Effect of Planar Interfaces on Nucleation in Melting and Crystallization

The effect of planar interfaces on nucleation (namely, on the work of critical cluster formation and their shape) is studied both for crystallization and melting. Advancing an approach formulated about 150 years ago by J. W. Gibbs for liquid phase formation at planar liquid–liquid interfaces, we show that nucleation of liquids in the crystal at crystal–vapor planar interfaces proceeds as a rule with a much higher rate compared to nucleation in the bulk of the crystal. Provided the surface tensions crystal–liquid (σcl), liquid–vapor (σlv), and crystal–vapor (σcv) obey the condition σcv=σcl+σlv, the work of critical cluster formation tends to zero; in the range σcv<σcl+σlv, it is less than one half of the work of critical cluster formation for bulk nucleation. The existence of a liquid–vapor planar interface modifies the work of critical cluster formation in crystal nucleation in liquids to a much less significant degree. The work of critical crystal cluster formation is larger than one half of the bulk value of the work of critical cluster formation, reaching this limit at σcv=σcl+σlv. The shape of the critical clusters can be described in both cases by spherical caps with a radius, R, and a width parameter, h. This parameter, h, is the distance from the cutting plane (coinciding with the crystal–vapor and liquid–vapor planar interface, respectively) to the top of the spherical cap. It varies for nucleation of a liquid in a crystal in the range (h/R)≤1 and for crystal nucleation in a liquid in the range 2≥(h/R)≥1. At σcv=σcl+σlv, the ratio (h/R) of the critical cluster for nucleation in melting tends to zero ((h/R)→0). At the same condition, the critical crystallite has the shape of a sphere located tangentially to the liquid–vapor interface inside the liquid ((h/R)≅2). We present experimental data which confirm the results of the theoretical analysis, and potential further developments of the theoretical approach developed here are anticipated.     

Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers

Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature T_g, whether there is an underlying thermodynamic transition at some finite temperature below T_g, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section discusses the specific case of polymeric glass-forming liquids, which usually have extremely high fragility. We emphasize the apparent difference between the glass transitions in polymers and small molecules. We also discuss the possible role of quantum effects in the glass transition of light molecules and highlight the recent discovery of the unusually low fragility of water. At the end, we formulate the major challenges and questions remaining in this field.     

Condensation and Crystal Nucleation in a Lattice Gas with a Realistic Phase Diagram

We reconsider model II of Orban et al. (J. Chem. Phys. 1968, 49, 1778–1783), a two-dimensional lattice-gas system featuring a crystalline phase and two distinct fluid phases (liquid and vapor). In this system, a particle prevents other particles from occupying sites up to third neighbors on the square lattice, while attracting (with decreasing strength) particles sitting at fourth- or fifth-neighbor sites. To make the model more realistic, we assume a finite repulsion at third-neighbor distance, with the result that a second crystalline phase appears at higher pressures. However, the similarity with real-world substances is only partial: Upon closer inspection, the alleged liquid–vapor transition turns out to be a continuous (albeit sharp) crossover, even near the putative triple point. Closer to the standard picture is instead the freezing transition, as we show by computing the free-energy barrier relative to crystal nucleation from the “liquid”.     

On Using the BMCSL Equation of State to Renormalize the Onsager Theory Approach to Modeling Hard Prolate Spheroidal Liquid Crystal Mixtures

Modifications to the traditional Onsager theory for modeling isotropic–nematic phase transitions in hard prolate spheroidal systems are presented. Pure component systems are used to identify the need to update the Lee–Parsons resummation term. The Lee–Parsons resummation term uses the Carnahan–Starling equation of state to approximate higher-order virial coefficients beyond the second virial coefficient employed in Onsager’s original theoretical approach. As more exact ways of calculating the excluded volume of two hard prolate spheroids of a given orientation are used, the division of the excluded volume by eight, which is an empirical correction used in the original Lee–Parsons resummation term, must be replaced by six to yield a better match between the theoretical and simulation results. These modifications are also extended to binary mixtures of hard prolate spheroids using the Boublík–Mansoori–Carnahan–Starling–Leland (BMCSL) equation of state.     

X-Ray kinetic study of glassy crystal formation in adamantane derivatives: TTT curves and crystal size effect

Abstract

The kinetics of polymorphic solid-state transformation in mixed adamantane compounds (CN_1?x Cl _x ADM: x = 0 and x = 0.25) have been studied by X-ray scattering. The classical form of the time-temperature-transformation TTT curves has been directly observed for the first time for the ordering supercooled plastic phases. For both compounds a considerable effect of crystal size on the kinetics has been observed. For x = 0.25 it leads to a continuous transition from Avrami to nucleation behaviour. These observations help us to understand the factors controlling nucleation and growth as well as to establish better operating conditions in order to form a glassy crystal.     

Kinetics of the Ordered Phase Growth across the Isotropic-Nematic Phase Transition in Liquid Crystal Azomethine Polymers during Cooling

Main-chain azomethine liquid crystal (LC) polymers varying in spacer length were studied using differential scanning calorimetry and polarizing optical microscopy. The nematic droplets appearing across the isotropic-nematic phase transition during cooling the polymer melt were treated statistically and their size distributions were described with equation based on the principles of irreversible thermodynamics. Kinetics of the ordered phase growth was described analytically with the universal law for cluster growth. Analysis of the mean LC droplet diameter as a function of time allowed recognition of two regimes of the ordered phase growth: i) nucleation and fast LC droplet growth and ii) consequent cluster coarsening. The length of the spacer in the polymer chain was shown to have an influence on the duration of the LC droplet growth regimes.     

酯类铁电液晶的相变和构型研究(英文)

：本文对酯类铁电液晶的相变和构型进行了研究和评述。它包括固相 近晶Ｃ,近晶Ｃ 近晶Ａ相变的性质,一级、二级相变的判据,固相 近晶Ｃ相变的临界弛豫,顺式、反式的构型转变和新相分析。     

Low-temperature phase behaviour of the binary system water–oxyethylated glycerol of polymerization degree n = 5 and intermolecular interactions in the system

In this paper, phase and glass transitions occurring in a binary system water–oxyethylated glycerol of polymerization degree n = 5 (OEG _{n =5}) are studied by differential scanning calorimetry (DSC) in the concentration range 0–100% (w/w) at temperatures lower than 273 K. The supplemented phase diagram of this system has been constructed for the first time. Concentration and temperature regions for the existence of a completely amorphous state and a heterogeneous crystalline/amorphous state have been determined. Processes of formation of crystalline and amorphous phases in the water–OEG _{n =5} system are studied by optical cryomicroscopy, both during cooling and subsequent heating. The cryomicroscopic data confirm the conclusions derived from the DSC data analysis. The number of strongly and weakly bound water molecules per OEG _{n =5} molecule has been calculated on the basis of the analysis of the supplemented phase diagram of the water–OEG _{n =5} system. The hydration number of an OEG _{n =5} molecule has been also determined by infrared spectroscopy.     

氟锆酸盐光纤玻璃中的析晶和分相现象

氟化物光纤玻璃中的夹杂物,分相颗粒和微晶颗粒等缺陷,造成光纤的散射损失,影响了实际制备工作.本文利用大视场光学显微镜,X射线衍射,透射电子显微镜和差热分析,研究了ZBLA氟化物玻璃中的分相和析晶现象,观察到了以成核生长机理进行的分相颗粒,析晶颗粒的成分是β-BaF_2·ZrF_4和α-BaF_2·2ZrF_4,通过DTA分析确定了这些相的析晶活化能.     

Phase transitions in melt-grown PbI2 crystals

Employing the method of zone- refining single crystals of PbI₂ of different purities were grown. Using X-ray diffraction technique their structure was found to be of the type 12R and in some cases 12R+4H. The crystals were re-examined after prolonged storage of several months at room temperature. Neither the crystals of very high purity nor of relatively less purity transformed. But thecrystals having intermediate purities did transform from 12R to 2H. This observation establishes a clear link between impurities and phase transitions. The results have been explained on the basis of nucleation and growth mechanism of stacking faults. Also it has been concluded that the presence of impurities is a must for the formation of polytypes.     

玻璃转变研究进展

玻璃转变是凝聚态物理基础理论中的一个重要问题和难题，是涉及动力学和热力学的众多前沿问题．玻璃转变的理论一直在不断的发展和更新．从20世纪50年代出现的自由体积理论到现在还在不断完善的模态耦合理论及其他众多理论，都只能解决玻璃转变中的某些问题．一个完整的玻璃转变理论仍需要人们作艰苦的努力．为了澄清混淆不清的玻璃转变概念，文章就玻璃转变的概念、研究内容和有关理论的发展进行简述．在分析了几个占主导地位的玻璃转变理论后，阐述了玻璃转变中需要进一步深入研究的问题．     

关于相变本构关系的一个一维模型

 以一含缺陷的一维杆模型模拟实际材料的相变得到了率相关相变本构关系。该本构预测的硫化镉（CdS）晶体的冲击相变波形,除了前驱波后的松弛沟外,在主要特征方面能很好地描述实验波形。预测的波形在形态上与Fe和锑化铟（InSb）的相变波形比较接近,说明这种本构可适合于更广泛的情况。     

西佛碱型液晶化合物相变过程中分子排列方式变化的拉曼光谱研究

用变温拉曼光谱对相变过程的研究表明,液晶化合物的初始晶态与熔融后缓慢降温得到的晶态并不吻合,两个状态下分子尾链的构象及刚性核部分的构象不同导致分子的聚集状态不同。西佛碱型液晶化合物VO10相变过程中,在晶态到液晶态相转变过程中,烷氧基尾链链内构象发生突变,同时有序性降低,刚性核部分两个苯环之间的二面角在相变点时发生明显变化,二面角加大。     

Stored energy in metallic glasses due to strains within the elastic limit

Room temperature loading of metallic glasses, at stresses below the macroscopic yield stress, raises their enthalpy and causes creep. Thermal cycling of metallic glasses between room temperature and 77 K also raises their enthalpy. In both cases, the enthalpy increases are comparable to those induced by heavy plastic deformation, but, as we show, the origins must be quite different. For plastic deformation, the enthalpy increase is a fraction (<10%) of the work done (WD) (and, in this sense, the behaviour is similar to that of conventional polycrystalline metals and alloys). In contrast, the room temperature creep and the thermal cycling involve small strains well within the elastic limit; in these cases, the enthalpy increase in the glass exceeds the WD, by as much as three orders of magnitude. We argue that the increased enthalpy can arise only from an endothermic disordering process drawing heat from the surroundings. We examine the mechanisms of this process. The increased enthalpy (‘stored energy’) is a measure of rejuvenation and appears as an exothermic heat of relaxation on heating the glass. The profile of this heat release (the ‘relaxation spectrum’) is analysed for several metallic glasses subjected to various treatments. Thus, the effects of the small-strain processing (creep and thermal cycling) can be better understood, and we can explore the potential for improving properties, in particular the plasticity, of metallic glasses. Metallic glasses can exhibit a wide range of enthalpy at a given temperature, and small-strain processing may assist in accessing this for practical purposes.     

On the Becker/Döring Theory of Nucleation of Liquid Droplets in Solids

Nucleation of liquid precipitates in semi-insulating GaAs is accompanied by deviatoric stresses resulting from the liquid/solid misfit. A competition of surface tension and stress deviators at the interface determines the nucleation barrier.The evolution of liquid precipitates in semi-insulating GaAs is due to diffusional processes in the vicinity of the droplet. The diffusion flux results from a competition of chemical and mechanical driving forces.The size distribution of the precipitates is determined by a Becker--Dö-ring system. The study of its properties in the presence of deviatoric stresses is the subject of this study. The main tasks of this study are: (i) We propose a new Becker/Döring model that takes thermomechanical coupling into account. (ii) We compare the current model with already existing models from the literature. Irrespective of the incorporation of mechanical stresses, the various models differ due to different environments where the evolution of precipitates takes place. (iii) We determine the structure of equilibrium solutions according to the Becker/Döring model, and we compare these solutions with those that result from equilibrium thermodynamics.     

变分累积展开方法中确定相变点的新方法

在计算有限温度SU(2)格点规范模型Polyakov线的过程中,考虑到变分累积展开法中有限距离相互作用效应与Monte Carlo中的有限体积效应的相似性,采用了新的方法来确定相变点.与已有的方法以及Monte Carlo模拟给出的结果比较,新方法给出的结果更接近Monte Carlo结果.     

A generic equation of state for liquid crystalline phases of hard-oblate particles

A generic equation of state (EoS) is developed for the hard cylindrical disc model to describe the isotropic phase of hard cut-sphere particles introducing a correction parameter to incorporate the negative contributions from higher-order virial coefficients. The isotropic–nematic–columnar phase diagram of hard cut-sphere fluids is investigated combining the new EoS with a scaled Onsager free energy for the nematic phase and an extended cell theory for columnar phase. By mapping the virial coefficients of an oblate spherocylinder on to those of the cylindrical disc (which are known algebraically), the new generic EoS is used to describe the isotropic and nematic phases of hard oblate spherocylinder particles. The predictions of the generic EoS are compared with available simulation data.