Molecular dynamics simulation for aggregation phenomena of nanocolloids

Nonequilibrium molecular dynamics (MD) method was used to study the dielectrophoresis (DEP) motion of nanocolloids in non-uniform electric field. By changing the electric field strength and system temperature, aggregation phenomena of nanocolloids was analyzed. Simulation results showed that at normal temperature, though the Brownian force can affect the motion of colloids, the attractive force will increase quickly with the distance between colloids down to 12σ , which makes colloids aggregate. When the Brownian force is weak to colloid's motion, for the enhancement of electric field strength, the DEP force of colloid will increase and so did the attractive force, which finally quickens the aggregate speed. Simulation results also showed that the temperature' enhancement will increase the Brownian force of colloids, hence disturbing the colloids aggregation. Moreover, the DLVO theory was used to study the motion of a pair of interactional colloids, both the potential energy and the attractive force versus distance of colloids were presented, then the latter graph was used to compare with another graph elicited by MD method. Results showed that the two graphs were nearly the same, indicating the MD model accorded with the theory.     

胶体粒子之间的等效相互作用

不同介观带电粒子(聚离子)的电双层之间的等效相互作用是目前软物质研究的前沿。本文主要评述胶体系统在这一领域的研究现状，包括标准Poisson—Boltzmann或平均场理论对于电双层研究的局限性和微离子之间空间相关性的重要性；尤其是相关性引起的聚离子之间等效相互作用为吸引的可能性，等效聚离子电荷(电荷重正化)和电荷规范化的计算方法，约束对于胶体粒子之间等效相互作用的影响和电双层溶剂粒度的统计描述，指出仍存在的问题和研究前景。     

Incoherent scatter spectra due to HF heating in the low ionosphere region

The electron velocity distribution function and the incoherent scatter spectra during ionospheric heating in low ionosphere region are presented with consideration of the elastic collision between electrons and neural particles and the excitation of rotation energy level. The effects of pump frequency and electric fields on the spectra are discussed. With the increase of electric field, the non-Maxwellian feature is enhanced, and with the increase of heating frequency, the non-Maxwellian feature is weakened. The non-Maxwellian factor will bring a large error in ionosphere parameters, which must be considered in the inversion. Supported by the National Natural Science Foundation of China (Grant No. 40310223), the National Key Laboratory of Electromagnetic Environment (LEME), and National Key Technologies R&D Program of China (Grant No. 2006BAB18B06)     

Large Eddy Simulation of a dilute particle-laden turbulent flow over a backward-facing step

Dilute gas-particle turbulent flows over a backward-facing step are numerically simulated by Large Eddy Simulation (LES) for the continuous phase and Lagrangian particle trajectory method for the particle phase. Predicted results of mean velocities and fluctuating velocities of both phases agree well with the experimental data, and demonstrate that the main characteristics of the flow are accurately captured by the simulations. Characteristics of separation and reattachments as well as essential features of the coherent structure are obtained, in which the processes of vortex roll up, growth, pairing and breaking up are shown in details. Particle dispersions are then investigated through particles’ instantaneous distributions in coherent structure as well as the mean and fluctuating properties of particle number density (PND). The predicted mean PND agree well with experiment results. For small particles, the instantaneous distributions show much preferential concentration, while their mean PND shows more uniform distribution in downstream region. On the contrary, for large particles, their instantaneous distributions are much uniform (without clear preferential concentration) due to less effect of large eddy coherent, while their mean PND across the section is not uniform for more particles are distributed in the main flow region. The preferential concentration of particles by the large-scale eddies can lead to a high fluctuating PND. Supported by the National Natural Science Foundation of China (Grant Nos. 19972036, 50172067) and the Research Committee of The Hong Kong Polytechnic University (Grant No. A-DP99)     

Molecular dynamics simulation of the test of single-walled carbon nanotubes under tensile loading

Molecular dynamics (MD) simulations were performed to do the test of sin-gle-walled carbon nanotubes (SWCNT) under tensile loading with the use of Bren-ner potential to describe the interactions of atoms in SWCNTs. The Young’s modulus and tensile strength for SWCNTs were calculated and the values found are 4.2 TPa and 1.40―1.77 TPa, respectively. During the simulation, it was found that if the SWCNTs are unloaded prior to the maximum stress, the stress-strain curve for unloading process overlaps with the loading one, showing that the SWCNT’s de-formation up to its fracture point is completely elastic. The MD simulation also demonstrates the fracture process for several types of SWCNT and the breaking mechanisms for SWCNTs were analyzed based on the energy and structure be-havior.     

Controllable synthesis of metal particles by a direct current electrochemical approach

Shapes of copper and silver particles were successfully controlled by using a very simple, effective direct-current electrochemical approach without introducing any additives or templates. A diverse range of shapes and also different inner structures were thus accessible. The products prepared at relatively high potentials have flowerlike morphologies and exhibit flakes as building blocks. The uniformly thick flakes intersect mutually, have smooth surfaces and outwardly wavy edges. The particle diameter and the flake density can be easily controlled by changing potential and/or deposition time. With a decrease of potential, the particles’ shapes changed from flower to bud, to sphere and to octahedron. Surface plasmon resonance (SPR) properties of the supported metal particles were investigated by UV-Vis diffuse reflectance spectra (UV-Vis DRS) and surface enhanced Raman scattering (SERS). It was found that the copper octahedra exhibited three characteristic bands, and SERS effect increases with the number of flakes within individual particles. Based on the experimental results, the mechanism for direct-current electrochemical growth of metal nanostructures was discussed. Supported by the National Basic Research Program of China (“973” Program) (Grant No. 2004CB619305) and the National Natural Science Foundation of China (Grant Nos. 50571044, 50831004)     

呼吸性粉尘电极化产生凝并行为的机理

依电介质极化理论，分析了电场中呼吸性粉尘的极化现象和极化粒子所受的感应力矩、感应力以及粒子间的相互作用能，进而探讨了粒子由极化而产生凝并行的机理。     

The cluster size transformation model of molten alloy under pulse electric field

Based on the electric dipole theory, the coupled field distribution of pulse electric field (PEF) with electric dipole field around the cluster in superheated molten alloy is simulated under the effect of PEF. For the difference of electro-migration, the atom of solute and solution will accumulate around the cluster, and then the smaller cluster may reorganize and grow up under the action of the coupled field. We also apply the electrostatic induction theory to analyze the bearing behavior of the half side of the cluster. The bigger the cluster is, the stronger the electrostatic force is, therefore, the bigger cluster’s stability is weak apparently. The study indicates that the cluster in the superheated molten has the homogeneous tendency under the effect of PEF. Supported jointly by the National Natural Science Foundation of China (Grant No. 50674054), and the College Key Laboratory Open Fund of Liaoning Province (Grant No. 200516203)     

Ultralight X-type lattice sandwich structure (II): Micromechanics modeling and finite element analysis

The methods of homogenization and finite elements are employed to predict the effective elastic constants and stress-strain responses of a new type of lattice structure, the X-structure proposed by the authors in a companion paper. It is shown that in most cases the predictions by the equivalent homogenization theory agree well with the experimental and 3-dimensional finite element calculated results. The theoretical and numerical study supports the argument that the X-structure is superior to the pyramid lattice structure in terms of mechanical strength. Supported by the National Basic Research Program of China (“973” Project) (Grant No. 2006CB601202), the National Natural Science Foundation of China (Grant Nos. 10632060, 10825210), the National “111” Project of China (Grant No. B06024) and the National High-Tech Research and Development Program of China (“863” Project) (Grant No. 2006AA03Z519)     

Application of time reversal mirror technique in microwave-induced thermo-acoustic tomography system

Microwave-induced thermo-acoustic tomography (MITAT) is a promising technique with great potential in biomedical imaging. It has both the high contrast of the microwave imaging and the high resolution of the ultrasound imaging. In this paper, the proportional relationship between the absorbed microwave energy distribution and the induced ultrasound source distribution is derived. Further, the time reversal mirror (TRM) technique based on the pseudo-spectral time domain (PSTD) method is applied to MITAT system. The simulation results show that high contrast and resolution can be achieved by the TRM technique based on PSTD method even for the received signals with very low signal-to-noise ratio (SNR) and the model parameter with random fluctuation. Supported by the National Natural Science Foundation of China (Grant No. 60771042), the National Hi-Tech Research and Development Program (“863” Project) (Grant No. 2007AA12Z159), 111 Project (Grant No. B07046), SiChuan Excellent Youth Foundation (Grant No. 08ZQ026-039), Program for New Century Excellent Talents in University of China and Program for Changjiang Scholars     

The event-driven constant volume method for particle coagulation dynamics

Monte Carlo (MC) method, which tracks small numbers of the dispersed simulation particles and then describes the dynamic evolution of large numbers of real particles, constitutes an important class of methods for the numerical solution of population balance modeling. Particle coagulation dynamics is a complex task for MC. Event-driven MC exhibits higher accuracy and efficiency than time-driven MC on the whole. However, these available event-driven MCs track the “equally weighted simulation particle population” and maintain the number of simulated particles within bounds at the cost of “regulating” computational domain, which results in some constraints and drawbacks. This study designed the procedure of “differently weighted fictitious particle population” and the corresponding coagulation rule for differently weighted fictitious particles. And then, a new event-driven MC method was promoted to describe the coagulation dynamics between differently weighted fictitious particles, where “constant number scheme” and “stepwise constant number scheme” were developed to maintain the number of fictitious particles within bounds as well as the constant computational domain. The MC is named event-driven constant volume (EDCV) method. The quantitative comparison among several popular MCs shows that the EDCV method has the advantages of computational precision and computational efficiency over other available MCs. Supported by the National Basic Research Program of China (“973” Project) (Grant No. 2006CB705800), the National Natural Science Foundation of China (Grant No. 20606015) and the Fok Ying Tong Education Foundation (Grant No. 114017)     

Experimental study of chemical concentration variation of ASP flooding

A physical modeling system of long slim tube was established. Several pressure measuring and sampling points were laid out at different positions along the tube. Through real-time measurements of pressures and chemical concentrations at different points, the mass transfer and chemical concentration of ASP flooding in porous media are studied. The concentration of chemicals declines gradually during the fluid flow from the inlet to the outlet of the model. The concentration increases in the front edge of the slug faster than the concentration decreases in the rear edge of the slug. The concentration variation of the chemicals is an asymmetrical and offset process. The order of motion velocities of the chemicals from fast to slow is polymer, alkali and surfactant. The motion lag and comprehensive diffusion are strong in the vicinity of the inlet, the motion velocities of the chemicals are high, the difference of flow velocities among the three chemicals is significant and the chromatographic separation of the chemicals is obvious. In the area near the outlet, the comprehensive diffusion and motion lag become weak, the concentrations of the chemicals decrease, the motion velocities of the chemicals are slow, the difference among the motion velocities of the chemicals becomes small, the chromatographic separation is not obvious, the adsorption and retention of chemicals gradually increase as the chemical slug moves further along the tube, the adsorption and retention of polymer is the most serious. Supported by the National Basic Research Program of China (“973” Project) (Grant No. 2005CB221300)     

Synthesis and oxidation behavior of boron-substituted carbon powders by hot filament chemical vapor deposition

Boron-substituted carbon powder, B _x C_1−x with x up to 0.17, has been successfully synthesized by hot filament chemical vapor deposition. The boron concentration in prepared B _x C_1−x samples can be controlled by varying the relative proportions of methane and diborane. X-ray diffraction, transmission electron microscopy, and electron energy loss spectrum confirm the successful synthesis of an amorphous BC₅ compound, which consists of 10–20 nm particles with disk-like morphology. Thermogravimetry measurement shows that BC₅ compound starts to oxidize approximately at 620°C and has a higher oxidation resistance than carbon. Supported by the National Natural Science Foundation of China (Grant Nos. 10474083, 50472051, 50532020, 50672081) and the National Basic Research Program of China (Grant No. 2005CB724400)     

An experimental study on the spray characteristics of diesel-dimethyl ether (DME) blended fuels by phase doppler anemometry

This paper presents an experimental study on the spray characteristics of diesel-dimethyl ether (DME) blended fuels by phase doppler anemometry (PDA). Blended fuels with DME mass fractions of 15%, 30% and pure diesel fuel were used to evaluate the effect of the DME concentration on the spray pattern, droplet size and velocity. The data for spray velocity vector and droplet size field were obtained. The experimental results reveal that the micro-explosive function exists in the jet of diesel-dimethyl ether (DME) blended fuels and the radial velocity of the blended fuels spray is larger than that of conventional diesel fuel spray near the nozzle exit. At the downstream part of the spray, the radial velocity and its attenuation rate of blended fuels are much more uniform and smaller than those of pure diesel spray. At the centerline of the spray, the attenuation rates of all spray axial velocities are similar. With the increase of DME concentration in the fuel, the spray angle and the exit velocity increase and the droplet size deceases. Supported by the National Natural Science Foundation of China (Grant No. 50806046) and the National Basic Research Program of China (“973” Project) (Grant No. 2007CB210007)     

A reaction-diffusion model for atomic oxygen interacting with spacecraft surface protective materials in low earth orbit environment

When hyperthermal atomic oxygen collides with a silicon surface, an ultrathin oxidation regime characterized by fractional atomic-oxygen anions having low diffusive and reactive barriers, along with their enhanced diffusion due to both the electric field and image potential, will form on the surface. In accordance with these properties, an attempt was made in the present study to modify the Almeida-Goncalves-Baumvol (AGB) model by setting the diffusivity and reaction rate constant to be diffusion-length dependence. According to the modified model, numerical parametric studies for oxidation thin growth were performed. The dependencies of the diffusion coefficient, the reaction rate constant, the attenuation length, and the adjustable parameter upon the translational kinetic energy, flux, temperature, and tangential flux of atomic oxygen were analyzed briefly via the fitting of the experimental data given by Tagawa et al. The numerical results confirmed the rationality of the modified diffusion-reaction model. The model together with the computer code developed in this study would be a useful tool for thickness evaluation of the protective film against the oxidation of atomic oxygen toward spacecraft surface materials in LEO environment. Supported by the National Natural Science Foundation of China (Grant No. 10572016)     

Interface Characteristics Between Colloidal Gold and Kaolinite Surface by XPS

The distribution of gold colloids in kaolinite and the interaction between gold and kaolinite surface were investigated by transmission electron mieroscotgy （TEM） and X-ray photoelectron spectroscopy （ XPS ）. There is strong interaction between the gold particles and the edge surfaces of kaolinite, in low pH solution, the edge surface of kaolinite is positively charged and electrostutic attrcactive force between colloide gold panicles and the positive edge surface of kaolinite woald facilitate the adsorption of colloidal gold particles onto the suface. TEM observation shows that the aggregate morphology of gold particles was dominated by particle-particle interaction and gold particles were adsorbed on the edge surface of kaolinite crystals , resulting from the electrostatic attractive force between colloidal gold particles and the positice surfaces of kaolinite. XPS data show that in Au4 f electron spectra there are four energy peaks related to gold, 83.8 eV, 85.7 eV, 87.5 eV, and 89.4 eV, respectively, which suggests that in chemical states there are metallic gold and Au bonded to O, similar to the form of Au2O3 , and composite Au2O3 is formed between the edge surface of kaolinite and colloidal gold surface.     

Mesomechanism of steel fiber reinforcement and toughening of reactive powder concrete

Reactive powder concrete (RPC) is a novel cement-based composite material with ultra-high strength. Embedding a certain amount of short steel fibers in the matrix can improve the RPC's toughness and overcome the disadvantage of high brittleness. In this paper, a number of direct uniaxial tension tests have been carried out with '8-shape' RPC200 specimens. The bond-slip process, mesoscopic structural variation and mechanical characteristics of a fiber pullout of the matrix have been investigated using the real-time SEM loading system and CCD observation techniques. The influence of the volume of embedded short steel fibers in matrix on the mesoscopic morphology of attachments on the surface of a pulled individual fiber, the initial cracking force, the ultimate pullout force, interfacial bond strength and the pullout rupture energy have been analyzed. A general formulation relating these quantities to the volume of fibers in matrix has been proposed. The components comprising the interfacial bond strength have been outlined. In addition, the contribution that fibers make to enhance and toughen the reactive powder concrete has been discussed. It is shown that there exists an optimal threshold of fiber volume ρv, opt =1.5% at which the bond performance of a fiber pullout of RPC behaves best.     

Photocatalytic performance of TiO<Subscript>2</Subscript> thin films connected with Cu micro-grid

Aiming at reducing the recombination of photo-induced carriers in semiconductor photocatalytic process, we prepared TiO₂ thin film with its surface modified by a connected Cu micro-grid via a microsphere lithography strategy, which showed higher photocatalytic activity than the pure TiO₂ film. The improvement of photocatalytic activity of Cu micro-grid to the TiO₂ film is due to the charge carrier separation and electron transfer by the conducting metal grid. The photocatalytic activity was improved as metal loading increased, which obtained the best performance at a certain loading amount, and then decreased at higher loading amount. This phenomenon was attributed to the metal’s bulk effect which could be explained by the relationship between the energetic positions and the metal cluster size. Supported by the National Natural Science Foundation of China (Grant Nos. 50672003, 50872005) and the National Basic Research Program of China (“973” Project) (Grant No. 2007CB613302) and the Fok Ying Tong Education Foundation (Grant No. 111050)     

Micro powder injection molding—large scale production technology for micro-sized components

Micro powder injection molding (μPIM), a miniaturized variant of powder injection molding, has advantages of shape complexity, applicability to many materials and good mechanical properties. Co-injection molding has been realized between metals and ceramics on micro components, which become the first breakthrough within the PIM field. Combined with the prominent characteristics of high features/cost ratio, micro powder injection molding becomes a potential technique for large scale production of intricate and three-dimensional micro components or microstructured components in microsystems technology (MST) field. Supported by National Basic Research Program of China (Grant No. 2004CB719802) and Hi-Tech Research and Development Program of China (Grant No. 2006AA03Z113)     

Deformation behavior of Cu-12wt%Al alloy wires with continuous columnar crystals in dieless drawing process

The microstructure and mechanical properties of Cu-12wt%Al alloy wires which are composed of continuous columnar crystals after dieless drawing forming at drawing speed of 1.0―1.4 mm/s and deformation temperature of 600―900℃ were analyzed, and deformation behavior of the alloy during dieless drawing forming was experimentally investigated. The results showed that in the abovemen-tioned conditions, recrystallization phenomenon was not found during dieless drawing forming. When a drawing speed of 1.0 mm/s was...