Mono-sized Particles Research Articles

Modeling of the densification rates of monosized and bimodal-sized particle systems during hot isostatic pressing (HIP)

The increasing importance of powder materials fabrication by use of the HIP has led to recent emphasis on analytical techniques for describing and understanding the process. Modeling of particle adhesion and consolidation during the HIP process has been achieved through the characterization of the particle system densiflcation behavior by the deformation of a representative particle due to forces transmitted through its interparticle contacts. Instead of assuming that all of these forces are equal, a model is developed based on forces which vary to produce a uniform rate of contraction (densiflcation) across a compact. Initial parameters of the systems are obtained from radial distribution data available for monosized and bimodal-sized particle systems. Deformation mechanism maps are predicted for superalloys under temperature conditions of 873–1373 K and pressure conditions of 1–1000 MPa by coupling the equations for athermal yield and power-law creep densification mechanisms. The deformation of large vs small particles in a bimodal-sized particle system is also analyzed and assumptions important in these models are discussed.

A study on viscosity of suspensions

AbstractFrom a simple kinetic viewpoint for the formation and rupture of links among primary particles, the pseudoplastic behavior of suspensions can be well described. At steady state, the formation and rupture of links reach a dynamic equilibrium, and the viscosity of suspension is proportional to the state of aggregates. It is assumed that the formation of links is due to the surface forces over particles, including van der Waals' force, and is independent of shear rate. The rupture of links is proportional to ṙm. Finally we get a very simple equation to predict the viscosity of suspension. With this simple kinetic viewpoint, the pseudoplastic behavior of suspensions composed of binary blends of monosize particles is also predictable.

STEADY-STATE EXPANSION CHARACTERISTICS OF MONOSIZE SPHERES FLUIDISED BY LIQUIDS

Abstract Velocity-voidage experiments conducted on beds of truly monosized and spherical plastic particles fluidised by solutions of glycerol, enable the change of behaviour at high expansion conditions to be correlated with the unhindered particle Reynolds number.

Mechanism of formation of MTiO3 (M=Sr or Ba) by the gel method

Strontium and barium titanates were obtained at room temperature and 60° C, respectively, by the gel method. Factors affecting the formation of the titanates, such as the nature of the reactants, temperature, pH and time were studied. It is shown that the mechanism of formation occurs through diffusion of alkaline earth cations inside the titanium dioxide (TiO2) gel particles. Thus, particle size and shape of the titanates can be controlled by determining the TiO2 gel characteristics. This is illustrated by synthesizing the titanates with spherical submicron monosized particles.

Formation of monosized spherical aluminum hydroxide particles by urea method

Monosized spherical aluminum hydroxide particles were first prepared by homogeneous precipitation using the urea method below its boiling temperature. To obtain monosized particles a careful control is needed of the precipitation process, such as factors like temperature, hold time and concentration of Al 3+. The formation chemistry of monosized particles is studied through the measurement of pH and concentration of Al 3+ during precipitation. It is found that when the nucleation limit of aluminum hydroxide is reached only once monosized particles are formed; and in contrast, when the nucleation limit is attained more than once it will cause a multisize distribution of particles. Besides factors like temperature and hold time, anions would also affect the formation of monosized particles; monosized particles can only be obtained with SO 2- 4, not with NO - 3 or Cl -.

High Tc Oxide Solid State Reaction Kinetics Via Complex Impedance Spectroscopy

AbstractImpedance spectra (5Hz ‐ 13MHz) were collected during the solid state reaction of Yba2Cu2O6+y from large monosized CuO particles imbedded in a finely divided Y2 O3 /BaCO3 matrix. No particle size effect was observed, but a large temperature effect was observed corresponding to an activation energy of approximately 1.8eV (175kJ/mol) over the range 700‐900°C.

Treatment of polished section data for detailed liberation analysis

A transformation equation can be used to describe the relationship between the one, two and three dimensional information regarding the composition of mineral particles of specified size. Linear or areal grade distribution f( g i ) can be transformed to an estimate of the volumetric grade distribution p( g) via a transformation function H( g i | g, Nn, …), a conditional probability function. The effect of the external particle strucuture (shape) and internal grain characteristics (grade, dispersion density, and grain size distribution) on the transformation matrix have been evaluated by computer simulation of randomly oriented, irregularly shaped, multiphase particles. Volumetric grade and dispersion density (number of grains per particle) are the most important variables which influence the transformation matrix. Least square minimization of fitted functions and the Phillips-Twomey inversion technique have been used to solve the transformation equation. Three examples, a computer simulated volumetric grade distribution and two experimental depth profiles of different monosize particle samples (iron ore and copper ore), provide evidence that such an approach can be useful for detailed liberation analysis.

Liberation modeling using automated image analysis

A population balance model describing the combined processes of size reduction and mineral liberation has been developed for batch grinding. Important model parameters including breakage distribution functions, breakage rate functions, and liberation functions have been determined experimentally by examining the mill product using a Zeiss SEM-IPS image analyzer. Areal assays, obtained from image analysis of monosized particle mounts, have been found to correspond quite closely to the actual chemical assays. As a result of the model parameter analysis, breakage distribution functions have been found to be independent of the grinding environment, while breakage rate functions appear to be sensitive to it. The model has been validated by simulating the batch grinding of a sphalerite ore from Tennessee. Excellent agreement between the model predictions and the experimental results is observed for both monosized and multisized feed materials. The liberation function has been found to be useful for analyzing the liberation mechanisms of composite particles.

Preparation of spherical, monosized Y 2O 3 precursor particles

Spherical, monosized yttria precursor particles were obtained by homogeneous precipitation in aqueous solutions by reaction with the thermal decomposition products of urea. Increasing [Y 3+ above 0.05 M resulted in a deviation from spherical morphology and caused agglomeration of particles. Over the concentration range studied, excess urea did not affect particle morphology, but increased the yield. Increasing aging time appeared to increase particle size as well as to improve yield, as long as the urea was not depleted. The approximate chemical composition of the precipitate was YOHCO 3. The YOHCO 3 particles formed were amorphous to X-rays, and underwent a two-stage thermal decomposition, first forming Y 2O 2CO 3 near 180°C, and then cubic Y 2O 3 above 610°C. Generation of CO 3 2− appeared to be crucial to the formation of the solid phase. Heating the aqueous yttrium solution with trichloroacetic acid (CCl 3COOH) instead of urea as the precipitating agent produced a solid phase, while heating the same solution with formamide (HCONH 2) substituted for urea formed no precipitate.

Preparation of model mesoporous carbons

The objective of this work was to prepare mesoporous carbon solids made of monosized spherical particles, produced from poly(acrylonitrile)(PAN) precursors, after oxidation in air and subsequent carbonization under an inert atmosphere. The precursors were obtained from PAN latices by slow drying in air. The PAN latices stabilized by ABA amphipathic block copolymers were prepared in n-hexane by means of dispersion polymerization. Transmission electron micrographs showed that the particles of the latex dispersions were small spheres with a narrow particle-size distribution. Scanning electron micrographs of the mesoporous carbon samples demonstrated that the particles were arranged either in a regular (orthorhombic) or in a random (disordered) type of packing. The resulting carbon particles were found to have suffered slight coalescence around points of contact. Surface roughness on the surface of the particles was also evident.

Effect of a sterically stabilizing surfactant on the nucleation, growth and agglomeration of monosized ceramic powders

Hydroxypropyl cellulose (HPC) has been shown to provide steric stabilization during the precipitation of TiO 2 from alcohol solution, allowing the precipitation of high solids loading of monosized particles by preventing agglomeration. The adsorption isotherm for HPC on TiO 2 in ethanol was determined to account for the effect of HPC on the nucleation, growth and agglomeration of particles during the hydrolysis of titanium tetraethoxide in ethanol. Agglomeration is essentially prevented by HPC.

Activation of resting, pure CD4+, and CD8+ cells via CD3. Requirements for second signals.

We studied the requirements for secondary activation signals in pure CD4+ and CD8+ T cells after stimulation with anti-CD3 antibodies. Stimulation of CD4+ or CD8+ cells with anti-CD3 monoclonal antibodies (MoAb) bound to polystyrene monosized particles never resulted in a proliferative response. However, DNA synthesis was observed when recombinant interleukin 2 (IL-2) or other secondary signals, such as those provided by phorbol myristate acetate (PMA) or autologous accessory cells (AC), were also added. These secondary signals were not in themselves capable of inducing DNA synthesis in the absence of particle-bound anti-CD3. We also found that the signals provided by AC may be dependent on the activation state of these cells. Thus, the effects of accessory cells were enhanced by a factor present in fetal calf serum (FCS), most likely endotoxin or lipopolysaccharide (LPS), which alone, however, were not able to activate T cells, even in the presence of particle-bound anti-CD3. Recombinant IL-1 over a broad dose range was unable to replace PMA or activated AC after stimulation with particle-bound anti-CD3. Purified CD4+ and CD8+ T cells behaved identically in all the experiments, indicating that the basic mechanisms for activation in the two T-cell subsets are identical.

Densification mechanism maps for hot isostatic pressing (HIP) of unequal sized particles

A model is developed for the mechanisms of densification of powder compacts of unequal sized powder particles subjected to isostatic pressures at elevated temperature. The regimes of deformation mechanisms examined are those of athermal plastic flow and power law creep. Specifically, a bimodal distribution of particle sizes is assumed and the model evaluates the magnitude of the interparticle contact areas and the interparticle contact stresses are for each type of particle. It is found that the smaller particles bear the burden of higher interparticle contact stresses and plastic strains on an average and, accordingly, there is no longer a single boundary on a densification mechanism map between the regions of athermal plastic flow and power law creep for the bimodal case but rather a separate boundary for each particle size. The results are discussed with respect to a previous densification model for monosized particles, and the implications for a full size distribution are analyzed based on the bimodal results.

Modelling shear-flocculation by population balances

A discretized population-balance approach based on a coalescence model is presented in this paper. The population-balance equation was tested for its validity by applying it to two special cases: constant collision-rate function and orthokinetic aggregation of monosized particles. Numerical solutions obtained for these two cases were compared with simplified solutions available in the literature. The size distribution of flocculated suspensions as observed in shear-flocculation in stirred tanks can be predicted by this method. An empirical collision-efficiency function obtained from small-scale laboratory tests was incorporated into the orthokinetic mechanism to limit floc growth to a maximum stable size. The size distribution of flocculated suspensions in a larger tank of similar geometry can be calculated from this method which was found to be satisfactory for the scheelite-sodium oleate system used in laboratory tests.

New model to calculate the choking velocity of monosize and multisize solids in vertical pneumatic transport lines

AbstractThe choking velocity is the minimum gas velocity required for the vertical dilute phase transport of a given solids flux. Experimental choking velocities obtained from the literature for both monosize and multisize particles were compared to choking velocities predicted with various published correlations. For monosize particles, Yang's correlation (1983) gave the most accurate predictions. For multisize particles, all the literature correlations gave rather poor predictions. Much more accurate predictions were obtained with a new simple physical model which assumes that an annulus forms along the pipe wall at low gas velocities. Choking then occurs when this annulus occupies approximately 25% of the total pipe cross‐sectional area.

Calculation of the ignition sensitivity of dust clouds of varying size distributions

For a cloud of flammable dust in air to ignite, the temperature of the air must be a specific value that depends upon properties of the dust material and of the dust cloud. First, a theoretical treatment is used to explain variations in experimental ignition temperatures in terms of particle size. The theory modifies Cassel and Liebman's method to take account of residence time of dust in experimental furnaces or in hot air. It is shown that it is possible for the ignition temperature of mono-sized coal particles (about 50 μm diameter) to be minimal under a limited residence time. The theory is extended to deal with dust clouds with a distribution in particle size. It is shown that there exists a range of size distributions for which the possibility of ignition is at a maximum. The calculated results are presented in the form of Rosin—Rammler charts indicating the distribution most sensitive to ignition.

Thermodynamics of swelling. Preparation and application of some composite, monosized polymer particles

The thermodynamic equations describing the equilibrium swelling of polymer, oligomer and polymer/oligomer particles are reviewed. The phenomenon of phase separation during swelling is discussed. Methods for preparation of monodisperse composite particles are described and the particle composition is discussed in terms of various relevant parameters. It is shown that crosslinked polymer particles may swell by a core and shell mechan sm which may increase the rate of swelling in the second step as compared to non-crosslinked part cles. Biochemical and biomedical applications of different composite particles are described.

Kinetics of magnetic flocculation in colloidal dispersions

A description is given of the kinetics of particle agglomeration in colloidal dispersions in applied magnetic fields, a phenomenon referred to as magnetic flocculation. The influence of monosize particles and dissimilar particle sizes on the position and field value for secondary flocculation minima is re-examined. The concept of isochronal volume is introduced through which a relaxation time parameter as well as a flocculation frequency is described for the case of magnetically induced agglomeration. The flocculation half life (the relaxation time parameter) and flocculation frequency both have characteristics similar to parameters used in the literature to describe rapid coagulation in the absence of a magnetic field.

Preparation of SiO2 Glass from Model Powder Compacts: II, Sintering

The sintering behavior of powder compacts formed from spherical, nearly monosized SiO2 particles was investigated. Highly ordered compacts sintered to high density and translucency at 1000°C. In contrast, less homogeneous samples prepared from flocculated suspensions remained highly porous after sintering under the same conditions. Densification kinetics were determined over the temperature range 900° to 1050°C for ordered compacts. The viscosity at each sintering temperature and the activation energy for viscous flow were determined using available sintering models. Sintering of ordered compacts is divided into several stages. Densification, mercury porosimetry, and electron microscopy results indicate that the first stage is dominated by the shrinkage of three‐particle pore channels, whereas the second stage primarily involves the shrinkage of four‐particle pore channels.

Theoretical upper limit of dust explosion in relation to oxygen concentration

On the basis of the uniform dispersion model of mono-sized particles, the present paper deals with the upper limit of explosible dust concentration in terms of oxygen concentration. This limit is defined as the maximum dust concentration resulting in the propagation of a flame which consumes a definite amount of oxygen allocated to one solid particle. Comparisons with the existing data are reasonable. An evaluation of this upper limit will be particularly useful in view of the scarcity of data arising from the difficulties inherent in experimental techniques.