High-Efficiency Unipolar Charger for Sub-10 nm Aerosol Particles Using Surface-Discharge Microplasma with a Voltage of Sinc Function

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: High-Efficiency Unipolar Charger for Sub-10 nm Aerosol Particles Using Surface-Discharge Microplasma with a Voltage of Sinc Function Eric Manirakiza, Takafumi Seto, Saho Osone, Kanta Fukumori & Yoshio Otani To cite this article: Eric Manirakiza, Takafumi Seto, Saho Osone, Kanta Fukumori & Yoshio Otani (2013) High-Efficiency Unipolar Charger for Sub-10 nm Aerosol Particles Using Surface- Discharge Microplasma with a Voltage of Sinc Function, Aerosol Science and Technology, 47:1, 60-68, DOI: / To link to this article: View supplementary material Accepted author version posted online: 12 Sep Published online: 12 Sep Submit your article to this journal Article views: 454 View related articles Citing articles: 5 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 03 December 2017, At: 11:02

2 Aerosol Science and Technology, 47:60 68, 2013 Copyright C American Association for Aerosol Research ISSN: print / online DOI: / High-Efficiency Unipolar Charger for Sub-10 nm Aerosol Particles Using Surface-Discharge Microplasma with a Voltage of Sinc Function Eric Manirakiza, Takafumi Seto, Saho Osone, Kanta Fukumori, and Yoshio Otani Department of Chemical and Material Engineering, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Japan Our group developed a high-performance unipolar charger for sub-10-nm particles using a surface-discharge microplasma by increasing the charging time and minimizing the electrostatic deposition loss of the charged particles. An investigation of the discharge voltages of various discharge voltage waveforms demonstrated that a sinc function of time, t, thatis,(sinωt)/ωt with a bias voltage, achieved a high extrinsic charging efficiency (a high yield of charged particles) by generating a high concentration of ions and suppressing the electrostatic deposition of the charged particles. In trial operation at the optimal discharge voltage and an aerosol flow rate of 2.5 L/min, the charger attained intrinsic and extrinsic charging efficiencies of 79.3 and 61.4% for 10-nm particles and intrinsic and extrinsic charging efficiencies of 48.0 and 34.6% for 5-nm particles, respectively. [Supplementary materials are available for this article. Go to the publisher s online edition of Aerosol Science and Technology to view the free supplementary files.] 1. INTRODUCTION High-efficiency electrical charging of aerosol particles is essential for the practical application of nanoparticles in various fields. The enhanced charging efficiency of nanoparticles allows highly sensitive detection of atmospheric aerosol particles, precise control of particle transport in nanostructuring processes, and high removal efficiency in contamination control. Various types of chargers have been developed to enhance the charging efficiency for nanoparticles using corona discharge Received 16 May 2012; accepted 18 August This research was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 4003), Impact of Aerosols on Plants and Human Health in East Asia, and Scientific Research (b) (No ) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Address correspondence to Takafumi Seto, Department of Chemical and Material Engineering, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa , Japan. seto@t.kanazawa-u.ac.jp 60 (Pui et al. 1988; Buscher et al. 1994; Medved et al. 2000; Hernandez-Sierra et al. 2003; Qi et al. 2007; Kimoto et al. 2010; Tsai et al. 2010; Chen et al. 2011; Li and Chen 2011) and surface-discharge microplasmas (Kwon et al. 2006, 2007; Osone et al. 2012) as ion sources. Marquard et al. (2006) reviewed the performances of various electrical aerosol chargers by assessing their charging effectiveness and the particle loss within them. One measure of effectiveness for a charger is its intrinsic charging efficiency. The intrinsic charging efficiency (η intr )isusually defined as the ratio of the concentration of charged particles downstream of a charger under a no-particle-loss condition to the concentration of neutral particles introduced into the charger. The intrinsic charging efficiency is the fraction of particles that physically collide with ions. The performance of a unipolar aerosol charger is sometimes associated with a specific charging parameter, that is, the product of ion concentration and charging time (Nt-product). Qi et al. (2007), for example, reported an η intr value of 60% for 10-nm particles at Nt = s/m 3. Given that high diffusional loss and electrostatic loss are inevitable in the charging of nanoparticles, another kind of charging efficiency, namely, extrinsic charging efficiency, is also employed. The extrinsic charging efficiency (η extr ) is generally defined as the ratio of the concentration of charged particles at the outlet of the charger to the concentration of neutral particles introduced into the charger. The definition of the extrinsic charging efficiency, however, varies from one research group to another. Some groups, for example, define η extr based on the particle concentration upstream of the charger (Kruis and Fissan 2001; Kimoto et al. 2010; Li and Chen 2011), while others define it based on the particle concentration downstream (Kwon et al. 2007). As we go on to explain later, we use the number concentration of particles entering the charger as a reference value in defining η extr for this study. The extrinsic charging efficiency is a crucial parameter in selecting a charger for the subsequent electrostatic classification of aerosol particles. One way to minimize the loss of charged particles is to control both the ion transport and particle transport within the charger. Wiedensohler et al. (1990), for example,

3 ELECTRICAL CHARGING OF SUB-10 NM PARTICLES 61 used bipolar ions generated by two α-ray sources ( 244 Curium) to develop a new type of charger in which the charged particles moved under the influence of an alternating electric field. Disappointingly, this charger had an η extr as low as 12% for 7-nm particles. A few years later, Buscher et al. (1994) developed a new type of charger with a square wave high voltage applied to a corona needle. Next, Chen and Pui (1999) succeeded in increasing the charging efficiency by using an electric field to separate the positive and negative ions and the sheath air for suppressing the particle deposition. Their charger achieved an η extr of 65% for 10-nm particles. In the 2000s, Medved et al. (2000) and Qi et al. (2007) applied an ion jet to corona-based chargers. Their success in charging nanoparticles with diameters of less than 10 nm was compromised, however, by the limitations in ion concentration and residence (charging) time for all corona-based chargers. Tsai et al. (2010) improved the charging efficiency using a corona-based charger with multiple discharging wires and sheath air near the charger s wall. They reported an η extr of about 40% for 10-nm particles. More recently, Li and Chen (2011) developed a charger with mechanisms in place to condition the electrical charges on particles by minimizing the overcharging of large particles. They reported an η extr of about 35% for 10-nm particles when the charger was operated at a volumetric flow rate of 3 L/min. Kimoto et al. (2010) developed the Small Mixing-type Unipolar Charger (SMUC), a new type of corona-based charger with greatly reduced particles loss for particles in the size range of 3 40 nm. The use of a smallvolume charging chamber in SMUC significantly enhanced the experimentally measured η extr for sub-10 nm aerosol particles. SMUC had the highest charging efficiency reported so far, with an η extr of 55 and 65% for negative and positive particles of 10 nm, respectively. Further analysis seems to be necessary, however, as the reported efficiencies were both higher and less size-dependent than the theoretical predictions. This type of charger and several of the other chargers reported previously (Buscher et al. 1994; Chen and Pui 1999; Qi et al. 2007; Tsai et al. 2010; Chien et al. 2011) require a sheath gas flow to generate an ion-rich gas jet. It thus remains difficult to charge sub-10-nm particles efficiently without relying on sheath gas or preventing the sheath gas from diluting the aerosol particles. In the above-mentioned charger by Chen and Pui (1999), for example, the aerosol was diluted to a factor of 4, hence the η extr on a particle-number-concentration basis would be one fourth of that reported in their paper (Hernandez-Sierra et al. 2003). Kwon et al. (2005, 2006, 2007) attempted to attain a high charging efficiency and low particle loss by developing a new type of aerosol charger with a surface-discharge to generate ions without the use of sheath gas flow (Surface-discharge Microplasma Aerosol Charger: SMAC). The surface-discharge unit (22 mm in length) was made of a thin mica film with a micro-patterned discharge and ground electrodes on each side. In the initial experiments with SMAC, the system was used to neutralize the aerosol charge (Kwon et al. 2005, 2006) and generate unipolar ions through the application of a DC-biased pulse voltage to the discharge electrode (Kwon et al. 2007). Osone et al. (2012) markedly improved the charging efficiency of this unipolar charger by introducing a longer surface-discharge unit (275 mm in length) for ion generation. Table 1 lists the TABLE 1 Comparison of the SMAC by Kwon et al. (2007), the SMAC by Osone et al. (2012), and the present charger SMAC by Kwon SMAC by Osone et al. (2007) et al. (2012) Present charger Dimensions of the charging zone SUS cylindrical tube SUS cylindrical tube SUS semi-cylindrical tube 36-mm-inner diameter 22-mm-inner diameter 15.6-mm-inner diameter 140-mm long 275-mm long 300-mm long Disposition of the ionizer chip Outside of the charging zone (ions supplied via a slit) Inside the charging zone (set into the charging zone) Inside the charging zone (set into the charging zone) Applied voltage DC-biased pulse DC-biased pulse DC-biased sin x/x wave (2 kv: peak-peak) (2 3 kv: peak-peak) (3 4.5 kv: peak-peak) Length of the ionizer chip 22 mm 275 mm 300 mm Volume of the charging zone cm cm cm 3 Effective charging volume 39.0 cm cm cm 3 Effective charging time (t) at 1.5 LPM 1.56 s 4.2 s 1.2 s (residence time) (5.7 s) (4.2 s) (1.2 s) Measured ion concentration (N) per m 3 N/A per m 3 Effective ion concentration (N) per m 3( 1) per m 3( 1) per m 3( 2) Nt-product s/m 3( 1) s/m 3( 1) s/m 3( 2) Intrinsic charging efficiency at 10 nm N/A 0.90 at 1.5 LPM 0.79 at 2.5 LPM Extrinsic charging efficiency at 10 nm 0.22 at 1.5 LPM (10.5 nm) N/A 0.61 at 2.5 LPM Estimated values from experimental charging efficiencies at 1.5 LPM ( 1) and 2.5 LPM ( 2).

4 62 E. MANIRAKIZA ET AL. basic characteristics of the two SMAC chargers in comparison with the charger we are proposing here. Osone et al. (2012) achieved an Nt-product of about s/m 3. Their charger attained an intrinsic charging efficiency (η intr ) equal to 90% for 10-nm particles when the particle charging time (t) was increased. While the microplasma-based aerosol chargers have a charging efficiency higher than corona-based chargers, the sub- 10-nm particles charged by the former inevitably demonstrated a high electrostatic charger loss. Kwon et al. (2007), for example, reported an η extr of about 6 and 22% for 4.5-nm and 10.5-nm particles, respectively. In the present study, we seek to optimize the applied voltage for microplasma generation with a view to meeting the current demand for a nanoparticle aerosol charger with both a high-charging efficiency and high throughput. To this end, we propose a novel type of microplasma-based charger for nanoparticles in the sub-10-nm size range. Our principal objective is to minimize the loss of particles charged at a relatively high charging efficiency, η intr. Our strategy was to achieve a low particle loss, that is, a high η extr, by optimizing the discharge voltage waveform. A sinc (sin x/x) voltage waveform yielded a high charging efficiency and a low particle-deposition loss by controlling the transport of both the ions and charged particles in the charger. We also investigated how the aerosol flow rate influenced the charging efficiencies (η extr, η intr ) with the optimal discharge voltage waveform. 2. EXPERIMENTAL EVALUATION 2.1. Overview of the Charger Figure 1a is a schematic of the microplasma-based aerosol charger we are proposing here. The charger has the same basic design as that reported by Osone et al. (2012), but with a longer FIG. 1. Schematic of (a) the microplasma-based unipolar charger and (b) a cross-section of the charging chamber. FIG. 2. Upper view of the ionizer chip and the disposition of electrodes. microplasma-based ionizer chip (300 mm in length) placed at the center of the charger to increase the Nt-product. As shown in Figure 2, the ionizer chip is composed of a micro-patterned discharge electrode and ground electrode glued on the respective sides of a dielectric barrier. The dielectric barrier is a rectangular mica sheet of 300 mm in length, 7.5 mm in width, and 0.13 mm in thickness. The discharge electrode and ground electrodes are made of 0.03-mm-thick stainless steel and patterned with triangles at recurring intervals. The distance between the apices of the triangles is 2.4 mm and the clearance between the mica-separated discharge and ground electrodes is 0.09 mm. When a high voltage (pulse or AC voltage) is applied to the electrodes, an electric discharge generates a microplasma in the vicinity of the apices of each electrode. As many as 115 apices are arranged along the length of each electrode. Concentrated ions are thus generated every 2.4 mm along the ionizer chip, which ensures a uniform and constant concentration of unipolar ions along the electrodes. Positives ions are generated when a positive biased AC or high-voltage pulse is applied to the electrode. The ionizer chip is set in the middle of a stainless steel cylinder pipe, as shown in Figure 1b. The tube has an inner diameter of 15.6 mm and length of 300 mm. The charging chamber has a volume of about 29 cm 3 and a residence time of about 0.49 s when the charger is operated at a volumetric flow rate of 3.5 L/min. For the sake of comparison, Table 1 lists the basic design characteristics and operating parameters of the SMAC presented by Kwon et al. (2007), the SMAC presented by Osone et al. (2012), and the charger we are proposing here. Our charger differs from the SMACs reported by Kwon et al. (2007) and Osone et al. (2012) in three important respects: a longer microplasma ionizer chip, a longer charging zone but smaller charging volume, and the application of the sinc function to drive the discharge for producing ions. As previously mentioned, the longer ionizer chip placed at the center of the semi-cylindrical tube provides a uniform ion-filled space in the charger for the assurance of a more effective mixing of ions and particles. As we will go on to explain, the use of sinc function voltage to drive the discharge led to the generation of a much higher ion concentration (N = per m 3 ; effective value, N = per m 3 ) than has been reported by Kwon et al. (2007) or Osone et al. (2012). Yet because of the small charging chamber volume in the current design, the aerosol residence time and effective charging time are shorter than those reported by Osone et al. (2012), which results in a smaller Nt-product (Nt = per m 3 ) (Table 1).

5 ELECTRICAL CHARGING OF SUB-10 NM PARTICLES 63 Another point we will go on to discuss is the application of a sinc wave voltage with a DC bias component in order to suppress electrostatic loss. Since the bias voltage affects the transport of both charged particles and ions, this charger may demonstrate variable Nt-products when certain discharge voltage parameters are used. Thus, the above-mentioned Nt-product value was the value measured under the test conditions that resulted in the lowest electrostatic loss of particles; that is, when an extrinsic charging efficiency (η extr ) of 61% was obtained for 10-nm particles at an aerosol flow rate of 2.5 L/min. Compared with the SMAC from Kwon et al. (2007), our new design turned out to have an improvement factor of about Experimental Setup Figure 3 is a schematic diagram of the experimental setup for evaluating the performance of the charger. Polydisperse uncharged silver (Ag) particles were generated by the evaporationcondensation method. The furnace temperature was set at a temperature between C, depending on the desired particle size (3 15 nm) and concentration ( particles/cm 3 ). Clean dry nitrogen gas (N 2 > 99%, O 2 < 0.7%, H 2 O < 3 ppm) was used as a carrier gas. The generated polydisperse neutral particles were passed through an 241 Am neutralizer at the flow rate of 1.5 L/min. A monodisperse test aerosol was prepared by classifying these particles using a nano differential mobility analyzer (Nano-DMA, TSI, Model 3085). The Nano- DMA aerosol-to-sheath flow rate ratio was 1/10. The output of the Nano-DMA was once again passed through an 241 Am neutralizer and then through a coaxial tubular electrostatic precipitator (ESP). A 2 kv dc voltage was supplied to the ESP for the complete removal of the charged particles. Thus, monodisperse neutral test particles in a diameter range of 3 to 10 nm were continuously introduced into the charger. As shown in Figure 3, an additional particle-free nitrogen gas was added to the main aerosol flow (1.5 L/min) to adjust the total aerosol flow rate to 2.5, 3.5, and 4.5 L/min. Figure 3 also shows the experimental system used to control the applied discharge voltage. The source voltage waveform ( 10 to +10 V) was generated using a Function Generator (FG; Wavetek, Model 395) programmed to control both a specific electrical waveform voltage and offset voltage component. The output voltage of the FG was amplified 500 times using a highvoltage amplifier (Matsusada Precision Inc., Model HEOPT- 5B20). The voltage amplitude (V 0 ), repetition rate (frequency, f ), and discharge current (I d ) were measured using a digital oscilloscope (Tektronix DP04032 Digital Phosphor Oscilloscope) coupled with a high-voltage probe and current probe. In section 3 we will go on to discuss the details of the applied waveform Evaluation of the Penetration and Charging Efficiencies The particle penetration through the charger was determined as the ratio of the particle concentration upstream of the charger to that downstream of the charger. Number concentrations of nanoparticles were measured using a condensation particle counter (CPC, TSI, Model 3776) capable of detection down to a minimum particle size of 2.5 nm. For conformance with the CPC sampling rate of 1.5 L/min, excess aerosol flow was discarded at the inlet of the CPC when the charger was operated at a flow rate higher than 1.5 L/min. Penetration, P OFF, was defined as the penetration when the charger was idle (OFF). P OFF is given by Equation (1), where n 0 in is the concentration of neutral particles at the charger inlet and n 0 OFF is the concentration of neutral of particles at the charger outlet when the charger is OFF. P OFF = n0 OFF n 0 in Figure 3 also depicts the system set up to evaluate the charging efficiency of the charger. In this study we defined the extrinsic charging efficiency as the ratio of the concentration of charged particles leaving the charger outlet to the concentration of neutral particles at the charger inlet. The intrinsic charging efficiency was defined as the ratio of the concentration of charged particles at the charger outlet when there was no loss of charged particles. The charger performance was evaluated using a Charger-ESP-CPC combination system (Figure 3). The intrinsic charging efficiency, η intr(exp), was calculated as: η intr(exp) = n0 in n0 in (1 P OFF) n out,on n 0 in where n out,on is the particle concentration at the ESP outlet when a sufficient DC voltage is supplied to the ESP to precipitate all of the charged particles. We should note that n 0 in was calculated using a penetration (P OFF ) measured in separate experiments. Thus, n 0 in was defined by n0 in = n0 out,off /P OFF, where n 0 out,off is the particle concentration at the outlet of the ESP when the FIG. 3. Schematic diagram of the experimental setup for the evaluation of the charging performance. [1] [2]

6 64 E. MANIRAKIZA ET AL. voltages of the ionizer and ESP were both OFF. Given the small dimension of the ESP, we assumed that the diffusional loss of particles inside the ESP was negligible. For a given particle size, the extrinsic charging efficiency was evaluated by a method similar to that described by Hernandez- Sierra et al. (2003). Hence, η extr(exp) = n out,off n out,on n 0 in where n out,off is the particle concentration at the ESP outlet when the voltage of the ionizer is ON but the voltage of the ESP is OFF. To determine the particle generation from the ionizer chip due to sputtering and chemical reaction, we also measured the particle concentration at the charger outlet without introducing test particles. The particle concentration in this experiment suddenly dropped from less than 10 particles/cm 3 at the starting point of discharge with a discharge amplitude voltage lower than V 0 = 3.5 kv, to 0/cm 3 after the discharge. 3. EXPERIMENTAL RESULTS AND DISCUSSION In the present work, sin x/x voltage waveform was used for the generation of unipolar ions. A unit sin x/x waveform is expressed as: [3] V (t) = [(V o sin x)/x] + V bias [4] x = 2πκft = 2πκt T where t [s] is the time, V 0 [V] is the peak voltage, V bias [V] is the offset voltage or bias voltage, f [Hz] is the frequency, T is the cycle time [s] (time interval between larger peaks), and κ is a constant (κ = 25 in the present work). Figure 4 shows the sin x/x waveform for f = 1.5 khz, V 0 = 3.05 kv, and FIG. 4. Typical negative sin /x voltage waveform and discharge current waveform when f = 1500 Hz, V o = 3.05 kv, and V bias =+850 V. [5] TABLE 2 Extrinsic charging efficiencies of 10 nm-particles obtained experimentally by various voltage waveforms at Q = 3.5 L/min. DC pulses are those used by Kwon et al. (2007), and the corresponding extrinsic efficiency is for 10.5-nm particles Extrinsic Bias charging Amplitude Frequency voltage efficiency Waveforms [kv] [khz] [kv] [%] ramp ( ) Triangle Square Sine ramp (+) DC pulses 1 ± sin x/x ( ) Kwon et al. (2007). V bias =+0.85 kv. The discharge occurs at the largest peak for a small fraction of a second. For the waveform shown in Figure 4, the discharge took place for about 27 µs at the largest peak and had a discharge current of I d = 15 ma (peak-peak). The short duration of the large voltage helped reduce the deposition loss of the ions and charged particles caused by electrical drifts. Meanwhile, an offset dc voltage (bias voltage) with a polarity opposite to the discharge voltage (i.e., the polarity of the generated ions) drew the ions and charged particles back to the discharge electrode. Table 2 shows the extrinsic charging efficiencies of 10-nm particles obtained experimentally by various waveforms under optimal charging conditions at Q = 3.5 L/min, in comparison with the pulse voltages reported by Kwon et al. (2007). As the table shows, we attained an extrinsic charging efficiency of about 66.7% for 10-nm particles with the sin x/x waveform (Figure 4) at an aerosol flow rate of 3.5 L/min. The table also lists the much lower extrinsic efficiencies obtained by various other waveforms such as square, sine, and ramp waves. Note, however, that we chose not to set similar waveform parameters (i.e., similar amplitudes, bias voltages, and frequencies) when comparing the various waveforms since the discharge conditions (breakdown voltage, etc.) appeared to be dependent on the waveform aspect itself. Consequently, we employed sin x/x as the discharge voltage waveform and optimized the parameters (f, V 0, V bias ) with the objective functions of extrinsic charging efficiency.

7 ELECTRICAL CHARGING OF SUB-10 NM PARTICLES 65 First, to optimize the sin x/x waveform parameters, we experimentally studied how the voltage amplitude (V 0 ) influenced the charging efficiencies (η extr and η intr ) at a constant frequency (f ) and constant bias voltage (V bias ). The increased voltage between the electrodes of the microplasma-based ionizer chip (Figure 2), V 0, results in the following: the local electric field around the apices of the electrodes reaches the threshold for gas breakdown, micro-discharges start to occur along the surface of the dielectric barrier, and electrons are emitted in abundance so as to dissociate the molecules. The polarity of the generated ions depends on the polarity of the voltage applied to the electrodes. During this experiment we observed a stable discharge, violet in color, around the apices of the discharge electrodes. Figures 5a d show the relationships between the discharge voltage amplitude V 0 (V 0 < 0) and charging efficiencies. Figures 5a and c are for D p = 5 nm and Figures 5b and d for D p = 10 nm. No bias voltage (V bias = 0 V) was applied in the two upper plots, Figures 5a and b, while a bias voltage of 800 (V bias = 800 V) was applied in the lower plots, Figures 5c and d. The frequency was fixed at f = 1500 Hz and the aerosol flow rate, Q, was 3.5 L/min in all cases. The breakdown voltage, V s, was about 2800 V, hence η intr(exp) starts to rise at V 0 = V s with V 0 and then plateaus, in Figures 5a and b. This is to be expected, as a higher V 0 leads to a higher concentration of ions and higher charge limit since Nt-product is attained at a high V 0. On the other hand, η extr(exp) in Figures 5a and b departs from zero at V s but gradually drops to zero with V 0 because a larger electric field inside the charger causes a higher deposition loss of charged particles. While a higher η intr(exp) could practically result in higher η extr(exp),theη extr(exp) values obtained for 5- and 10-nm particles were comparable, as shown in Figures 5a and b. We attribute this result to a statistical error, as the larger electric field at V bias = 0 V significantly reduced the concentration of charged particles at the charger outlet. If we compare η extr(exp) in the upper row (V bias = 0 V) and lower row (V bias =+800 V) of Figure 5, the bias voltage of 800 V doubles η extr(exp) for D p = 5 nm and triples η extr(exp) for D p = 10 nm. This suggests that a positive bias voltage opposite to the polarity of the discharge voltage is effective in reducing the loss of charged particles. Figure 6 shows the charging efficiencies of 10-nm particles as a function of bias voltage, V bias. The amplitude of the discharge voltage, the frequency, and the flow rate were all kept constant (V 0 = 3050 V, f = 1500 Hz, and Q = 3.5 L/min). As the figure shows, η intr(exp) remains almost constant up to V bias = 950 V and then suddenly drops, while η intr(exp) increases with V bias and then shows the same trend as η intr(exp) for V bias > 950 V. Yun et al. (1997) reported that an external electric field applied to the bipolar ion generation zone affects the transport of ions and, as a consequence, the ion concentration. At bias voltages smaller than 950 V, the ions removed from the bipolar ion generation zone by the electric field are replenished by newly generated FIG. 5. Effect of amplitude voltage on the charging efficiencies at constant V bias = 0V,f = 1500 Hz for (a) 5-nm and (b) 10-nm particles and at constant V bias =+800 V, f = 1500 Hz for (c) 5-nm and (d) 10-nm particles.

8 66 E. MANIRAKIZA ET AL. FIG. 6. Effect of bias voltage (offset voltage), V bias, on the charging efficiencies of 10-nm particles at f = 1500 Hz, V 0 = 3.05 kv, and Q = 3.5 L/min. ions. The ion concentration therefore stays constant. But when the external electrical field exceeds a certain value, the rate of new ion generation cannot match the rate of ion removal from the bipolar ion generation zone. The ion concentration therefore remains the same for V bias < 950 V, which gives the same η intr(exp) regardless of the bias voltage, but decreases for V bias > 950 V, which gives a lower η intr(exp). The extrinsic charging efficiency, η extr (exp), on the other hand, increases up to V bias > 950 V due to the retreat of the charged particles back to the discharge electrode under the force of the electrical field, but decreases at higher bias voltages (V bias > 950 V) via the same mechanism described for η intr(exp). Note that the maximum extrinsic charging efficiency in Figure 6 is about 0.7 at V bias = 950 V. The graph in Figure 7 plots the charging efficiencies of 10-nm particles as a function of frequency, f. The amplitude of discharge voltage, the bias voltage, and the flow rate were all kept constant (V 0 = 3200 V, V bias = 800 V, and Q = 3.5 L/min). The plot shows a slight decrease of η intr(exp) as the frequency, f, rises. As we notice from Equations (4) and (5), the discharge FIG. 7. Effect of frequency, f, on the charging efficiencies of 10-nm particles at V 0 = 3.2 kv, V bias = 800 V, and Q = 3.5 L/min. voltage, V(t), decreases as the frequency rises. Thus, the decrease of η intr(exp) at higher frequencies is caused by the shortage of ions as the discharge voltage drops. The extrinsic charging efficiency, η extr(exp), on the other hand, rises together with f up to a peak at f = 1600 Hz and then decreases at about the same rate as η intr(exp). Because T (= 1/f ), the period of time during which the bias voltage stays effective, gets shorter as the frequency, f, increases, the bias voltage has a significantly reduced effect on the suppression of electrostatic loss. Meanwhile, T increases as f increases, hence η extr(exp) decreases due to the attraction of charged particles to the discharge electrode, as already discussed above. We can therefore find an optimal frequency for obtaining the lowest possible charged particle loss due to amplitude voltage and the highest possible bias voltage effect in suppressing the charged particle loss. This value fell in the range of 1400 to 1600 Hz under the experimental conditions used here. The particle transport inside the charger takes place in tandem with the particle charging by way of diffusion and the electric field. We predicted the particle transport in the charger by dividing the charger into elementary volumes and assuming that the migration of charged particles, diffusional loss of uncharged particles, and charging of uncharged particles take place independently in each incremental volume (sections A, B, and C of the online supplemental information). For simplicity, we considered an incremental charging volume approximately equal to the charger volume swept by the flow during a single discharge-cycle period (T). By analyzing the particle trajectories on the assumption that charged particles move under the effect of the electric field governed by the sinc function, we identified Vbias crt crt (Vbias = 950 V), a critical bias voltage at which particles move in a straight line through the charger without loss. For a bias voltagev bias >Vbias crt, the effect of the bias voltage is dominant, thus, the charged particles are attracted toward the discharge electrode. For a bias voltage V bias <Vbias crt, the effect of the discharge voltage is dominant and the charged particles are repelled toward the wall. Therefore, the electrostatic loss were minimum at V bias = Vbias crt, but became significant at bias voltages lower or higher than Vbias crt (Figure 6). The ratio of charged particles lost solely due to electrostatic effects was evaluated from the particle-limiting trajectories at various bias voltages. Note, however, that the particles are supposed to be charged at the inlet of a given elementary charging volume in order to determine the particle-limiting trajectories. Because the uncharged particles are charged as they penetrate the charging volume, it was important for us to evaluate the fraction of charged particles at a specific distance from the charger inlet. We assumed here that the uncharged particles entering the elementary charging volume were instantaneously charged to a fraction predicted by diffusion charging theory (Fuchs 1963). By considering the whole charger as a succession of identical elementary volumes, the charging efficiencies were evaluated based on the material balance within one elemental charging volume. The material balance was evaluated by calculating the charging probability of particles and the

9 ELECTRICAL CHARGING OF SUB-10 NM PARTICLES 67 FIG. 8. Experimental result of the (a) intrinsic charging efficiency and (b) extrinsic charging efficiency against the particle diameter at various aerosol flow rates: Q = 2.5 L/min, Q = 3.5 L/min, Q = 4.5 L/min, V 0 = 3.05 kv, V bias =+850 V, and f = 1500 Hz. The lines are the theoretical values of the intrinsic charging efficiency (η intr ) and extrinsic charging efficiency (η extr ). ratios of both the electrostatic loss of charged particles (limiting particle trajectories) and the diffusional loss of charged and uncharged particles (Gormely and Kennedy 1949; Alonso and Alguacil 2007). Figures 8a and b compare the predicted intrinsic and extrinsic charging efficiencies (η intr and η extr ) with the charging efficiency experimentally obtained under the following conditions: flow rates of Q = 2.5, 3.5, and 4.5 L/min, discharge voltage V 0 = 3.05 kv, V bias =+850 V, and f = 1500 Hz. The ion concentration for the predictions was estimated from the ion current detected using a high-sensitivity digital electrometer (ADVANTEST, R8240 Digital Electrometer). The digital electrometer was connected to an ion-collecting aluminum plate exposed over the surface of a microplasma-based ionizer (section C of the supplemental information). The measured concentration of generated negative ions was about N = per m 3. As the figures show, the use of the measured value of N = per m 3 for the prediction led to an overestimation of the charging efficiency. This can be attributed to the reduction of the ion concentration due to the diffusional loss, electrostatic loss, and neutralization of the ions. To avoid this overestimation we treated the ion concentration as a fitting parameter that gave the best-fit for experimental data on the intrinsic charging efficiency (section B of the supplemental information). The best-fitting values for the intrinsic charging efficiency were respectively obtained at N = , , and per m 3 at Q = 2.5, 3.5, and 4.5 L/min, respectively, for V 0 = 3.05 kv, V bias =+850 V, and f = 1500 Hz. The theoretical lines of η intr (D p < 10 nm) were calculated using Equation (S.11) (section B of the supplemental information). As we see from Figure 8a, η intr(exp) decreases with an increase of the flow rate, Q. ForD p = 5 nm, for example, η intr(exp) is 47.9, 39.4, and 38.0%, respectively, at Q = 2.5, 3.5, and 4.5 L/min. This can be attributed to the reduction of Nt-product as the charging time and aerosol flow rate decrease. From Figure 8b we find that the theoretically plotted lines for the extrinsic charging efficiency, η extr (Equation (S.12)), are less dependent on the aerosol flow rate. The low dependency of intrinsic charging efficiency on the aerosol flow rate may be explained by the competition of two mechanisms, that is, the decrease in charging efficiency and the reduction of particle deposition loss due to the increase in the aerosol flow rate. Figure 8a and b also reveal that the deviations of experimental data from the predicted lines are more pronounced for particles smaller than 5 nm. Interferences impeding particle transfer and charging processes may have to be taken into account when predicting the charging efficiency for these small particles. The initial aim of this study was to enhance the charging efficiency of particles in the sub-10-nm range. Then, upon achieving high ion concentrations and high charging efficiencies as we went along, we decided that it would be more interesting to investigate the charge distribution of particles larger than 10 nm. In this study, we applied the tandem differential mobility analyzer system, TDMA (Kim et al. 2005). Two identical Nano-DMAs (TSI, Model 3085) adjusted to the same operating settings (1.5 L/min of inlet aerosol and 15 L/min) were used.

10 68 E. MANIRAKIZA ET AL. The experimental result proved that less than 10% of the 10- nm charged particles were doubly charged and the rest (more than 90%) were singly charged. On the other hand, a significant fraction of the multiply charged particles was detected among 20-nm particles (up to 4-charges) and 50-nm particles (up to 7-charges). This implies that highly concentrated unipolar ion conditions were established in the present charger (see section E of the supplemental information for details). We will be analyzing the mechanisms of the multiple nanoparticle charging by this charger in a future study. 4. CONCLUSION In this study, we developed a high-efficiency, highthroughput microplasma-based aerosol charger for sub-10-nm particles by minimizing the electrostatic loss of the charged particles. We obtained the following conclusions: (1) A discharge voltage waveform of the sinc function, sin x/x, with a bias voltage was capable of generating a high concentration of ions and suppressing electrostatic loss of the charged particles. (2) There is an optimal voltage for achieving high extrinsic efficiencies, that is, high yields of charged particles, for each particle size. (3) With the optimal discharge voltage set, an intrinsic charging efficiency of 79.3% and an extrinsic charging efficiency of 61.4% were obtained for 10-nm particles charged at a flow rate of 2.5 L/min. (4) An intrinsic charging efficiency of 48.0% and an extrinsic charging efficiency of 34.6% were obtained for 5-nm particles charged at a flow rate of 2.5 L/min. (5) The volumetric flow rate through the charger studied in the present work did not have a significant effect on the charging efficiencies. REFERENCES Alonso, M., and Alguacil, F. J. (2007). 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