Volume Ratio Of Particles Research Articles

Size control and catalytic activity of bio-supported palladium nanoparticles

The development of nanoparticles has greatly improved the catalytic properties of metals due to the higher surface to volume ratio of smaller particles. The production of nanoparticles is most commonly based on abiotic processes, but in the search for alternative protocols, bacterial cells have been identified as excellent scaffolds of nanoparticle nucleation, and bacteria have been successfully employed to recover and regenerate platinum group metals from industrial waste. We report on the formation of bio-supported palladium (Pd) nanoparticles on the surface of two bacterial species with distinctly different surfaces: the gram positive Staphylococcus sciuri and the gram negative Cupriavidus necator. We investigated how the type of bacterium and the amount of biomass affected the size and catalytic properties of the nanoparticles formed. By increasing the biomass:Pd ratio, we could produce bio-supported Pd nanoparticles smaller than 10nm in diameter, whereas lower biomass:Pd ratios resulted in particles ranging from few to hundreds of nm. The bio-supported Pd nanoparticle catalytic properties were investigated towards the Suzuki-Miyaura cross coupling reaction and hydrogenation reactions. Surprisingly, the smallest nanoparticles obtained at the highest biomass:Pd ratio showed no reactivity towards the test reactions. The lack of reactivity appears to be caused by thiol groups, which poison the catalyst by binding strongly to Pd. Different treatments intended to liberate particles from the biomass, such as burning or rinsing in acetone, did not re-establish their catalytic activity. Sulphur-free biomaterials should therefore be explored as more suitable scaffolds for Pd(0) nanoparticle formation.

Uptake of Ozone to Deliquesced KI and Mixed KI/NaCl Aerosol Particles

The kinetics of uptake of ozone to deliquesced potassium iodide (KI) aerosol particles has been investigated in an aerosol flow tube at 72-75% relative humidity, room temperature, and atmospheric pressure. The observed loss of ozone was further analyzed in terms of a numeric model to explicitly track the iodide concentration in the particles. This allowed retrieving a value alpha(b) = 0.6 +/- (0.5)(0.4) for the bulk accommodation coefficient (alpha(b)). The second order rate constant in the bulk phase agreed with available literature (k(b) = (1.0 +/- 0.3) x 10(9) M(-1) s(-1)) even for the high ionic strength conditions of the present experiments. As long as iodide remained in excess, the average uptake coefficient was gamma = (1.10 +/- 0.20) x 10(-2). Different experiments were performed where the iodide to chloride ratio, the ozone concentration, and the surface to volume ratio of particles were varied. In combination, the results obtained indicate that uptake was driven by fast bulk accommodation and reaction in the bulk for all conditions investigated. The results further suggest that ozone uptake is not limited by the bulk accommodation coefficient alpha(b) under atmospheric conditions.

Evaluation of sand grain crushing in the sand dollar Mellita tenuis (Echinoidea: Echinodermata)

Sand dollars in the genus Mellita have been postulated to use their teeth to crush sedi- ments comprised of quartz. If so, sand dollars may have a greater impact on nearshore siliciclastic environments than generally thought. The present study compared the size distributions of quartz grains in the guts and immediate habitat of the common sand dollar Mellita tenuis in summer and winter in Port Saint Joseph Bay, Florida. In both seasons, sand dollars had significantly greater fre- quencies of the smaller size classes of grains in their guts. The hypothesis that sand dollars crushed quartz sand grains with their teeth was evaluated using light microscopy. We compared the degree of angularity of ingested grains with those collected from surrounding sediments. Among 6 individu- als examined (3 in summer and 3 in winter), only half (2 in summer and 1 in winter) had guts contain- ing more angular grains than those in the surrounding sediments. A comparison of more angular grains isolated from sand dollar guts with artificially broken grains using scanning electron microscopy indicated that gut-derived grains lacked freshly broken surfaces. In a laboratory study, after 72 h of feeding upon quartz sediments with the smallest size classes (<250 µm) removed, sand dollar guts were full but contained only trace amounts of sediments <250 µm. We conclude that the smaller size classes of grains found in the guts of M. tenuis are not the result of grain breakage. Rather, grains are selected pre- or post-ingestion, perhaps to optimize the surface to volume ratio of particles so as to maximize ingestion of microbial films that coat sand grains.

Effect of SiC Particles on the Machining of Aluminum/SiC Composite

The objective of this study is to experimentally investigate the effect of reinforced SiC particles on the machining of Aluminum/SiC composite (15% volume ratio of SiC particles with average grain size 15m). Aluminum/SiC composite and aluminum metal were milled by a tungsten carbide endmill in this study. Based on the surface observation and surface roughness inspection, it is found that the machining parameters of Aluminum/SiC composite have optimum values, and that the surface roughness of aluminum/SiC composite is smaller than that of aluminum metal. when feedrate and depth of cut are smaller than limited values, satisfactory surface finish can be attainable, however, as the depth of cut and feedrate increases, the microcracks are first initiated at the interface of SiC particles and aluminum matrix, and then periodically macrocracks are formed on the machined surface, The damage mechanism during the machining of aluminum/SiC are discussed in this paper.

Ink-Jet Printing of Cu−Ag-Based Highly Conductive Tracks on a Transparent Substrate

We have developed a Cu-Ag-based mixed metal conductive ink from which highly conductive tracks form on a flexible substrate after annealing at low temperature. Addition of small Ag particles significantly improves the particle packing density by filling the interstices formed between the larger Cu particles, which in turn facilitates better conductivity compared to pure Cu metal film. The particle size and volume ratio of the Ag particles added should be carefully controlled to achieve maximum packing density in the bimodal particle system, which is consistent with the theoretical considerations of the Furnas model. In addition, we demonstrate direct writing of complex patterns that exhibit high conductivity upon annealing at sufficiently low temperature (175-210 degrees C) to not damage the transparent plastic substrate such as polyethersulphone (PES).

Experimental and numerical investigations on the performance of dehumidifying desiccant beds composed of silica-gel and thermal energy storage particles

Enhanced efficiency of the adsorption process in the dehumidifier is a key element for improved performance of desiccant cooling systems. Due to the exothermic nature of the adsorption process, the dehumidification and cooling capacity are limited by significant temperature changes in the adsorption column. In the present study, the effects of integration of sensible and latent heat storage particles in the desiccant bed for in situ management of released adsorption heat are investigated. For this purpose, column experiments are performed using an initially dry granular bed made of silica-gel particles or a homogeneous mixture of silica gel and inert sensible or latent heat storage particles. The packed bed is subject to a sudden uniform air flow at selected values of temperature and humidity. Also, a packed bed numerical model is developed that includes the coupled non-equilibrium heat and moisture transfer in the solid and gas phases. Investigations of the heat and mass transfer characteristics are reported using the composite structure and the results are compared with the base case of simple silica gel bed. Improved desiccant cooling system performance can be obtained by appropriate adjustment of desiccant cycle operation and proper choice of the volume ratio of thermal energy storage particles.

Quantitative Optical Trapping of Single Gold Nanorods

We report a quantitative analysis of the forces acting on optically trapped single gold nanorods. Individual nanorods with diameters between 8 and 44 nm and aspect ratios between 1.7 and 5.6 were stably trapped in three dimensions using a laser wavelength exceeding their plasmon resonance wavelengths. The interaction between the electromagnetic field of an optical trap and a single gold nanorod correlated with particle polarizability, which is a function of both particle volume and aspect ratio.

On Linear Elastic Fragmentation Mechanics Under Hydrostatic Compression

In this letter we apply the Griffith's energy balance of Linear Elastic Fracture Mechanics to predict the hydrostatic compressive strength of granular matter, as a first step towards a Linear Elastic Fragmentation Mechanics. The result is a simple law for the critical pressure, in strict analogy to the Griffith's strength prediction, found to be mainly a function of the surface to volume ratio of particles. The influence of compaction and fragmentation evolution on the compressive strength is also quantified. Thus, this letter presents one possible approach to fragmentation mechanics.

Strain effects on percolation conduction in conductive particle filled composites

The effect of uniaxial and multiaxial mechanical strain on the electrical conductivity of particle filled polymer composites is investigated in the framework of concentration-driven percolation. For composites consisting of low aspect ratio, rigid conductive particles in a compliant polymer matrix, a simple argument leads to the conclusion that the effective volume fraction of conductive particles (the ratio of total particle volume to the total volume of the deformed composite) plays a dominant role, with conductivity remaining isotropic despite the directional bias of the strain state. As such, conductivity is expected to exhibit classical power, law-dependence on concentration, which in this case takes the form of a strain-dependent effective volume fraction. Consideration of deformation effects on particle agglomerates suggest, however, that particle-to-particle network connections are likely to be affected most significantly along directions experiencing the most severe strains, introducing a directional bias in network connectivity at a higher length scale. To assess the importance of this possible directional bias, random resistor network models are used to study the conductivity of uniaxially strained composites. For conservative assumptions on the severity of the bias in bond probabilities, network conductivities exhibit approximately isotropic, concentration-driven behavior for moderate strains, supporting the predictive utility of the simple percolation conduction-effective volume fraction approach. Further corroboration is provided by experiments in the literature on silicone-graphite composites subjected to uniaxial compressive strain, where good agreement is obtained through moderate strains for the theoretically correct value of the conduction exponent in concentration-driven percolation.

Design of oxide nanoparticles by aqueous chemistry

The elaboration of nanoparticles designed for technological applications in various fields such as catalysis, optics, magnetism, electronics… needs the strict control of their characteristics, especially chemical composition, crystalline structure, size, and shape. These characteristics bring the physical properties (color, magnetism, band gap…) of the material, and also the surface to volume ratio of particles which is of high importance when they are used as a chemically active or reactive support, in catalysis for instance. The nanoparticles may have also to be surface functionalized by various species, and/or dispersed in aqueous or non aqueous media. We will show that the aqueous chemistry of metal cations is a very versatile and attractive way for the design of oxide nanomaterials, allowing the control of size, shape, and crystalline structure for polymorphic materials. Aqueous surface chemistry, including adsorption of various species, may be used to modify the morphology of nanoparticles. In some cases, redox processes can be involved to control the morphology of nanoparticles. Technologically important nanomaterials such as titania, alumina, and iron oxides are studied.

Increase of SiC sublimation growth rate by optimizing of powder packaging

In this paper, a comprehensive model for silicon carbide (SiC) sublimation crystal growth considering powder packaging is presented. This model is based on heat and mass transfer of porous powder charge with different sizes of the particle and accounts for induction heating, powder charge sublimation, vapor transport, and porosity evolution in a SiC sublimation crystal growth system. The mechanism of vapor transport is proposed by introducing a driving force arising from temperature difference between powder charge and seed in the growth system. Powder porosity evolution and sublimation rate variations are predicted based on vapor transport mechanism. Effects of powder geometry, such as particle sizes, volume and size ratios of different particles, and driving forces on the sublimation rate are studied. In addition, simulation results for powder sublimation with and without a central hole are presented. The results indicate that the sublimation rate can be increased by optimizing the powder packaging, or by creating a hole in the center of packed powder.

Cell optimization for fast contact detection in the discrete element method algorithm

The objective of this paper is to find out a rule of cell optimization for fast contact detection in the discrete element method (DEM) algorithm. The contact detection process was executed by using the conventional cell model or distinct cell model (DCM) under several particle conditions having different particle size ratio and volume ratio, and its time was measured. The conventional cell model is suitable for the conditions that particle size ratio, d large/ d small, is small and its volume ratio, V small/ V 0, is also small. The optimum cell condition of the conventional cell model is that the mean number of particle in a cell is 1.0–5.0, regardless of the particle size ratio and its volume ratio. Contact detection using DCM, which has several grids for different sized particles, is much faster than that of the conventional model when particles have a large size ratio. The size of the larger cell does not affect the contact detection speed; however, the size of the smaller cell is important for the fast contact detection and its size is determined to be about 3.0 times the particle radius. Therefore, the optimum cell condition is estimated from the particle size ratio and its volume ratio, and a large-scale DEM will be possible.

Zwanzig model of multi-component mixtures of biaxial particles: y 3 theory re-visited

The paper considers the thermodynamic and phase ordering properties of a multi-component Zwanzig mixture of hard rectangular biaxial parallelepipeds. An equation of state (EOS) is derived based on an estimate of the number of arrangements of the particles on a three- dimensional cubic lattice. The methodology is a generalization of the Flory–DiMarzio counting scheme, but, unlike previous work, this treatment is thermodynamically consistent. The results are independent of the order in which particles are placed on the lattice. By taking the limit of zero lattice spacing, a translationally continuous variant of the model (the off-lattice variant) is obtained. The EOS is identical to that obtained previously by a wide variety of different approaches. In the off-lattice limit, it corresponds to a third-level y-expansion and, in the case of a binary mixture of square platelets, it also corresponds to the EOS obtained from fundamental measure theory. On the lattice it is identical to the EOS obtained by retaining only complete stars in the virial expansion. The off-lattice theory is used to study binary mixtures of rods (R 1 − R 2) and binary mixtures of platelets (P 1 − P 2). The particles were uniaxial, of length (thickness) L and width D. The aspect ratios Γ i = Li /Di of the components were kept constant (Γ1 R = 15, Γ1 P = 1/15 and Γ2 R = 150, Γ2 P = 1/150), so the second virial coefficient of R 1 was identical to P 1 and similarly for R 2 and P 2. The volume ratio of particles 1 and 2, v 1/v 2, was then varied, with the constraints that viR = viP and Results on nematic–isotropic (N − I) phase coexistence at an infinite dilution of component 2, are qualitatively similar for rods and platelets. At small values of the ratio v 1/v 2, the addition of component 2 (i.e. a thin rod (e.g. a polymer) or a thin plate) results in the stabilization of the nematic phase. For larger values of v 1/v 2, however, this effect is reversed and the addition of component 2 destabilizes the nematic. For similar molecular volumes of the two components strong fractionation is observed: shorter rods and thicker platelets congregate in the isotropic phase. In general, the stabilization of the ordered phase and the fractionation between the phases are both weaker in the platelet mixtures. The calculated spinodal curves for isotropic-isotropic demixing are noticeably different between the R 1 − R 2 and the P 1 − P 2 systems. The platelet mixtures turn out to be stable with respect to de-mixing up to extremely high densities. The values of the consolute points for the R 1 − R 2 blends are remarkably similar to those obtained using the Parsons–Lee approximation for bi-disperse mixtures of freely rotating cylinders with similar aspect ratios [S. Varga. A. Galindo, G. Jackson, Mol. Phys., 101, 817 (2003)]. In a number of R 1 − R 2 mixtures, phase diagrams exhibiting both N − I equilibrium and I − I de-mixing were calculated. The latter is pre-empted by nematic ordering in all the cases studied. Calculations show the possible appearance of azeotropes in the N − I coexistence domain.

Mineral particle size as a control on aerosol iron solubility

Aerosol iron solubility is a major uncertainty in the global biogeochemical cycle of iron and, via its impact on ocean productivity, the carbon cycle and their influence on global climate. Previous studies have reported widely different values for this solubility (0.01 – 80%). Here we show that the primary control on aerosol iron solubility is the surface area to volume ratio of mineral aerosol particles, which changes during atmospheric transport as mineral aerosol concentration decreases due to preferential removal of larger particles (assuming particle morphology to be relatively constant with particle size). This important result indicates that aerosol iron solubility is not fixed, but will change predictably as an inverse function of dust concentration on both spatial and temporal (e.g. glacial – interglacial) scales.

Dimensionless heat transfer correlations for high viscosity fluid-particle mixtures in cans during end-over-end rotation

Data on heat transfer coefficients associated with heat transfer in canned particulates in high viscosity Newtonian and non-Newtonian fluids were used for developing dimensionless correlations. Conventional calculation methods usually pose calculation problems under such conditions and hence apparent h ap and U a were used. In order to accommodate non-Newtonian fluids, generalized Reynolds (GRe) and generalized Prandtl numbers (GPr) were used in the developed correlations. The effects of can size and particle material were accounted by including the ratio of the length of the can to the diameter of the can ( L c/ D c) and the ratio of the particle density to the liquid density ( ρ P/ ρ l). For h ap in non-Newtonian fluids, Nusselt number ( Nu) was correlated with GRe, GPr, Froude number ( Fr), L c/ D c, the ratio of particle volume to the can volume ( V p/ V c) and ρ p/ ρ l. For U a in non-Newtonian fluids, Nu was related to GRe, GPr, Fr, ρ p/ ρ l and V p/ V c. For h ap in Newtonian fluids, Nu was correlated to Re, Pr, Fr, L c/ D c and ρ P/ ρ l. For U a in Newtonian fluids, Nu was correlated to Re, Pr and Fr. Combinations of the rotation radius, the diameter of the can and the particle radius were chosen as the characteristic length for the correlations. The developed equations showed a good agreement with experimental data.

A study on thermal conductivity of a mixture of magnesia particles and molten nitrate by a transient hot wire method

AbstractIn this study, the effective thermal conductivity of a mixture of magnesia particles and molten nitrate used in a high temperature thermal energy storage system was investigated by a transient hot wire method. The effective thermal conductivity of the mixture was around 2.0W/(m·K) in the temperature range of 423 K to 703 K, although it decreased about 5% with increasing temperature. This value was about 10 times larger than that of the packed bed of magnesia particles including air. The effective thermal conductivity increased about 3% with a 1% increase in the volume ratio of magnesia particles in the mixture with molten nitrate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(4): 245–253, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20116

Radar scattering by aggregate snowflakes

AbstractThe radar scattering properties of realistic aggregate snowflakes have been calculated using the Rayleigh–Gans theory. We find that the effect of the snowflake geometry on the scattering may be described in terms of a single universal function, which depends only on the overall shape of the aggregate and not the geometry or size of the pristine ice crystals which compose the flake. This function is well approximated by a simple analytic expression at small sizes; for larger snowflakes we fit a curve to our numerical data. We then demonstrate how this allows a characteristic snowflake radius to be derived from dual wavelength radar measurements without knowledge of the pristine crystal size or habit, while at the same time showing that this detail is crucial to using such data to estimate ice water content. We also show that the ‘effective radius’, characterizing the ratio of particle volume to projected area, cannot be inferred from dual wavelength radar data for aggregates. Finally, we consider the errors involved in approximating snowflakes by ‘air–ice spheres', and show that for small enough aggregates the predicted dual wavelength ratio typically agrees to within a few percent, provided some care is taken in choosing the radius of the sphere and the dielectric constant of the air–ice mixture; at larger sizes the radar becomes more sensitive to particle shape, and the errors associated with the sphere model are found to increase accordingly. Copyright © 2006 Royal Meteorological Society.

Bulk Scattering Properties for the Remote Sensing of Ice Clouds. Part I: Microphysical Data and Models

Abstract This study reports on the use of in situ data obtained in midlatitude and tropical ice clouds from airborne sampling probes and balloon-borne replicators as the basis for the development of bulk scattering models for use in satellite remote sensing applications. Airborne sampling instrumentation includes the two-dimensional cloud (2D-C), two-dimensional precipitation (2D-P), high-volume precipitation spectrometer (HVPS), cloud particle imager (CPI), and NCAR video ice particle sampler (VIPS) probes. Herein the development of a comprehensive set of microphysical models based on in situ measurements of particle size distributions (PSDs) is discussed. Two parameters are developed and examined: ice water content (IWC) and median mass diameter Dm. Comparisons are provided between the IWC and Dm values derived from in situ measurements obtained during a series of field campaigns held in the midlatitude and tropical regions and those calculated from a set of modeled ice particles used for light-scattering calculations. The ice particle types considered in this study include droxtals, hexagonal plates, solid columns, hollow columns, aggregates, and 3D bullet rosettes. It is shown that no single habit accurately replicates the derived IWC and Dm values, but a mixture of habits can significantly improve the comparison of these bulk microphysical properties. In addition, the relationship between Dm and the effective particle size Deff, defined as 1.5 times the ratio of ice particle volume to projected area for a given PSD, is investigated. Based on these results, a subset of microphysical models is chosen as the basis for the development of ice cloud bulk scattering models in Part II of this study.

Viscosity of Electrostatically Stabilized Dispersions of Monodispersed, Bimodal, and Trimodal Silica Particles

Effect of particle size and polydispersity on the viscosity and maximum packing fraction of aqueous colloidal dispersions has been studied. For dispersions of mono‐sized particles, the results indicate that there is a linear relationship between the log(η) (viscosity) and particle size at a fixed shear rate and volume fraction of solids. However, there is a particle diameter at which there is a decrease in the dependency of viscosity on particle size as the slope of the linear plots of log(η) versus particle diameter changes to a smaller value. Preliminary calculations indicate that this particle size may correspond to a separation distance at which electrostatic energy as compared with the thermal energy of the particles can be ignored. In the case of bimodal dispersions, the viscosity is affected by both absolute size and the ratio of the two sizes. The effect of particle size ratio on the viscosity was investigated using bimodal dispersions of the same size coarse particles, but fines of different sizes. There is a critical volume ratio below which bimodal dispersions of larger size ratios show lower viscosities than systems of smaller size ratios. Above this volume ratio of the two sizes, the trend becomes reversed and the fines will have a dominant effect on the viscosity behavior of the bimodal system. Statistically designed experiments were carried out using trimodal mixtures of monodispersed silica particles and it was shown that tridispersed suspensions demonstrate similar behavior as bidispersed suspensions, with a minimum in viscosity observed as a function of particle volume ratio.

Increased Cytokine Secretion in Patients with Failed Implants Compared with Patients with Primary Implants

All total joint replacements generate wear debris; yet, some implant prostheses fail while others survive despite the presence of ultrahigh molecular weight polyethylene particulate. It was hypothesized that patients with failed hip implants who have osteolysis will secrete higher inflammatory cytokines than patients receiving total joint replacements. Our study evaluated the peripheral blood monocyte response to varying polyethylene particle volume ratios through cytokine quantification in two patient populations: patients having revision surgery for failed total hip replacements (failed implant group) and patients having primary total hip surgery for osteoarthritis of the hip (primary implant group). We observed elevation of all three proinflammatory cytokines tested (interleukin-6, interleukin-1, and tumor necrosis factor-alpha) in response to polyethylene particulate challenge when compared with the controls in both patient groups. The population with failed implants also had a higher absolute cytokine response to polyethylene exposure compared with the control patients having primary implants. These findings suggest that patients with failed implants have a greater inflammatory cytokine response to polyethylene than seen in patients with primary implants.