Universal scaling of dielectric response of various liquid crystals and glass-forming liquids

Abstract

We present a new generalized scaling relationship accounting both for the real and imaginary parts of the complex permittivity data. The generalized scaling procedure has been successfully used for various relaxation processes in liquid crystals (4-bromobenzylidene-4′-pentyloxyaniline, 4-bromobenzylidene-4′-hexyloxyaniline, 4′-butyl-4-(2-methylbutoxy)-azoxybenzene, 4-ethyl-4′-octylazoxybenzene), and in glass-forming liquids (glycerol, propylene carbonate, salol, cresolphthalein-dimethylether). As it is shown, one obtains common master-curve for liquid-like phases (isotropic liquid, cholesteric, nematic, smectic A), solid-like phases (smectic B, conformationally disorder crystal) and supercooled liquid phase.     

An existence of universality in the dynamics of two types of glass-forming liquids - fragile liquids and strong liquids

By employing a simplified nonlinear memory function proposed recently by the present author, a universal equation for a collective-intermediate scattering function derived based on the time-convolutionless mode-coupling theory is numerically solved to study the dynamics of glass-forming liquids. The numerical calculation is done based on the simulation results performed on two types of liquids, fragile liquids and strong liquids. Those are then shown to be uniquely determined by the long-time collective diffusion coefficient $D(q_m)$, where $q_m$ is a first peak position of a static structure factor for a whole system. Thus, there exists such a universality that there is only one solution for different liquids of a same type at a given value of D. This may be consistent with the fact that strong liquids are structurally quite different from fragile liquids. Finally, it is emphasized that such a universality must be helpful to predict $q_m$ from experimental data.     

Dynamic criticality in glass-forming liquids

We propose that the dynamics of supercooled liquids and the formation of glasses can be understood from the existence of a zero-temperature dynamical critical point. To support our proposal, we derive a dynamic field theory for a generic kinetically constrained model, which we expect to describe the dynamics of a supercooled liquid. We study this field theory using the renormalization group (RG). Its long time behavior is dominated by a zero-temperature critical point, which for d>2 belongs to the directed percolation universality class. Molecular dynamics simulations seem to confirm the existence of dynamic scaling behavior consistent with the RG predictions.     

The importance of the activation volume for the description of the molecular dynamics of glass-forming liquids

High pressure dielectric measurements were carried out on hydrogen bonded d-glucose and two van der Waals peracetyl saccharides, i.e. α pentaacetyl glucose and α octaacetyl maltose. In this study we found that after removing H bonds, the molecular dynamics of both modified saccharides is very sensitive to pressure, as reflected by a large value of the pressure coefficient of the glass transition temperature, equal to 270 K GPa(-1) and 280 K GPa(-1) for α pentaacetyl glucose and α octaacetyl maltose, respectively. On the other hand, dT(g)/dP for d-glucose is much lower, equal to 67 K GPa(-1). Our result confirms the general rule that the hydrogen bonding glass-forming liquids exhibit much lower values of dT(g)/dP compared to the van der Waals systems. Additionally, on the basis of results reported herein and also recent literature data for polyalcohols, we point out that the activation volume correlates fairly well with the molecular volume in the case of hydrogen bonding liquids. On the other hand, much larger values of the activation volumes at T(g) with respect to the molecular volumes were found for both peracetyl saccharides.     

金属玻璃形成液体的热力学特性

通过分析规则熔体的热力学模型，计算了典型金属玻璃的熔体混合焓ΔH^mix和混合熵ΔS^mix.结合临界冷却速率，归纳出典型金属玻璃形成液体的热力学特性,并提出基于原子尺寸、元素组成以及元素之间混合焓等参数的形成大块金属玻璃的成分判定方法.结果表明，当ΔH^mix<-15 kJ·mol^-1且ΔS^mix>0.6 J·K^-1mol^-1时，合金易于形成大块金属玻璃.金属玻璃的临界冷却速率R_c具有明显的尺寸效应，其值与熔体的ΔS^mix值呈指数关系，可以用R_c＝42.24×10⁴exp(-13.91ΔS^mix)+19.66粗略判断.运用该方法成功设计并制备出远离原有Zr基大块金属玻璃形成区域(55at%—65at%Zr)的Zr₄₀Al₁₀Ni₁₅Cu₃₅和四元Fe-B基Fe₅₃Co₅Nd₁₂B₃₀大块金属玻璃. 关键词： 混合焓 混合熵 大块金属玻璃 玻璃形成能力     

Generalized Vogel law for glass-forming liquids

A model for non-Arrhenius structural and dielectric relaxation in glass-forming materials is based on defect clustering in supercooled liquids. Relaxation in the cold liquid is highly hindered, and assumed to require the presence of a mobile defect to loosen the structure near it. A mild distribution of free-energy barriers impeding defect hopping can generate a wide distribution of waiting times between relaxation events. When the mean waiting time is longer than the time of an experiment, no characteristic time scale exists. This case directly yields the Kohlrausch-Williams-Watts (KWW) relaxation law. A free-energy mismatch between defect and nondefect regions produces a defect-defect attraction, which can lead to aggregation. This may occur in defect-rich fragile liquids which also exhibit Vogel kinetics. Defect aggregation and correlation in the high-temperature region above the critical consolute temperatureT _c is described using the Ornstein-Zernike theory of critical fluctuations. For a defect correlation length divergence (T-T _c)^-/2, a generalized Vogel law for the structural relaxation time results: =₀exp[B./(T-T _c)^1.5] In the mean-field limit (=1) this provides as good an account of dielectric and structural relaxation in glycerol,n-propanol, andi-butyl bromide as does the original Vogel law, and for the mixed salt KNO₃–Ca(NO₃)₂ and B₂O₂ it also describes kinetics over their entire temperature ranges. A breakdown of the Vogel law in the immediate vicinity ofT _g is avoided, and the need to invoke extra low-temperature mechanisms to explain an apparent return to Arrhenius behavior is removed.This paper is dedicated to Prof. N. G. van Kampen on the occasion of his 67th birthday.     

金属玻璃液体中的强脆转变现象

强脆转变是玻璃形成液体在从低温到高温升温过程中由强性液体转变为脆性液体的现象,反之从高温到低温冷却过程即为脆强转变.由于其意味着液体的结构发生了某种快速、非连续的变化,强脆转变现象成为异常动力学的典型代表.自1999年《Nature》杂志首次报道了水的强-脆转变现象之后,液体的强脆转变现象就作为凝聚态物理和材料科学领域中的前沿和热点问题被广泛关注.越来越多的研究表明,强脆转变现象在金属玻璃形成液体中普遍存在.为阐明金属玻璃强-脆转变现象对于深入理解玻璃转变本质、探讨液固遗传微观结构特征、揭示晶化过程相互竞争规律、提高玻璃形成能力、促进金属玻璃制备和处理工艺标准化等方面的重要意义,综合评述了强脆转变现象在金属玻璃形成液体中的普遍性、特殊性、定量表征、热力学表现以及结构起源等研究领域的最新进展,并指出了该领域今后的发展方向.     

Hydrogen-induced dynamic slowdown of metallic glass-forming liquids

Dynamics of hydrogen doped Cu_(50) Zr_(50) glass-forming liquids are investigated by using the newly developed modified embedded atomic method(MEAM) potential based on molecular dynamics simulations. We find that the doping of hydrogen atoms slows down the relaxation dynamics, reduces the fragility of supercooled melts, and promotes the occurrence of glass transitions. The dynamic slowdown is suggested to be closely related to the effect of hydrogen atoms on locally ordered structure of melts. With increasing concentration of hydrogen, the five-fold symmetry associated with Cu-and Zr-centered polyhedrons is lowered, on the other hand, the local order featuring metal hydrides is enhanced. The latter dominates the dynamic behaviors of glass-forming liquids, especially for Zr atoms, and results in the dynamic slowdown.     

Properties of a soft-sphere liquid from non-Newtonian molecular dynamics

A soft-sphere, inverse-12 liquid is simulated in both the isokinetic-isochoric and the isokinetic-isobaric ensemble using nonequilibrium molecular dynamics. The simulation for the isobaric ensemble is discussed in detail. The non-Newtonian characteristics of the liquid are clearly demonstrated; namely, the shear-rate-dependent pressure and density (shear dilatancy), the shear-rate-dependent shear viscosity (shear thinning), and evidence of normal pressure differences. For the first time, it is clearly shown that a significant component of isobaric shear thinning is due to shear dilatancy. The isochoric and isobaric results are checked for consistency. Simple empirical relations for the equation of state and transport properties of the fluid are presented.This is a publication in part of the National Institute of Standards and Technology (formerly NBS) and is not subject to copyright.     

Dynamic singularity in multicomponent glass-forming metallic liquids

In the liquid state, glass-forming Ni59.5Nb40.5 and Ni60Nb34.8Sn5.2 alloys exhibit an extraordinarily high packing fraction. The self-correlation functions measured using quasielastic neutron scattering clearly show the slowing down of microscopic dynamics with an increase in packing fraction. The self-diffusivity in liquid Ni60Nb34.8Sn5.2 decreases by about 2 orders of magnitude within a temperature range of 360 K. For these highly fragile systems, the critical packing fraction obtained form the analysis of incoherent data is in excellent agreement with the prediction made by mode-coupling theory. Our results provide the first experimentally observed value for the critical packing fraction in glass-forming metallic liquids.     

A statistical-mechanical theory of self-diffusion in glass-forming liquids

A statistical-mechanical theory of self-diffusion in glass-forming liquids is presented. A non-Markov linear Langevin equation is derived from a Newton equation by employing the Tokuyama-Mori projection operator method. The memory function is explicitly written in terms of the force-force correlation functions. The equations for the mean-square displacement, the mean-fourth displacement, and the non-Gaussian parameter are then formally derived. The present theory is applied to the glass transitions in the glass-forming liquids to discuss the crossover phenomena in the dynamics of a single particle from a short-time ballistic motion to a long-time self-diffusion process via a β (caging) stage. The effects of the renormalized friction coefficient on self-diffusion are thus explored with the aid of analyses of the simulation results by the mean-field theory proposed recently by the present author. It is thus shown that the relaxation time of the renormalized memory function is given by the β-relaxation time. It is also shown that for times longer than the β-relaxation time the dynamics of a single particle is identical to that discussed in the suspensions.     

Relation between thermodynamics and kinetics of glass-forming liquids

Vitrification of a supercooled liquid is often characterized by the hypothetical kinetic instability point, the Vogel-Fulcher temperature T0, and the thermodynamic one, the Kauzmann temperature T(K). The widely believed relation T0 congruent with T(K) is regarded as the supporting evidence of a direct connection between the thermodynamics and kinetics of glass-forming liquids. Here we demonstrate that T(K)/T(0) systematically increases from unity with a decrease in the fragility, contrary to the common belief. This systematic deviation may be explained by a synergistic effect between the weaker cooperativity and the stronger tendency of short-range ordering in stronger glass formers.     

Nearly logarithmic decay of correlations in glass-forming liquids

Nearly logarithmic decay of correlations, which was observed for several supercooled liquids in optical-Kerr-effect experiments [Phys. Rev. Lett. 84, 2437 (2000)]; Phys. Rev. Lett. 90, 197401 (2003)]], is explained within the mode-coupling theory for ideal glass transitions as a manifestation of the beta-peak phenomenon. A schematic model, which describes the dynamics by only two correlators, one referring to density fluctuations and the other to the reorientational fluctuations of the molecules, yields for strong rotation-translation coupling response functions in agreement with those measured for benzophenone and Salol for the time interval extending from 2 ps to about 20 and 200 ns, respectively.     

Cooperative shear model for the rheology of glass-forming metallic liquids

A rheological law based on the concept of cooperatively sheared flow zones is presented, in which the effective thermodynamic state variable controlling flow is identified to be the isoconfigurational shear modulus of the liquid. The law captures Newtonian as well as non-Newtonian viscosity data for glass-forming metallic liquids over a broad range of fragility. Acoustic measurements on specimens deformed at a constant strain rate correlate well with the measured steady-state viscosities, hence verifying that viscosity has a unique functional relationship with the isoconfigurational shear modulus.     

Mössbauer investigations on glass-forming organic liquids

Glycerol forms a molecular glass near 180K. Fe²⁺ dissolved in glycerol allows the study of the dynamics of the system by Mössbauer spectroscopy. Recently it has been shown that the Mössbauer spectra can be understood in a way consistent with the results of dielectric and ultrasonic viscoelastic relaxation measurements. A jump diffusion model of Sinqwi and Sjolander with a jump rate distribution according to Davidson and Cole allowed us to fit the Mössbauer spectra of Fe in glycerol. First attempts to compare mode coupling theory with Mössbauer spectra are reported.     

Anelastic to plastic transition in metallic glass-forming liquids

The configurational properties associated with the transition from anelasticity to plasticity in a transiently deforming metallic glass-forming liquid are studied. The data reveal that the underlying transition kinetics for flow can be separated into reversible and irreversible configurational hopping across the liquid energy landscape, identified with beta and alpha relaxation processes, respectively. A critical stress characterizing the transition is recognized as an effective Eshelby "backstress," revealing a link between the apparent anelasticity and the "confinement stress" of the elastic matrix surrounding the plastic core of a shear transformation zone.