火焰法合成纳米TiO2颗粒生长过程

在新型火焰反应器生产纳米TiO2的过程中使用TEM微栅在不同火焰高度位置处进行原位取样分析,得到生长过程中纳米TiO2颗粒的粒径和形态. TiO2颗粒经历了成核、生长、聚并、烧结的过程. 调节反应物浓度为7.9×10-5~5.7×10-3 mol/L,研究了不同反应物浓度对纳米颗粒生长过程的影响,高前驱体浓度形成较高的单体浓度,使颗粒间碰撞几率增加,从而得到粒径较大的颗粒,产物粒径17~85 nm. 调节CH4和O2流量,改变温度场,研究温度对颗粒生长过程的影响,在相同反应物浓度条件下,较高的温度下形成分散性好、一次粒径为63 nm的颗粒,而在较低的温度下形成的颗粒一次粒径为35 nm,但颈部烧结严重;增加喷嘴气流速度减小了反应停留时间,颗粒粒径从63 nm减小到36 nm.     

EFFECT OF AGING ON PARTICLE SIZE AND OXIDATION RATE IN THE FORMATION OF NEEDLE-LIKE GOETHITE PARTICLES BY AIR OXIDATION OF AQUEOUS FERROUS HYDROXIDE SUSPENSION

Needle-like goethite particles were synthesized by air oxidation of an alkaline suspension of ferrous hydroxide using a standard-type bubble column of 10cm in internal diameter and 64 cm in height at a constant temperature of 40°C. Specifically, the effect of aging step by N2 stream prior to the oxidation step was investigated on both the oxidation rate and the final particle size based on the major axis of needle-like goethite particle. The time dependence of the fractional oxidation of total ferrous species with incorporated aging step exceeded that in the absence of aging step. The velocity of N2 in the aging step had negligible influence on the time dependence of the fractional oxidation. For the aging period exceeding 30min, the mean particle size was almost independent of the aging period. The mean size could be correlated considerably well with the oxidation rate, that is, decreased with increasing oxidation rate with and without incorporated aging step.     

SYNTHESIS OF ACICULAR GOETHITE PARTICLES BY OXIDATION OF AQUEOUS FERROUS HYDROXIDE SUSPENSION IN A BUBBLE COLUMN WITH DRAFT TUBE

Acicular goethite particles were synthesized by oxidation of an alkaline suspension of ferrous hydroxide using a bubble column with draft tubes of different diameters at a constant temperature of 40°C. The oxidation rate increased with decreasing draft tube diameter. The mean size based on the major axis of acicular particle decreased with increasing oxidation rate and decreasing draft tube diameter. The major axis was found to grow preferentially compared with the minor axis. The mean size of major axis was correlated well to the modified reaction rate defined as a product of the reaction rate per unit volume of the reactor multiplied by the ratio of the column diameter to the draft tube diameter, irrespective of the draft tube diameter.     

Particle size distribution and molecular size distribution of polymers in soap-free emulsion polymerization of styrene

Polystrene (PS) particles, generated from soap-free emulsion polymerization of styrene monomer, water, and potassium persulfate, were investigated by photon correlation spectroscopy (PCS) and TEM. The particle size distribution (PSD) was quite uniform. From the data of PCS, it could be said that lots of particles flocculated in the final stage of reaction. It was also deduced from the molecular weight distribution (MWD), measured by GPC, that, during the early stage of reaction, molecules with low molecular weight (<4000) might exist and the particles were perhaps formed through micellar-type nucleation mechanism. When initiator concentration increased, reaction rate increased but weight average molecular weight, tensile modulus, and elongation decreased. The number density of particles was found to be proportional to 0.49 power of initiator concentration. However, monomer concentration did not seem to have any great effect on all of them above.     

Particle Nucleation in Semi-batch Surfactant-free Emulsion Polymerization of Acrylic Monomers

Based on the coagulative particle nucleation mechanism, a two-step model has been developed for the semi-batch surfactant-free emulsion polymerization of butyl acrylate, or butyl acrylate in the presence of a small amount of acrylic acid. During Stage 1, precursor particles are first formed by phase separation of the oligomeric radicals in the aqueous phase. These precursor particles are extremely unstable and tend to aggregate until a stable (primary) particle size is achieved. The size of the newly formed particles is controlled mainly by limited coagulation among the precursor particles in this stage. During the early part of Stage 2, the rate of aggregation of these precursor particles to form a primary particle may be slower than the rate of coagulation of the primary particles. This factor would lead to a decrease in the particle concentration with time. Later on, the particle concentration starts to level off and finally reaches a steady value due to the effect of limited coagulation. In this stage, the particle growth is achieved mainly by polymerization of the feed monomer inside the particles. This model has been assessed by a number of experiments. The reaction variables selected for this purpose are the concentrations of initiator, acrylic acid and NaHCO₃, and the agitation speed. The proposed model predicts the experimental data of the particle concentration and particle size reasonably well. © of SCI.     

Mechanism of seeded dispersion polymerization of methyl methacrylate using submicron polystyrene seed particles

Submicron polystyrene (PS) latex particles were used as seed in seeded dispersion polymerization of methyl methacrylate (MMA) to investigate the particle nucleation and aggregation behavior in this type of polymerization. The PS seed particles were located and tracked during the reaction using a refractive index matching technique. The number of PS seed particles present in the poly(methyl methacrylate) (PMMA) particles was investigated in detail throughout the reaction. The change in the distribution of PMMA particle populations containing different numbers of seed particles indicated that intensive nucleation and aggregation occurred during the early stage of the reaction until a transition point of 8.7% conversion was attained under the reaction conditions studied. The size of the large particles at this point was around 1 μm. These particles were regarded as mature particles that did not aggregate with other mature particles. Meanwhile, immature particle were still generated continuously from the continuous phase. These immature particles could not survive the aggregation process to grow to become mature particle, but instead, were captured by the mature particles. Therefore, the total number of the mature particles remained constant from this point until the end of the reaction. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

FORMATION OF MAGNETITE FINE PARTICLES BY CHEMICAL ABSORPTION INTO AQUEOUS SUSPENSIONS OF FERROUS HYDROXIDE

The formation of magnetite particles by oxidation of aqueous suspensions of Fe(OH)₂ with dissolved oxygen has been found to be composed of (I) the formation step of ferric hydroxo-complex by oxidation of FeOH' ion in the liquid film around the gas bubble, and (ii) the formation step of magnetite by the coprecipitation of FeOH⁺ and ferric hydroxo-complex in the thin layer adjacent to the Fe(OH)₂ particle in the bulk of suspension medium. The oxidation reaction rate can be expressed apparently by zero-order with respect to the content of total Fe(II) and by first-order with respect to the concentration of dissolved oxygen up to the fractional oxidation of 0.9.Only octahedral particles whose size ranges from O.14μm to 0.20 μmare formed so long as the oxidation reaction finishes within 50 min. When the reaction completion time becomes longer than about 90 min, almost particles are of cubic shape with a mean size of about 0.20 fim.     

Measurement of Electrical Charge on Diesel Particles

Nanoparticles (D _p < 50 nm), which are formed as diesel engine exhaust cools and dilutes, constitute minority of total particle mass but majority of total particle number. There are several different theories to explain the nucleation of nanoparticles from diesel exhaust. The two main theories are homogeneous binary nucleation of sulfuric acid and water, and ion-induced nucleation. This study examined the ion-induced nucleation theory. In order to test the ionic nucleation theory, the charged fraction of the diesel particles were measured as a function of particle size using regular diesel fuel in this study. A very small amount of charge was found for the diesel nanoparticles in the nuclei mode, whereas there was a large charged fraction for the diesel particles in the accumulation mode. If ion-induced nucleation were the dominant mechanism for the nucleation of nanoparticles from diesel exhaust, one would expect a significant charge on the nuclei mode particles. The results from this study suggest that ion-induced binary nucleation is at least not a dominant mechanism for the nucleation of diesel exhaust when using regular diesel fuel.

This study also examined the influence of metal additives on nucleation and particle charging. The metal additives examined are of the type used to enhance particle oxidation in diesel particulate filters. When used, the additives led to a large increase in the concentration of solid particles in the nuclei mode, and significantly raised the level of particle charge for particles of all sizes. When additives were used, some of the solid particles in the nuclei mode carried a charge. We believe that these metal related particles form early enough in the combustion process to be charged by ions present during and shortly after combustion.     

Measurements of Kelvin-Equivalent Size Distributions of Well-Defined Aerosols with Particle Diameters > 13 nm

During the 1979 workshop of the working group on ultrafine aerosols, different experimental techniques for measuring the number concentration and size of ultrafine aerosol particles were compared. In the present paper we report on a comparison of different particle size measuring techniques for ultrafine aerosols. Well-defined monodisperse aerosols with electrical mobility particle diameters ranging from 13 to 100 nm were generated using an electrical aerosol classifier. Kelvin-equivalent size distributions of these aerosols were determined by means of a process-controlled expansion chamber, the size-analyzing nuclei counter (SANC). To this end the considered aerosol was humidified and the number concentration of the droplets growing in the expansion chamber was measured for stepwise increase in supersaturation. At a quite well defined critical supersaturation, a significant increase in the measured droplet concentration, and thus the onset of heterogeneous nucleation, was observed. By means of the Kelvin-Gibbs equation this critical supersaturation is related to the Kelvin-equivalent diameter of the aerosol particles. Measurements were made on NaCl and dioctyl phthalate (DOP) aerosols. For NaCl particles the Kelvin diameter was found to be larger by a factor of about 4 than the electrical mobility diameter, as determined by the electrostatic aerosol classifier. This is explained by the solubility of the NaCl particles. For DOP particles, however, the Kelvin diameter agrees quite well with the electrical mobility diameter. The Kelvin size distributions were found to be quite narrow, indicating a high monodispersity of the generated aerosol as well as a satisfactory size resolution of the SANC. Thus different experimental techniques, based on completely different principles, yielded similar measurement results.     

新型氯乙烯非均相聚合动力学和成粒机理

对以正丁烷（But)为反应介质的新型氯乙烯（VC）非均相聚合动力学和成粒机理进行了研究，根据VC-But二元体系气液平衡方程，由聚合过程气相压力或组成变化计算VC聚合转化率，VC非均相聚合的诱导期不明显，自动加速现象一般发生在聚合前、中期，后期聚合速率较小，新型VC非均相聚合PVC树脂的体粒径与悬浮PVC树脂相当，数均粒径较小，PVC颗粒由基本不熔结的初级粒子组成，颗粒内部初级粒子分布密度大，粒径大，孔隙率高；而在颗粒表层初级粒子分布密度高，粒径小，孔隙率低；树脂的增塑剂吸收率远大于悬浮PVC树脂，根据PVC树脂的颗粒特性和PVC与VC／But混合液的溶解度参数差异，推断聚合成粒机理为：PVC分子链在很低转化率时就从聚合介质中沉析出来并聚集形成微区，初级粒子和颗粒；后期成粒过程包括颗粒内部初级粒子的增长和向表层的离心聚集，颗粒对新形成的大分子链及其初级聚集体的捕捉等。     

Formation of monodisperse polyacrylamide particles by dispersion polymerization: particle size and size distribution

Highly monodisperse polyacrylamide microparticles were directly prepared by dispersion polymerization in aqueous alcoholic media initiated by 2,2′‐azobisisobutyronitrile using poly(N‐vinylpyrrolidone) as a steric stabilizer. Monomer conversion was studied dilatometrically and polymer molecular weight was determined viscometrically. The hydrodynamic diameter of polymer particles and its distribution were measured with a dynamic laser light scattering spectrometer. The number of the nuclei produced in the early stage of the polymerization was found to be constant during the remainder of the polymerization. The influences of various polymerization parameters, such as initiator concentration, monomer concentration, stabilizer content, medium polarity, and polymerization temperature on the particle size and size distribution were systematically investigated. Copyright © 2003 Society of Chemical Industry     

Morphologies and properties of NiO particles prepared from NiCl<Subscript>2</Subscript>·6H<Subscript>2</Subscript>O by spray pyrolysis

Nickel oxide particles were prepared by spray pyrolysis of aqueous solution of NiCl₂·6H₂O. In the reactor the salt droplets were first converted to hollow particles by drying and then they were collapsed by oxidation to reduce their size. Each oxide particle was composed of many small nuclei with voids among them due to extremely low rate of sintering. The particle size decreased with the temperature as the sintering and crystallization proceeded. The size as well as the crystallinity of the particles increased with the initial salt concentration. When the salt droplets were preliminarily dried in diffusion dryer before entering the reactor, the collapse of the particles was considerably reduced, resulting in lower hollowness and higher sphericity. Numerical simulation on the drying of the droplets provided insight on the initial stage of spray pyrolysis.     

Formation of fine magnetite particles by oxidation of aqueous suspensions of ferrous hydroxide

Fine magnetite particles were prepared by oxidation of aqueous ferrous hydroxide suspensions in a bubble column equipped with an internal concentric draft tube. The oxidation reaction rate could be expressed apparently as zero-order with respect to the concentration of total ferrous species and first-order with respect to the concentration of dissolved oxygen. The final particle size generally decreased with increasing reaction rate. The geometric Standard deviation of size-distribution ranged from 1.20 to 1.35 under the present experimental conditions, and increased with an increase in reaction rate.     

COUNTING EFFICIENCY OF THE API AEROSIZER

The Aerosizer (Amherst Process Instruments, Inc. Hadley MA) is a time-of-flight instrument frequently used to measure the size distribution of an aerosol. However, if the Aerosizer’s counting efficiency, defined as the number of particles counted divided by the total number entering the instrument, is not 100% or varies with particle size, the resulting size distribution will be inaccurate.Experiments were conducted to determine the effect of particle diameter, particle concentration, photomultiplier tube (PMT) voltage, and model type on the Aerosizer’s counting efficiency. To calculate counting efficiency, the number of particles between 0.3 and 10 μm recorded by the Aerosizer was divided by the number of particles of the same size collected on each stage of a cascade impactor.Particle diameter, aerosol concentration, Aerosizer model, PMT voltage, and the diameter interaction terms influenced counting efficiency. Counting efficiencies were less than 1% for particles smaller than 0.45 μm, and more than 100% for particles larger than 7 μm. Increasing the PMT voltage increased the counting efficiency for the smaller particles, but also created false, larger particles. Counting efficiency decreased as count rate increased for count rates greater than 20,000 particles per second. The Aerosizer LD counted particles more efficiently than the Aerosizer Mach 2 because of improved laser and optics systems. Four regression models that relate counting efficiency to the salient operating parameters were developed, one for each combination of Aerosizer model and photomultiplier tube voltage studied.     

白云石制备的纳米氢氧化镁的性能及其影响因素

探讨了白云石碳化法制备纳米级氢氧化镁的工艺条件,研究了沉淀剂、反应温度对纳米级氢氧化镁形貌的影响,以及表面活性剂对纳米级氢氧化镁分散性的改善,并对反应机理进行了阐述.结果表明:以氨水为沉淀剂所得的纳米氢氧化镁近似六边形的薄片状,平均粒径为16nm,其结构稳定性优于以氢氧化钠为沉淀剂的产品.当反应温度为30℃时,Mg(OH)_2形成细小晶核,薄片的厚度为5～7nm,晶粒粒径为10～15nm;反应温度为50℃时,晶核开始生长为大晶粒,但排列无规则;反应温度为70℃时,Mg(OH)_2薄片的厚度增至10nm左右,晶粒粒径为10～20nm,具有规则排列的完整晶粒:反应温度为80℃时,Mg(OH)_2晶粒具有不规则排列.加入表面活性剂聚乙二醇或十二烷基硫酸钠可以提高纳米粒子的分散性.当表面活性剂聚乙二醇用量为氢氧化镁的3.0%(以质量计,下同)时,纳米氢氧化镁的分散性最好,片层的厚度约为10 nm,平均粒径为20 nm.当表面活性剂十二烷基硫酸钠用量为氢氧化镁的4.0%时,纳米氢氧化镁具有较好的分散效果,平均粒径为20nm.     

基于ERT技术的硫酸钡颗粒形成过程研究

采用电阻层析成像技术（ERT）实时监测反应器内横截面电导率变化,进而考查络合法形成球形硫酸钡颗粒的过程中溶液电导率变化趋势。在拟均相条件下,根据ERT实时监测、采集、分析硫酸钡颗粒生成过程中电导率变化情况,得出电导率变化曲线,并采用扫描电子显微镜方法观察随着反应时间变化硫酸钡颗粒的形态变化情况。实验结果表明,使用ERT监测电导率变化可以得出不同浓度条件下电导率变化趋势及其浓度条件下的最适宜反应时间,并验证了采用电阻层析技术（ERT）对研究硫酸钡颗粒的形成具有较高的灵敏度,可用于研究硫酸钡颗粒的形成最适宜时间;用扫描电子显微镜观察络合法合成硫酸钡最开始形成一种梭型颗粒,随着钡离子的缓慢释放,硫酸钡颗粒不断生长,最终得到粒度均匀、分散性良好的球形硫酸钡颗粒。     

Preparation of Nanosized Hematite Particles by Mechanical Activation of Goethite Samples

Nanosized single crystals of hematite with a very narrow particle size distribution were prepared by mechanical activation of two different goethite samples. Both goethite samples transformed completely into hematite after 70 h grinding time. Transmission electron microscopy showed that the final particles were spherical in shape and of ∼17 nm average particle size. This particle size was coincident with that estimated from specific surface measurements, indicating that the hematite samples consisted of nonporous and nonaggregated particles. The crystallite size, calculated from the broadening of the XRD peaks, in the hematite samples indicated that particles consisted of single crystals. No influence of the precursor was observed in the products, so both goethite samples yielded identical rounded single crystals with a narrow particle size distribution.     

Generation and Quantification of Ultrafine Particles through Terpene/Ozone Reaction in a Chamber Setting

Terpene/ozone reactions produce gas- and condensed-phase products and thus contribute to both indoor and outdoor aerosol. These reactions may be important in indoor settings, where terpenes are generated from indoor sources and ambient ozone can reach significant levels. Moreover, airway irritation has been observed in mice exposed to terpene oxidation products (OPs). The aim of this study was to characterize a system for generating and quantifying ultrafine particles formed through terpene/ozone reactions in preparation for inhalation toxicology experiments. Two common monoterpenes, f -pinene and d -limonene, and a hemiterpene, isoprene, were investigated. Ozone and gas-phase terpene were introduced continuously into a reaction flow tube, from which reaction products entered a plexiglass chamber. Particle number, mass, and size distribution (～15-750 nm) were monitored in the chamber for various reactant concentrations and air exchange rates (AERs). In all experiments, ozone was the limiting reagent and the reaction rate was much more rapid than the AER. Particles formed rapidly and in high concentrations in the pinene and limonene systems. Particle formation was slower in the isoprene system and fewer particles were formed; moreover, particle diameters were smaller. In all 3 systems, progressive growth of particles was observed due to condensation and coagulation processes. The isoprene system displayed instability with respect to aerosol characteristics and did not reach steady-state conditions. In the pinene system, ozone concentration was a strong predictor of steady-state particle number and mass concentration and particle diameter. The particle number was greater at higher AERs, but particles were smaller. This study is the first to incorporate measurement of ultrafine particles formed from terpene/ozone reactions into a controlled exposure chamber setting. Following system characterization, we will conduct mouse exposures to further investigate the respiratory effects of gas- and particle-phase terpene OPs.     

煤自燃氧化放热效应的影响因素分析

任何一种煤都具有氧化放热性是煤自燃的根本原因,煤自燃氧化放热效应的主要影响因素包括煤分子结构、温度、氧浓度和松散煤松粒度。通过总结前人提出的煤化学结构模型,参照现代研究的成果,提出了描述煤自燃的煤分子化学结构模型。从煤分子结构上看,主要是煤分子中的非芳香结构侧链、桥链与氧发生反应,煤的氧化放热效应主要取决于煤表面活性分子种类、数量和空间结构特性。氧化放热强度和耗氧速率分别是描述煤氧化放热效应和煤氧复合速度的主要指标,利用化学动力学理论和煤低温自然发火实验测算出不同煤温时的氧化放热强度,利用程度升温实验测算出不同氧浓度时的耗氧速率,从定性和定量两个方面分析了氧浓度、温度和松散煤体粒度对煤氧化放热效应的影响。     

Controlling factors of the particle size of spherical silica synthesized by a dry process

Silica is an important inorganic compound used in many materials, including quartz products and resin fillers. To synthesize silica by a dry process, combustion methods using organosilicon compounds as raw monomers have been extensively studied. However, few studies have considered mass production or compared preparation methods using different types of organosilicon compounds and the same equipment. In this study, we used six starting materials in a dry process and examined the effect of monomer species on particle shape and size. The amount of raw monomer supplied was adjusted so that the calorific value of combustion was the same for all the raw monomers. We focused on the calorific value because it affects the construction cost of industrial-scale production plants. For example, when the calorific value is high, the manufacturing plant is large and costly because the heat generated after the reaction must be removed. We obtained spherical silica particles by a dry process using six monomers. The particle size depended on both the basic unit of the monomer and the required oxygen (RO) ratio of the primary burner gas. This combination was essential in determining the speed of the initial reaction, number of nuclei, and residence time. Consequently, we established a factor that determines the size of spherical silica particles and successfully adjusted the particle size.