Showing papers on "Particle-size distribution published in 2000"

Effect of a crystallite size distribution on X‐ray diffraction line profiles and whole‐powder‐pattern fitting

Abstract: A distribution of crystallite size reduces the width of a powder diffraction line profile, relative to that for a single crystallite, and lengthens its tails. It is shown that estimates of size from the integral breadth or Fourier methods differ from the arithmetic mean of the distribution by an amount which depends on its dispersion. It is also shown that the form of `size' line profiles for a unimodal distribution is generally not Lorentzian. A powder pattern can be simulated for a given distribution of sizes, if it is assumed that on average the crystallites have a regular shape, and this can then be compared with experimental data to give refined parameters defining the distribution. Unlike `traditional' methods of line-profile analysis, this entirely physical approach can be applied to powder patterns with severe overlap of reflections, as is demonstrated by using data for nanocrystalline ceria. The procedure is compared with alternative powder-pattern fitting methods, by using pseudo-Voigt and Pearson VII functions to model individual line profiles, and with transmission electron microscopy (TEM) data.

Detection and Analysis of Aerosol Particles by Laser-Induced Breakdown Spectroscopy

Abstract: Laser-induced breakdown spectroscopy (LIBS) was evaluated as a means for quantitative analysis of the size, mass, and composition of individual micron-to submicron-sized aerosol particles over a range of well-characterized experimental conditions. Conditional data analysis was used to identify LIBS spectra that correspond to discrete aerosol particles under low aerosol particle loadings. The size distributions of monodisperse particle source flows were measured using the LIBS technique for calcium- and magnesium-based aerosols. The resulting size distributions were in good agreement with independently measured size distribution data. A lower size detection limit of 175 nm was determined for the calcium- and magnesium-based particles, which corresponds to a detectable mass of approximately 3 femtograms. In addition, the accuracy of the LIBS technique for the interference-free analysis of different particle types was verified using a binary aerosol system of calcium-based and chromium particles.

Characterization of Particles from a Current Technology Heavy-Duty Diesel Engine

Abstract: The physical properties, chemical properties, and morphology of particles from a current technology diesel engine run under steady-state conditions were determined during five campaigns across three calendar years. Concentrations of particulate matter, NOx, HC, CO, and CO2 measured at a fixed sampling position were found to be repeatable between different campaigns. However, particle size distributions and number concentrations were significantly affected by dilution conditions. Particle density, estimated using a volume size distribution estimated from an ELPI together with measured mass, was found to be below 1 g/cm3 at nonzero loads, but this is explained by an overestimation of volume by the ELPI due to particle fractal properties. Particle phase SO42-, NO3-, PAH, organic carbon, and elemental carbon were found to vary with engine operating modes. Particles emitted from a modern diesel engine had similar cluster structures to those from old engines but consisted of smaller basic particles when analyze...

Effect of Particle Size of Chemical Mechanical Polishing Slurries for Enhanced Polishing with Minimal Defects

Abstract: In this study the effects of oversize particle contamination in chemical mechanical polishing (CMP) slurries were investigated on the silica CMP process The limits of light scattering technique were established in detecting coarse particles in a commercial silica CMP slurry using two different methods The detection limits were set by observing the shift in particle size distribution curve or by the appearance of an additional peak in the particle size distribution curve of the baseline slurry when a known amount of coarser particles were added to it Simultaneously, polishing tests were conducted by spiking the base slurry with coarser sol‐gel silica particles at the established detection limits It was observed that the contamination of larger particles not only created surface damage but also changed the material removal rate The mechanism of polishing in the presence of larger size particles is discussed as a function of particle size and concentration © 2000 The Electrochemical Society All rights reserved

Measurement of number, mass and size distribution of particles in the atmosphere

Abstract: Typical size distributions for airborne particles are described and the significance of the ultrafine fraction highlighted. Size distributions may be expressed in terms of either mass (volume), surface area or number, and the interpretation of each is discussed together with appropriate measurement methods. The sources of ultrafine particles in the atmosphere include both primary emissions and secondary particles formed through homogeneous nucleation processes within the atmosphere. Examples of measurements of atmospheric ultrafine particles are given, highlighting situations with high concentrations of primary ultrafine particles and also situations where gas–to–particle conversion through homogeneous nucleation gives rise to bursts of new particle production. Finally, the relationship between particle mass and number within the atmosphere at a polluted site is examined.

Permeability Reduction of Soil Filters due to Physical Clogging

Abstract: Soil filters, which are commonly used to provide stability to the base soils in subsurface infrastructure, are prone to long-term accumulation of fine micron-sized particles. This causes reduction in the permeability, which in turn may lead to intolerable decreases in their drainage capacity. In this paper, the extent of this reduction is addressed using results from both experimental and theoretical investigations. In the experimental phase, a sandy soil commonly used as a filter or drainage layer was subjected to pore fluids containing polystyrene or kaolinite particles, and their permeability reductions were determined in terms of the pore fluid suspension parameters. In the theoretical phase of the investigation, a representative elemental volume of the soil filter was modeled as an ensemble of capillary tubes and the permeability reduction due to physical clogging was simulated using basic principles of flow in cylindrical tubes. The results from the experimental and theoretical investigations were in good agreement. In general, the permeability reduced by more than one order of magnitude, even when the migrating particles were smaller than the majority of the soil filter pores. The concentration of particles in the pore stream affected the rate at which the permeability reduced. Self-filtration of particles, which is prominent at higher flow rates, may itself lead to a 20% reduction in the permeability for these sands.

Film Formation of Latex Blends with Bimodal Particle Size Distributions: Consideration of Particle Deformability and Continuity of the Dispersed Phase

Abstract: Latex dispersions having a well-controlled, bimodal particle size distribution are gaining attention because they potentially enable control of the dispersion rheology, the film formation characteristics, and the final film properties. Here we study the film formation of dispersions with a bimodal particle size distribution (large:small size ratio of ca. 6:1) and with varying concentrations of the two particle sizes. We also compare the film formation of blends containing only deformable (i.e., “soft”) particles with blends containing both soft and nondeformable (i.e., “hard”) particles. We use ellipsometry as a noninvasive tool for studying film morphology as film formation proceeds. We interpret our ellipsometry data using a physical model of the morphology based on atomic force microscopy observation. Electron microscopy of film cross sections provides information about the bulk morphology. We measure void content and surface roughness in blend films as a function of the concentration of large particle...

A numerical study of the particle size distribution of an aerosol undergoing turbulent coagulation

Abstract: Coagulation and growth of aerosol particles subject to isotropic turbulence has been explored using direct numerical simulations. The computations follow the trajectories of 262 144 initial particles as they are convected by the turbulent flow field. Collision between two parent particles leads to the formation of a new daughter particle with the mass and momentum (but not necessarily the energy) of the parent particles. The initially monodisperse population of particles will develop a size distribution over time that is controlled by the collision dynamics. In an earlier study, Sundaram & Collins (1997) showed that collision rates in isotropic turbulence are controlled by two statistics: (i) the radial distribution of the particles and (ii) the relative velocity probability density function. Their study considered particles that rebound elastically; however, we find that the formula that they derived is equally valid in a coagulating system. However, coagulation alters the numerical values of these statistics from the values they attain for the elastic rebound case. This difference is substantial and must be taken into consideration to properly predict the evolution of the size distribution of a population of particles. The DNS results also show surprising trends in the relative breadth of the particle size distribution. First, in all cases, the standard deviation of the particle size distribution of particles with finite Stokes numbers is much larger than the standard deviation for either the zero-Stokes-number or infinite-Stokes-number limits. Secondly, for particles with small initial Stokes numbers, the standard deviation of the final particle size distribution decreases with increasing initial particle size; however, the opposite trend is observed for particles with slightly larger initial Stokes numbers. An explanation for these phenomena can be found by carefully examining the functional dependence of the radial distribution function on the particle size and Stokes number.

Vertical aerosol fluxes measured by the eddy covariance method and deposition of nucleation mode particles above a Scots pine forest in southern Finland

Abstract: Particle number fluxes were measured by the eddy covariance method above a Scots pine forest in southern Finland. The observed number fluxes included particles with sizes down to 10 nm in diameter. The errors of measured particle number fluxes resulting from the technical limitations of particle counter and atmospheric fluctuations of water content are estimated. Simultaneous aerosol size distribution measurements were used to obtain information on particle sizes contributing to the flux. During new particle formation events the particle fluxes were dominated by nucleation mode particles, allowing the deduction of the deposition velocities characteristic to those particles. The corresponding deposition velocities were within a range from 5 to 40 mm s−1.

Fabrication and physical properties of self-organized silver nanocrystals

Abstract: A simple method is used to prepare highly monodispersed silver nanoparticles in the liquid phase, which starts from an initial synthesis in functionalized AOT reverse micelles. To narrow the particle size distribution from 43 to 12.5% in dispersion, the particles are extracted from the micellar solution. The size-selected precipitation method is used. The nanocrystallites dispersed in hexane are deposited on a support. A monolayer made of nanoparticles with spontaneous compact hexagonal organization is observed. The immersion of the support on the solution yields to the formation of organized multilayers arranged as microcrystal in a face-centered-cubic structure. We compare the optical properties of spherical particles organized in a two- and three- dimensional structure with isolated and disordered particles. When particles, deposited on cleaved graphite, are arranged in a hexagonal array, the optical measurements under p-polarization show a new high-energy resonance, which is interpreted as a collective effect, resulting from optical anisotropy due to the mutual interactions between particles. We support this interpretation by numerical calculations performed for finite-size clusters of silver spheres. For disordered particles, a low-energy resonance appears instead of the high-energy resonance observed for spherical and organized particles. This is interpreted as optical shape anisotropy due to the asymmetrical arrangement of particles. The tip of a scanning tunneling microscope (STM) may be used as an extremely localized source of low-energy electrons to locally excite photon emission from a variety of metal films. The detection of locally excited luminescence at the junction of an STM tip provides access to electron dynamic properties at the surface, which makes it possible to study luminescence phenomena of nanometer-sized structures. The photon intensity emitted from electrically isolated silver nanoparticles self-organized as a 2D network on a gold (111) substrate is analyzed. We observed unexpectedly strong variations of photon-emission efficiency from isolated nanoparticles, depending on how tightly they are embedded within the network site. The quenching site observed in the STM photon emission map is interpreted as an enhanced interaction of electrons with surface photon modes.

Numerical study of stress distribution in sheared granular material in two dimensions

Abstract: We simulate the response of dense granular material to shear. Our simulations use a micromechanical model which includes realistic material models for each deformable grain, and a Coulomb friction model for interactions between grains. We measure the probability density function (PDF) governing the volume distribution of stress for monodisperse and polydisperse samples, circular and polygonal grains, and various values of microscopic friction coefficients, yield stresses, and packing fractions. Remarkably, PDF's are similar in form for all cases simulated, and similar to those observed in experiments with granular materials under both compression and shear. Namely, the simulations yield an exponential probability of large stresses above the mean. The relationship between distributions of boundary tractions and volume distributions of stress is discussed. The ratio of normal and tangential components of traction on the boundary defines a bulk frictional response, which is shown to increase with the intergranular friction coefficient. However, the bulk friction is always larger than the intergranular friction for densely packed samples. Bulk friction is also strongly dependent on grain size distribution and shape. New observations of force-chain banding during recrystallization, of slip systems in monodisperse samples, and of the effects of plastic yield, are also presented.

Method for producing ceramic particles and agglomerates

Abstract: A method for generating spherical and irregularly shaped dense particles of ceramic oxides having a controlled particle size and particle size distribution. An aerosol containing precursor particles of oxide ceramics is directed into a plasma. As the particles flow through the hot zone of the plasma, they melt, collide, and join to form larger particles. If these larger particles remain in the hot zone, they continue melting and acquire a spherical shape that is retained after they exit the hot zone, cool down, and solidify. If they exit the hot zone before melting completely, their irregular shape persists and agglomerates are produced. The size and size distribution of the dense product particles can be controlled by adjusting several parameters, the most important in the case of powder precursors appears to be the density of powder in the aerosol stream that enters the plasma hot zone. This suggests that particle collision rate is responsible for determining ultimate size of the resulting sphere or agglomerate. Other parameters, particularly the gas flow rates and the microwave power, are also adjusted to control the particle size distribution.

A Bulk Parameterization of the Ice Particle Size Distribution and the Optical Properties in Ice Clouds

Abstract: A new parameterization has been developed that assumes that nonprecipitating particles obey the Heymsfield–Platt power-law (H–P particles) and that the precipitating particles obey the Marshall–Palmer distribution (M–P particles). The parameterization defines a critical ice content for the onset of precipitation particles and allows the number of ice crystals, the extinction coefficient, and the effective diameter of the crystals for the cloud layer in the model to be diagnosed. The implementation of the new parameterization in a model unifies the microphysical assumptions used to calculate the optical properties and precipitation. If it is assumed that the number of H–P particles at cloud top is much larger than the number of M–P particles in southeastern Australia frontal systems, then the observed number of ice crystals at cloud top agrees well with the diagnosed number of H–P particles at cloud top. A simulation of the passage of a cold front is used to test the parameterization. The modeled ...

Particle size distribution and dislocation density determined by high resolution X-ray diffraction in nanocrystalline silicon nitride powders

Abstract: Two silicon nitride powders were investigated by high resolution X-ray diffraction. The first sample was crystallized from the powder prepared by the vapour phase reaction of silicon tetrachloride and ammonia while the second was a commercial powder produced by the direct nitridation of silicon. Their particle size and dislocation density were obtained by the recently developed modified Williamson‐Hall and Warren‐Averbach procedures from X-ray diffraction profiles. Assuming that the particle size distribution is log-normal the size distribution function was calculated from the size parameters derived from X-ray diffraction profiles. The size distributions determined from TEM micrographs were in good correlation with the X-ray results. The area-weighted average particle size calculated from nitrogen adsorption isotherms was in good agreement with that obtained from X-rays. The powder produced by silicon nitridation has a wider size distribution with a smaller average size than the powder prepared by vapour phase reaction. The dislocation densities were found to be between about 10 14 and 10 15 m 2 . Published by Elsevier Science S.A. All rights reserved.

Fine hollow glass sphere and method for preparing the same

Abstract: It is an object of the present invention to provide hollow glass microspheres having particle properties of a low particle density and grain size properties of a sharp grain size distribution, having a high homogeneity and having such strength that they are less likely to fracture during processing. In the present invention, a glass material containing a foaming agent and containing B 2 O 3 in an amount of from 9 to 20%, is subjected to wet grinding to obtain a slurry having an average particle size of at most 3 μm, droplets thereof are heated, to obtain hollow glass microspheres having an average particle size of at most 15 μm, a maximum particle size of at most 45 μm, a particle density of at most 0.5 g/cm 3 and a grain size gradient of at most 2.

Effect of particle size distribution on pile tip resistance in calcareous sand in the geotechnical centrifuge

Abstract: Until recently, the micro mechanical origins of soil behaviour have remained illusive, but it is now known that that the constitutive behaviour of a soil is largely determined by its particle size distribution. This paper examines the specific boundary problem associated with the penetration of a model pile into two different gradings of dry calcareous sand in a geotechnical centrifuge, in or- der to establish the effect of the inclusion of fine particles on the pile end bearing resistance. The first grading of sand comprised particles smaller than 0.5mm; the sec- ond grading contained particles of nominal size d such that 0.15mm