Molecular dynamics study of structure and glass forming ability of Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy

In this study, the temperature effects on the structural evolution of theZr₇₀Pd₃₀ binary alloy in the glassy and liquid states werestudied using the molecular dynamics simulations based on the many-body type tight-bindingpotential. We considered the following properties in detail: the temperature dependence ofthe volume, the partial and total pair distribution functions and the simulated glasstransition temperature. The effects of the cooling rates on the glass transitiontemperature were examined. The Wendt-Abraham parameter was calculated to determine theglass transition temperature of Zr₇₀Pd₃₀ glassy alloy. The pair analysis technique ofHoneycutt-Andersen was applied to define local atomic arrangements produced from moleculardynamics simulations. The results show that the icosahedral ordering in glassy state hasbeen composed during quenching period, and the simulated glass transition temperature andthe total pair distribution functions are in good agreement with the experimental data.     

Glass-transition temperature of water: a simulation study

We report a computer simulation study of the glass transition for water using the extended simple point charge potential. To mimic the difference between standard and hyperquenched glass, we generate glassy configurations with different cooling rates, and we calculate the temperature dependence of the specific heat on heating. The absence of crystallization phenomena allows us, for properly annealed samples, to detect in the specific heat the simultaneous presence of a weak prepeak ("shadow transition") and an intense glass transition peak at higher temperature. Our results support the view-point that the glass transition temperature is higher than the conventionally accepted value 136 K. We also compare our simulation results with the Tool-Narayanaswamy-Moynihan phenomenological model.     

Yielding behavior of glassy polymers. I. Free-volume model

Rigid, glassy polymers show a diversity of tensile behavior-ranging from apparently brittle to ductile. To delineate some of the factors that control the toughness or impact resistance of these polymers, the yielding behavior of poly (methyl methacrylate) (PMMA) was studied. Results of other workers have shown that the cold flow exhibited by many glassy polymers can be explained qualitatively by a free-volume model. The treatment assumes that molecular flow is permitted when the free volume increase, resulting from the dilatational component of the applied stress, is sufficient to bring the total free volume to that characteristic of the polymer liquid. The present study refines this approach by introducing an “effective temperature,” defined as that hypothetical temperature at which the glass would have an equilibrium free volume equal to the total free volume of the nonequilibrium glass at temperature T. Equations are derived which more satisfactorily describe the temperature and strain-rate dependences of the tensile yield strain of PMMA glass from -10° to 90°C at rates between 0.015 and 120%/sec.     

Time-temperature superposition and relaxational behavior in polymeric glasses

It is shown that a free-volume treatment of the relaxational behavior of a polymer can be extended into the glassy region. A modified form of the WLF equation is derived in which the temperature is replaced by T_e, a parameter related to the “frozen” free volume in the glass and defined such that Te = T at equilibrium. Measurements of the isothermal volume contraction of polystyrene and poly(methyl methacrylate) between ?20 and + 95°C are used to estimate the “frozen” free volume and to calculate the temperature dependence of log(a_T) below Tg. The calculated shift factors are compared to experimental values for the glassy state, and good agreement is obtained by selecting an arbitrary, but reasonable, equilibrium glass volume-temperature curve. The slope of this equilibrium glass curve is smaller than the experimental volume curve at some finite cooling rate. The data indicate that the glass is not an “iso-free volume” state and that the relaxation mechanisms in the glass are controlled primarily by the free volume, at least in the vicinity of Tg. A quantitative definition of the role of free volume in the glassy state requires evaluation of the quantities ? log a_T/?V)_T,P and (? log a_T/?T)_v,p; sufficient data of this type are not presently available.     

非晶材料玻璃转变过程中记忆效应的热力学

低温下处于非平衡态的非晶材料升温到玻璃转变以上,要先后发生弛豫和回复最终达到平衡过冷液态,其中弛豫过程中释放的能量在回复过程中以等量的方式获取,表现出明显记忆行为.本文基于氧化物、金属与小分子等多种非晶形成体系,全面探讨了在围绕玻璃转变的一个冷却加热循环过程中的焓弛豫特征,建立了弛豫谱,发现弛豫焓在数值上与熔化焓密切相关.基于弛豫焓与非晶材料动力学Fragility之间的关联,展示了非晶体系在动力学极限(m=175)条件下的玻璃转变热力学基本特征,与热力学二级相变进行了对比.研究深化了对非晶弛豫与玻璃转变热力学的理解.     

On the aging characterization of poly(ethylene terephthalate) by positron spectroscopy

Positron spectroscopy was used to study the free volume properties of glassy poly(ethylene terephthalate) as a function of temperature and aging time. The experimental results indicate that the glass transition temperature increases with increasing aging time. This is confirmed by differential scanning calorimetry measurements. From the aging time dependence of the intensity of the ortho-positronium (o-Ps) component I₃ and the fractional free volume, we observe that the concentration of free volume holes and the fractional free volume show a continuous decrease at a fixed aging temperature, which suggests that structural relaxation and segmental rearrangement occur during physical aging.     

Atomic mechanism of vitrification process in simple monatomic nanoparticles *

Glass formation in simple monatomic nanoparticles has been studied by molecular dynamics simulations in spherical model with a free surface. Models have been obtained by cooling from the melt toward glassy state. Atomic mechanism of glass formation was monitored via spatio-temporal arrangement of solid-like and liquid-like atoms in nanoparticles. We use Lindemann freezing-like criterion for identification of solid-like atoms which occur randomly in supercooled region. Their number grows intensively with decreasing temperature and they form clusters. Subsequently, single percolation solid-like cluster occurs at temperature above the glass transition. Glass transition occurs when atoms aggregated into this single percolation cluster are in majority in the system to form relatively rigid glassy state. Solid-like domain is forming in the center of nanoparticles and grows outward to the surface. We found temperature dependence of potential energy, mean-squared displacement (MSD) of atoms, diffusion constant, incoherent intermediate scattering function, radial distribution function (RDF), local bond-pair orders detected by Honeycutt-Andersen analysis, radial density profile and radial atomic displacement distributions in nanoparticles. We found that liquid-like atoms in models obtained below glass transition have a tendency to concentrate in the surface layer of nanoparticles. However, they do not form a purely liquid-like surface layer coated nanoparticles.     

Experimental evidence for interplay of dynamic heterogeneity and finite-size effect in glassy polymers

Despite two decades of extensive research, direct experimental evidence of a dynamical length scale determining the glass transition of confined polymers has yet to emerge. Using a recently established experimental technique of interface micro-rheology we provide evidence of finite-size effect truncating the growth of a quantity proportional to a dynamical length scale in confined glassy polymers, on cooling towards the glass transition temperature. We show how the interplay of variation of polymer film thickness and this temperature-dependent growing dynamical length scale determines the glass transition temperature, which in our case of 2–3 nm thick films, is reduced significantly as compared to their bulk values.     

金属玻璃Tg附近内耗理论分析

本文根据自由体积模型和固体材料的力学理论分析并讨论了金属玻璃在玻璃转变温度T_g附近的内耗行为。当考虑到体系中固状区和液状区的比例之后,得出了应力与应变之间的关系式。理论上把内耗和模量变化与玻璃转变过程有机地结合起来。证明了在T_g附近可以出现具有弛豫型特征的内耗降,并在玻璃转变过程中模量明显下降。理论与实验结果基本相符。 关键词：     

The effect of excess volume on molecular mobility and on the mode of failure of glassy poly(ethylene terephthalate)

To gain further understanding regarding the modes of molecular motion which contribute to the ductility and toughness of glassy polymers, and the factors that influence such molecular mobility, a study has been undertaken of the effect of the amount of excess volume in glassy poly(ethylene terephthalate) on the mode of failure observed in tensile tests and on the associated level of molecular motion. The results of tensile stress-strain studies on films annealed at temperatures below the glass transition temperature indicate that there is a relationship between the mode of failure and the level of excess volume trapped in the glassy polymer. From dynamic mechanical loss studies of the annealed films, however, it is evident that the modes of motion associated with the major subsidiary dynamic mechanical absorption, i.e., the γ-relaxation process, are relatively unaffected by the loss of free volume caused by the densification of the glassy polymer on annealing. Further, these studies suggest that the limited segmental mobility associated with the onset of the glass transition phenomenon may be significant with regard to the ductile behavior of the glassy polymer.     

冷却速率对Lennard-Jones体系凝固过程中结构与动力学性质的影响

用分子动力学模拟方法研究了五种不同冷却速率对Lennard-Jones体系凝固过程中结构与动力学性质的影响。采用两种不同的方法来确定玻璃转变温度Tg,并且对结晶温度Tc、径向分布函数g(r)、均方位移函数MSD与扩散系数D、平均配位数进行比较分析。结果表明：冷却速率影响Lennard-Jones体系凝固过程中的结构。当使用足够高的冷却速率冷却时,体系发生玻璃化转变,而且冷却速率越快,玻璃转变温度越高;当冷却速率较小时,体系形成晶体,而且冷却速率越慢,结晶温度越高,结晶程度也越高。同时发现,冷却速率对扩散系数和平均配位数也有很大影响,二者在体系发生玻璃转变时都有一个缓变的过程,表明了过冷液相区的存在。     

In situ solidification of ionic liquid [EMIM][EtOSO3] from melt under high pressure

In situ solidification of 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][EtOSO₃] from melt under high pressure has been investigated by using Raman spectroscopy. The results indicate that [EMIM][EtOSO₃] might experience a phase transition at about 2.4 GPa upon compression, which could be identified as solidification to a superpressurized glass by pressure broadening of the sharp ruby R₁ fluorescence line. Upon cooling, it solidifies as a glassy state rather than crystallizes at low temperature down to 93 K. These facts are suggestive of a phase transition of liquid to a superpressurized glass induced by compression in [EMIM][EtOSO₃], which is similar to the glassy state at low temperature.     

Anomalous electrical conductivity behavior at elevated pressure in the protic ionic liquid procainamide hydrochloride

Using broadband dielectric spectroscopy, we investigated the effect of hydrostatic pressure on the conductivity relaxation time τ{σ} of the supercooled protic ionic liquid, procainamide hydrochloride, a common pharmaceutical. The pressure dependence of τ{σ} exhibited anomalous behavior in the vicinity of the glass transition T{g}, manifested by abrupt changes in activation volume. This peculiar behavior, paralleling the change in temperature dependence of τ{σ} near T{g}, is a manifestation of the decoupling between electrical conductivity and structural relaxation. Although the latter effectively ceases in the glassy state, free ions retain their mobility but with a reduced sensitivity to thermodynamic changes. This is the first observation of decoupling of ion migration from structural relaxation in a glassy conductor by isothermal densification.     

Colloidal glass transition observed in confinement

We study a colloidal suspension confined between two quasiparallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. For higher volume fraction samples (closer to the glass transition), the onset of confinement effects occurs at larger length scales.     

Impact of Chlorine Substitution on Spin–Lattice Relaxation of Triarylmethyl and 1,4-Benzosemiquinone Radicals in Glass-Forming Solvents Between 25 and 295 K

Spin–lattice relaxation rates measured by long-pulse saturation recovery in glassy solvents for chlorinated aromatic radicals: perchlorotriphenylmethyl radical, 2,5-dichloro-3,6-dihydroxy-1,4-benzosemiquinone, and tetrachloro-1,4-benzosemiquinone, were compared with relaxation rates for non-chlorinated analogs. The impact of the quadrupolar chlorines is small and less than the effects of changing the rigidity of the glass. The temperature dependence of relaxation rates below the glass transition temperature could be modeled as the sum of contributions from the direct, Raman, and local mode processes.     

Hysteresis in vortex glass transitions of the first order

The distribution of relaxation times in a glass is very broad. As a result the free energy of a glass slowly approaches its equilibrium value but never reaches it. This slow relaxation explains why, in the hysteresis cycle, the melting tempetature is higher than the solidification temperature.

We estimate the width of this hysteresis for a general type of transition into the glassy state. We apply these estimates to the special case of the vortex melting transition and find good agreement with recent experiments. We show how strong disorder changes the character of the vortex melting transition and evaluate the position of the tricritical point.     

Small activation entropy bestows high-stability of nanoconfined D-mannitol

It has been a long-standing puzzling problem that some glasses exhibit higher glass transition temperatures(denoting high stability) but lower activation energy for relaxations(denoting low stability). In this paper, the relaxation kinetics of the nanoconfined D-mannitol(DM) glass was studied systematically using a high-precision and high-rate nanocalorimeter.The nanoconfined DM exhibits enhanced thermal stability compared to the free DM. For example, the critical cooling rate for glass formation decreases from 200 K/s to below 1 K/s; the Tg increases by about 20 K–50 K. The relaxation kinetics is analyzed based on the absolute reaction rate theory. It is found that, even though the activation energy E~*decreases,the activation entropy S~*decreases much more for the nanoconfined glass that yields a large activation free energy G~*and higher thermal stability. These results suggest that the activation entropy may provide new insights in understanding the abnormal kinetics of nanoconfined glassy systems.     

Local atomic structures of single-component metallic glasses

In this study we examine the structural properties of single-component metallic glasses of aluminum. We use a molecular dynamics simulation based on semi-empirical many-body potential, derived from the embedded atom method (EAM). The radial distribution function (RDF), common neighbors analysis method (CNA), coordination number analysis (CN) and Voronoi tessellation are used to characterize the metal’s local structure during the heating and cooling (quenching). The simulation results reveal that the melting temperature depends on the heating rate. In addition, atomic visualization shows that the structure of aluminum after fast quenching is in a glassy state, confirmed quantitatively by the splitting of the second peak of the radial distribution function, and by the appearance of icosahedral clusters observed via CNA technique. On the other hand, the Wendt-Abraham parameters are calculated to determine the glass transition temperature (T_g), which depends strongly on the cooling rate; it increases while the cooling rate increases. On the basis of CN analysis and Voronoi tessellation, we demonstrate that the transition from the Al liquid to glassy state is mainly due to the formation of distorted and perfect icosahedral clusters.     

Glass Transition Temperature of Water： from Simulations of Diffusion and Excess Entropy

We report a computer simulation study of the glass transition of water with SP2 potential. The temperature dependences of the diffusion coefficient and the excess entropy on cooling process are calculated. It is found that both the diffusion coefficient and the excess entropy show a break point at 160K. Our results support the viewpoint that the glass transition temperature is 160K. According to the calculated viscosity, we obtain a fragility index of water to be 326, which is much larger than the value accepted before.