NUMERICAL SIMULATION OF THE GROWTH OF NANOPARTICLES IN A FLAME CVD PROCESS

The growth of titania nanoparticles in a flame CVD process has been simulated by computational fluid dynamics, based on the change rate of particle number density due to their collisions calculated from an integral collision kernel. The assumptions made on constant particle volume density nv (nd^3), constant density of particle surface area ns (nd^2), and constant entity nd^2.5 in coagulation process have been examined. Comparisons have been made on particle size distribution between measurement results and predictions from present model of particle growth and Kruis model of particle dynamics for titania nanoparticles synthesized by the flame CVD process. Effects of operational parameters such as O2 mole fraction and particle number density on mean particle size and size distribution have been discussed.     

NANOPARTICLE AEROSOL SCIENCE AND TECHNOLOGY： AN OVERVIEW

As a new scientific discipline, nanoparticle aerosol science and technology （NAST） deals with the formation, properties and behavior of nanoparticles in gases. Driven by its practical applications in many different fields, NAST has been undergoing rapid development. A conceptual framework of the discipline, with its own basic principles, experimental methods and computational techniques, is now taking shape. This paper presents an overview of the current status and research needs of the new discipline. The presentation begins with a discourse on the relationship among various particle systems, which occur frequently in nature and industry. The properties and behavior of nanoparticle aerosols are then discussed, with emphasis on the key roles played by particle size and morphology. Similar to fluid dynamics, NAST is an enabling discipline in the sense that it has provided the concepts and methodology needed for the development of many other fields. Applications of nanoparticle aerosol science and technology are highlighted in three important areas： （1） aerosol processes for synthesis of nanoparticles, （2） atmospheric nanoparticles and global climate, and （3） dosimetry of inhaled nanoparticles. These fields have features in common insofar as nanoparticie aerosols follow the same basic laws of physics and chemistry.     

Hollow spherical titanium dioxide nanoparticles for energy and environmental applications

Hollow spherical titanium dioxide(TiO_2) nanoparticles possess unique properties toward energy and environmental applications,because of the intrinsic properties of TiO_2 and benefits induced by their hollow structure.A detailed understanding of TiO_2 hollow spheres will promote their use in sustainable energy and environmental applications.This perspective details current methods for synthesizing hollow spherical TiO_2 nanoparticles,and their performance in dye-sensitized solar cells,photocatalysts,and batteries.This perspective will promote the design and innovative thinking on the application of hollow spherical metal oxide nanoparticles.     

Micromechanical modeling of the effect of particle size difference in dual phase steels

Dual phase (DP) steels having a microstructure consisting of martensite islands, referred to as particles, dispersed in a ferrite matrix have received a great deal of attention due to their useful combination of high strength, high work hardening rate and ductility, all of which are favorable properties for forming processes. The martensite particles display two distinct deformation mechanisms, depending on their size. Small particles are reported in the literature to undergo no measurable plastic deformation and thus can be described as rigid particles dispersed in a matrix of ferrite. On the other hand, large particles reportedly experience a small degree of plastic deformation, which has a significant influence on the mechanism of deformation of such materials. Although most micromechanical models assume a uniform particle size, a distribution of sizes in DP-steels is a more realistic assumption. In this work, a micromechanical model is developed to capture the effect of particle size differences on the mechanical behavior of DP-steels. It is shown that the difference becomes most significant when the ratio of the small to large particle size is approximately 1/2. At low volume fractions of martensite, the effect of a distribution of particle sizes is negligible, but at intermediate and high volume fractions of martensite the interaction due to the size difference becomes quite important. The model displays the intrinsic ability of capturing the steep rise in the strain-hardening rate observed in DP-steels. The model also successfully predicts the mechanisms involved in the deformation process in the DP-steels in agreement with experimental observations reported in the literature.     

Controllable preparation of particles with microfluidics

This paper reviews recent development and achievements in controllable preparation of nanoparticles, micron spherical and non-spherical particles, using microfluidics. A variety of synthesis strategies are presented and compared, including single-phase and multiphase microflows. The main structures of microfluidic devices and the fundamental principles of microflows for particle preparation are summarized and identified. The controllability of particle size, size distribution, crystal structure, morphology, physical and chemical properties, is examined in terms of the special features of microfluidic reactors. An outlook on opinions and predictions concerning the future development of powder technology with microfluidics is specially provided.     

Ag2S纳米粒子原位合成以增强PAI涂层机械和摩擦学性能

借助单源前驱体热分解在聚酰胺酰亚胺(PAI)涂层中原位合成了硫化银(Ag₂S)纳米粒子,并通过调节单源前驱体的含量进一步调控纳米粒子尺寸. 采用X射线衍射仪和高分辨场发射扫描电镜对原位合成Ag₂S纳米粒子的物相结构、形貌、尺寸和尺寸分布进行了表征和分析;详细研究了Ag₂S纳米粒子对PAI涂层机械性能和摩擦学性能的影响;对其增强机制进行了探讨. 结果表明:PAI涂层中原位合成的Ag₂S纳米粒子粒径较小而且分散均匀,且调节单源前驱体能有效调控Ag₂S纳米粒子的尺寸和尺寸分布. Ag₂S纳米粒子的原位引入(优化质量分数为5.0%)有效改善了PAI涂层的机械性能和摩擦学性能,其摩擦学性能的增强归因于机械强度的提高和诱导转移膜的形成.      

Scalable gas-phase processes to create nanostructured particles

The properties of nanoparticles are often different from those of larger grains of the same solid material because of their very large specific surface area. This enables many novel applications, but properties such as agglomeration can also hinder their potential use. By creating nanostructured particles one can take optimum benefit from the desired properties while minimizing the adverse effects. We aim at developing high-precision routes for scalable production of nanostructured particles. Two gas-phase synthesis routes are explored. The first one - covering nanoparticles with a continuous layer - is carried out using atomic layer deposition in a fluidized bed. Through fluidization, the full surface area of the nanoparticles becomes available. With this process, particles can be coated with an ultra-thin film of constant and well-tunable thickness. For the second route - attaching nanoparticles to larger particles - a novel approach using electrostatic forces is demonstrated. The micron-sized particles are charged with one polarity using tribocharging. Using electrospraying, a spray of charged nanoparticles with opposite polarity is generated. Their charge prevents agglomeration, while it enhances efficient deposition at the surface of the host particle. While the proposed processes offer good potential for scale-up, further work is needed to realize large-scale processes.     

Hollow spherical titanium dioxide nanoparticles for energy and environmental applications

Hollow spherical titanium dioxide (TiO₂) nanoparticles possess unique properties toward energy and environmental applications, because of the intrinsic properties of TiO₂ and benefits induced by their hollow structure. A detailed understanding of TiO₂ hollow spheres will promote their use in sustainable energy and environmental applications. This perspective details current methods for synthesizing hollow spherical TiO₂ nanoparticles, and their performance in dye-sensitized solar cells, photocatalysts, and batteries. This perspective will promote the design and innovative thinking on the application of hollow spherical metal oxide nanoparticles.     

Nanoparticle coagulation in a planar jet via moment method

Large eddy simulations of nanoparticle coagulation in an incompressible pla- nar jet were performed.The particle is described using a moment method to approximate the particle general dynamics equations.The time-averaged results based on 3000 time steps for every case were obtained to explore the influence of the Schmidt number and the Damkohler number on the nanoparticle dynamics.The results show that the changes of Schmidt number have the influence on the number concentration of nanoparticles only when the particle diameter is less than 1 nm for the fixed gas parameters.The number concentration of particles for small particles decreases more rapidly along the flow di- rection,and the nanoparticles with larger Schmidt number have a narrower distribution along the transverse direction.The smaller nanoparticles coagulate and disperse easily, grow rapidly hence show a stronger polydispersity.The smaller coagulation time scale can enhance the particle collision and coagulation.Frequented collision and coagulation bring a great increase in particle size.The larger the Damkohler number is,the higher the particle polydispersity is.     

Key synthesis of magnetic Janus nanoparticles using a modified facile method

Inorganic/organic poly（methylmethacrylate-acrylic acid-divinylbenzene） iron oxide Janus magnetic nanoparticles（P（MMA-AA-DVB）/Fe3O4） with strong magnetic domains and unique surface functionalities were prepared using a solvothermal process.The P（MMA-AA-DVB） nanoparticles were prepared via soapfree emulsion polymerization and used as a precursor for preparing Janus nanoparticles.The morphology and magnetic properties of the magnetic Janus nanoparticles formed were characterized using a laser particle size analyzer,transmission electron microscopy,Fourier transform infrared spectroscopy,vibrating sample magnetometry,and thermogravimetric analysis.The synthesized P（MMA-AA-DVB）/Fe3O4 magnetic Janus nanoparticles were characterized by a Janus structure and possessed a stable asymmetric morphology after being dually functionalized.The particle size,magnetic content,and magnetic domain of the P（MMA-AA-DVB）/Fe3O4 magnetic Janus nanoparticles were 200 nm,40%,and 25 emu/g,respectively.The formation mechanism of the Janus nanoparticles was also investigated,and the results revealed that the reduction of Fe3＋ ions and growth of Fe3O4 took place on the surface of the P（MMA-AA-DVB） polymeric precursor particles.The size of the Janus particles could be controlled by narrowing the size distribution of the P（MMA-AA-DVB） precursor nanoparticles.     

Segregation of particulate solids: Experiments and DEM simulations

Segregation of granular materials is a complex phenomenon, difficult to measure quantitatively and to predict. Discrete element method (DEM) can be a useful tool to predict segregation effects and to support the industrial design. In this context, a very challenging idea is the characterization of the granular solids to provide the key parameters needed for a successful DEM simulation of segregation processes. Rolling friction, sliding friction and the coefficient of restitution are the critical parameters to be studied. These microscopic simulation parameters are calibrated by comparing the macroscopic behavior of granular matter in standard bulk experiments, which have the advantage of being highly repeatable and reliable.An experimental method is presented to characterize free surface segregation. The effects of different particle properties, particularly, shape and size, on segregation of cohesionless materials were investigated. From the experiments, particle size demonstrated a stronger effect on segregation than particle shape. Finally, the corresponding DEM simulations of the segregation experiments were presented. The parameters obtained by calibration were validated by the comparison of the modeled segregation behavior with the experimental results. Thus, calibrated DEM simulations are capable of predicting segregation effects.     

Particulate size effects in the particle-reinforced metal-matrix composites

The influences of particle size on the mechanical properties of the particulate metal matrix composite are obviously displayed in the experimental observations. However, the phenomenon can not be predicted directly using the conventional elastic-plastic theory. It is because that no length scale parameters are involved in the conventional theory. In the present research, using the strain gradient plasticity theory, a systematic research of the particle size effect in the particulate metal matrix composite is carried out. The roles of many composite factors, such as: the particle size, the Young's modulus of the particle, the particle aspect ratio and volume fraction, as well as the plastic strain hardening exponent of the matrix material, are studied in detail. In order to obtain a general understanding for the composite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered to check the strength dependence of the smooth or non-smooth particle surface. Finally, the prediction results will be applied to the several experiments about the ceramic particle-reinforced metal-matrix composites. The material length scale parameter is predicted. The project supported by the National Natural Science Foundation of China (19891180, 19925211) and by the Chinese Academy of Sciences (KJ951-1-201) and “Bai Ren” plan     

Particle manipulation via integration of electroosmotic flow of power-law fluids with standing surface acoustic waves (SSAW)

Standing surface acoustic wave (SSAW) based microfluidic devices have shown great promise toward fluid and particle manipulation applications in medicine, chemistry, and biotechnology. In this article, we present an analytical model for investigating continuous manipulation of particles (both synthetic and biological) within electroosmotic flow of non-Newtonian bio-fluids in a microfluidic channel under the influence of standing surface acoustic waves (SSAW). The particles are injected along the center of channel into the electroosmotically driven flow of power-law fluids, wherein their transport through the SSAW region is dictated by the hydrodynamic, electrophoretic, and acoustic forces. We first present a mathematical model to analyze the characteristics of electroosmotic flow of non-Newtonian power-law fluids in a hydrophobic slit microchannel. Next, we investigate the trajectories of particles in the flow field due to the combined effect of electroosmotic, electrophoretic, and acoustophoretic forcing mechanisms. The effect of key parameters such as particle size, their physical properties, input power, flow rate, and flow behavior index on the particle trajectories is examined while including the effect of the channel walls. The presented model delineates the methodologies of improving SSAW-based particle separation technology by considering the fluid rheology as well as the surface properties of the channel walls. Therefore, we believe that this model can serve as an efficient tool for device design and quick optimizations to explore novel applications concerning the integration of electroosmotic flows with acoustofluidic technologies.     

Magnetic nanoparticles for environmental and biomedical applications: A review

Engineered magnetic nanoparticles (MNPs) hold great potential in environmental, biomedical, and clinical applications owing to their many unique properties. This contribution provides an overview of iron oxide MNPs used in environmental, biomedical, and clinical fields. The first part discusses the use of MNPs for environmental purposes, such as contaminant removal, remediation, and water treatment, with a focus on the use of zero-valent iron, magnetite (Fe₃O₄), and maghemite (γ-Fe₂O₃) nanoparticles, either alone or incorporated onto membrane materials. The second part of this review elaborates on the use of MNPs in the biomedical and clinical fields with particular attention to the application of superparamagnetic iron oxide nanoparticles (SPIONs), which have gained research focus recently owing to their many desirable features such as biocompatibility, biodegradability, ease of synthesis and absence of hysteresis. The properties of MNPs and their ability to work at both cellular and molecular levels have allowed their application in vitro and in vivo including drug delivery, hyperthermia treatment, radio-therapeutics, gene delivery, and biotherapeutics. Physiochemical properties such as size, shape, and surface and magnetic properties as well as agglomeration of MNPs and methods to enhance their stability are also discussed.     

Segregation of particulate solids＂ Experiments and DEM simulations

Segregation of granular materials is a complex phenomenon, difficult to measure quantitatively and to predict. Discrete element method （DEM） can be a useful tool to predict segregation effects and to support the industrial design. In this context, a very challenging idea is the characterization of the granular solids to provide the key parameters needed for a successful DEM simulation of segregation processes. Rolling friction, sliding friction and the coefficient of restitution are the critical parameters to be studied. These microscopic simulation parameters are calibrated by comparing the macroscopic behavior of granular matter in standard bulk experiments, which have the advantage of being highly repeatable and reliable. An experimental method is presented to characterize free surface segregation. The effects of different particle properties, particularly, shape and size, on segregation of cohesionless materials were investi- gated. From the experiments, particle size demonstrated a stronger effect on segregation than particle shape. Finally, the corresponding DEM simulations of the segregation experiments were presented. The parameters obtained by calibration were validated by the comparison of the modeled segregation behav- ior with the experimental results. Thus, calibrated DEM simulations are capable of predicting segregation effects.     

Continuous synthesis of iron oxide (Fe3O4) nanoparticles via thermal decomposition

Iron oxide nanoparticles have become of great interest in the medical field for their potential uses in areas such as biomagnetic imaging and hypothermia cancer treatment. Traditionally, particles for these applications are produced through batch-based methodologies. Herein, we demonstrate an alternative continuous flow production method for the synthesis of Fe₃O₄ iron oxide nanoparticles. Advantages of continuous flow over the batch method include consistent formation of uniformly spherical particles, thorough mixing of reactants, and capacity for high-volume particle production. In this study, a continuous flow reaction mechanism was proposed in which stoichiometric control of reactants had the potential to control final particle size. The project was conducted under the supposition that the iron oleate/ligand ratio in the precursor was the greatest size control factor, with a higher ratio resulting in smaller particles. The resulting particles produced by this continuous method were characterized by high-resolution transmission electron microscopy, X-ray diffraction, and magnetometry.     

Molecular dynamics simulation of diffusion of nanoparticles in mucus

The rapid diffusion of nanoparticles(NPs) through mucus layer is critical for efficient transportation of NPs-loaded drug delivery system. To understand how the physical and surface properties of NPs affect their diffusion in mucus, we have developed a coarse-grained molecular dynamics model to study the diffusion of NPs in modeled mucus layer. Both steric obstruction and hydrodynamic interaction are included in the model capable of capturing the key characteristics of NPs' diffusion in mucus. The results show that both particle size and surface properties significantly affect the diffusivities of NPs in mucus. Furthermore, we find rodlike NPs can gain a higher diffusivity than spherical NPs with the same hydrodynamic diameter. In addition, the disturbed environment can enhance the diffusivity of NPs. Our findings can be utilized to design mucus penetrating NPs for targeted drug delivery system.     

Effect of interphase layers on the overall elastic and conductive properties of matrix composites. Applications to nanosize inclusion

For a composite with thin interface layers between inclusions and the matrix, the effective elastic properties and the effective conductivity (thermal or electric) are almost unaffected by the layers, provided (1) the layer thickness is much smaller than the inclusion sizes and (2) the contrast between the properties of the layers and either of the phases is not overly high. For composites with nanoparticles, the interface thickness may be comparable to the particle sizes. Therefore, the effect of interfaces on the overall properties may be substantial. The controlling parameters are (1) the ratio of the interface thickness to particle sizes and (2) variability of the properties across the interface thickness. Explicit expressions constructed in the present work show that the overall elastic/conductive properties are affected, mostly, by the interface thickness (normalized to the size of the core particle) and are much less sensitive to the extent of the variation and its exact character. Similarities and differences between the elasticity and the conductivity problems are discussed.     

Prediction of nanoparticle transport and deposition in bends

Nanoparticle transport and deposition in bends with circular cross-section are solved for different Reynolds numbers and Schmidt numbers. The perturbation method is used in solving the equations. The results show that the particle transport patterns are similar and independent of the particle size and other parameters when suspended nanoparticles flow in a straight tube. At the outside edge, particle deposition is the most intensive, while deposition at the inside edge is the weakest. In the upper and lower parts of the tube, depositions are approximately the same for different Schmidt numbers. Curvatures of tube, Reynolds number, and Schmidt number have second-order, forth-order, and first-order effects on the relative deposition efficiency, respectively.