Formation and evolution of metastable bcc phase during solidification of liquid Ag: a molecular dynamics simulation study

On the basis of the quantum Sutton-Chen potential, the rapid solidification processes of liquid silver have been studied by molecular dynamics simulation for four cooling rates. By means of several analysis methods, the competitions and transitions between microstructures during the cooling processes have been analyzed intensively. It is found that there are two phase transitions in all simulation processes. The first one is from liquid state to metastable (transitional) body-centered cubic (bcc) phase. The initial crystallization temperature T(ic) increases with the decrease of the cooling rate. The second one is from the transitional bcc phase to the final solid phase. This study validates the Ostwald's step rule and provides evidence for the prediction that the metastable bcc phase forms first from liquid. Further analyses reveal that the final solid at 273 K can be a mixture of hexagonal close-packed (hcp) and face-centered cubic (fcc) microstructures with various proportions of the two, and the slower the cooling rate is, the higher proportion the fcc structure occupies.     

高分子结晶理论的新概念与新进展

回顾了传统的高分子结晶成核与生长模型,指出了该模型在应用中遇到的一些问题;同时总结了Strobl根据近年小角X射线散射结果提出的高分子结晶新机理-中介相机理.介绍了Strobl等构建的热动力学图解对熔体、中介相和片晶的转变过程,阐述了各相间的平衡转变温度、潜在的转变热以及表面自由能,说明了处于熔体和晶体之间的中介相的热动力学性质是理解高分子结晶过程的重要依据.     

Atomistic simulation of the homogeneous nucleation and of the growth of N2 crystallites

We report on a computer simulation study of the early stages of the crystallization of molecular nitrogen. First, we study how homogeneous nucleation takes place in supercooled liquid N(2) for a moderate degree of supercooling. Using the umbrella sampling technique, we determine the free energy barrier of formation for a critical nucleus of N(2). We show that, in accord with Ostwald's rule of stages, the structure of the critical nucleus is predominantly that of a metastable polymorph (alpha-N(2) for the state point investigated). We then monitor the evolution of several critical nuclei through a series of unbiased molecular dynamics trajectories. The growth of N(2) crystallites is accompanied by a structural evolution toward the stable polymorph beta-N(2). The microscopic mechanism underlying this evolution qualitatively differs from that reported previously. We do not observe any dissolution or reorganization of the alpha-like core of the nucleus. On the contrary, we show that alpha-like and beta-like blocks coexist in postcritical nuclei. We relate the structural evolution to a greater adsorption rate of beta-like molecules on the surface and show that this transition actually starts well within the precritical regime. We also carefully investigate the effect of the system size on the height of the free energy barrier of nucleation and on the structure and size of the critical nucleus.     

Size-dependent phase stability of a molecular nanocrystal: a proxy for investigating the early stages of crystallization

We make the link between the size-dependent phase stability of a nanocrystal and the phase-transition behavior of emerging crystallites during the earliest stages of crystallization, by using the former as a proxy for the latter. We outline an extension of the classical nucleation theory to describe crystal nucleation and subsequent transformations of competing polymorphic phases that characterize Ostwald's rule of stages. The theoretical framework reveals that the relative stability of the competing phases is a function of cluster size, which in turn varies with time, and therefore explains the complex transformation behavior observed for some systems. We investigated the stability of a nanocrystal of dl-norleucine by means of molecular simulation as a proxy for post-nucleation phase-transformation behavior in emerging crystallites. The simulations reveal that, for nanocrystals, the surface energy of the transition state of a transformation can dominate the barrier to phase change, thus causing metastable phases to be stabilized, not because they are thermodynamically stable, but rather due to kinetic hindering. Therefore, in the context of the earliest stages of crystal growth, not only does phase stability vary as a function of cluster size, and hence time, but thermodynamically feasible transformations are also prone to kinetic hindering.     

Role of liquid polymorphism during the crystallization of silicon

Using molecular simulation, we establish the pivotal role played by liquid polymorphs during the crystallization of silicon. When undercooled at a temperature 20% below the melting point, a silicon melt is under the form of the highly coordinated, high-density liquid (HDL) polymorph. We find that crystallization starts with the formation, within the HDL liquid, of a nanosized droplet of the least stable liquid polymorph, known as the almost tetracoordinated low-density liquid (LDL) polymorph. We then show that the crystalline embryo forms within the LDL droplet, close to the interface with the surrounding HDL liquid, thereby following a pathway associated with a much lower free energy barrier than the direct formation of the crystalline embryo from the HDL liquid would have required. This implies that, for substances exhibiting liquid polymorphs, theories, like the classical nucleation theory, and empirical rules, like Ostwald's rule, should be modified to account for the role of liquid polymorphs in the nucleation process.     

Kinetics study of crystallization with the disorder-bcc-fcc phase transition of charged colloidal dispersions

Structure transformation (disorder-bcc-fcc) in charged colloidal dispersions, as a manifestation of the Ostwald's step rule, was confirmed by means of reflection spectrum (RS) measurements in our previous study. By taking advantage of a reflection spectrum containing plenty of information about the crystallization behaviors, time-dependent changes of parameters associated with the crystal structure and composition during the disorder-bcc-fcc transition are reported by treating the data from RS in this article. In addition, Avrami's model is adopted to analyze the transition process and investigate the transition rate. On the basis of the above investigations, associated kinetic features of crystallization with the disorder-bcc-fcc transition are described.     

Cyclodextrin-based isolation of Ostwald's metastable polymorphs occurring during crystallization

We describe a novel approach for the selective isolation of Ostwald's intermediate metastable polymorphs occurring during an early stage of crystallization, by utilizing the inclusion complex formed with a cyclic oligosaccharide derivative, 2,6-di-O-methyl-beta-cyclodextrin.     

Molecular simulation of the homogeneous crystal nucleation of carbon dioxide

We report on a molecular simulation study of the homogeneous nucleation of CO2 in the supercooled liquid at low pressure (P = 5 MPa) and for degrees of supercooling ranging from 32% to 60%. In all cases, regardless of the degree of supercooling, the structure of the crystal nuclei is that of the Pa3 phase, the thermodynamically stable phase. For the more moderate degree of supercooling of 32%, the nucleation is an activated process and requires a method to sample states of high free energy. In this work, we apply a series of bias potentials, which promote the ordering of the centers of mass of the molecules and allow us to gradually grow crystal nuclei. The reliability of the results so obtained is assessed by studying the evolution of the nuclei in the absence of any bias potential, and by determining their probability of growth. We estimate that the size of the critical nucleus, for which the probability of growth is 0.5, is approximately 240 molecules. Throughout the nucleation process, the crystal nuclei clearly exhibit a Pa3 structure, in apparent contradiction with Ostwald's rule of stages. The other polymorphs have a much larger free energy. This makes their formation highly unlikely and accounts for the fact that the nucleation of CO2 proceeds directly in the stable Pa3 structure.     

Polymorphism in 2,4,6-trinitrotoluene crystallized from solution.

An examination has been made of the role of solvent type in the definition of the polymorphic nature of 2,4,6-trinitrotoluene precipitated from solution. A combination of calorimetric and structural techniques including in situ crystallization studies using synchrotron radiation has shown that the variations in polymorphic form following precipitation from solution do not arise specifically from any stereospecific guidance that the nature of the solvent might impose on the structural form. Rather the differences are linked to the variations in solubility and hence supersaturation which might build up prior to nucleation and growth and to the isolation of the metastable orthorhombic phase from the solvent. The final conclusion is that the changes fit well with Ostwald's Law of Stages with the orthorhombic form always precipitating initially followed by its conversion to the stable monoclinic form. The previously observed tendency for some solvents to yield one or the other form then becomes attributable to kinetics in solution rather than structural control. It can be associated with the solubility of the material in the solvent used and the role of this factor in a solvent-mediated phase transformation. On this basis rules can be formulated for the isolation of the metastable forms of this and similarly related polymorphic systems.     

Dual roles of borax in kinetics of calcium sulfate dihydrate formation

An additive is not exclusively retardant or promoter for a crystallization system. The kinetic studies of calcium sulfate dihydrate (CSD) crystal growth demonstrated that borax played dual roles in the reaction, which accelerated CSD formations at the low concentration levels but inhibited the crystal growth at the high ones. In situ atomic force microscopy studies revealed that borax modulated the CSD crystallization via two different pathways: promoted the secondary nucleation to increase the step density on the growing crystal faces but simultaneously retarded the spread of these growth steps by the Langmuir adsorption. These two contradictory factors were incorporated in the crystallization, and their balance was regulated by the borax concentration. Both the macroscopic and microscopic experimental data nicely displayed the crystallization model of birth and spread that was able to account for the behaviors of borax in CSD formations.     

Order-parameter-based Monte Carlo simulation of crystallization

A Monte Carlo simulation method is presented for simulation of phase transitions, with emphasis on the study of crystallization. The method relies on a random walk in order parameter Phi(q(N)) space to calculate a free energy profile between the two coexisting phases. The energy and volume data generated over the course of the simulation are subsequently reweighed to identify the precise conditions for phase coexistence. The usefulness of the method is demonstrated in the context of crystallization of a purely repulsive Lennard-Jones system. A systematic analysis of precritical and critical nuclei as a function of supercooling reveals a gradual change from a bcc to a fcc structure inside the crystalline nucleus as it grows at large degrees of supercooling. The method is generally applicable and is expected to find applications in systems for which two or more coexisting phases can be distinguished through one or more order parameters.     

Cross-nucleation between ROY polymorphs

Cross-nucleation between polymorphs is a newly discovered phenomenon important for understanding and controlling crystal polymorphism. It contradicts Ostwald's law of stages and other theories of crystallization in polymorphic systems. We studied the phenomenon in the spontaneous and seeded melt crystallization of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY), currently the most polymorphic system of known structures. We observed extensive and sometimes selective cross-nucleation between ROY polymorphs. Certain polymorphs could not nucleate without the aid of others. The new polymorph was found to be more or less thermodynamically stable than the initial one but to always grow faster than or as fast as the initial one. The temperature and surface characteristics of the seed crystals affected the occurrence of cross-nucleation. Our results show that the pathway of crystallization in polymorphic systems is not determined solely by the initial nucleation, but also by cross-nucleation between polymorphs and the different growth rates of polymorphs. This study identified a new metastable polymorph of ROY, the 10th of the family.     

Molecular Insights into the Effect of Nitrogen Bubbles on the Formation of Tetrahydrofuran Hydrates

In this work, a molecular dynamics simulation was conducted to study the microscopic mechanism of how nitrogen bubbles affect the formation of THF hydrates at the molecular level. The results obtained reveal that the nitrogen bubble can promote the formation of THF hydrates. In the system with a nitrogen bubble, more THF-filled cages were generated, and the crystal structure was more orderly. The promotion of nitrogen bubbles on hydrate crystallization comes from the dissolution of nitrogen molecules. Some of dissolved nitrogen molecules can be enclosed in small hydrate cages near the nitrogen bubble, which can serve as stable sites for hydrate crystal growth, resulting in the fact that THF-filled cages connected with N₂-filled cages are much more stable and have a long lifetime. The results in this work can help to understand the promotion effect of micro- and nano-air bubbles on the crystallization of THF hydrates.     

Synchrotron radiation macrobeam and microbeam X-ray diffraction studies of interfacial crystallization of fats in water-in-oil emulsions

Using macrobeam and microbeam techniques, we performed synchrotron radiation X-ray diffraction (SR-XRD) analyses of fat crystallization in water-in-oil (W/O) emulsion, in combination with DSC and polarized optical microscopic observation. Particular focus was on the crystallization of the fats around water droplets in the W/O emulsion systems using food emulsifiers of polyglycerol polyricinoleate (PGPR) alone (PGPR emulsion), and PGPR and monobehenoylglycerol (MB) (PGPR+MB emulsion). We obtained the following results: (1) macrobeam SR-XRD confirmed that adding MB promoted fat crystallization during cooling, (2) microbeam SR-XRD indicated that the lamellar planes of fat crystals near the water and oil interfaces are arranged almost parallel to the interface planes in both PGPR emulsion and PGPR+MB emulsion, and (3) adding MB resulted in the formation of tiny fat crystals because it promoted crystallization, which occurred both in the bulk oil phase and at the W/O interfaces. The present study is the first to apply microbeam SR-XRD to observe the microscopic features of fat crystallization in W/O emulsion, following fat crystallization in the oil droplets in the oil-in-water (O/W) emulsion (Arima, S.; Ueno, S.; Ogawa, A.; Sato, K. Langmuir 2009, 25, 9777-9784).     

Low-temperature adiabatic calorimetry of salol and benzophenone and microscopic observation of their crystallization: finding of homogeneous-nucleation-based crystallization

An adiabatic calorimetric and direct microscopic observation of crystallization at low temperatures was performed for salol and benzophenone. Both of the materials in the supercooled-liquid phase exhibited a low-temperature crystallization proceeding in the glass-transition temperature ( T_g) region. The crystallization was observed to proceed as the advance of the crystal front into the liquid phase, and stopped suddenly atT =  227 K and T ≈  215 K for salol and benzophenone, respectively. This anomalous temperature dependence of the crystal-growth rate is a characteristic feature of the homogeneous-nucleation-based (HNB) crystallization, which has been reported for o -terphenyl and triphenylethylene, and the presently observed low-temperature crystallization in salol and benzophenone was concluded to be the HNB crystallization, in which the crystal growth is brought about by the coalescence of crystal nuclei to the crystalline phase on the liquid-crystal interface. The substances exhibiting the HNB crystallization were revealed to have almost the same characteristics with respect to configurational entropy, and undergo the HNB crystallization in the same range of temperature normalized by T_g. These results indicate that the HNB crystallization is potentially a universal phenomenon in fragile liquids, and imply that the process is closely related to the liquid structure changing with temperature.     

Growth dynamics of isotactic polypropylene single crystals during isothermal crystallization from a miscible polymeric solvent

The present article presents a spatiotemporal growth of isotactic polypropylene (iPP) single crystals, melt crystallized from a polymeric solvent, i.e., poly (ethylene octene) copolymer that is known to be miscible with iPP. Optical and atomic force microscopic investigations reveal that the melt grown single crystals of iPP develop in the form of two parallel rows of crystal lamellae, but these crystals merge at the tips. To elucidate the mechanism of these emerging parallel rows of iPP crystals, a phase field model pertaining to solidification phenomena has been employed that involves a nonconserved crystal order parameter and a chain-tilting angle. This phase field model is based on the free energy of crystallization, having an asymmetric double well, and a tensorial surface free energy of the crystal interface coupled with a curvature elastic free energy that is possessed by the solid-liquid interface. The spatiotemporal simulation of iPP single crystal growth has been carried out on a square lattice based on the finite difference method for spatial steps and an explicit method for temporal steps with a periodic boundary condition. The appearance of the seemingly twin crystal is captured in the simulation, which may be attributed to the sector demarcation that is taking place in the anisotropically growing single crystal of iPP.     

On the formation of calcium carbonate thin films under Langmuir monolayers of stearic acid

In this publication, we describe the growth of thin films of calcium carbonate beneath Langmuir monolayers of stearic acid. The size and shape of the crystalline structures were systematically studied by means of different microscopic techniques including Brewster angle microscopy, atomic force microscopy and scanning electron microscopy. In a series of experiments, we explored the calcium carbonate crystallization process for different lipid monolayers and subphases. The observed phenomena support a crystallization process which is induced by a thin, film-like structure of a precursor phase. The basic processes of crystal and aggregate formation can be represented by a simple model which is based on electrostatic interactions between the surfactant film and the inorganic calcium carbonate structures.     

Use of Aggregation‐Induced Emission for Selective Detection of Phase Transformation during Evaporative Crystallization of Hexaphenylsilole

Crystallization of organic molecules is quite complicated because the crystallization process is governed by weak intermolecular interactions. By exploiting aggregation‐induced emission (AIE), we attempted to realize the selective detection of phase transformation during the evaporative crystallization of hexaphenylsilole (HPS), which shows different fluorescent colors in the amorphous and crystalline phases. No fluorescence emission was observed in the HPS solution immediately after dropping on the glass substrate due to the non‐radiative deactivation induced by intramolecular rotational or vibrational motion, suggesting that HPS exists as a monomer in solution. As time elapsed after dropping, green emission first appeared, which changed to blue after solvent evaporation, because of phase transformation from the amorphous state to the crystalline state. This phenomenon supports not only the two‐step nucleation model involving an intermediate such as a liquid‐like cluster prior to nucleation but also the real‐time detection of Ostwald's rule of stages during evaporative crystallization.     

Stability and growth of nuclei during the crystallization of polymers

During the crystallization of linear flexible chain molecules a boundary phase necessarily develops which has a volume of its own. The boundary phase, however, is not autonomous so that Gibbs' phase rule loses its validity. The 2-phase system {crystal; boundary phase} is bivariant, the 3-phase system {crystal, boundary phase; melt} is monovariant. At quasistatic cooling the ability of the crystal nuclei to grow demands the formation of loose loops or tie-molecules. This leads to the final state of the crystallization to be an arrested equilibrium state. The internal equilibrium state cannot be reached under normal circumstances.Dedicated to H.-G. Kilian on the occasion of his 60th birthday.