Mechanism of formation and nanostructure of Stöber silica particles

The formation of silica nano-?and microparticles has been studied during growth by the modified St?ber-Fink-Bohn (SFB) method. It has been experimentally found that the density and fractal structure of particles vary with size as they grow from 70 to 2200?nm. We propose a model of particle structure which is a dense primary particle core and is composed of concentric secondary particle shells terminating in dense primary particle layers.     

非简谐振动对纳米Ag微粒表面能的影响

测量了纳米Ａｇ微粒的熔化温度，求出了不同粒径微粒的表面能，从微观上探讨了表面能随粒径和温度的变化规律，讨论了原子作非简谐振动对它们的影响，结果表明：原子振动的非简谐效应对纳米Ａｇ微粒的表面能有重要贡献。     

An ellipsoidal particle–finite element method for hypervelocity impact simulation

A number of coupled particle–element and hybrid particle–element methods have been developed for the simulation of hypervelocity impact problems to avoid certain disadvantages associated with the use of pure continuum‐based or pure particle‐based methods. To date these methods have employed spherical particles. In recent work a hybrid formulation has been extended to the ellipsoidal particle case. A model formulation approach based on Lagrange's equations, with particle entropies serving as generalized coordinates, avoids the angular momentum conservation problems which have been reported with ellipsoidal smooth particle hydrodynamics models. Copyright © 2003 John Wiley & Sons, Ltd.     

Analysis of the effect of particle size on polymorphic quantitation by Raman spectroscopy

Raman spectroscopy has been widely used to monitor various aspects of the crystallization process. Although it has long been known that particle size can influence Raman signal, relatively little research has been conducted in this area, in particular for mixtures of organic materials. The aim of this study was to investigate the effect of particle size on quantification of polymorphic mixtures. Several sets of calibration samples containing different particle size fractions were prepared and Raman spectra were collected with different probes. Calibration models were built using both univariate and multivariate analysis. It was found that, for a single component system, Raman intensity decreased with increasing particle size. For mixtures, calibration models generated from the same particle size distribution as the sample yielded relatively good predictions of the actual sample composition. However, if the particle sizes of the calibration and unknown samples were different, prediction errors resulted. For extreme differences in particle sizes, prediction errors of up to 20% were observed. Prediction errors could be minimized by changing the sampling optics employed.     

背包问题的混合粒子群优化算法

经典的粒子群是一个有效的寻找连续函数极值的方法，结合遗传算法的思想提出的混合粒子群算法来解决背包问题，经过比较测试，6种混合粒子群算法的效果都比较好，特别交叉策略A和变异策略C的混合粒子群算法是最好的且简单有效的算法，并成功地运用在投资问题中。对于目前还没有好的解法的组合优化问题，很容易地修改此算法就可解决     

Optimizing the effect of reinforcement,particle size and aging on impact strength for Al 6061-red mud composite using Taguchi technique

Aluminium 6061-red mud composite has been successfully casted by using stir casting. Fairly uniform distribution of red mud reinforcement has been observed by SEM micrographs. The presence of red mud particles inside the matrix has been confirmed by energy dispersive spectroscopy. Impact testing has been done on impact testing machine with a range of 1–300 J (Charpy), 1–175 (Izod) and least count of 0.5 J. Impact strength shows a decreasing trend with increase in percentage reinforcement, aging time and is increased with increase in particle size. ANOVA analysis found that the effect of particle size and aging time are significant but the effect of percentage reinforcement has not been found significant. From the analysis, it can be observed that the particle size influences the impact strength most followed by aging time and percentage reinforcement. Optimum value of impact strength has been predicted by using Taguchi technique and confirmed experimentally by confirmation experiment results.     

The effect of annealing on the structure of cobalt modified iron oxide particles. Part II

A new high coercivity cobalt modified iron oxide particle series has been prepared by annealing in a nitrogen atmosphere. The dependence of coercivity (Hc), Hc-aging, temperature dependence of Hc, and magnetic tape properties have been studied as a function of annealing temperature. X-ray photoelectron spectroscopy (XPS) has been used to correlate the changes in magnetic properties to the cobalt concentration gradient near the particle surface. As in Part I of the study, most of the changes observed are consistent with diffusion of cobalt from the surface into the particle core.     

Preparation and microstructure of VC0.875 nanopowder

Nanocrystalline VC_0.875 nonstoichiometric vanadium carbide powder has been prepared by high-energy milling. The crystal structure, microstructure, morphology, and particle size distribution of the starting and milled powders have been studied by X-ray diffraction, laser diffraction, and scanning electron microscopy. The particle size of the VC_0.875 nanopowder has been calculated as a function of milling time in a model of the milling process. Comparison of experimental data and calculation results demonstrates that 10-h milling of vanadium carbide powder with an average particle size 6 μm yields nanopowder 40 to 80 nm in average particle size.     

Fabrication of fine-grained spherical tungsten powder by radio frequency (RF) inductively coupled plasma spheroidization combined with jet milling

Fine-grained spherical tungsten powder with particle size from 6 μm to 11 μm has been fabricated by a well-designed method with two simple steps. A pretreatment of tungsten powder was carried out in the first step. Complete dispersion, effective classification and favorable shape modification have been achieved by jet milling process. Accordingly, monodisperse tungsten powders with narrow particle size distribution were obtained. On the basis of the existing calculation and simulation results, plasma spheroidization parameters have been optimized. Thus, fine-grained spherical tungsten powder with narrow particle size distribution has been prepared by subsequent radio frequency (RF) inductively coupled plasma spheroidization. Furthermore, the presented method can be useful in fabricating spherical tungsten powder of different particle size with narrow particle size distribution as well as for process control in large-scale continuous production.     

Transverse Motion of a Particle with an Oscillating Charge and Variable Mass in a Magnetic Field

The problem of motion of a particle with an oscillating electric charge and variable mass in an uniform magnetic field has been solved. Three laws of mass variation have been considered: linear growth, oscillations, and stepwise growth. Analytical expressions for the particle velocity at different time dependences of the particle mass are obtained. It is established that simultaneous consideration of changes in the mass and charge leads to a significant change in the particle trajectory.     

A Monte-Carlo Approach to Cellular Pattern Evolution for Binary Alloys during Directional Solidification

A Monte-Carlo approach of cellular pattern evolution during directional solidification of a binary alloy has been carried out by imposing a non-uniform probability distribution of particle emission from the particle source, as an approach to the contribution of solute redistribution to the interface evolution. Both the bulk and interface diffusion have been involved in the simulation,and coarsening of the cellular patterns with time has been clearly revealed. The remarkable influence of the interface relaxation and the solute redistribution on the profile of solidifying interface has been presented.     

纳米二氧化锰的可控制备及其电化学储能机理研究

通过添加十二烷基溴化氨(CTAB)利用液相沉淀法制备了不同形貌的纳米二氧化锰,用扫描电镜、X射线衍射、孔结构以及循环伏安法等分析研究了CTAB添加量对二氧化锰形貌和电化学性能的影响.结果表明,随着CTAB添加量的增加,二氧化锰形貌从长棒状到均匀球状发生规律变化,此外随着颗粒尺寸变小,二氧化锰比容量从162F/g提高到了...     

Modeling electrowinning process in an expanded bed electrode

A theoretical model has been developed to describe the flow behavior of conducting particles in a fluidized bed electrode for electro winning of metal ions present in the dilute solution. Model equations have been developed for potential and current distributions and mass transfer rates. The influence of operating parameters on particle growth has been critically examined. It has been observed from the present investigation that the particle size increased with electrolysis time. The present model simulations have been compared with the experimental data reported in the literature and observed that the model predictions satisfactorily match with the reported experimental findings.     

The synthesis of colloidal zeolite hydroxysodalite sols by homogeneous nucleation

Colloidal suspensions of discrete particles of microcrystalline hydroxysodalite have been synthesized from clear homogeneous solutions wherein the average particle size determined by dynamic light scattering is 37 nm with a narrow particle-size distribution. Particle-size analysis performed with various methods yield results that are in agreement. The growth-limiting nutrient has been identified as alumina. As a result, the midsynthesis addition of alumina results in further particle growth to 48 nm with the corresponding increase in zeolite yield. Hence, the fine control of particle size and thereby zeolite yield without the formation of a secondary particle population is shown to be possible. Direct measurements of zeolite yield and particle size allow for a detailed evaluation of the events that occur during crystallization.     

Mathematical modeling of the heat treatment and combustion of a coal particle. I. Heating stage

Mathematical modeling of the heat treatment and subsequent combustion of a coal particle as a multistage process has been carried out. The basic parameters of the following sequential stages of this process have been calculated by approximate-analytic dependences: heating of particles; their drying; yield of volatiles, their ignition and combustion; and burning out of the coke residues. A detailed parametric analysis of the influence of the physical and regime characteristics of the process on the burning mechanism of a coal particle (with the example of coal from the Shivee-Ovoo deposit in Mongolia) has been performed. The conditions for effective burning of a single coal particle as the main element of the whole process in the furnace have been determined.     

Influence of particle size on electrical transport properties of La0.67Sr0.33MnO3 manganite system

A systematic investigation of lanthanum-based manganite, La0.67Sr0.33MnO3, has been undertaken with a view to understand the influence of varying particle sizes on electrical transport properties. With a view to obtain materials with varying particle size, they were prepared by sol-gel route, sintering at four different temperatures. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD data has been analyzed by Rietveld refinement technique and it has been confirmed that the materials have rhombohedral crystal structure with R3c space group. Metal-insulator transition temperatures (Tp) were found to decrease continuously with decreasing particle size where as ferro to paramagnetic transition temperatures (Tc) are found to remain constant. The magnetoresistance (MR) values are found to increase with decreasing particle size. With a view to understand the conduction mechanism, the electrical resistivity data have been analyzed both in the ferromagnetic metallic (T < Tp) as well as high temperature paramagnetic insulating (T > Tp) regions.     

Particle Trajectory in a Bidirectional Vortex Flow

In this research particle trajectory in a bidirectional vortex flow has been numerically predicted and the results experimentally validated. Scale analyses of forces show their order of magnitudes and give a criterion to recognize the order of magnitude of exerting forces on the particle. The particle has been assumed to be a rigid sphere. Initial velocity, diameter, density, and position of entering particle are assumed to be known. If the particle length scale is considered not to be comparable with the chamber length and if particle number density is low, then influence of particle on the flow field is negligible and a one-way solution is applicable. The governing equation is converted to a set of nonlinear, coupled, second-order ODE and solved by a numerical scheme. Results show that higher density, larger diameter, and higher initial axial velocity tend to move the particles further in the axial direction. Also, the maximum axial movement of the particle occurs when the initial radial velocity is zero and there is an optimum entrance position that provides a maximum traveling trajectory for particles. Increasing initial z-direction velocity component and density will result in increasing traveling trajectory.     

韧性颗粒增韧脆性材料的桥接变形计算模型

在脆性材料中弥散加入韧性颗粒可显著提高基体材料的断裂韧性。为了计算材料断裂韧性的增量，本文提出了新的基体与颗粒界面颈缩剥离变形假设，建立了应力与位移之间的关系式。文中还给出了精确的简化计算式。     

DEM investigation of particle anti-rotation effects on the micromechanical response of granular materials

The importance of particle rotation to the mechanical behavior of granular materials subject to quasi-static shearing has been well recognized in the literature. Although the physical source of the resistance to particle rotation is known to lie in the particle surface topography, it has been conveniently studied using the rolling resistance model installed typically on spherical particles within the DEM community. However, there has been little effort on assessing the capability of the rolling resistance model to produce more realistic particle rotation behavior as exhibited by irregular-shaped particles. This paper aims to eliminate this deficiency by making a comprehensive comparison study on the micromechanical behavior of assemblies of irregular-shaped particles and spherical particles installed with the rolling resistance model. A variety of DEM analysis techniques have been applied to elucidate the full picture of micromechanical processes occurring in the two types of granular materials with different particle-level anti-rotation mechanisms. Simulation results show that the conventional rheology-type rolling resistance models cannot reproduce the particle rotation and strain localization behavior as displayed by irregular-shaped materials, although they demonstrate clear effects on the macroscopic strength and dilatancy behavior, as have been adequately documented in the literature. More insights into the effects of particle-level anti-rotation mechanism are gained from an in-depth inter-particle energy dissipation analysis.