Fragile Liquids Research Articles

Physicochemical properties and theoretical studies of novel fragile ionic liquids based on N-allyl-N,N-dimethylethylammonium cation

A series of novel ionic liquids (ILs) based on N-allyl-N,N-dimethylethylammonium (N112A+) are synthesized and characterized. Their physical properties and electrochemical stabilities are investigated and discussed toward the anion constituents. Herein, four anions are studied including one planar non-fluorinated anion (DCA−) and three fluorinated anions (TFSI−, OTf−, TFA−). The experimental properties are compared to those predicted by density functional theory (DFT) calculations. All salts are found to be low melting compounds while being liquid at room temperature. N112A-TFSI is the most thermally stable (Td = 329 °C) and N112A-TFA is the least stable (Td = 158 °C). The temperature effect on ILs transport properties is determined and discussed using the ionicity and Angell's fragility concepts. Among the good and fragile prepared ILs, N112A –DCA exhibits outstanding transport properties thanks to the low dynamic viscosity (19.97 mPa·s) and high ionic conductivity (19.20 mS·cm−1) reached at 298 K. These suitable properties are explained, from theoretical calculations, by the lower intermolecular interactions of ion-pairs in N112A –DCA and quantified by the decreased value of dispersion ion-pairs interaction energy (−6 kJ·mol−1). The electrochemical stability window (ESW) of selected ILs is strongly affected by the anion structure. The ESW values are found to decrease in the following order N112A-TFSI (4.40 V) > N112A-OTf (3.80 V) > N112A-DCA (3.00 V) > N112A-TFA (2.18 V). Taking into account the most likely oxidation and reduction reactions, the ESWs are discussed and validated by the density-based solvent model (SMD). The experimental values of ESWs are very close to the predicted ones.

Prediction of elemental glass-transition temperatures of metals from thermophysical properties of liquids

Abstract Since extremely high quench rates (1012 to 1014 K/s) are normally necessary to form elemental metallic glasses, very few are known. Even when synthesized, very little is known about their properties because of the limited quantities of material produced under such extreme conditions. In particular, the glass transition temperature, Tg, of most of the elemental glasses are unknown. Here, predictions of Tg for many elemental metals are made based on the properties of the liquids. The basis for these predictions comes from a number of recently observed correlations in metallic alloy liquids between the liquid fragility, thermal expansion coefficient, rate of change of liquid structure with temperature, and cohesive energy. Using those quantitative correlations and the properties of elemental liquids, T g ′ s for several metallic elements are predicted. Where experimental data are available, they are in reasonable agreement with these predictions.

Liquid fragility determination of oxide glass‐formers using temperature‐modulated DSC

AbstractGlass‐forming liquids exhibit a pronounced diversity in the viscosity‐temperature relation. This has been characterized by the liquid fragility index to quantify the extent of the non‐Arrhenian flow. Precise and accurate determination of liquid fragility is important for understanding a range of phenomena and controlling industrial glass melting processes. In this study, we use temperature‐modulated differential scanning calorimetry (TM‐DSC) to determine liquid fragility of a wide range of oxide compositions, including tellurites, borates, and silicates. We compare our fragility data to those determined using viscometry and the Moynihan DSC approach. We find that TM‐DSC is a useful method for determination of fragility, as it exhibits higher sensitivity and provides an easier and more reliable determination of characteristic temperatures compared to the Moynihan approach. Moreover, TM‐DSC is faster and requires smaller sample volume compared to the viscometric approach. However, we also observe that TM‐DSC tends to either overestimate or underestimate the fragility of very strong and highly fragile compositions, respectively.

Observation of a dynamical crossover in the shear relaxation processes in supercooled selenium near the glass transition.

Shear relaxation in supercooled selenium liquid near its glass transition over the viscosity range of 106 Pa s-1012 Pa s is studied using oscillatory parallel plate rheometry. The results demonstrate the presence of a slow, Debye-like relaxation process and a fast, cooperative relaxation process that are attributed, respectively, to the Se-Se bond scission/renewal dynamics and the segmental motion of selenium chains. The slow process displays a nearly-Arrhenius temperature dependence, while the fast process is strongly non-Arrhenius, and their combined contribution to viscosity is estimated using the Maxwell relation. The slow process is found to be coupled to viscous flow over the entire viscosity range. In contrast, the fast process becomes a major contributor to viscosity, and hence, to fragility only near Tg. This dynamical crossover is likely a fundamental characteristic of fragile liquids that represents a temperature dependent evolution of their free energy landscape. The fragility of supercooled selenium liquid appears to be remarkably closely linked to the temperature dependence of the shear modulus of the slow process, thus validating the prediction of the elastic shoving model.

U-based binary strong glass forming system

Fundamental issues of U-based binary metallic glasses (MGs) have been scarcely studied due to their low thermostability and glass forming ability (GFA). In this work, the glass formation in U–Ni system was reexamined. It is found that currently designed U–Ni glassy alloys in the range of 54–75% U show a pronounced thermal glass transition feature for the first time among binary U-based systems, and also have the glass transition temperature (Tg) above 600 K and much stronger thermostability than other similar U-based glasses. The Tg presence contributes to the evaluation of their glass transition activation energy, liquid fragility, viscosity as well as the Kauzmann temperature. Moreover, the fragility value of the typical alloy U69.2Ni30.8 is about 20, almost the same as those of the oxide glass formers SiO2 and GeO2, illustrating that U–Ni is a binary strong glass forming system. The reasons for the remarkable Tg presence and low fragility together with the GFA level of U-based MGs are discussed.

Fragility and aging behavior of SixSe1-x glasses and liquids.

The composition dependence of the fragility of SixSe1-x liquids with 0.05 ≤ x ≤ 0.33 is determined using the calorimetric method and is found to be rather similar to that characteristic of their Ge analogues. In addition, the nature and the time scale of the structural relaxation of the Si25Se75 glass during aging at 40 K below Tg are measured using Raman spectroscopy. The structural relaxation in this glass, which belongs to the so-called intermediate phase, involves progressive conversion of the doubly edge-shared SiSe4/2 tetrahedra E2 into singly edge-shared E1 and corner-shared E0 tetrahedra upon lowering of temperature. This tetrahedral speciation can be expressed in the form of the reaction 2 E2 → E0 + E1. The time scale of this tetrahedral conversion reaction corresponds well with that of shear relaxation. This result is inconsistent with the claim made previously in the literature that intermediate phase compositions do not undergo aging. Moreover, when taken together, the fragility and the structural relaxation results suggest that the constraint counting scheme typically adopted in the literature for edge- vs. corner-shared tetrahedra in chalcogenide networks may need to be revised. A rigid-polytope based constraint counting approach is shown to be more consistent with the experimental results.

Dramatic slowing down of the conformational equilibrium in the silyl derivative of glucose in the vicinity of the glass transition temperature.

The vitrification process is usually preceded by a significant change (around 6-8 decades) in the viscosity, structural relaxation times, or diffusion that occurs in a relatively small range of temperatures in fragile liquids. Along with this phenomenon, conformations of the molecules vary as well. In fact, this process is studied in bulk polymers and high molecular weight materials deposited in the form of thin films. On the other hand, spatial rearrangement of small glass formers in the supercooled liquid state has not been intensively investigated, so far. Herein, data obtained from measurements carried out using various experimental techniques on supercooled 1,2,3,4,6-penta-O-(trimethylsilyl)-d-glucopyranose (S-GLU) have revealed that rotations of silyl moieties along with the deformation in the saccharide ring are significantly slowed down in the vicinity of the glass transition temperature (Tg). These intramolecular reorganizations affect the structural relaxation time, atomic pair distribution function, integrated intensity, as well as a number of bands and signals observed, respectively, in the Raman and NMR spectra. Data reported herein offer a better understanding of the conformational variation and time scale of this process in the complex and flexible molecules around the Tg.

Reducing the Gap between the Activation Energy Measured in the Liquid and the Glassy States by Adding a Plasticizer to Polylactide.

The kinetic fragility of a glass-forming liquid is an important parameter to describe its molecular mobility. In most polymers, the kinetic fragility index obtained from the glassy state by thermally stimulated depolarization current is lower than the one determined in the liquid-like state by dielectric relaxation spectroscopy, as shown in this work for neat polylactide (PLA). When PLA is plasticized to different extents, the fragility calculated in the liquid-like state progressively decreases, until approaching the value of fragility calculated from the glass, which on the other hand remains constant with plasticization. Using the cooperative rearranging region (CRR) concept, it is shown that the decrease of the fragility in the liquid-like state is concomitant with a decrease of the cooperativity length. By splitting the fragility calculated in the liquid, in two contributions: volume and energetic, respectively, dependent and independent on cooperativity, we observed that the slope of the fragility plot in the glass is equivalent to the energetic contribution of the fragility in the liquid. It is then deduced that the difference between the slopes of the relaxation time dependence calculated in both glass and liquid is an indicator of the cooperative character of the segmental relaxation when transiting from liquid to glass. As the main structural consequence of plasticization lies in the decrease of interchain weak bonds, it is assumed that these bonds drive the size of the CRR. In contrast, the dynamics in the glass are independent on plasticization structural effects.

Intrinsic correlation of the plasticity with liquid behavior of bulk metallic glass forming alloys

Nowadays, plasticizing has become a critical issue for the engineering application of bulk metallic glasses (BMGs). What is the origin of plastic flow and how to evaluate the plasticity of BMGs with a unified standard are still open questions. Here we report a universal fragility criterion to scale the plasticity of BMGs from the equilibrium liquids and supercooled liquids. Based on our experimentally measured and previously reported viscosities at superheated and supercooled region for 51 glasses, the plasticity (ε) of BMGs has been found to positively correlate to the supercooling liquid fragility (m) with the equation of ε = 92exp[− 73/(m−15)], where ε is the plastic strain and m the fragility near glass transition temperature, but negatively correlate to the fragile-to-strong (F-S) transition tendency characterized by F-S transition factor and temperature. The intrinsic mechanism of the plasticity can be well interpreted in the frame of structural relaxation theory. This work provides a new insight into the microscopic essence of the plasticity of glassy solids and a universal approach to scale the fluidity of metallic glasses.

Evaluation of glass forming ability of Zr–Nb alloy systems through liquid fragility and Voronoi cluster analysis

In the present study, a molecular dynamics (MD) simulation-based investigation has been carried out for Zr-Nb binary system to analyze structural evolution during the glass formation along with its underlying physics for identifying suitable composition having better glass forming ability (GFA). The analysis is performed through the determination of liquid fragility, coordination numbers, and Voronoi cluster analysis. Results from Angell’s plot have shown that Zr50Nb50 specimen is a strong liquid and has the highest glass forming ability. It is revealed from Voronoi cluster analysis, that Zr50Nb50 metallic glass has a higher population fraction of icosahedral cluster, which also confirms its better glass forming ability. Apart from 〈0 0 12 0〉 icosahedral polyhedra, some Zr-centered clusters like 〈0 1 10 2〉, 〈0 2 8 5〉 and 〈0 2 8 4〉 are also found to be the key factors with respect to slowing down the dynamics of the system during liquid to glass transition. In addition, a pathway for Zr50Nb50 metallic glass formation is presented to show different stages of structural transformation from liquid to glass state.

Analysis of crystal growth and viscosity in Ge-Sb-Se-Te undercooled melts

A comprehensive analysis on crystal growth and viscosity is presented for the Ge2Sb2Se5-xTex (x = 0.5 and 1) undercooled melts in this article. Temperature dependence of the measured viscosities (107.5–1012.5 Pa·s) can be described by a simple exponential Arrhenius type equation in the relatively narrow temperature region in which the measurements were performed. Fragility of the measured materials are all roughly equal to 43, which means that these materials belong to the so called “intermediate” liquids (lying in the middle between strong and fragile liquids) and cannot be described by simple Arrhenius type equation if the temperature region is expanded from glass transition to melting, which is important for description of crystal growth. Therefore, MYEGA equation is used to extrapolate the viscosity data into the temperature region of studied crystal growth. Nucleation and crystal growth started on a surface of studied samples. The isothermal growth rate of the formed surface crystalline layer was measured at various temperatures. Analysis of the growth data revealed a crystal growth driven by liquid-crystal interface kinetics. Therefore, the standard crystal growth models were used for description of crystal growth rates in the studied systems.

Thermal Behavior and Slow Relaxation Dynamics in Amorphous Efavirenz: A Study by DSC, XRPD, TSDC, and DRS

The analysis of the thermal behavior of efavirenz showed a high glass-forming ability and good glass stability of this glass-forming liquid at room temperature. No polymorphic forms were formed either by cold crystallization or by recrystallization from solvent acetone. The determination of the dynamic fragility by the differential scanning calorimetry, thermally stimulated depolarization currents (TSDC), and dielectric relaxation spectroscopy (DRS) techniques is unanimous in suggesting efavirenz as a moderately fragile liquid. With DRS, secondary relaxations were detected, however, with weak intensities that did not allow the respective kinetic analysis; in contrast, TSDC allows clearly resolving the components of the secondary β-relaxation below Tg, with activation energies distributed between about 75 and 90 kJ mol−1 and Arrhenius prefactors of the order of 10−13 s. In this regard, the TSDC technique proved to be more effective compared to DRS in characterizing the secondary relaxation. The glass forming ability and glass stability found for efavirenz have been discussed in terms of various thermodynamic and kinetic parameters such as the reduced glass transition temperature, Tgred, the dynamic fragility, m, the stretching exponent, βKWW, the melting entropy, ΔSfus, and the molecular stiffness. The exceptionally low value of efavirenz fusion entropy was highlighted as a key feature of the thermal behavior of this glass-forming liquid.

Relations between the sub-molecula and the cooperative scales in the molecular dynamics of the glass-forming liquids

The molecules of the supercooled liquids in the cooperative molecular dynamics are partitioning to specific dynamic units, known as “beads”. The beads have been defined as the smallest units, forming the heat capacity of the supercooled liquids. The beads appear as a sub-molecular scale in the dynamics of the glass-forming liquids. In the Adam-Gibbs theory the number of molecules, rearranging simultaneously, has been defined as the size of the cooperative rearranging region and these regions form the second larger molecular scale in molecular dynamics. Due to a noticeable discrepancy in estimating the number of beads in the molecules by the empirical rules as well as by theoretical models the relations between these two scales are not well known. Recently, a new thermodynamic method for estimating the number of the beads in the molecules and a rule for the examination of their temperature dependence have been suggested. By these methods 48 low-molecular supercooled liquids are investigated and the relations between these two molecular scales and the structure of the molecules have been found. The estimated number of configuration beads is compared with the number of the structural units (the chemical groups, the ions and the atoms), with the assigned number by the empirical rules and with the ones calculated, based on a Lennar-Jones model of the random first order transition theory. A constant and two kinds of temperature dependence of the number of the beads in the molecules of different liquids have been revealed. A molecular basis, forming the fragility of the supercooled liquids, has been found. From the results obtained, the investigated liquids are classified in several groups following the logical line in the formation of the beads.

Correlating ultrafast calorimetry, viscosity, and structural measurements in liquid GeTe and Ge15Te85

Two distinct trends in the temperature dependence of viscosity, measured directly and inferred from calorimetry by analyzing crystallization kinetics, can be correlated with the temperature evolution of the height of the first peak of the x-ray total structure factor for liquid GeTe and ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$. The phase-change chalcogenide GeTe is a high-fragility liquid with the kinetic fragility value of 76, at the glass-transition temperature, being between those for liquid (Ag,In)-doped ${\mathrm{Sb}}_{2}\mathrm{Te}$ and ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. The viscosity of the high-temperature liquid shows Arrhenius kinetics on cooling to the melting point, and the structure factor conforms to the fragile liquid. For liquid ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$, the temperature evolution of the structure factor suggests a transition in the temperature range of about 100 K above the melting. The crystallization shows a wide range of Arrhenius kinetics in the supercooled liquid region. This finding combined with the dynamic viscosity measurements is interpreted by invoking a weak fragile-to-strong crossover on cooling the liquid ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$. The differences in structures and dynamics of liquid GeTe and ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$ appear closely correlated to their distinctly different crystallization mechanisms.

Modeling of delayed elasticity in glass

We have developed a numerical model based on finite element analysis (FEA) with a viscoelastic material model coupling stress relaxation and structural relaxation, using the Mauro-Allan-Potuzak (MAP) non-equilibrium viscosity equation as the shift function. A modeling study of the delayed elasticity behavior in glass under different equilibrium viscosity and non-equilibrium viscosity conditions is conducted. The delayed elastic response is found to be well described by a stretched exponential function with three parameters: the maximum delayed elasticity response, the retardation time of delayed elasticity response, and the stretching exponent of delayed elasticity response. The delayed elasticity magnitude is seen to increase with lower values of the stretching exponent bstress. At equilibrium viscosity, the retardation time shows a linear relationship with the stress relaxation time. However, when the temperature drops sharply in the non-equilibrium viscosity cases, the delayed elastic response may be frozen resulting in a lower magnitude for the delayed elasticity and the retardation time is not linear any more with the stress relaxation time. The delayed elasticity stretching exponent is seen to vary slightly at different relaxation times and normalized delayed elasticity response can roughly be collapsed into a single master curve. The impact of liquid fragility is also studied.

Density fluctuations with fractal order in metallic glasses detected by synchrotron X-ray nano-computed tomography

We report density fluctuations spanning from nano-to micrometer scale in the three-dimensional (3D) space in metallic glasses (MGs) detected with synchrotron X-ray nano-computed tomography, which decouple with the chemical homogeneity. The MG quenched from fragile supercooled liquid exhibits a more heterogeneous microstructure with a gradient density fluctuation. Severe inhomogeneous deformation rejuvenates the microstructure of the MG with the creation of a large amount of low-density regions, which percolate into shear-band-like layers. Ramified 2.5-dimensional fractal low-density regions exist in all the MGs with the correlation length increasing with microstructural heterogeneity. The formation of the density fluctuations with fractal order in the MGs is discussed based on the percolation of low-density regions and shear transformation zones during the glass transition and deformation processes. Our results directly demonstrate the existence of fractal low-density entropic droplets in MGs affected by fragilities and thermomechanical histories, which could be significant for understanding and controlling the disordered structure of amorphous solids.

A possible structural signature of the onset of cooperativity in metallic liquids.

It is widely, although not universally, believed that there must be a connection between liquid dynamics and the structure. Previous supporting studies, for example, have demonstrated a link between the structural evolution in the liquid and kinetic fragility. Here, new results are presented that strengthen the evidence for a connection. By combining the results from high-energy synchrotron X-ray scattering studies of containerlessly processed supercooled liquids with viscosity measurements, an accelerated rate of structural ordering beyond the nearest neighbors in the liquid is demonstrated to correlate with the temperature at which the viscosity transitions from Arrhenius to super-Arrhenius behavior. This is the first confirmation of predictions from several recent molecular dynamics studies.

Structural impact of nitrogen incorporation on properties of alkali germanophosphate glasses

AbstractThe structure, atomic packing density, calorimetric glass transition, and hardness of mixed sodium–lithium germanophosphate oxynitride glasses with varying Ge/P and N/P ratios were investigated. The combined influences of nitridation and mixed network former effect (MNFE) on the glass structure were analyzed using Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and 31P nuclear magnetic resonance (NMR) spectroscopy. Evidence for the existence of germanium in a higher coordination state, i.e., five‐ or sixfold coordination, was obtained by performing XPS analysis of the oxide glasses, with indication of conversion to tetrahedral coordination upon nitridation. Raman spectroscopy measurements implied that the germanate network was modified upon nitridation, including the removal of ring‐like germanate structures and P–O–Ge mixed linkages. The partial anionic N‐for‐O substitution gave rise to the linear dependence of the glass transition temperature (Tg) and hardness (HV) on nitrogen content (expressed as N/P ratio), especially for lower Ge/P ratio. However, nitridation also caused an unexpected increase in liquid fragility and decrease in density. This suggests that the governing structural parameter for property evolution in such LiNaGePON glasses is not only the increased degree of cross‐linking of the phosphate chains, but rather the short‐ and intermediate‐range structural modifications within the germanate component of the oxynitride glasses.

Femtosecond Raman-Induced Kerr Effect Study of Temperature-Dependent Intermolecular Dynamics in Molten Bis(trifluoromethylsulfonyl)amide Salts: Effects of Cation Species.

In this study, we have investigated the effects of cation structures on the temperature dependence of the intermolecular vibrational dynamics of ionic liquids using femtosecond Raman-induced Kerr effect spectroscopy. The ionic liquids used in this study are bis(trifluoromethylsulfonyl)amide [NTf2]- salts of the cations 1-butyl-3-methylimidazolium [C4MIm]+, 1-butyl-1-methylpyrrolidinium [Pyrr14]+, 1-butylpyridinium [C4Py]+, butyldiethylmethylammonium [N1224]+, triethyloctylammonium [N2228]+, and triethyloctylphosphonium [P2228]+. All of the ionic liquids show temperature-dependent low-frequency spectra. A difference in the temperature dependence between the spectra of the aromatic and nonaromatic cation based ionic liquids is especially significant. In the case of the aromatic cation based ionic liquids [C4MIm][NTf2] and [C4Py][NTf2], the spectral intensities in the low-frequency region below ca. 50 cm-1 increase and the high-frequency components at ca. 80 cm-1 shift to lower frequencies with rising temperature. In contrast, the ionic liquids based on nonaromatic cations only exhibit an increase in the low-frequency region below ca. 50 cm-1 with increasing temperature, while the high-frequency region of the spectra above ca. 50 cm-1 shows little change with variation of the temperature. These results suggest that the presence or absence of aromatic rings is the main factor in determining the temperature-dependent spectral features, particularly in the high-frequency region. We also found that the alkyl chain length and central atoms of the nonaromatic quaternary cations do not have much influence on the temperature-dependent spectral features. The first moments of the aromatic cation based ionic liquids are a little more sensitive to temperature than those of the nonaromatic cation based ionic liquids. The temperature-dependent viscosities and fragilities of the ionic liquids have also been examined.

Optimum Combination of Thermoplastic Formability and Electrical Conductivity in Al–Ni–Y Metallic Glass

Both thermoplastic formability and electrical conductivity of Al–Ni–Y metallic glass with 12 different compositions have been investigated in the present study with an aim to apply as a functional material, i.e. as a binder of Ag powders in Ag paste for silicon solar cell. The thermoplastic formability is basically influenced by thermal stability and fragility of supercooled liquid which can be reflected by the temperature range for the supercooled liquid region (ΔTx) and the difference in specific heat between the frozen glass state and the supercooled liquid state (ΔCp). The measured ΔTx and ΔCp values show a strong composition dependence. However, the composition showing the highest ΔTx and ΔCp does not correspond to the composition with the highest amount of Ni and Y. It is considered that higher ΔTx and ΔCp may be related to enhancement of icosahedral SRO near Tg during cooling. On the other hand, electrical resistivity varies with the change of Al contents as well as with the change of the volume fraction of each phase after crystallization. The composition range with the optimum combination of thermoplastic formability and electrical conductivity in Al–Ni–Y system located inside the composition triangle whose vertices compositions are Al87Ni3Y10, Al85Ni5Y10, and Al86Ni5Y9.