Measurement and simulation of soot agglomerates and aggregates

Soot clusters produced by partial oxidation of acetylene show three distinct phases - primary particles, agglomerates of the primary particles, and aggregates of the agglomerates. The dominant mode of soot formation can be controlled by the concentration of acetylene. Comparison of optical microscopy with inertial measurements indicate that the aggregates are not fractal (f_HB = 3) but porous with a density of 0.094. Those simulations for the smallest aerodynamic diameter suggest a transition from aggregates which are fractal to agglomerates. Algorithms based on generalized Menger sponges are developed to simulate the transition from agglomerates to aggregates.     

Aggregate of nanoparticles: rheological and mechanical properties

The understanding of the rheological and mechanical properties of nanoparticle aggregates is important for the application of nanofillers in nanocompoistes. In this work, we report a rheological study on the rheological and mechanical properties of nano-silica agglomerates in the form of gel network mainly constructed by hydrogen bonds. The elastic model for rubber is modified to analyze the elastic behavior of the agglomerates. By this modified elastic model, the size of the network mesh can be estimated by the elastic modulus of the network which can be easily obtained by rheology. The stress to destroy the aggregates, i.e., the yield stress (σ _y ), and the elastic modulus (G') of the network are found to be depended on the concentration of nano-silica (ϕ, wt.%) with the power of 4.02 and 3.83, respectively. Via this concentration dependent behavior, we can extrapolate two important mechanical parameters for the agglomerates in a dense packing state (ϕ = 1): the shear modulus and the yield stress. Under large deformation (continuous shear flow), the network structure of the aggregates will experience destruction and reconstruction, which gives rise to fluctuations in the viscosity and a shear-thinning behavior.     

Influence of shear flow on the preparation of polymer layered silicate nanocomposites

In this paper the influence of melt-processing on the final polymer/layered silicate nanocomposite morphology is discussed. In particular the role of shear forces on the transformation of the original large clay agglomerates is of interest. Several polymer nanocomposites were prepared by melt-extrusion, involving polycaprolactone, poly(ethylene oxide), polyamide-12 or polyamide-6 as the matrix polymer. The nanocomposite morphology was characterised by X-ray diffraction and transmission electron microscopy and the clay tactoid morphology with polarised optical microscopy and scanning electron microscopy. The development of the tactoid and nanocomposite morphology during melt-mixing under shear was studied time-resolved by optical microscopy in conjunction with a rheometer and synchrotron X-ray scattering together with a Couette type flow cell. The shear forces in the melt-preparation of polymer layered mineral nanocomposites facilitate the break-up of large-sized agglomerates, whereas the extent of further exfoliation of the mineral layers is determined by the compatibility between the polymer matrix and the mineral layers rather than by shear forces.     

Processing—Morphology regulation of epoxy/layered-silicate nanocomposites

Polymer/layered-silicate nanocomposites have unique and hierarchical structures that can provide improvements to the properties of polymeric materials. Controlling the dispersion of the nanomaterials through processing greatly influences the resulting morphology and the resulting properties of the nanocomposite. In this article, the dispersion behavior of organic layered silicates (OLS) as a function of the processing procedure is reported. The behavior of the OLS in all stages of processing—in the solvent, the epoxy prepolymer, and in the epoxy through cure—is discussed. On the basis of understanding of the dispersion behavior of the OLS in the epoxy resin at each stage of processing, a different processing procedure can be designed and used so that the morphology of the epoxy/layered-silicate nanocomposite can be regulated. Mild low-shear processing resulted in an intercalated nanocomposite with large-size aggregates (> 10 μm), and high-shear processing resulted in an intercalated nanocomposite with relatively small-size aggregates (0.5–3 μm), whereas the high-shear and ultrasonication processing procedures gave rise to an exfoliated nanocomposite. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Fractal dimension of fumed silica: Comparison of light scattering and electron microscope methods

Due to the enormous increase in nanopowder production, it becomes necessary to find and develop adapted characterization techniques. In the case of nanostructured agglomerates, the structure of these particles has a direct impact on flowing, and handling, but also on end-use final product properties. In this work, a fractal approach is used to characterize the agglomerate structure using two different, commercially available and widely used, methods: static light scattering (SLS) and image analysis of scanning electron microscope (SEM) photographs of the aggregates. Fumed silica aggregates are used for this comparison. The results by image analysis show that fumed silica aggregates have a two-level structure, made of compact aggregates of open aggregates of nanoparticles. This structure is not detected by SLS. For such a structure, SLS seems to be less accurate than image analysis method, although it could be an interesting technique in more simple cases, since it is a much less time-consuming technique.     

Dispersion characteristics and rheology of organoclay nanocomposites based on a segmented main-chain liquid-crystalline polymer having side-chain azopyridine with flexible spacer

The dispersion characteristics and rheology of organoclay nanocomposites based on a main-chain liquid-crystalline polymer having side-chain azopyridine with flexible spacer (PABP) were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and oscillatory shear rheometry. In the preparation of nanocomposites via solution blending under vigorous stirring, two commercial organoclays (Southern Clay Products) were employed: one (Cloisite 30B) treated with a surfactant (MT2EtOH) having hydroxyl groups, and the other (Cloisite 20A) treated with a nonpolar surfactant (2M2HT) having hydrogenated tallow. Also prepared, for comparison, were nanocomposites prepared by mixing PABP with natural clay (montmorillonite, MMT). The following observations were made. (i) PABP/Cloisite 30B nanocomposite has featureless XRD patterns and a very high degree of dispersion of Cloisite 30B aggregates as determined from TEM. (ii) PABP/Cloisite 20A nanocomposite has shown a conspicuous XRD reflection peak and intercalation of Cloisite 20A aggregates as determined from TEM. (iii) PABP/MMT nanocomposite has shown XRD patterns, which are virtually the same as the XRD patterns of neat PABP with a slightly increased gallery distance, and it has very poor dispersion of MMT aggregates in the matrix of PABP. The observed high degree of dispersion of Cloisite 30B aggregates in PABP/Cloisite 30B nanocomposite is attributable to the formation of hydrogen bonds between the pyridyl group of side-chain azopyridine and the hydroxyl groups in the surfactant MT2EtOH residing at the surface of Cloisite 30B. The presence of hydrogen bonds in the PABP/Cloisite 30B nanocomposite was confirmed by in situ Fourier transform infrared (FTIR) spectroscopy. It was observed via polarized optical microscopy that the liquid crystallinity of PABP in the PABP/Cloisite 30B nanocomposites was more or less intact with a very high degree of dispersion of Cloisite 30B aggregates. Oscillatory shear flow measurements of the organoclay nanocomposites prepared support the conclusions drawn from XRD, TEM, and FTIR spectroscopy.     

Sensitivity of nanoparticles’ stability at the point of zero charge (PZC)

Since bare-nanoparticles (NPs) without stabilizers in aqueous phase are stabilized by electrostatic forces, pH control is easily used to influence the surface charge and hence the formation of aggregates or agglomerates. Therefore, herein, the sensitivity of bare-NPs’ stability in aqueous phase was analyzed close to the point of zero charge (PZC) using six model NPs. The results demonstrated that the sensitivity of NPs’ agglomeration/aggregation could be estimated by the diameter ratio (R_D) between the primary and secondary particle sizes. Namely, if the primary particle size was almost identical to the hydrodynamic diameter (HDD), the agglomeration proceeded aggressively.     

Small angle light scattering study of improved dispersion of carbon nanofibers in water by plasma treatment

Ultrathin polymer film is deposited on the surfaces of vapor-grown carbon nanofibers by a plasma polymerization using acrylic acid as a monomer. Small angle light scattering is used to investigate the dispersion behavior of the carbon nanofibers suspended in water and provides information on the mechanism by which plasma treatment assists dispersion. Both plasma-treated and untreated nanofibers exhibit a hierarchical morphology consisting of small-scale aggregates that agglomerate to form fractal clusters that eventually precipitate. The time evolution of small-scale aggregation and large-scale agglomeration is studied by fitting the scattering data to a unified model. The morphology of the small-scale aggregates is also studied by extracting the size distribution from the angle-dependence of the scattered intensity, using the maximum entropy method in conjunction with a simplified tube form factor. The aggregates are side-by-side bundles of individual nanofibers or more complex structures. Plasma treatment not only contributes to breaking up of the small-scale aggregates into smaller sizes but also inhibits their agglomeration. For untreated fibers, large agglomerates appear immediately after sonication and their size remains almost unchanged during the precipitation process. For treated fibers, precipitation dominates during the first 8 h, leaving small entities in suspension which form agglomerates after a few days.     

Alkali-silica reactivity of large silica fume-derived particles

Pozzolanic materials, including silica fume, are commonly added to concrete to reduce expansion due to alkali-silica reaction (ASR). It has been noted, however, that commercial silica fume is not always adequately dispersed, and large agglomerates may be present. These large particles have been hypothesized to act as amorphous silica aggregates, thereby participating in an expansive reaction with the alkalis present in cement paste pore solution. If such were the case, some silica fume particles would actually aggravate expansion due to ASR rather than suppress it. The present investigation characterizes the microstructure and morphology of agglomerated and sintered silica fume particles and compares their effects on alkali-silica-related expansion. While a 5% replacement of moderately reactive sand with sintered silica fume aggregates caused significant expansion under accelerated testing conditions (modified ASTM C1260), the replacement with large agglomerates of densified silica fume decreased expansion compared with control mortar bars containing only sand. Both the sintered aggregates and the agglomerates reacted with the pore solution; one reaction was expansive, while the other was not.     

Mechanosynthesis of complex oxides and preparation of mixed conducting nanocomposites for catalytic membrane reactors

Nanopowders of LaGaO₃- and LaMnO₃-based complex perovskites (P) and ceria-based fluorites (F) were prepared by mechanosynthesis. Compatible nanocomposites F + P and P + P with mixed ion and electron conducting (MIEC) properties were prepared and sintered at moderate temperatures up to dense ceramics. The obtained materials were studied by means of XRD, SEM, TEM, electrical conductivity measurements, temperature programmed (TP) reduction/oxidation and preliminary estimations of permeability were obtained. A new strategy based on the advantages of the mechanochemical ceramic approach is proposed to design multilayer ceramic membranes for CMR. Casting technology and one-step sintering were used for the production of thin film membranes with MIEC properties on porous substrates. The coarse fraction of as-milled powders from agglomerates with density 70% was used for the porous substrate, and fine fractions of aggregates with sizes <1 μm were used in preparation of composites for thin dense films. Ceria-based composites prepared by the Pechini route and/or mechanochemical method are proposed as materials for protecting thin films.     

Effect of drawing on the molecular orientation,polymorphism, and properties of melt‐spun nanocomposite fibers based on nylon 6 with polyhedral oligomeric silsesquioxane,montmorillonite, and their combination

In this work, binary and ternary nanocomposite systems based on nylon 6 with montmorillonite (MMT), polyhedral oligomeric silsesquioxane (POSS), and their combination were prepared using a melt‐compounding process. In the transmission electron microscope (TEM) images, the MMT was found to be generally well dispersed in all materials resulting in its good chemical compatibility with nylon 6, affording intercalated disordered microstructures. On the other hand, the TEM images showed that POSS formed micron‐size crystalline agglomerates possibly resulting from a lack in chemical compatibility with nylon 6. These nanocomposite systems were melt‐spun into fibers, and the relevant structure–property relationships that occur during the cold drawing process was established by correlating the tensile properties to the changes in crystallinity, polymorphic crystal forms, and molecular orientation. The properties of the resulting fibers were found to be rather skewed and significantly affected by the polymer/nanoparticles interface. The agglomeration of POSS and POSS–MMT particles coupled with the weak nylon 6/POSS interface, reflected on the tensile properties of the nylon 6/POSS and nylon 6/MMT‐POSS fibers which underperformed. Some nanocomposite fiber systems offered significant improvements in modulus without excessively compromising the extensibility of the fibers. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Morphology of Highly Dispersing Precipitated Silica: Impact of Drying and Sonication

Light and X-ray scattering are used to examine the structure of two commercial precipitated silicas (Zeosil 1165 and Ultrasil 7005) and one developmental precipitated silica, Dimosil 288. All three products have a four-level hierarchical structure consisting of primary particles, aggregates, hard agglomerates and soft agglomerates, with Dimosil 288 showing the clearest evidence of the four structural levels. The impact of sonication and drying protocol was explored by light scattering for Dimosil 288. With the exception of the large-scale soft agglomerates, all the structural levels are robust under sonication of aqueous suspensions. Sonication breaks down soft agglomerates leaving hard structures approximately 11 μm in radius-of-gyration. Drying plays a critical role in hardening the soft agglomerates. If the product is never dried, sonication reduces the agglomerate size to 3.5 μm, which is identified as the size of the hard agglomerate. Although agglomerates larger than 3.5 μm are present in the never-dried product, they are friable. Large-scale agglomerates are marginally more robust when dried at 150 °C compared to room temperature drying. Given the similarity of mechanical properties of rubbers filled with these silicas, it appears that the four-level structure with friable large-scale soft agglomerates is a characteristic of highly dispersing silica products.     

Numerical simulation of flow behavior of agglomerates in gas–cohesive particles fluidized beds using agglomerates-based approach

Flow behavior of gas and agglomerates is numerically investigated in fluidized beds using a transient two-fluid model. It is assumed that the particles move as agglomerates rather than single particles in the gas–cohesive particles fluidized beds. The present model is coupled a modified kinetic theory model proposed by Arastoopour (2001) with an agglomerate-based approach (ABA). The interaction between gas and agglomerates is considered. The agglomerates properties are estimated from the ABA. Predictions are compared with experimental data measured by Jiradilok (2005) in a bubbling fluidized bed and Li and Tong (2004) in a circulating fluidized bed. The distributions of velocity, concentration and diameter of agglomerates, and pressure drop are numerically obtained. The influences of the contact bonding energy on the distributions of velocity and concentration of agglomerates are analyzed.     

Strength properties of polyimideamide nanocomposite fibers in terms of their porous and supermolecular structure

The strength properties of fibers made from polyimideamide (PIA) nanocomposite were investigated and the effect of the presence of MMT in the fiber‐forming polymer on the porous structure and supermolecular structure of fibers was analyzed. It was found that lower strength properties (tenacity, elongation at break) of PIA nanocomposite fibers, as compared with those ones of fibers without montmorillonite (MMT), are connected with a lower deformability of the polymer during drawing stage and the collapse of MMT galleries, confirmed by WAXS investigations. This results in the formation of agglomerates that are weakly connected with the fiber‐forming polymer. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 339–344, 2007     

Shear deagglomeration of solid aggregates suspended in viscous liquids

The paper deals with the deagglomeration process in viscous fluids. The process is that in which aggregates of particles suspended in fluids are broken in a shear field. A mathematical model is proposed to predict the equilibrium particle size distribution as a function of shear stress, the initial particle size distribution and the agglomerate strength distribution. Experimental work was performed to test the model. A concentric cylinder apparatus provided the shear field and a procedure was developed to determine the agglomerate size distribution. Artificial agglomerates were prepared by a novel method. Within the limitations of experimental and sampling errors, the calculations agree with experimental results.     

胶粒聚集程度对ZrO2—MgO超细粉末团聚状态的影响

依据共沉淀过程的动力学分析,研究了共沉淀反应最初阶段胶粒的聚集程度与粉末团聚状态之间的关系。实验结果表明:粉末中的团聚体起源于沉淀反应过程,胶粒的聚集程度决定着粉末的团聚状态。因而,可以通过控制胶粒的聚集特征来控制粉末的团聚状态。根据实验,提出了用以表征粉末中团粒聚集特征的聚集系数的概念。     

Simulation of structure and mobility of aggregates formed by simultaneous coagulation,sintering and surface growth

In this work, a new model for the simulation of nanostructured aggregates by simultaneous coagulation, sintering and surface growth is presented. Coagulation is treated as cluster–cluster agglomeration along the line connecting the center of mass of both agglomerates and is implemented using a Monte Carlo algorithm. Sintering is modeled as successive overlapping of spheres which cause reduction in the surface area based on a rate law for surface reduction. Surface growth is modeled as an increase in primary particle diameter, e.g. as a result of surface reactions. The evolved aggregates are analyzed by calculating their fractal dimension, radius of gyration, mobility diameter and mobility shape factor. It is found that the aggregates structure tends to be more compact when introducing the surface growth in shorter time comparing to the coagulation-sintering step only. Fractal dimension and the mobility shape factor of the resulting aggregates are correlated to an effective dimensionless time that combines the characteristic times of these three fundamental mechanisms. It is shown that the mobility diameter in the free molecular regime is not proportional to the radius of gyration. A power law relation that correlates the aggregates projected area and the equivalent number of primary particles is found to be in a very good agreement with estimates published in literature.     

超细粉在声场导向管喷流床中的聚团尺寸预测模型

超细粉的流化性能与聚团尺寸密切相关。通过分析超细粉聚团在声场导向管喷流床中的形成过程,提出了高速射流的剪切作用和聚团间的碰撞作用是决定聚团尺寸的主要原因。在此基础上,结合聚团在射流剪切过程和聚团间碰撞过程中的力平衡分析,建立了声场导向管喷流床中聚团尺寸分布的预测模型;并运用这一模型成功预测了不同射流气速下,超细TiO₂颗粒在声场导向管喷流床中的聚团平均直径和聚团尺寸分布。     

Efficient Dispersion of Magnetite Nanoparticles in the Polyurethane Matrix Through Solution Mixing and Investigation of the Nanocomposite Properties

Recent studies on inorganic/polymer nanocomposites have shown enhancements in thermal, mechanical, and chemical properties over the neat polymer without compromising density, toughness, and processibility. When nanoparticles are incorporated into the polymer matrix, significant enhancements in thermal and mechanical properties of the nanocomposite are observed. The present study is focused on the preparation and characterization of nanosize magnetite-reinforced PU composites, which induces magnetic properties to a specific thermoplastic polyurethane elastomer. The nanocomposites are prepared and the effects of magnetite content on thermal, mechanical, and magnetic properties of the nanocomposites are evaluated. Ultrasonication was used to disperse the nanoparticles and break up any large clumps and aggregates and followed by mechanical mixing. The magnetic nanocomposites were characterized by FT-IR spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). Characterization of the magnetic nanocomposite by FT-IR showed a successful incorporation of magnetite nanoparticles into the polymeric matrix. TGA and magnetometry of the magnetic nanocomposites revealed the amount of magnetite that was incorporated into the polymeric phase. Finally, the corresponding magnetization behavior of the nanocomposites was studied.     

The influence of Cd(ZnO) on the structure,optical and thermal stabilities of polyvinyl chloride nanocomposites

The doping of nanomaterials into polymers creates novel nanocomposite materials with desired properties. The influence of these nanoscale fillers on the structure and optical and thermal stabilities of polymer are the crucial clue to introduce these novel nanocomposites to service life applications. In this work, cadmium doped zinc oxide (Cd_0.5Zn_0.5O) nanopowders with a uniform particle size of around 10 nm have been synthesized, purified, and blended with polyvinyl chloride (PVC) by solution mixing to prepare PVC/Cd_0.5Zn_0.5O nanocomposite films. The structure and morphology of PVC/Cd_0.5Zn_0.5O nanocomposite films have been characterized using transmission electron microscopy. The results showed that as the amount of nanoscale fillers increases the nanocrystals tend to aggregates and the size of these aggregates increases from 25 to 100 nm. The UV–vis spectra showed that the prepared PVC/Cd_0.5Zn_0.5O nanocomposite films are highly transparent. The transparency at higher concentrations slightly decreased as a consequence of light scattering due to large aggregates. The thermogravimetric results showed that a small amount of Cd_0.5Zn_0.5O nanopowders (<0.6 wt%) without further surface modification can greatly improve the thermal stability of PVC. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers