Effect of Particle Size on the Ice Nucleating Ability of AgI?AgBr?CuI System

The effect of particle size on the ice nucleating behaviour of AgI-AgBr-CuI system has been studied for the first time. Six samples of different molar ratios and five sizes in each sample are considered in the present study. It is found that the nucleation temperature is the highest for particle size in the range of 60 ± 5um.     

Preparation of low-density xerogels from mixtures of TEOS with substituted alkoxysilanes. II. Viscosity study of the sol–gel transition

Mixtures of TEOS with substituted methoxysilanes generate low-density xerogels due to a nucleation mechanism involving the substituted alkoxysilane. The sol–gel transition of these mixtures was followed by rheological characterisation. The transition from sol to gel takes place in a few minutes at ambient temperature. For the series exhibiting nucleation by the additive, the gel time goes through a slight minimum when the ratio of additive/main reagent increases. The elastic modulus increases with increasing ratio of additive/main reagent as the particle size decreases because of the nucleation mechanism by the additive. Samples with smaller particles exhibit the highest modulus for equal silica concentrations.     

Particle size of powders under hydrothermal conditions

Various non‐oxide (CuI, AgI, AgCl, PbS, CuS and ZnS) and oxide (ZnO, TiO₂, SnO₂, CeO₂ and ZrO₂) powders were prepared under hydrothermal conditions to investigate the effects of temperature, pH and precursors on the particle size of powders. It was found that the particle sizes of PbS, CuS and ZnS powders were much smaller than that of CuI, AgI and AgCl powders prepared under the same conditions. The particle sizes of TiO₂, SnO₂, CeO₂ and ZrO₂ powders are much smaller than that of ZnO powders prepared under the same conditions. It is concluded that the solution conditions have a certain effect on the particle size of powders under the hydrothermal conditions. The particle size of powders increased with the rising of temperature. Additional factors affecting the particle size were uncovered through studying the nucleation mechanism. The particle size was mainly related to the Madelung constant and the electric charge number of ions. Powders with smaller particle size resulted from systems that possessed the larger Madelung constant and ionic charge number. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Improvement of the theory of heterogeneous crystallization of metals and choice of the nanomodifier particle sizes

A numerical analysis of the joint effect that the radii of the nucleus and the insoluble impurity particle have on the nucleation work and the nucleation rate of crystallization centers has been performed. The extreme dependence of these parameters on the impurity particle size has been established for the first time and scientifically justified practical recommendations on the choice of nanomodifier particle sizes have been elaborated.     

Self-seeded, position-controlled InAs nanowire growth on Si: A growth parameter study

In this work, the nucleation and growth of InAs nanowires on patterned SiO(2)/Si(111) substrates is studied. It is found that the nanowire yield is strongly dependent on the size of the etched holes in the SiO(2), where openings smaller than 180?nm lead to a substantial decrease in nucleation yield, while openings larger than ≈500nm promote nucleation of crystallites rather than nanowires. We propose that this is a result of indium particle formation prior to nanowire growth, where the size of the indium particles, under constant growth parameters, is strongly influenced by the size of the openings in the SiO(2) film. Nanowires overgrowing the etched holes, eventually leading to a merging of neighboring nanowires, shed light into the growth mechanism.     

The role of the main silica precursor and the additive in the preparation of low-density xerogels

The incorporation of an additive during sol-gel synthesis reduces shinkage during ambient drying. The following additives have been studied: 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltriethoxysilane (AES) and 3-(2-aminoethylamino)propyltriethoxysilane (EDAES) and the main silica precursors were tetraethylorthosilicate (TEOS) and tetrapropylorthosilicate (TPOS). When the additive contains methoxy groups (EDAS), it acts as a nucleation agent of the silica particles and exactly the same properties (pore volume, specific surface area, particle and aggregate size) are obtained whether the main reagent is TEOS or TPOS. The nucleation mechanism is based on the difference in reactivity between additive and main reagent. In case of nucleation by the additive, the nucleation agent fixes the properties whatever the main silica precursor is. When both the additive and the main reagent contain ethoxy groups (series AES-TEOS and EDAES-TEOS), there is no nucleation mechanism by the additive, and the silica particle size remains nearly constant. With less reactive main reagent (series AES-TPOS and EDAES-TPOS), pore volumes up to 17 cm³/g have been obtained with pore sizes up to nearly 10 μm and very big particles (∼100 nm). The absence of nucleation by the additive for the couples AES-TPOS and EDAES-TPOS could be due to the fact that the difference in reactivity between ethoxy groups and propoxy groups is not sufficient to initiate the nucleation mechanism by the additive. In the absence of nucleation by the additive, the main reagent plays a role: highly porous materials with very large pores are prepared with TPOS.     

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.     

Growth behavior and photoluminescence properties of ZnO nanowires on gold nano-particle coated Si surfaces

ZnO nanowires arrays were synthesized on gold coated Si by the hydrothermal technique. The density of the ZnO nanowires array is found to be inversely proportional to the particle size of gold films. The initial nucleation of ZnO nanowires is found to nucleate at the boundaries of the gold nano-particle films. The density of the ZnO nanowires can be controlled through the particle size of the gold nano-particle films. Anomalous photoluminescence properties were observed from the as-grown ZnO nanowires.     

Nucleation kinetics of selenium (+4) precipitation from an acidic copper sulphate solution

The removal of selenium from copper sulphate solution prior to the electrowinning of copper is desirable in order to minimise contamination of the copper cathodes by selenium and other impurities. The selenium removal is effected by a precipitation process that takes place under high supersaturation conditions, which favour nucleation over any other particle formation processes. There is currently no fundamental information on the nucleation kinetics of this important process. In this study, the nucleation kinetics of selenium (+4) precipitation from an acidic copper sulphate solution was determined using the classical nucleation theory (CNT). Experiments were carried out by varying the levels of supersaturation from 8.66×10¹⁵ to 4.33×10¹⁷ at a temperature of 95 °C under atmospheric pressure. The nucleation rates for four different levels of supersaturation, the nucleation work and the nucleus size were determined. The kinetic constant A was found to be 3.92×10²⁷ m⁻³ s⁻¹, which shows that the nucleation process takes place through a homogeneous mechanism. The associated thermodynamic parameter (B) was determined to be 8.98×10⁰⁴.     

Magnetocaloric effects in magnetic nanoparticle systems: A Monte Carlo study

A superparamagnetic nanoparticle system is a good candidate for implementing the magnetocaloric effect, due to its suitable properties as a frozen ferrofluid able to follow a thermodynamic cycle. For such aim, we have concentrated on the study of the entropy dependence on both the particle size and sample concentration using a Monte Carlo simulation. We have found that for a given sample concentration there exists a particle size for the larger entropy increase, and reciprocally for a given particle volume there exists a sample concentration able to produce the larger entropy change.     

Synthesis of monodispersed cubic magnetite particles through the addition of small amount of Fe into Fe(OH)2 suspension

Magnetite particles were synthesized through a process including dissolution of Fe(OH)₂ and precipitation of an oxidized phase in aqueous solution. The Fe³⁺ ion was added at the beginning of the synthesis to accelerate the formation of magnetite, control the particle size and improve the monodispersibility. It was found that the addition of Fe³⁺ ion affected the nucleation and the formation of magnetite particles significantly. Magnetite nanoparticles with small particle size and narrow size distribution were obtained. Furthermore, high magnetic properties were obtained in small particle size. The particle size and magnetic properties increased through the increase of Fe²⁺/Fe³⁺ ratio.     

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.     

Preparation and characterization of superfine ammonium perchlorate (AP) crystals through ceramic membrane anti-solvent crystallization

In this paper, a novel ceramic membrane anti-solvent crystallization (CMASC) method was proposed for the safe and rapid preparation ammonium perchlorate (AP) crystals, in which the acetone and ethyl acetate were chosen as solvent and anti-solvent, respectively. Comparing with the conventional liquid anti-solvent crystallization (LASC), CMASC which successfully introduces ceramic membrane with regular pore structure to the LASC as feeding medium, is favorable to control the rate of feeding rate and, therefore, to obtain size and morphology controllable AP. Several kinds of micro-sized AP particles with different morphology were obtained including polyhedral-like, quadrate-like to rod-like. The effect of processing parameters on the crystal size and shape of AP crystals such as volume ratio of anti-solvent to solvent, feeding pressure and crystallization temperature were investigated. It is found that higher volume ratio of anti-solvent to solvent, higher feeding pressure and higher temperature result in smaller particle size. Scaning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the resulting AP crystals. The nucleation and growth kinetic of the resulting AP crystals were also discussed.     

纳米TiO2薄膜的制备与光催化性能研究

以四氯化钛为前驱体,采用水解沉淀法,在不同的水解温度下,在硅藻土基多孔陶瓷上负载纳米TiO2薄膜,结合XRD和TEM对负载的纳米TiO2粒径进行了表征。对比分析了测试方法及水解温度对纳米TiO2粒径的影响。以罗丹明B为目标降解物,考察了水解温度对光催化剂活性的影响。结果表明:样品经550℃煅烧后,TiO2薄膜为锐钛矿型;水解温度为50℃时TiO2粒径小于水解温度为75℃时所负载纳米TiO2粒径;两种方法所测TiO2粒径有一定差异:dXRD>dTEM;水解温度为50℃所负载纳米TiO2薄膜,紫外光照300 min,对罗丹明B的去除率为89.9%,而75℃时,样品在紫外光照330 min,对罗丹明B的去除率为78.6%。     

Nucleation of GaN on sapphire substrates at intermediate temperatures by Hydride Vapor Phase Epitaxy

A novel approach to deposit GaN layers directly on a sapphire substrate by Hydride Vapor Phase Epitaxy is presented. The two‐step deposition process includes the growth of GaN nucleation layers at intermediate temperatures in the range of 750 – 900 °C and subsequent high‐temperature overgrowth at about 1040 °C. Closed and non‐closed nucleation layers with a thickness of up to 2 μm were produced and characterized by scanning and transmission electron microscopy, micro‐Raman spectroscopy and X‐ray diffraction. A growth temperature of 780 °C is found to be optimal with respect to density and size distribution of nucleation islands. Raman measurements performed on the nucleation layers reveal nearly zero residual stress indicating effective stress relaxation on cooling down from growth temperature. The results of first overgrowth experiments demonstrate the possibility to grow 10 μm thick, crack‐free GaN layers of high crystalline quality on the nucleation layers. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

煅烧制度对Lu2Ti2O7粉体生长形貌的影响

采用熔盐法制备Lu2Ti2O7粉体,研究了不同煅烧工艺对粉体形貌的影响,确定了适用于Lu2Ti2O7粉体合成的工艺条件,并在利用XRD、SEM等方法分析表征样品的基础上,提出了初步的粉体形貌生长转变机制。实验结果表明:形核速率和生长速率对粉体的颗粒尺寸和形状影响很大,过快的形核速率和生长速率有利于多面棱柱状晶粒的生长,反之则有利于八面体形状的晶粒生长。     

外加氩气流下脉冲激光烧蚀制备纳米硅晶粒成核生长动力学研究

为研究纳米硅晶粒成核生长动力学过程,采用脉冲激光烧蚀(PLA)技术,在室温,50～200 Pa的氩气氛围中,通过引入垂直于烧蚀羽辉轴线的外加气流,在水平放置的衬底上沉积了一系列纳米Si晶薄膜.扫描电子显微镜( SEM)、拉曼(Raman)散射和X射线衍射(XRD)检测结果表明,未引入气流时,衬底上相同位置处晶粒尺寸随气体压强的增大逐渐减小;在距靶1 ～2cm范围内引入气流后,尺寸变化规律与未引入气流时相反.通过分析晶粒尺寸及其在衬底上的位置分布特点,结合流体力学模型和热动力学方程,分析得出在激光能量密度一定的条件下,环境气体压强、烧蚀粒子温度和密度共同影响着纳米晶粒的成核生长.     

新型有机非线性光学晶体-L-苹果酸脲晶体生长的初步探讨

本文初步探讨了新型非线性光学晶体-L-苹果酸脲的晶体生长,研究结果表明,L-苹果酸脲晶体在甲醇、乙醇和水三种介质中的溶解度温度系数均较大,同温度下L-苹果酸脲晶体在水中的溶解度最大,无水甲醇次之,无水乙醇中最小.L-苹果酸脲晶体在无水甲醇中的成核自由能比在无水乙醇中的低得多,因此在无水甲醇中易于成核,晶核数多,晶体尺寸小,而在无水乙醇介质中,虽然成核相对比较困难一些,但有利于制备大的单晶,且发现低温有利于大晶体的生长.因此宜选择无水乙醇为晶体生长介质.L-苹果酸脲晶体在介质中以非均匀成核方式在试管壁成核长大,晶体呈棱柱状,生长过程中呈现台阶生长的特征.     

Crystallization-in-emulsion process of a melted organic compound: In situ optical monitoring and simultaneous droplet and particle size measurements

The crystallization-in-emulsion process allows the production of solid particles exhibiting specific features. Here, the batch crystallization process carried out by cooling a melted oil dispersed as an oil-in-water emulsion was studied. Two experimental set-ups allowing the in situ visualization of the nucleation and growth phenomena occurring in the dispersed liquid phase were developed. Observations in quiescent medium of motionless droplets having a diameter of few tens of micrometers showed that primary nucleation started on the inner surface of the droplets. The fast growth of the crystals consumed all the liquid contained within each droplet and was confined within each droplet by the oil–water interface. Solid polycrystalline particles similar in size to the parent droplets were produced. Dynamic tracking of the transient evolution of the size distributions of the two populations of droplets and solid particles during the cooling process in a stirred vessel was carried out using an in situ optical probe. It was shown that the droplets crystallized very progressively during cooling, starting with the largest droplets and ending with the smaller size droplets since the induction time of primary nucleation was dependent on droplet volume. In dilute conditions (1% wt% of dispersed phase) each droplet was converted into a single solid particle. Secondary nucleation based on inter-droplet collisions was not observed in these conditions.     

Size-dependent nucleation and growth kinetics model for potassium chloride—Application in Qarhan salt lake

The nucleation process and crystallization kinetics of KCl were studied in order to investigate the problems in KCl production in Qarhan salt lake. The correlation between particle size and KCl crystal growth rate was studied in a continuous mixed-suspension mixed-product removal (MSMPR) crystallizer. Classical theory of primary nucleation was used to study the homogeneous and heterogeneous nucleation mechanism of KCl crystallization. Several size-independent growth models and size-dependent growth models, such as Bransom, C-R, MJ2, ASL, and MJ3, were examined by the population balance theory. Results showed that MJ3 model closely fitted the experimental data, the mean relative deviation was 0.94%, the crystal growth rate G=2068{1−exp[−1.081×10−5(L+961.882)]}.