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Evaluation of the performance of a soft X-ray charger for the bipolar charging of nanoparticles
Particuology 18 (2015) 165–169
Contents lists available at ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Short communication
Evaluation of the performance of a soft X-ray charger for the bipolar charging of nanoparticles Young Hun Yoon a , Choonkeun Bong b , Dae Seong Kim c,∗ a
Department of Integrated Environmental Systems, Pyeongtaek University, 3825 Seodongdae-ro, Pyeongtaek-Shi, Gyeonggi-do 450-701, Republic of Korea GreenSolus Co., #804, Byoksan Digital Valley 3-Cha, 271 Digital-ro, Guro-gu, Seoul 152-775, Republic of Korea c Global Frontier Center for Multiscale Energy Systems, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 31 October 2013 Received in revised form 3 April 2014 Accepted 1 May 2014 Keywords: Soft X-ray Neutralizer Bipolar charger Nanoparticle TDMA
a b s t r a c t The use of soft X-rays in a neutralizer represents an alternative technique that could replace conventional radioactive sources. In this study, we evaluated the charging characteristics of a soft X-ray aerosol neutralizer. In addition, the results from the evaluation of the soft X-ray charger were compared with results obtained using a neutralizer incorporating an 241 Am radioactive source. The tandem differential mobility analyzer technique was used previously to determine the size-dependent positive, negative, and neutral charge fractions of a soft X-ray neutralizer. This technique was used to show that the neutral fractions obtained using the soft X-ray charger agreed well with the predictions of bipolar diffusion charging theory, and that the soft X-ray charger could be used as a neutralizer for a scanning mobility particle sizer system. © 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
Introduction The bipolar diffusion charging of particles can be used to classify nanoparticles of a certain size using electrical mobility-based techniques, or to count particle numbers using an electrometer. The differential mobility analyzer (DMA) with a condensation particle counter (CPC) is an important instrument for measuring particle size distributions, and for classifying airborne particles into narrow size fractions. Raw data from a DMA can be converted into a size distribution if the bipolar charge distribution for the aerosol is known. Hence, the size distribution will be representative only if the bipolar charge distribution is accurately described. Bipolar charge distributions measured by Adachi, Kousaka, and Okuyama (1985) and Wiedensohler, Lütkemeier, Feldpausch, and Helsper (1986) supported the diffusion charging theories proposed by Fuchs (1963). Wiedensohler (1988) obtained the bipolar charge distribution for particles in the submicron size range, and his results were used to solve the problem of Boltzmann’s law and Fuchs’ charge distribution, in which the positive and negative charging ratios of particles were assumed to be identical.
∗ Corresponding author. Tel.: +82 2 880 1694. E-mail address: [email protected] (D.S. Kim).
In general, an equilibrium charge distribution can be achieved in an aerosol via the collision of the aerosol particles with bipolar ions generated by radioactive sources such as 241 Am, 85 Kr, and 210 Po (Cooper & Reist, 1973; Fuchs, 1963). Radioactive sources have been widely used, because of their ease of use. However, the application of radioactive sources has significant disadvantages, not least in that radioactive sources are subject to severe legal restrictions in some countries. In an attempt to provide alternatives to radioactive sources, some researchers have attempted to apply different physical principles to attain an equilibrium charge distribution. Stommel and Riebel (2004) developed a corona discharger for charging submicron particles. This method was demonstrated to be a good alternative to radioactive sources in terms of the equal production of positive and negative ions. However, the loss of charged particles due to electrostatic effects, and the nanoparticle generation caused by the electrode sputtering and the chemical reactions of the gaseous species were problematic. Kwon, Sakurai, Seto, and Kim (2006) investigated the charging characteristics of a surfacedischarge microplasma aerosol charger (SMAC) by measuring the discharge across the dielectric barrier. The charging probability of the SMAC for 10–200-nm-sized particles was determined for each particle size, and these values were compared with the charging probability of the radioactive source and the theoretically predicted values. However, it is possible that the nanoparticle generation
http://dx.doi.org/10.1016/j.partic.2014.05.005 1674-2001/© 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
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Fig. 1. Schematic illustration of the soft X-ray charger. The units are mm.
resulting from the electrode sputtering or the chemical reactions influenced the results. There is therefore a need for an alternative neutralizer that can mitigate the aforementioned problems, which include the loss of charged particles due to electrostatic effects, and the generation of nanoparticles via the sputtering of the discharging electrode and the chemical reactions of gaseous species. The use of soft X-rays in a neutralizer represents an alternative charging technique that provides relief from the expensive safety precautions and severe legal restrictions that are imposed in some countries for the use of radioactive sources. Shimada, Han, Okuyama, and Otani (2002) demonstrated soft X-rays as a desirable alternative to radioactive sources; they used soft X-rays to generate bipolar ions for aerosol charging. Lee, Kim, Shimada, and Okuyama (2005) applied a soft X-ray charger for size distribution measurements on polydisperse NaCl particles. They observed almost equal ion mobilities for positive and negative ions generated by soft X-ray discharging, and therefore obtained almost identical size distributions for positively and negatively charged particles. Kim, Kim, Kwon, and Park (2011) developed and evaluated soft X-ray chargers by measuring the charge fractions (positive, negative, and neutral) of ultrafine NaCl particles in the size range of 20–100 nm. They demonstrated that a soft X-ray charger was efficient for charging nanoparticles. In this study, we evaluated the charging characteristics of a soft X-ray aerosol neutralizer (a soft X-ray charger). In addition, the results from the evaluation of the soft X-ray charger were compared with the results obtained using a neutralizer incorporating an 241 Am radioactive source. The tandem differential mobility analyzer (TDMA) technique (Kim et al., 2011) was used to determine the size-dependent positive, negative, and neutral charge fractions, and the particle losses to evaluate the performance of the soft X-ray neutralizer. Experimental Fig. 1 shows a schematic diagram of the soft X-ray charger. The length and diameter of the chamber were 95 and 30 mm, respectively. The overall dimensions of the soft X-ray neutralizer were the same as those of the type C instrument used by Kim et al. (2011). The soft X-ray emitter (soft X-ray ionizer SXH-10H, Sunje Hi-Tek, Busan, Korea) generated soft X-rays from a circular beryllium window. The soft X-ray emitter was tightly attached to the aluminum chamber, and the soft X-rays were emitted from the opening window of the emitter with a solid angle of approximately 130◦ , to prevent the soft X-rays from radiating outside of the desired area.
The soft X-ray emitter produced both positive and negative ions; the negative and positive ions charged the particles in a bipolar fashion. Fig. 2 shows the experimental setup used to measure the size-dependent neutralization probability, and the positive and negative charge fractions of the initially neutralized particles. The experimental system comprised a clean air supply, an atomizer, a diffusion dryer, two neutralizers (241 Am neutralizers), test neutralizers (the soft X-ray charger and the 241 Am neutralizer), two differential mobility analyzers (TSI DMA, KRISS Controller; ISO 15900, 2009), two electrostatic precipitators (ESPs), a mixing tube, and a condensation particle counter (TSI CPC). Sodium chloride (NaCl) particles were used as test aerosols. An NaCl solution was atomized using compressed, clean air, and NaCl particles were obtained after the atomized solution was passed through the diffusion dryer. The sampling flow rate was maintained at 1 L/min. The 1st DMA generated monodisperse particles with sizes in the range from approximately 20 to 200 nm, and the selected particles were neutralized using an 241 Am neutralizer. The charged particles produced by the neutralizer were removed by the ESP, and only neutralized particles entered the test neutralizer. In the case of the tests performed using a flow rate higher than 2 L/min, a mixing tube was applied after the ESP, to make up for the lack of air. Filtered air was introduced into the entrance of the mixing tube, and was then mixed with the test particles that were introduced into the Venturi tube. After this mixing, the flow entered the test neutralizer. Particles were then introduced into the 2nd DMA and the CPC, or the ESP and the CPC. The combination of the 2nd DMA and the CPC was used to measure the positive or negative charge fraction, while the combination of the ESP and the CPC was used to determine the neutral fraction (Kim et al., 2011). With the combination of the DMA and the CPC, only charged particles (positively or negatively charged) survived passage through the DMA, because a negative or positive voltage was supplied in the DMA. Because the size-selected particles (i.e., monodisperse particles) passed through the second DMA and the CPC, the charge distribution could be calculated from the entire electrical mobility distribution, allowing the charge fraction to be calculated. With the combination of the ESP and the CPC, the ESP removed only the charged particles, and the remaining neutral particles could be counted by the CPC. Thus, the neutral fraction of particles could be obtained using this combination. In this work, these two methods were used to evaluate the particle charge distribution of the neutralizer. The particle loss in the soft X-ray charger was determined by measuring the particle number concentrations before and after the neutralizer.
Results and discussion In aerosol neutralizers, nanoparticle generation and particle losses are very important issues. Motivated by this, in this study, experimental tests were carried out at a flow rate of 1 L/min to investigate nanoparticle generation and particle losses. No significant particle generation was observed in the soft X-ray charger over the 24-h test period, and the number of particles generated was always below 0.01/cm3 . The particle losses were calculated by comparing the particle concentrations before and after the samples had passed through the soft X-ray charger. The results showed that the soft X-ray charger displayed a particle penetration value in excess of 97% for the measured particle size range (20–200 nm). Particle losses typically occur because of the Brownian diffusion of ultrafine particles (Kim, Park, Song, Kim, & Lee, 2003).
Y.H. Yoon et al. / Particuology 18 (2015) 165–169
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Fig. 2. The experimental set-up used to evaluate the aerosol neutralization probability and the charge distribution.
To investigate the particle size distortion in the soft X-ray charger, we compared the geometric mean diameter (Dg ) measured via scanning mobility particle sizer (SMPS) using a soft X-ray charger with the data obtained using an 241 Am neutralizer. The Dg,X-ray (soft X-ray charger) to Dg,Am (241 Am neutralizer) size ratio was approximately 1 (within ±3%) over the whole of the investigated size range. These results showed that size distortion did not occur in the soft X-ray charger. As shown in Fig. 3, the neutral fractions obtained in the soft Xray charger and the 241 Am neutralizer obtained using a sampling flow rate of 1 L/min were compared with Wiedensohler’s approximations (1988), for different particle sizes. The neutral fractions for the soft X-ray charger agreed well with the fractions obtained using the 241 Am neutralizer. The neutral fractions decreased as the particle size increased. In addition, the neutral fraction obtained using the soft X-ray charger showed good agreement with the numerical scheme fitting of Wiedensohler (1988) for bipolar diffusion charging. Fig. 4 shows the experimentally determined neutral fractions obtained in the soft X-ray charger and the 241 Am neutralizer as a function of flow rate, together with the Wiedensohler’s approximations, for various particle sizes. The neutral fractions of Wiedensohler’s approximations for bipolar diffusion charging are 71.03%, 58.1%, 42.36%, and 37.32% for 30-, 50-, 100-, and 130nm particles, respectively. As shown in Fig. 4, the neutral fractions in the soft X-ray charger decreased as the flow rate increased. In
Fig. 3. The neutral fractions obtained in the soft X-ray charger and the 241 Am neutralizer for different particle sizes.
the soft X-ray charger, the neutral fraction became constant in the range of approximately 0.3–2 L/min, and the fractions were very close to the values predicted by bipolar diffusion charging theory (Fuchs, 1963; Wiedensohler, 1988). For example, the neutral fractions of the soft X-ray charger were 71.1%, 58.1%, 41.5%, and 36.6% for 30-, 50-, 100-, and 130-nm particles, respectively, at a flow rate of 1 L/min. However, for flow rates higher than 3 L/min, the neutral fractions were too low, so the fractions did not follow the predictions of bipolar charging theory. In the tests performed using flow rates higher than 2 L/min, a mixing tube was applied after the ESP to make up for the lack of air. It was found that some of the particles were bipolarly charged again after the mixing was performed in the mixing tube. The higher the flow rate, the less chance there was to obtain charge equilibrium; consequently, the neutral fraction was lower because the particles were charged bipolarly at the entrance. Thus, it was concluded that the soft X-ray charger was effective for flow rates of less than 2 L/min. In the case of the 241 Am neutralizer, the neutral fractions were constant in the flow rate range from 0.3 to 7 L/min, and the determined fractions were very close to those predicted by bipolar diffusion charging theory, as shown in Fig. 4. It was therefore concluded that the 241 Am neutralizer would be useful for an SMPS system.
Fig. 4. The experimentally determined neutral fractions obtained in the soft Xray charger and the 241 Am neutralizer as a function of flow rate, together with Wiedensohler’s approximations, for various particle sizes.
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Fig. 5. Charge distributions obtained using the soft X-ray neutralizer for 30- (a), 50- (b), 100- (c), and 130-nm particles (d).
Fig. 5 shows the size-dependent charge distributions measured in the soft X-ray charger for 30-, 50-, 100-, and 130-nm particles, using a sampling flow rate of 1 L/min. The positive, negative, and neutral charge fractions of 30-, 50-, 100-, and 130-nm particles are shown in Fig. 5(a)–(d), respectively. The neutral fraction for the 30-nm particles was 71.1%, and the negative and positive fractions for singly charged particles were 15.9% and 12.7%, respectively. As shown in Fig. 5(a), there were very few doubly charged 30-nm particles. Similar to the 30-nm particles, the neutral fraction was relatively low for the 50-nm particles (58.1%), and the fraction of doubly charged particles was very small (1% for the negative fraction, and 0.7% for the positive fraction). The negative and positive fractions for the singly charged particles were 22.5% and 17.7%, respectively (see Fig. 5(b)). In the case of the 100-nm particles, the neutral fraction was somewhat lower (41.5%), but the fractions for the singly charged particles were far higher (27.5% for the negative fraction, and 21.6% for the positive fraction), in comparison with the values measured for the 30- and 50-nm particles. In addition, as shown in Fig. 5(c), quite a few doubly charged particles were generated in this particle
size range (with values of 5.7% for the negative fraction, and 3.7% for the positive fraction). The charge distribution for the 130-nm particles was similar to that measured for the 100-nm particles, as shown in Fig. 5(d). The neutral fraction for the 130-nm particles was 36.6%, and the negative and positive fractions for the singly charged particles were 28% and 22.1%, respectively. In the case of the doubly charged particles, the negative and positive fractions were 8.2% and 5.1%, respectively. The results showed that the positive and negative charge fractions increased, and the neutral fractions decreased, as the particle size increased. In addition, doubly charged particles were generated in the large particle size range, and the negative fraction was somewhat higher than the positive fraction. It was demonstrated that the proportion of negative ions in the bipolar charger was greater than the proportion of positive ions, and consequently, the negative charging ratio was higher than the positive charging ratio (asymmetric charging); this was because positive and negative ions are generally different in terms of their electrical mobility and mass (Adachi et al., 1985; Hoppel & Frick, 1990; Wiedensohler & Fissan, 1991).
Y.H. Yoon et al. / Particuology 18 (2015) 165–169
The long-term stability of the soft X-ray charger was determined using a sampling flow rate of 1 L/min. The results showed that the uncertainty in the stability of the soft X-ray charger over a period of 6 months was less than 1.5%, the data obtained agreed well with the predictions of bipolar diffusion charging theory (Fuchs, 1963; Wiedensohler, 1988) over the whole measurement period. It was concluded that the charge distributions of negatively and positively charged particles measured using the soft X-ray charger, as well as the neutral fractions, agreed well with the predictions of bipolar diffusion charging theory, and that the soft X-ray charger could be applied as a neutralizer for sampling flow rates lower than 2 L/min. Conclusions Positive, negative, and neutral charge fractions were evaluated to determine the charging characteristics of the soft X-ray charger used in this work. The results were compared with the predictions of bipolar diffusion charging theory and results obtained using an 241 Am neutralizer. It was shown that the neutral fractions obtained in the soft Xray charger agreed well with the predictions of bipolar diffusion charging theory (Fuchs, 1963; Wiedensohler, 1988) and the data obtained using the 241 Am neutralizer. The results indicated that the soft X-ray charger was effective for flow rates lower than 2 L/min, while the 241 Am neutralizer showed good efficiency over the whole of the measured flow rate range (0.3–7 L/min). In addition, the results showed that doubly charged particles were generated in the large particle size range, and the negative fraction was somewhat higher than the positive fraction (Adachi et al., 1985; Hoppel & Frick, 1990; Wiedensohler & Fissan, 1991). In conclusion, the particle charge distributions and the neutral fractions obtained using the soft X-ray charger agreed well with the predictions of bipolar diffusion charging theory; in addition, no nanoparticle generation was observed, and the particle penetration exceeded 97%. Thus, the soft X-ray charger could be used as a neutralizer for an SMPS system.
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Acknowledgement This work was supported by the Korean Government (Ministry of Environment) Eco-innovation Program (EI 401-112-017 and EI 2013000160001). References Adachi, M., Kousaka, Y., & Okuyama, K. (1985). Unipolar and bipolar diffusion charging of ultrafine aerosol particles. Journal of Aerosol Science, 16, 109–123. Cooper, D. W., & Reist, P. C. (1973). Neutralizing charged aerosols with radioactive sources. Journal of Colloid and Interface Science, 45, 17–26. Fuchs, N. A. (1963). On the stationary charge distribution on aerosol particles in a bipolar ionic atmosphere. Pure and Applied Geophysics, 56, 185–193. Hoppel, W. A., & Frick, G. M. (1990). The nonequilibrium character of the aerosol charge distributions produced by neutralizers. Aerosol Science and Technology, 12, 471–496. ISO 15900. (2009). Determination of particle size distribution – Differential electrical mobility analysis for aerosol particles. British standard, BS ISO 15900-(E). Kim, D. S., Kim, Y. M., Kwon, Y. T., & Park, K. (2011). Evaluation of a soft X-ray unipolar charger for charging nanoparticles. Journal of Nanoparticle Research, 13, 579–585. Kim, D. S., Park, S. H., Song, Y. M., Kim, D. H., & Lee, K. W. (2003). Brownian coagulation of polydisperse aerosols in the transition regime. Journal of Aerosol Science, 34, 859–868. Kwon, S. B., Sakurai, H., Seto, T., & Kim, Y. J. (2006). Charge neutralization of submicron aerosols using surface-discharge microplasma. Journal of Aerosol Science, 37, 483–499. Lee, H. M., Kim, C. S., Shimada, M., & Okuyama, K. (2005). Bipolar diffusion charging for aerosol nanoparticle measurement using soft X-ray charger. Journal of Aerosol Science, 36, 813–829. Shimada, M., Han, B., Okuyama, K., & Otani, Y. (2002). Bipolar charging of aerosol nanoparticles by a soft X-ray photoionizer. Journal of Chemical Engineering of Japan, 35, 786–793. Stommel, Y. G., & Riebel, U. (2004). A new corona discharge-based aerosol charger for submicron particles with low initial charge. Journal of Aerosol Science, 35, 1051–1069. Wiedensohler, A. (1988). An approximation of the bipolar charge distribution for particles in the submicron size range. Journal of Aerosol Science, 19, 387–389. Wiedensohler, A., & Fissan, H. J. (1991). Bipolar charge distributions of aerosol particles in high-purity argon and nitrogen. Aerosol Science and Technology, 14, 358–364. Wiedensohler, A., Lütkemeier, E., Feldpausch, M., & Helsper, C. (1986). Investigation of the bipolar charge distribution at various gas conditions. Journal of Aerosol Science, 17, 413–416.
Contents lists available at ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Short communication
Evaluation of the performance of a soft X-ray charger for the bipolar charging of nanoparticles Young Hun Yoon a , Choonkeun Bong b , Dae Seong Kim c,∗ a
Department of Integrated Environmental Systems, Pyeongtaek University, 3825 Seodongdae-ro, Pyeongtaek-Shi, Gyeonggi-do 450-701, Republic of Korea GreenSolus Co., #804, Byoksan Digital Valley 3-Cha, 271 Digital-ro, Guro-gu, Seoul 152-775, Republic of Korea c Global Frontier Center for Multiscale Energy Systems, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 31 October 2013 Received in revised form 3 April 2014 Accepted 1 May 2014 Keywords: Soft X-ray Neutralizer Bipolar charger Nanoparticle TDMA
a b s t r a c t The use of soft X-rays in a neutralizer represents an alternative technique that could replace conventional radioactive sources. In this study, we evaluated the charging characteristics of a soft X-ray aerosol neutralizer. In addition, the results from the evaluation of the soft X-ray charger were compared with results obtained using a neutralizer incorporating an 241 Am radioactive source. The tandem differential mobility analyzer technique was used previously to determine the size-dependent positive, negative, and neutral charge fractions of a soft X-ray neutralizer. This technique was used to show that the neutral fractions obtained using the soft X-ray charger agreed well with the predictions of bipolar diffusion charging theory, and that the soft X-ray charger could be used as a neutralizer for a scanning mobility particle sizer system. © 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
Introduction The bipolar diffusion charging of particles can be used to classify nanoparticles of a certain size using electrical mobility-based techniques, or to count particle numbers using an electrometer. The differential mobility analyzer (DMA) with a condensation particle counter (CPC) is an important instrument for measuring particle size distributions, and for classifying airborne particles into narrow size fractions. Raw data from a DMA can be converted into a size distribution if the bipolar charge distribution for the aerosol is known. Hence, the size distribution will be representative only if the bipolar charge distribution is accurately described. Bipolar charge distributions measured by Adachi, Kousaka, and Okuyama (1985) and Wiedensohler, Lütkemeier, Feldpausch, and Helsper (1986) supported the diffusion charging theories proposed by Fuchs (1963). Wiedensohler (1988) obtained the bipolar charge distribution for particles in the submicron size range, and his results were used to solve the problem of Boltzmann’s law and Fuchs’ charge distribution, in which the positive and negative charging ratios of particles were assumed to be identical.
∗ Corresponding author. Tel.: +82 2 880 1694. E-mail address: [email protected] (D.S. Kim).
In general, an equilibrium charge distribution can be achieved in an aerosol via the collision of the aerosol particles with bipolar ions generated by radioactive sources such as 241 Am, 85 Kr, and 210 Po (Cooper & Reist, 1973; Fuchs, 1963). Radioactive sources have been widely used, because of their ease of use. However, the application of radioactive sources has significant disadvantages, not least in that radioactive sources are subject to severe legal restrictions in some countries. In an attempt to provide alternatives to radioactive sources, some researchers have attempted to apply different physical principles to attain an equilibrium charge distribution. Stommel and Riebel (2004) developed a corona discharger for charging submicron particles. This method was demonstrated to be a good alternative to radioactive sources in terms of the equal production of positive and negative ions. However, the loss of charged particles due to electrostatic effects, and the nanoparticle generation caused by the electrode sputtering and the chemical reactions of the gaseous species were problematic. Kwon, Sakurai, Seto, and Kim (2006) investigated the charging characteristics of a surfacedischarge microplasma aerosol charger (SMAC) by measuring the discharge across the dielectric barrier. The charging probability of the SMAC for 10–200-nm-sized particles was determined for each particle size, and these values were compared with the charging probability of the radioactive source and the theoretically predicted values. However, it is possible that the nanoparticle generation
http://dx.doi.org/10.1016/j.partic.2014.05.005 1674-2001/© 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
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Fig. 1. Schematic illustration of the soft X-ray charger. The units are mm.
resulting from the electrode sputtering or the chemical reactions influenced the results. There is therefore a need for an alternative neutralizer that can mitigate the aforementioned problems, which include the loss of charged particles due to electrostatic effects, and the generation of nanoparticles via the sputtering of the discharging electrode and the chemical reactions of gaseous species. The use of soft X-rays in a neutralizer represents an alternative charging technique that provides relief from the expensive safety precautions and severe legal restrictions that are imposed in some countries for the use of radioactive sources. Shimada, Han, Okuyama, and Otani (2002) demonstrated soft X-rays as a desirable alternative to radioactive sources; they used soft X-rays to generate bipolar ions for aerosol charging. Lee, Kim, Shimada, and Okuyama (2005) applied a soft X-ray charger for size distribution measurements on polydisperse NaCl particles. They observed almost equal ion mobilities for positive and negative ions generated by soft X-ray discharging, and therefore obtained almost identical size distributions for positively and negatively charged particles. Kim, Kim, Kwon, and Park (2011) developed and evaluated soft X-ray chargers by measuring the charge fractions (positive, negative, and neutral) of ultrafine NaCl particles in the size range of 20–100 nm. They demonstrated that a soft X-ray charger was efficient for charging nanoparticles. In this study, we evaluated the charging characteristics of a soft X-ray aerosol neutralizer (a soft X-ray charger). In addition, the results from the evaluation of the soft X-ray charger were compared with the results obtained using a neutralizer incorporating an 241 Am radioactive source. The tandem differential mobility analyzer (TDMA) technique (Kim et al., 2011) was used to determine the size-dependent positive, negative, and neutral charge fractions, and the particle losses to evaluate the performance of the soft X-ray neutralizer. Experimental Fig. 1 shows a schematic diagram of the soft X-ray charger. The length and diameter of the chamber were 95 and 30 mm, respectively. The overall dimensions of the soft X-ray neutralizer were the same as those of the type C instrument used by Kim et al. (2011). The soft X-ray emitter (soft X-ray ionizer SXH-10H, Sunje Hi-Tek, Busan, Korea) generated soft X-rays from a circular beryllium window. The soft X-ray emitter was tightly attached to the aluminum chamber, and the soft X-rays were emitted from the opening window of the emitter with a solid angle of approximately 130◦ , to prevent the soft X-rays from radiating outside of the desired area.
The soft X-ray emitter produced both positive and negative ions; the negative and positive ions charged the particles in a bipolar fashion. Fig. 2 shows the experimental setup used to measure the size-dependent neutralization probability, and the positive and negative charge fractions of the initially neutralized particles. The experimental system comprised a clean air supply, an atomizer, a diffusion dryer, two neutralizers (241 Am neutralizers), test neutralizers (the soft X-ray charger and the 241 Am neutralizer), two differential mobility analyzers (TSI DMA, KRISS Controller; ISO 15900, 2009), two electrostatic precipitators (ESPs), a mixing tube, and a condensation particle counter (TSI CPC). Sodium chloride (NaCl) particles were used as test aerosols. An NaCl solution was atomized using compressed, clean air, and NaCl particles were obtained after the atomized solution was passed through the diffusion dryer. The sampling flow rate was maintained at 1 L/min. The 1st DMA generated monodisperse particles with sizes in the range from approximately 20 to 200 nm, and the selected particles were neutralized using an 241 Am neutralizer. The charged particles produced by the neutralizer were removed by the ESP, and only neutralized particles entered the test neutralizer. In the case of the tests performed using a flow rate higher than 2 L/min, a mixing tube was applied after the ESP, to make up for the lack of air. Filtered air was introduced into the entrance of the mixing tube, and was then mixed with the test particles that were introduced into the Venturi tube. After this mixing, the flow entered the test neutralizer. Particles were then introduced into the 2nd DMA and the CPC, or the ESP and the CPC. The combination of the 2nd DMA and the CPC was used to measure the positive or negative charge fraction, while the combination of the ESP and the CPC was used to determine the neutral fraction (Kim et al., 2011). With the combination of the DMA and the CPC, only charged particles (positively or negatively charged) survived passage through the DMA, because a negative or positive voltage was supplied in the DMA. Because the size-selected particles (i.e., monodisperse particles) passed through the second DMA and the CPC, the charge distribution could be calculated from the entire electrical mobility distribution, allowing the charge fraction to be calculated. With the combination of the ESP and the CPC, the ESP removed only the charged particles, and the remaining neutral particles could be counted by the CPC. Thus, the neutral fraction of particles could be obtained using this combination. In this work, these two methods were used to evaluate the particle charge distribution of the neutralizer. The particle loss in the soft X-ray charger was determined by measuring the particle number concentrations before and after the neutralizer.
Results and discussion In aerosol neutralizers, nanoparticle generation and particle losses are very important issues. Motivated by this, in this study, experimental tests were carried out at a flow rate of 1 L/min to investigate nanoparticle generation and particle losses. No significant particle generation was observed in the soft X-ray charger over the 24-h test period, and the number of particles generated was always below 0.01/cm3 . The particle losses were calculated by comparing the particle concentrations before and after the samples had passed through the soft X-ray charger. The results showed that the soft X-ray charger displayed a particle penetration value in excess of 97% for the measured particle size range (20–200 nm). Particle losses typically occur because of the Brownian diffusion of ultrafine particles (Kim, Park, Song, Kim, & Lee, 2003).
Y.H. Yoon et al. / Particuology 18 (2015) 165–169
167
Fig. 2. The experimental set-up used to evaluate the aerosol neutralization probability and the charge distribution.
To investigate the particle size distortion in the soft X-ray charger, we compared the geometric mean diameter (Dg ) measured via scanning mobility particle sizer (SMPS) using a soft X-ray charger with the data obtained using an 241 Am neutralizer. The Dg,X-ray (soft X-ray charger) to Dg,Am (241 Am neutralizer) size ratio was approximately 1 (within ±3%) over the whole of the investigated size range. These results showed that size distortion did not occur in the soft X-ray charger. As shown in Fig. 3, the neutral fractions obtained in the soft Xray charger and the 241 Am neutralizer obtained using a sampling flow rate of 1 L/min were compared with Wiedensohler’s approximations (1988), for different particle sizes. The neutral fractions for the soft X-ray charger agreed well with the fractions obtained using the 241 Am neutralizer. The neutral fractions decreased as the particle size increased. In addition, the neutral fraction obtained using the soft X-ray charger showed good agreement with the numerical scheme fitting of Wiedensohler (1988) for bipolar diffusion charging. Fig. 4 shows the experimentally determined neutral fractions obtained in the soft X-ray charger and the 241 Am neutralizer as a function of flow rate, together with the Wiedensohler’s approximations, for various particle sizes. The neutral fractions of Wiedensohler’s approximations for bipolar diffusion charging are 71.03%, 58.1%, 42.36%, and 37.32% for 30-, 50-, 100-, and 130nm particles, respectively. As shown in Fig. 4, the neutral fractions in the soft X-ray charger decreased as the flow rate increased. In
Fig. 3. The neutral fractions obtained in the soft X-ray charger and the 241 Am neutralizer for different particle sizes.
the soft X-ray charger, the neutral fraction became constant in the range of approximately 0.3–2 L/min, and the fractions were very close to the values predicted by bipolar diffusion charging theory (Fuchs, 1963; Wiedensohler, 1988). For example, the neutral fractions of the soft X-ray charger were 71.1%, 58.1%, 41.5%, and 36.6% for 30-, 50-, 100-, and 130-nm particles, respectively, at a flow rate of 1 L/min. However, for flow rates higher than 3 L/min, the neutral fractions were too low, so the fractions did not follow the predictions of bipolar charging theory. In the tests performed using flow rates higher than 2 L/min, a mixing tube was applied after the ESP to make up for the lack of air. It was found that some of the particles were bipolarly charged again after the mixing was performed in the mixing tube. The higher the flow rate, the less chance there was to obtain charge equilibrium; consequently, the neutral fraction was lower because the particles were charged bipolarly at the entrance. Thus, it was concluded that the soft X-ray charger was effective for flow rates of less than 2 L/min. In the case of the 241 Am neutralizer, the neutral fractions were constant in the flow rate range from 0.3 to 7 L/min, and the determined fractions were very close to those predicted by bipolar diffusion charging theory, as shown in Fig. 4. It was therefore concluded that the 241 Am neutralizer would be useful for an SMPS system.
Fig. 4. The experimentally determined neutral fractions obtained in the soft Xray charger and the 241 Am neutralizer as a function of flow rate, together with Wiedensohler’s approximations, for various particle sizes.
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Fig. 5. Charge distributions obtained using the soft X-ray neutralizer for 30- (a), 50- (b), 100- (c), and 130-nm particles (d).
Fig. 5 shows the size-dependent charge distributions measured in the soft X-ray charger for 30-, 50-, 100-, and 130-nm particles, using a sampling flow rate of 1 L/min. The positive, negative, and neutral charge fractions of 30-, 50-, 100-, and 130-nm particles are shown in Fig. 5(a)–(d), respectively. The neutral fraction for the 30-nm particles was 71.1%, and the negative and positive fractions for singly charged particles were 15.9% and 12.7%, respectively. As shown in Fig. 5(a), there were very few doubly charged 30-nm particles. Similar to the 30-nm particles, the neutral fraction was relatively low for the 50-nm particles (58.1%), and the fraction of doubly charged particles was very small (1% for the negative fraction, and 0.7% for the positive fraction). The negative and positive fractions for the singly charged particles were 22.5% and 17.7%, respectively (see Fig. 5(b)). In the case of the 100-nm particles, the neutral fraction was somewhat lower (41.5%), but the fractions for the singly charged particles were far higher (27.5% for the negative fraction, and 21.6% for the positive fraction), in comparison with the values measured for the 30- and 50-nm particles. In addition, as shown in Fig. 5(c), quite a few doubly charged particles were generated in this particle
size range (with values of 5.7% for the negative fraction, and 3.7% for the positive fraction). The charge distribution for the 130-nm particles was similar to that measured for the 100-nm particles, as shown in Fig. 5(d). The neutral fraction for the 130-nm particles was 36.6%, and the negative and positive fractions for the singly charged particles were 28% and 22.1%, respectively. In the case of the doubly charged particles, the negative and positive fractions were 8.2% and 5.1%, respectively. The results showed that the positive and negative charge fractions increased, and the neutral fractions decreased, as the particle size increased. In addition, doubly charged particles were generated in the large particle size range, and the negative fraction was somewhat higher than the positive fraction. It was demonstrated that the proportion of negative ions in the bipolar charger was greater than the proportion of positive ions, and consequently, the negative charging ratio was higher than the positive charging ratio (asymmetric charging); this was because positive and negative ions are generally different in terms of their electrical mobility and mass (Adachi et al., 1985; Hoppel & Frick, 1990; Wiedensohler & Fissan, 1991).
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The long-term stability of the soft X-ray charger was determined using a sampling flow rate of 1 L/min. The results showed that the uncertainty in the stability of the soft X-ray charger over a period of 6 months was less than 1.5%, the data obtained agreed well with the predictions of bipolar diffusion charging theory (Fuchs, 1963; Wiedensohler, 1988) over the whole measurement period. It was concluded that the charge distributions of negatively and positively charged particles measured using the soft X-ray charger, as well as the neutral fractions, agreed well with the predictions of bipolar diffusion charging theory, and that the soft X-ray charger could be applied as a neutralizer for sampling flow rates lower than 2 L/min. Conclusions Positive, negative, and neutral charge fractions were evaluated to determine the charging characteristics of the soft X-ray charger used in this work. The results were compared with the predictions of bipolar diffusion charging theory and results obtained using an 241 Am neutralizer. It was shown that the neutral fractions obtained in the soft Xray charger agreed well with the predictions of bipolar diffusion charging theory (Fuchs, 1963; Wiedensohler, 1988) and the data obtained using the 241 Am neutralizer. The results indicated that the soft X-ray charger was effective for flow rates lower than 2 L/min, while the 241 Am neutralizer showed good efficiency over the whole of the measured flow rate range (0.3–7 L/min). In addition, the results showed that doubly charged particles were generated in the large particle size range, and the negative fraction was somewhat higher than the positive fraction (Adachi et al., 1985; Hoppel & Frick, 1990; Wiedensohler & Fissan, 1991). In conclusion, the particle charge distributions and the neutral fractions obtained using the soft X-ray charger agreed well with the predictions of bipolar diffusion charging theory; in addition, no nanoparticle generation was observed, and the particle penetration exceeded 97%. Thus, the soft X-ray charger could be used as a neutralizer for an SMPS system.
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