Particle Mass Median Diameter Research Articles

A novel crucible-less inert gas atomisation method of producing titanium powder for additive manufacturing

ABSTRACTA novel low-cost gas atomisation technology producing spherical titanium powder (wire induction heating gas atomisation, WIGA) has been developed for additive manufacturing. Combined with the gas atomisation principle, the characteristics of WIGA were analysed. The effects of the gas pressure, metal temperature and the wire-feeding speed on the particle size of the titanium powder were studied. The results indicated that the decreases in mass median particle diameter (D50) and the increases in efficiency of fine size powders occurred with the increase in gas atomisation pressure and melting temperature and with the decrease of wire-feed speed. The optimum parameters are that the main gas pressure (P0) is 4.0 MPa, the degree of superheat of the metal melt is 350°C and the wire-feed speed is 50 mm s−1. On the condition, the D50 of titanium powder was 40.2 μm and powder morphology was spherical. Satellites rarely existed on the surface of particles.

Convection induced by vibrating rod in fine-powder bed

In this study, vibration-induced convection was studied experimentally using a fine powder with a mass median particle diameter of 8μm. A cylindrical rod arranged vertically in a powder container was vibrated horizontally with simple harmonic motion at a frequency of 300Hz using a piezoelectric vibrator. For a vibration amplitude of 10μm, particles around the cylindrical rod were consolidated to a certain extent due to gravity; however, for a vibration amplitude of 70μm or more, a pair of convection rolls formed on both sides of the vibrating rod. The strength of the convection was quantified from the particle velocity distribution in the convection rolls, and the relationship between the convection strength and vibration amplitude was elucidated. In addition, the air-pressure distribution in the powder bed was measured showing that the convection strength correlates with the characteristic positive pressure, i.e., the average value of positive pressure measurements. Elliptical motion and circular motion as well as simple harmonic motion were applied to the cylindrical rod by adding two harmonic motions in directions orthogonally crossing each other with a phase difference of π/2rad. The convection of the particles varied according to the Lissajous trajectory of the cylindrical rod. Even for simple harmonic motion, heaps of a pair of convection cells overlapped each other. In the case of elliptical motion, the overlapping portion of the heaps became larger. In the case of circular motion, the two heaps were integrated into one circular heap, and there were no effects of the circumferential angle on the particle velocity and the characteristic positive pressure.

Characterization of 17-4PH stainless steel powders produced by supersonic gas atomization

17-4PH stainless steel powders were prepared using a supersonic nozzle in a close-coupled gas atomization system. The characteristics of powder particles were carried out by means of a laser particle size analyzer, scanning electron microscopy (SEM), and the X-ray diffraction (XRD) technique. The results show that the mass median particle diameter is about 19.15 μm. Three main types of surface microstructures are observed in the powders: well-developed dendrite, cellular, and cellular dendrite structure. The XRD measurements show that, as the particle size decreases, the amount of fcc phase gradually decreases and that of bcc phase increases. The cooling rate is inversely related to the particle size, i.e., it decreases with an increase in particle size.

DETERMINATION OF THE INTERNAL EXPOSURE HAZARD FROM PLUTONIUM WORK IN AN OPEN FRONT HOOD

The particle size and quantity of a plutonium dioxide surrogate escaping an open front hood during routine work was measured. Aerosols were collected using a cascade impactor, and the amount of surrogate on each stage of the impactor was measured using neutron activation analysis. The aerodynamic mass median diameter of particles in the breathing zone of a worker was found to be 0.35 ± 5.1 μm. During a 2-h period when normal work activities were mimicked, an average of 3% of the material or 11.13 ± 0.54 μg escaped the hood. This was calculated to be equivalent to 10,000 ± 487 Bq (²³⁹Pu). The activity in the breathing zone of the worker is estimated, based on other research, to be 36 times less than this, or approximately 280 Bq. Worker occupancy factors were considered and a committed effective dose equivalent of 5 mSv was calculated. The cost-benefit analysis showed the use of the open front hood as a reasonable protective measure. Although worker exposure may approach the International Commission on Radiological Protection limit, the cost of the ergonomic injuries caused by work in a glove box is 5,000 times greater than the dose received by the worker. Protective measures such as respiratory protection should be evaluated on a case by case basis to keep worker exposure as low as reasonably achievable.

Experimental investigation and modelling of aluminum dusts explosions in the 20 L sphere

An experimental investigation was carried out on the influences of dust concentration, particle size distribution and humidity on aluminum dust explosion. Tests were mainly conducted thanks to a 20 L explosion sphere. The effect of humidity was studied by storing the aluminum particles at constant relative humidity until the sorption equilibrium or by introducing water vapour in the explosion vessel. The tested particles sizes ranged from a volume median diameter of 7 to 42 μm and the dust concentrations were up to 3000 g m −3. Among other results, the strong influence of the particle size was pointed out, especially when the Sauter mean diameter is considered. These results stressed the predominance of the specific surface area on the mass median particle diameter. The effect of water on aluminum dust explosion was decoupled: on the one hand, when water adsorption occurs, hydrogen generation leads to an increase of the explosion severity; on the other hand, when the explosion of dried aluminum powder occurs in a humid atmosphere, the inhibiting effect of humidity is put forward. A model based on mass and heat balances, assuming a shrinking core model with chemical reaction limitation, leads to a satisfactory representation of the pressure evolution during the dust explosion.

Particle Emissions from Chrome Plating

Aerosol particles formed from the bursting of small gas bubbles emitted from the anode and cathode electrodes during electrolytic chrome plating were measured with a laboratory apparatus. The measured aerosol particle emission factors were about 5.15 mg particles/amp-hour or 3.81 mg Cr+ 6/amp hour. The cathode gases generated more particles on a gas volume basis with 20 mg particles/liter hydrogen gas evolved from the cathode compared to 1 mg particles/liter oxygen gas evolved from the anode. With about 35% more cathode hydrogen gas evolved than anode oxygen gas evolved and with the much greater particle emissions caused by the cathode hydrogen on a gas volumetric basis, the aerosol particle emissions from the cathode bubble bursting were about 97% of the total particle mass emissions. The particle size distributions measured from the cathode had a mass median aerodynamic diameter of about 38 μ m whereas the anode mass median particle diameter was about 8 μ m. The anode particle mass size distribution was...

Comparative study of ground and sonicated vermiculite

The aim of this work is to compare the effects of grinding and ultrasonic treatment on vermiculite. Sonication produces a drastic change in particle size (mass median particle diameter 2.4 μm, layer thickness 38 nm), while the resulting material is crystalline as assayed by X-ray diffraction patterns. The TEM study shows that the sonicated vermiculite consists of nanometric flakes. On the other hand, grinding produces particles with medium diameter in the range of about 9 μm and very broad particle size distribution. X-ray and TEM studies of ground sample show an important alteration with grinding time. Prolonged grinding of vermiculite produces the loss of long-range order and eventually an amorphous product is obtained. The results show that grinding treatment produces a decrease of particle size, amorphization and agglomeration of the particles, whereas the ultrasound treatment only produces a decrease of particle size.

A SEMI-EMPIRICAL APPROACH TO PREDICT THE TOTAL COLLECTION EFFICIENCY OF ELECTROSTATIC PRECIPITATORS

Using empirical data for the effective migration velocity, the Deutsch-Anderson equation has been widely used for calculation of the total collection efficiency and design of electrostatic precipitators (ESPs). Based on a normal particle size distribution, the present paper develops a semi-empirical equation for the total collection efficiency of ESPs. The equation is then simplified and extended to other particle size distributions. The simplified equation shows that the total collection efficiency increases with increasing mass median diameter of particles and with increasing electrical field strengths. The effective migration velocity is found to be directly proportional to the mass median diameter and to the electrical field strengths for particle charging and collection. Moreover, an example is given to illustrate how to use the present simplified model in the design of ESPs.

The effect of solid particle mass loading on the pressure drop of HEPA filters

The increase in pressure drop across a HEPA filter has been measured as a function of solid particle mass loading using three materials with different particle morphologies. Sodium chloride, ammonium chloride and aluminum oxide particles in several different size distributions were generated as challenge aerosols. For each particle size distribution, the mass of material loaded per unit area of filter per unit pressure decrease for a given filtration velocity (1/specific resistance) was measured. In all cases, specific resistance of the filter cake increased as the mass median particle diameter decreased. The experimental data for the specific resistance were found to be well correlated with the increase of the mass median particle diameter and independent of the particle density. The advantages of this approach over other methods are the use of a more convenient and easily measured particle diameter and the described approach does not require the measurement or prior knowledge of the particle cake porosity. This new correlation can be used to predict either the final pressure difference across a loaded HEPA filter or the maximum mass that can be loaded onto a filter for a specified pressure difference.

The effect of particle size distribution on the antithixotropic and shear thickening properties of coal–water dispersions

The shear thickening rheology of coal–water suspensions prepared from six different naturally generated particle size distributions with mass median particle diameter d50 from 0.8 to 28 μm were investigated. Refined coal samples were thoroughly deflocculated with an anionic dispersant in water at particle concentrations from 47.7 to 56.4 vol %. The variations in flow behavior of the dispersions were observed in Couette flow as a function of shear rate in ramp tests and of time as a constant rate of shear was applied. Ramp test results indicated a decrease in the severity of shear thickening with increasing particle size distribution. Discontinuous shear thickening that occurred with the smaller particle sized samples was found to be approximated by a single function of the particle concentration reduced by the maximum packing fraction. The experimental results from constant shear rate experiments were compared in terms of the rate of antithixotropy, the extent of stress increase, and the total energy dissipated prior to shear thickening, as determined from characteristic points in the stress‐time rheograms. Significant differences in the shape of the antithixotropic curves were observed for the different particle size distributions. The magnitude of stress increase observed during the antithixotropic event was found to diminish with increasing particle size distributions which limited the detection of the antithixotropic event at the larger sizes. The extent of stress increase for the smaller particle sized samples, i.e., d50 < 3 μm, was such that the antithixotropic shear thickening appeared to dominate the flow behavior. A bimodal mixture of particles at d50 = 0.77 and 7.9 μm was also examined. An 8% increase in volumetric particle concentration was observed when compared to a monomodal suspension with equivalent d50 and average viscosity. No antithixotropic tendencies were observed with the bimodal suspension.

Minimum dc electric field requirements for removing powder layers from a conductive surface

An electrostatic powder dispenser was constructed to dispense particles without the use of a carrier gas. This device consisted of two contoured, outer stainless-steel plates that were electrically grounded and a flat, inner copper grid that was electrified and contained a central powder reservoir. Experiments were performed to investigate the levitation of various powders from the reservoir in the presence of an applied dc electric field. Eleven materials including metals, oxides, and conductively coated oxides were studied under vacuum and atmospheric conditions. The electric field required to remove particles from the powder reservoir was found to be a function of particle density and size. An equation was developed that predicted the minimum voltage necessary to remove conductive particles larger than 10 μm in diameter from a conductive surface in a vacuum environment: E=[4.85(ρD)1/2 +0.362]×105 , where E is the field strength (V/m), ρ is the particle density (kg/m3 ), and D is the mass median particle diameter (m). For particles in this size range, gravitational and electrostatic forces appeared to dominate, whereas for particles with a mass median diameter less than 10 μm, adhesive forces appeared to dominate. This equation was also found to hold for the removal of glass beads in air. A semiquantitative model was developed that was consistent with experimental results. This model calculated the force and charge induced on the particles in an electric field while taking into account the neighboring particles.

Characterization of the deposition of particles from the atmosphere to a flat plate

The airborne concentration of coarse particles ( > 1 μm diameter) was measured with a Rotary Impactor simultaneously with the measurement of particle dry deposition flux with a smooth surrogate surface with a sharp edge, mounted on a wind vane. The deposition surface was designed to provide minimum air flow disruption and thus provide an estimate of the lower limit for dry deposition flux. The deposited particles were weighed and counted. Microscopic count data generated the size distribution of particles collected on the deposition plate. The results demonstrated that 99% of the mass deposited on the plate was due to particles > 2 μm diameter. Because the measured dry deposition mass was due to atmospheric coarse particles and the measured airborne concentration was also due to coarse particles, the two data sets were combined to evaluate the dynamics of atmospheric coarse particle deposition. Deposition velocity was calculated by dividing the particle flux by the airborne coarse particle concentration. The deposition flux was 3.0 μg m −2s −1 for 17 samples taken over a 1-month period in an urban area. The deposition velocity averaged 11.7cm s −1. The deposition flux, deposition velocity and coarse particle Mass Median Diameter (MMD) were all shown to increase as wind speed increased. There was a large difference between day and night samples. Night samples had lower airborne concentrations (47%), lower deposition flux (65%) and lower deposition velocities (40%). These differences were accompanied by a 47% decrease in wind speed and a 30% decrease in the MMD of airborne coarse particles. This is in agreement with deposition model results where deposition velocity decreases when wind speed and particle size decrease.

The Hamilton Study: estimating exposure to ambient suspended particles.

In the industrial city of Hamilton, Ontario, Canada, we recently carried out an epidemiological study of the effect of environmental factors on respiratory health in 3500 elementary, schoolchildren. The level and size distribution of suspended particles in ambient air was measured from 24-h samples taken at 6-day intervals from a network of 29 hivol TSP samplers, and nine Andersen 2000 4-stage cascade impactors. Exposure was computed by generating a 3-dimensional response surface (in TSP and in easting and northing geographical coordinates) using a linear regression model of the form: TSP = (1 + E + N)2, based on monthly geometrical mean data for all sites. From the response surface generated for a given month, TSP levels were predicted by the model for all schools by specifying their geographical coordinates. The yearly exposure for a given child was determined from the arithmetic mean of the predicted values for 12 monthly TSP levels. A similar procedure was employed for calculation of the exposure to the "fine" (≤3.3 μm) and "coarse" (>3.3 μm) size fraction, as well as the aerodynamic mass median diameter of particles from the network of cascade impactors. Results of the measurements showed that gradients for TSP up to approximately 10 μg/m3/km exist over the city covering distances from 5 to 10 km. The range of 1 yr mean exposure values calculated for each child was from 30.5 μg/m3 to 74.5 μg/m3. Comparable figures for particle size were up to 0.3 μm AMMD (aerodynamic mass median diameter)/km and annual mean particle size exposure from 2.69 to 3.53 μm AMMD.

The delivery of therapeutic aerosols through endotracheal tubes.

We used an in vitro model system to examine the sites of deposition of aqueous therapeutic aerosols administered through 3-mm, 6-mm, and 9-mm endotracheal tubes (commonly used in infants, children, and adults) at clinically relevant inspiratory flow rates. Aerosol was delivered to the endotracheal tube via a "T" piece and a 90 degree elbow. Aerosol exiting the endotracheal tube passed through an appropriately sized Plexiglas model of the trachea and mainstem bronchi, and then into an 80-liter bag. Aerosol deposited in the "T" and elbow, endotracheal tube, large airway model, and collection bag was quantitated separately using 0.1% uranine as a tracer. Study of a conventional aerosol typical of those in common clinical use (aerodynamic mass median diameter = 3.95 microns) showed that most of the aerosol delivered into each endotracheal tube was deposited before leaving the mainstem bronchi. Substitution of an alternative nebulizer that produced a much smaller aerosol particle size (aerodynamic mass median diameter = 0.54 micron) dramatically decreased aerosol deposition in the "T" and elbow, endotracheal tube, and large airway model, and increased the quantity of aerosol penetrating beyond the mainstem bronchi up to ninefold. The mass median particle diameter of the conventional aerosol was reduced during endotracheal tube and large airway passage by poorly defined aerodynamic mechanisms that selectively removed larger particles. The smaller submicron aerosol was not similarly affected. Thus, conventional therapeutic aerosols appear to penetrate poorly through endotracheal tubes. Use of smaller particle size aerosols in treatment of intubated patients may be an effective way to circumvent this problem.

Respiratory tract deposition of polydisperse aerosols in humans.

Total and regional deposition of polydisperse aerosols in the human respiratory tract are studied theoretically. The size distribution of the aerosol is assumed to be lognormal. For a given mass median particle diameter, mass deposition fraction is found to vary with the geometric standard deviation of the aerosol. The departure of the deposition pattern in various regions of the respiratory system from that of a monodisperse aerosol is interpreted in terms of the average mobility effect and deposition limitation effect of the polydisperse aerosol together with the sequential filtering effect of the respiratory tract.

Respiratory Tract Deposition of Polydisperse Aerosols in Humans

Total and regional deposition of polydisperse aerosols in the human respiratory tract are studied theoretically. The size distribution of the aerosol is assumed to be lognormal. For a given mass median particle diameter, mass deposition fraction is found to vary with the geometric standard deviation of the aerosol. The departure of the deposition pattern in various regions of the respiratory system from that of a monodisperse aerosol is interpreted in terms of the average mobility effect and deposition limitation effect of the polydisperse aerosol together with the sequential filtering effect of the respiratory tract.

Theoretical investigation of pressure drop in combined cyclone and fabric filter systems

Computer simulations were conducted to investigate potential pressure drop reductions obtainable by combining cyclones, as pre-collectors, with fabric filters. The Leith-Licht model was used to characterize cyclone emissions and the specific resistance ( K 2) of the fabric filter dust cake was calculated from an empirical correlation. Several important dimensionless groups were identified and evaluated. One group, the product of the ratio of the dust cake specific resistances expected with and without the cyclone and the mass penetration of the cyclone, ( K 2/ K 2) Pn, indicates whether a pressure drop reduction is possible. A correlation was developed for this group as a function of the size properties of the inlet dust (particle mass median diameter and geometric standard deviation) and the cyclone particle cut diameter. Expressions were derived for the break-even time, the duration of filtration with the cyclone needed to show a pressure drop reduction in comparison with filtration without the cyclone. It is shown that in previously reported experiments and simulations indicating an advantage for the combined cyclone-fabric filter system, filtration cycles were typically longer than the break-even time; those showing no improvement typically had filtration times shorter than the break-even time.

Factors influencing trace metal, sulfate and hydrogen ion concentrations in rain

Precipitation collected as wetfall-only and primarily on an event basis at a site in the Tennessee Valley is a dilute sulfuric acid solution containing trace metals whose concentrations exhibit log normal distributions. The concentrations of H +, SO 4 2−, Pb, Mn and Zn exhibit maxima in the warmer months in response to synoptic meteorologic conditions. These conditions result in elevated air concentrations during air stagnation and in generally lower rainfall amounts per event and hence less dilution of the scavenged material. Dilution is indicated by significant negative correlation coefficients between concentrations and rainfall amount, the rainfall parameter exerting the strongest influence on concentrations. Concentrations are negatively correlated with rainfall duration and intensity to a lesser extent. The relationship between concentration and rainfall amount can be described by a negative exponential, while that between wet deposition and rainfall amount is approximately a 0.6-power relationship. Calculated scavenging ratios generally increase with increasing atmospheric particle mass-median diameter of each element and are on the order of 100 for Pb, 1000 for SO 4 2− and Zn, and 5000 for Cd and Mn.

The dependence of open field burning emissions and plume concentrations on meteorology, field conditions and ignition technique

A program of field and laboratory measurements of emissions from the burning of agricultural residues (primarily cereal straw and stubble) and plume behavior is described. Relationships investigated include the dependence of total emissions and plume concentrations on fuel management, field ignition technique and ambient meteorological conditions. Total particulate emissions (by mass) were found to increase strongly with increasing residue moisture content but are reduced to one half through the use of a backfire field ignition technique versus the head fire ignition method. Increased emissions from headfires is attributable to smoldering in the burned over areas. Backfire fuel consumption rates are typically about one-fifth that for headfires, resulting in a further reduction of particle concentrations in backfire plumes. Increased fuel loading reduces emissions, especially for headfires. The mass median diameter of particles from either fire ignition technique is less than 0.2 μm and in the average about one half of these particles are chloroform soluble. CO and gaseous hydrocarbon emissions were also found to be directly proportional to residue moisture content, but independent of field ignition technique. Plume rise is maximized (ground level concentration minimized) by using headfires in light winds (<4 m s −1). At higher wind speeds, limited plume rise and increasing fumigation increases ground level concentrations but this can be minimized through the use of backfires. As the effluent plume ages, the size spectrum shifts to smaller diameters suggesting significant evaporation with time. The results of fuel drying studies and cost analyses for the various techniques are also presented.

A Five-year Inhalation Study with Natural Uranium Dioxide (UO2) Dust-II. Postexposure Retention and Biologic Effects in the Monkey, Dog and Rat

Inhalation studies show that dogs, monkeys, and rats can breathe a natural uranium dioxide aerosol of approximately 1 mu m mass median particle diameter at a mean concentration of 5 mg U/m/sup 3/ (25 x Threshold Limit Value or 28 x Maximum Permissible Concentration in Air) for periods as long as 5 yr with little evidence of serious injury (L. J. Leach et al., Health Phys. 18: postexposure periods during which pulmonary neoplasia developed in a high percentage of the dogs examined 2 to 6 yr after exposure. Pulmonary and tracheobronchial lymph node fibrosis, consistent with radiation effects, apparently dose dependent, and more marked in monkeys than in dogs was also noted. No evidence of uranium toxicity was found in records of body weights, mortality, various hematologic parameters, or the histologic condition of the kidneys. (auth)