Die compaction of powders is a process which involves filling a die with powder, compression of the powder using rigid punches to form a dense compact, and ejection from the die. The choice of powder composition and selection of process parameters determine the microstructure and final properties of the compacts. The practical issues in the powder-forming industries (powder metallurgy, ceramics, hard metals, pharmaceuticals, detergents, etc.) are related to mechanical strength, control of microstructure, avoidance of cracks and defects, content uniformity, etc. We review the modelling strategies used for powder compaction. The main focus is on the constitutive model development for finite element analysis. Knowledge of the following input factors is required: 1.constitutive equations which describe the deformation of a volume of powder under the loads applied during compaction 2.friction interaction between powder and tooling 3.geometry of die and punches 4.pressing schedule, e.g. sequence of punch motions 5.initial conditions that relate to the state of the powder in the die after die fill The constitutive model and friction relate to fundamental properties of the material and are reviewed in more detail. The methodologies used for model calibration are also described. The remaining factors (geometry, pressing schedule and initial conditions) are specific to particular problems. Their relative effect is discussed by presenting examples for a range of powder materials. We show how compact microstructure can be manipulated by changing the factors discussed above and illustrate the effect of microstructure on final properties. The model predictions are validated using experimental data. The use of numerical analysis in powder formulation design and optimisation of the process parameters is discussed.
This paper summarises some of our recent work on the heat transfer of nanofluids (dilute liquid suspensions of nanoparticles). It covers heat conduction, convective heat transfer under both natural and forced flow conditions, and boiling heat transfer in the nucleate regime. The results show that, despite considerable data scattering, the presence of nanoparticles enhances thermal conduction under macroscopically static conditions mainly due to nanoparticle structuring/networking. The natural convective heat transfer coefficient is observed to decrease systematically with increasing nanoparticle concentration, and the deterioration is partially attributed to the high viscosity of nanofluids. However, either enhancement or deterioration of convective heat transfer is observed under the forced flow conditions and particle migration is suggested to be an important mechanism. The results also show that the boiling heat transfer is enhanced in the nucleate regime for both alumina and titania nanofluids, and the enhancement is more sensitive to the concentration change for TiO2 nanofluids. It is concluded that there is still some way to go before we can tailor-make nanofluids for any targeted applications.
This paper gives a review on the plasma synthesis of nanoparticulate powders. The concept of plasma synthesis is used quite comprehensively, it covers all processes where charged particles are observed. Therefore, the topic of this paper ranges from high-temperature processes and microwave processes to the laser and flame synthesis of nanopowders. For each of the processes discussed in this paper, the product characteristics are explained. This may be used as guidance for the selection of a process. The presentation of the paper is quite basic; this is to give people working in industry on daily problems a chance to see what is going on in this field. There is a vast amount of literature in the field of plasma synthesis. The literature was therefore selected in a way to demonstrate basic phenomena and not to give a comprehensive review of the literature.
Experimental procedures for characterizing the wetting behavior of powders are reviewed. The fundamental processes involved in wetting–passage from one state of two-phase equilibrium (solid/gas and liquid/gas) through a three-phase condition (solid/liquid/gas) to a second two-phase condition (solid/liquid and liquid/gas)–are evaluated. A brief discussion of the use of chemical agents such as surfactants to control wetting/dewetting behavior is also included. Characterization procedures, ranging from direct measurement of three-phase contact angles to indirect measures based on observation of the behavior of particles at liquid/gas interfaces are described. For large pieces of solid, contact angles can be determined by direct observation of liquid drops or gas bubbles in contact with polished surfaces. The problems associated with applications of this approach to packed beds of powder are discussed. A procedure for estimating apparent contact angles from the relative partitioning of small particles across an interface is presented. Estimation procedures based on static and dynamic measurements of liquid penetration into powder beds are evaluated. While indirect methods do not generally provide values of well-defined quantities such as contact angles, when appropriately selected they can yield quantitative information directly relevant to practical applications. Various indirect methods including film flotation, the Hallimond tube, bubble pick-up, induction time, immersion/sink time, imbibition time and wetting rate are described. Investigation of wetting phenomena at the molecular scale using techniques such as atomic force microscopy is discussed.
Epidemiological studies have shown that the occupational exposure of crystalline silica can lead to silicosis and other lung injury. In this paper, we briefly summarize the various research works that have been conducted related to crystalline silica-induced toxicity. Firstly, a short description of the structure of quartz is presented, followed by the various types of silicosis that can be caused by inhalation of crystalline silica particles. Methods to characterize the particles and mechanisms of particle-cell interactions are also reported. The effect of physicochemical properties such as size, shape and surface functionality that influence the toxicity mechanisms are discussed, followed by a brief description on the effects of free radical generation due to quartz inhalation on biological targets (proteins, lipids and DNA). Finally, methods reported to modify the surface and reduce the quartz-induced toxicity are discussed.
In order to keep pace with Moore’s law, multilevel metallization has become the process of choice. Having a planar wafer surface before every successive step in multilevel metallization is important. The Chemical Mechanical Planarization (CMP) process is used in the semiconductor industry to achieve planar surfaces at every step of the multi-level metallization. In CMP, planarization is achieved primarily due to material removal by the use of abrasive particle slurries. In addition to the slurry chemistry, slurry performance is also dictated by the properties of the abrasive particles. A better understanding of the properties of abrasive particles and particle abrasion mechanisms will lead to better CMP. Some of the challenges in particle modeling, slurry stabilization and particle induced lubrication as well as recent developments in engineered particles for CMP are discussed in this paper.
This paper presents a review of the flow pattern in hydrocyclones from a fluid dynamics perspective. Measurements attempt to establish the velocity and pressure distribution inside the equipment. With the experimental values of the velocity distributions, mass and linear momentum equations were solved using different approximations. The review starts with discussion of the principal experimental measurements in hydrocyclones and continues obtaining theoretical solutions to the equations with the simplest inviscid approach combining a free vortex with a sink, with rotational inviscid models, with models based on the exact solution of the Reynolds equation for highly idealized conditions and ends with solutions using different mathematical techniques such as asymptotic expansions, similarity solutions and boundary layer flows. When developments of the models can be found in the original papers they were not reproduced here, but in some cases they were include to relate them to a given fluid dynamic solution type.
The packing efficiency of granular materials is an important consideration in a variety of industrial settings. Here, the density relaxation problem is studied using three dimensional Monte Carlo simulations, which model the effect of regular taps applied to vessel having a planar floor filled with hard spheres. Results show that the equilibrium bulk solids fraction depends strongly upon the intensity of the taps, with relatively lower intensity taps producing more dense systems. A broad range of solids fractions are generated, starting from a loose configuration, rather like a ‘poured’ assembly, to a relatively dense structure having local crystalline order. Reasonably good agreement of the computed coordination number with experiments in the literature is found. Results show an enhanced tap-induced ordering effect of the floor on the local microstructure, which is reflected in the radial distribution function. For energetic taps, the solids fraction evolution fit well to a hyperbolic tangent model, while results at low intensity taps are described by an inverse log law.
The shape of pulverized bituminous coal particles (vitrinites) was determined by optical and laser light scattering. Vitrain samples were collected from obvious tree remains located in the ceilings of two Appalachian coal mines. Wet sieving produced narrow size cuts. The particles were determined to be oblong or blocky in shape, with average length-to-width ratio of 1.7 and sphericity of 0.78. They were analogous in shape to a square ended, rectangular “house brick”. The two bituminous coals and different size cuts of each coal had essentially the same shape parameters. Characteristic heating times and terminal velocities were higher by 22 and 20%, respectively compared to spherical particles.
Solid fuel particles will become increasingly important in the future. Present energy conversion systems for solid fuels are too inefficient. New energy conversion systems for solid fuels with higher energy conversion efficiencies are possible. Fuel cell technology is a key-technology in these new conversion systems. The direct carbon fuel cell (DCFC) operates on carbon particles obtained from a variety of solid fuel feedstocks. The DCFC is the only fuel cell designed to directly oxidize carbon particles in a special anode chamber. The particles are generally graphite structure with high purity. The electrolyte used is the high temperature solid oxide, molten carbonate or hydroxide electrolyte. Since a pure stream of CO2 is produced the stream can easily be sequestered and disposed. Pure carbon dioxide produced as a by-product would also have a market in many industries. A well defined technology roadmap identifying key research and development (R&D) issues is necessary to provide a framework for the development of these systems and to prevent entrenchment in inherently inefficient technologies. This review paper describes the direct carbon fuel cell and its system, how it works, the developmental status, the characteristics of the carbon particles needed, and the research and development issues for the technology.
Valuable information on the phenomenology of mechanical activation will be obtained from the accurate characterization of wearing processes that take place within mechanochemical reactors. Favoring the contamination of reactant powders with material from a reactor’s debris, for a long time such processes represented one of the greatest limitations to practical mechanochemistry applications. We will focus on two related aspects of mechanochemical processing by grinding in a ball mill: (i) nanoscale wear of the treated substances and of the milling tools (balls and container wall); and (ii) deposition of a powder coating on the surface of the milling tools (self-lining phenomenon). A new technology called abrasive-reactive wear (ARW) has been developed that utilizes wear debris as an integral component of the reaction system rather than treating it as a harmful impurity. This technology is applied to the preparation of nanocomposites and to the processing of mineral raw materials and industrial byproducts. This review includes preparation experiments, material characterization, ARW kinetics and simulation. Besides developing new technologies, ARW will contribute to a better understanding of the mechanochemical or mechanical alloying process in general.
Aggregation during crystallization and precipitation processes often leads to complex-shaped particle aggregates. As an alternative to low-dimensional deterministic population balance models, where assumptions on the particle shape must be made, stochastic or so-called Monte Carlo methods can be employed. In previous work a hierarchical characterization of aggregates has been proposed (Briesen, AIChE J., 52, 2436–2446, 2006), which allows the use of different levels of detail for modelling the different rate processes as primary particle growth or particle aggregation. With that hierarchical characterization, the detailed geometry of aggregates becomes accessible for rate process modelling and product characterization. Here, this framework is extended to investigate size-dependent collision rates and aggregation efficiencies. The results show that the aggregate structures can be modelled by the interplay of shear rate and the growth rate at the particle necks in a mechanistic way. Future work will address the comparison with experimental data and alternative model formulations.
Due to the increasing production and development of nanoparticles, it has become necessary to control the exposure to ultrafine particles when handling nanopowders. The use of dustiness tests makes it possible to compare the ability of a given powder to re-suspend particles, and to determine the effect of the different external (powder handling method) or internal parameters (powder properties). A dustiness test associated with an electrical low-pressure impactor (ELPI) device is proposed to study the free fall of nanopowders. Titanium dioxide (TiO2) and fumed silica (SiO2) are the studied nanopowders. The free falling of nanopowders in the test chamber generates bimodal aerosols corresponding to the re-suspension of the micrometric agglomerates that constitute the nanopowders and to the breakage and/or erosion of these agglomerates leading to ultrafine aggregates. The presence of ultrafine aggregates was checked by scanning electron microscopy (SEM). When the height of fall and the dropped mass of the powders are increased, the aerosol concentration increases. Aerosols are mostly generated by the impact of nanopowders on the floor of the experimental chamber. Fumed silica is dustier than titanium dioxide, and its agglomerates break more easily.
Discrete element models of fluidized beds allow the accurate reproduction of many aspects of the flow of the fluid and solid phases on both microscopic and macroscopic scales. In the present paper, we first discuss some of the basics of the DEM-CFD model, along with an illustration of areas where it proved successful. Then, a discrete element computational code is used to analyse in detail the key mechanisms governing the mixing of solids in fluidized beds. Air fluidization of a mixture of glass ballotini and steel shots is simulated, and the steady-state concentration profiles are successfully validated with experimental data. By changing the density of the heavier component, the effect of the gas velocity in combination with the density ratio is investigated in terms of an equilibrium degree of mixing and a characteristic time to reach this condition. The maximum mixing achievable is found to depend strongly on the difference of density. For mixtures with a density ratio close to 3, full mixing is practically impossible due to the presence of a non-mixable region, rich in the heavy component, at the bottom of the bed.
Two processes have been developed for the enhancement of bioavailability of a poorly-soluble active substance, Eflucimibe by associating it with γ-CD (γ-cyclodextrin). In the first process (process a), Eflucimibe was added to an aqueous slurry of CD, in a kneading device. The evolution of the transformation was followed by DSC, FTIR, Eflucimibe dissolution kinetics, as well as semi-solid state change of the mixture. An optimization of the process was performed and a prevision of the scaling-up was made using dimensionless numbers. This process is simple and robust. It can be compatible at the industrial scale with a good economy and appropriate control. In the second process (process b), Eflucimibe and CD are co-crystallized using an anti-solvent process, dimethylsulfoxide being the solvent and supercritical carbon dioxide being the anti-solvent. Then, the co-crystallized powder is held in a static mode under supercritical conditions for several hours. A final stripping step, is used to extract the residual solvent. The coupling of the first two steps brings about a significant synergistic effect to improve the dissolution rate of the drug. Both processes resulted in a strong acceleration of the in vitro dissolution rate of the drug. Finally, in an in vivo test, these two processes appeared to be very effective, process (a) and (b) giving respectively an 8-fold and 11-fold increase in bioavailability.
Silanising zeolites can result in significant beneficial changes to their catalytic and sorptive properties. It is, however, necessary to carefully control the reaction conditions when silanising a zeolite. Apart from the different effects of using vapour or liquid deposition procedures and static or flow systems, the deposition temperature and the number of silanisation/calcination cycles are of great importance. By careful control of these conditions, it is possible to systematically modify the diffusional properties of the zeolite while at the same time inertizing the external surface acidity. The diffusional changes are more likely due to a blockage of pore entrances, resulting in a greater diffusion pathway, than to a controlled narrowing of the pore openings. By careful control of the number of silanisation/calcination cycles, it is possible to systematically change the diffusional properties. The amount of Si deposited/nm2 is a good indicator of the process of silanisation. Silanised zeolites are able to significantly increase the yield of particular isomers as a result of the diffusional constraints.
The five-year METI/NEDO’s nanoparticle project started in 2001. In this study, various nano-sized particles, e.g. Au, Ag, GaN, ZnO, FePt, CdSe, Y2O3:Eu, (Y,Gd)3Al5O12:Ce, ZnS:Mn, etc., were prepared by gas-phase methods (thermal and plasma CVD) and by liquid-phase methods (spray pyrolysis, spray drying as well as sol-gel method) using continuous reactors. Nanoparticles and nanoparticle/polymer composite materials were also prepared using polymeric precursor/processing techniques. Using these preparation methods, non-agglomerated and highly-functional nanoparticles were successfully produced in controlled sizes ranging from around 100 nm to a single nanometer with good stoichiometry and high crystallinity.
†This paper, appeared originally in Japanese in the 72nd Annual Meeting of the Society of Chemical Engineers, Japan, Session ID: K120 (Kyoto, 2007), is published in KONA Powder and Particle Journal with the permission of the editorial committee of the Society of Chemical Engineers, Japan.
We developed a self-assembly process of SiO2 particles to fabricate desired patterns of colloidal crystals having high feature edge acuity and high regularity. A micropattern of colloidal methanol prepared on a self-assembled monolayer in hexane was used as a mold for particle patterning, and slow dissolution of methanol into hexane caused shrinkage of molds to form micropatterns of close-packed SiO2 particle assemblies. We further developed spherical particle assemblies and micropatterns of them. Hydrophilic regions of a patterned self-assembled monolayer were covered with methanol solution containing SiO2 particles and immersed in decalin. Particles were assembled to form spherical shapes and consequently, micropatterns of spherical particle assemblies were successfully fabricated through self-assembly. This result is a step toward the realization of nano/micro periodic structures for next-generation photonic devices by a self-assembly process.
†This report published originally in Japanese in J. Soc. Powder Technology, Japan, 43, 362-371 (2006), is published in KONA with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Nano-ordered composite materials consisting of organic polymers and inorganic materials have been attracting attention for the purpose of the creation of high-performance or high-functional polymeric materials. Especially, the word of “polymer hybrid” claims the blends of organic and inorganic components at nano-level dispersion. By using this idea, an enhancement of mechanical strength of organic polymers with silica particles is possible. High transparency of this material is another important property and indispensable for development of optical waveguides, optical biosensors, non-linear optical materials, and contact lenses. Hybrid materials are also potential candidates for catalysts and gas separation membranes. The sol-gel reaction makes possible to incorporate the organic polymer segments in the network matrix of inorganic materials. The high homogeneity of the hybrid strongly suggests that the organic polymer segments and inorganic one are blended at the nano-meter level. The organic polymer nano-hybrids can be considered not only as the combination of organic polymer and inorganic materials, but as quite new materials.
†This paper, published originally in Japanese in The Micrometrics, 50, 11-15 (2006/2007), is published in KONA with the permission of the editorial committee of the Micrometrics.
The natural graphite ground in vacuum atmosphere by a vibration ball mill is found to have the porous nanostructure consisting of the agglomeration of primary particles of approximately 20nm in size. Such nanostructured graphite particles were used as a raw material to experientially investigate the hydrogen desorption characteristics of the products ground in high pressure hydrogen atmospheres by 3 types of milling machines including a ball mill, a vibration ball mill and planetary ball mill. We experimentally investigated the relationship between the nanostructures and the hydrogen desorption temperature of the products obtained by grinding the nanostructured graphite in hydrogen atmosphere. The micropores and mesopores in the nanostructured graphite prepared by a vibration ball milling in vacuum atmosphere were almost maintained during the further grinding by the ball milling in hydrogen atmosphere. The starting temperature of hydrogen desorption of the ground products of the nanostructured graphite, which was prepared by a vibration ball mill in vacuum atmosphere, decreased to 470K from 600K, which had been previously reported.
†This paper, published originally in Japanese in J. Soc. Powder Technology, Japan, 42, 185-191 (2005), is published in KONA with the permission of the editorial committee of the Soc. Powder Technology, Japan.
The relations between normal stress and normal strain and that between shear stress and shear strain, which are the constitution relationships, have been obtained in a simple and general particulate field by the numerical simulation using Distinct Element Method. We calculate the dynamics of particulate matters using the continuum model based on these relations. Furthermore, the numerically calculated constitutive relations are formulated in order to use them for the continuum model simulation. The constitution relationships presented in this work confirm the following. 1.Stresses are functions of strains and packing ratios. 2.The constitution relationship of particulate matters has a hysteresis because of the change in the internal state of the particulate matters. 3.Particulate matters are deformed more easily by the shear stress than the normal stress.
†This paper, published originally in Japanese in J. Soc. Powder Technology, Japan, 42, 116-124 (2005), is published in KONA with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Based on the fluid penetration, an evaluation method of the non-uniformity of a particle bed structure was developed. The newly developed apparatus has a cubic sample box that can be set at any direction. Pressure drops through the packed bed in the same direction as the packing direction and the cross direction of that were measured in the apparatus. It was found that the fluid penetration resistance in gravitational direction (=packing direction) was higher than that in cross direction to gravity. The differences between them depend on the particle size and shape. In order to represent the differences as the apparent alignment of packed bed structure, the experiments using the model packed bed constructed by uniform circular bars having a diameter of 4mm were conducted. Through the experiments, correlation between aligned angle and the ratio of specific surface area of the cross direction to that in the gravitational direction was obtained. By means of the correlation, the non-uniformity of the packed bed structure can be evaluated as the apparent aligned angle.
†This paper, published originally in Japanese in J. Soc. Powder Technology, Japan, 42, 613-618 (2005), is published in KONA with the permission of the editorial committee of the Soc. Powder Technology, Japan.