Mixing selectivity in bicomponent,bipolar aggregation

The selectivity of aggregation in mixtures of two charged aerosols containing chemically dissimilar nanoparticles is studied by means of a newly developed direct simulation Monte Carlo method. This method allows to trace changes in complex multidimensional systems, in this case describing particle size, charge and aggregate composition. A new procedure was developed for estimating the effective collision diameter of an aggregate composed of primary particles of any size. Three model systems were studied: polydisperse aerosols with initially bipolar charge distribution, unipolarly charged polydisperse aerosols and quasi-monodisperse oppositely charged aerosols. The study is focused on the aggregate composition's dependence on the initial size and charge distribution. It was found that the use of bipolarly charged aerosols does not increase the selectivity of mixing whereas unipolarly, oppositely charged aerosols reach more rapidly a more homogeneous distribution of components within the aggregates. In the last case, the addition of one more elementary charge to the particles roughly doubles the fraction of bicomponent, $\text{1}\text{:}\text{1}$ mixed nanoaggregates and accelerates the process.     

Particle charge-size distribution measurement using a differential mobility analyzer and an electrical low pressure impactor

We introduce a particle charge-size distribution measurement method using a differential mobility analyzer and an electrical low pressure impactor in tandem configuration. The main advantage of this type of measurement is that it is suitable for a wide range of particle sizes, from approximately 30 nm up to a micrometer, and for high charge levels, which have been problematic for previously used methods. The developed charge measurement method requires information on the particle effective density, and the accuracy of the measurement is dependent on how well the particle effective density is known or estimated. We introduce the measurement and calculation procedures and test these in laboratory conditions. The developed method has been tested using narrow and wide particle size distributions of a known density and well-defined particle charging states. The particles have been produced by the Singly Charged Aerosol Reference (SCAR) and an atomizer and charged with the previously well-characterized unipolar diffusion chargers used in the Nanoparticle Surface Area Monitor (NSAM) and in the Electrical Low Pressure Impactor (ELPI+). The acquired charge-size distributions are in good agreement with the reference values in terms of the median charge levels and widths of the charge distributions.

Copyright © 2017 American Association for Aerosol Research     

Heat generation during pulse operation of prototype aluminium electrolytic capacitors

Measurements were made of heat generation in aluminium electrolytic capacitor sandwiches operating under pulse charge and discharge. With commercial anode foils the fraction of input power dissipated as heat in the dielectric is proportional to theDF of the oxide dielectric. There is also a dependence on the applied voltage that is thought to be due to frictional losses arising from expansion and relaxation of the oxide in the pulsed field. This field dependence is specific to the commercial etched aluminium foils and is absent when the oxide dielectric is grown on smooth substrate, indicating a dependence of the stresses in the oxide on the etch structure. The oxide degrades under isothermal pulse operation at 60° C, with bothDF and heat generation increasing proportionally. Oxide reformation brings these properties back to their initial values, or lower, and stabilizes the dielectric during subsequent pulse operation. An amorphous anodic oxide dielectric deposited at room temperature on commercial etched foils had different characteristics. The fractional heat dissipation was equal to the lowest observed with the commercial oxides, but was independent of oxideDF. Long term isothermal pulsing produced only slight degradation of the oxide properties.     

Study on the pressure distribution at the bottom of a pressed powder bed by pressure-detecting sheets

A method using pressure-detecting sheets is proposed for determining the pressure distribution at the bottom of a pressed powder bed. This method is not very accurate but is practically feasible.The relation between the bottom pressure σ and the radial distance r from the centre of the bed can be expressed empirically by

The dependence of the constants a and b on the applied pressure P, the radius of the cylindrical container R, the thickness of the pressed bed H, and the roughness of the cylinder wall, has been given.     

Long–Pulse Explosive Compaction of a Diamond Powder

The problem of producing bulk specimens by explosive compaction of a synthetic diamond powder without binders and catalytic additions is considered. The method of long–pulse explosive compaction with the use of multilayered explosive charges with a total mass up to 760 kg is used in experiments. The physicochemical properties of the resulting compacts are studied in detail. It is shown that, for relatively low loading pressures, long–pulse explosive compaction makes it possible to produce specimens whose hardness is equal to that usually obtained under higher (by one order of magnitude) shock pressures with a pulse duration of 1 sec. In contrast to short–pulse loading, long–pulse loading ensures a considerable decrease in the cracking of the compacts.     

Charge distribution uncertainty in differential mobility analysis of aerosols

The inference of particle size distributions from differential mobility analyzer (DMA) data requires knowledge of the charge distribution on the particles being measured. The charge distribution produced by a bipolar aerosol charger depends on the properties of the ions produced in the charger, and on the kinetics of charge transfer from molecular ions or ion clusters to the particles. A single parameterization of a theoretically predicted charge distribution is employed in most DMA analyses regardless of the atmospheric conditions being probed. Deviations of the actual charge distribution from that assumed in the data analysis will bias the estimated particle size distribution. We examine these potential biases by modeling measurements and data inversion using charge distributions calculated for a range of atmospheric conditions. Moreover, simulations were performed using the ion-to-particle flux coefficients predicted for a range of properties of both the particles and ions. To probe the biases over the full range of particle sizes, the measurements were simulated through an atmospheric new particle formation event. The differences between the actual charge distribution and that according to the commonly used parametrization resulted in biases as large as a factor of 5 for nucleation-mode particles, and up to 80% for larger particles. Incorrect estimates of the relative permittivity of the particles or not accounting for the temperature and pressure effects for measurements at 10 km altitude produced biases in excess of 50%; three-fold biases result from erroneous estimates of the ion mobility distribution. We further report on the effects of the relative permittivity of the ions, the relative concentrations of negative and positive ions, and truncation of the number of charge states considered in the inversion.

Copyright © 2017 American Association for Aerosol Research     

Possibilities of Controlling the Shaped–Charge Effect by Electromagnetic Actions

This paper deals with electromagnetic actions that allow one to control the shaped–charge effect at different stages of shaped–charge operation. A decrease in penetration of the shaped–charge jet is attained by passage of a powerful electric current pulse through it, production of an axial magnetic field in the shaped–charge liner immediately before shot, and production of a magnetic field in the conducting target material that is transverse to the direction of jet propagation. The action on a jet by low–frequency and high–frequency longitudinal magnetic fields and mild current action are directed toward increasing the penetration capability of a shaped–charge jet by increasing its ultimate elongation. Results of experimental and theoretical studies of different versions of electromagnetic actions are analyzed, and the associated physical effects are considered.     

Current levelling behaviour under pulse current electroplating conditions: The role of reaction pseudocapacitance

Despite growing interest in pulse current electroplating, published theories have not evolved beyond treatments of simple, single-step reaction kinetics. This omission of multiple-step kinetics and associated reaction pseudocapacitance is responsible for the inability to interpret mechanistically several key pulse current electrodeposition process characteristics.A complex kinetics charge transfer model is developed to describe the enhanced geometric control of deposit thickness achievable with judiciously chosen pulse current control conditions. This semiquantitative analysis demonstrates that deposit thickness levelling is primarily the result of the strong potential dependence of the reaction pseudocapacitance, together with proper choices of pulse current control variables. For any system of interest, a well defined on-time current,I _A, and on-time and off-time periods,t _A andt _B, are optimum for minimizing relative deposit thickness variation.     

Characteristics of Aerosol Charge Distribution by Surface-Discharge Microplasma Aerosol Charger (SMAC)

The surface discharge on a dielectric barrier induced by dc pulses was successfully utilized as a stable bipolar ion source for neutralizing submicron aerosol particles where the concentration of positive and negative ions could be adjusted independently (a surface-discharge microplasma aerosol charger: SMAC; Kwon et al., 2005 Kwon, S. B., Sakurai, H., Seto, T. and Kim, Y. J. 2005. Charge Neutralization of Submicron Aerosols Using Surface-Discharge Microplasma. J. Aerosol Sci., in press[CSA] [Google Scholar]). The aim of this study was to determine the charge distribution obtained by the SMAC, which has been qualitatively presented in our previous study, and to investigate the effect of unequal bipolar ion concentration on the charge distribution. For this purpose, we performed quantitative analysis of the charge distribution of monodisperse particles in the size range of 30–200 nm acquired by the SMAC and compared the charge distributions with calculated charge fractions obtained from the diffusion charging theory. The ion parameters were calculated by measuring the ion mobility of positive and negative ions and they were used to obtain the analytic solutions of charge distribution. The charge distributions obtained by the equal or unequal concentration of bipolar ions adjusted by the SMAC showed general agreement with the diffusion charging theory.     

Pulse electrodeposition of titanium on carbon steel in the LiF–NaF–KF eutectic melt

Electrodeposition of titanium was carried out in the K₃TiF₆–LiF–NaF–KF melt using both direct (DC) and unipolar pulse current (PC) techniques. Dense and smooth titanium coatings were obtained by PC plating at 750 °C whereas DC plating led to rough and dendritic deposits. The best results were obtained using a 100C cm^–2 pulse charge and a cathodic current density of 50 and 75mA cm^–2. The cathodic current efficiency was in the range 60–65%. The titanium deposits obtained under such conditions behaved similarly to CP-titanium in NaCl and HNO₃ solutions at room temperature.     

Shock‐Wave Initiation of Detonation of Double‐Base Propellants

In the known experimental system active charge–target–HE charge to be tested, critical pressures of shock waves initiating detonation of doublebase propellant charges are determined. TNT charges of various density were used as active HE, and copper plates 5 mm thick were used as targets. The pressure of the shock wave acting on the propellant versus the TNT density was constructed; this dependence being known the critical pressure can be readily determined with only the density of the active charge available. It was found that doublebase propellants are close to liquid HE in terms of sensitivity to shock waves; the critical pressure is 6.0–9.0 GPa for a charge diameter of 40 mm and decreases with increasing diameter. By an example of the NDT2 propellant, it is shown that the use of factorypacked propellants in line charges may lead to failure in transfer of detonation from one propellant charge to another.     

One-point method for determination of number-average molecular weight

The following well-known equation permits the ready determination of M _n from a single osmotic pressure measurement at a known concentration, if the second virial coefficient is previously given: On this basis, the one-point method was investigated to determine the number-average molecular weight. It was found that this method was applicable to commercial polymers. However, in this application, the dependence of Γ₂ on molecular weight distribution has to be kept in mind.     

Some aspects of nonisothermal crystallization of polymers. III. Crystallization during melt spinning

Crystallization during melt spinning is studied as an example of the nonisothermal crystallization of polymers. The following equation is derived, taking the temperature distribution within a filament into consideration: where T = temperature, X = crystallinity, κ = thermal diffusivity, V = velocity, ΔH = heat of crystallization, and C_p = specific heat at constant pressure. The assumptions and the procedure for a numerical calculation of crystallinity and temperature within a running filament are described, and some results of calculation are illustrated. The results are compared with those obtained by a simpler calculation in which the radial temperature distribution is neglected. The simpler method proved useful in connection with x-ray measurements.     

Potentiostatic pulse technique to determine the efficiency of hydrogen absorption by alloys

A potentiostatic pulse technique was used to determine the efficiency of hydrogen entry and trapping in two precipitation-hardened (Inconel 718 and Incoloy 925) and two work-hardened (Inconel 625 and Hastelloy C-276) nickel-base alloys in an acetate buffer (1 M acetic acid/1 M sodium acetate) containing 15 p.p.m. As₂O₃. The data were examined as a function of charging time and overpotential. The trapping efficiency increases with charging time (f _c) and is linearly dependent on (t _c)^1/2 for sufficiently short times. The range of linearity is determined by the apparent trapping constant for the alloy. The entry efficiency is independent of charging time but it can vary with overpotential depending on the value of the charge transfer coefficient () for the hydrogen evolution reaction. In the case of Inconel 718 and Hastelloy C-276, the value of was 1.4, which is consistent with a mechanism for hydrogen evolution involving fast discharge followed by slow electrochemical desorption.     

Axial dispersion in metal foams and streamwise-periodic porous media

We present a study on the axial dispersion in metal foams and laser sintered reactors. Commercially available metal foams of 20 and 30 ppi are compared to a designed streamwise-periodic structure in terms of axial dispersion coefficients and pressure drops. Therefore tracer pulse experiments were performed and post processed by means of a deconvolution method. The Peclet number Pe_p based on the pore size is ranging from 5×10⁴ to 8×10⁵ which is attributed to the increased velocities due to the high porosity of the material compared to fixed bed reactors. The attained dispersion coefficients ranging from 1.3×10⁻⁴ to demonstrate the trend of packed beds and common packing materials and increase monotone with the Peclet number Pe_p. The pressure drop versus the interstitial bulk velocity follows the Forchheimer equation and can be described by the conventional Ergun model for all investigated porous media. The parameters obtained correspond to values found in literature. The results of this study show the high potential of foam reactors for catalyst driven reactions. They provide the same or even a higher surface area per volume of catalyst bed while inducing a much smaller pressure drop than corresponding fixed beds.     

Modern Applications of Electron Microscopy to Catalysis

The active sites of ultra-dispersed Pt/-Al₂O₃ catalysts are studied using high-resolution electron microscopy, Z-contrast and dark field. In addition we have calculated using a method based in density functional theory the electrostatic potential and charge distribution of the active sites. It is conclude that the most likely Pt clusters that are formed correspond to Pt₁₃ and Pt with icosahedral and decahedral structure. It is shown that this is consistent with the electron microscopy data.