Enhancement of Extrinsic Charging Efficiency of a Nanoparticle Charger with Multiple Discharging Wires

A unipolar charger with multiple discharging wires has been developed and investigated to enhance the extrinsic charging efficiency of nanoparticles by using sheath air near the wall of the charger. The applied voltage of the charger ranged from +4.0 to +10 kV, corresponding to corona current from 0.02 to 119.63 μA. Monodisperse NaCl particles of 10 ～ 50 nm and Ag particles of 2.5 ～ 10 nm in diameter were produced to test the performance of the charger with multiple discharging wires and to investigate the particle loss at different sheath flow rates, corona voltages and sheath air velocities. Results showed that the optimal efficiency in the charger was obtained at +9 kV applied voltage, 10 L/min aerosol flow rate and 20 L/min sheath air flow rate. The extrinsic charging efficiency increased from 2.86% to 86.3% in the charger as the particle diameter increasing from 2.5 to 50 nm. The TDMA (tandem-differential mobility analyzer) technique was used to investigate the charge distribution, and the charge distributions in the exit were obtained at the optimal operating condition.     

Experimental study of a new corona-based unipolar aerosol charger

A corona-based unipolar aerosol charger has been constructed, and its performance has been systematically evaluated. The prototype consists of completely separated corona ionization and charging chambers. With this configuration the electrostatic loss of charged particles is eliminated, and particle loss by diffusion and the space charge effect is minimized by the angular injection of the ionizer flow and the rapid exit of charged particles. The charger performance was optimized by varying different operational parameters, i.e., total and ionizer flowrates, and ion concentration. It was found that operation with one corona ionizer gave higher extrinsic charging efficiency than operation with two ionizers. The corona-discharge current has negligible effect on the charging performance. Operating the charger at a total flowrate of 5 lpm, with 1.0 lpm flow in each of the two ionizers, gave the highest extrinsic charging efficiency. Further, the performance of prototype charger was not compromised even at a total flowrate of 10 lpm. The charger provides higher extrinsic charging efficiency than other corona-based unipolar chargers. Extrinsic charge distributions for particles of different sizes were at last measured by the tandem-DMA technique.     

A novel bipolar charger for submicron aerosol particles using carbon fiber ionizers

A simple and novel bipolar charging device using carbon fiber ionizers was developed to neutralize submicron aerosol particles without the generation of ozone. The ion currents of the positive and negative ions generated by carbon fiber ionizers were so chosen as to optimize particle neutralization. The particle penetration, charging probability and charge distribution resulting from the charger were investigated and compared to those from a Kr-85 radioactive neutralizer for the particles in the size range of 20–120 nm. Size distributions for various laboratory-generated aerosols (sodium chloride, ammonium nitrate, ammonium sulfate and glutaric acid) neutralized by the charger were also investigated and compared to those obtained without neutralization. Particle penetration in the charger was over 90% for particles larger than 20 nm. Charging probability and charge distribution for the charger were in good agreement with those from Kr-85 neutralizer and with theoretical estimations. Size distributions observed for the charger and Kr-85 neutralizer were also in good agreement for particles of different concentrations and various chemical compositions. The newly developed bipolar carbon fiber charger can neutralize submicron particles, at least as effectively as currently available radioactive neutralizers and with negligible ozone generation which is its major advantage.     

Maximizing the singly charged fraction of sub-micrometer particles using a unipolar charger

Particle charging via the mixing of aerosols with unipolar ions typically results in multiple charges on particles. Particle classification and sizing, based on the electrical mobility, ideally requires all the particles being singly charged to the performance enhancement. In this study, we explored the feasibility of maximizing the singly charged fraction of particles via the control of the N_it product in a unipolar charger. The feasibility was first investigated by modeling unipolar diffusion charging. It was found that the singly charged fraction of monodisperse particles could be maximized by the control of the N_it product. A corona-based unipolar charger was also constructed to study the maximization of the singly charged fraction of monodisperse particles. It was found that a wider range of ion concentration in the charging zone could be obtained by the variation of ion-driving voltage compared to that by changing the corona-discharge current. The maximum singly charged fraction of monodisperse particles in various sizes was characterized when the charger was operated at the flow rates of 1.5 and 3.0 lpm. It was evidenced that the current charger could be conditioned to achieve a higher singly charged fraction of particles than that by bipolar chargers in the particle size range of 20–200?nm, particularly in the ultrafine particle size range. The control of N_it product in the charging zone of a unipolar charger offers a simple and effective means to enhance the singly charged fraction of particles in a given size range.

Copyright © 2019 American Association for Aerosol Research     

Improvement of the Nanoparticle Charging Efficiency of a Single-Wire Corona Unipolar Charger by Using Radial Sheath Airflow: Numerical Study

A single-wire corona unipolar charger with radial sheath air was proposed to enhance the nanoparticle charging efficiency. The charger consists of an insulated Teflon tube (inner diameter = 6.35 mm) with a 6 mm-long grounded porous metal tube placed at its center from which radial sheath air is introduced, and a discharge gold wire of 50 μm in the outer diameter and 6 mm in the effective length. The performance of the charger was evaluated and optimized numerically. The effect of the position of the sheath air opening on reducing charged particle loss was found to be important and two designs were studied. In design 1, both ends of the 6 mm wide sheath air opening are aligned with the ends of the 6 mm-long discharge wire, while in design 2 the sheath air opening is shifted 2 mm toward the left of the leading edge of the wire. At the same operating condition, design 2 was found to have less electrostatic loss than design 1 because of its smaller deposition region for charged particles. Compared to two unipolar chargers with the highest extrinsic charging efficiency for particles smaller than 10 nm in diameter, design 2 operated at the applied voltage of +3.5 kV, aerosol flow rate of 0.5 L/min, and sheath airflow rate of 0.7 L/min has a comparable extrinsic charging efficiency of 17.2%–70.5% based on particle number for particles ranging from 2.5 to 10 nm in diameter.

Copyright 2013 American Association for Aerosol Research     

Charging of Ultra-Fine Aerosol Particles by an Ozone-Free Indirect UV Photo-Charger

Particle charging by indirect photoemission may be an alternative to charging methods based on corona discharge or on bipolar charging by radioactive ion sources. An indirect charger using a low energy UV radiation is introduced and characterized in detail. Depending on the carrier gas, photoelectrons or ions formed by electron-attachment charge the particles by diffusion charging. The achieved charging efficiency is in the range of 20–70% for particle sizes of 20 to 100 nm. It can easily be modulated by a small voltage applied to the photoemitter. To achieve stable electron emission, glassy carbon with its very inert surface is used as photoemitter. Particle losses are very small. The charge distribution has been measured by a tandem DMA setup. The experimental results are compared with the theory of unipolar diffusion charging based on the Fuchs’ combination probability of ions with the particles. The charger characterization has been performed by carbon particles in a size range of 20–100 nm, produced by a spark discharge generator.

Copyright 2013 American Association for Aerosol Research     

Unipolar Charging of Nanosized Aerosol Particles Using Soft X-ray Photoionization

A unipolar charging device based on a soft X-ray (<9.5 keV) photoionization was developed to investigate the charging efficiency of aerosol nanoparticles. Unipolar charging using a 241 Am charger was also evaluated as a comparison with the characteristics obtained by X-ray charging. The production rate and the concentration of ions generated by the X-ray and 241 Am unipolar chargers were estimated from ion current measurements. Theoretical calculations by the unipolar diffusion charging theory were also carried out and the calculated data were compared with the experimental results. For acquiring a high number of standard nanoparticles, the classification of monodisperse nanoparticles from polydisperse aerosol particles using the X-ray unipolar charger and a differential mobility analyzer (DMA) was also evaluated. The ion production rate of the X-ray unipolar charger was at least 5.5 times higher than that of the 241 Am unipolar charger and the ion concentration was about three times higher. Therefore, the X-ray unipolar charger showed a higher capability for charging aerosol particles of 10-40 nm size in diameter than the 241 Am charger. The charging state of particles produced by the X-ray unipolar charger was in good agreement with theoretical calculations. The X-ray unipolar charger developed herein has potential for use in charging a high number concentration of nanoparticles for use in nanotechnology investigations.     

The Effect of Particle Pre-Existing Charge on Unipolar Charging and Its Implication on Electrical Aerosol Measurements

We investigated the effect of particle pre-existing charges on unipolar charging. Particles carrying a defined number and polarity of pre-existing charges were used to study the unipolar charging process in a unipolar diffusion charger with positive ions. It was found that the particles initially carrying negative charges have almost the same amount of positive charges as the initially uncharged particles after passing the test charger; and the particles initially carrying more positive charges have more final charges. An analytical solution of a model for particle charge distribution of initially charged particles was provided for unipolar charging based on Fuchs' theory and the birth-and-death theory. The N _ion t value used in this model was obtained by fitting the experimental data of average charge on particles for initially uncharged particles. The results from the analytical solution show very good agreements with experimental data regarding the relationship between the pre-existing charge and the final charge on particles (50–200 nm in this study). Experimental tests of the response of Nanoparticle Surface Area Monitor (NSAM) against initially charged particles demonstrated that NSAM could have a large response deviation (more than 20% in the tested charge level) depending on the particle size and the amount of pre-existing positive charges on particles. Modeling of NSAM response showed similar deviation and predicted that when pre-existing charge is high enough, the NSAM response can be as large as 5 and 9 times of the uncharged particle response for alveolar and tracheobronchial surface area concentration, respectively.     

A numerical study of the influence of ion–aerosol mixing on unipolar charging in a laminar flow tube

A theoretical investigation has been carried out to examine the effect of the initial ion–aerosol mixing state on the resulting charged fraction (extrinsic charging efficiency) of ultrafine particles in a circular tube under laminar flow conditions. Particle and ion loss to the walls by diffusion and electrostatic dispersion were taken into account in the calculations. Four different mixing states were considered at the inlet section of the charging tube: (I) uniform mixing of ions and neutral particles; (II) a core of ions surrounded by an envelope of neutral particles; (III) a core of neutral particles surrounded by an envelope of ions; and (IV) the inlet section split up into two circular segments, one containing neutral particles alone, the other only ions. The numerical results show that the extrinsic charging efficiency strongly depends on the initial mixing state, the maximum efficiency being attained for mixing state II, a state which can be easily realized in practice by injecting ions and particles into the charger through two coaxial cylinders. The worst case is that of mixing state III, the reverse of II, in which the neutral particles are injected through the inner cylinder, and the unipolar ions through the annular space between the injection tubes. The main reason underlying the observed behavior is the strong dependency of the ions loss rate to the wall on their initial spatial distribution.     

Estimating aerosol particle charging parameters using a Bayesian inversion technique

A Bayesian inversion routine is described in which data from tandem differential mobility analysis (TDMA) can be used to determine fundamental parameters for charging models as a function of particle size. The measurement and inversion techniques were verified using simulated data for particles in the 50–500 nm range undergoing unipolar diffusion charging as described by Fuchs’ limiting sphere theory, for which the fundamental charging parameter is the product of the unipolar ion concentration and residence time within the charger, the so-called nt product. Under conditions where the average particle charge is greater than one unit charge, the inversion routine is precisely and accurately able to determine the nt value. The inversion routine breaks down, however, when the average number of charges per particle is well below unity, i.e. for low nt values or for small particle sizes. Incorporation of charging efficiency data in addition to TDMA data can allow for use of the inversion routine when the average number of charges per particle is low. With its limitations known, the inversion routine was applied to determine the nt value for the unipolar charger used in the TSI electrical aerosol detector (EAD), model 3070A, for particles in the 50–200 nm size range. The effective nt value within the EAD charger decreased with decreasing particle size, implying that charged particle losses occur within the EAD charger. The described inversion routine is unique in its ability to determine size dependent charge model parameters, and can be utilized with any given charging model. It is expected that this technique will be of use in advancing the understanding of aerosol particle charging models and in future design of unipolar chargers.     

Study of a Nanoparticle Charger Containing Multiple Discharging Wires in a Tube

Abstract

A unipolar charger containing multiple discharging wires in a tube (inner diameter: 50 mm) was developed and tested in order to increase the aerosol flow rate and the charging efficiency of nanoparticles. Four gold wires of 25 µm in diameter and 15 mm in length were used as the discharging electrodes to generate positive ions (N_i) from 2.72 × 10⁸ ions/cc to 3.87 × 10⁹ ions/cc in concentration at the discharging voltage of + 4.0 ～ + 10 KV. Monodisperse NaCl particles of 10 ～ 50 nm in diameter were used to test the charging efficiency and the particle loss of charged particles with different aerosol flow rates, corona voltages and sheath flow rates. The sheath air near the tube wall was found to increase the extrinsic charging efficiency, and the highest efficiency was obtained at + 6.0 KV discharging voltage, 10 L/min aerosol flow rate and 9 L/min sheath flow rate. The extrinsic charging efficiency increased from 10.6% to 74.2% when the particle diameter was increased from 10 to 50 nm. The TDMA (tandem differential mobility analyzer) method was used to determine the charge distribution and the mean charge per particle and it was found that the Fuchs charging theory corrected for the extrinsic charging efficiency matched with the experimental data very well.     

Charge distributions of aerosol dioctyl sebacate particles charged in a dielectric barrier discharger

The collection efficiencies of submicron aerosol particles using a two-stage, dielectric barrier discharge (DBD) type electrostatic precipitator have been reported previously [Byeon et al. (2006). Collection of submicron particles by an electrostatic precipitator using a dielectric barrier discharge. Journal of Aerosol Science, 37, 1618–1628]. In this paper, the charge distributions of aerosol dioctyl sebacate (DOS) particles, which had a mobility equivalent diameter of 118, 175, and 241 nm and were charged in a DBD charger, were examined using a tandem differential mobility analyzer (TDMA) system at applied voltages of 9–11 kV and frequencies of 60–120 Hz. The mean number of elementary charges for positively or negatively charged particles increased slightly with increasing applied voltage or frequency. However, the number of elementary charges increased significantly with increasing particle size. At any applied voltage and frequency, the charge distributions of these particles of these sizes indicated asymmetric bipolar charging. The positive-to-negative charge ratios were 10.4, 4.7, and 3.0 for particle sizes of 118, 175, and 241 nm, respectively, at a DBD voltage and frequency was 9 kV and 60 Hz, respectively. Fluorometric analysis showed that average positive-to-negative charge ratios were 11.5, 4.9, and 3.7 for particle sizes of 118, 175, and 241 nm, which agrees well with the TDMA results. Further fluorometric analyses with larger particles (514 and 710 nm) and higher frequencies (1 and 2 kHz) showed that the positive-to-negative charge ratio reached almost unity with increasing particle size or frequency.     

Design and development of a self-contained personal electrostatic bioaerosol sampler (PEBS) with a wire-to-wire charger

Here, we present a concept of a personal electrostatic bioaerosol sampler (PEBS), which is an open channel collector consisting of a novel wire-to-wire particle charger and a collection section housing a double-sided and removable metal collection plate and two quarter-cylinder ground electrodes. The charger consists of a tungsten wire (25.4 mm long and 0.076 mm in diameter) connected to high voltage and positioned in the center of the charging section (a cylinder 50.8 mm long and 25.4 mm in diameter); a ring of stainless steel wire 0.381 mm in diameter surrounds the hot electrode at its midpoint and is grounded. The newly designed wire-to-wire charger produces lower ozone concentrations compared to traditional wire-to-plate or wire-to-cylinder charger designs. The particles captured on the collection plate are easily eluted using water or other fluids. The sampler was iteratively optimized for optimum charging and collection voltages, and collection electrode geometry. When tested with polystyrene latex particles ranging from 0.026 µm to 3.1 µm in diameter and 10 L/min collection flow rate, the sampler's collection efficiency was approximately 70%–80% at charging and collection voltages of +5.5 kV and ?7 kV, respectively. The PEBS showed this collection efficiency at sampling times ranging from 10 min to 4 h. Preliminary tests with Bacillus atrophaeus bacterial cells and fungal spores of Penicillium chrysogenum showed similar collection efficiency. The use of a unique wire-to-wire charger resulted in ozone production below 10 ppb. Due to low ozone emissions, this sampler will allow maintaining desirable physiological characteristics of the collected bioaerosols, leading to a more accurate sample analysis.

© 2017 American Association for Aerosol Research     

Modeling and Validation of Nanoparticle Charging Efficiency of a Single-Wire Corona Unipolar Charger

A single-wire corona unipolar aerosol charger with a sheath air to avoid particle loss was designed and experimental charging efficiencies were obtained at a fixed aerosol flow rate of 1 L/min using monodisperse silver nanoparticles of 2.5 to 20 nm in diameter. The charger has a cylindrical casing of 30 mm in inner diameter in which a gold wire of 50 μm in diameter and 2 mm in length is used as the discharge electrode. A two-dimensional (2-D) numerical model was developed to predict nanoparticle charging efficiency in the unipolar charger. Laminar flow field was solved by using the Semi-Implicit Method for Pressure Linked Equations (SIMPLER method), while electric potential and ion concentration fields were solved on the basis of Poisson and convection–diffusion equations, respectively. The charged particle concentration fields and charging efficiencies were then calculated on the basis of the convection–diffusion equation in which ion–particle combination coefficient was calculated by Fuchs diffusion charging theory (Fuchs, N. A. (1963 Fuchs, N. A. 1963. On the Stationary Charge Distribution on Aerosol Particles in a Bipolar Ionic Atmosphere. Geophys. Pura. Appl., 56: 185–193. [Crossref] , [Google Scholar]). On the Stationary Charge Distribution on Aerosol Particles in a Bipolar Ionic Atmosphere. Geophys. Pura. Appl., 56:185–193). Good agreement between predicted and experimental extrinsic charging efficiencies was obtained. Numerical results showed the advantage of using sheath air to minimize charged particle loss and indicated the location where major charged particle loss occurred. It is expected that the present model can be used to facilitate the design of more efficient corona-wire unipolar charger in the future.     

Electrical Agglomeration of Aerosol Particles in an Alternating Electric Field

A laboratory scale test system has been designed and constructed to study the electrical agglomeration of charged aerosol particles as a method to increase the fine particle collection efficiency of electrostatic precipitators. The system consists of test aerosol generator, aerosol charger, agglomerator chambers, and aerosol measurement equipment. Air atomizing nozzles and the TSI six-jet atomizer have been used as the test particle generators. The test particles have been charged by a corona discharge. Two types of agglomerator chambers have been investigated. In one agglomerator the gas flows between two parallel plates, across which the alternating high voltage is applied. The other agglomerator is a quadrupole structure with cylindrical electrodes positioned between the grounded plates. Particle concentration and size distribution measurements have been carried out downstream of the agglomerator with agglomerator voltage on and off. Particle concentrations and size distributions have been measured with differential mobility analyzer (DMA) and a Berner low pressure impactor. These measurements show that agglomeration causes about a 4%-8% decrease in the fine particle concentration when the total mass concentration is between 1 and 2 g/m³. There was no difference between the results measured with the parallel plate and the quadrupole agglomerator.     

Multiple charging of ultrafine particles in a corona charger

The charge distribution on ultrafine aerosol particles in the size range below 35 nm has been measured using a corona-based unipolar charger, in which ion generation and aerosol charging take place simultaneously in the region around a sharp-point discharge electrode. The mean number of charges per particle predicted by Fuchs’ diffusion charging theory is in relatively good agreement with the experimental results, and this implies that diffusion charging is the predominant mechanism in spite that the electric field in the charging region is very high. Since a steady state is unattainable in unipolar charging, the charge distribution depends on the geometry and operating conditions of each particular charger. When the present device operates under the conditions (nt-product) which yield the maximum charging efficiency, double charge appears on particles with diameter as small as 15 nm. At larger values of nt, 32 nm particles can acquire up to five charges. The critical particle diameter above which multiple charging occurs is about four times smaller than for bipolar radioactive chargers. In order to use corona charging in aerosol particle size measurement by electrical methods, the required mobility data inversion is thus straightforward for particle diameter below about 15 nm, but becomes quite complex for larger sizes.     

Design and Collection Efficiency of a New Electrostatic Precipitator for Bioaerosol Collection

We have developed and tested a new bioaerosol sampler in which airborne microorganisms are collected by electrostatic means. In this sampler, 2 ionizers charge the incoming particles if they carry insufficient electric charge for efficient collection. The organisms are then subjected to a precipitating electric field and are collected onto 2 square agar plates positioned along the flow axis. Tests with nonbiological NaCl particles versus B. subtilis var. niger (BG) spores and vegetative cells have shown that airborne microorganisms are collected more efficiently than nonbiological particles, even when the microorganisms have first passed through an electric charge neutralizer with no additional charging applied. The difference was attributed to the natural charges contained in cell membranes or spore coats of the microorganisms. Charge-neutralized BG spores and vegetative cells were collected at 4 L/min with efficiencies close to 80%, depending on the precipitation voltage, versus 50-60% for NaCl test particles. When incoming BG spores were charged with positive ions and then collected by a precipitating voltage of + 1,300 V, about 80% of the incoming spores were collected and more than 70% of incoming spores formed colonies. These experiments with BG spores have also indicated that there were no significant particle losses inside the sampler. The collection efficiency of biological and nonbiological particles increased to 90-100% when the particles were externally charged and the precipitating voltage was increased to more than - 4,000 V. It has also been shown that the aerosolized BG spores (used as anthrax simulants for bioaerosol sensors) carry a net negative electric charge. Thus the collection efficiency depends on the polarity of the electric field applied across the agar plates. These findings indicate that the collection of airborne microorganisms is possible by electrostatic precipitation without prior electric charging if the microorganisms already carry electric charges. These are usually high immediately after their release into the air.     

Free electron charging of ultrafine aerosol particles

Experiments of free electron charging of aerosol particles in the free molecule regime are reported. Monodisperse ultrafine silver particles of 5–30 nm were exposed to known concentrations of low energy electrons produced by ionization by -particles in a unipolar aerosol charger. The kinetic energy of the electrons was varied by changing the electric field intensity in the charger (i.e. between 93 and 279 V cm⁻¹). The range of Knudsen number for aerosol charging (i.e. the ratio of the electron mean free path to the particle radius) was from 30 to 261. The charged fraction was measured as a function of particle size in high-purity helium and nitrogen under different charging conditions. The experimental results for the combination coefficient between neutral particles and electrons suggested a free-molecule diffusion charging mechanism which was dependent on the electron mobility, transverse diffusion coefficient, mean free path, and mean kinetic energy (i.e. electron temperature). The functional dependence was similar to that given by the ionic charging theories of Natanson (1960, Sov. Phys. 5, 538–551), and Fuchs (1963, Geofis. Pura Appl. 56, 185–193) when the appropriate electron properties were used. The electron charging models of O'Hara et al. (1989, J. Aerosol Sci. 20, 313–330), and Zagnit'ko et al. (1989, Russ. J. Phys. Chem. 63, 883–888) did not fit the experimental results because they were not derived for the free-molecule regime. A modified Fuchs charging theory that uses an empirical accommodation coefficient for the electrons at the surface of the particle was used to fit the experimental results. Good agreement was found by using an accommodation coefficient of 0.4 for both helium and nitrogen.     

An Improved Method for Charging Submicron and Nano Particles with Uniform Charging Performance

An improved method for charging submicron and nano silver particles with uniform charging performance was developed. Monodisperse silver particles were grown into microdroplets through condensation. The aerodynamic diameter and GSD of the condensed droplets were the same regardless of their original diameter. The diameter of the droplets increased from 1.7μm to 2.5 μm as the temperature of the saturator increased from 45°C to 55°C. They were charged by an indirect corona-based charger, in which the ion-generation zone is followed by a particle-charging zone through which the condensed droplets pass. The charges of the droplets were controlled by varying the droplet size, ion concentration, and strength of electric field in the charger. The solvent of the charged droplets was evaporated in an evaporator. The size distribution of the evaporated particles was measured by SMPS spectrometer and compared with their original size distribution. The particles after evaporation were slightly larger than their original particles, due to recondensation. The total charge and number concentration of the particles were measured by aerosol electrometer and CPC, to calculate the average charge. Their electrical mobility distribution was measured by SMPS spectrometer without a neutralizer, to calculate the charge distribution and average charge of the evaporated particles. The results showed the average charges of the particles were similar, regardless of initial diameter and manner of calculation. The charge distributions of the evaporated particles were identical, except for 16.9 nm particles. Ion evaporation phenomenon of particles smaller than 40 nm in diameter was not detected.