flux dynamics in YBCO films with crossed columnar defects around the ab-plane

We systematically investigated the influence of columnar defects (CDs) on the flux dynamics around B ∣∣ ab in YBa2Cu3O7-δ (YBCO) films, where two types of the crossed CD configurations with the crossing angle relative to the c-axis, θi = ±30° and ±60°, were prepared by using 200 MeV Xe ion irradiation. The crossed CDs with θi = ±30° induce a peak of critical current density Jc centered at B ∣∣ c, which suggests a typical entanglement state of flux lines. The Jc at B ∣∣ ab, by contrast, is insensitive to the crossed CDs with θi = ±60°, which are crossing at ±30° around the ab-plane. In addition, the behaviors of the glass–liquid transition temperature Tg and the dynamic critical exponent z indicate that the VG state develops at B ∣∣ ab not only for θi = ±30° but also for θi = ±60°. The difference in the contribution of CDs to the flux dynamics for between θi = ±30° and ±60° also stands out at B ∣∣ CDs, where the magnetic field for θi = ±60° is more tilted toward the ab-plane: the Jc peak at B ∣∣ CDs occurs in any magnetic field for θi = ±60°, whereas the correlated pinning state disappears in high magnetic fields for θi = ±30°. These results suggest that the pinning by CDs is angle-dependent due to the flux line structure with strong line tension around B ∣∣ ab: CDs around the ab-plane can trap flux lines along their long axis only in much narrower angular-range, in comparison with CDs around the c-axis.

Liquid, glass, and crystalline indomethacin studied by Brillouin scattering

Indomethacin (IMC) is a medicine available in various countries worldwide. The longitudinal acoustic (LA) and transverse acoustic (TA) modes of liquid, glass, and γ-form crystalline IMC were studied by Brillouin scattering spectroscopy on the basis of the backward scattering and reflection-induced ΘA scattering geometies. Upon heating of glassy IMC, the elastic anomaly at the liquid–glass transition temperature Tg = 315 K was clearly observed. With further heating of the supercooled liquid state, crystallization and melting into an equilibrium liquid phase were also observed. The temperature dependence of the elastic constants of a γ-form IMC crystal was determined using the observed velocities of an LA and two TA modes below the melting temperature Tm = 433 K.

On the Possibility of Calculating Entropy, Free Energy, and Enthalpy of Vitreous Substances.

A critical analysis for the arguments in support of, and against, the traditional approach to thermodynamics of vitreous state is provided. In this approach one presumes that there is a continuous variation of the entropy in the glass-liquid transition temperature range, or a “continuous entropy approach” towards 0 K which produces a positive value of the entropy at T → 0 K. I find that arguments given against this traditional approach use a different understanding of the thermodynamics of glass transition on cooling a liquid, because it suggests a discontinuity or “entropy loss approach” in the variation of entropy in the glass-liquid transition range. That is based on: (1) an unjustifiable use of the classical Boltzmann statistics for interpreting the value of entropy at absolute zero; (2) the rejection of thermodynamic analysis of systems with broken ergodicity, even though the possibility of analogous analysis was proposed already by Gibbs; (3) the possibility of a finite change in entropy of a system without absorption or release of heat; and, (4) describing the thermodynamic properties of glasses in the framework of functions, instead of functionals. The last one is necessary because for glasses the entropy and enthalpy are not functions of the state, but functionals, as defined by Gibbs’ in his classification.

On relative merits of the criteria of glass formation and effects of ultraviscous liquid properties

A liquid is said to become glass when cooled to a temperature T infinitesimally below the temperature Tl→g. According to the glass-formation criteria, either, (I) τs=CI/qc at Tl→g, or, (II) (dτs/dT) ≅ CII/qc, or, (III) (τs/T)≅CIII/qc, where τs is taken as equal to τα, the relaxation time of the α-process, qc is the cooling rate, and CI=16.67K, CII≅1, and CIII≅1. Criterion I is generally used in glass phenomenology, criterion II was related to the effective enthalpy of activation in the Tl→g range and recommended over criterion I, and criterion III was used for discussing the pressure dependence of Tl→g. We investigate their merits by comparing formally the criteria-based τs at Tl→g in an ideal case against τα at T=Tl→g, and discuss the effects of using the glass-liquid transition temperature, Tg→l, the fictive temperature, Tf, and the midpoint temperature of the Cp-T plots as alternatives to Tl→g. The three criteria yield basically different results. Moreover, Tl→g differs from the kinetic freezing temperature of the α-relaxation process, and CI varies with both qc and the material. We suggest that contributions from several properties of ultraviscous liquids affect the shape of the Cp-T scans from which Tl→g is determined. The findings have consequences for interpretations in which τs is used as τα.

Formation temperature of ultra-stable glasses and application to ethylbenzene

Ultra-stable thin glass films are obtained when they are deposited at a temperature Td below the glass transition temperature Tg. An enthalpy excess Δɛ×ΔHm is frozen during deposition inducing an isothermal liquid–glass transition temperature Tsg equal to Td and a liquid-to-liquid transition temperature Tgf above Tg depending on Δɛ. The enthalpy excess, the ultra-stable glass enthalpy saving, and the optimum deposition temperature Tsg leading to the maximum density, are predicted for ethylbenzene, knowing only the molar melting heat ΔHm of crystals, their melting temperature Tm, Tg, and the specific heat jump at Tg.

Glass transition of aqueous solutions involving annealing-induced ice recrystallization resolves liquid-liquid transition puzzle of water.

Liquid-liquid transition of water is an important concept in condensed-matter physics. Recently, it was claimed to have been confirmed in aqueous solutions based on annealing-induced upshift of glass-liquid transition temperature, . Here we report a universal water-content, , dependence of for aqueous solutions. Solutions with vitrify/devitrify at a constant temperature, , referring to freeze-concentrated phase with left behind ice crystallization. Those solutions with totally vitrify at under conventional cooling/heating process though, of the samples annealed at temperatures to effectively evoke ice recrystallization is stabilized at . Experiments on aqueous glycerol and 1,2,4-butanetriol solutions in literature were repeated, and the same samples subject to other annealing treatments equally reproduce the result. The upshift of by annealing is attributable to freeze-concentrated phase of solutions instead of ‘liquid II phase of water’. Our work also provides a reliable method to determine hydration formula and to scrutinize solute-solvent interaction in solution.

Effect of incongruent crystallization on glass–liquid transition features of a bulk metal glass

It is known that most multi-component glasses cold-crystallize incongruently on heating through the temperature range of their ultraviscous melt. If the incongruent melt's composition changes with time, its viscosity, η, and the glass–liquid transition temperature, Tg, would change. Since the η, relaxation time, and expansion coefficient of a liquid in its partially crystallized mixture cannot be determined, we used scanning calorimetry to study the liquid–glass–liquid transition during thermal cycling of the incongruently crystallizing Ce66Al10Cu20Co4 glass. Its Tg is 358K for 20K/min and 354K for 10K/min heating rates, and its ultraviscous melt crystallized incongruently when heated beyond the hysteresis peak of its heat capacity scan. Its sample that had been aged for nine years at ambient conditions had a higher crystallization-onset temperature than an un-aged sample. Delayed enthalpy gain on heating of the aged glass is ∼1/5th of the enthalpy lost on its crystallization. Crystallization of the melt occurred on both the heating and cooling paths of a thermal cycle and Tg of the un-aged glass increased as the volume fraction of the compositionally different glass, fgl, decreased. The increase was by 8K after the 24th cycle of 20K/min, and by 11K after the 13th cycle of 10K/min cooling-heating. The highest Tg values reached differed by ∼1K, which indicates that closely similar Tgs may be reached if the total time period for thermal cycling (at different rates) is kept the same. As fgl approached its limiting low value, increase in Tg per cycle itself increased, and the Tg-endotherm became unusually broad, which we attribute to diverse energies of interactions between the incongruent liquid and the crystal-surface atoms that dominate at the lowest fgl.

Kinetic-freezing and unfreezing of local-region fluctuations in a glass structure observed by heat capacity hysteresis.

Fluctuations confined to local regions in the structure of a glass are observed as the Johari-Goldstein (JG) relaxation. Properties of these regions and their atomic configuration are currently studied by relaxation techniques, by electron microscopy, and by high-energy X-ray scattering and extended x-ray absorption fine structure methods. One expects that these fluctuations (i) would kinetically freeze on cooling a glass, and the temperature coefficient of its enthalpy, dH/dT, would consequently show a gradual decrease with decrease in T, (ii) would kinetically unfreeze on heating the glass toward the glass-liquid transition temperature, Tg, and dH/dT would gradually increase, and (iii) there would be a thermal hysteresis indicating the time and temperature dependence of the enthalpy. Since no such features have been found, thermodynamic consequences of these fluctuations are debated. After searching for these features in glasses of different types, we found it in one of the most stable metal alloy glasses of composition Pd40Ni10Cu30P20. On cooling from its Tg, dH/dT decreased along a broad sigmoid-shape path as local-region fluctuations kinetically froze. On heating thereafter, dH/dT increased along a similar path as these fluctuations unfroze, and there is hysteresis in the cooling and heating paths, similar to that observed in the Tg-endotherm range. After eliminating other interpretations, we conclude that local-region fluctuations seen as the JG relaxation in the non-equilibrium state of a glass contribute to its entropy, and we suggest conditions under which such fluctuations may be observed.

Stress-induced glass transitions

In 1995, Chudnovsky and coworkers (Zhou, Chudnovsky, Bosnyak, & Sehanobish, 1995) interpreted cold drawing in polymeric glasses as a dual glass transition, first from glass state to a liquid, and then, after drawing turned the glass into a non-isotropic oriented polymeric liquid, from oriented liquid to a new glass state. On the microscopic scale, a glass is a system with local arrangements of particles frozen. At the glass–liquid transition temperature, these arrangements become changeable at an observable time scale. Assuming that cold drawing is caused by slip-stick rearrangements in small clusters throughout the material, we investigate the relations of conventional glass transition to the onset of cold drawing; our theory is based on a generalization of Eyring’s ideas of stress-dependent rearrangement activation energies.

Correlated vortex pinning in slightly orthorhombic twinned Ba(Fe1−xCox)2As2single crystals: Possible shift of the vortex-glass/liquid transition

The interest in twin-boundary (TB) planes as a source of vortex pinning has been recently renewed with the discovery of the new iron-arsenide pnictide superconductors. In the family of compounds Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ a structural transition from a tetragonal to orthorhombic lattice takes place for compounds with $xl{x}_{\mathrm{cr}}\ensuremath{\sim}0.065$. Approaching the critical doping, domain structure shrinks with sizes ultimately becoming comparable to vortex cores. In this work we investigate the changes in anisotropy produced by subtle differences in the Co doping level, in the neighborhood of the structural transition, in good-quality single crystals. Using a scaling approach we are able to determine the angular regions where correlated or uncorrelated disorder prevails. In the tetragonal samples ($xg{x}_{\mathrm{cr}}$) there is no twinning and we find good agreement with the expected scaling function under uncorrelated disorder, with small anisotropy values similar to those reported in the literature. We show that in the orthorhombic samples ($xl{x}_{\mathrm{cr}}$), TBs act as correlated disorder in a broad angular range. We propose that the observed angular dependence could be due to an increase in the vortex liquid-glass transition temperature.

Ultra-slow dynamics in low density amorphous ice revealed by deuteron NMR: indication of a glass transition

The postulated glass-liquid transition of low density amorphous ice (LDA) is investigated with deuteron NMR stimulated echo experiments. Such experiments give access to ultra-slow reorientations of water molecules on time scales expected for structural relaxation of glass formers close to the glass-liquid transition temperature. An involved data analysis is necessary to account for signal contributions originating from a gradual crystallization to cubic ice. Even if some ambiguities remain, our findings support the view that pressure amorphized LDA ices are of glassy nature and undergo a glass-liquid transition before crystallization.

Surface Viscoelasticity of an Organic Interlayer Affects the Crystalline Nanostructure of an Organic Semiconductor and Its Electrical Performance

We demonstrated that the viscoelasticity of a dielectric surface affected the overlying pentacene crystalline nanostructures and the electrical performances of pentacene-based field-effect transistors (FETs). The surface viscoelasticities of the gate dielectrics were systematically controlled by varying the polymer chain lengths of polystyrene brushes (b-PSs) and the substrate temperature during pentacene deposition. The b-PSs were chosen as a model surface because the glass–liquid transition affected neither the surface energy nor the surface roughness. Moreover, the glass–liquid transition temperature increased with increasing b-PS chain length. The liquid-like b-PS chains disturbed the surface arrangement of the pentacene molecules, which reduced the organization of the crystalline structures, yielding smaller grains during the early stages of pentacene growth. The dramatic changes in the film morphology and crystalline nanostructures above the b-PS glass–liquid transition resulted in noticeable change...

Atomic Mechanism of Liquid–Glass Transition for Ca7Mg3 Alloy

The atomic mechanism of liquid-glass transition for Ca(7)Mg(3) alloy during the rapid quenching processes is studied by the molecular dynamics simulations. The temperature dependences of structural, thermodynamic, and dynamic properties during the liquid-glass transition have been investigated. It is found that onset temperatures where these different properties begin to deviate from the equilibrium liquid are identical and near the melting temperature T(m). The liquid-glass transition temperatures in structure (T(g)(Str)) and dynamics (T(g)(Dyn)) are identical and higher than the calorimetric one (T(g)(Cal)), which are consistent with many experiments and computer simulations. The solid- and liquid-like atoms are defined by the Debye-Waller factor. It reveals that the solid-like atoms hold lower potential and higher degree of local order. On the basis of the evolution of solid-like atoms, the atomic mechanisms in structure, thermodynamics, and dynamics transition are systematically elucidated, which are consistent with the potential energy landscape.

Effect of pressure on thermal conductivity and pressure collapse of ice in a polymer-hydrogel and kinetic unfreezing at 1 GPa

We report a study of aqueous solutions of poly(vinylalcohol) and its hydrogel by thermal conductivity, κ, and specific heat measurements. In particular, we investigate (i) the changes in the solution and the hydrogel at 0.1 MPa observed in the 350-90 K range and of the frozen hydrogel at 130 K observed in the range from 0.1 MPa to 1.3 GPa, and (ii) the nature of the pressure collapse of ice in the frozen hydrogel and kinetic unfreezing on heating of its high density water at 1 GPa. The water component of the polymer solution on cooling either first phase separates and then freezes to hexagonal ice or freezes without phase separation and the dispersed polymer chains freeze-concentrate in nanoscopic and microscopic regions of the grain boundaries and grain junctions of the ice crystals in the frozen state of water in the hydrogel. The change in κ with temperature at 1 bar is reversible with some hysteresis, but not reversible with pressure after compression to 0.8 GPa at 130 K. At high pressures the crystallized state collapses showing features of κ and specific heat characteristic of formation of high density amorphous solid water. The pressure of structural collapse is 0.08 GPa higher than that of ice at 130 K. The slowly formed collapsed state shows kinetic unfreezing or glass-liquid transition temperature at 140 K for a time scale of 1 s. Comparison with the change in the properties observed for ice shows that κ decreases when the polymer is added.

Two-dimensional flux pinning in multilayered MgB 2/Ni thin films prepared by electron beam evaporation

Multilayered MgB 2/Ni thin films on Si substrates by electron beam evaporation were fabricated to enhance the critical current density J c in magnetic fields parallel to the surface. The magnetic field dependence of J c of the multilayered MgB 2/Ni thin films for B//surface shows a peak at the matching field. In the field angular dependence of J c, a sharp peak appears at B//surface. These results indicate that Ni layers which have ferromagnetism act as two-dimensional pinning centers in multilayered MgB 2/Ni thin films. The vortex glass–liquid transition temperature T g and the pinning parameter m were evaluated from the E– J characteristics. The slope of the magnetic field dependence of m in the multilayered MgB 2/Ni thin film under the matching field is larger than that over the matching field. In the T g/ T c– B phase diagram, the remarkable enhancement of T g for B//surface appears around the matching field. These features are characteristic behaviors for two-dimensional pinning centers.