Mixing in powders generally results from relative motion of groups of particles – convective mixing – or of individuals – diffusive mixing. Segregation or demixing occurs when the motion of individual particles is biased according to their particular characteristics – size, shape, composition etc. In the absence of such bias, individual motion invariably leads to homogenization of the mixture. Relationships between mixing/segregation processes and the external and interparticle forces responsible for causing or opposing relative motion are reviewed. Specific examples of mixing and segregation in flow over surfaces, in rotating cylinders and other applications are described.
This paper presents a review of the process of sedimentation of individual particles and suspensions of particles. Using the solutions of the Navier-Stokes equation with boundary layer approximation, explicit functions for the drag coefficient and settling velocities of spheres, isometric particles and arbitrary particles are developed.
The role of structure-energy properties of the mechanically induced defects of the crystal structure of heterogeneous catalysts is considered here. The main concepts of the effect of mechanochemical activation on the activity and selectivity of catalysts are theoretically stated. The most promising research trends are presented.
The possibilities of preparing advanced powders for thermal spraying functional coatings by the method of self-propagating high-temperature synthesis are discussed in this review. Besides important economical and ecological benefits, the method allows the formation of powders with improved or unique structure and properties in size ranges and with an external morphology suitable for different thermal spray processes. A number of novel powders and recent achievements are presented.
Direct write technology assists the electronic industry in their effort to miniaturize electronic circuits and enhance speed of printing capability. The technology allows printing various patterns without employing a mask or a resist with an enhanced speed with the aid of computer. This paper describes the current status of the synthesis of conductive inks, a key to success in this technology. There are numerous factors to overcome in manufacturing inks which meet all the necessary conditions of conductivity, viscosity, and stability. In order to achieve the required conductivity, nano-particles used in the ink have to be made from gold, silver or copper. Inherent problems involved in these metal powders include high melting point, coagulation, impurities, cost, and, in the case of copper, oxidation. The direction of research currently being carried out in meeting and overcoming various challenges in this technology is reviewed and discussed. Other salient applications of nano-sized metal powders are also briefly examined.
The diversity and potentials of the aerosol route for making functional materials at the nano size level are reviewed. Among the methods currently used for nanophase processing, synthesis through dispersion phase (aerosol) enables generation of ultrafine, either single or complex powders with controlled stoichiometry, chemical and phase content provided by high surface reaction, high heating and cooling rates and short residence time. It represents a “bottom-up” chemical approach and provides control over a variety of important parameters for particle processing. This may favors to the formation of either amorphous, nanocrystalline or metastable phases implying a huge impact in the search for advanced functional materials having novel and unique structures and properties. Particularly, the opportunities of the hot wall aerosol synthesis, i.e. spray pyrolysis, for the generation of ultrafine spherical particles with uniformly distributed components, phases and nano-clustered inner structure and luminescence properties is demonstrated with various analyzing techniques like XRPD, FE-SEM, HR-TEM, STEM and nanotomography. Following the initial attempts, a more detailed aspect of the several phosphor particles generation based on Gd2O3:Eu, Y2O3:Eu, (Y1-xGdx)2O3:Eu and Y3Al5O12:Ce is reviewed highlighting the research activities in the Institute of Technical Sciences of SASA, Serbia.
Structural joining is new concept of materials processing to create novel functional materials with special patterns and morphologies. Nanometer sized ceramics particles are bound continuously with resin materials solidified by laser beam irradiations to form micrometer order structures exactly. In this paper, fabrication processes of alumina photonic crystals with a periodic diamond structure will be introduced. Periodic arrangements of dielectric constants can control electromagnetic waves in terahertz frequency ranges through spatial wave diffractions. Three dimensional dielectric lattices were designed by using graphical application of a computer aided designing (CAD) software, and acrylic diamond structures with alumina nanoparticles dispersion were formed by using micro stereolithography (μ-STL) of a computer aided manufacturing (CAD) system. Fabricated precursors were dewaxed and sintered in the air to obtained full ceramics photonic crystals. The terahertz wave properties were measured by terahertz time domain spectroscopy (TDS) device. A complete photonic band gap to reflect the terahertz wave perfectly was observed, and showed good agreement with a theoretical simulation of plane wave expansion (PWE) method. Moreover, localization of the terahertz wave were observed in point or plane defects introduced into the diamond photonic crystals trough an electromagnetic field analysis of transmission line modeling (TLM) method.
Nanoparticles are now an indispensable material for science and technology such an in materials, medicals, and cosmetics areas. Controlling the dispersion stability of nanoparticles in various liquid media is an essential issue to control the properties of the final products. At the beginning of this review, we will introduce several reasons why it is difficult to control the stability of nanoparticles in liquid media. Then, we will briefly review the surface modification techniques to overcome the difficulties of handling nanoparticles in liquids. Two types of surface modification concepts, post-synthesis surface modification and in-situ surface modification, which is a surface modification on manufactured particles and surface modification during the particle synthesis, respectively, will be introduced.
Smart powder processing stands for novel powder processing techniques that create advanced materials with minimal energy consumption and environmental impacts. Particle bonding technology is a typical smart powder processing technique to make advanced composites. The technology has two main unique features. Firstly, it creates direct bonding between particles without any heat support or binders of any kind in the dry phase. The bonding is achieved through the enhanced particle surface activation induced by mechanical energy, in addition to the intrinsic high surface reactivity of nanoparticles. Using this feature, desired composite particles can be successfully fabricated. The second feature of this technology is its ability to control the nano/micro structure of the assembled composite particles. As a result, it can custom various kinds of nano/micro structures and can produce new materials with a simpler manufacturing process in comparison to wet chemical techniques. In this paper, its application examples for making advanced materials will be explained. These two features lead to the achievement of minimizing energy consumption and environmental impacts when producing advanced materials. By making use of the particle bonding principle, a new one-pot processing method to synthesize nanoparticles without applying extra heat was developed. Furthermore, by carefully controlling the bonding between different kinds of materials in the composite particles, effective separation of elemental components can be achieved. It leads to the development of a novel technique for recycling advanced composite materials and turns them to high-functional applications. In this paper, these approaches will also be introduced. It is our goal to signify the particle bonding technology as a potential advanced processing technique for producing powder materials.
Coal is an important energy resource for meeting the further demand for electricity, as coal reserves are much more abundant than those of other fossil fuels. In pulverized coal fired power plants, it is very important to improve the technology for the control of environmental pollutants such as NOx, SOx and ash particles including unburned carbon. With the remarkable progress in the performance of computers, it is strongly expected that the computational fluid dynamics (CFD) would be a tool for the development and design of such suitable combustion furnaces and burners for the pulverized coal combustion. The focus of this review is to highlight our recent progress of CFD of the pulverized coal combustion in terms of Reynolds-Averaged Navier-Stokes (RANS) simulation and Large-Eddy Simulation (LES) together with some of future perspectives.
In this work, a new laser Chemical Vapor Pyrolysis (LaCVP) reactor is presented. The concepts behind the new reactor’s design aim to improve and scale up the production of powders compared to conventional used laser CVP systems. For these purposes, the reaction zone has been studied in relation to the nozzle and the laser beam geometry. As a result, a new reaction zone comprising a rectangular nozzle and rectangular laser beam has been developed. The new reactor, including the new reaction zone, has great flexibility towards different type of materials that can be produced. Moreover, the advanced design of the reactor allows flexibility towards the use of various types of equipment, such as, nozzles, hoods, sampling equipment, etc. First encouraging results of silicon synthesis using the new reactor are presented. The silicon nanoparticles were synthesized from silane (SiH4) precursor in a N2 atmosphere. The products have been characterized with TEM, XRD, TGA and FTIR analyses and compared with a commercial available one (Aldrich). The synthesized products show a narrow size distribution and small particle size, in comparison with the commercial material. Future work will include the further development of the system and investigation of the role of the experimental parameters on the product characteristics.
The kneading of a powder generates a product made of three phases (solid/liquid/gas) that interact through many interfaces. The respective proportions of each phase can vary according to the process parameters and the applied stress. After the initial operation of kneading, the medium can also be subjected to other stresses such as packing and thermal stresses or mechanical drying which will induce deep modifications in the relative proportions of the three phases. According to the twin influence of process strain and the nature of the three phases, the rheological behavior of the granular material can vary from that of a rigid solid to that of a more or less deformable plastic paste. The objective of this work is to depict, on a phase diagram, the potential states of each fraction constituting an unsaturated wet granular medium and their potential connectivity. From an experimental point of view, sorption isotherms and capillary retention curves allow determination of the two fluid phases state (liquid and gas). The hydrotextural diagram can be considered as a tool for the analysis and understanding of the mechanisms occurring during the processing (kneading and packing) or drying of wet granular media.
The principles of high-temperature reactive particle formation in flames are characterized by a sequence of partly interacting rate processes in the gas flow, while the necessary energy is delivered by the exothermic combustion reaction heating the flow to high temperatures. A complete description of the precursor decomposition kinetics and the subsequent oxidation/hydrolysis reactions is rarely obtained, while the properties of the products manufactured such as size, morphology, phase composition, and crystallography are decisively influenced by these parameters. A precise understanding and control of the initial steps is therefore required to open up the possibility of tuning particle properties. In the present study, the formation of oxidic particles in flame reactors is presented. It will be shown that the stoichiometry and crystallography of oxides such as ZnO, SnO2 and TiO2, and therefore their physical and chemical properties, can be adjusted depending on the reaction conditions. In addition to the synthesis of pure materials, coated particles as well as nanocomposites are accessible when a few requirements are fulfilled. In the case of immiscible oxides such as TiO2 and SiO2, composites consisting of separate phases are produced, while the formation of composites from miscible compounds usually requires a two-step process that tends to produce poorly mixed materials. Nevertheless, in the case of kinetically controlled synthesis, a one-step formation of nanocomposites from miscible oxides can be realized when the kinetics of precursor decomposition and particle formation of the participating oxides are quite different. This results in materials that exhibit new properties according to the used oxides. As an example, the one-step formation of homogeneously dispersed superparamagnetic Fe2O3 in fumed silica will be shown. Chemically, this material behaves like common silica but due to the superparamagnetic characteristics of the embedded iron oxide, it can be heated in a contactless manner by means of an alternating magnetic field. Applications focusing on contactless hardening and bonding become apparent.
At small Knudsen numbers (continuum regime) and sufficiently small Reynolds numbers, the friction resistance of particles with the carrier gas is described by means of the Stokes formula assuming no slip of the gas molecules at the particle surface. However, at high Knudsen numbers, this assumption does not hold anymore and the deviation from the continuum character is considered with the slip or Cunningham correction. In the present work, measurements of the separation curve of nanoparticles in a single-stage low-pressure impactor are presented to systematically investigate the disagreement of classical theory of friction resistance of nanoparticles in low-pressure flows. At the same time, the impaction process of nanoparticles is simulated with a CFD code based on the classic theory. An effective Cunningham correction is determined as a function of the Knudsen number from fitting the calculated separation curve to the measured one. Moreover, the so far unresolved finding of too low densities determined for metal nanoparticles by low-pressure impaction is explained for the first time.
A laboratory often is faced with the task of calibrating particle inertial classifiers or investigating how instruments, or systems, respond to particles of specific sizes. Since, over time, multiple technicians may be involved in the calibrations, it is imperative that the procedures be uniform. For this purpose, “Good Laboratory Practice” (GLP) procedures as outlined in Code of Federal Regulations (CFR), Volume 21, Part 58 can be followed. There are three main features of GLP that can be employed in calibrations with particles. They are: 1) all equipment that are used for measuring flow rates, pressure, volume, etc. are calibrated to NIST traceable standards, 2) a calibration protocol is written, and 3) Standard Operating Procedures (SOPs) are defined for all steps of the calibration process. The SOPs include management structure, document control, data handling, procedures for training personnel, calibration of the equipment, and the actual calibration for the device in question. Although these GLP procedures are applied to nearly all calibrations performed in our laboratory, this paper will emphasize procedures suitable for the calibration of inertial classifiers, specifically cascade impactors.
The state of dispersion is a vital aspect of powder technology impacting both traditional and emerging technologies in many diverse areas of interest including health care, industry, the environment, and the military. In such applications, the properties and quality of the resultant product will be directly and noticeably affected by the degree to which the particles are dispersed. In any operation where satisfactory particle dispersion is imperative, a reduction in dispersion, or increased agglomeration, leads to poor efficiency and ultimately results in lower yields and increased cost. As powder technology moves into the nano-age, the challenges of maintaining and improving particle dispersion become increasingly important. Decreasing particle size into the submicron range can significantly increases the effect of surface forces leading to an increase in the cohesive forces of the powder, thus, making an understanding of dispersion even more essential. In order to develop the knowledge base to understand and control aerosol dispersion, a fundamental study into significant powder characteristics and environmental conditions that influence dispersion has been conducted. This paper discusses the results of testing various techniques employed to improve the dispersion of sub-micron aluminum flakes such as using a fumed silica spacer to reduce interparticle contact area and, in turn, attractive van der Waals forces, as well as examining the effect of dissemination pressure on the state of dispersion of the particles. This research was conducted using statistical experimental design in order to efficiently study significant factors.
Fundamental understanding of the deposition of powders into dies is essential to optimizing processes such as compaction. Toward this end, models were developed and verified to simulate the filling characteristics for three deposition methods into shallow dies. Understanding of the filling process for shallow dies represents the key first step towards gaining fundamental knowledge of filling deep dies. The three different filling methods modeled were: the feed shoe, rotational rainy, and point feed, which represent the commonly used methods to fill dies and containers. A free flowing, spray dried battery powder mixture (d50=600 μm) was filled into a circular shallow die (35 mm diameter x 6.5 mm deep) at 20 mm/s feed shoe speed equivalent to 26 g/min filling rate. A physics based explanation of the filling process, i.e., pressure vs. time for a specific location at the bottom of the die, is included that provides a rational basis for the use of Chapman-Richards model. In order to evaluate the goodness of the model, the average root mean square error (RMSE) and the mean value of average relative difference (ARD) of the models were calculated. The results showed that 1) the RMSE for feed shoe, rotational rainy, and point feed were 0.16 dm (dm=decimeter, fill head equivalent of measured pressure), 0.44 dm, and 0.32 dm, respectively, whereas, the ARD for feed shoe, rotational rainy, and point feed were 7%, 16%, and 11%, respectively; 2) the deposition profile for feed shoe and rainy fill were sigmoidal in shape, while for the point feed it was linear.
Aqueous-based C/LiFePO4 positive electrodes with high-rate capabilities are described with an intended application in lithium ion batteries. Pastes consisting of C/LiFePO4 particles, sodium carboxymethyl cellulose (CMC), water-soluble elastomeric binder (WSB) and poly (acrylic acid) (PAA) were prepared in an aqueous medium and tape-cast onto an aluminum foil. It was found that the incorporation of PAA significantly decreased the apparent viscosity of the C/LiFePO4 paste as well as shifted particle size distribution to lower value, which resulted in an improvement of the C/LiFePO4 dispersion properties. A correlation was made between the dispersion properties and the electrochemical properties. The electrochemical properties indicated that the electrode with PAA exhibited a discharge specific capacity above 70 mAhg−1 at 20C, which was about seven times higher than the discharge specific capacity without PAA at 20C which showed a discharge specific capacity of only 10 mAhg−1. This was attributed to the formation of a resistance-imparting agglomeration, which is consistent with the dispersion properties of C/LiFePO4 particles.