NUCLEATION AND GROWTH IN THE DIFFUSION CLOUD CHAMBER II STEADY STATE NUCLEATION

A model describing nucleation and growth in a thermal diffusion cloud chamber presented earlier in this Journal to explain oscillatory nucleation is extended to the case of steady state nucleation. The model is applied to nucleation rate data for ethanol vapor obtained using a thermal diffusion cloud chamber. A correction is made for the deleterious effects of vapor depletion and latent heat generation which accompany droplet growth thus extending the accessible range of nucleation rate measurements by several orders of magnitude.     

NUCLEATION AND GROWTH IN THE DIFFUSION CLOUD CHAMBER. I. OSCILLATORY NUCLEATION

A model is presented describing vapor depletion and latent heat effects accompanying the nucleation and growth of droplets during homogeneous nucleation in a thermal diffusion cloud chamber. The model explains successfully the oscillatory nucleation phenomenon reported earlier by the authors in this Journal.     

NUCLEATION AND GROWTH IN THE DIFFUSION CLOUD CHAMBER. I. OSCILLATORY NUCLEATION

A model is presented describing vapor depletion and latent heat effects accompanying the nucleation and growth of droplets during homogeneous nucleation in a thermal diffusion cloud chamber. The model explains successfully the oscillatory nucleation phenomenon reported earlier by the authors in this Journal.     

NUCLEATION OF THIOUREA ADDUCT CRYSTALS WITH CYCLOHEXANE-METHYLCYCLOPENTANE SYSTEM

To study the homogeneous nucleation of thiourea adducts, the metastable zone widths for the thiourea adducts formed with guests of cydohexane, methylcyclopentane (MCP) and their mixtures were determined. The influence of moderate agitation rate and saturation temperature on the maximum allowable undercooling (i.e., metastable zone width) was studied. The metastable zone width is markedly affected by the cooling rate and the agitation rate. MCP in guests roles to retard the nucleation rate of cyclohexane-MCP-thiourea adducts by causing k2 to decrease. The order of the nucleation in the nucleation process of thiourea adducts is in the range between 3 and 6, and appears to be dependent on the saturation temperature and the guest concentration. The nucleation rate constant (i.e., frequency factor (k₂/N_obs) has shown to exclusively depend on the concentration of guest and the temperature. The retarding effect of MCP in the thiourea adduction was found to be caused by, not only the increase of surface energy, but the decrease of k₂/N_obs     

GROWTH AND NUCLEATION KINETICS IN BATCH CRYSTALLIZATION OF TRICLOSAN

Batch crystallization of triclosan, a broad spectrum antibacterial agent, was carried out under isothermal conditions (at 10°C) with n-hexane as the solvent. Supersaturation was generated by cooling a seeded solution. Samples were collected at periodic intervals. Triclosan concentration in solution was determined by UV-spectrophotometry and particle size distribution by optical microscopy. Growth and nucleation kinetic parameters were obtained. The data indicated size independent growth. Growth kinetic parameters based on triclosan concentration in solution were in reasonable agreement with growth kinetic parameters obtained from cumulative size distribution data. Growth and nucleation kinetic orders were comparable to those reported in the literature.     

GROWTH AND NUCLEATION KINETICS IN BATCH CRYSTALLIZATION OF TRICLOSAN

Batch crystallization of triclosan, a broad spectrum antibacterial agent, was carried out under isothermal conditions (at 10°C) with n-hexane as the solvent. Supersaturation was generated by cooling a seeded solution. Samples were collected at periodic intervals. Triclosan concentration in solution was determined by UV-spectrophotometry and particle size distribution by optical microscopy. Growth and nucleation kinetic parameters were obtained. The data indicated size independent growth. Growth kinetic parameters based on triclosan concentration in solution were in reasonable agreement with growth kinetic parameters obtained from cumulative size distribution data. Growth and nucleation kinetic orders were comparable to those reported in the literature.     

SECONDARY NUCLEATION OF SUCROSE BY FLUID SHEAR IN AQUEOUS SOLUTIONS

The motivation for this work was based on two hypotheses: subcritical nuclei of sucrose may be generated by secondary nucleation using a fluid shear field acting on the surface of a growing seed crystal in aqueous solutions, and a near-dynamic equilibrium distribution of embryos may be attained. These were supported experimentally. The value of a σ-based model parameter, a, was obtained by describing the system by a Boltzman- like distribution model. The number concentration of subcritical nuclei was well described by the model. Despite differences in experimental method and in the basic nature of the solute, sucrose, the behavior of the system and results were similar to those obtained in the earlier work using aqueous salt solutions [Sung et al. (1973), Jagannathan et al. (1980)].     

SECONDARY NUCLEATION OF SUCROSE BY FLUID SHEAR IN AQUEOUS SOLUTIONS

The motivation for this work was based on two hypotheses: subcritical nuclei of sucrose may be generated by secondary nucleation using a fluid shear field acting on the surface of a growing seed crystal in aqueous solutions, and a near-dynamic equilibrium distribution of embryos may be attained. These were supported experimentally. The value of a σ-based model parameter, a, was obtained by describing the system by a Boltzman- like distribution model. The number concentration of subcritical nuclei was well described by the model. Despite differences in experimental method and in the basic nature of the solute, sucrose, the behavior of the system and results were similar to those obtained in the earlier work using aqueous salt solutions [Sung et al. (1973), Jagannathan et al. (1980)].     

INVESTIGATIONS INTO THE NUCLEATION BEHAVIOUR OF METHANE GAS HYDRATES

Details on the design and construction of a high-pressure, temperature controlled sapphire cell are reported for the study of clathrate hydrate nucleation under pristine experimental conditions. Laser light scattering optics and detection equipment have been successfully integrated with the apparatus to monitor the development of the incipient phase and characterize the early stages of hydrate growth. Experiments on the scattering behaviour of the nucleation process have revealed that within the detection limit of the technique, the hydrate phase was not present in sufficient size or number prior to its catastrophic appearance. A series of investigations to identify nucleation process variables have verified the existence of an aqueous phase thermal history. In addition, the efficacy of perturbations on inducing hydrate formation has been established. These studies are consistent with literature on the formation of ice and the crystallization of inorganic salts and sugars. The data support an argument that the nucleation process is non-deterministic due to a probabilistic element within the nucleation mechanism     

碳化硅超细粉末形核生长过程的研究

用热化学气相合成法制备的超细SiC粉末的组织结构,保留了许多与形核生长过程直接有关的许多特点,为用高分辨电子显微术研究其形核生长过程提供了有利条件。据此,本文讨论了SiC超细粉末形成的主要机制——均相形核生长过程。它可分为5个主要方面:非晶相核的形成;SiC旋涡状生长及受阻;亚稳的SiC旋涡的析晶;聚结;表面非晶SiC的形成。另外,也分析了固态Si上的SiC的异相成核、外延以及固态Si的碳化过程。     

MULTIPLE OSCILLATIONS AT HIGH RATES OF HOMOGENEOUS NUCLEATION

Regular, sustained oscillatory nucleation has been observed in supersaturated vapor systems involving only homogeneous nucleation and droplet growth. The oscillation frequency is approximately 4 hz and appears to be modulated with secondary, lower frequency oscillations possible arising from hydrodynamic instabilities in the working fluid pool below the nucleation region.     

MULTIPLE OSCILLATIONS AT HIGH RATES OF HOMOGENEOUS NUCLEATION

Regular, sustained oscillatory nucleation has been observed in supersaturated vapor systems involving only homogeneous nucleation and droplet growth. The oscillation frequency is approximately 4 hz and appears to be modulated with secondary, lower frequency oscillations possible arising from hydrodynamic instabilities in the working fluid pool below the nucleation region.     

Fe-C-H_2O体系中金刚石成核特性

用国产SPD6×1200型六面顶压机,在Fe粉末触媒和石墨(Fe-C)体系中添加不同比例的H2O,高温高压条件下合成出了金刚石单晶。通过扫描电子显微镜和Mossbauer谱研究了合成样品中金刚石的成核状态和Fe-C-H2O体系中H2O对金刚石成核的影响。结果表明:在5.7GPa,1600℃条件下,当H2O的质量含量少于5%时,Fe-C-H2O体系可合成出金刚石;随着Fe-C-H2O体系中H2O含量的增加,样品中金刚石成核量逐渐减少;当H2O含量达到5%时,金刚石将不能成核生长。在合成过程中,Fe-C-H2O体系中的Fe与H2O首先发生氧化还原反应,生成FeO和FeHx。与Fe-C体系相比,H2O对金刚石成核具有明显的抑制作用,而且随着H2O含量的增加,抑制作用越明显。讨论了H2O对金刚石成核和生长抑制的机理。     

溶剂超声波协同法用于有机物系结晶快速成核

<正> 引言 有机物系结晶大多属于溶液群体结晶范畴,是工业结晶的重要组成部分.晶核是过饱和溶液中新生成的微小晶体粒子,是晶体生长过程必不可少的核心. A.VanHook在论述晶核的生成时指出,声波辐照由于具有强烈的定向效应,有补充和加强形成临界晶核所需的波动作用,故能加速起晶过程.高大维提出了蔗糖溶液的溶剂超声波协同成核法,该法已被利用来进行味精、含水α-葡萄糖、木糖醇等的成核研究并获得成功,所需的成核溶剂、超声波强度及刺激时间因不同物质而异.与传统的晶种制备方法(如粉碎机破碎法、球磨法、晶浆中留一部分晶体作为晶种等)相比,该法具有简单、快速、晶核数目容易控制的优点.同时还具有晶核形状好、投种量小并减少结晶生长过程中用于整晶的能耗、成品质量高等优点.溶剂超声波协同成核法的原理是溶析效应与超声波的机械(分散)效应、空化效应的结合.从溶液成核速率公式可看出表面张力σ、粘度η和过饱和度S对成核速率J的关系     

不同Br~-浓度下铂电极上镀银层的氧化动力学

本文采用循环伏安法研究了镀银铂电极在溴化钾溶液中的成核过程及氧化动力学,结果表明:AgBr的形成包括三个步骤,首先在银表面某些吸附Br~-的特殊位置上以平面方式成核,然后经过二个三维的成核步骤。在较低浓度的KBr溶液中,AgBr的形成由溶液中的Br~-迁移控制。在较高浓度的溶液中,由于生成的AgBr膜是高度松散的,速度控制步骤转化为在膜孔内Br~-的迁移。随着AgBr的生成以及AgBr的溶解沉积作用,膜孔被堵塞,同时反应速度转化为由AgBr膜中的Ag~+迁移控制。电镜结果也支持了以上结论。     

NUCLEATION AND GROWTH OF OXIDE NANOPARTICLES PREPARED BY INDUCTION THERMAL PLASMAS

The reactive evaporation method of injected metal powders was investigated for preparing oxide nanoparticles. The method has advantages such as evaporation is enhancement of the injected powder owing to exothermic reaction heat of the metal powder oxidation in induction thermal plasmas. Tangential gas flow injection to the plasma tail flame controls the diameter and the yield of the oxide nanoparticles. The purpose of this research is to investigate the effect of the injection gas flow on the preparation mechanism of oxide nanoparticles in thermal plasmas by experimental work and numerical analysis. An increase in the flow rate of the injection gas leads to an increase in the diameter as well as the yield of the prepared nanoparticles. Numerical analysis for nucleation and growth provides the preparation mechanism of the oxide nanoparticles; however, a more sophisticated model should be developed.     

显影液中银的成核过程

本文用受阻显影技术和电镜研究方法研究了显影剂量对显影液中银成核速度和核分散度的影响。随着显影液中显影剂量的减小,卤化银颗粒上银的成核速度明显减慢,单个颗粒上显影中心的数目明显增加。实验结果表明显影液中银的成核过程和曝光后的潜影形成过程在机理上是有差别的。文章讨论了显影过程中的一些基本问题:可显性、临界尺寸和显影选择性等等。     

FORMATION OF SULPHURIC ACID AEROSOLS AND CLOUD CONDENSATION NUCLEI: AN EXPRESSION FOR SIGNIFICANT NUCLEATION AND MODEL COMPRARISON

A new analytical expression has been derived to predict atmospheric conditions where homogeneous water–sulphuric acid nucleation will have a significant effect on aerosol and cloud condensation nuclei population. In the expression, the condensational sink due to pre-existing aerosol particles and source due to chemical production of sulphuric acid have been taken into account. The analytical expression has been derived using a sectional aerosol dynamic model including nucleation, condensation, coagulation, deposition and sulphuric acid formation in the gas phase. In the present study we have also compared the sectional model with modal and monodisperse models. All models may be used for predicting the onset of significant new particle formation. However, the computationally more efficient models—monodisperse, modal, and sectional with low number of sections—over- or underpredict particle formation in some situations.     

化学气相反应法合成SiC超细粉的成核和生长过程的研究

借助于ＴＥＭ观察，讨论了化学气相反应法合成ＳｉＣ超细粉的成长机理，分析结果显示，由于气体流速不同，合成的ＳｉＣＦ超细粉颗粒尺寸和颗粒形貌有较大变化，颗粒尺寸和颗粒形貌的变化成核和生长机理决定，一般涉及到均匀成术怜惜是相成核，团聚和熔融。