Absence of thermodynamic phase transition in a model glass former

The glass transition can be viewed simply as the point at which the viscosity of a structurally disordered liquid reaches a universal threshold value. But this is an operational definition that circumvents fundamental issues, such as whether the glass transition is a purely dynamical phenomenon. If so, ergodicity gets broken (the system becomes confined to some part of its phase space), but the thermodynamic properties of the liquid remain unchanged across the transition, provided they are determined as thermodynamic equilibrium averages over the whole phase space. The opposite view claims that an underlying thermodynamic phase transition is responsible for the pronounced slow-down in the dynamics at the liquid-glass boundary. Such a phase transition would trigger the dynamic standstill, and then be masked by it. Here we perform Monte Carlo simulations of a two-dimensional system of polydisperse hard disks far within its glassy phase. The approach allows for non-local moves in a way that preserves micro-reversibility. We find no evidence for a thermodynamic phase transition up to very high densities; the glass is thus indistinguishable from the liquid on purely thermodynamic grounds.     

Bond-controlled configurational entropy reduction in chemical vitrification

Glass formation is usually viewed in terms of physical vitrification: a liquid in a metastable state is cooled or compressed so as to avoid crystallization. However, glasses may also be formed by chemical vitrification, a process involving progressive polymerization of the constituent molecules via the formation of irreversible chemical bonds. The formation of most of the materials used in engineering plastics and the hardening of natural and synthetic resins are based on chemical vitrification. Despite the differences in the molecular processes involved in chemical and physical vitrification, surprising similarities are observed in the slowing down of the dynamics and in the thermodynamical properties of the resulting glasses. Explaining such similarities would improve general understanding of the glass transition and may disclose its universal nature. Here we report dielectric and photon-correlation measurements that reveal the origin of the similarity in the dynamical behaviour of physical and chemical glass formers. We find that the evolution of their configurational restrictions proceeds in a similar manner. In particular, we make a connection between the reduction in configurational entropy and the number of chemical bonds, a quantity that can be controlled in experiments.     

The relationship between fragility, configurational entropy and the potential energy landscape of glass-forming liquids

Glass is a microscopically disordered, solid form of matter that results when a fluid is cooled or compressed in such a manner that it does not crystallize. Almost all types of materials are capable of glass formation, including polymers, metal alloys and molten salts. Given such diversity, general principles by which different glass-forming materials can be systematically classified are invaluable. One such principle is the classification of glass-formers according to their fragility. Fragility measures the rapidity with which a liquid's properties (such as viscosity) change as the glassy state is approached. Although the relationship between the fragility, configurational entropy and features of the energy landscape (the complicated dependence of energy on configuration) of a glass-former have been analysed previously, a detailed understanding of the origins of fragility is lacking. Here I use simulations to analyse the relationship between fragility and quantitative measures of the energy landscape for a model liquid whose fragility depends on its bulk density. The results reveal that fragility depends on changes in the vibrational properties of individual energy minima in addition to their total number and spread in energy. A thermodynamic expression for fragility is derived, which is in quantitative agreement with kinetic fragilities obtained from the liquid's diffusivity.     

A thermodynamic connection to the fragility of glass-forming liquids

Although liquids normally crystallize on cooling, there are members of all liquid types (including molecular, ionic and metallic) that supercool and then solidify at their glass transition temperature, Tg. This continuous solidification process exhibits great diversity within each class of liquid-both in the steepness of the viscosity-temperature profile, and in the rate at which the excess entropy of the liquid over the crystalline phase changes as Tg is approached. However, the source of the diversity is unknown. The viscosity and associated relaxation time behaviour have been classified between 'strong' and 'fragile' extremes, using Tg as a scaling parameter, but attempts to correlate such kinetic properties with the thermodynamic behaviour have been controversial. Here we show that the kinetic fragility can be correlated with a scaled quantity representing excess entropy, using data over the entire fragility range and embracing liquids of all classes. The excess entropy used in our correlation contains both configurational and vibration-related contributions. In order to reconcile our correlation with existing theory and simulations, we propose that variations in the fragility of liquids originate in differences between their vibrational heat capacities, harmonic and anharmonic, which we interpret in terms of an energy landscape. The differences evidently relate to behaviour of low-energy modes near and below the boson peak.     

Direct observation of molecular cooperativity near the glass transition

The increasingly sluggish response of a supercooled liquid as it nears its glass transition (for example, refrigerated honey) is prototypical of glassy dynamics found in proteins, neural networks and superconductors. The notion that molecules rearrange cooperatively has long been postulated to explain diverging relaxation times and broadened (non-exponential) response functions near the glass transition. Recently, cooperativity was observed and analysed in colloid glasses and in simulations of binary liquids well above the glass transition. But nanometre-scale studies of cooperativity at the molecular glass transition are lacking. Important issues to be resolved include the precise form of the cooperativity and its length scale, and whether the broadened response is intrinsic to individual cooperative regions, or arises only from heterogeneity in an ensemble of such regions. Here we describe direct observations of molecular cooperativity near the glass transition in polyvinylacetate (PVAc), using nanometre-scale probing of dielectric fluctuations. Molecular clusters switched spontaneously among two to four distinct configurations, producing random telegraph noise. Our analysis of these noise signals and their power spectra reveals that individual clusters exhibit transient dynamical heterogeneity and non-exponential kinetics.     

铝、锆液固态转变和动力学性质的第一性原理分子动力学研究

采用第一性原理分子动力学(Ab Initio Molecular Dynamics,AIMD)方法研究了Al和Zr液固转变过程中的能量、偶关联函数、结构因子和键对分布的变化规律,获得了不同温度下两种金属液体的扩散系数和黏度.结果表明,AIMD计算得到的液态金属偶关联函数、结构因子和扩散系数与实验测量数据符合得很好.在冷速为5.0×1013和2.5×1013K/s时,液态Al分别在730K附近发生玻璃化转变或者形成有一定缺陷的fcc晶体结构.在平均冷却速率为4.3×1013和2.0×1014K/s的条件下,液态Zr在1200K时分别开始转变为热力学上亚稳定的bcc结构和玻璃相.Zr的液态和玻璃态结构中二十面体和bcc类型短程序是其主要拓扑短程序.     

金属玻璃的液态结构及其动力学胶体模拟

研究金属玻璃对于改善性能,开发新的合金体系有重要意义。由于实验手段的局限,目前为止还未能直接观测到液态结构的演化过程。胶体体系可以得到直观可视的粒子图像,已被应用于模拟研究原子体系的物理变化过程。通过不同的胶体体积浓度来模拟金属玻璃熔体,分析了不同胶体液态体系中的团簇结构,并对胶体粒子的扩散规律进行了探讨。结果表明液态体系的浓度越高,粒子的扩散速率越慢,局域特征结构发生变化,并在高浓度液态结构中观察到了结构的局域有序性动力学阻滞形成的笼子效应,为胶体体系液态微结构随浓度的演化提供了直接的实验证据。     

非晶聚合物构型熵的温度关系

基于无序非晶材料“关联蕈排区域”的构型熵理论，运用构型熵与热容龟差的关系式、弛豫时间与活化势垒的基本公式，及聚合物量热学的实验结果和热容量差与温度的基本关系，研究了4种情况下构型熵、介电弛豫指数和关联重排区域大小的温度变化关系，研究结沦表明：聚合物的构型熵随温度的上升均增大，并由同一物质非品态的热容量与晶态的热容量之差决定，其变化限制在热容量差为恒定及与温度成倒数的范围之内。     

Perspective on Materials Genome~

The author’s perspective on Materials Genome is presented in this paper through several related projects.Current thermodynamic and kinetic databases of multicomponent materials consist of Gibbs energy functions and atomic mobility of individual phases as functions of temperature,composition,and sometimes pressure,i.e.,with the individual phases based on crystal structures as the genome(building blocks)of materials.It is articulated that if an individual phase has its internal configurations,such as magnetic spin configurations and ferroelectric polarization,change significantly with respect to temperature,stress,and magnetic and electric fields,then those individual configurations instead should be considered as the genome of the individual phase.The‘‘mutation’’of an individual phase is governed by the entropy of mixing among the individual stable and metastable configurations,named as microstate configurational entropy,and responsible to anomalies in individual phases.Our ability to tailor the properties of those individual configurations as a function of compositions is the key for the design of materials.     

论动态统计信息理论

将现有的静态统计信息理论拓展至动态过程,建立以动力学系统的信息和信息熵的演化规律为主题及其应用的动态统计信息理论.从态变量演化方程出发,推导出表述信息熵和信息的演化规律的动态信息熵密度和动态信息密度的非线性演化方程.这两个方程表明:信息熵密度随时间的变化率是由其在坐标空间和态变量空间的漂移、扩散和产生三者引起的;而信息密度随时间的变化率则是由其在坐标空间和态变量空间的漂移、扩散和减损三者引起的.给出了漂移信息流和扩散信息流的表达式、信息减损率和信息熵产生率的简明公式.证明了动力学系统内的信息减损(或增加)率等于它的信息熵产生(或减少)率,信息扩散与信息减损同时发生.得到了反映传递过程动态特性的动态互信息公式,它在信道长度与信号传递速度之比趋于零的极限情况下变为现有的静态互信息公式.这些结果都是从信息和信息熵演化方程统一得出的,未增加新的假设.作为这些理论公式的应用,研究和计算了布朗运动的漂移扩散传递、热缺陷产生动力学及分子马达3个动力学课题的信息熵和信息的变化式,给出了高斯信道的动态互信息公式.     

Liquid fragility——A key togoing deep into materials of glassy states

“Liquid fragility“ is a concept that has been widely used in the investigation on the glass community,though it was presented less than two decades ago. The concept enables the comparison between the glass-forming liquids with different dynamic characters by using a general criterion, in which the temperature scale is reduced by the glass transition temperature. In order to illuminate the significance of the concept in the fields of the glass transition,structural relaxation process and the structure of supercooled liquids, the accomplished progress and the faced challenges are summarized from different aspects such as on the correlation between dynamics and thermodynamic characters of condensed matters, on the energy landscape, on the nonexponential relaxation and on the theoretical model of microstructure and medium-range order. The tendency of investigation in “liquid fragility“ is also evaluated.     

Structural transformation in supercooled water controls the crystallization rate of ice

One of water's unsolved puzzles is the question of what determines the lowest temperature to which it can be cooled before freezing to ice. The supercooled liquid has been probed experimentally to near the homogeneous nucleation temperature, T(H) ≈ 232 K, yet the mechanism of ice crystallization-including the size and structure of critical nuclei-has not yet been resolved. The heat capacity and compressibility of liquid water anomalously increase on moving into the supercooled region, according to power laws that would diverge (that is, approach infinity) at ~225 K (refs 1, 2), so there may be a link between water's thermodynamic anomalies and the crystallization rate of ice. But probing this link is challenging because fast crystallization prevents experimental studies of the liquid below T(H). And although atomistic studies have captured water crystallization, high computational costs have so far prevented an assessment of the rates and mechanism involved. Here we report coarse-grained molecular simulations with the mW water model in the supercooled regime around T(H) which reveal that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water explains its anomalous thermodynamics and also controls the rate and mechanisms of ice formation. The results of the simulations and classical nucleation theory using experimental data suggest that the crystallization rate of water reaches a maximum around 225 K, below which ice nuclei form faster than liquid water can equilibrate. This implies a lower limit of metastability of liquid water just below T(H) and well above its glass transition temperature, 136 K. By establishing a relationship between the structural transformation in liquid water and its anomalous thermodynamics and crystallization rate, our findings also provide mechanistic insight into the observed dependence of homogeneous ice nucleation rates on the thermodynamics of water.     

Zr53Cu18.7Ni12Al16.3过冷熔体的原子结构以及动力学性质的研究

本文采用第一性原理分子动力学方法,模拟了Zr53Cu18.7Ni12Al16.3过冷液体的原子结构,以及其对动力学性质的影响.通过对Al原子周围的偏对分布函数的分析,发现该过冷液体的原子结构可用以Al原子为中心的团簇来表征.这种局域结构稳定程度的空间不均匀性,导致了过冷液体的动力学在空间分布上的不均匀性,即出现速度较慢的原子.这种慢原子的出现,使得该过冷液体在过冷相区内发生动力学迟缓,从而抑制了晶化.     

生理时间序列的一种符号化分析方法

提出了一种分析生理时间序列的方法,即对重构的相空间进行符号化分析.因生理时间序列通常是非平稳的,为去除时间序列中的局部趋势、提取时间序列的波形特征,相空间中的向量被归一化,从而具有相同的均值和标准差.然后,引入最大拓扑熵（MTE）原则来寻找相空间的适当划分以实现向量的符号化.采用Logistic映射和人体运动信号检验的结果表明,用MTE原则比用最大熵原则得到的划分更接近最优划分.原时间序列的波动特征用出现的字模式个数与所有可能的字模式个数之比以及字模式分布概率的Shannon熵来描述.对人体运动信号的分析结果表明,该方法能够有效区分不同生理状态下的时间序列.     

Phonon interpretation of the 'boson peak' in supercooled liquids

Glasses are amorphous solids, in the sense that they display elastic behaviour. In crystalline solids, elasticity is associated with phonons, which are quantized vibrational excitations. Phonon-like excitations also exist in glasses at very high (terahertz; 10(12) Hz) frequencies; surprisingly, these persist in the supercooled liquids. A universal feature of such amorphous systems is the boson peak: the vibrational density of states has an excess compared to the Debye squared-frequency law. Here we investigate the origin of this feature by studying the spectra of inherent structures (local minima of the potential energy) in a realistic glass model. We claim that the peak is the signature of a phase transition in the space of the stationary points of the energy, from a minima-dominated phase (with phonons) at low energy to a saddle-point-dominated phase (without phonons). The boson peak moves to lower frequencies on approaching the phonon-saddle transition, and its height diverges at the critical point. Our numerical results agree with the predictions of euclidean random matrix theory on the existence of a sharp phase transition between an amorphous elastic phase and a phonon-free one.     

Structural relaxation in supercooled water by time-resolved spectroscopy

Water has many kinetic and thermodynamic properties that exhibit an anomalous dependence on temperature, in particular in the supercooled phase. These anomalies have long been interpreted in terms of underlying structural causes, and their experimental characterization points to the existence of a singularity at a temperature of about 225 K. Further insights into the nature and origin of this singularity might be gained by completely characterizing the structural relaxation in supercooled water. But until now, such a characterization has only been realized in simulations that agree with the predictions of simple mode-coupling theory; unambiguous experimental support for this surprising conclusion is, however, not yet available. Here we report time-resolved optical Kerr effect measurements that unambiguously demonstrate that the structural relaxation of liquid and weakly supercooled water follows the behaviour predicted by simple mode-coupling theory. Our findings thus support the interpretation of the singularity as a purely dynamical transition. That is, the anomalous behaviour of weakly supercooled water can be explained using a fully dynamic model and without needing to invoke a thermodynamic origin. In this regard, water behaves like many other, normal molecular liquids that are fragile glass-formers.     

高压下BeSe相变和热力学性质第一性原理研究

利用密度泛函理论研究了BeSe的相变、晶格动力学和热力学性质.计算得到的晶格常数、体弹模量以及其对压强的一阶偏导与实验值符合较好.焓的计算结果表明在压强为57.68GPa时BeSe会发生从闪锌矿(ZB)结构到砷化镍(NiAs)结构的相变,与实验值56±5GPa较吻合.BeSe的声子色散曲线计算表明零压下ZB结构满足动力学稳定性,而在高压下ZB结构的动力学稳定性将消失.基于准简谐近似方法,还成功预测了BeSe的热容、热膨胀系数和熵随着压强和温度的变化关系.     

分子高激发振动态的经典非线性性质

该文阐述如何运用经典非线性力学的概念,来理解分子高激发振动态的谱学性质.内容包括:莫尔斯振子,单摆的动力学和共振的关系,力学体系的构成单元是单摆,一个共振对应于一个守恒量,混沌,共振的重叠导致混沌的产生等.我们的出发点是运用二次量子化算子构成的代数哈密顿量,经由海森伯对应而得到的陪集空间上的动力学体系.从此哈密顿量,可以得到动力学势.而动力学势和经典的不动点关系密切,并且量子态就处在由动力学势所包围,分成的几个量子环境中.从动力学势,可以依据它的对称性,方便推得局域模式的存在,最后我们利用这些非线性力学的概念,来分析DCO分子高激发振动态的解离问题.     

热致型液晶聚合物的玻璃态结构及形成热力学和动力学

利用热台偏光显微镜、差示扫描量热法(DSC)、广角X-射线衍射(WAXD)等分析测试手段,对聚1,4-苯二氧乙酸-1,8-二氧代辛撑基二苯酚酯的织态结构及相态进行了研究。结果表明,该聚合物在经过一次加热冷却循环后。生成了液晶玻璃态。对相转变的转变熵值的分析表明,上述结论是正确的。并在热力学与动力学共同作用的基础上,讨论了热致型液晶聚合物液晶玻璃态的形成,提出了固化温度大于结晶温度是液晶玻璃态产生的必要条件。