SAPO-34分子筛晶化机理及晶化动力学研究

对SAPO-34分子筛晶化过程中预相形成、诱导期内晶核生成、晶体生长和晶化过程的研究进行综述。SAPO-34分子筛晶化过程首先形成不稳定的层状预相结构,进而发展为具有有序排列晶格骨架的晶核。结晶热力学控制晶相结构,晶化动力学控制晶体成核和生长速率。影响晶化动力学的关键因素是温度和浓度,成核速率和晶体生长速率互相竞争控制晶粒大小。晶化过程的Si取代机理和Si分布影响分子筛酸性。晶化动力学研究结果表明,温度升高,结晶速率增加,成核时间缩短。     

The nucleation period for crystallization of colloidal TPA-silicalite-1 with varying silica source

The effect of varying silica source on the nucleation and crystallization of TPA-silicalite-1 was investigated. A direct experimental method, involving a two-stage varying-temperature synthesis, was used to determine the nucleation period for colloidal crystals of TPA-silicalite-1 with different silica sources, including tetraethoxysilane (TEOS) and amorphous silica (Ludox TM and Ludox LS). For syntheses performed at 60°C with TEOS as silica source, the duration of the nucleation was about 72 h, and a very rapid increase in the crystal population occurred during the initial crystallization time. However, with the amorphous silica sources (Ludox TM or Ludox LS), the duration of the nucleation period was extended to about 120 h, and the nucleation profile consisted of a self-accelerating nucleation rate at the beginning of the nucleation period. The two-stage synthesis method could be used to determine the nucleation profile for the various silica sources. However, this technique overestimated the crystal concentration at the earliest stage of nucleation with amorphous silica. The use of amorphous silica gave rise to a broader crystal size distribution compared to that of TEOS. However, it was found that for both TEOS and amorphous silica the vast majority of the nucleation occurred during an induction period when little or no crystal growth was observed. In addition, Raman spectroscopy revealed structural differences between Ludox TM and Ludox LS which may account for differences in the nucleation processes observed for these two amorphous silicas.     

Crystal Nucleation in Soda–Lime–Silica Glass at Temperatures below the Glass Transition Temperature

The effect of crystal nuclei formed during the production of glass on the kinetics of nucleation of crystals in it at temperatures below the glass transition temperature is studied. A glass composition of 22.4Na2O ⋅ 28.0CaO ⋅ 49.6SiO2 (mol %) is studied (glass transition temperature 540°C). Using the development method, the dependence of the number of nucleated crystals on the heat treatment time is determined at a temperature of 500°C. The nuclei of crystals formed during the production of glass lead to an unusual, in the form of a step, initial section of the curve N(t). The rate of stationary nucleation of crystals with a long time of nucleation heat treatment exceeding the induction period of crystal nucleation is determined. The reasons for the unusual initial nature of the dependence N(t) are discussed. The data obtained make it possible to expand the temperature dependences of the stationary rate and the induction period of crystal nucleation for the given glass toward low temperatures.     

均相成核—水热法制备纳米氧化锌

制备纳米氧化锌的关键是成核与生长控制。采用尿素为均相沉淀剂 ,结合水热处理 ,温度 130℃ ,反应时间为 3～ 5h制备了粒径小 ,分布窄的纳米氧化锌。Zn2 +的均相成核与其浓度、OH- 的浓度有关 ,体系中Zn(OH) 2 的过饱和度越大 ,则成核数量越多 ,所制备的氧化锌颗粒越小 ;与以氢氧化锌胶体为前驱体的共沉淀技术相比 ,均相沉淀技术有利于氧化锌纳米晶粒均匀生长 ;此外 ,添加有机物作分散剂可以减弱微小晶粒间的叠合生长 ,有利于制备均一性较好的纳米氧化锌。     

Effects of ultrasonic radiation on induction period and nucleation kinetics of sodium sulfate

The effects of ultrasound on crystal nucleation and particle size distribution of sodium sulfate were investigated via determining the induction period and particle size. Crystal nucleation parameters and equations for primary nucleation were calculated. The experimental results show that the induction time decreases under the ultrasound irradiation, therefore, we can get a shorter induction period at a higher supersaturation level. Based on these observations, the growth mechanism of sodium sulfate is continuous growth because the value of the surface entropy factor f is smaller than 3. The induction period was observed shorter and particle size was smaller when the ultrasonic radiation time increased. Crystal growth improved with the longer crystallization time.     

Temperature effects on the transition from nucleation and growth to Ostwald ripening

Condensation, involving nucleation, growth, and ripening from a metastable state, is an important but complex phase transition process. The effect of physical parameters, including temperature, on condensation dynamics, the competition between homogeneous and heterogeneous (seeding) nucleation, and the separation of polymorphs are among several issues of practical interest. We present a model based on population dynamics that describes the time evolution of the particle size distributions for condensation of the fluid phase and consequent decline in supersaturation. The crucial effect of interfacial curvature on energy, and hence on particle size (Gibbs-Thomson effect), causes larger particles to be less soluble, so that smaller particles dissolve and eventually vanish (denucleate). Numerical solutions of the governing equations show the transition from nucleation and growth to ripening occurs over a relatively long time period. The influence of temperature on these phenomena is primarily through its effect on interfacial energy, growth rate coefficients, and equilibrium solubility. Temperature programming is proposed as a potential method to control the size distribution during the phase transition. The model suggests conditions to suppress homogeneous nucleation by seeding. We also explore how a temperature program for cooling crystallization based on different properties of the crystal forms can separate two crystal polymorphs.     

Programmed cooling crystallization of potassium sulphate solutions

The crystal size distribution from a batch cooling crystallizer is predicted by the numerical solution of a mathematical model which uses empirical kinetics of nucleation and crystal growth. The predictions clearly point out the potential advantages of controlled cooling at a constant nucleation rate for improving the product crystal size over that obtained by either natural or linear cooling.Experimental runs following programmed cooling curves for seeded potassium sulphate solutions showed reasonable agreement with the theoretical predictions. A size dispersion of the crystals was observed which contributes to a slight deviation from theory. Nevertheless, controlled cooling significantly reduced the quantity of nuclei formed and improved the product crystal size distribution.     

The role of solute conformation,solvent–solute and solute–solute interactions in crystal nucleation

The induction time for nucleation can differ based on the solutions used to conduct a crystallization, which can in turn impact the efficiency and economics of a crystallization process, the crystal size distribution, the morphology and ultimately functionality of the final product. Establishing a link between the nucleation pathway/solution structure and nucleation induction time is essential to achieve improved comprehension of the process of crystal nucleation from solution. In this study, the role of solute conformation, solvent–solute interaction, and solute–solute interaction in nucleation was examined using tolbutamide as a model compound in toluene and toluene–alcohol solutions. Through a combination of induction time experiments, attenuated total reflection Fourier transformed infrared spectroscopy, nuclear magnetic resonance spectroscopy, molecular dynamics simulations, and quantum chemical calculations, it was found that not only solvent–solute interactions but also solute–solute interactions and structural similarities between molecular self-assemblies in the solution and synthons in the crystal structure, can significantly influence the nucleation induction time.     

Modelling nucleation in wet granulation

Nucleation is the first stage in any granulation process where binder liquid first comes into contact with the powder. This paper investigates the nucleation process where binder liquid is added to a fine powder with a spray nozzle. The dimensionless spray flux approach of Hapgood et al. (Powder Technol. 141 (2004) 20) is extended to account for nonuniform spray patterns and allow for overlap of nuclei granules rather than spray drops. A dimensionless nuclei distribution function which describes the effects of the design and operating parameters of the nucleation process (binder spray characteristics, the nucleation area ratio between droplets and nuclei and the powder bed velocity) on the fractional surface area coverage of nuclei on a moving powder bed is developed. From this starting point, a Monte Carlo nucleation model that simulates full nuclei size distributions as a function of the design and operating parameters that were implemented in the dimensionless nuclei distribution function is developed. The nucleation model was then used to investigate the effects of the design and operating parameters on the formed nuclei size distributions and to correlate these effects to changes of the dimensionless nuclei distribution function. Model simulations also showed that it is possible to predict nuclei size distributions beyond the drop controlled nucleation regime in Hapgood's nucleation regime map. Qualitative comparison of model simulations and experimental nucleation data showed similar shapes of the nuclei size distributions. In its current form, the nucleation model can replace the nucleation term in one-dimensional population balance models describing wet granulation processes. Implementation of more sophisticated nucleation kinetics can make the model applicable to multi-dimensional population balance models.     

Modeling of a Continuous Crystallization Process for Solvent Dewaxing

Abstract

A steady 1-D parallel flow model was established for a continuous solvent dewaxing process; the model equations were numerically solved to obtain the wax supersaturations, amount of crystals, crystal sizes and crystal size distributions in the wax-oil-solvent solution along the flow direction. The effects of the operating conditions, including temperature distributions, multiple dilutions, and the composition of dewaxing solvent, the system's characteristics, including nucleation constant, nucleation order, and wax compound, and the crystallizer dimensions on the formed wax crystals were examined and discussed in detail. The system's nucleation constant was determined by an experimentally obtained recovery of wax; further predictions are in acceptable agreement with the results from a pilot plant.     

Thermal Decomposition of Brucite: I, Electron and Optical Microscope Studies

The decomposition of brucite was studied directly in electron and optical microscopes. A small amount of decomposition caused marked changes in the physical state of the brucite crystal. The decomposition was interpreted as a normal nucleation and growth process in which magnesium oxide nuclei formed coherently with the brucite matrix. This process accounts for: ( a ) the structural relations between parent and product phases, ( b ) the small crystallite size of the product, ( c ) the cracking process occurring in the initial stages of decomposition, and ( d ) the range of d spacings observed for the magnesium oxide formed during the decomposition.     

A general probabilistic model of electrochemical nucleation

A general probabilistic model is developed for the random time at which crystals appear in electrochemical nucleation. Using the model, formulae are found for the distribution functions, the density functions, and the moments of crystal radii during nucleation and growth. Similar formulae are found for crystal volumes and for electrical currents.The subsampling of crystals using small elements of electrode surface is also considered. General formulae are obtained for the moments of electrical currents observed on microelectrodes, and a special case is considered, namely, nucletion consisting of a Poisson point process along the time axis.In the final section of the paper, a model of electrochemical nucleation is developed which takes into account the fact that active sites on electrode surfaces may have a distribution of activities towards electrochemical nucleation.     

Effect of the addition of poly(d-lactic acid) on the thermal property of poly(l-lactic acid)

Thermal property and crystallization behavior of PLLA blended with a small amount of PDLA (1-5 wt%) were studied. PDLA molecules added in PLLA formed stereocomplex crystallites in the PLLA matrix. When the blend was cooled to a temperature below T_m of PLLA, stereocomplex crystallites acted as nucleation sites of PLLA and enhanced the crystallization of PLLA significantly (heterogeneous nucleation). Such crystallization enhancement was not observed when the blend with lower PDLA content was cooled from 240 °C at which both PLLA crystal and the stereocomplex disappeared. Low molecular weight PDLA isolated in the matrix of PLLA did not form a stereocomplex crystallite with a large surface area enough to act as a nucleation site. On the other hand, high molecular weight PDLA chains formed a large stereocomplex crystallite. With increasing PDLA content, stereocomplex crystallites were more easily formed and they acted as nucleation sites. PLLA crystal near the stereocomplex crystallites has an incomplete structure and showed a melting peak at a lower temperature than pure PLLA crystal.     

环丙沙星在溶液中固液表面能的测定

采用激光法测定了环丙沙星在纯水和水－乙醇溶液（水体积分数23％）中的溶解度，介稳区和不同条件下的诱导期。根据经典成核理论和诱导期理论，推导了固液表面能与诱导期的理论关系，应用该理论关系计算了环丙沙星在溶液中的固液表面能，为环丙沙星初级成核与晶体生长研究提供了重要的参数。     

泡沫塑料加工过程中的气泡成核理论(Ⅰ)——经典成核理论及述评

介绍了泡沫塑料加工过程中气泡成核的经典理论,给出了几种成核方式的临界成核半径、所需克服的吉布斯(Gibbs)自由能及成核速率的计算公式;评述了经典成核理论的发展,包括自由体积和气体过饱和度对气泡成核的影响,并分析指出了经典成核理论对动态聚合物熔体气泡成核解释存在的不足。     

泡沫塑料加工过程中的气泡成核理论（I）——经典成核理论及述评

介绍了泡沫塑料加工过程中气泡成核的经典理论，给出了几种成核方式的临界成核半径、所需克服的吉布斯(Gibbs)自由能及成核速率的计算公式；评述了经典成核理论的发展，包括自由体积和气体过饱和度对气泡成核的影响，并分析指出了经典成核理论对动态聚合物熔体气泡成核解释存在的不足。     

Modeling nonisothermal crystallization in a BaO∙2SiO2 glass

The accuracy of a differential thermal analysis (DTA) technique for predicting the temperature range of significant nucleation is examined in a BaO∙2SiO₂ glass by iterative numerical calculations. The numerical model takes account of time-dependent nucleation, finite particle size, size-dependent crystal growth rates, and surface crystallization. The calculations were made using the classical and, for the first time, the diffuse interface theories of nucleation. The results of the calculations are in agreement with experimental measurements, demonstrating the validity of the DTA technique. They show that this is independent of the DTA scan rate used and that surface crystallization has a negligible effect for the glass particle sizes studied. A breakdown of the Stokes-Einstein relation between viscosity and the diffusion coefficient is demonstrated for low temperatures, near the maximum nucleation rate. However, it is shown that accurate values for the diffusion coefficient can be obtained from the induction time for nucleation and the growth velocity in this temperature range.     

Structural Characteristics of p-Type Porous Silicon and their Relation to the Nucleation and Growth of Pores

The influence of the anodization time on some structural characteristics (dissolved mass, maximum porous layer thickness, porosity, crystallite size, etc.) of p-type porous silicon has been investigated. It is shown that chemical dissolution of the porous layer during the anodization must be taken into account in order to correct some of the experimental data. Measurements of the anode potential have allowed to distinguish an early stage of the porous layer formation due to nucleation of pores. Equations based on pore nucleation and growth processes explain well the evolution of the maximum layer thickness with anodization time.     

Transparent Nano Crystalline Glass-Ceramics by Interface Controlled Crystallization

A prerequisite for the preparation of nano glass-ceramics is an increase in viscosity of the residual glassy matrix phase during the course of the crystallization process. This results in a deceleration of the crystal growth process due to increasing stresses, which finally may lead to a total freezing of the crystallization process. In principle, two routes for the preparation of nano glass-ceramics exist: in one of these routes, the first step is a phase separation in which a droplet phase is formed. Inside this droplet phase, the crystals are precipitated. In the other route, a preceding phase separation does not occur and the nucleation takes place in a homogeneous glassy phase. In both cases, the crystal growth velocity is high until the crystals reach a size of some nanometers or some ten nanometers, then the crystal growth velocity decreases strongly.     

Modeling the crystallization of proteins and small organic molecules in nanoliter drops

Drop‐based crystallization techniques are used to achieve a high degree of control over crystallization conditions in order to grow high‐quality protein crystals for X‐ray diffraction or to produce organic crystals with well‐controlled size distributions. Simultaneous crystal growth and stochastic nucleation makes it difficult to predict the number and size of crystals that will be produced in a drop‐based crystallization process. A mathematical model of crystallization in drops is developed using a Monte Carlo method. The model incorporates key phenomena in drop‐based crystallization, including stochastic primary nucleation and growth rate dispersion (GRD) and can predict distributions of the number of crystals per drop and full crystal size distributions (CSD). Key dimensionless parameters are identified to quickly screen for crystallization conditions that are expected to yield a high fraction of drops containing one crystal and a narrow CSD. Using literature correlations for the solubilities, growth, and nucleation rates of lactose and lysozyme, the model is able to predict the experimentally observed crystallization behavior over a wide range of conditions. Model‐based strategies for use in the design and optimization of a drop‐based crystallization process for producing crystals of well‐controlled CSD are identified. © 2009 American Institute of Chemical Engineers AIChE J, 2010