Controlling polymorphism during the crystallization of an atomic fluid

We use molecular dynamics simulations to shed light on polymorph selection during the crystallization of the Lennard-Jones fluid. By varying pressure at fixed supercooling, we form large crystallites either of the stable face centered cubic form or of the metastable body centered cubic form and even fine-tune the fractions of stable and metastable polymorphs in the crystallite. We demonstrate that the conditions of crystallization, leading to large bcc crystallites, lie within the occurrence domain of the metastable bcc polymorph. We also find that the predominantly fcc crystallites contain a notable amount of the hexagonal close packed form, due to the cross nucleation of the hcp form on the fcc form. By varying temperature at fixed pressure, we prevent cross nucleation and form pure fcc crystallites. Our results reveal that polymorph selection may take place, and be controlled, during the growth step.     

Evidence for out-of-equilibrium crystal nucleation in suspensions of oppositely charged colloids

We report a numerical study of the rate of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free-energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for subcritical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation.     

亚稳相的高压暴露

 很久以前,便有人指出,气态冷凝成固态时,要连续经历液相及各种高温相,才达到平衡结晶相。但是,液态及高温相往往需靠很大的冷却速度才能冻结下来,这在当时对绝大多数合金,是不可能的。近些年,随着超急冷等技术的进步,关于非晶等亚稳相得研究十分活跃。当超过一定临界冷却速度时,液态合金可固化为非晶态。虽然,亚稳结晶相较非晶应更容易冻结,但是,由于产生各种亚稳相所需的过冷条件各不相同,以及对冷却速度的选择不能是任意的,因此有时它们较非晶还难于形成。与液相凝固过程相似,非晶合金的晶化也服从构型最小重排原理,即在晶化完成之前,存在某些亚稳相变态阶段。但是,限于热力学上的不稳定性及动力学因素,在常压下这些亚稳相同样是难以发现的。作者根据对多种合金系的研究,提出高压暴露亚稳相的设想,并利用非晶等亚稳相的高压变态过程,将进行液态急冷时的速度控制方式,改为便于掌握的高压退火方式,来获得新亚稳相。本文对压力暴露亚稳相的原理和实践,加以论述。     

Non-uniformities in polymer/liquid crystal mixtures

We theoretically model the nucleation of nematic droplets during phase ordering in mixtures of a flexible polymer and a low-molecular-weight liquid crystal. By appealing to classical nucleation theory (CNT), we calculate the energy barrier to nucleation and the size of a critical nucleus. We study the influence of a metastable intermediate phase on the nucleation of the nematic. Below a triple point in the phase diagram, there are two distinct mechanisms for the formation of a nematic nucleus: 1) direct nucleation from the isotropic phase and 2) nucleation via a precursor metastable isotropic phase. We calculate the crossover concentration as a function of temperature, delineating the regions of the phase diagram in which each mechanism prevails. In the latter case, the presence of a hidden metastable isotropic-isotropic binodal may either promote or delay the nucleation of a nematic phase. Received 9 August 2002 RID="a" ID="a"e-mail: matuyama@chem.mie-u.ac.jp     

在熔融金属溶液中石墨在金刚石稳定区结晶的可能性

本文给出了在超高压高温条件下,碳的亚稳相石墨从熔融金属溶液中结晶的实验现象,并以此为例分析了亚稳相从溶液结晶的热力学可能性。提出了促使亚稳相结晶的过饱和的概念,指出:溶液体系中的温度差是亚稳相结晶的推动力。重点给出在一个结晶条件下亚稳相石墨可能结晶的临界温度差的计算。 关键词：     

Undercooling and metastable phase formation in a Bi95Sb5 melt

High undercoolings have been obtained in bulk Bi₉₅Sb₅ alloy melts by the cyclic superheating and cooling technology. The highest undercooling that was achieved in this paper is 121 K. The influence of various processing factors on the undercooling behavior is examined. Undercooling of 121 K leads to the formation of a metastable solid phase with the tetragonal crystal structure. The phase selection and the metastable phase formation have been discussed based on the classical nucleation theory. A criterion that contains the relative melting temperature, the relative molar volume of the solid, the relative structure-dependent factor, and the undercooling has been developed to interpret the formation of the metastable tetragonal phase. Received: 12 January 2000 / Accepted: 28 March 2000 / Published online: 13 July 2000     

Phase transitions in condensed media at a finite rate of formation of a metastable state

The influence of the finite rate of formation of a metastable state on the kinetics of the first-order phase transition is analyzed. The conditions determined by the thermodynamic parameters and the cooling rate of the system under consideration are derived. Under these conditions, the formation of the metastable state can be treated either as an instantaneous process, when nucleation occurs at the end of the cooling stage, or as a slow process, when intensive nucleation of a new phase proceeds within the cooling stage. An equation describing the time and temperature that correspond to intensive nucleation of new-phase particles is obtained. The nucleation stage of the new phase takes place in the immediate vicinity of the temperature determined from this equation. All the other parameters, which determine the kinetics of the initial and transient stages of the phase transition, are calculated with respect to this temperature. As an example, all the relationships for a weak solid solution are presented.     

非晶一次晶化过程微观组织演化和生长动力学的相场法研究

在晶化物理模型中添加扩散系数对晶化过程的影响, 采用相场方法研究初始形核率和初始形核半径对一次晶化过程中微观组织和生长动力学的影响。结果表明: 随着初始形核率的增加, 相同时间内非晶一次晶化的晶粒数量逐渐增加, 晶粒尺寸逐渐减小。晶化分数随着演化时间和初始形核率的增加逐渐增大, 初始形核率越大, 相同演化时间内的晶化分数越高。不同初始形核半径情况下, 非晶一次晶化过程中的晶粒数量和尺寸随着演化时间的增加基本保持不变。晶化分数随着演化时间的增加而增大。不同初始形核率和初始形核半径情况下所对应的生长指数均小于1, 表明初始形核率和初始形核半径对晶化方式无影响, 均为一次晶化。改变初始形核率和初始形核半径可调控一次晶化微观组织结构, 而晶粒尺寸及晶化分数直接关系到合金性能。     

Salt crystallization as damage mechanism in porous building materials--a nuclear magnetic resonance study

Salts can damage building materials by chemical reactions or crystallization, which is a serious threat to cultural heritage. In order to develop better conservation techniques, more knowledge of the crystallization processes is needed. In a porous material, the size of a salt crystal is limited by the sizes of the pores. It has been predicted that as a consequence, the solubility of a salt increases with decreasing pore size. This increase seems to be related to an increase of the stress generated by a crystal on the pore wall. It has been suggested that the resulting stress could become high enough to induce failure. We have studied the crystallization of salts in porous materials with well-defined pore sizes. Samples were saturated at 40 degrees C with saturated Na2SO4 and Na2CO3 solutions. Next we have cooled the samples to 0 degrees C and waited for nucleation. After nucleation occurred, the solubility in the porous material was measured with nuclear magnetic resonance (NMR) as a function of the temperature. The measurements on Na2CO3 indeed show an increase in solubility with a decrease in pore size. For Na2SO4, we did not observe a pore size-dependent solubility. However, we have to remark that these results show a metastable crystal phase. The results can be used to calculate the actual pressure exerted by the crystals onto the pore wall.     

Ultrasound-assisted intensified crystallization of L-glutamic acid: Crystal nucleation and polymorph transformation

In this paper, the crystallization of L-glutamic acid with application of ultrasound was explored in detail, including the process of nucleation, polymorphic control and polymorphic transformation. The induction time and metastable zone widths (MSZWs) were measured with and without ultrasound during the nucleation process. The induction time and MSZWs were decreased by ultrasound and the induction time was further decreased by higher ultrasonic power. The calculated nucleation parameters (such as interfacial energy, critical nucleus size and critical Gibbs energy) showed an obvious decrease in the presence of ultrasound, indicating that the nucleation was enhanced with application of ultrasound. By adjusting the ultrasonic power in the quench cooling process, the difference in nucleation temperatures would determine the distribution of polymorphs. In further, the polymorphic transformation was investigated quantitatively, and to the best of our knowledge, it was the first time to study the transformation kinetics with ultrasound using Avrami-Erofeev model. In the transformation process, the crystallization mechanism of the stable form was modified by ultrasound. The ultrasound eliminated the nucleation element in the rate-limiting step and facilitated the crystal growth of stable form. Thus, the ultrasound has a profound influence on L-glutamic acid crystallization.     

Influence of Polycarbonate on the Crystallization Behavior of Poly(Trimethylene Terephthalate)/Polycarbonate Blends

To determine the factors influencing the retardation of the crystallization of poly(trimethylene terephthalate) (PTT) when PTT is blended with polycarbonate (PC), different PTT/PC blends were prepared via the melt mixing method. The relationships between the crystallization behavior and blend composition, as well as the phase morphology, were investigated. The results showed that the predominant reason for the retardation in crystallization is due to the PC content and phase morphology. The PC influences the crystallization of PTT via two methods. First, it retards PTT crystallization. Secondly, the PC exhibits a nucleation effect on the PTT crystallization which is, however, much weaker compared to the negative effect PC exerts with regards to PTT crystallization. When the processing temperature and shear rate remains unchanged, the two effects of PC determine the crystallization behavior of the blend. The phase morphology, which is strongly dependent on the mixing temperature and the shear rate, and which is also related to mixing time, had an appreciable impact on PTT crystallization. In the case of similar adhesion with the interface, a finer PC phase domain would show a slightly stronger nucleation effect on PTT crystallization.     

Enhancement of lysozyme crystallization under ultrasound field

With the increasing demand for biopharmaceuticals, a method to crystallize biomolecule products with high quality, high yield and uniform size distribution as well as regular crystal habit is needed. In this work, ultrasound was used as a nucleation accelerator to decrease the energy barrier for lysozyme crystal formation. Crystallization experiments on egg-white lysozyme were carried out with and without ultrasound. The effect of ultrasound on induction time, metastable zone width, crystal size and morphology and process yield was investigated in detail. The nucleation-promoting effect produced by ultrasound is illustrated by the reduction of metastable zone width and induction time. By inducing faster nucleation, ultrasound leads to protein crystals grow at lower supersaturation levels with shorter induction time. It was found that ultrasound could result in uniform size distribution of the product due to the preventing of aggregation. However, long time continuous application of ultrasound could result in smaller particle size. Hence, ultrasonic-stop method was found to be a more appropriate strategy to enhance the crystallization process of proteins such as lysozyme.     

Ni51Zr49非晶合金晶化过程的电子显微镜研究

用透射电子显微镜对Ni₅₁Zr₄₉非晶合金的晶化过程进行了研究,发现晶化初期首先从非晶基体上析出的是具有C心单斜点阵的NiZr＇相,a=0.3268nm,b=0.4101nm,c=0.5224nm,β=71.8°。与稳定相NiZr有很明确的取向关系:[100]_NiZr＇∥[100]_NiZr [001]_NiZr＇∥[110]_NiZr。它可以看成是NiZr相的亚稳态或超结构。随温度升高,逐渐转变成NiZr相。温度再高时,在非晶基体上还会析出少量Ni₁₀Zr₇相。晶化初期,无论是NiZr还是NiZr＇相,都是以多次孪晶的形式析出。孪晶有这两种基本形式,一种是以NiZr或NiZr＇的[021]方向为轴的180°孪晶,一种是以NiZr的[001]或NiZr＇的[010]方向为轴的36°旋转孪晶,其它孪晶为这两种孪晶方式的重复与组合。随着温度的升高,孪晶逐渐减少。 关键词：     

Self-organization of spatially inhomogeneous structures during volume crystallization of polydisperse systems

The kinetic instability arising during crystallization of metastable supercooled binary melts (produced by applying laser radiation to respective binary compounds) and culminating in the formation of ordered structures is studied. The nonlinear self-organization of inhomogeneous structures in a polydisperse system is statistically simulated using (i) a self-consistent set of kinetic equations for the cluster distributions in space and size and (ii) balance equations for the temperature and concentration of dissolved impurities. Allowance is made for cluster growth and evaporation, dependence of the phase transition temperature on the curvature of the cluster surface and impurity concentration, diffusion mobility of the clusters, and their withdrawal from the nucleation zone. The domain of the kinetic instability and the scale of the resulting inhomogeneous structures are determined.     

Dynamics of transient metastable states in mixtures under coupled phase ordering and chemical demixing

We present theory and simulation of simultaneous chemical demixing and phase ordering in a polymer-liquid crystal mixture in conditions where isotropic-isotropic phase separation is metastable with respect to isotropic-nematic phase transition. In the case the mechanism is nucleation and growth, it is found that mesophase growth proceeds by a transient metastable phase that surround the ordered phase, and whose lifetime is a function of the ratio of diffusional to orientational mobilities. In the case of spinodal decomposition, different dynamic regimes are observed depending on the mobility ratio: metastable phase separation preceding phase ordering, phase ordering preceding phase separation, or simultaneous phase ordering and phase separation. Not only the overall dynamics but also the final structure of the material can be different for each kinetic regime.     

The application of power ultrasound to reaction crystallization

The action of power ultrasound in controlling the supersaturation, nucleation and crystal growth during the acid-base reaction crystallization of 7-amino-3-desacetoxy cephalosporanic acid (7-ACDA) has been investigated. The experimental results show that ultrasound can mix the reaction more efficiently and uniformly than conventional agitation methods. Insonation leads to the reduction of both the induction period and metastable zone width of the crystallization, so that the process of crystallization can be more efficiently controlled than without insonation. Agglomeration is also greatly reduced. Adjusting the ultrasonic parameters can modulate the crystal size and size distribution. The mechanism of the sonoprocess has been analyzed and comments are made on the potential applicability of sonication in large-scale crystallization practice.     

Formation of NI80P20 solid solution phase under pressure

A Ni-P solid solution phase was obtained by quenching of melts under a pressure of 4.5 GPa. This was considered as a metastable high pressure phase. Despite the lack of thermodynamic parameters for Ni₈₀, P₂₀ alloy under pressure, the degree of undercooling, nucleation frequency and crystal growth velocity were calculated. We conclude that metastable phases with the same composition as the melting phase, such as supersaturated solid solution phase and amorphous phase, are easily prepared by high-pressure quenching.     

Low-temperature crystallization of the Fe−Co−B amorphous alloys

Low-temperature crystallization of the melt-spun amorphous ribbon Fe₆₄Co₂₁B₁₅ was measured by the⁵⁷Fe Mössbauerin situ scanning method in the temperature range of about 300 to 350°C. Least-squares fit of the time-dependent α-Fe?Co isothermal absorption intensity revealed up to three superimposed exponential processes. Taking into account that this method indicates the content of the crystallized metallic phase only, then the first two (nucleation and primary crystallization) processes are followed by a rather slow third one, which could only mean the decay of metastable (Fe, Co)₃B having a thermally activated character as well. Besides confirming the previously reported activation enthalpies for the two faster processes, that of the slowest one was determined as (450±10) kJ/mol.     

Determination of the colloidal crystal nucleation rate density

Colloidal suspensions of charged latex microspheres in water exhibit liquid-like or crystalline ordering depending on particle interaction and concentration. By virtue of large particle spacing and slow dynamics, colloidal systems offer a unique opportunity to study interfacial structure and dynamics. This paper presents the first reported experimental study of the nucleation rate density, c, of an nonequilibrium (supercooled) colloidal liquid to colloidal crystal first order phase transition. Local and global observations of colloidal crystals growing from a metastable colloidal liquid were used to determine c. Microscopic local observations revealed homogeneous nucleation and constant interface velocity growth of quasispherical crystallites in the bulk and heterogeneous nucleation of a crystalline sheet with lower growth velocity at the cell wall. Complementary global observations of the recrystallization transition made by measuring the time dependence of the suspension transparency (the fraction of transmitted laser light) determined c by fitting this curve to a model based on an extension of Avrami's theory of crystallization.     

Quantitative characterization of critical nanoclusters nucleated on large single molecules

First order phase transitions involve nucleation, formation of nanoscale regions of a new phase within a metastable parent phase. Using the heterogeneous nucleation theorem we show how clusters formed by nucleation on single molecules evolve from the gas phase and determine the critical size beyond which condensation starts to form aerosol particles. Our experiments reveal the activation of molecules into droplets to happen via formation of critical clusters substantially larger than the seed molecule. The nanosized critical clusters were found to be well predicted by the Kelvin-Thomson relation pointing directly to the key step in the phase transition.