Fragile Liquids Research Articles

High density liquid structure enhancement in glass forming aqueous solution of LiCl.

We investigate using molecular dynamics simulations the dynamical and structural properties of LiCl:6H2O aqueous solution upon supercooling. This ionic solution is a glass forming liquid of relevant interest in connection with the study of the anomalies of supercooled water. The LiCl:6H2O solution is easily supercooled and the liquid state can be maintained over a large decreasing temperature range. We performed simulations from ambient to 200 K in order to investigate how the presence of the salt modifies the behavior of supercooled water. The study of the relaxation time of the self-density correlation function shows that the system follows the prediction of the mode coupling theory and behaves like a fragile liquid in all the range explored. The analysis of the changes in the water structure induced by the salt shows that while the salt preserves the water hydrogen bonds in the system, it strongly affects the tetrahedral hydrogen bond network. Following the interpretation of the anomalies of water in terms of a two-state model, the modifications of the oxygen radial distribution function and the angular distribution function of the hydrogen bonds in water indicate that LiCl has the role of enhancing the high density liquid component of water with respect to the low density component. This is in agreement with recent experiments on aqueous ionic solutions.

Communication: Observation of ultra-slow relaxation in supercooled selenium and related glass-forming liquids.

The rheological behavior of supercooled Se, As10Se90, and As20Se80 liquids is studied in the linear regime as a function of frequency, extending over nearly 11 orders of magnitude, using oscillatory parallel plate rheometry. While the viscoelastic response of the As20Se80 liquid is characterized by a single relaxation time scale, the Se and the As10Se90 liquids display two distinct relaxation processes, both of which are coupled to viscosity, although their time scales differ by nearly 3-5 orders of magnitude. The ultra-slow relaxation process appears to be related to the dynamics of -Se-Se-Se- chain segments in the structure of these liquids, with characteristic time scale and shear modulus that are dependent on the average chain length. The fast mode, on the other hand, is associated with the glassy modulus and is tentatively assigned to a Johari-Goldstein β-process. These results, when taken together, are consistent with the presence of a hierarchical free energy landscape that characterizes the dynamics of the fragile Se and As10Se90 liquids.

Revealing the Link between Structural Relaxation and Dynamic Heterogeneity in Glass-Forming Liquids.

Despite the use of glasses for thousands of years, the nature of the glass transition is still mysterious. On approaching the glass transition, the growth of dynamic heterogeneity has long been thought to play a key role in explaining the abrupt slowdown of structural relaxation. However, it still remains elusive whether there is an underlying link between structural relaxation and dynamic heterogeneity. Here, we unravel the link by introducing a characteristic time scale hiding behind an identical dynamic heterogeneity for various model glass-forming liquids. We find that the time scale corresponds to the kinetic fragility of liquids. Moreover, it leads to scaling collapse of both the structural relaxation time and dynamic heterogeneity for all liquids studied, together with a characteristic temperature associated with the same dynamic heterogeneity. Our findings imply that studying the glass transition from the viewpoint of dynamic heterogeneity is more informative than expected.

Temperature dependence of the elastic properties of lead-borate glasses using Brillouin spectroscopy

Macroscopic elastic properties of different compositions of lead-borate [xPbO:(1-x)B2O3] glasses were investigated in the temperature range from 293 to 1173 K using Brillouin light scattering technique. Temperature dependence of the sound velocities of each of the glass composition remains almost constant in the glassy state and exhibit a rapid reduction in velocity above the glass transition temperature, Tg. The slopes of the temperature-dependence of the sound velocities above Tg infers the fragility in the glass-forming liquids, and exhibits a decreasing trend for x > 0.4. The scaled sound velocities within the glassy regime were found to fall on two separate master curves: one for the samples with 0.2 ≤ x ≤ 0.6, and another with x = 0.7, 0.8. This shows that the variation of the glass structure with temperature varies uniquely for the first and second set of samples. It further implies that the temperature dependence of the elastic behaviour within the glassy state is independent of the amount of modifier up to x = 0.6, and marks the regime in x above which the Pb–Pb correlations increase and PbO acts as a glass former. The first set of samples exhibits the presence of residual stresses owing to more open-volume structure compared to the second set of samples which do not exhibit residual stresses.

Analysis and characterization of the transition from the Arrhenius to non-Arrhenius structural relaxation in fragile glass-forming liquids

We have studied the transition from an Arrhenius-like to a non-Arrhenius-like structural relaxation behavior in fragile glass-forming liquids. This transition is denoted by the temperature TA that usually occurs above the melting point Tm and the dynamic crossover temperature TB. Recent studies reveal that TA is a characteristic temperature related with the dynamical properties of the system. However, its unambiguous determination is not easy. In this work, a method to obtain the temperature TA from the experimental data of α-relaxation time is presented. The obtained TA is compared with the cooperativity onset temperature Tx extracted from the bond strength–coordination number fluctuation model. The result reveals that TA is close to Tx for fragile liquids. From the result of the present analyses combined with the linear relation Tx $$\propto$$ T0, where T0 is the Vogel temperature, the Arrhenius crossover phenomenon in fragile liquids is linked to the low-temperature structural relaxation dynamics.

Parametric study of temperature-modulated differential scanning calorimetry for high-temperature oxide glasses with varying fragility

Differential scanning calorimetry (DSC) has proven to be a highly versatile technique for understanding the glass transition, relaxation, and crystallization behavior of inorganic glasses. However, the approach is challenging when probing glass samples that exhibit overlapping transitions or low sensitivity. To overcome these problems, temperature-modulated DSC (TM-DSC) can be utilized, in which a sinusoidal heating rate is superimposed on the linear heating rate known from standard linear DSC. Until recently, it has only been possible to perform TM-DSC measurements on commercial instruments at temperatures below 973 K, which is insufficient for many oxide glasses of industrial interest, particularly silicate glasses. However, recent commercially available software now enables TM-DSC measurements to be performed at temperatures far exceeding 973 K. To investigate the suitability of using TM-DSC to study glass transition and relaxation behavior in high-temperature silicate systems, we have performed systematic TM-DSC measurements on three different oxide glass systems with varying glass transition temperature and liquid fragility. We find that relatively large underlying heating rates (2–5 K/min) and modulation amplitudes (4–5 K) are needed in order to obtain data with high signal-to-noise ratios. For these combinations of experimental parameters, we also observe a linear response as found using Lissajous curves. Overall, this study suggests that TM-DSC is a promising technique for investigating the dynamics of high-temperature oxide glass systems with a wide range of liquid fragilities.

Structure and Liquid Fragility in Sodium Carbonate.

The relationship between local structure and dynamics is explored for molten sodium carbonate. A flexible fluctuating-charge model, which allows for changes in the shape and charge distribution of the carbonate molecular anion, is developed. The system shows the evolution of highly temperature-dependent complex low-dimensional structures which control the dynamics (and hence the liquid fragility). By varying the molecular anion charge distribution, the key interactions responsible for the formation of these structures can be identified and rationalized. An increase in the mean charge separation within the carbonate ions increases the connectivity of the emerging structures and leads to an increase in the system fragility.

Predicting glass-to-glass and liquid-to-liquid phase transitions in supercooled water using classical nucleation theory

Glass-to-glass and liquid-to-liquid phase transitions are observed in bulk and confined water, with or without applied pressure. They result from the competition of two liquid phases separated by an enthalpy difference depending on temperature. The classical nucleation equation of these phases is completed by this quantity existing at all temperatures, a pressure contribution, and an enthalpy excess. This equation leads to two homogeneous nucleation temperatures in each liquid phase; the first one (Tn- below Tm) being the formation temperature of an “ordered” liquid phase and the second one corresponding to the overheating temperature (Tn+ above Tm). Thermodynamic properties, double glass transition temperatures, sharp enthalpy and volume changes are predicted in agreement with experimental results. The first-order transition line at TLL = 0.833 × Tm between fragile and strong liquids joins two critical points. Glass phase above Tg becomes “ordered” liquid phase disappearing at TLL at low pressure and at Tn+ = 1.302 × Tm at high pressure.

Molecular simulations of fragility of linear and ring polymers

Nowadays, investigation on fragility in glass-forming liquids is a hot topic in the field of condensed matter physics, since the understanding of its microscopic mechanism is useful to uncover the underlying mechanism of the glass transition. In the present paper, we performed dynamic Monte Carlo simulations to explore the correlations between fragility index and conformational changes in linear and ring polymer chains. Compared with ring chains, linear chains exhibit a higher fragility index. In addition, for both ring and linear chains, during approaching the glass transition temperature (Tg), the fraction of segments with trans-conformation increased significantly. To reveal the effect of conformational changes on segmental dynamics during relaxation, a new parameter, the aggregation region of segments with trans-conformation (ARST), is proposed. The maximum ARST has the lowest mobility, which determines the relaxation time of polymers. The fragility index has a strong relationship with the maximum ARST. For linear chains, during approaching Tg, the maximum ARST grew more abruptly, resulting in the larger fragility index.

Crystal growth and viscous flow in barium disilicate glass

Decoupling between the diffusion coefficients that control crystal growth (DU) and viscous flow (Dη) in deeply supercooled glass-forming liquids is one of the most celebrated, still unsolved, phenomena in glass science because it strongly impacts the analysis and prediction of crystallization kinetics. To further understand the decoupling phenomenon, we measured viscosity and crystal growth rates (of the crystalline surface layer growth and internally nucleated crystals) of a supercooled liquid with barium disilicate composition. We performed all measurements using glass samples from the same batch, a rare case that we call a “clean” experiment. This type of analysis is uncommon, and its aim is to minimize the uncertainty due to the composition variation that is inherent in laboratory glass production. The decoupling temperature (Td), the glass transition temperature (Tg), the melting point (Tm), and the liquid fragility index were obtained and successfully used to test a recently derived relationship which allows one to estimate Td by knowing only Tm and the viscosity curve. We also show, for the first time, that the reduced decoupling parameter, which gauges the degree of deviation between DU and Dη, does not depend on the crystal growth mechanism for the cases of Normal or Screw Dislocation growth.

Pathways for diffusion in the potential energy landscape of the network glass former SiO2.

We study the dynamical behaviour of a computer model for viscous silica, the archetypal strong glass former, and compare its diffusion mechanism with earlier studies of a fragile binary Lennard-Jones liquid. Three different methods of analysis are employed. First, the temperature and time scale dependence of the diffusion constant is analysed. Negative correlation of particle displacements influences transport properties in silica as well as in fragile liquids. We suggest that the difference between Arrhenius and super-Arrhenius diffusive behaviour results from competition between the correlation time scale and the caging time scale. Second, we analyse the dynamics using a geometrical definition of cage-breaking transitions that was proposed previously for fragile glass formers. We find that this definition accurately captures the bond rearrangement mechanisms that control transport in open network liquids, and reproduces the diffusion constants accurately at low temperatures. As the same method is applicable to both strong and fragile glass formers, we can compare correlation time scales in these two types of systems. We compare the time spent in chains of correlated cage breaks with the characteristic caging time and find that correlations in the fragile binary Lennard-Jones system persist for an order of magnitude longer than those in the strong silica system. We investigate the origin of the correlation behaviour by sampling the potential energy landscape for silica and comparing it with the binary Lennard-Jones model. We find no qualitative difference between the landscapes, but several metrics suggest that the landscape of the fragile liquid is rougher and more frustrated. Metabasins in silica are smaller than those in binary Lennard-Jones and contain fewer high-barrier processes. This difference probably leads to the observed separation of correlation and caging time scales.

Evaluating the correlation between liquid fragility and glass-forming ability in the extremely strong Ce-based bulk metallic glasses

This investigation focuses on liquid fragility (m) and its correlation with glass-forming ability (GFA) of Ce-Ga-Cu, Ce-Al-Cu-Ni and newly developed Ce-Ga-Cu-Ni bulk metallic glass (BMG) systems. The present Ce-based BMGs belong to the extremely strong glass-forming system, whose fragility m values lie in a strong liquid range of 18–33. The quantitative correlation between m and GFA in the present Ce-based BMGs cannot be described well by previous criteria proposed by Senkov [Phys. Rev. B 76, 104202 (2007)], Wang [Appl. Phys. Lett. 100, 261913 (2012)], and Johnson [Nat. Commun. 7, 10313 (2016)]. By considering both the stability of the liquid phase and the stability of the competing crystalline phases, it is found that the stability of the competing crystalline phases plays a dominant role in the determination of GFA of strong Ce-based BMGs. And, the influence of reduced glass transition temperature (Trg) on GFA would be submerged by that of fragility m when m is below ∼30. The results help in further understanding the underlying correlation between fragility and GFA in metallic glasses, especially for the strong glass-forming liquids.

On the Fragility of Bulk Metallic Glass Forming Liquids

In contrast to pure metals and most non-glass forming alloys, metallic glass-formers are moderately strong liquids in terms of fragility. The notion of fragility of an undercooling liquid reflects the sensitivity of the viscosity of the liquid to temperature changes and describes the degree of departure of the liquid kinetics from the Arrhenius equation. In general, the fragility of metallic glass-formers increases with the complexity of the alloy with differences between the alloy families, e.g., Pd-based alloys being more fragile than Zr-based alloys, which are more fragile than Mg-based alloys. Here, experimental data are assessed for 15 bulk metallic glasses-formers including the novel and technologically important systems based on Ni-Cr-Nb-P-B, Fe-Mo-Ni-Cr-P-C-B, and Au-Ag-Pd-Cu-Si. The data for the equilibrium viscosity are analyzed using the Vogel–Fulcher–Tammann (VFT) equation, the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) equation, and the Adam–Gibbs approach based on specific heat capacity data. An overall larger trend of the excess specific heat for the more fragile supercooled liquids is experimentally observed than for the stronger liquids. Moreover, the stronger the glass, the higher the free enthalpy barrier to cooperative rearrangements is, suggesting the same microscopic origin and rigorously connecting the kinetic and thermodynamic aspects of fragility.

Study of dynamical heterogeneities in colloidal nanoclay suspensions approaching dynamical arrest

The dynamics of aqueous Laponite clay suspensions slow down with increasing sample waiting time (tw). This behavior, and the material fragility that results, closely resemble the dynamical slowdown in fragile supercooled liquids with decreasing temperature, and are typically ascribed to the increasing sizes of distinct dynamical heterogeneities in the sample. In this article, we characterize the dynamical heterogeneities in Laponite suspensions by invoking the three-point dynamic susceptibility formalism. The average time-dependent two-point intensity autocorrelation and its sensitivity to tw are probed in dynamic light scattering experiments. Distributions of relaxation time scales, deduced from the Kohlrausch-Williams-Watts equation, are seen to widen with increasing tw. The calculated three-point dynamic susceptibility of Laponite suspensions exhibits a peak, with the peak height increasing with evolving tw at fixed volume fraction or with increasing volume fraction at fixed tw, thereby signifying the slowdown of the sample dynamics. The number of dynamically correlated particles, calculated from the peak-height, is seen to initially increase rapidly with increasing tw, before eventually slowing down close to the non-ergodic transition point. This observation is in agreement with published reports on supercooled liquids and hard sphere colloidal suspensions and offers a unique insight into the colloidal glass transition of Laponite suspensions.

Experimental and ab initio molecular dynamics study of the structure and physical properties of liquid GeTe

GeTe is a prototypical phase-change material employed in data storage devices. In this work, the atomic structure of liquid GeTe is studied by x-ray and neutron diffraction in the temperature range from 1197 to 998 K. The dynamic viscosity is measured from 1273 to 953 K, which is 55 K below the solidification point, using an oscillating-cup viscometer. The density of liquid GeTe between 1293 and 973 K is determined by the high-energy $\ensuremath{\gamma}$-ray attenuation method. The experiments are complemented with ab initio molecular dynamics (AIMD) simulations based on density functional theory (DFT). Compatibility of the AIMD-DFT models with the diffraction data is proven by simultaneous fitting of all data sets in the frame of the reverse Monte Carlo simulation technique. It is shown that octahedral order dominates in liquid GeTe, although tetrahedral structures are also present. The viscosity of the equilibrium and weakly undercooled liquid GeTe obeys the Arrhenius law with a small activation energy of the order of 0.3 eV, which is indicative of a highly fragile liquid. The calculated density of states and electronic wave functions point to the existence of a pseudogap and localized electron states within the gap in the equilibrium liquid near the melting point as well as in the undercooled liquid.

Strength of the repulsive part of the interatomic potential determines fragility in metallic liquids.

The dynamical behaviour of liquids is frequently characterized by the fragility, which can be defined from the temperature dependence of the shear viscosity, η (ref.). For a strong liquid, the activation energy for η changes little with cooling towards the glass transition temperature, Tg. The change is much greater in fragile liquids, with the activation energy becoming very large near Tg. While fragility is widely recognized as an important concept-believed, for example, to play an important role in glass formation-the microscopic origin of fragility is poorly understood. Here, we present new experimental evidence showing that fragility reflects the strength of the repulsive part of the interatomic potential, which can be determined from the steepness of the pair distribution function near the hard-sphere cutoff. On the basis of an analysis of scattering data from ten different metallic alloy liquids, we show that stronger liquids have steeper repulsive potentials.

Recent progress in understanding high temperature dynamical properties and fragility in metallic liquids, and their connection with atomic structure

Abstract

Mixed alkaline‐earth effect on the mechanical and rheological properties of Ca–Mg silicate glasses

AbstractIn this article, we investigate the mixed alkaline‐earth effect in a silicate glass series with varying the molar ratio of [MgO]/([CaO]+[MgO]). This effect manifests itself as a minimum in Vickers microhardness (HV), coefficient of thermal expansion (CTE), and isokom temperatures at 1012(Tg) and 102 Pa·s, and as a maximum in liquid fragility. To probe the structural origin of the mixed alkaline‐earth effect in CTE and Hv, we conducted the Raman measurements. In contrast to the aluminosilicate glasses, the present glass series exhibit a negative deviation of shift of peak position at ~1100 cm−1 from a linear additivity, indicating the role of the aluminum speciation in affecting the vibration modes. By fitting the Vogel–Fulcher–Tamann equation to the high‐temperature viscosity data, we found a near‐linear increase of the fractional free volume with the gradual substitution of Ca by Mg, confirming the dynamic structural mismatch model describing the mixed modifier effect. This work gives insight into the mixed modifier effect in glassy systems.

Calorimetric studies on Ge23Se67Sb10-0.5%RbI glass

Understanding the crystallization mechanism of chalcogenide glass can help in directing the subsequent annealing processing, thus tuning the microstructures and physical properties. In this work, the transformation kinetics of Ge23Se67Sb10-0.5%RbI was investigated under non-isothermal conditions. From VFT equation, the ideal glass transition temperature was determined as T0g=427.74K. The fragility index m was estimated to be 64.74, indicating a relatively fragile liquid. From the classical JMA theory, the activation energy was determined as 222.9kJ/mol, growth index was 1.26, while from the model considering the impingement modes(MCI) activation energy was determined as 90.99kJ/mol and growth index was 1.38, which indicated that the growth mode of crystallization of the present glass was between one dimension and two dimension. The result shows that addition of RbI can reduce the growth dimension and activation energy.

Poor glass-forming ability of Fe-based alloys: Its origin in high-temperature melt dynamics

The relatively poor glass-forming ability (GFA) of Fe-based alloys has limited the mass production of Fe-based bulk metallic glasses (MGs), and hence their applications. To clarify the origin of the low GFA of the Fe-based alloys, we investigate the dynamic behaviors of these alloy systems well above their respective liquidus temperatures. By using a torsional oscillating viscometer we have observed a dynamic transition, where the slope of the viscosity-temperature relation of the Fe-based metallic liquid increases upon cooling, and a viscosity hysteresis occurs between the heating and cooling processes. By using the concept of fluid cluster and supercooled liquid fragility in metallic liquids, it has been found that this dynamic transition makes the Fe-based supercooled liquids become more unstable, which leads to the poor GFA of Fe-based alloys. Further, it has been found that the degree of the dynamic transition relates to the different GFA among Fe-based alloys. By taking into account the thermodynamic effect accompanied with the dynamic transition in Fe-based melts, a possible scenario has been proposed from the aspect of the underlying structural evolution during cooling above the liquidus temperature.