The method of obtaining ultradispersed powders from the gas phase of PTFE pyrolysis products is presented in this review. The fact that nanoaerosols form in the gas medium was stated. The monoparticles with various structures are formed from nanoaerosols. The monoparticles in their turn organize composite formations. The ultradispersed powder consists of two PTFE fractions (low- and high-molecular). The differences of fraction structures are revealed by various physical methods. The industrial-scale production of the obtained powder was demonstrated.
Traditionally, the generation of nanoparticles for technological applications has been mostly performed by classical wet chemistry or lithographic methods, and their size has been commonly determined in situ by electron microscopy techniques. Advances in aerosol technology over the past 30 years have provided methods that enable the generation and measurement of nanosize building blocks, and have opened up new opportunities in the assembly of nanostructured materials and nanodevices. This article provides a brief review on state-of-the-art techniques for generating nanoparticles of well-defined size and chemical composition in view of applications in nanotechnology. Covering atomization techniques from the liquid phase and nanoparticle synthesis from the gas phase, we discuss the advantages and limitations of each method. Considering the advantages of on-line methods that aerosols instruments offer, we describe the most efficient techniques for measuring the size distributions of airborne nanosize particles. Finally, we provide a brief discussion on existing and emerging applications of aerosol-based nanotechnology.
As diesel emission regulations become more stringent, diesel particulate filters (DPFs) have become possibly the most important and complex diesel emission control device. This paper provides an update on the science and technology of diesel particulate emission control, drawing from the results of many research projects carried out by the authors in collaboration with the automotive industry and its suppliers. Both fundamental as well as application-oriented approaches are presented to study the physicochemical characteristics of diesel soot particles and soot deposits formed in DPFs, which are viewed as multifunctional separators/reactors. Theroretical and experimental aspects of filtration efficiency, pressure drop, ash accumulation and soot reactivity (with emphasis on catalyst-assisted soot oxidation) are addressed, employing systems ranging from small-scale filter samples to full-scale devices installed in the exhaust system of diesel engines. Properly combined, the current state of knowledge, experimental methods and simulation approaches all provide a rational and systematic route for enhancing the design and reliability of future diesel particulate emission control systems.
Over the last decade, the progress in numerical modelling of hydrocyclones has advanced at a rapid pace. A summary of the recent advancements is undertaken in the paper to summarize the major directions and developments in hydrocyclone modelling. The important aspects that contribute to the accurate simulation of general internal swirling flows and other characteristic features occurring in hydrocyclones modelling are covered. The relevant issues are: the formulation of the governing equations (classical versus stress divergence form), the imposition of boundary conditions especially regarding the open part of the outlet boundary conditions and the problem of proper representation of the computational domain. The complex flow pattern that occurs in hydrocyclones invalidates several assumptions in the standard turbulence models. Challenges still remain in predicting accurately the effects of turbulence anisotropy. A performance review of selected equations closures is presented with insight on their potential and limitations. The flow predictions obtained using the Reynolds Stress Model (RSM) and Large Eddy Simulation (LES) turbulence models are examined. The predictions of the free surface flow and air core dimensions have yet to be fully resolved. The same concerns the problem of simulating large number of particles. The recent important developments related to the solid/gas and sold/liquid interaction problems are presented together with intriguing challenges one need to overcome in order to numerically handle multi-component flows. The Lagrangian and multi-continuum Eulerian techniques are considered. The attempt to systemize various approaches for multi-component flows according to their accurateness and relevance to the various flow regimes in hydrocyclones is undertaken. The issue of experimental validation is crucial to provide confidence in hydrocyclone flow-field models and the current state of experimental data is examined. Finally, suggestions for the future direction of hydrocyclone modelling are highlighted.
Important issues that arise in the acquisition, presentation and interpretation of particle size data are discussed. Presentation of size distributions as relative quantity versus size, representation of quantity by number, mass, etc., and procedures for inter-conversion of the different forms are described. Definitions of various kinds of average sizes are presented. Limitations on their use and the importance of precise definition are emphasized. Definitions of size for irregular particles, the role of particle shape and implications with regard to comparability of analytical results based on different principles and procedures are evaluated. Different types of measurement procedures are classified according to whether they involve measurements on individual particles, on separated classes of particles or on complete assemblages. The restrictions and constraints, such as size limitations and resolution which apply to these types, are discussed.
Chemical Mechanical Planarization (CMP) is a process that is now routinely used to planarize metal as well dielectric films during the fabrication of integrated circuits. This process uses slurries comprised of fine abrasive particles, such as silica, ceria or alumina. The stability and performance of the slurries are influenced by the size and properties of these particles, which in turn are influenced by their synthesis route. Stability is important to improve slurry lifetime and minimize defects such as microscratching caused by particle agglomerates or large particle counts (LPC) during polishing. The rheological behavior of slurry affects the friction at the pad-particle-wafer interface and alters the material removal rate. It is, thus, necessary to carefully engineer the slurry characteristics to achieve good removal rate and planarity without causing defects. This paper reviews the published literature on the synthesis of abrasive particles used in the preparation of CMP slurries as well as stability and rheological characteristics of slurries made from these particles. A brief account of particle-film interactions and importance of LPC in wafer scratching is also provided.
As the atomic force microscope (AFM) becomes a more accessible tool in the laboratory, researchers involved in the development of pharmaceutical formulations are taking advantage of its unique capabilities. In this work we review the uses of the AFM in the development of hydrofluoroalkane-based pressurized metered-dose inhaler formulations. Perspectives and limitations of the AFM and related techniques in this area also discussed.
The effects of various fundamental forces on the adhesion of fine dust particles are reviewed. The particle-size and distance variation of surface-energy related (e.g., van der Waals) forces are compared to similar relations for static-electric image-forces for tribo-charged particles near (or contacting) conducting surfaces. The van der Waals force (between macroscopic spheres), a patch-charge image-force and static-electric image-forces all exhibit an inverse square variation with distance; however, these forces have dramatically different ranges-of-effect. The very short-range nature of van der Waals forces (of order 10nm) is a major reason that most real contacts, involving non-smooth surfaces, exhibit adhesion forces that are substantially lower than values predicted for smooth particles. Based on studies of Lunar and Martian regolith stimulant powders, triboelectric charges on fine particles appear to scale linearly with particle size. It is shown that below some threshold size, the adhesion (to conducting surfaces) of charged dust particles possessing such a linear charge-to-size scaling relationship, may be dominated by image-charge forces, instead of surface-energy related interactions. This is counter to what might have been expected from a cursory examination of the fundamental force relations, which would suggest van der Waals adhesion forces would dominate for small particle sizes.
Cellulose and saccharide are commonly used filler-binder. This summary refers to the technology for high performance filler-binder of direct compression, using these two materials. In case of saccharide filler-binder, spray dry technology and size controlling of primary particle were the key technology. In case of cellulose filler-binder, two different approaches were made. One was co-processing approach and the other was particle shape controlling approach. Some applications using high performance filler-binder were also introduced.
Small particles adsorbed to gas-liquid interfaces are widely observed and utilized in many products and various industrial sections, such as flotation, washing, food, water treatment, treatment of radioactive wastes, and distillation in the petroleum industry. After a dormant period of nearly half a century, there is a revival of interests in studying their behaviour. Colloidal particles with a suitable wettability can strongly adsorb to gas-liquid interfaces, and can work as effective stabilizers for foams and liquid marbles (dry liquids). This article presents an overview of current research activities that center on foams and liquid marbles stabilized with well-defined colloidal particles. It is organized into three parts: i) brief discussions of small particles adsorbed to gas-liquid interfaces and their characterizations; ii) demonstrations of aqueous foams stabilized with inorganic or organic particles; and iii) demonstrations of liquid marbles (dry liquids) stabilized with colloidal particles.
Dry blended fertilizers are known to segregate. Furthermore, researchers have documented that the size of blended fertilizers is the most dominant physical property contributing towards segregation. Additionally, it is known that flowability is also affected by the size and moisture content of blended fertilizers. Therefore, segregation and flowability of binary size mixtures were studied at three different equilibrium relative humidity conditions 40%, 50%, and 60% with the goal to evaluate the feasibility to mitigate segregation using moisture content. To that end, binary size mixtures were prepared using coarse and fine size urea of size ratio 2.0 and 1.7 mixed in weight proportions 33:67 and 50:50, respectively (commonly found in 10-10-10 blends). Urea is the most hygroscopic and expensive component of the blended fertilizers. Percolation segregation was quantified using the Primary Segregation Shear Cell (PSSC-II). Based on experimental results using the PSSC-II, the segregated fines mass, normalized segregation rate (NSR), and segregation rate (SR) of fines for binary urea mixtures were higher at equilibrium relative humidity of 40% vs. 50% and 60%. The NSR is defined as the amount of fines percolated from the total initial fines in the binary mixture based on the total time of PSSC-II operation (kg/kg-h). For size ratios 2.0 and 1.7, only 2.8% and 7.0% decrease in NSRs were recorded for the increase in relative humidity by 10 points (from 40% to 50%), respectively, whereas 36.0% and 45.0% decrease in NSRs were recorded for increase in relative humidity by 20 points (from 40% to 60%), respectively (P<0.5). Additionally, the flowability of binary size mixtures was quantified using a true Cubical Triaxial Tester (CTT). For size ratios 2.0 and 1.7, angle of internal friction increased from 31.3° to 35.9° to 39.0° and 27.4° to 32.0° to 36.0° when relative humidity increased from 40% to 50% to 60%, respectively. The angle of internal friction values were significantly different (P<0.05) but cohesion values, at different relative humidity conditions were not significantly different (P>0.05). Based on experimental results, relative humidity, if implemented carefully, could be used as a tool to mitigate segregation in blended fertilizers.
The complete characterization of powder flow properties with shear cells is a long and time-consuming process that requires specially trained operators or costly automated instruments. For these reasons, in industrial practice, the use of simpler and less extensive measurement by uniaxial compaction testers is often preferred. However, previous studies in the literature indicate that the results of the two techniques are not directly comparable due to the different stress state conditions achieved in the two testers. In this study, an experimental campaign to measure the flow function of five different powders with a ring shear tester (RST) and a uniaxial compaction tester (UCT) was performed. Different flowability results that arose for the more cohesive powders are explained by the wall friction effect in the UCT. Re-evaluation of the results accounting for the wall friction gave substantial agreement between the two experimental techniques for a calcium carbonate powder and only at low consolidation levels for the other four food powders. Phenomena other than wall friction seem to appear within these powders tested at high consolidation levels. The comparison between the results of the two techniques suggests that straightforward extrapolation of the UCT flow functions to a low consolidation condition can lead to an underestimation of powder cohesion.
The mechanics of particle interactions and the prevailing level of compressive stresses and shear strains are affected by the scale of operation, which in turn affects the granule structure, strength and functional properties. This may be the main reason why the current scaling relationships are ineffective from a viewpoint of product engineering. In a research programme supported by the EPSRC and four industrial organizations, i.e. Borax Europe, Hosokawa Micron BV, Pfizer Global Research and Development, and Procter and Gamble, we have addressed the following topics: Development of methodologies for quantifying the structure of granules in terms of internal voidage and composition distributions, strength, shape, size and density. Identification of the parameters that affect the structure of granules by using fundamental theories of microscopic contact mechanics of particles using DEM, macroscopic granular flow dynamics and kinetics of wetting. Experimental work across several length scales (1 L, 5 L, 50 L and 250 L) to aid the analysis of the process. In this paper, an overview of findings and their implications for granulation practice is presented.
The development of an aerosol mass spectrometer for the analysis of biological aerosols is described. The working principles of the aerosol mass spectrometer are particle sizing, particle selection and particle analysis by matrix-assisted laser desorption/ionization mass spectrometry (MALDI). The instrument has the capability of selecting only those aerosol particles that emit fluorescence when excited with 266 nm laser light, which differentiates the biological particles from virtually all other particles likely to be present in an aerosol. The implementation of a new ion source and delayed extraction has resulted in the capability of obtaining high-quality mass spectra of single bioaerosol particles. Isotopic resolution was obtained for a low-mass peptide. The sensitivity limit of the instrument was determined to be 1 zeptomole. The suitability of the aerosol mass spectrometer for the analysis of bacterial aerosol particles is demonstrated with an aerosol containing vegetative cells of the bacterium Escherichia coli when prepared off-line. The mass spectrum obtained has good resolution and covers a mass range up to 15 kDalton.
Spray drying of suspensions leads most often to free-flowing powder products of good quality. However, this process is energy-consuming, especially because of the high amounts of liquid to be evaporated from the sprayed suspension. Indeed, in order to ensure a good quality and stability of spraying, the amount of solvent must be determined so as to avoid excessive suspension viscosities. For a given amount of solvent, it is also possible to improve the viscosity of concentrated suspensions by deflocculation using appropriate dispersants. Through the use of an adequate dispersant, we showed that it is possible to obtain fully dispersed suspensions and to optimize their dryness in order to reduce the moisture content to be evaporated. The effect of different salts and the molecular weight of polyacrylic acid is investigated to improve the formulation of concentrated calcite suspensions with the focus on spray drying. A new continuous process, developed to enable the drying of highly concentrated slurries at low temperature, is also described. This process is based on the coupling of coating/attrition of a suspension in a fluidized bed of inert particles, and it strongly increases the surface area available for heat and mass transfers. Further analysis of this process has been carried out addressing the drying kinetics in a drying pipe. It shows a strong influence of the support wettability as well as the surface tension of suspension on the drying kinetics of suspension films. The surface tension of the suspension and humidity were found to be crucial parameters to optimize the powder recovery.
Fluidized bed medium separation (FBMS) was applied to separate objects with a small specific gravity difference by precisely controlling the apparent specific gravity of gas-solid fluidized bed with binary particle systems. The binary systems of particles with various specific gravities and diameters were employed, and the apparent specific gravity of fluidized bed, gfb, and fluctuation, Δgfb, were measured by changing the volume mixing ratio of heavier particle, Vhp, and the superficial air velocity, u0. It was found that gfb can be controlled by Vhp and u0 because gfb varies with the total mass of particles and bed expansion. On the other hand, since Δgfb is determined by the extent of segregation of fluidized particles and fluidization intensity, Δgfb decreases with reducing the extent of the segregation and the vertical fluctuation of fluidized bed surface. The experimental results showed that practical fluidized conditions for FBMS are in the wide range of gfb ≈ 0.5∼3.8 and Δgfb ≤ about 0.1. Under these fluidized conditions, plastics with a small difference in specific gravity (0.04∼0.24) were almost completely separated at the top and bottom of the fluidized bed.
†This paper, appeared originally in Japanese in J. Soc. Powder Technology, Japan 43, 567-576 (2006), is published in KONA Powder and Particle Journal with the permission of the editorial committee of the Soc. Powder Technology, Japan.
A single particle is brought into contact with a metal target, and the force acting on the particle is measured by using atomic force microscope (AFM). By focusing on measuring the force ‘curve’, rather than looking only at the maximum adhesive force, electrostatic interaction was successfully observed by separating other interactions such as liquid bridge and intermolecular forces. To evaluate the force curve, the force curve with an approximation of disk-to-disk interaction based on image force was numerically calculated and a good analytical approximation was developed. The fact of successful agreement between the observed force curve and the theory revealed that the force curve observed can be surely attributed to the electrostatic interaction, and that the amount of charge on the particle and the radius of the charged (contact) area can be estimated from the analysis. The order of magnitude of the measured charge density was 10−2C/m2, which is much greater than that obtained with impact charging experiment as 10−4C/m2. From this it was concluded that the force curve measurement with AFM can capture the net amount of the charge generated before charge relaxation due to gas discharge taking place. In the experiment using 8 kinds of metal (Al, Au, Cr, Ni, Pt, Ti, Zn, and Zr) and polystyrene particle, the net charge generated was shown to be compared fairly well with the conventional simple condenser model based on metal-to-metal contact model in terms of contact potential difference in its order of magnitude. Although a clear correlation was not obtained between the measured charge density and the work functions of metal targets because of a large scatter in the data, a strong relationship between the charge density and contact area was found. The underlying mechanisms for the relationship is not known at this moment; however the finding gives a good hint for the next attempt.
†This paper, appeared originally in Japanese in J. Soc. Powder Technology Japan 43, 174-180 (2006), is Published in KONA Powder and Particle Journal with the permission of the editorial committee of the Soc. Powder Technology, Japan.
The effect of electrostatic and hydrophobic properties of microbes in anaerobic sludge on immobilization to support materials was examined. The most popular aceticlastic methanogen, Methanosaeta concilii, was uncharged and hydrophobic. Methanosarcina barkeri of a methyltrophic methanogen, and acidogens cultivated selectively from anaerobic sludge, were negatively charged and hydrophobic. Immobilized microbes on support materials were incubated with sodium acetate. Methanogens were dramatically immobilized to bamboo charcoal, in contact with hydrophilic alumina. Methanosaeta-like microbes were immobilized to bamboo charcoal. These results indicate that the hydrophobic and negatively-charged support material that can suppress the immobilization of microbes except for Methanosaeta species is suitable for selective immobilization of Methanosaeta species, which is the most important microbe in methane fermentation.
†This paper, appeared originally in Japanese in J. Soc. Powder Technology, Japan, 43, 653-659 (2006), is published in KONA Powder and Particle Journal with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Differential mobility analysis enables particle size distribution measurements of submicron aerosols with resolution well beyond that attainable in other size regimes of aerosols or other particulate media. The pages that follow present a tutorial introduction to differential mobility analysis, describing the underlying physics, the differential mobility classifiers and associated detectors, data analysis, data presentation, and some of the challenges in using this powerful measurement method.