Fragile Liquids Research Articles

Structure-topology-property correlations of sodium phosphosilicate glasses.

In this work, we investigate the correlations among structure, topology, and properties in a series of sodium phosphosilicate glasses with [SiO2]/[SiO2 + P2O5] ranging from 0 to 1. The network structure is characterized by (29)Si and (31)P magic-angle spinning nuclear magnetic resonance and Raman spectroscopy. The results show the formation of six-fold coordinated silicon species in phosphorous-rich glasses. Based on the structural data, we propose a formation mechanism of the six-fold coordinated silicon, which is used to develop a quantitative structural model for predicting the speciation of the network forming units as a function of chemical composition. The structural model is then used to establish a temperature-dependent constraint description of phosphosilicate glass topology that enables prediction of glass transition temperature, liquid fragility, and indentation hardness. The topological constraint model provides insight into structural origin of the mixed network former effect in phosphosilicate glasses.

Anomalous Enthalpy Relaxation in Vitreous Silica

It is a challenge to calorimetrically determine the glass transition temperature (Tg) of vitreous silica. Here we demonstrate that this challenge mainly arises from the extreme sensitivity of the Tg to the hydroxyl content in vitreous silica, but also from the irreversibility of its glass transition when repeating the calorimetric scans. It is known that the liquid fragility (i.e., the speed of the viscous slow-down of a supercooled liquid at its Tg during cooling) has impact on enthalpy relaxation in glass. Here we find that vitreous silica (as a strong system) exhibits striking anomalies in both glass transition and enthalpy relaxation compared to fragile oxide systems. The anomalous enthalpy relaxation of vitreous silica is discovered by performing the hperquenching-annealing-calorimetry experiments. We argue that the strong systems like vitreous silica and vitreous Germania relax in a structurally cooperative manner, whereas the fragile ones do in a structurally independent fashion. We discuss the origin of the anomalous enthalpy relaxation in the HQ vitreous silica.

Structure-property relations in calcium aluminate glasses containing different divalent cations and SiO2

Calcium aluminate glasses are transparent in the infrared region, exhibit ultralow optical losses, and possess excellent mechanical properties. However, these systems exhibit poor glass-forming ability, and it is therefore important to understand their composition-structure-property relations to enable the design of new compositions with optimized properties and glass-forming ability. In this paper, we study the structure-property relationships in CaO–Al2O3 based glasses with different additives (SiO2, MgO, SrO, BaO, and ZnO). Our results reveal a gradual change in all the measured properties (molar volume, hardness, crack resistance, glass transition temperature, and liquid fragility) upon the addition of SiO2 to the eutectic 64CaO–36Al2O3 composition and the partial substitution of CaO for different divalent cation oxides. The relation between the measured liquid fragility index and the glass-forming ability is also investigated. We discuss these phenomena based on structural information obtained by Raman spectroscopy and topological considerations.

The kinetic fragility of liquids as manifestation of the elastic softening.

We show that the fragility m , the steepness of the viscosity and relaxation time close to the vitrification, increases with the degree of elastic softening, i.e. the decrease of the elastic modulus with increasing temperature, in a universal way. This provides a novel connection between the thermodynamics, via the modulus, and the kinetics. The finding is evidenced by numerical simulations and comparison with the experimental data of glassformers with widely different fragilities (33 ≤ m ≤ 115), leading to a fragility-independent elastic master curve extending over eighteen decades in viscosity and relaxation time. The master curve is accounted for by a cavity model pointing out the roles of both the available free volume and the cage softness. A major implication of our findings is that ultraslow relaxations, hardly characterised experimentally, become predictable by linear elasticity. As an example, the viscosity of supercooled silica is derived over about fifteen decades with no adjustable parameters.

Viscosity and fragility of the supercooled liquids and melts from the Fe–Co–B–Si–Nb and Fe–Mo–P–C–B–Si glass-forming alloy systems

The dynamic viscosity of four Fe-based bulk metallic glass-forming alloys, [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 (alloy A), {[(Fe0.5Co0.5)0.75B0.2Si0.05]0.96Nb0.04}99.5Cu0.5 (alloy B), Fe74Mo4P10C7.5B2.5Si2 (alloy C) and (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 (alloy D), was investigated as a function of temperature in the supercooled liquid region, as well as above the melting point. The alloy B is Cu-added alloy A, while the alloy D was obtained upon fine-tuning the alloy C composition. All these alloys may form bulk metallic glasses upon copper mold casting. The viscosities in the supercooled liquid region were calculated using the data obtained upon parallel plate rheometry measurements, as well as upon differential scanning calorimetry (DSC). The values of the supercooled fragility parameter m, 61, 66, 52 and 60 for the alloys A, B, C and D respectively, indicate that these alloys are intermediate glass formers. The behavior of the same alloys, in the molten state, was studied using a high temperature torsional oscillation cup viscometer. The values of the corresponding fragility parameter M was calculated as 5.03, 5.91, 4.25, 4.93 for the alloys A, B, C and D, respectively. They confirm the supercooled liquid behavior and predict that the alloys A and C may form glasses easier than the fine-tuned compositions B and D. Angell plot is constructed for the entire range of viscosities and the values from both regions, i.e. above melting point and supercooled liquid region, fit well with the model.

Correlation between density of metallic glasses and dynamic fragility of metallic glass-forming liquids

We study the density evolution upon isothermal annealing far below the glass transition temperature of various metallic glasses (MGs) with different fragilities. We show that the density variation, which reflects the structural heterogeneity of MGs, has correlations with the Poisson's ratio and the kinetic fragility of their supercooled liquid state. The found relationship between the structure heterogeneity of metallic glasses and dynamic heterogeneity of their supercooled liquids has implications for understanding the structural heterogeneity and heredity, mechanical behaviors, and nature of metallic glass.

Physics of the Jagla model as the liquid-liquid coexistence line slope varies.

The slope of the coexistence line of the liquid-liquid phase transition can be positive, negative, or zero. All three possibilities have been found in Monte-Carlo simulations of a modified spherically symmetric two-scale Jagla model. Since the liquid-liquid critical point frequently lies in a region of the phase diagram that is difficult to access experimentally, it is of great interest to study critical phenomena in the supercritical region. We therefore study the properties of the Widom line, defined in the one-phase region above the critical point as an extension of the coexistence line near which the loci of various response functions extrema asymptotically converge with each other. This phenomenon is predicted by the scaling theory according to which all response functions can be expressed asymptotically in the vicinity of a critical point as functions of the diverging correlation length. We find that the method of identifying the Widom line as the loci of heat capacity maxima becomes unfruitful when the slope of the coexistence line approaches zero in the T-P plane. In this case, the specific heat displays no maximum in the one-phase region because, for a horizontal phase coexistence line, according to the Clapeyron equation, the enthalpy difference between the coexisting phases is zero, and thus the critical fluctuations do not contribute to enthalpy fluctuations. The extension of the coexistence line beyond the critical point into the one-phase region must in this case be performed using density fluctuations. Although the line of compressibility maxima bifurcates into a symmetrical pair of lines, it remains well-defined. We also study how the glass transition changes as the slope of the coexistence line in the T-P plane approaches zero. We find that for the case of positive slopes, diffusivity shows a fragile-to-strong transition upon crossing the Widom line, while for horizontal slope, diffusivity shows the behavior typical for fragile liquids.

Correlation Between Superheated Liquid Fragility And Onset Temperature Of Crystallization For Al-Based Amorphous Alloys

Abstract Amorphous alloys or metallic glasses have attracted significant interest in the materials science and engineering communities due to their unique physical, mechanical, and chemical properties. The viscous flow of amorphous alloys exhibiting high strain rate sensitivity and homogeneous deformation is considered to be an important characteristic in thermoplastic forming processes performed within the supercooled liquid region because it allows superplastic-like deformation behavior. Here, the correlation between the superheated liquid fragility, and the onset temperature of crystallization for Al-based alloys, is investigated. The activation energy for viscous flow of the liquid is also investigated. There is a negative correlation between the parameter of superheated liquid fragility and the onset temperature of crystallization in the same Al-based alloy system. The activation energy decreases as the onset temperature of crystallization increases. This indicates that the stability of a superheated liquid can affect the thermal stability of the amorphous alloy. It also means that a liquid with a large superheated liquid fragility, when rapidly solidified, forms an amorphous alloy with a low thermal stability.

Strain induced fragility transition in metallic glass.

Relaxation dynamics are the central topic in glassy physics. Recently, there is an emerging view that mechanical strain plays a similar role as temperature in altering the relaxation dynamics. Here, we report that mechanical strain in a model metallic glass modulates the relaxation dynamics in unexpected ways. We find that a large strain amplitude makes a fragile liquid become stronger, reduces dynamical heterogeneity at the glass transition and broadens the loss spectra asymmetrically, in addition to speeding up the relaxation dynamics. These findings demonstrate the distinctive roles of strain compared with temperature on the relaxation dynamics and indicate that dynamical heterogeneity inherently relates to the fragility of glass-forming materials.

Structural and topological aspects of borophosphate glasses and their relation to physical properties.

We establish a topological model of alkali borophosphate and calcium borophosphate glasses, which describes the effect of both the network formers and network modifiers on physical properties. We show that the glass transition temperature (Tg), Vickers hardness (HV), liquid fragility (m), and isobaric heat capacity jump at Tg (ΔCp) of these glasses are related to the network topology, which is determined by structure of the glass. Therefore, we also demonstrate that the temperature dependent constraint theory can quantitatively explain the mixed network former effect in borophosphate glasses. The origin of the effect of the type of network modifying oxide on Tg, HV, m, and ΔCp of calcium borophosphate glasses is revealed in terms of the modifying ion sub-network. The same topological principles quantitatively explain the significant differences in physical properties between the alkali and the calcium borophosphate glasses. This work has implications for quantifying structure-property relations in complex glass forming systems containing several types of network forming and modifying oxides.

Linking structure to fragility in bulk metallic glass-forming liquids

Using in-situ synchrotron X-ray scattering, we show that the structural evolution of various bulk metallic glass-forming liquids can be quantitatively connected to their viscosity behavior in the supercooled liquid near Tg. The structural signature of fragility is identified as the temperature dependence of local dilatation on distinct key atomic length scales. A more fragile behavior results from a more pronounced thermally induced dilatation of the structure on a length scale of about 3 to 4 atomic diameters, coupled with shallower temperature dependence of structural changes in the nearest neighbor environment. These findings shed light on the structural origin of viscous slowdown during undercooling of bulk metallic glass-forming liquids and demonstrate the promise of predicting the properties of bulk metallic glasses from the atomic scale structure.

Rigidity and soft percolation in the glass transition of an atomistic model of ionic liquid, 1-ethyl-3-methyl imidazolium nitrate, from molecular dynamics simulations--Existence of infinite overlapping networks in a fragile ionic liquid.

The typical ionic liquid, 1-ethyl-3-methyl imidazolium nitrate (EMIM-NO3), was examined by molecular dynamics simulations of an all-atomistic model to show the characteristics of networks of cages and/or bonds in the course of vitrification of this fragile glass-former. The system shows changes of dynamics at two characteristic temperatures, TB (or Tc) and the glass transition temperature Tg, found in other fragile glass forming liquids [K. L. Ngai and J. Habasaki, J. Chem. Phys. 141, 114502 (2014)]. On decreasing temperature, the number of neighboring cation-anion pairs, NB, within the first minimum of the pair correlation function, g(r)min, increases. On crossing TB (>Tg), the system volume and diffusion coefficient both show changes in temperature dependence, and as usual at Tg. The glass transition temperature, Tg, is characterized by the saturation of the total number of "bonds," NB and the corresponding decrease in degree of freedom, F = [(3N - 6) - NB], of the system consisting of N particles. Similar behavior holds for the other ion-ion pairs. Therefore, as an alternative, the dynamics of glass transition can be interpreted conceptually by rigidity percolation. Before saturation occurring at Tg, the number of bonds shows a remarkable change at around TB. This temperature is associated with the disappearance of the loosely packed coordination polyhedra of anions around cation (or vice versa), related to the loss of geometrical freedom of the polyhedra, fg, of each coordination polyhedron, which can be defined by fg = [(3NV - 6) - Nb]. Here, 3Nv is the degree of freedom of NV vertices of the polyhedron, and Nb is number of fictive bonds. The packing of polyhedra is characterized by the soft percolation of cages, which allows further changes with decreasing temperature. The power spectrum of displacement of the central ion in the cage is found to be correlated with the fluctuation of Nb of cation-cation (or anion-anion) pairs in the polyhedron, although the effect from the coordination shells beyond the neighboring ions is not negligible.

Thermal Properties of Alkyl-triethylammonium bis $$\{$$ { (trifluoromethyl)sulfonyl $$\}$$ } imide Ionic Liquids

In this work, we address the thermal properties of selected members of a homologous series of alkyltriethylammonium bisf(trifluoromethyl)sulfonylgimide ionic liquids. Their phase and glass transition behavior, as well as their standard isobaric heat capacities at 298.15 K, were studied using differential scanning calorimetry (DSC), whereas their decomposition temperature was determined by thermal gravimetry analysis. DSC was further used to measure standard molar heat capacities of the studied ionic liquids and standard molar heat capacity as a function of temperature for hexyltriethylammonium, octyltriethylammonium, and dodecyltriethylammonium bisf(trifluoromethyl)sulfonylg- imide ionic liquids. Based on the data obtained, we discuss the influence of the alkyl chain length of the cation on the studied ionic liquids on the measured properties. Using viscosity data obtained in a previous work, the liquid fragility of the ionic liquids is then discussed. Viscosity data were correlated by the VTF equation using a robust regression along a gnostic influence function. In this way, more reliable VTF model parameters were obtained

Applying Vogel–Fulcher–Tammann relationship in crystallization kinetics of amorphous alloys

The Vogel–Fulcher–Tammann relationship is a generic equation for describing the transport properties of supercooled fragile liquids. Coupling this reasonable equation, a kinetic model is derived in the present work for describing the crystallization kinetics of metallic glass in supercooled liquid region. In the modeling, a constant value for apparent activation energy is substituted by a temperature-dependent one, so that a model is carried forward to describe more general solid-state phase transformation kinetics. Then, recipes of kinetic analysis are proposed, and information about liquid fragility is obtained. Applying the newly proposed model, the evolution of apparent activation energy during the crystallization of many amorphous alloys can be explained. Finally, the crystallization kinetics of Zr55Cu30Al10Ni5 bulk metallic glass, respectively measured by isothermal and non-isothermal differential scanning calorimetry, is studied and interpreted using the present model.

Impact of Yb3+/Er3+ ions on crystallization of phosphosilicate glass melts

Transparent glass ceramics were prepared from the phosphosilicate system by melt-quenching devitrification (MQD) method, i.e., nanocrystals spontaneously form during cooling of the melts. Introduction of 2.5 wt.% Yb2O3 and 0.5 wt.% Er2O3 into the glass melt induced the change of type and concentration of crystals. The comparison of rheological and thermodynamic properties of both undoped and Yb3+/Er3+ doped melts showed that addition of Yb3+/Er3+ oxides caused increase of liquid fragility, and degree of medium-range order. In addition, the thermodynamic barriers for nucleation ΔG* as a function of reduced temperature T/Tm were calculated with an assumption of wetting angle θ=90°, Yb3+/Er3+ doped melt tended to firstly nucleate as compared to undoped melt at small undercooling.

Role of elastic deformation in determining the mixed alkaline earth effect of hardness in silicate glasses

Glasses deform permanently as a result of indentation and the total resistance to deformation consists of three individual resistances, i.e., those to elastic deformation, densification, and plastic flow. The link between Vickers hardness and the resistances to densification and plastic flow has been investigated previously, but the link between the resistance to elastic deformation and hardness has not yet been studied. In this work, we investigate the link between elastic deformation during indentation and Vickers hardness in a series of mixed magnesium-barium boroaluminosilicate glasses. We show that the mixed alkaline earth effect manifests itself as deviations from linearity in shear modulus, Poisson's ratio, glass transition temperature, liquid fragility index, hardness, volume of densification, and volume of plastic flow. We find no correlation between the elastic part of the indentation and hardness, and we thus infer that elastic deformation does not play a dominant role in determining the mixed alkaline earth effect of hardness. However, interestingly, we find a strong correlation between Poisson's ratio, volume of plastic flow, and hardness, by which the minimum in hardness could be explained in terms of a minimum in shear viscosity.

Atomic Distribution in Catalytic Amorphous Metals

The atomic distribution in catalytically active metallic glass alloys, Pd43Cu27Ni10P20 and Pt57.5Cu14.7Ni5.3P22.5, was investigated using three‐dimensional atom probe microscopy. Atom probe analysis showed uniform distribution of constituent elements for both the starting amorphous alloys, with no phase separation. Both the crystallized alloys showed eutectic microstructure with a very sharp interface (~0.5 nm as determined from atom probe). The atomic distribution in the devitrified state is explained based on the “fragile liquid” behavior for these noble‐metal glassy alloys.

Relationship between particle elasticity, glass fragility, and structural relaxation in dense microgel suspensions.

"Fragile" glassy materials, which include most polymeric materials and organic liquids, exhibit a steep and super-Arrhenius dependence of relaxation time with temperature upon the glass transition and have been extensively studied. Yet, a full understanding of strong glass formers that exhibit an Arrhenius dependence on temperature is still lacking. In this work, we have investigated the glassy dynamics of poly(N-isopropylacrylamide) (PNIPAM) microgel particles of varied elasticity in dense aqueous suspensions, giving rise to a full spectrum of strong to fragile glass-forming behaviors. We have observed the dependence of particle motions and structural relaxation on particle volume fraction can be weakened by decreasing particle elasticity, due to particle deformation and the resulting interparticle elastic interaction upon intimate particle contacts at high particle concentration. Both measured α-relaxation time scales and dynamic length scales for cooperative rearranging motions of microgels in suspensions show similarly dependence on particle volume fraction and elasticity, thereby quantifying the glass fragility of dense microgel suspension of varied particle elasticity.

Critical V2O5/TeO2 Ratio Inducing Abrupt Property Changes in Vanadium Tellurite Glasses.

Transition metal containing glasses have unique electrical properties and are therefore often used for electrochemical applications, such as in batteries. Among oxide glasses, vanadium tellurite glasses exhibit the highest electronic conductivity and thus the high potential for applications. In this work, we investigate how the dynamic and physical properties vary with composition in the vanadium tellurite system. The results show that there exists a critical V(2)O(5) concentration of 45 mol %, above which the local structure is subjected to a drastic change with increasing V(2)O(5), leading to abrupt changes in both hardness and liquid fragility. Electronic conductivity does not follow the expected correlation to the valence state of the vanadium as predicted by the Mott-Austin equation but shows a linear correlation to the mean distance between vanadium ions. These findings could contribute to designing optimum vanadium tellurite compositions for electrochemical devices. The work gives insight into the mechanism of electron conduction in the vanadium tellurite systems.

Heterophase liquid states: Thermodynamics, structure, dynamics

An overview of theoretical results and experimental data on the thermodynamics, structure and dynamics of the heterophase glass-forming liquids is presented. The theoretical approach is based on the mesoscopic heterophase fluctuations model (HPFM) developed within the framework of the bounded partition function approach. The Fischer cluster phenomenon, glass transition, liquid-liquid transformations, parametric phase diagram, cooperative dynamics and fragility of the glass-forming liquids is considered.