Effect of the Growth Treatment on Two-Stage Nucleation Experiments

Numerical simulations are presented that document the strong effect of a previously underappreciated portion of two-stage thermal treatments used in the study of nucleation processes: the heat-up process whereby samples are heated from nucleation conditions to growth conditions. The simulations indicate that two limiting regimes exist, dependent on (a) the cluster size distribution of as-quenched glasses, (b) the temperatures used for nucleation and growth, and (c) the rates of heating and cooling: (1) all clusters larger than the critical size at growth conditions ( n ^*_gr) will grow to macroscopic size (the standard case); and (2) all clusters larger than the critical size at nucleation conditions ( n ^*_nuc) will grow to macroscopic size. In addition, cases in which the effective critical size ( n ^*_eff) is intermediate between n ^*_gr and n ^*_nuc can also occur. Cases in which n ^*_eff < n ^*_gr is manifested during nucleation experiments as an abrupt boost in crystal number density during the heat-up from nucleation to growth conditions, as all clusters larger than n ^*_eff are rapidly flushed past n ^*_gr. For the system studied herein, this can lead to a 10⁶-fold increase in final number density within seconds to a few minutes. Finally, the importance of structural relaxation for this process is demonstrated by examining a case in which the nucleation temperature is below the nominal glass transition temperature.     

二氧化钛微米球形颗粒的制备与研究

以硫酸钛为原料,尿素为均一沉淀剂,制备出单分散的二氧化钛微米球形颗粒,研究不同反应物浓度和溶剂对二氧化钛形貌的影响。结果表明,制备出二氧化钛微米球形粒子的最佳实验条件为:硫酸钛的浓度为0.2mol·L-1,尿素的浓度是2mol·L-1,表面活性剂(DBS)的浓度是0.005～0.01mol·L-1,溶剂为丙酮或乙醇。     

成核剂在多元醇还原法超细磁粉制备中的应用

主要探讨多元醇还原法制备CoNi类超细磁粉过程中 ,使用Pt或Ag等贵金属作成核剂 ,有利于获得粒度均匀、尺寸可控制的颗粒材料。讨论了贵金属的种类、用量以及合金组分对颗粒材料结构和性能的影响。为制备纳米级、亚微米级的优质超细合金颗粒材料提供了可行方法     

Nucleation,Growth, and TEM Analysis of Metal Particles and Clusters Deposited in UHV

It has been established for some time [1, 2] that small supported metal particles can be reproducibly prepared in a controlled way by ultrahigh vacuum (UHV) deposition onto planar supports. Such planar deposits have been employed in the past to study the physical [3–11] and chemical [12–20] properties of mesoscopic metal aggregates. In these studies, the emphasis has usually been placed on pointing out the differences in particle properties with respect to the bulk properties of the same material. Although it has often been found that the electronic properties reach bulk values at surprisingly small cluster sizes of about 30 to 50 atoms, it has also been observed that the chemical properties can change over a much wider particle size range [21] for structure-sensitive reactions [22]. Furthermore, it is generally realized that particle size alone is usually not the only important parameter but that the influence of the support in terms of electronic, structural, chemical, and morphological particle properties can be essential. which demonstrates the importance of experimental attempts to control the support properties as well as the particle properties.     

Nucleation, Growth, and TEM Analysis of Metal Particles and Clusters Deposited in UHV

It has been established for some time [1, 2] that small supported metal particles can be reproducibly prepared in a controlled way by ultrahigh vacuum (UHV) deposition onto planar supports. Such planar deposits have been employed in the past to study the physical [3-11] and chemical [12-20] properties of mesoscopic metal aggregates. In these studies, the emphasis has usually been placed on pointing out the differences in particle properties with respect to the bulk properties of the same material. Although it has often been found that the electronic properties reach bulk values at surprisingly small cluster sizes of about 30 to 50 atoms, it has also been observed that the chemical properties can change over a much wider particle size range [21] for structure-sensitive reactions [22]. Furthermore, it is generally realized that particle size alone is usually not the only important parameter but that the influence of the support in terms of electronic, structural, chemical, and morphological particle properties can be essential. which demonstrates the importance of experimental attempts to control the support properties as well as the particle properties.     

混炼工艺对天然橡胶胶料中氧化锌粒子微细化的影响

分析两个普通氧化锌样品的粒子形态,考察优化混炼工艺对天然橡胶胶料中两个氧化锌样品粒子的微细化作用。结果表明:普通氧化锌粒子聚集形成聚集体,2~#氧化锌聚集体粒子尺寸明显大于1~#氧化锌;胶料混炼时将氧化锌先于其他小料加入,能够使氧化锌粒子微细化,但效果不明显;采用一段母炼法混炼,能大幅度减小氧化锌粒径,改善硫化胶的综合性能。     

显示剂实验橡胶在密炼机内的混炼性能

本文在密炼机内 ,用顺丁橡胶加入聚四氟乙烯显示剂 ,研究转子的螺旋角 (从 5°～15°)变化对胶料分散性能的影响 ,证明了螺旋角越大混炼效果越好的结论。     

Heterogeneous Vapor-to-Liquid Nucleation of Water on Individual Glass Particles

An apparatus that combines upward thermal diffusion with electrostatic levitation was used to study vapor-to-liquid nucleation on individual micron-sized clean glass particles. A vertical gradient in relative humidity was established in the chamber with the lower region being undersaturated while the upper region was supersaturated. Particles underwent vertical oscillatory motion when a fixed electric field was applied in excess of that required simply to achieve levitation. The period and the amplitude of oscillation were measured at varying electric field intensities. The balancing electric potential for several particles at a given elevation was also measured. The critical supersaturation, the radius, and the electric charge of the particles were then determined by using the equations of motion and condensational growth. Variations in the surface charge density for soda-lime glass particles of nominally the same size largely accounted for the particle-to-particle variability in the critical supersaturation. Cycle-to-cycle variations were also examined. For most particles, these variations were small with no general trends.     

Nucleation of Ethylene Glycol Vapor and Growth of Sub-10-nm Particles in Nanoparticle Size Magnifier

We investigated the rates of heterogeneous and homogeneous nucleation of ethylene glycol vapor onto sub-10-nm particles in a newly developed condensation device called nanoparticle size magnifier (NanoPSM). The saturation ratio in the NanoPSM is precisely controlled by vapor-feeding system and mixing section, which are designed based on an earlier particle size magnifier (PSM) developed by Okuyama et al. (1984). Size-classified NaCl nanoparticles smaller than 10 nm in mobility diameter are used as heterogeneous nuclei for the condensation of ethylene glycol vapor. The activation efficiency and growth rate of the activated nuclei are determined by a pulse height analysis using an optical particle counter (OPC). A computer fluid dynamics (CFD) simulation is employed to calculate the profiles of the gas velocity, temperature, vapor concentration, and resulting supersaturation in the NanoPSM. Annular high-supersaturation region is generated around the mixing boundary between cold aerosol and hot vapor. The experimental activation efficiency is 50% for 4.5-nm and 0.8% for 2 nm NaCl particles, through the subsequent growth of droplets to 2 μm in diameter. The experimental data are in fairly good agreement with the predicted activation efficiencies based on the classical Kelvin-Thomson theory when the local profiles of supersaturation are taken into account.     

Effect of Second-Phase Particles on Grain Growth in Calcite

Grain growth in polycrystalline caicite (CaCO₃) at 800°C and 300-MPa confining pressure is inhibited by the addition of Al₂O₃ particles; volume fractions of second phase (f) ranged from 0.02 to 10.0 vol%, and several powders with different average particle size were used. The stable grain size reached during heat treatment was inversely proportional to l/f^m where m varied from 0.3 to 0.55 — agreeing with results from other grain growth experiments in ceramics and metals, and with models developed for particles located at grain boundaries and grain corners, but not agreeing with models developed for random dispersions. The dependence of the stable grain size on second-phase particle size in these experiments was less systematic, possibly because of variations in the particle size distributions used, or because of particle agglomeration effects. In the single-phase aggregates, the growth kinetics were consistent with a normal grain growth equation with n=3, although uncertainties were large.     

Nucleation and Growth of Titania Nanoparticles Prepared by Pulsed Injection Metal Organic Chemical Vapor Deposition from a Single Molecular Precursor

Nanometer-sized titania particles were deposited by the pulsed liquid injection chemical vapor deposition technique from a single molecular metal-organic precursor. A liquid solution of titanium (IV) isopropoxide was used, Ar was the carrier gas, and titania nanoparticles were deposited on the hot wall of the reactor fused quartz tube, under supersaturated conditions. Energy-dispersive spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy studies reveal the formation of amorphous titania-agglomerated nanoparticles (40–50 nm). XRD patterns after different post-annealing processes reveal a high anatase–rutile transformation temperature, comparable to that of micrometer-sized titania, attributed to lattice strains in the nanoparticle structure, as calculated from the X-ray data. The growth dynamics involved in the nucleation and formation of titania nanoparticles is discussed in terms of the so-called chemical vapor synthesis or chemical vapor condensation.     

均一、单分散二氧化钛系列球形胶体颗粒的可控制备研究

TiO2作为一种重要的半导体材料，具有光催化活性高、化学性质稳定、折射率高等特性，因而在光催化、陶瓷、涂料、化妆品及光电子器件等领域有着十分广泛的应用前景．尤其是近年来，核．壳结构的复合颗粒以及空心和介孔TiO2胶体颗粒的出现，使TiO2材料增添了许多独特的优异性能，相应研究成为热点．本论文主要研究了二氧化钛系列球形胶体颗粒的可控制备与性能：     

Coagulation of Particles Through Rapid Mixing

As is often assumed, rapid mix effectively distributes the dosed coagulant in suspension to promote subsequent flocculation of suspended particles and coagulant molecules. This study determined that the role of rapid mix is considerably more complex than conventionally assumed. Particles in the raw water samples from the Banxing Water Works of Taipei City were coagulated at high shear environment, during which flocs of size 40–70 μm were produced. The produced floc interior was compacted, and residual turbidity and the amount of dissolved natural organic matter (NOM) in suspension were substantially reduced. The subsequent stage of rapid mix slightly reduces floc size, further compacts the floc interior, and expels fine particles and some adsorbed NOM from the flocs. A sudden reduction in shearing rate produced large sized flocs with not so compact structures. Moreover, this action released fine particles from flocs into the suspension and enhanced adsorption of NOM onto solid surfaces.     

Coagulation of Particles Through Rapid Mixing

As is often assumed, rapid mix effectively distributes the dosed coagulant in suspension to promote subsequent flocculation of suspended particles and coagulant molecules. This study determined that the role of rapid mix is considerably more complex than conventionally assumed. Particles in the raw water samples from the Banxing Water Works of Taipei City were coagulated at high shear environment, during which flocs of size 40-70 μm were produced. The produced floc interior was compacted, and residual turbidity and the amount of dissolved natural organic matter (NOM) in suspension were substantially reduced. The subsequent stage of rapid mix slightly reduces floc size, further compacts the floc interior, and expels fine particles and some adsorbed NOM from the flocs. A sudden reduction in shearing rate produced large sized flocs with not so compact structures. Moreover, this action released fine particles from flocs into the suspension and enhanced adsorption of NOM onto solid surfaces.     

Kinetics of Nucleation and Crystal Growth

This paper deals with homogeneous and heterogeneous primary nucleation and the atomistic model of heterogeneous primary and secondary nucleation. Three aspects of crystal growth mechanisms will be taken into account: growth by two-dimensional nucleation, the so-called birth and spread model, spiral growth without and with consideration of the influence of diffusion, and finally adhesive growth. An equation for the estimation of the growth rate of imperfect crystals including dislocations, published by Mersmann [1], is discussed. The influence of additives on nucleation and crystal growth, respectively, will be exemplarily explained for several model systems. The effect of growth rate dispersion (GRD) is described. Therefore, some experimental results concerning growth rates and different ways to influence growth are represented and discussed. Furthermore, a model explaining the effect of GRD is represented.     

Understanding the Filler Effect on the Nucleation and Growth of C‐S‐H

The “filler effect”, due to the physical presence of mineral additions in cement, is mainly known to accelerate the hydration of the clinker component. Previously, this was attributed to the surface of the filler providing nucleation sites for C‐S‐H as there is a clear dependence on the surface provided by the filler particles. Our results reveal that the increase in nucleation is quite low compared to the area provided. Based on the isothermal calorimetry experiments and SEM images, we demonstrate that the most important parameter is the interparticle distance. We propose that this is mainly the result of the shearing conditions rather than extra surface available for C‐S‐H as formerly assumed. Quantitatively slag and fly ash behave very similarly to quartz. Limestone, on the other hand, seems also to stimulate C‐S‐H nucleation giving it a higher efficiency in accelerating clinker hydration.     

微晶釉中晶粒的成核与生长

微晶玻璃材料由于其优良的力学性能而被广泛使用,近年来的一个研究热点就是使用微晶釉来取代传统釉从而改善陶瓷制品的表面性能。微晶釉主要是通过控制釉原料在烧成过程的析晶而得到的,因此深入地了解晶体的成核与长大过程是很重要的。本文主要综述了釉中常见的三种晶体,硅酸锆、氧化钛和堇青石的析晶过程,同时也简单探讨了析晶的机理以及晶体和结构对釉面性能的影响。     

Unusual Temperature Dependence of Heterogeneous Nucleation of Water Vapor on Ag Particles

Copyright 2013 American Association for Aerosol Research     

Effect of Nucleation on Transformation Toughening

A simple model is presented to account for the effect of martensitic nucleation in ZrO₂ particles on theoretical predictions of toughness enhancement during transformation toughening. The model allows for shear and volumetric strain contributions during nucleation, whereas only volumetric strain contributes to toughness enhancement. It is predicted that toughness enhancement results for both stationary and growing cracks. The predicted toughness enhancement exceeds the theoretical prediction for the case when only volumetric strain is allowed to operate. The important material property is identified to be the ratio of volumetric to shear strain during nucleation of the martensitic product in the transformation from tetragonal to monoclinic symmetry of the ZrO₂ particles.     

Bubble-Induced Nucleation and Crystal Growth in Glass

Evidence that bubbles can cause annular nucleation and subsequent crystal growth on the surface of Bao-Al₂O₃-TiO₂-SiO₂, glasses is presented. The crystalline rings were identified as hexacelsian, BaAl₂Si₂O₈ (the hexagonal or high-temperature polymorph of celsian). Size, character, and suggestions as to the genesis of these rings are given.