Showing papers on "Particle published in 1999"

Measuring Thermal Conductivity of Fluids Containing Oxide Nanoparticles

Abstract: Oxide nanofluids were produced and their thermal conductivities were measured by a transient hot-wire method. The experimental results show that these nanofluids, containing a small amount of nanoparticles, have substantially higher thermal conductivities than the same liquids without nanoparticles. Comparisons between experiments and the Hamilton and Crosser model show that the model can predict the thermal conductivity of nanofluids containing large agglomerated Al{sub 2}O{sub 3} particles. However, the model appears to be inadequate for nanofluids containing CuO particles. This suggests that not only particle shape but size is considered to be dominant in enhancing the thermal conductivity of nanofluids.

Electrodynamics of Noble Metal Nanoparticles and Nanoparticle Clusters

Abstract: In this paper we examine the electrodynamics of silver nanoparticles and of clusters of nanoparticles, with an emphasis on extinction spectra and of electric fields near the particle surfaces that are important in determining surface-enhanced Raman (SER) intensities. The particles and clusters are chosen to be representative of what has been studied in recent work on colloids and with lithographically prepared particles. These include spheres, spheroids, truncated tetrahedrons, and clusters of two or three of these particles, with sizes that are too large to be described with simple electrostatic approximations but small compared to the wavelength of light. The electrodynamics calculations are mostly based on the discrete dipole approximation (DDA), which is a coupled-finite element approach which produces exact or nearly exact results for particles of arbitrary size and shape if fully converged. Mie theory results are used to study the validity of the DDA for spherical particles, and we also study the validity of the modified long wavelength approximation (MLWA), which is based on perturbative corrections to the electrostatic limit, and of the single dipole per particle approximation (SDA). The results show how the dipole plasmon resonance properties and the electric field contours around the particle vary with particle shape and size for isolated particles. For clusters of particles, we study the effect of interparticle spacing on plasmon resonance characteristics. We also show that the quadrupole resonance is much less sensitive to particle shape and interparticle interactions than the dipole plasmon resonance. These results provide benchmarks that will be used in future comparisons with experiment.

Effects of mixing on extinction by carbonaceous particles

Abstract: Reported values for the absorption cross section of particulate carbon per unit mass range from under 4 to over 20 m2/g, and the intermediate value of 10 m2/g is used by many as a standard gram-specific absorption cross section for atmospheric soot. In order to better understand the possible variations in absorption by atmospheric carbon, we reevaluated its optical properties in terms of the material composition and morphology of soot and the electrodynamics of spherules agglomerated into loose (ramiform) aggregates. Primary particles ranging in composition from paracrystalline graphite to low-density air/graphite volume mixtures are considered. The effects on extinction efficiency of aggregation and of internal mixing of carbon with sulfate are considered in detail. We also compare our results with estimates of specific absorption of internally mixed soot that are based on several homogeneous mixing rules (effective medium approximations), On the basis of our modeling of the optical properties of aggregates of graphitic carbon grains, we conclude that 10 m2/g may be over 50% too high in many cases, and we suggest that the mass absorption coefficient for the light-absorbing carbon in diesel soot at a wavelength of 0.550 μm may often be less than 7 m2/g, although variations in optical constants and, especially, the specific gravity of the absorbing material make it difficult to assign a specific numerical value. Adhesion of carbon grains to sulfate droplet surfaces is expected to enhance their absorption by no more than about 30%. Soot randomly positioned within droplets, however, can display averaged absorption enhancement factors of about 2.5–4 for hosts with refractive indices ranging from 1.33–1.53, respectively, and radii ≳0.20 μm. Nonetheless, calculations indicate that for realistic dry particle populations, αa < 10 m2/g for graphitic carbon in the atmosphere unless (1) most of it is encapsulated, and (2) the geometric mean radius of the hosts is larger than about 0.06 μm (which corresponds to a mass median diameter of 0.34 μm). These results suggest the importance of the determination of the physical state of the soot particles and their immediate environment when ascribing characteristic values for their absorption and scattering efficiencies.

Bilayer Surfactant Stabilized Magnetic Fluids: Synthesis and Interactions at Interfaces

Abstract: Aqueous magnetic fluids were synthesized by a sequential process involving the chemical coprecipitation of Fe(II) and Fe(III) salts with ammonium hydroxide (NH4OH) followed by resuspension of the ultrafine particles in water using fatty acids. This procedure produced Fe3O4 nanoparticles stabilized against agglomeration by bilayers of n-alkanoic acids with 9−13 carbons encapsulating the metal particles. The magnetic properties and particle size and size distributions of these magnetic fluids, characterized by transmission electron microscopy and superconducting quantum interference device, indicated the formation of single-domain nanoparticles of mean diameter ∼9.3 and ∼7.5 nm, respectively; the difference in values determined by the two methods implies the presence of a nonmagnetic layer on the particle surface. Thermogravimetric analysis measurements showed the existence of two distinct populations of surfactants on the particle surface, each having surfactant coverage of ∼21−24 A2/molecule, that was con...

Spatial correlations of mobility and immobility in a glass-forming Lennard-Jones liquid

Abstract: Using extensive molecular dynamics simulations of an equilibrium, glass-forming Lennard-Jones mixture, we characterize in detail the local atomic motions. We show that spatial correlations exist among particles undergoing extremely large (``mobile'') or extremely small (``immobile'') displacements over a suitably chosen time interval. The immobile particles form the cores of relatively compact clusters, while the mobile particles move cooperatively and form quasi-one-dimensional, stringlike clusters. The strength and length scale of the correlations between mobile particles are found to grow strongly with decreasing temperature, and the mean cluster size appears to diverge near the mode-coupling critical temperature. We show that these correlations in the particle displacements are related to equilibrium fluctuations in the local potential energy and local composition.

Impact of high velocity cold spray particles

Abstract: This article presents experimental data and a computational model of the cold spray solid particle impact process. Copper particles impacting onto a polished stainless steel substrate were examined in this study. The high velocity impact causes significant plastic deformation of both the particle and the substrate, but no melting was observed. The plastic deformation exposes clean surfaces that, under the high impact pressures, result in significant bond strengths between the particle and substrate. Experimental measurements of the splat and crater sizes compare well with the numerical calculations. It was shown that the crater depth is significant and increases with impact velocity. However, the splat diameter is much less sensitive to the impact velocity. It was also shown that the geometric lengths of the splat and crater scale linearly with the diameter of the impacting particle. The results presented will allow a better understanding of the bonding process during cold spray.

Airborne minerals and related aerosol particles: effects on climate and the environment.

Abstract: Aerosol particles are ubiquitous in the troposphere and exert an important influence on global climate and the environment. They affect climate through scattering, transmission, and absorption of radiation as well as by acting as nuclei for cloud formation. A significant fraction of the aerosol particle burden consists of minerals, and most of the remainder— whether natural or anthropogenic—consists of materials that can be studied by the same methods as are used for fine-grained minerals. Our emphasis is on the study and character of the individual particles. Sulfate particles are the main cooling agents among aerosols; we found that in the remote oceanic atmosphere a significant fraction is aggregated with soot, a material that can diminish the cooling effect of sulfate. Our results suggest oxidization of SO2 may have occurred on soot surfaces, implying that even in the remote marine troposphere soot provided nuclei for heterogeneous sulfate formation. Sea salt is the dominant aerosol species (by mass) above the oceans. In addition to being important light scatterers and contributors to cloud condensation nuclei, sea-salt particles also provide large surface areas for heterogeneous atmospheric reactions. Minerals comprise the dominant mass fraction of the atmospheric aerosol burden. As all geologists know, they are a highly heterogeneous mixture. However, among atmospheric scientists they are commonly treated as a fairly uniform group, and one whose interaction with radiation is widely assumed to be unpredictable. Given their abundances, large total surface areas, and reactivities, their role in influencing climate will require increased attention as climate models are refined.

Particle Velocity and Deposition Efficiency in the Cold Spray Process

Abstract: Copper powder was sprayed by the cold gas-dynamic method. In-flight particle velocities were measured with a laser two-focus system as a function of process parameters such as gas temperature, gas pressure, and powder feed rate. Mean particle velocities were uniform in a relatively large volume within the plume and agreed with theoretical predictions. The presence of a substrate was found to have no significant effect on in-flight particle velocities prior to impact. Cold-spray deposition efficiencies were measured on aluminum substrates as a function of particle velocity and incident angle of the plume. Deposition efficiencies of up to 95% were achieved. The critical velocity for deposition was determined to be about 640 m/s for the system studied.

Shutter mode microencapsulated electrophoretic display

Abstract: An electrophoretic display element includes a capsule having a first, larger surface and a second, smaller surface. The capsule contains a suspending fluid and at least one particle dispersed within said suspending fluid. Application of a first electrical field causes the particle or particles to migrate towards the first, larger surface of the capsule, causing it to take on the visual appearance of the particles. Application of a second electrical field causes the particle or particles to migrate towards the second, smaller surface, allowing the capsule to take on the visual appearance of the dispersing fluid or of a substrate or electrode positioned behind the display element. Displays may be fabricated from multiple display elements arranged on a substrate.

Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation

Abstract: The use of “equivalent” spheres to represent the scattering and absorption properties of nonspherical particles has been unsatisfactory in the past because the sphere of equal volume has too little surface area and thus too little scattering, whereas the sphere of equal area has too much volume giving too much absorption. Their asymmetry factors are also too large. These problems can largely be avoided if the real cloud of nonspherical particles is represented by a model cloud of spheres where the model cloud contains the same total surface area as well as the same total volume. Each nonspherical particle is then represented not by just one sphere but rather by a collection of independent spheres that has the same volume-to-surface-area (V/A) ratio as the nonspherical particle. To demonstrate the broad utility of this approach, we show results for ice, whose absorption coefficient varies with wavelength by 8 orders of magnitude. Randomly oriented infinitely long circular cylinders are used as a test case because an exact solution is available for all size parameters. The extinction efficiency and single-scattering coalbedo are closely approximated by the values for equal-V/A spheres across the ultraviolet, visible, and infrared from 0.2 to 50 μm wavelength; the asymmetry factor is matched somewhat less well. Errors in hemispheric reflectance, absorptance, and transmittance are calculated for horizontally homogeneous clouds which cover the range of crystal sizes and optical depths from polar stratospheric clouds through cirrus clouds to surface snow. The errors are less than 0.05 at all wavelengths over most of this space.

Growing Small Silver Particle as Redox Catalyst

Abstract: Growing small particles of silver have been observed to be more efficient catalysts than stable colloidal particles. These growing particles catalyze the borohydride reduction of several organic dy...

Effects of Particle Size of TiO2 on Photocatalytic Degradation of Methylene Blue in Aqueous Suspensions

Abstract: The influence of particle size of TiO2 on the photocatalytic degradation of methylene blue (MB) in a suspended aqueous solution has been studied. The results suggested that the adsorption rate and adsorbability of MB on suspended TiO2 particles increased as the particle sizes of TiO2 decreased. Photocatalytic activity of TiO2 also increased as the particle size of TiO2 became smaller, especially when the particle size is less than 30 nm. The half-life (t0.5) of the photocatalytic degradation of MB also decreased as the particle sizes of TiO2 decreased. The first-order reaction rate constant for photodegradation of MB increased as the particle size of TiO2 decreased. The initial degradation rate of MB in a suspended model was higher than that of a fixed-bed model. This will overcome the difficulty of preparation of ultrafine TiO2 catalyst particles. Once the problem of separation of fine TiO2 particles is solved, a TiO2 suspended photoreactor could be provided on an industrial basis.

Shape representation of axi‐symmetrical, non‐spherical particles in discrete element simulation using multi‐element model particles

Abstract: A new method of representing non‐spherical, smooth‐surfaced, axi‐symmetrical particles in discrete element (DE) simulation using model particles comprising overlapping spheres of arbitrary size whose centres are fixed in position relative to each other along the major axis of symmetry of the particle is presented. Contact detection and calculation of force‐deformation and particle movement is achieved using standard DE techniques modified to integrate the behaviour of each element sphere with that of the multi‐element particle to which it belongs. The method enables the dynamic behaviour of particles of high aspect ratio and irregular curvature (in two dimensions) to be modelled. The use of spheres to represent a particle takes advantage of the computational speed and accuracy of contact detection for spheres, which should make the method comparable in computational efficiency to alternative schemes for representing non‐spherical particles.

Size and Composition Distribution of Fine Particulate Matter Emitted from Wood Burning, Meat Charbroiling, and Cigarettes

Abstract: A dilution source sampling system is augmented to measure the size-distributed chemical composition of fine particle emissions from air pollution sources. Measurements are made using a laser optical particle counter (OPC), a differential mobility analyzer/condensation nucleus counter (DMA/CNC) combination, and a pair of microorifice uniform deposit impactors (MOUDIs). The sources tested with this system include wood smoke (pine, oak, eucalyptus), meat charbroiling, and cigarettes. The particle mass distributions from all wood smoke sources have a single mode that peaks at approximately 0.1−0.2 μm particle diameter. The smoke from meat charbroiling shows a major peak in the particle mass distribution at 0.1−0.2 μm particle diameter, with some material present at larger particle sizes. Particle mass distributions from cigarettes peak between 0.3 and 0.4 μm particle diameter. Chemical composition analysis reveals that particles emitted from the sources tested here are largely composed of organic compounds. N...

Chemical and thermal freeze-out parameters from 1 A to 2 0 0 A GeV

Abstract: The present knowledge about hadrons produced in relativistic heavy ion collisions is compatible with chemical freeze-out happening when the energy density divided by the particle density reaches the value of 1 GeV. This observation is used to determine the energy dependence of the chemical freeze-out parameters ${T}_{\mathrm{ch}}$ and ${\ensuremath{\mu}}_{B}^{\mathrm{ch}}$ for beam energies varying between 1 and $200A$ GeV. The consequences of this energy dependence are studied for various particle ratios. Predictions for particle ratios at beam energy $40A$ GeV are presented. The conditions for thermal freeze-out are also determined. These correspond either to an energy density of 45 ${\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{f}\mathrm{m}}^{3}$ or to a particle density of $0.05/{\mathrm{fm}}^{3}.$

Formulation and physical characterization of large porous particles for inhalation.

Abstract: Purpose. Relatively large (>5 µm) and porous (mass density < 0.4 g/cm3) particles present advantages for the delivery of drugs to the lungs, e.g., excellent aerosolization properties. The aim of this study was, first, to formulate such particles with excipients that are either FDA-approved for inhalation or endogenous to the lungs; and second, to compare the aerodynamic size and performance of the particles with theoretical estimates based on bulk powder measurements. Methods. Dry powders were made of water-soluble excipients (e.g., lactose, albumin) combined with water-insoluble material (e.g., lung surfactant), using a standard single-step spray-drying process. Aerosolization properties were assessed with a Spinhaler TM device in vitro in both an Andersen cascade impactor and an AerosizerTM.. Results. By properly choosing excipient concentration and varying the spray drying parameters, a high degree of control was achieved over the physical properties of the dry powders. Mean geometric diameters ranged between 3 and 15 µm, and tap densities between 0.04 and 0.6 g/cm3. Theoretical estimates of mass mean aerodynamic diameter (MMAD) were rationalized and calculated in terms of geometric particle diameters and bulk tap densities. Experimental values of MMAD obtained from the AerosizerTM most closely approximated the theoretical estimates, as compared to those obtained from the Andersen cascade impactor. Particles possessing high porosity and large size, with theoretical estimates of MMAD between 1−3 µm, exhibited emitted doses as high as 96% and respirable fractions ranging up to 49% or 92%, depending on measurement technique. Conclusions. Dry powders engineered as large and light particles, and prepared with combinations of GRAS (generally recognized as safe) excipients, may be broadly applicable to inhalation therapy.

Laboratory studies of particle nucleation: Initial results for H2SO4, H2O, and NH3 vapors

Abstract: Particle formation in the binary H2SO4-H2O vapor system was studied at 295 K in a series of experiments employing a flow reactor. The concentration of H2SO4 was detected by chemical ionization mass spectrometry, and an ultrafine particle condensation nucleus counter was used to count the newly nucleated particles. Results yield a particle formation rate that is approximately proportional to [H2SO4] raised to the eighth power and to [H2O] raised to the fifth power. The power dependencies measured here are significantly different from those determined in previous experimental work, and furthermore, the water dependence is markedly different from that predicted from current theories. The effect of adding ammonia vapor to the binary system was investigated; concentrations of NH3 in the many tens of parts per trillion by volume range were observed to promote dramatically the rate of particle nucleation.

Estimation of mean size and shape of small metal particles by EXAFS

Abstract: The possibility to estimate the mean size and shape of metal particles from their average coordination number, which is typically obtained from EXAFS (extended X-ray absorption fine structure) analysis, is evaluated. While the number of neighbors in the first and second coordination shell were found to be not significantly dependent on the particle shape, and thus can be used to estimate the particle size, a substantial influence of the particle shape was found to be present for the coordination numbers of higher neighboring shells. Consequently, a method using the coordination number of the first or second shell (N1 or N2) to estimate the average particle size and that of a higher shell (N3 or N4) to estimate the particle shape is presented.

Kinetic spray coatings

Abstract: Coatings have been produced by entraining metal powders in an air flow which is accelerated by a de Laval type of nozzle. The particles are not melted or thermally softened prior to impingement onto the substrate. The coating process depends primarily on the kinetic energy of the incident powders. The coatings have low oxide content and low thermal stress, and can exhibit relatively low porosity and high adhesion. The mechanism by which the coatings are formed is not well understood, and it is the goal of this work to provide some insights into this mechanism. We have produced a new high-velocity spray apparatus which allows the spray parameters to be controlled and monitored for the first time. This, together with our simulations of air and particle velocities and temperatures, has provided new information on the coating process. Al, Cu, and Fe powders were sprayed onto Al, brass, Cu, and steel substrates. A threshold behavior was observed for coating deposition as a function of nozzle inlet air temperature, with a roughly linear behavior above the threshold. Results are obtained as a function of nozzle inlet air pressure and temperature, powder feed rate, and nozzle–substrate stand-off distance. The effect of the choice of substrate metal was relatively weak in our experiments. Results seem consistent with necessary inelastic processes such as plastic deformation and/or partial melting of the powder particles upon collision with the substrate. More research is needed to define the relative importance of these phenomena or of other possible mechanisms.

Physical and Chemical Properties of Magnetite and Magnetite-Polymer Nanoparticles and Their Colloidal Dispersions

Abstract: The properties of polymer-coated magnetite nanoparticles, which have the potential to be used as effective magnetic resonance contrast agents, have been studied. The magnetite particles were synthesized by using continuous synthesis in an aqueous solution. The polymer-coated magnetite nanoparticles were synthesized by seed precipitation polymerization of methacrylic acid and hydroxyethyl methacrylate in the presence of the magnetite nanoparticles. The particle size was measured by laser light scattering. It was shown that the particle size, variance, magnetic properties, and stability of aqueous magnetite colloidal dispersion strictly depend on the nature of the stabilizing agent. The average hydrodynamic radius of the magnetite particles was found to be 5.7 nm in the stable aqueous colloidal dispersion. An inclusion of the magnetite particle into a hydrophilic polymeric shell increases the stability of the dispersion and decreases the influence of the stabilizing agent on the magnetic and structural properties of the magnetite particles as was shown by X-ray diffraction and Mossbauer and IR spectroscopy, as well as by vibrating sample magnetometry. The variation in the polymeric shell size and the polymer net density can be useful tools for evaluation of the polymer-coated magnetite particles as effective contrast agents.

Investigation of ultrafine particle formation during diesel exhaust dilution

Abstract: Measurements of the size distribution of particles emitted from a modern heavy duty diesel engine using fuel with a sulfur content of between 0.03 and 0.05% by mass have been made under constant engine operating conditions, but with variations in the humidity of dilution air and dilution rato prior to particle size measurement. The results show clearly that the measured size distribution is crucially dependent upon the conditions of dilution, hence real difficulties for comparison of data between different investigators. Conditions of high dilution ratio and high relative humidity both tend to favor the production of nanoparticles, especially within the range below 50 nm diameter. Application of homogeneous nucleation theory shows that nanoparticle production during dilution is qualitatively consistent with the production of sulfuric acid, but the predicted nucleation rates are lower than those measured, in common with studies of nucleation in the atmosphere. Chemical analysis of size-fractionated particles shows enhancement of sulfate concentrations in humid dilution conditions and at high dilution ratios consistent with the above mechanism. The possible role of semivolatile organic compounds in these processes has not been investigated. (A)