Transition from nucleation and growth to Ostwald ripening

Condensation phase transitions from metastable fluids occur by nucleation with accompanying cluster growth and eventual Ostwald ripening. In closed batch systems the supersaturation declines as clusters nucleate and grow, causing an increase in the stable critical nucleus size. During ripening the subcritical clusters are considered to dissolve spontaneously, so that their mass is released to contribute to further growth of remaining clusters. A model based on the dynamics of cluster size distributions (CSDs) represents this sequence of phenomena in terms of four-dimensionless parameters: initial supersaturation, power on the cluster-mass expression for the growth rate coefficient, and parameters for nucleation rate and Gibbs-Thomson effect. A numerical solution of the scaled governing equations for CSD and supersaturation shows the transition from nucleation and growth to ripening, which can occur over a relatively long time period, thus separating the stages. The asymptotic ripening stage shows a power law increase in average cluster mass as the CSD evolves to (minimum) polydispersity index of unity.     

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.     

Strategies and practices for suppressing abnormal grain growth during liquid phase sintering

Abnormal grain growth (AGG), where a small number of grains grow to sizes much larger than the neighboring matrix grains, is a frequent occurrence in liquid phase sintering of ceramics and cermets. As AGG can be detrimental to the material properties, a considerable amount of research on the nature, causes and suppression of AGG has been carried out. In this review, we outline the mixed control theory of grain growth and the principle of microstructural evolution that have been developed by Kang and coworkers over the last two decades. The theory and the principle, which are based on theories of crystal growth from a liquid, state that grain growth behavior is controlled by the nature of the solid-liquid interfaces, either atomically rough (macroscopically rounded) or smooth (macroscopically faceted). For grains with atomically rough solid-liquid interfaces, growth is controlled by diffusion of solute through the liquid phase and normal grain growth always occurs. For grains with faceted solid-liquid interfaces (or a mixture of rough and faceted interfaces), growth is interface reaction-controlled and diffusion-controlled below and above a critical driving force for growth, respectively. Depending on the relative values of the critical driving force for growth Δg_c and the maximum driving force for the largest grain in the system Δg_max, pseudo-normal, abnormal, and stagnant grain growth can take place. Based on this theory and principle, we present strategies for suppressing AGG by adjusting Δg_c and Δg_max to avoid AGG and examples of the successful use of these strategies.     

合成镁白云石中CaO和MgO晶粒生长动力学

应用ＴＰ速率等式和线性截距法研究了经１５００℃至１７００℃烧成的两个合成镁白云石体系中ＣａＯ和ＭｇＯ的晶粒生长，采用回归分析和最小二乘法求得ＣａＯ和ＭｇＯ晶粒的生长指数和晶位生长活化能。结果表明，体系中ＭｇＯ晶粒的生长速率总比ＣａＯ的要大，而加入少量ＣｅＯ２添加剂的合成镁白云石中ＣａＯ和ＭｇＯ晶粒生长速率比无添加剂的要快。     

青霉素亚砜结晶生长与成核动力学

利用Mydlarz 和 Jones 模型（MJ2）,对乙酸丁酯中青霉素亚砜的成核与生长动力学进行研究。通过矩量法对MJ2模型进行处理后,利用晶体产品的粒度分布计算得到青霉素亚砜的生长速率与成核速率,然后利用最小二乘法拟合回归求解出成核与生长动力学方程参数。通过实验设计考察了过饱和度、温度与搅拌速度对青霉素亚砜晶体成核和生长过程的影响。研究表明青霉素亚砜晶体生长速率随过饱和度比的增加呈现指数型增长,确定青霉素亚砜晶体生长属于晶体表面生长控制过程。由于高速搅拌会增加青霉素亚砜晶体的破碎,促进了二次成核过程,随着搅拌速度的增加,晶体生长速率出现小幅下滑,而成核速率则明显升高。青霉素亚砜成核与生长动力学研究将有助于工业生产过程优化。     

分散液相强化微溶气体吸收的研究

在气升式搅拌反应器中,利用水对微溶气体CO2(常压和室温条件)的物理吸收过程,考察加入辛醇(有机分散相)对传质过程中体积传质系数和增强因子的影响。研究辛醇加入体积分数(1%—5%)、搅拌速率(200—700 r/min)、气体流速(350—700 L/min)变化对吸收效果的影响,用插值函数微商法确定体积传质系数。结果表明,向水体系中加入少量辛醇可以明显改善吸收效果,体积传质系数随辛醇加入体积分数的增加呈现先增加后减小的趋势,当辛醇加入的体积分数为4%左右时吸收效果最好。增强因子与CO2分配系数、扩散系数有关,与吸收效果呈线性关系,最大增强因子为2.76。     

The kinetics and morphology of polymer dispersed liquid crystals by doped crylic acid

It is widely appreciated that electro‐optical activity in polymer dispersed liquid crystals (PDLCs) depends on phase separation of polymer and liquid crystals (LCs). The morphology of the LCs domains depends on the detail of the chemical and physical processes active during the formation of domains. This work discusses two‐phase morphology in an acrylate‐based system that developed during polymerization induced phase separation. UV/VIS spectrometer is used for monitoring the polymerization of the PDLCs by real‐time scattering. The doped crylic acid accelerated the speed of polymerization. The electro‐optical properties of PDLCs films are measured by Polarimeter (PerkinElmer Model 341). The lower threshold voltage was obtained by doped crylic acid at suitable ratio. The polarizing optical microscopy and Fourier transform infrared image system are used for depicting the morphology of LC droplets in polymer matrix. The sizes and dispersion of LC droplets were influenced by doped crylic acid which accelerated the speed of polymerization. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Simultaneous determination of nucleation and crystal growth kinetics of gypsum

The chemical treatment of industrial effluent, containing sulphuric acid, is typically completed by neutralising it with lime through solid/liquid separation. The mineral precipitation processes developed by Veolia Environnement use sludge recirculation. The production of a high sludge density characterises these processes, at a minimum of at least 25% weight in solids. The first laboratory tests studied the precipitation reaction, confirming the production of gypsum (CaSO₄·2H₂O). Then the precipitation mechanisms, nucleation and crystal growth kinetics, are determined in order to establish a model which predicts population density in reactor in accordance with fluid dynamics. The determined nucleation and growth kinetics are then used to feed a reaction model, coupling computational fluid dynamics (CFD) and population balance modelling to simulate the precipitation process.     

Cell growth kinetics and ice nucleation activity of pseudomonas syringae

The bacterium Pseudomonas syringae causes freezing of supercooled water at temperatures close to 0°C. In this study, the growth kinetics and ice nucleation activity (INA) of the bacterium P. syringae pv. syringae cit 7 were investigated in a batch and continuously operated laboratory scale bioreactor system. Under continuous culture conditions, the bacterial INA decreased with an increase in dilution rate. The maximum biomass and INA productivities were obtained at a dilution rate of 0.054 hr^?1 at 25°C and pH 7.0. The results of this investigation can be applied towards the large scale production of P. syringae bacteria for snow making and other commercial applications of ice nucleation.     

Effects of high concentrations of liquid phase and magnesia on the grain growth of alumina

Grain growth phenomenon of alumina in the presence of various quantities of liquid phase and magnesia was investigated at 1600°C, and the data were regressed by a statistical approach. The statistical result indicated that liquid phase, compared with magnesia, was a major factor in determining the grain growth mechanism of alumina. The alumina phase developed into faceted grains when the liquid phase was so abundant as to penetrate all the grain boundaries and junctions of alumina, whether with the doping of magnesia or not. This observation implied that magnesia, when co-existing with a large quantity of liquid phase, did not effectively reduce the degree of anisotropy in interfacial energy of alumina. Nevertheless, when its concentration was high enough to result in the precipitation of MgAl₂O₄, it avoided the evolution of elongated alumina grains by pinning the fast moving boundaries.     

Effect of step free energy on delayed abnormal grain growth in a liquid phase-sintered BaTiO3 model system

The changes in solid/liquid interface structure and grain growth behavior with oxygen partial pressure (PO₂) were systematically studied during liquid-phase sintering of 8TiO₂/2SiO₂-added BaTiO₃. As the PO₂ of the sintering atmosphere increased, the grain boundaries and solid/liquid interfaces showed increased faceting, indicating an increase in step free energy. This increase in PO₂ and step free energy caused a change in grain growth behavior as a function of sintering time. When samples were sintered in H₂, abnormal grain growth (AGG) occurred from the beginning, resulting in a coarse microstructure with a large average grain size. With increasing PO₂, the incubation time necessary for AGG also increased. Finally, for samples sintered in air, AGG did not occur even after 100 h. These changes in incubation time for abnormal grain growth demonstrate the effect of changing the step free energy on the microstructural development during liquid phase sintering of ceramic systems.     

Modeling hydration kinetics based on boundary nucleation and space-filling growth in a fixed confined zone

A semi-analytical model of hydration kinetics based on the boundary nucleation and space-filling growth for early hydration and a diffusion-controlled kinetics for late hydration is proposed. The objective is to develop a simple model that can be coupled with more complex approaches required for the estimation of properties at early-age, but without neglecting some key mechanisms driving the kinetics. The particle size distribution (PSD) of cement is explicitly accounted for. The impingements are modeled by means of an exponential boundary condition in which a limited reaction zone is defined. Numerical solutions for the model are obtained and compared to experimental data. The model can reproduce some relevant features regarding hydration kinetics such as calorimetric data and the influence of the w/c ratio on late kinetics.     

Kinetics of the dissolution and growth of a finely dispersed solid phase in stirred apparatuses

A mathematical model is considered for the kinetics of dissolution and growth of solid particles by the diffusion mechanism in apparatuses with the stirring of a liquid phase. The mathematical model of the kinetics is based on the concept of the pseudolaminar boundary layer on the surface of a particle. The reliability of the developed mathematical model is verified by experimental data on rubber crumb degassing and silver halide crystallization in stirred apparatuses.     

旋转圆环内分散气泡对液相表观黏度的影响

泡状液体广泛存在于化工、食品和生物医学等领域,深刻理解气泡对液相流变特性的影响对于产品质量改进和过程强化具有重要意义。文中以旋转流变仪作为物理模型,基于VOF和动网格相结合的方法,详细研究了分散气泡对液相表观黏度的影响。目前的研究发现:在相同气泡体积分数下,当毛细数较小时,由于表面张力,气泡几乎保持为球形,气泡的加入使流线扭曲,从而导致液相的相对黏度增大(即η_r1);然而,当毛细数较大时,气泡由圆形变为细长的椭圆形,并且沿最大法线方向气泡所占比例增加,气泡的加入使液相的相对黏度减小(即η_r1);而且对于相同毛细数的工况,体积分数越大,气泡对液相表观黏度的影响也越大。     

Analysis of abnormal grain growth of oriented LiCoO2 prepared by slip casting in a strong magnetic field

The relationship between the development of the crystallographic orientation and the grain growth behavior were studied. The degree of orientations of the green compacts and sintered samples were evaluated by the Lotgering factor. The f(0 0 l) of all the samples were drastically increased with the increasing applied magnetic field strength. The f(0 0 l) of the samples sintered at 1223 K were improved in comparison to those of the green compacts. However, the f(0 0 l) value of the samples sintered at 1273 K were not increased at 4 T or lower. To characterize the grain growth process, these samples were analyzed using electron backscatter diffraction (EBSD). The sintered samples prepared in the magnetic field at 4 T or lower showed abnormal grain growth. The samples with an applied magnetic field of 8 T or higher had no abnormal grain growth. It was revealed that the orientation angle of the particles has an effect on the grain growth.     

Inhibition of abnormal grain growth in BaTiO3 by addition of Al2O3

The effect of additions of up to 1 mol% Al₂O₃ on abnormal grain growth in BaTiO₃ samples sintered at 1200 and 1250 °C has been studied. Samples with and without additions of 0.4 mol% TiO₂ were prepared. For the samples without added TiO₂, addition of 0.1 mol% Al₂O₃ increases the number density of abnormal grains, with further additions reducing the number density. The initial increase in number density is caused by Al₂O₃ forming a solid solution with BaTiO₃ and releasing TiO₂ to the grain boundaries. This excess TiO₂ then reacts with BaTiO₃ to form Ba₆Ti₁₇O₄₀, which promotes {1 1 1} twin formation and abnormal grain growth. Further additions of Al₂O₃ react with BaTiO₃, Ba₆Ti₁₇O₄₀ and excess TiO₂ to form Ba₄Al₂Ti₁₀O₂₇ and BaAl₂O₄ second phases, neither of which are growth sites for abnormal grains. For the samples with added TiO₂, addition of Al₂O₃ decreases the number of abnormal grains due to the Al₂O₃ reacting with the excess TiO₂ and BaTiO₃ to form Ba₄Al₂Ti₁₀O₂₇ and BaAl₂O₄ instead of Ba₆Ti₁₇O₄₀.     

纳米氧化铁的微波制备及其晶粒生长动力学

采用微波焙烧法和常规焙烧方法分别制备了一种新型的无机非金属材料纳米氧化铁,并研究了氧化铁晶粒生长动力学。使用扫描电镜、热重分析仪、傅里叶变换红外光谱仪和X射线衍射仪对前驱体和纳米氧化铁进行表征。结果表明:在相同焙烧温度和时间下,微波焙烧氧化铁的晶粒尺寸要明显大于常规焙烧方式,同时,微波制备的试样颗粒大小更均匀。微波焙烧和常规焙烧下氧化铁的晶粒生长平均动力学指数分别是4.493和5.133,晶粒生长的平均活化能分别为24.30 k J/mol和30.43 k J/mol。表明微波焙烧有利于晶粒生长,晶粒的平均生长速率较高。     

Nucleation and growth kinetics of graphene layers from a molten phase

Nucleation and growth of graphene layers from Ni–C melts were investigated. It is shown that upon cooling of supersaturated liquids, graphite will grow either with flake or sphere morphology depending on the solidification rate and degree of supersaturation. At small solidification rates, graphite crystals are normally bounded by faceted low index basal and prismatic planes which grow by lateral movement of ledges produced by 2D-nucleation or dislocations. At higher growth rates, however, both interfaces become kinetically rough, and growth becomes limited by diffusion of carbon to growing interface. The roughening transition from faceted to non-faceted depends on the driving force and nature of growing plane. Due to high number of C–C dangling bonds in prismatic face, its roughening transition occurs in smaller driving force. As such, at intermediate rates, the prismatic interfaces become rough and grow faster while the basal plane is still faceted, leading to formation of flake graphite. At higher growth rates, both interfaces grow with a relatively similar rate leading to initiation of graphite sphere formation, which later grow by a multi-stage growth mechanism. An analytical model is developed to describe the size and morphology of graphite as a function of solidification parameters.     

Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.