Influence of decomposition parameters on agglomeration process and total soda content in precipitated Al(OH)3

Grain size composition of precipitated Al(OH)₃ is dependent on the mechanism of decomposition process of the caustic solution which determines crystal growth process, agglomeration process and secondary nucleation. Because the literature data shows that the growth rate is very low, the agglomeration process plays an important role in increasing the initial particle size. On the other hand, the agglomeration process enables the inclusion of impurities by the grain boundary of sticking Al(OH)₃ particles, above all, the inclusion of soda Na₂O.

In this paper we investigate the influence of caustic soda concentration Na₂O_(c), that is the supersaturation of the solution, seed charge and seed grain size on the agglomeration and secondary nucleation processes and total soda content in precipitated Al(OH)₃. The results have shown that the factor which causes the increase of the agglomeration process also causes the increase of total soda content in precipitated Al(OH)₃.     

Influence of solution constituents, solution conditioning and seeding on the crystalline phase of aluminium hydroxide using in situ X-ray diffraction

Filtering, drying and aging of crystalline solids can affect crystalline phase(s) and therefore ex situ X-ray diffraction is not necessarily indicative of the in situ crystalline components. This paper examines some of the factors affecting the crystallised phase of aluminium hydroxides in highly caustic solutions. Caustic aluminate solutions were prepared using several different methodologies. Where solutions were prepared from aluminium metal, gibbsite and bayerite were observed to be the crystallisation products. Where gibbsite was used as the aluminium source in the liquors, the crystallised phase was found to be a function of solution conditioning. Solutions prepared at 100°C and at atmospheric pressure crystallised to form only gibbsite. These same solutions subsequently heated to 160°C for 16 h crystallised as gibbsite and bayerite. It has been concluded from these results that the caustic aluminate solutions made using gibbsite were not dissolved on the molecular scale (although optically clear) if the solutions were heated to only 100°C. Further heating at elevated temperatures completed the dissolution process so that the species in solution were similar to those found in solutions made from aluminium metal. Solutions seeded with either bayerite or gibbsite crystallised to form only gibbsite. Only crystallisation in the bulk solution and not on the seed surfaces was recorded. In some cases seeding altered the expected crystallisation products, i.e. from gibbsite and bayerite to only gibbsite.     

Crystallisation Mechanism in Gradient Temperature Zone Melting

Theory of zone recrystallization in the framwork of already known models of normal crystal growth, growth by screw dislocations and by two-dimensional nucleation is discussed. By mathematical treatment, different from Tillers approach, analytical expressions for supersaturation at the crystallisation interface, for superheating at the dissolution interface, growth rate and some other parameters have been obtained for both cases of normal and screw dislocations growth. It is possible to determine the growth mechanism.     

Nucleation kinetics in deionized charged colloidal suspension

A systematic experimental study on the nucleation, crystallization and crystal-growth of one-component charged colloidal particles (122 nm diluted in pure water with densities between 0.5 μm⁻³ < n_p < 5 μm⁻³) is present by means of time resolved static light scattering spectroscopy revealing the heterogeneous and homogenous nature of the crystallization. The interactions between the charged colloidal particles are sufficiently strong to cause crystallization which described in terms of Debye-Hückel approximation. Crystallization starts always with the formation of compressed structurally heterogeneous precursor domains. The results show that the heterogeneous nucleation at the cell walls starts simultaneously with the homogeneous bulk nucleation and the rate density of the heterogeneous nucleation appears slightly higher. It has been also found that the overall crystallization consists of at least a two-step nucleation process involving formation of early stage nuclei or crystal precursor then followed by the main crystallization. The induction time, the number density of nuclei and the growth rate of crystals, is strongly dependent on particle concentration and on whether the nucleation are homogeneous in cell center or heterogeneous on cell walls.     

Mechanisms controlling primary crystallisation of Ga20Te80 glasses

The kinetic study of the crystallisation process of Ga₂₀Te₈₀ glass from isothermal and continuous heating calorimetric data have been performed applying a recently developed procedure. The kinetic information was complemented with X-ray diffraction measurements. With this scope, three crystallisation patterns, with three-dimensional isotropic growth have been analysed: (i) site saturation and interface controlled growth. (ii) homogeneous nucleation with interface controlled growth and (iii) homogeneous nucleation with two simultaneous modes of crystal growth (interface- and diffusion-controlled). A complex model with two simultaneous modes of three-dimensional isotropic crystal growth with decreasing homogeneous nucleation and soft impingement has been applied for modelling primary crystallisation of the Ga₂₀Te₈₀ glass. The model goes beyond the isokinetic hypothesis when coupling isothermal and continuous heating kinetic data. The apparent activation energy E_a = (2.06 ± 0.03) eV/at obtained for the primary crystallisation of the phase Te is shown to correspond to an activation energy for nucleation E_I = (2.85 ± 0.03) eV/at and an interface controlled activation energy for growth E_u = (1.90 ± 0.03) eV/at at the crystallisation onset.     

Temperature control of protein crystal nucleation

The thermal variant of the classical nucleation‐growth‐separation principle is shown, both theoretically and experimentally, to be a reliable tool for studying protein crystal nucleation. The classical nucleation theory is used to elucidate the temperature dependence of crystal nucleus size. A one‐to‐one ratio of the number density of nuclei formed to crystals grown to visible size is achieved using the nucleation‐growth‐separation method. The experiments conducted in such a way show that new nuclei are prevented from appearing while avoiding any crystal loss due to dissolution. The same method is used to study experimentally the interval of growth temperatures where the number density of (nucleated) crystals is relatively insensitive to the growth temperature. It is argued that this temperature interval corresponds to the width of the so‐called metastable zone.     

The crystallisation of the yttrium–sialon glass: Y15.2Si14.7Al8.7O54.1N7.4

The development of microstructure during crystallisation of a glass with composition Y_15.2Si_14.7Al_8.7O_54.1N_7.4 has been studied by analytical and high resolution transmission electron microscopy. Crystal nucleation at temperatures in the range 965–1050°C occurs by the heterogeneous nucleation of lenticular-shaped yttrium, silicon and aluminium containing crystals on silicon-rich clusters that formed during glass preparation. The lenticular crystals have a wide range of composition after heat treatment at 1050°C; the yttrium cation percentage varies around that of the expected B-phase composition Y₂SiAlO₅N but the aluminium content is lower and the silicon content generally significantly higher than that. The crystals display the hexagonal crystal structure of B-phase, although the results from EDX analysis imply that the atomic arrangement of the lattice is not the previously proposed B-phase structure. Crystal growth during prolonged heat treatment at 1050°C occurs to a significant extent by coalescence.     

药物结晶中的经典与非经典结晶路径

晶体的结晶路径分为经典结晶路径和非经典结晶路径。经典结晶路径往往涉及一些简单的化学结构,晶体的成核、生长是通过单体的依次添加实现的,经过长期研究,目前研究人员已对其有了较为深刻的理解并形成了一套相对完善的理论体系;但对于非经典的结晶路径,由于涉及复杂中间态粒子的形成和多步结晶过程,尚未获得全面、统一的理论支持。在药物结晶领域,有机分子构象自由度的引入增加了体系的复杂性,分子间弱的相互作用使得固态药物分子存在多晶型现象。由于药物的理化性质及生物利用度与其晶型息息相关,同时,药物结晶过程中出现的一些复杂中间态粒子往往会改变最终得到的药物晶型,因此迫切需要加强对药物晶体成核和生长路径的研究,以期发展能实现对药物晶体成核和生长过程主动控制的方法。本文简要综述了目前药物在溶液或熔体中结晶的经典与非经典结晶路径,包括奥斯特瓦尔德阶段定律、独立成核、交叉成核。从溶液化学的角度看,分子在浓溶液中会通过自组装形成结构合成子,成核与溶液中的生长单元、结构合成子密切相关。从分子水平上探索溶液中有关分子运动的信息、分析各体系下晶核与结构合成子之间的关系是区分两种结晶路径的关键所在,非经典结晶在药物结晶领域是机遇也是挑战。     

Advancements (and challenges) in the study of protein crystal nucleation and growth; thermodynamic and kinetic explanations and comparison with small-molecule crystallization

This paper reviews advancements and some novel ideas (not yet covered by reviews and monographs) concerning thermodynamics and kinetics of protein crystal nucleation and growth, as well as some outcomes resulting therefrom. By accounting the role of physical and biochemical factors, the paper aims to present a comprehensive (rather than complete) review of recent studies and efforts to elucidate the protein crystallization process. Thermodynamic rules that govern both protein and small-molecule crystallization are considered firstly. The thermodynamically substantiated EBDE method (meaning equilibration between the cohesive energy which maintains the integrity of a crystalline cluster and the destructive energies tending to tear-up it) determines the supersaturation dependent size of stable nuclei (i.e., nuclei that are doomed to grow). The size of the stable nucleus is worth-considering because it is exactly related to the size of the critical crystal nucleus, and permits calculation of the latter. Besides, merely stable nuclei grow to visible crystals, and are detected experimentally. EBDE is applied for considering protein crystal nucleation in pores and hydrophobicity assisted protein crystallization. The logistic functional kinetics of nucleation (expressed as nuclei number density vs. nucleation time) explains quantitatively important aspects of the crystallization process, such as supersaturation dependence of crystal nuclei number density at fixed nucleation time and crystal size distribution (CSD) resulting from batch crystallization. It is shown that the CSD is instigated by the crystal nucleation stage, which produces an ogee-curve shaped CSD vs. crystal birth moments. Experimental results confirm both the logistic functional nucleation kinetics and the calculated CSD. And even though Ostwald ripening modifies the latter (because the smallest crystals dissolve rendering material for the growth of larger crystals), CSD during this terminal crystallization stage retains some traces of the CSD shape inherited from the nucleation stage. Another objective of this paper is to point-out some biochemical aspects of the protein crystallization, such as bond selection mechanism (BSM) of protein crystal nucleation and growth and the effect of electric fields exerted on the process. Finally, an in-silico study on crystal polymorph selection is reviewed.     

Nucleation in quiet supersaturated ADP solutions

The nucleation study has been made with supersaturated ADP solution. Rate of nucleation as indicated by the induction period was measured for non-agitated system over the temperature range 20°C–45°C. The nucleation rate increases with increase in temperature but supersaturation has the dominant effect as predicted by classical nucleation theory. However, attempts to analyse the results in accordance with classical theory were not entirely successful, but it is shown how the assumption of (a) a variation of crystal surface energy with temperature and (b) the influence of heterogeneous nucleation can account for the discrepancies. Activation energies, surface free energies and sizes of crystal nuclei were determined for nuclei of ammonium dihydrogen orthophosphate over the range of temperature 20°C–45°C.     

The dissolution and crystallisation of amphoteric metal hydroxides from sodium hydroxide solutions: Ionic equilibria,crystalline phases and crystallisation mechanisms

The dissolution and recrystallisation of beryllium, zinc, cadmium and tin (II) hydroxides and chromium (III), iron(III), aluminium, scandium, yttrium, gallium and indium hydroxides, from sodium hydroxide solutions of concentrations C = 1 to 20 M at ambient temperatures, are surveyed. The different inic equilibria in metal hydroxide-sodium hydroxide systems are examined: the phases crystallising from different sodium hydroxide solutions are tabulated and crystallisation mechanisms are analysed.     

Impact of amorphous Ge thin layer at the amorphous Si/Al interface on Al-induced crystallization

We investigated the impact of an amorphous Ge (a-Ge) thin layer inserted at the amorphous Si (a-Si)/Al interface on Al-induced crystallization. In situ observation of the growth process clarified that the nucleation rate is drastically reduced by insertion of a-Ge, which led to increase in the average size of crystal grains. This was interpreted as resulting from decrease in the driving force of crystallization, mainly due to the larger solubility of Ge in Al than that of Si in Al. The obtained films were SiGe alloys with lateral distribution of Ge content, and its origin is discussed based on the two-step nucleation process.     

Thermodynamics of Crystal Nucleation in an External Electric Field

The influence of electric field on crystal nucleation in a saturated solution has been studied both theoretically and experimentally. The classical equations for nucleation have been used to determine the free energy of formation, critical radius of the cluster and the concentration of the critical nuclei. The theory shows that an externally applied electric field can modify the free energy of formation of a crystalline cluster in its aqueous solution. The impact of the field will be stronger on large molecules. Two experimental set ups have been designed to study the nucleation of crystals in saturated aqueous solutions, in the presence of electric fields. Experiments conducted using a metal coordination compound bis‐ thiourea zinc chloride show that electric fields of strength around 10⁵ V/m would increase its nucleation.     

Protein crystallization in low gravity by step gradient diffusion method.

Two-step crystallization experiments were conducted in low gravity employing a liquid-liquid diffusion method in an effort to eliminate problems associated with protein crystal growth under the supersaturating conditions required for nucleation. Experiments were performed in diffusion cells formed by the sliding of blocks on orbit. Step gradient diffusion experiments consisted of first exposing protein solutions in diffusion half-wells for brief periods to initiating buffer solutions of high precipitant concentrations to induce nucleation followed by expoure of the same protein solutions to solutions of lower precepitant concentration to promote growth of induced nuclei into crystals. To avoid convective disturbances that occur when solutions of discrepant densities are interfaced at normal gravity, crystallization of hen egg-white lysozyme and rabbit skeletal muscle aldolase by step gradient diffusion was investigated in low gravity on four NASA space shuttle flights. In general, the largest ctystals of both proteins formed at the highest initiating precipitant concentration used, which is consistent with nuclei formation upon brief exposure to high precipitant concentration, and that these nuclei are competent for sustained growth at lower precipitant concentration. The two-step approach dissociates nucleation events from crystal growth allowing parameters affecting nucleation kinetics such as time, precipitant concentration and temperature of nucleation to be varied separately from conditions used for post-nucleation growth.     

Effect of the Temperature Fluctuation of the Melt on β‐BaB2O4 (BBO) Crystal Growth

In this paper the effect of the growth temperature fluctuation, for instance, the transient furnace temperature variation due to a short‐term electric power supply interruption on BBO crystal growth was investigated based on the theory of temperature wave transmitting in melt and the boundary layer theory of melt. It was found that the critical width of the temperature pulse to avoid the temperature wave penetrating through the boundary layer and reaching to the growth interface at a constant rotation speed (9∼4 r/min) is 69∼150 s and the corresponding amplitude of the temperature pulse is high more than 60 °C due to the large thickness of the velocity boundary layer of the melt. This result indicates that a small transient temperature fluctuation has no significant effect on the crystal quality, and therefore implies that not only transport processes but interface growth kinetics, a two‐dimensional nucleation growth mode at the interface may also dominate the crystal growth.     

Batch Precipitation of Potash Alum Crystals by Mixing Aqueous Solutions of the Component Salts

The effects of temperature, supersaturation, excess of reacting component salts and method of precipitation on the batchwise precipitation of potash alum crystals, by mixing aqueous solutions of potassium sulphate and aluminium sulphate, have been investigated. From the primary characteristics of the process (crystal size, size distribution and yield) relative crystal growth rates and relative nucleation rates are calculated and interrelation of these rates with the corresponding supersaturation leads to a value of the nucleation order. It is concluded that secondary nucleation plays an important role in the batch precipitation of potash alum crystals.     

Induction time in batch crystallisation

Three theoretical models of nucleation in batch crystallisation experiments are presented and compared. The first model, based on the assumption that the new particles are detected immediately after their birth, can be used for assessing the width of metastable zones for practical purposes of industrial crystallisation and crystalliser design. The second one is based on the conception that a certain period of time is needed for crystals, newly born in a solution, before they grow up to become visible; they cannot be detected during this period, though nucleation has already taken place. The third model assumes that all the subcrystals, present in the solution, must attain the visible size before the process of nucleation can be observed. The authors given preference to the second model for evaluating the effective parameters of nucleation.     

大尺寸晶体快速生长理论与技术的研究进展

“如何突破大尺寸晶体材料的制备理论和技术”是中国科协发布的2021年度的十大前沿科学问题之一,揭示晶体生长机制和突破生长关键技术是大尺寸功能晶体发展的两个趋势。在原子分子尺度上,晶体生长可以是有势垒的热激活过程,也可以是无势垒的超快结晶过程,这与具体的体系以及晶面有关。从界面属性角度来看,光滑界面是以台阶拓展的方式生长;粗糙界面没有明显的固-液分层,通过局部原子固化进行生长。本文从晶体生长理论模型、生长技术及其应用实例,以及分子动力学方法在晶体生长中的应用等方面探讨了近些年大尺寸晶体快速生长理论和技术的研究进展。目前有多种方法制备大尺寸晶体,但普遍存在制备的晶体质量差和性能不稳定等问题。需要突破对晶体生长微观机制上的认识,建立机制与温度、流速等外界因素的内在联系。而利用机器学习力场以及分子动力学模拟方法,建立固-液界面,模拟晶体生长,将是探究晶体生长微观机制的一种有效方式。     

Heterogeneous versus bulk nucleation of lysozyme crystals

Heterogeneous (on‐glass) protein crystal nucleation was separated from the bulk one in systems of thin protein solution layers, confined between two glass plates of custom made quasi two‐dimensional all‐glass cells, as well as by applying forced protein solution flow. Two commercial samples of hen‐egg‐white lysozyme, Seikagaku and Sigma were used as model proteins. Applying the classical technique of separation in time of nucleation and growth stages with protein solution layers of thickness 0.05 cm we found that the on‐glass crystal nucleation prevailed highly with Seikagaku HEWL, while on the opposite, bulk nucleated crystals represented the main crystal fraction in Sigma solution. Also using 0.05 cm solution layers nucleation rates were measured separately for the on‐glass and bulk protein crystals. The process was investigated by varying solution layer thicknesses as well, from 0.05 down to 0.01, 0.0065 and 0.002 cm. Studying the influence of the forced protein solution flow on HEWL crystal nucleation the classical double‐pulse technique was modified by separating the nucleation and growth stages not only in time, but simultaneously also in place. In this case we found that the ratio of on‐glass formed crystal nuclei to bulk nuclei depended on the flow velocity, but in different manner with Seikagaku HEWL and Sigma HEWL. A plausible explanation of our experimental results is that the bulk crystal nucleation occurs on foreign surfaces as well, e.g. on rests of source biomaterial, which are always present in the protein solutions. Moreover, biomaterial seems to be more active nucleant than glass. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)