Characterization of Pyrogenic Powders with Conventional Particle Sizing Technique: I. Prediction of Measured Size Distributions

Pyrogenic powders consist of fractal like aggregates with nanosized primary particles. The formation of such aggregates, their hydrodynamic behavior and their optical properties are in principle well understood. Even so, there is only little experience in interpreting results from particle sizing of such materials. Dramatic differences in size distribution obtained from different measurement techniques give frequently rise to confusion on the “true” aggregate size. However, such differences can be attributed to the different particle properties used for size measurement and to the different types of quantities, by which the frequency of the individual size fractions are weighted. For two conventional sizing techniques, Dynamic Light Scattering and Optical Centrifugation Analysis, the influence of the structural properties on the relevant optical and hydrodynamic aggregate properties is discussed on the basis of virtual aggregates as well as of empirical data for pyrogenic powders. Finally measurable size distributions are predicted in a case study.     

Dynamics and stability of a linear cluster of spherical magnetic nanoparticles

Magnetic particles freely moving in a fluid may organize themselves into dense phases, bulk clusters, or linear chains. The dynamics of particles forming a chain is analyzed theoretically taking into account the magnetic dipole interaction as well as the Van der Waals molecular interaction. The vibrational spectrum has two branches (the magnetic branch associated with the rotation of the magnetic moment of a particle and the elastic branch associated with the displacement of particles). In the case of particles with constant mass density and magnetic moment, which is interesting for applications, these two modes are in fact independent; i.e., the effects of mode hybridization are weak. However, these effects can be manifested for hollow particles. From analysis of the vibrational spectrum, the criterion for the chain stability to a transition to a denser phase is established.     

Bridging the pressure and materials gaps: high pressure sum frequency generation study on supported Pd nanoparticles

Infrared-visible sum frequency generation vibrational spectroscopy is applied for the first time to monitor CO stretching vibrations on alumina supported Pd nanoparticles in a pressure range from 10(-7) to 200 mbar. The adsorption behavior of Pd aggregates with 3 and 6 nm mean size is dominated by surface defects and two different adsorption sites (twofold bridging and on-top) were identified. The CO adsorption site occupancy on Pd nanocrystals is mainly governed by the gas phase pressure while the structure of the particles and their temperature have a smaller influence.     

Monofunctional gold nanoparticles: synthesis and applications

The ability to control the assembly of nanoparticle building blocks is critically important for the development of new materials and devices. The properties and functions of nanomaterials are not only dependent on the size and properties of individual particles, but also the interparticle distance and interactions. In order to control the structures of nanoassemblies, it is important to first achieve a precise control on the chemical functionality of nanoparticle building blocks. This review discusses three methods that have been reported recently for the preparation of monofunctional gold nanoparticles, i.e., nanoparticles with a single chemical functional group attached to each particle. The advantages and disadvantages of the three methods are discussed and compared. With a single functional group attached to the surface, one can treat such nanoparticles as molecular building blocks to react with other molecules or nanoparticles. In other words, by using appropriate chemical reactions, nanoparticles can be linked together into nanoassemblies and materials by covalent bonds, similar to the total chemical synthesis of complicated organic compounds from smaller molecular units. An example of using this approach for the synthesis of nanoparticle/polymer hybrid materials with optical limiting properties is presented. Other potential applications and advantages of covalent bond-based nanoarchitectures vs. non-covalent interaction-based supramolecular self-assemblies are also discussed briefly in this review.     

光散射聚集速率测定中T矩阵方法的应用

聚集速率是评估胶体体系特性及稳定性的关键参数, 静态光散射和动态光散射则是测量聚集速率的两个重要方法. 然而, 用静态光散射和动态光散射测量聚集速率时, 需要知道有关单粒子和双粒子聚集体光散射特性的数据. 为此, 通常需要把动、静两种方法结合, 才能消去这个数据. 以前各种近似理论曾用来解决这个问题, 但因粒子尺寸和形状的限制, 结果并不理想. 而T矩阵方法可以不受粒子大小和形状的限制计算其光散射特性. 本工作用T矩阵方法直接计算静态光散射和动态光散射所必须的粒子散射特性, 并将该法得到的聚集速率与动静态光散射结合法得到的聚集速率进行了比较, 两者结果很接近. 本工作为简化静态光散射和动态光散射测量聚集速率, 扩展其应用范围开辟了新途径. 关键词： T矩阵')" href="#">T矩阵 光散射法 聚集速率     

On‐line Determination of Aggregate Size and Morphology in Suspensions

Information concerning the aggregation state of fine solid particles is an important element for process control and monitoring of product quality in many applications of industrial slurries. This work deals with the application of different in‐line methods to the characterization of silica aggregate size and morphology. All of these methods exploit turbidity signals, obtained by various means including: from analysis of turbidity fluctuations in homogeneous suspension and from overall turbidity decrease during particle settling. This work also presents the opportunity to report progress in morphological and optical models of small aggregates. As a result of these models, the morphological characteristics of the aggregates along with the number of their constituting particles are derived from experimental results. Similarities between the different methods are examined and discussed.     

Effect of the primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates

Depending on the application of nanoparticles, certain characteristics of the product quality such as size, morphology, abrasion resistance, specific surface, dispersibility and tendency to agglomeration are important. These characteristics are a function of the physicochemical properties, i.e. the micromechanical properties of the nanostructured material. The micromechanical properties of these nanostructured agglomerates such as the maximum indentation force, the plastic and elastic deformation energy and the strength give information on the product properties, e.g. the efficiency of a dispersion process of the agglomerates, and can be measured by nanoindentation. In this study a Berkovich indenter tip was used for the characterisation of model aggregates out of sol–gel produced silica and precipitated alumina agglomerates with different primary particle morphologies (dimension of 15–40 nm). In general, the effect of the primary particle morphology and the presence or absence of solid bonds can be characterised by the measurement of the micromechanical properties via nanoindentation. The micromechanical behaviour of aggregates containing solid bonds is strongly affected by the elastic–plastic deformation behaviour of the solid bonds and the breakage of solid bonds. Moreover, varying the primary particle morphology for similar particle material and approximately isotropic agglomerate behaviour the particle–particle interactions within the agglomerates can be described by the elementar breaking stress according to the formula of Rumpf.     

Light scattering from colloids

This paper presents a review of light scattering results on static and dynamic properties of ordered colloidal suspensions of charged polystyrene particles and fractal colloidal aggregates. Our studies on static structure factor,S(Q), of ordered monodisperse colloidal suspensions and binary mixtures of particles with different particle diameters, measured by angle-resolved Rayleigh scattering will be discussed. This will include determination of bulk modulus using gravitational compression and observation of colloidal glass (inferred from splitting of the second peak inS(Q)). Dynamic light scattering, with real time analysis of scattered intensity fluctuations, is used to get information about Brownian dynamics of the particles. Recent advances in the field of light scattering from colloidal aggregates which show fractal geometry will also be discussed.     

Intercomparison of Inversion Algorithms for Particle‐Sizing Using Mie Scattering

The applicability of different inversion algorithms to retrieve a size distribution of particles in air from light scattering is examined. The investigation is focused on an optical measurement setup with an elliptical mirror as the main optical element. In order to evaluate the capabilities of the individual inversion methods, light scattering by spherical particles is simulated in the size ranges of 0.1 – 10 μm and 0.05 – 1 μm. The distribution of the particle diameters is modeled with three different parametric functions, i.e., RRSB, logarithmic‐normal and a more specific distribution from an ultrasonic nebulizer. Different kinds of noise, e.g., additive and/or multiplicative, are applied in different levels to the simulated scattering measurement to include real physical measurement conditions. The convergence properties of the scattering simulation are investigated with respect to the number of size classes, and thus, information concerning the size resolution required to simulate a measurement for a given particle size distribution is obtained. Further parameters of interest are the minimum angular resolution of the measurements, the number of size classes of the retrieved particle size distribution and the measured polarization of the scattered light.     

Active diffusion of motor particles

The movement of motor particles consisting of one or several molecular motors bound to a cargo particle is studied theoretically. The particles move on patterns of immobilized filaments. Several patterns are described for which the motor particles undergo nondirected but enhanced diffusion. Depending on the walking distance of the particles and the mesh size of the patterns, the active diffusion coefficient exhibits three different regimes. For micrometer-sized motor particles in water, e.g., this diffusion coefficient can be enhanced by 2 orders of magnitude.     

Collective diffusion in a two-dimensional liquid composed of Janus particles

The collective diffusion of anisotropic particles in liquids plays a crucial role in many processes, such as self-assembly. The patchy particle, which is usually nearly spherical in shape, is an important anisotropic particle with different properties from other anisotropic particles like the ellipsoid liquid crystal particles. In the present study, molecular dynamics simulations are performed to study the collective diffusion of a two-dimensional anisotropic liquid system composed of Janus particles. The static structures and diffusion behaviours of anisotropic and isotropic Lennard-Jones liquids are compared. The long-time diffusion behaviour of an anisotropic liquid of nearly spherical Janus particles is found to be similar to that of an isotropic liquid because the orientation of the particles disappears over long-term averaging. The anisotropic properties of the Janus particles are mainly reflected in the spatial correlation of particle orientations and mid-time diffusion behaviour. The difference between nearly spherical anisotropic particles and rod-like particles is also discussed in this paper.     

Tensile properties and interfacial interactions of bimodal hard/soft latex blends

In this work, the effect of composition, particle size and particle size ratio on the tensile properties of well-characterized hard/soft latex blends was investigated. Four blends of hard/soft latices, with varying particle sizes (either small or large), and volume fractions of 100/0, 80/20, 60/40, 50/50, 40/60, 20/80 and 0/100 were studied. The stress at break increased and the strain at break decreased as the amount of hard particles in the blend increased. A simple model, introduced by Pukanszky for filled polymers and polymer blends, proved to be a very useful tool for evaluating the tensile properties of the latex blends. Parameter B of the model could be related to the specific surface of the dispersed hard particles and the particle size ratio. Increasing the specific surface of the dispersed hard particles resulted in an increase in parameter B. The influence of particle size ratio on parameter B was shown to depend on the formation of aggregates.     

Aggregation,fracture initiation,and strength of PP/CaCO3 composites

Polypropylene/CaCO₃ composites were homogenized in a twin-screw compounder and then injection molded into tensile bars. Six different fillers were used in a wide range of average particle sizes between 0.08 and 12 μm. Tensile and flexural properties were measured by standard techniques, while impact resistance was determined by instrumented impact testing. Structure was characterized by light and electron microscopy, while failure initiation and propagation was studied with in situ high-voltage electron microscopy. The results showed that aggregation of particulate fillers occurs when their particle size is smaller than a critical value. This critical size depends on component properties and processing conditions. Strength and impact resistance usually decrease with increasing number of aggregates. The presence of aggregation can be detected by the use of a simple semiempirical model. Comparison of samples prepared by two different technologies showed that twin-screw extrusion and injection molding leads to relatively homogeneous composites, which was indicated by smaller deviations of the properties from theoretical predictions. In spite of the acceptable dispersion, impact resistance showed a large standard deviation, probably determined by the local variation of structure. In composites containing relatively large particles, the dominating micromechanical deformation process is debonding, while in the presence of extensive aggregation of small particles, cracks are initiated inside and propagate through aggregates. Mixed-mode failure may also occur at certain intermediate panicle sizes.     

Vibrational modes of free nanoparticles: from atomic to continuum scales

Vibration analysis of free standing silicon nanoparticles, with sizes ranging from 0.732 to 4.223 nm, are calculated using two different methods: molecular mechanics and classical continuum elasticity. Three different geometries are studied: cubes, spheres, and tetrahedrons. Continuum mechanics methods provide good estimates of the lowest natural frequency of particles having at least 836 (R>1.5 nm) and 800 (R>1.28 nm) atoms for cube- and tetrahedron-shaped nanostructures, respectively. Equations for vibrational frequencies of smaller particles as a function of size are proposed. The vibrational modes obtained from both methods were practically the same for the sphere- and tetrahedron-shaped particles with a large number of atoms. However, for the cube geometry only the shape of the modes corresponding to the lowest couple of frequencies occur in the same order. In general, vibrational modes shapes obtained using both methods are the same although the order in which they appear may be shifted.     

Implementation of an advanced fixed sectional aerosol dynamics model with soot aggregate formation in a laminar methane/air coflow diffusion flame

An advanced fixed sectional aerosol dynamics model describing the evolution of soot particles under simultaneous nucleation, coagulation, surface growth and oxidation processes is successfully implemented to model soot formation in a two-dimensional laminar axisymmetric coflow methane/air diffusion flame. This fixed sectional model takes into account soot aggregate formation and is able to provide soot aggregate and primary particle size distributions. Soot nucleation, surface growth and oxidation steps are based on the model of Fairweather et al. Soot equations are solved simultaneously to ensure convergence. The numerically calculated flame temperature, species concentrations and soot volume fraction are in good agreement with the experimental data in the literature. The structures of soot aggregates are determined by the nucleation, coagulation, surface growth and oxidation processes. The result of the soot aggregate size distribution function shows that the aggregate number density is dominated by small aggregates while the aggregate mass density is generally dominated by aggregates of intermediate size. Parallel computation with the domain decomposition method is employed to speed up the calculation. Three different domain decomposition schemes are discussed and compared. Using 12 processors, a speed-up of almost 10 is achieved which makes it feasible to model soot formation in laminar coflow diffusion flames with detailed chemistry and detailed aerosol dynamics.     

Mechanical resistance of a compact nanoparticle in a gaseous medium

Resistance of fullerenes in air is investigated under standard conditions. The general problems of resistance of compact nanosized particles in gaseous media are solved at different temperatures and pressures. The resistance of the nanosized particle moving in a gaseous medium is calculated using the deterministic approach, direct methods of molecular dynamics, and main theorems of classical mechanics (including the theorem on the momenta of a particle and a particle ensemble). Theoretical results obtained are in complete agreement with the Lapple experimental data on the resistance of ultradispersed particles. Moreover, they describe well the resistance of particles having much larger sizes for which the molecular dynamics model disregarding collisions of molecules seems to be inapplicable.     

相干振动动力学和高分辨非线性光谱学：空气/二甲亚砜液体界面的比较

研究了空气/二甲亚砜界面C?H伸缩振动的自由诱导衰减的相干振动动力学和亚波数高分辨宽带和频振动光谱．对于特定分子体系,频率域光谱测量和时间域动力学测量原则上应获得相同的信息．但对具有耦合或者重叠在一起的若干振动模式的分子体系,通过时域或者频域测量以获取光谱和动力学信息细节均非易事．对于振动光谱并非过于复杂的空气/二甲亚砜界面,基于亚波数高分辨宽带和频振动光谱的频域测量较超快时域测量更有益于获取界面结构和相干动力学定量信