Novel powder packing theory with bimodal particle size distribution-application in superalloy

Powder packing behavior plays an important role in determining sintering ability of powder and the resultant performance of materials. In this study, a novel powder packing theory with bimodal particle size distribution is proposed by considering the loosening effect, wall effect and wedging effect. This theory is applied in PM nickel base superalloy by using mixture of coarse particles and fine particles. Microstructures of alloy sintered by vacuum hot pressing (HP) are observed by optical microscope (OM) and electron backscatter diffraction (EBSD). The prediction result by this theory is in good agreement with the experimental results. The enhanced sintering ability of powder containing appropriate fractions of coarse particle and fine particle is ascribed to the filling of fine particles to the voids between coarse particles, which enhanced the density of sample after sintering. Tensile behavior and the fracture morphology of alloys with various particle distributions are analyzed in details, suggesting the higher reliability of the present theory.     

细颗粒添加组分流态化研究进展

介绍了细颗粒聚团流态化的类型;从添加组分的种类、添加量以及粒径大小3个方面,综述了近些年来国内外研究者对细颗粒添加组分流态化的研究进展,进一步阐述了细颗粒中添加磁性大颗粒后在外加磁场作用下的流化性能的研究情况,重点指出了细颗粒添加组分流态化研究中存在的问题和发展方向,提出应开展细颗粒聚团与其添加颗粒之间的相互作用机理的研究,以得出定量的关系,从而为工业应用提供理论依据。     

Percolation Segregation in Binary Size Mixtures of Spherical and Angular-Shaped Particles of Different Densities

Percolation segregation in binary size mixtures for two particulate types, urea (spherical) and potash (angular), were studied. Materials chosen are major raw ingredients of blended fertilizer that represent two extremes based on shape and density. In this study, the coarse and fine particles were classified using particle sizes larger and smaller than 2,000 μm, respectively. Three coarse mean sizes (3,675 μm, 3,075 μm, and 2,580 μm) for both spherical and angular particles and three fines mean sizes (2,180 μm, 1,850 μm, and 1,550 μm) for angular particles and two fines mean sizes (2,180 μm and 1,850 μm) for spherical particles were selected for tests. Size ratio for binary size mixture is defined as the ratio of mean size of coarse to fine particles. Binary mixed samples of coarse and fine particles were placed into the shear box of the primary segregation shear cell (PSSC-II) very gently to avoid segregation. Percolation segregation was quantified using PSSC-II. Based on experimental results, the segregated fines mass, normalized segregation rate (NSR), and segregation rate of fines for binary mixtures were higher for larger size ratios as expected (2.4 > 2.0 > 1.7). The NSR is defined as the amount of fines percolated from initial fines present in the binary mixture based on total time of PSSC-II operation (kg/kg-h). Segregation rate was the highest and lowest for mixing ratios 33:67 and 67:33, respectively, when coarse mean size was 3,675 μm, where mixing ratio for binary mixtures is the ratio of the mass of coarse particles to the mass of fine particles. For the same size ratio, segregated fines mass for coarse-fine size combinations in the binary mixtures of urea and potash were significantly different (p < 0.05). Segregated fines mass of potash and urea particles was significantly different for the same size ratio and the same coarse sizes (p < 0.05). Percent segregated fines of angular particles (59%) was higher than that of spherical particles (45%) for the size ratio 2.0 and coarse mean size of 3,675 μm.     

Precipitate growth activation energy requirements in the duplex size γ′ distribution in the superalloy IN738LC

The precipitate growth features in the duplex size (fine and coarse) precipitate distribution, obtained by quenching the alloy from the holding temperature of 1140°C, was studied by treating the alloy for various times in vacuum at selected temperatures in the range 800–1100°C. It was found that both the fine and the coarse precipitate particles grow with time, apparently aided by the particle movement in the matrix and coalescence, by the so-designated Precipitate Agglomeration Mechanism (PAM). At 1100°C the fine particles grew to the size of the coarse particles in about 100 hours and a near single coarse size of about 840 nm was obtained in the matrix. The activation energies for the growth of both the fine and coarse particles were calculated using particle sizes at different temperatures after 25 h of aging and at 1040 and 1100°C for different aging times. These were found to decrease continuously with an increase in size of the particles, meaning that the coarse particles grow more easily than the fine particles, requiring less activation energies. This behavior could be attributed to the greater attractive force with which the coarser particles would attract the finer particles.     

Thermal conductivity of Al–SiC composites with monomodal and bimodal particle size distribution

The thermal conductivity of aluminum matrix composites having a high volume fraction of SiC particles is investigated by comparing data for composites fabricated by infiltrating liquid aluminum into preforms made either from a single particle size, or by mixing and packing SiC particles of two largely different average sizes (170 and 16 μm). For composites based on powders with a monomodal size distribution, the thermal conductivity increases steadily from 151 W/m K for particles of average diameter 8 μm to 216 W/m K for 170 μm particles. For the bimodal particle mixtures the thermal conductivity increases with increasing volume fraction of coarse particles and reaches a roughly constant value of 220 W/m K for mixtures with 40 or more vol.% of coarse particles. It is shown that all present data can be accounted for by the differential effective medium (DEM) scheme taking into account a finite interfacial thermal resistance.     

球磨转速对Nd:YAG透明陶瓷的显微结构及光学性能的影响

以高纯商业Y₂O₃、α-Al₂O₃和Nd₂O₃粉体为原料, 以TEOS(正硅酸乙酯)和MgO为烧结助剂, 采用固相反应和真空烧结技术制备了1.0at%Nd:YAG透明陶瓷。系统研究了球磨转速(球磨时间10 h)对混合粉体的尺寸以及对陶瓷样品致密化行为、显微结构和光学性能的影响。结果表明: 通过球磨过程可以充分细化原料粉体的颗粒; 随着球磨转速的提高, 陶瓷烧结时样品中的气孔能更好地排除。但是球磨转速过高时, 陶瓷烧结体中存在少量的富铝第二相会降低样品的光学透过率。当球磨转速为130 r/min时, 真空烧结(1760℃×50 h)所得Nd:YAG透明陶瓷的微结构均匀致密, 几乎没有晶界和晶内气孔存在, 样品在1064 nm处的直线透过率高达83%。     

Liquid phase sintering of bimodal size distributed alumina powder mixtures

Fine and coarse alumina powder mixtures (non-additive specimen) and those containing the additive formed liquid phase during firing (additive specimen) were compacted and fired at 1400–1600°C. Liquid phase sintering proceeded markedly at 1400–1500°C and additive specimens had much higher relative density than non-additive specimens at 1500°C. As the liquid phase sintering proceeded, the open pore volume decreased abruptly, but the open pore size changed depending on the packing structure. The open pore size decreased in the specimens where the fine particles formed matrix structure, while it increased in the specimens where the coarse particles formed skeletal structure. At 1600°C all additive specimens having different mixing ratios of fine and coarse powders had similar microstructure and the same relative density of 97%. However, spherical large pores were formed and remained in all additive specimens even at 1600°C. The bending strength of those specimens was about 400 MPa.     

AGGREGATION DURING THE DISSOLUTION OF DIAZEPAM IN INTERACTIVE MIXTURES

The dissolution of micronized diazepam (1.0-10.0%) in interactive mixtures with lactose-povidone, Emdex®, TabBase® and Compactrol® as carriers was investigated using the USP paddle method and distilled water as the dissolution medium. Dissolution rate of the binary diazepam-carrier mixtures increased using more soluble carriers such as lactose-povidone and decreased as the diazepam concentration of the mixtures increased. The data were interpreted by considering dissolution from both dispersed and aggregated particles and modelled using monoexponential and biexponential equations allowing the estimation of reciprocal dissolution rate constants for dispersed and aggregated particles (k_d and k_a and initial aggregate concentrations (C_a). The estimated k_d parameters were independent of carrier and diazepam concentration while the ka. parameters varied and were dependent on the aggregate size distribution in the interactive mixtures studied. The degree of aggregation increased markedly with increasing diazepam concentration and was greatest for the less soluble carrier, Compactrol®. Ternary surfactant interactive mixtures containing diazepam and sodium lauryl sulphate (100:2) adhered to the carrier surface were developed and demonstrated improved dissolution rates which were attributed to the deaggregation effect of the surfactant in the aggregate microenvironment. The effect was most noticeable at 10 percent drug loadings where the surfactant concentration was greatest and where both the k_a and C_a parameters were minimized.     

Surface Characterisation df Sucrose and Antibiotics Powder Mixes with Relevance to Mixing Theories

Adhesional ordered mixing is not applicable in real systems because ordered interaction between drug and excipient particles can not be achieved. In contrary, Interactive Mixture approach is more applicable because it allows the powder mixtures to be described according to the state of homogeneity achieved which is dependent on mixing variables.

Surface characteristics of powder mixture particles are important to study in order to understand the interparticulate interactions between drug and excipient particles.

Indentations on excipient particles act as mechanical entrapment sites for drug particles which result in areas containing highly localised drug content. Consequently, the state of homogeneity of powder mixtures is possibly affected. Furthermore, the release of particles which are entrapped will be different from those held by interparticulate forces on the plain surfaces of excipient particles. This is particularly important when the excipient particles are insoluble and drug particles are poorly soluble.     

Study of the shear behavior of binary granular materials by DEM simulations and experimental triaxial tests

This paper aims at studying the shear behavior of mixtures of fine and coarse particles by classical triaxial tests. The work is performed both on experimental tests and computer simulations by discrete element method. The comparisons between experimental and simulation results on monosized and binary samples show that the DEM model can reproduce deviatoric curves satisfactorily in experimental conditions. The shear behavior of monosized and binary systems with the same initial void ratio differs significantly, suggesting that the state of compaction of the system is more influential than the initial void ratio. Comparison between compacted and uncompacted samples confirms that compaction increases the shear strength of granular matter. At the particle scale, the coordination number decreases with the augmentation of the volume fraction of coarse particles. The average rotation velocity of fine particles is higher than coarse particles, but their particle stress tensor is smaller than coarse ones.     

Influence of coarse particles on microstructure of aluminum nitride sintered body

Aluminum nitride (AlN) is used for quick diffusion and elimination of heat that is generated from electronic devices, such as power modules used for hybrid cars and micro processing units (MPUs) of computers. AlN provides high thermal conductivity, and it is known that its sintering performance and sintered body characteristics vary with the quality of AlN raw powder. When two types of commercially available AlN raw powder produced by the same reductive nitriding method were compared, the sintering performance and the thermal conductivity of sintered compacts processed from low-price AlN material powder were found to be lower than those of sintered compacts processed from high-price, high-purity, evenly granulated, and fine AlN material powder. As one of the causes of the foregoing, the effect of coarse particles contained in AlN material powder was investigated. The investigation results indicated that the coarse particles were AlN and the powder with the coarse particles removed by sifting out with sieves provided sintering performance and sintering behavior similar to those of high-price and high-purity AlN material powder. It was therefore found that the coarse particle constituted a sintering inhibiting factor. This paper reports the investigation results.     

Segregation Quantification of Two-Component Particulate Mixtures: Effect of Particle Size, Density, Shape, and Surface Texture

Segregation, which occurs during handling, processing, and storage of particulate material, is highly dependent on properties such as particle size, size distribution (for continuous mixtures) or size ratio (for binary mixtures), particle density, shape, and surface texture. Quantification of the relationship of material properties to segregation becomes an important link in understanding and controlling segregation. Due to lack of well-developed equipment in the market, quantification of segregation of multicomponent particulate mixtures is currently a challenge. In this study, the effects of particle size, density, shape, and surface texture of two-component particulate mixtures (glass beads and mash poultry feed) on segregation were quantified with the use of the second generation of primary segregation shear cell (PSSC-II) developed at Penn State. It was concluded that (1) irregularly shaped (nonspherical) coarse particles or higher porosity of coarse component of a binary mixture lead to higher segregation potential; (2) the higher the density and smoother the surface of the fine component of a binary mixture, the higher the segregation potential; and (3) the fine particle properties, to a certain extent, determine the particle size-related effects such as absolute size and size ratios, i.e., if fine particle properties of a binary mixture change, the size-related effect on segregation potential would definitely change.     

Segregation Quantification of Two-Component Particulate Mixtures: Effect of Particle Size,Density, Shape,and Surface Texture

Segregation, which occurs during handling, processing, and storage of particulate material, is highly dependent on properties such as particle size, size distribution (for continuous mixtures) or size ratio (for binary mixtures), particle density, shape, and surface texture. Quantification of the relationship of material properties to segregation becomes an important link in understanding and controlling segregation. Due to lack of well-developed equipment in the market, quantification of segregation of multicomponent particulate mixtures is currently a challenge. In this study, the effects of particle size, density, shape, and surface texture of two-component particulate mixtures (glass beads and mash poultry feed) on segregation were quantified with the use of the second generation of primary segregation shear cell (PSSC-II) developed at Penn State. It was concluded that (1) irregularly shaped (nonspherical) coarse particles or higher porosity of coarse component of a binary mixture lead to higher segregation potential; (2) the higher the density and smoother the surface of the fine component of a binary mixture, the higher the segregation potential; and (3) the fine particle properties, to a certain extent, determine the particle size–related effects such as absolute size and size ratios, i.e., if fine particle properties of a binary mixture change, the size-related effect on segregation potential would definitely change.     

DEM analysis of particle adhesion during powder mixing for dry powder inhaler formulation development

Understanding the adhesive interactions between active pharmaceutical ingredient (API) particles and carrier particles in dry powder inhalers (DPIs) is critical for the development of formulations and process design. In the current study, a discrete element method, which accounts for particle adhesion, is employed to investigate the attachment processes in DPIs. A critical velocity criterion is proposed to determine the lowest impact velocity at which two elastic autoadhesive spherical particles will rebound from each other during impact. Furthermore, the process of fine API particles adhering to a large carrier in a vibrating container is investigated. It was found that there are optimal amplitude and frequency for the vibration velocity that can maximise the number of particles contacting with the carrier (i.e. the contact number). The impact number and detachment number during the vibration process both increase with increasing vibration amplitude and frequency while the sticking efficiency decreases as the amplitude and frequency are increased.     

The effect of particle morphology on the physical stability of pharmaceutical powder mixtures: the effect of surface roughness of the carrier on the stability of ordered mixtures

The effect of particle morphology of the components on the physical stability of ordered mixtures was determined for a model system comprised of a mixture of micronized aspirin and a monodisperse carrier. Spray-dried lactose, crystallized lactose, microcrystalline cellulose, and dextrate were used as carriers. The surface texture of the carriers was quantified in terms of the ratio of the perimeter of the particles to that of an idealized shape at a constant magnification. Mixtures containing highly textured carriers segregated to a lesser extent than those containing smoother textured carriers. This was postulated to be due to the presence of a higher concentration of surface asperities on the coarse carriers that can constitute potentially strong adhesion sites for the fine component because of their higher energy relative to adjacent areas on the surface. The effect of the addition of a ternary component, magnesium stearate, on the stability of the above mixtures was studied. The observed differences in the segregation response were attributed to electrostatic charge effects.     

The Effect of Particle Morphology on the Physical Stability of Pharmaceutical Powder Mixtures: The Effect of Surface Roughness of the Carrier on the Stability of Ordered Mixtures

The effect of particle morphology of the components on the physical stability of ordered mixtures was determined for a model system comprised of a mixture of micronized aspirin and a monodisperse carrier. Spray-dried lactose, crystallized lactose, microcrystalline cellulose, and dextrate were used as carriers. The surface texture of the carriers was quantified in terms of the ratio of the perimeter of the particles to that of an idealized shape at a constant magnification. Mixtures containing highly textured carriers segregated to a lesser extent than those containing smoother textured carriers. This was postulated to be due to the presence of a higher concentration of surface asperities on the coarse carriers that can constitute potentially strong adhesion sites for the fine component because of their higher energy relative to adjacent areas on the surface. The effect of the addition of a ternary component, magnesium stearate, on the stability of the above mixtures was studied. The observed differences in the segregation response were attributed to electrostatic charge effects.     

Study on a large-scale discrete element model for fine particles in a fluidized bed

The discrete element method (DEM) is widely used to comprehend complicated phenomena such as gas–solid flows. This is because the DEM enables us to investigate the characteristics of the granular flow at the particle level. The DEM is a Lagrangian approach where each individual particle is calculated based on Newton’s second law of motion. However, it is difficult to use the DEM to model industrial powder processes, where over a billion particles are dealt with, because the calculation cost becomes too expensive when the number of particles is huge. To solve this issue, we have developed a coarse grain model to simulate the non-cohesive particle behavior in large-scale powder systems. The coarse grain particle represents a group of original particles. Accordingly, the coarse grain model makes it possible to perform the simulations by using a smaller number of calculated particles than are physically present. As might be expected, handling of fine particles involving cohesive force is often required in industry. In the present study, we evolved the coarse grain model to simulate these fine particles. Numerical simulations were performed to show the adequacy of this model in a fluidized bed, which is a typical gas–solid flow situation. The results obtained from our model and for the original particle systems were compared in terms of the transient change of the bed height and pressure drop. The new model can simulate the original particle behavior accurately.     

Dependence of oxidation rate of WC powder on particle size

Isothermal oxidation experiments on WC powders revealed a systematic dependence of oxidation rate on powder particle size. Oxidation was followed by measuring the change in mass of the WC powder as WC is converted to WO₃. Fine powders oxidized more quickly than coarse powders because for the same initial mass the fine powder had a larger surface area. Measurement of the change in mass with time were shown to resolve differences in mean size of 0.1 m, and possibly less, between separate batches of powder. A theoretical expression for the change in mass with time of spherical particles has been derived which compares well with experimental measurements and which can also be used with appropriate assumptions to calculate the initial powder-size distribution.     

Predicting porosity of binary mixtures made out of irregular nonspherical particles: Application to natural sediments

Predicting porosity or packing density of sediments made of coarse and fine components of arbitrary geometry is critical to many science and engineering applications. Well-established analytical models for packing of spheres express porosity of the binary mixture as a function of fine-to-coarse particle size ratio. Nevertheless, the applicability of such models to natural granular materials is limited given the nonspherical and irregular nature of the particles whose packing depends on both particle size and shape. The objective of this study is to develop a model that predicts the porosity of binary mixtures made up of irregular nonspherical particles. We modified a previously developed linear sphere-packing model so that it takes into account the effect of both the particle size and shape. As an input, the modified model uses the coarse-to-fine particles specific surface area ratio instead of using the particle size ratio required by the sphere-packing model. We tested the modified model by predicting the porosities of a binary mixture composed of coarse and fine calcite aggregates. We further validate the model by using published data on the porosity of binary mixtures made of synthesized, cubical and cylindrical particles. Our model predictions show good agreement with the measured porosity.     

A METHOD FOR THE PREPARATION OF RAW POWDER FOR THE TANTALUM CAPACITOR: INFLUENCE OF PARTICLE SIZE AND BINDER CONTENT

Raw powder for use in the tantalum solid electrolytic capacitor was prepared by different processes, that is, the wet method (currently used conversion method) and the dry method (the proposed method). The properties of prepared powder from the dry method were compared with those of the wet method. The present report has focused on the relationship between the conditions of binder preparation and the properties of the prepared powder, such as the particle size distribution, angle of repose, and the tap density, respectively. Furthermore, the utility of the dry method was investigated.

Based on the results, it has been clarified that the properties of powder prepared by the dry method differ from those of the wet method. In the case of the dry method, it is suggested that the mechanism of preparation of the fine binder (PMMA powder) differs from that of the coarse binder. Furthermore, based on the experimental results, it has been suggested that the dry method could be used as the preparation method of the raw powder used in the tantalum solid electrolytic capacitor.