Crystallization of Spheres

Simplified computer models are used to gain insight into more complex real systems. In a reversion of this protocol, a colloidal suspension of submicron spherical particles, approximately hard and uniform, was recently crystallized in space and analyzed for crystal type. The objective was to establish how, and to what structure, hard spheres crystallize without gravity. Computational statistical thermodynamics predicts an equilibrium constant between fcc and hcp of order unity. The microgravity experiments, however, resulted in a random hybrid close-packed structure (rhcp) such that long-range order is two-dimensional. Here we report the mechanism from idealized computer experiments for crystallization of spheres from the metastable fluid. Model systems of up to N=64,000 spheres with infinite spatial periodicity have been crystallized in runs of up to 10 billion collisions. When the fluid, initially in a metastable supercooled state at 58% packing, is allowed to nucleate and freeze, a variety of structures emerges. There are three identifiable stages of structural growth: (i) initial nucleation of fcc, rhcp, and also bcc-like (body-centered cubic) local structures; (ii) rapid growth of all incipient nucleites to random stacked two-dimensional hexagonal (rhcp) grains, plus some fcc, to fill the volume; and (iii) relatively slow dissolution of unstable rhcp faces at grain boundaries. Eventually, stable nucleites emerge comprising hexagonal layers, arranged so as to contain predominantly either fcc arrangements of spheres or rhcp, in roughly 50% proportions.     

Crystallization in Confinement

Many crystallization processes of great importance, including frost heave, biomineralization, the synthesis of nanomaterials, and scale formation, occur in small volumes rather than bulk solution. Here, the influence of confinement on crystallization processes is described, drawing together information from fields as diverse as bioinspired mineralization, templating, pharmaceuticals, colloidal crystallization, and geochemistry. Experiments are principally conducted within confining systems that offer well-defined environments, varying from droplets in microfluidic devices, to cylindrical pores in filtration membranes, to nanoporous glasses and carbon nanotubes. Dramatic effects are observed, including a stabilization of metastable polymorphs, a depression of freezing points, and the formation of crystals with preferred orientations, modified morphologies, and even structures not seen in bulk. Confinement is also shown to influence crystallization processes over length scales ranging from the atomic to hundreds of micrometers, and to originate from a wide range of mechanisms. The development of an enhanced understanding of the influence of confinement on crystal nucleation and growth will not only provide superior insight into crystallization processes in many real-world environments, but will also enable this phenomenon to be used to control crystallization in applications including nanomaterial synthesis, heavy metal remediation, and the prevention of weathering.     

化学沉淀法制备的α-Fe2O3非晶纳米线的晶化研究

以AAO为模板,采用化学沉淀法制备α-Fe2O3纳米线,经TEM和XRD分析表明,所制备的α-Fe2O3纳米线长度约为5 μ m,直径约为40～50nm,且为非晶态,该非晶纳米线的晶化温度约为850℃,明显高于α-Fe2O3纳米粉体的晶化温度.     

Crystallization of cystine

Cystine is an amino acid, which is a constituent of the urinary stones. It also is deposited in eyes, thyroid glands, bone marrow and white blood corpuscles. Crystals of cystine have been grown by both solution and gel methods. Small bunched hexagonal crystals, along with many other morphological crystals of size 0.3 mm across, were obtained. The crystals were analysed by X-ray diffraction and IR analyses.     

气体水合物结晶引导时间和结晶区域的实验研究

本文对制冷剂HFC152a,HFC134a,HCFC141b所形成的单一气体水合物和混合气体水合物在不同外部条件下的结晶引导时间和结晶区域进行了实验研究,实验表明,气体水合物的结晶引导时间在水浴温度在3度到4度之间时,变化最大,在这个温度范围内,水浴温度每降低1度,引导时间缩短了约10分钟,水浴温度降到3度以下时。引导时间变化比较平缓,致冷剂的比份为20％＝25％之间时,引导时间变化最大,比份每降低一个百分点,引导时间约减小1分钟,同时实验研究了水合物的结晶区域,发现扰动速度是影响水合物结晶成核的一个重要因素,扰动速度只有超过200rpm时,结晶才有可能发生。     

用于蛋白质结晶的形核剂研究综述

获取蛋白质晶体是蛋白质三维结构解析、医疗药品生产、自组装纳米体系构建等过程中重要的步骤。例如,利用X射线衍射技术对蛋白质进行三维结构解析时,首先需要通过结晶条件筛选,获得质量较高的蛋白质晶体,进而进行衍射得到蛋白质结构相关信息。蛋白质结晶需要经历从未饱和区经亚稳区至形核区的形核过程以及从形核区到亚稳区的生长成熟过程。在整个蛋白质结晶过程中,形核过程是至关重要的一步。均相形核过程中,结晶体系中各个部分形核概率相同,当蛋白质结晶体系中溶液的过饱和度足够克服形核势垒时,在形核区发生成核,因而在低浓度的结晶溶液体系中,均相形核存在一定的局限性。形核剂的添加使蛋白质晶体异相形核,相较于均相形核其需要克服的阻力小,形核势垒低。因而形核剂的使用对于难结晶蛋白或者起始浓度过低的蛋白质结晶具有重要意义。随着结构生物学的发展,形核剂在蛋白质结晶中的研究仍是结晶方法学领域的热点问题。多孔微球对蛋白质分子的吸附作用有利于无序蛋白质分子团簇的形成,进而促进蛋白质形核。添加多孔微球不但可以增加结晶条件筛选数,也可以提高晶体质量。促进蛋白质分子有序排列的形核剂籽晶的使用,使晶体的形核生长过程始终处于结晶体系溶液浓度较低的状态,而交联的籽晶因为稳定性更高而更有应用前景。新型交互扩散结晶板中,蛋白质结晶体系通过一个较缓慢的交互扩散过程实现蛋白质结晶溶液浓度的变化,并且结晶体系可达到共平衡,因而能显著提高蛋白质晶体结晶条件筛选数和晶体质量:蛋白酶K结晶条件数由39个提升至47个,分辨率由1.66提升至1.54。利用基底材料的一些特性,如静电作用、疏水作用和氢键,可以起到促进蛋白质分子聚集的功能,从而促进形核。本文从物理作用和化学作用两个角度详细总结了形核剂对蛋白质结晶的影响,并展望了该领域的发展前景及研究方向。     

Crystallization of glassforming melts under hydrostatic pressure and shear stress: Part I Crystallization catalysis under hydrostatic pressure: possibilities and limitations

This is the first part of a thorough study of the kinetics of melt crystallization under applied static pressure, P, and under shear stress. The thermodynamic and kinetic consequences of increased external pressure on nucleation rate, non-steady-state time lag, rate of crystal growth and overall crystallization kinetics in undercooled melts are analysed. Two types of undercooled liquids (with either positive or negative volume dilatation upon crystallization) are considered. Particular attention is given to the effect of pressure on the specific interface energy, σ, at the crystal/melt phase boundary. Using an appropriate thermodynamic model it is shown that for one-component systems, (∂σ/∂p)<0 is to be expected as a rule. Thus an additional decrease of the thermodynamic barrier of nucleation in pressurized melts is to be expected. However, it is also shown that the increase of melt viscosity with pressure in most cases reduces the effect of this decrease. Thus increased pressure has a limited effect as a nucleation catalyst. The possibilities in this respect are analysed and conditions under which static pressure may lead to enhanced crystallization are outlined. This revised version was published online in November 2006 with corrections to the Cover Date.     

Crystallization: Key thermodynamic,kinetic and hydrodynamic aspects

Crystallization is extensively used in different industrial applications, including the production of a wide range of materials such as fertilizers, detergents, food and pharmaceutical products, as well as in the mineral processing industries and treatment of waste effluents. In spite of the wide-spread use of crystallization, a clear understanding of the thermodynamic, kinetic and hydrodynamic aspects of the design methodologies are not yet well established. More often than not crystallization is still considered an art especially in fine-chemicals, pharmaceuticals and life-sciences sector. It is essential to understand and relate key thermodynamic, kinetic and hydrodynamic aspects to crystallizer performance, not just in terms of yield but also in terms of product quality (characterized by particle size distribution, morphology, polymorphism and the amount of strain as well as the uptake of solvent or impurities in the crystal lattice). This paper attempts to do that by critically reviewing published experimental and modelling studies on establishing and enhancing state-of-the-art thermodynamic, kinetic and hydrodynamic aspects of crystallization. Efforts are made to discuss and raise points for emerging modelling tools needed for a flexible design and operation of crystallizers and crystallization processes that are needed to meet the ever increasing demand on precise product specifications. Focus is on bringing out the trends which can be used as perspectives for future studies in this field.     

Crystallization of polyphenylene sulfide

The properties of molded PPS parts are dependent on the crystalline morphology developed during processing. Even though processed under identical conditions, the crystalline morphology may differ owing to the differences in the crystallization process. The crystallization behaviour of a polymer is known to depend on its molecular architecture which in turn depends upon polymerization process. Thus the study of the crystallization behaviour of polymer with reference to its molecular architecture is essential for obtaining product with desirable properties. In the present paper, the crystallization behaviour of two grades of polyphenylene sulfide was investigated using differential scanning calorimetry (DSC). An attempt has been made to explain the differences in the crystallization behaviour of PPS samples on the basis of the differences in their molecular architecture. The structural differences of PPS manifest themselves in terms of the depression in the equilibrium melting point, retardation of nucleation and overall crystallization rate and coarsening of spherulitic texture.     

Spherical Crystallization of Celecoxib

ABSTRACT

Celecoxib exhibits poor flow properties and compressibility. Spherical crystallization of celecoxib was carried out using the solvent change method. An acetone:dichloromethane (DCM):water system was used where DCM acted as a bridging liquid and acetone and water as good and bad solvent, respectively. Hydroxypropylmethylcellulose (HPMC) was used to impart strength and sphericity to the agglomerates. The effect of amount of bridging liquid and speed of agitation was studied using 3² factorial design. Primary properties of the agglomerates were evaluated by infrared spectroscopy, powder X-ray diffraction, and differential scanning calorimetry. The effect of variables on micromeritic, mechanical, compressional, and dissolution behavior was evaluated by response surface methodology. Particle size, bulk density, mean yield pressure (MYP), and drug release were found to be significantly affected by either of the two variables. Interaction of variables significantly affected the MYP.     

Crystallization of phosphate glasses

The effects of refining time on the transformation temperature, viscosity, and rate of crystallization of a simple lead phosphate glass of nominal composition 60 P₂O₅, 30 PbO, 10 K₂O have been determined. As the hydroxyl content is reduced and the cross-link density increased by continued refining, the transformation temperature and melt viscosity rise. At corresponding temperatures above T _g the viscosity increases and the rate of crystallization decreases with cross-link density, but crystallization rates at isoviscous temperatures are shown to be independent of cross-link density.     

石英纤维析晶行为

为了提高石英纤维/ SiO₂ 复合材料的强度, 避免因石英纤维析晶而造成复合材料力学性能的降低, 本文中详细地研究了石英纤维的析晶行为。采用FT-IR 技术测试了石英纤维的化学键结构, 应用DSC 和XRD 技术综合分析了石英纤维的析晶过程, 并结合SEM、TEM 观察了石英纤维的析晶现象及产物。测试结果表明, 石英纤维结构中含有大量的Si —OH 键; 析晶过程受结构重排控制较为明显, DSC 曲线中存在明显的结构重排吸热峰。XRD 测试显示, 石英纤维析晶产物为单一α-方石英相, 起始析晶温度为950 ℃, 1400 ℃左右达到析晶最大量, 温度明显低于石英玻璃的析晶温度。Si —OH 键、杂质以及界面等因素是石英纤维析晶温度低于石英玻璃的主要原因。      

Crystallization kinetics of Mo-N

Thin film samples of molybdenum were electron-beam evaporated at 80 K in a 6.6×10^–4 Pa partial pressure of nitrogen and subsequently analysed for structure, superconductivity and crystallization behaviour. X-ray diffraction of as-deposited samples showed a typical amorphous structure and fcc and bcc phases upon heating. A superconducting transformation at 7.0 to 7.2 K was detected by the resistivity technique. Isothermal DSC scans obtained for a range of temperatures were calibrated and integrated to produce transformed volume (x)-time data. All plots of In [-In (1–x)] against Int have two linear regions with slopes ofn ₁3.6 for 0<x<0.5 andn ₂ 2.0 for 0.7<x<1; wheren is the principal parameter in the Johnson-Mehl-Avrami theory of the crystallization transformation. The relationship of this binary pattern of crystallization to the observation of two apparent crystallization peaks in dynamic DSC scans of several samples of electron-beam deposited Mo-N and to the appearance of both Mo-bcc and Mo₂N-fcc phases in the X-ray patterns of the crystallized samples is discussed. Possible patterns of simultaneous or successive modes of nucleation and growth, including combinations of varying nucleation rates, different dimensionals of growth, and interfacial and diffusion controlled growth, are considered.     

Crystallization of fluorozirconate glasses

The crystallization of a number of glasses of the fluorozirconate family has been studied using powder X-ray diffraction and differential scanning calorimetry, as a function of time and temperature of heating. The main crystalline phases were β-BaZrF₆ and β-BaZr₂F₁₀. Stable and metastable transformations to the low temperature α phases were also investigated. The size of crystallites in fully devitrified glasses was calculated to be $\text{～ 600}\text{A}\text{?}$ from line broadening of the X-ray diffraction peaks.     

Crystallization of aluminium orthophosphate

Aluminium orthophosphate crystals were crystallized by a hydrothermal technique using various solvents. The influence of nutrient materials and solvents on the rate of crystallization, solubility, morphology and quality of the crystals was studied. The crystals obtained were subjected to analysis by X-ray diffraction (XRD), scanning electron microscopy (SEM) and IR spectroscopy.     

Crystallization in Ni-Si-B glass: the influence of dispersoid additions

Crystallization processes in an Ni-Si-B metallic glass have been examined both for the glass alone and for the glass containing a distribution of fine crystalline particles. In the absence of dispersoids, nucleation of crystals occurs uniformly throughout the glass volume on heating following an initial slow, transient period. Nucleation occurs preferentially at the glass-dispersoid interface for the particle-containing glasses. The rate of nucleation at the dispersoid-particle interface depends on the nature of the particle and can best be correlated with the degree of misfit between the substrate and nucleating crystal for the planes and orientation relationships considered. The rate of nucleation at the dispersoid surface is shown to be completely explained on the basis of the classical heterogeneous nucleation model. Nucleation in the bulk occurs many orders of magnitude faster than predicted by classical homogeneous nucleation: the implication of this on the interpretation of nucleation is discussed.