The measurement of charging efficiencies and losses of aerosol nanoparticles in a corona charger

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Journal of Electrostatics 64 (2006) 203–214 www.elsevier.com/locate/elstat

The measurement of charging efficiencies and losses of aerosol nanoparticles in a corona charger M. Alonso, M.I. Martin, F.J. Alguacil National Center for Metallurgical Research (CSIC), Avenida Gregorio del Amo, 8, 28040 Madrid, Spain Received 8 February 2005; received in revised form 13 May 2005; accepted 27 May 2005 Available online 19 July 2005

Abstract A methodology is proposed for the measurement of a number of parameters relevant to the performance evaluation of aerosol corona chargers. These parameters are intrinsic and extrinsic charging efficiencies, and diffusion and electrostatic particle losses. The methodology is essentially the same as that used in earlier works, except that free ions are removed just after the charger outlet in order to minimize the extent of possible after-charging effects which might lead to measurement errors. However, the experimental results show that after-charging is negligible and that, consequently, practically all the effective ion–particle collisions take place before the aerosol leaves the charger. Formation of new particles during corona discharge, which could in principle be an additional cause of measurement error, has not been observed in the working voltage range of the charger. Particle diffusion and electrostatic losses have been measured at varying values of the voltage applied to the charger: for a given particle size, diffusion loss decreases and electrostatic loss increases as the charger voltage is increased. The intrinsic charging efficiency increases with particle size and charger voltage. In contrast, the extrinsic charging efficiency, which is the parameter of importance in practice, attains a maximum value for a given charger voltage in such a manner that this optimum voltage depends on particle size. r 2005 Published by Elsevier B.V. Keywords: Aerosol; Charger; Unipolar charging; Charging efficiency; Particle loss

1. Introduction Increasing research efforts are being directed to the development of high-efficiency electrical chargers for nanometer-sized aerosol particles. There are several fields in which nanoaerosol charging is important, e.g., aerosol particle size distribution measurement by electrical techniques, deposition of nanoparticles in selective positions in microelectronic components, contamination control, etc. In the case of relatively large particles, bipolar chargers are sufficiently efficient and produce a more or less well-known equilibrium charge distribution. During bipolar charging, two competing simultaneous processes take place: charging of neutral Corresponding author. Tel.: +34 91 553 8900x315; fax: +34 91 534 7425. E-mail address: [email protected] (M. Alonso).

0304-3886/$ - see front matter r 2005 Published by Elsevier B.V. doi:10.1016/j.elstat.2005.05.008

particles and neutralization of charged ones. As the particle size decreases, recombination of charged particles with ions of opposite polarity (i.e. neutralization) becomes predominant over charging of neutral particles. Particularly, the charging/discharging rate ratio for nanometer particles is so small [1] that the attainable charging efficiencies are deceptively low: less than 5% for particle diameters below 10 nm [2–6]. Loss of charged particles by recombination is avoided if the aerosol is made to flow through a cloud of unipolar ions. The simplest means to achieve an unipolar ion cloud is by DC corona discharge and, in fact, the use of corona ionizers (CIs) for aerosol charging goes back to Rohmann as far ago as 1923 (cited in [7]). Many other researchers have later employed corona type ionizers for aerosol charging [8–10]. Knowledge of the charging efficiency attainable in a given charger is needed, for instance, for particle size

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distribution measurement by electrical mobility techniques. The reason is that one has to know the charge distribution as a function of particle size in order to transform the experimentally measured mobility distribution into particle size distribution. However, in spite of that long history, reports on the charging efficiencies of unipolar chargers are quite scarce. Among these we may cite the work of Adachi et al. [3], who measured the efficiency for particle diameters above 10 nm using a unipolar radioactive charger in which ions of a given polarity were separated by means of a relatively weak electric field from a population of bipolar ions generated by a 241Am radioactive source. Experimental values of charging efficiency for particles under 10 nm have been reported by Buscher et al. [11] using corona discharge as the ion source; and by Wiedensohler et al. [12], who used 244Cm to generate bipolar ions in two boxes separated by a charging zone, and an alternating electric field to draw unipolar ions from the boxes into the charging zone. More recently, Chen and Pui [13] have developed a unipolar charger also using radioactive sources, and a stream of sheath air surrounding the aerosol in order to reduce particle losses within the charger. These authors, using a sheath-to-aerosol flow rate ratio of 3, obtained extrinsic charging efficiencies as high as 65% for 10 nm particles, measured on a particle number basis, i.e., calculating the fraction of neutral particles which acquire a net charge. However, in practice one is usually interested in the number concentration of charged particles at the charger outlet. Therefore, one must take into account that the aerosol is diluted by a factor of 4 in the charger of Chen and Pui, so that their reported efficiencies be divided by 4 to express them on a particle number concentration basis (hence, about 16% efficiency for 10 nm particles). Kruis and Fissan [14] have designed a unipolar charger similar to that described in [12], except that the ions in the two-generation boxes are produced by corona discharge. The present authors have recently shown that it is not necessary to use these relatively complicated designs in order to obtain reasonably high charging efficiencies; indeed, a simple grounded cylinder with a coaxial needle electrode suffices to achieve efficiencies practically identical to those attainable with the much more complex devices described above [15]. In our previous work [15] we evaluated the performance of an aerosol corona charger which basically consists of a cylindrical tube with tapered ends, and divided into three sections. The first, tapered, section and the second are made of methacrylate, while the end, also tapered, section is metallic and grounded. The second section contains a drilled circular piece perpendicular to the aerosol flow. This piece is used to hold a stainless steel sharp-pointed electrode coaxial with the cylinder. In this previous work we measured the

extrinsic charging efficiency (fraction of originally neutral particles which become charge and penetrate the charger without being lost to the walls) as a function of particle size (up to 10 nm), corona voltage, and aerosol flow rate (between 2 and 8 lpm). The two main results found in that work can be summarized thus: the extrinsic charging efficiency increased with particle diameter, and was about one order of magnitude higher than that attainable in radioactive bipolar chargers; and, for a given particle size, the extrinsic efficiency decreased with aerosol flow rate. In the present work, we have modified the aerosol inlet geometry as well as the manner of holding the discharge electrode of our previous corona charger (the drilled piece into which the inner electrode was held is absent in the present design). Besides the device modification, we have now also estimated the nt-product (mean ion number concentration
mean aerosol residence time), and an additional quantity, the intrinsic charging efficiency as a function of particle size and ntproduct, which were not dealt with in our former investigation. Furthermore, the particle size range examined has been extended up to 30 nm (mobility equivalent particle diameter). More importantly, in the present report we address two key issues, namely, the possible formation of additional particles during corona discharge, and the presence of free unattached ions downstream of the charger which might promote further charging and, hence, could lead to measurement errors in the experimental evaluation of the charger performance.

2. Methodology to evaluate charging efficiencies and particle losses The performance evaluation of any charger requires neutral monodisperse particles as the test aerosol. Usually, the basic experimental setup consists of three devices connected in series: the charger, an electrostatic precipitator (ESP) to remove the charged fraction of particles when it is operated (or to let them pass when it is switched off), and a particle counter (for instance, a condensation nucleus counter (CNC)) capable of measuring the number concentration of charged particles as well as that of neutral ones. This basic setup is illustrated in Fig. 1, which also shows a simplified scheme of the processes occurring within the charger. The need to measure number concentration of charged and neutral particles excludes the possibility of using an electrometer (EM) as the particle detector. In the CNC, also called condensation particle counter (CPC), nanometer-sized particles are grown to micrometer size by condensing a supersaturated vapor on them; the grown particles are then optically detected [16,17].

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The aim is to express the unknown concentrations N’s as functions of the experimentally measured concentrations C’s. To this end, one has to consider that particle losses take place, by diffusion all along the experimental line and, in addition, by the electrostatic mechanisms in the charger and the ESP. First, when no voltage is applied to either the charger or to the ESP, there are only neutral particles in the system, and we can write Fig. 1. Illustration of the particle charging and wall deposition processes occurring within the aerosol charger.

N 0IN represents the number concentration of neutral monodisperse particles of known size at the charger inlet. When the charger is operating under a positive and sufficiently high DC voltage, a fraction N þ =N 0IN of these particles become unipolarly charged, while the rest of the particles, N 0 , remain neutral. Each of these two fractions of particles suffers losses to the wall. In the case of neutral particles, the only possible mechanism of particle loss is by diffusion; the number concentration of neutral particles lost by diffusion is denoted as N 0D in Fig. 1. For unipolarly charged particles, there are three possible mechanisms of particle loss: diffusion, electrostatic dispersion (space-charge), and direct electrostatic precipitation caused by the applied electric field. The number concentration of charged particles lost by þ diffusion is represented by N þ D , while N E denotes the number concentration of charged particles lost by the combined mechanisms of electrostatic dispersion and direct electrostatic precipitation along the field lines. As a result of these ‘‘internal’’ processes occurring within the charger, there appear at the outlet a certain number concentration of surviving neutral ðN 0OUT Þ and unipolarly charged ðN þ OUT Þ particles. In order to evaluate the charging efficiency of particles of a given size, three particle number concentration measurements are carried out: (i) the concentration C 1 measured by the particle counter (CNC) when the voltages of the test charger and the ESP are both turned off; (ii) the number concentration C 2 obtained when the charger is being operated at a prescribed voltage, and the ESP is run at a voltage V IR large enough to eliminate the free unattached ions but low enough to prevent electrostatic deposition of the charged particles (a more detailed explanation is given later); and (iii) the particle number concentration C 3 measured when the voltage of the charger is turned on, and the ESP is operated at a voltage V PR sufficiently high so as to remove the charged particles. The calculation of the charging efficiencies and particle losses from these three experimentally measured concentrations was done using a set of steady-state balance equations based on the diagram shown in Fig. 1.

C 1 ¼ PCHOFF PESPOFF PCNC N 0IN .

(1)

Here P denotes neutral particle penetration; the subscripts CH, ESP, and CNC refer to the charger, the ESP, and the CNC, respectively, and the subscript OFF means that no voltage is being applied to the corresponding unit. This is very important: penetration through the charger depends on whether the charger is in operation mode or not, because in one case (OFF) there are neutral particles alone and the only possible mechanism of particle loss is diffusion, whereas in the other case (ON) there are also charged particles present and two additional particle loss mechanisms, as noted before. Therefore, when one speaks about particle loss in the corona charger, it has to be made clear if it is operating as a charger or simply as a geometrically identical dummy unit. The second measurement is done with the charger ON and the ESP at a voltage V IR for ion (but not particle) removal. Therefore, it follows that C 2 ¼ PESP2OFF PCNC ðN 0OUT þ N þ OUT Þ.

(2)

In writing Eq. (2), we are implicitly assuming that particle penetration through the ESP and the particle counter are both independent of the charging state of the particles, that is, it is being assumed that charged particles are lost to the same extent as neutral ones (recall that V IR is too low for charged particle precipitation). There are two reasons that justify this assumption. First, the number concentration of unipolarly charged particles was well below 104 cm3 in all the experiments, so that space-charge (electrostatic dispersion) is insignificant. For instance, at the flow rate (2 lpm) employed in the experiments to be described below, the mean aerosol residence time in the ESP was t ¼ 2:4 s; the particle loss fraction by electrostatic dispersion, given by [18] ½1  1=ð1 þ eZnt=
0 Þ is only about 0.01 for 3 nm particles with number concentration of 104 cm3, and even smaller for larger particles. (In the former expression, e is the elementary charge, Z the particle electrical mobility, n the particle number concentration, t the mean residence time, and
0 the permittivity of a vacuum.) Second, in the case of diffusion losses alone, it has been demonstrated experimentally that penetration through laminar flow tubes for neutral and charged particles of a given size are the same, at least in the case of nanometer-sized aerosols

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[19], which means that image forces can be neglected in this particle size range. It is important to point out that when the charger is in operation mode, free unattached ions may be present in the outlet stream along with charged and neutral particles. Therefore, if one measures the concentration C 2 as in former investigations [13–15], i.e. with the voltage of the ESP turned off, there exists the possibility of further particle diffusion charging downstream of the charger itself. It is to minimize this possibility that we have varied the usual method by applying a certain voltage ðV IR Þ to the ESP, large enough to remove the free ions, but low enough to avoid electrostatic precipitation of the charged particles; the appropriate value of V IR can be selected from experiments as will be shown below. Even doing so, it is clear that a minimum precipitator length is needed to complete the removal of free ions, so that the possible after-charging effect cannot be completely avoided, that is, free unattached ions must flow along with aerosol particles downstream of the charger for a certain period of time; the most one can do is to minimize the length of the latter. Nevertheless, as will be explained later in detail, the experimental results were practically unaffected by the presence of free unattached ions beyond the charger or, in other words, there is no significant difference in using V ESP ¼ 0 or V ESP ¼ V IR to measure C 2 . Finally, when the corona charger is operated, and a voltage V PR is also applied to the ESP for charged particle elimination (and, of course, for ion elimination too), one has C 3 ¼ PESPON PCNC N 0OUT .

(3)

The required value V PR in order to precipitate out all of the charged particles can be found from previous measurements. In the last equation, PESPON refers to the penetration of neutral particles through the precipitator when the latter operates at the voltage V PR . As long as image forces can be neglected, and this is a reasonable assumption for nanometer particles as stated above, the presence of a non-ionizing electric field should not affect the diffusional deposition of neutral particles, so that, PESPON ¼ PESPOFF . Considering now the unknown concentrations N’s, two more relations can be described. First, as is obvious from the diagram shown in Fig. 1, we have N 0OUT ¼ PCHOFF N 0 , because even when the charger is ON neutral particles can only be lost by diffusion. Also, N 0IN ¼ N 0 þ N þ . These two equations lead to N þ ¼ N 0IN 

N 0OUT PCHOFF

.

(4)

When the charger is ON, charged particles are mainly lost by the electrostatic mechanisms (space-charge and, most importantly, direct precipitation), rather than by diffusion, because the particle electrical migration

velocity is quite high. Indeed, consider a particle located at a distance x from the grounded electrode to where charged particles migrate driven by the electric field. The time required for the particle to reach the wall by electrical migration is tE ¼ x=ZE, where Z is the particle electrical mobility, and E the electric field. The time required for the same particle to reach the wall by Brownian diffusion is of the order tD  x2 =2D, where D is the particle diffusion coefficient. Hence, tD =tE  ZEx=2D ¼ eEx=2kT, where e is the elementary charge, k is Boltzmann’s constant, and T the absolute temperature. In the charger used in this work, the electric field is of the order of 105–106 V m1; on its part, x is of the order of 103 m. The diffusion-to-electrostatic characteristic deposition time ratio is thus tD =tE 4103 . Alternatively, we can estimate the ratio D=ZV , where V is the charger voltage; for singly charge particles, D=ZV ¼ kT=eV , which represents the ratio of thermal energy to the kinetic energy acquired by the charged particle accelerated by the electric field. At a typical voltage of 3 kV, kT=eV  105 . Whichever way one looks at it, it is clear that diffusion loss of charged particles can be safely neglected or, using the nomenþ clature of Fig. 1, N þ E bN D . Hence, one can write the following last balance equation þ Nþ ¼ Nþ E þ N OUT .

(5)

The above system of five equations allows a determination of the five unknown concentrations N’s in terms of the measured values of C’s, and of the particle penetrations through the corona charger, the ESP and the CNC. It will be seen, however, that actually we only need to measure the particle penetration through the charger because, according to the system of Eqs. (1)–(5), the other two penetrations cancel out automatically. We now define charging efficiencies and particle loss fractions. The intrinsic charging efficiency
i is defined as the fraction of originally neutral particles which become charged within the charger, regardless of whether they subsequently may or not survive the various particle loss processes that take place in the charging device. Hence,
i ¼

Nþ . N 0IN

(6)

The extrinsic charging efficiency
e , which is the parameter of interest in a practical application, is defined as the fraction of originally neutral particles which emerge out of the charger carrying at least a unit of charge:
e ¼

Nþ OUT . N 0IN

(7)

As for particle electrostatic losses, there are two possible definitions, both of them applicable to an operating charger. One can define the electrostatic particle loss either based on the total original aerosol

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population LE ¼

3. Experimental

Nþ E , N 0IN

(8)

or else based on the charged fraction of particles alone, L0E ¼

Nþ E . Nþ

(9)

Likewise, diffusion losses can be defined in terms of the original incoming aerosol, LD ¼

N 0D , N 0IN

(10)

or based on the fraction of particles which do not get charged, L0D ¼

N 0D . N0

(11)

From all the above equations, the following relations result:
i ¼

C1  C3 C1

ðintrinsic efficiencyÞ,


e ¼ PCHOFF

C2  C3 C1

LE ¼
i 
e

ðelectrostatic lossÞ,

L0E ¼

LE
e ¼1
i
i

ðextrinsic efficiencyÞ,

ðrelative electrostatic lossÞ,

LD ¼ ð1 
i Þð1  PCHOFF Þ ðdiffusion loss; charger ONÞ, L0D ¼ 1  PCHOFF

(12)

(13) (14) (15)

ð16Þ

ðdiffusion loss; charger OFFÞ. (17)

Finally, the total particle loss is L¼1

207

N 0OUT þ N þ OUT ¼ LE þ LD . N 0IN

(18)

Note that the fraction of particles lost by diffusion to the wall of a corona charger in operation (i.e. voltage ON), given by Eq. (16), is 1 
i times the diffusion loss fraction when the charger is OFF. When no particles get charged,
i ¼ 0, and diffusional penetration is the same as would be obtained with a geometrically identical unit in which no electric field is applied, or with the charger operating at a voltage below that required for corona onset. But as soon as the voltage reaches a value large enough to promote particle charging,
i 40, and diffusion losses become smaller when the charger is ON (though, of course, because there is an electric field acting on the charged particles, the electrostatic deposition mechanisms will produce additional losses).

The methodology described in the preceding section was applied to evaluate the performance of a corona charger which is being manufactured and commercialized by RAMEM S.A. Fig. 2 shows a sketch of the CI. It consists essentially of an inner stainless steel electrode ending in a sharp tip, to which a DC high voltage is applied (typically of the order of 3–3.5 kV). The electrode is coaxial with a grounded metal cylinder whose inner wall has a conical shape. The distance between the electrode tip and the cone apex is 1.75 mm. The neutral aerosol enters the charger through the annular gap shown in the drawing. This annular gap can be varied between 0 and 6 mm in 1 mm steps. For the experiments carried out in the course of this work, the aerosol flow rate was kept at 2 lpm. A few preliminary experiments showed that the charger can still give relatively high charging efficiencies at aerosol flow rates up to 10 lpm, with a tendency similar to that found with our previous ionizer [15], that is, the efficiency decreases with increasing flow rate. The essential difference between the present charger and the former one is that the latter (see Fig. 1 in [15]) contained a central piece of teflon to hold the electrode, and the aerosol passed through a series of small orificies drilled in the piece. Diffusion losses in the former device were about 25% for 3 nm particles and 7% for 10 nm particles (for a flow rate through the charger of 2 lpm, and when no voltage is being applied). In the present design, the electrode-holding piece has been removed. As a result, the aerosol must enter through the lateral wall. It was expected that this arrangement would help to reduce diffusion losses but, as measurements have shown, it turns out that the particle loss by diffusion is much the same as before. Nevertheless, the fabrication of the present device is easier to carry out. A flow diagram of the experimental setup utilized to evaluate the charger performance is shown in Fig. 3. A NaCl aerosol generated by a conventional aerosol in stainless steel teflon

aerosol out 34

3.5

30

20

Fig. 2. Sketch of the corona charger (dimensions in mm).

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A ECAG

CI

DMA

KN

PPC B TEST CHARGER

bypass

C CNC

ESP D

EM Fig. 3. Flow diagram of the experimental setup for evaluating the charger performance. ECAG ¼ evaporation–condensation aerosol generator; CI ¼ corona ionizer; DMA ¼ differential mobility analyzer; KN ¼ Kr-85 neutralizer; PPC ¼ parallel plate condenser; ESP ¼ electrostatic precipitator; CNC ¼ condensation nucleus counter; EM ¼ electrometer.

evaporation–condensation (ECAG) technique, similar to that used by Scheibel and Porstendorfer [20], was used as the source of test particles. These were charged in a CI similar to that sketched in Fig. 2. Charged particles were classified according to their electrical mobility in a differential mobility analyzer (DMA, TSI short column, flow rates: inlet aerosol ¼ sampling aerosol ¼ 2 lpm; sheath ¼ excess ¼ 20 lpm). The unipolarly charged particles of the desired size were passed through a 85Kr neutralizer (KN, TSI 3077) where the unipolarly charged aerosol is brought into contact with bipolar air ions generated by the radioactive source. The monodisperse aerosol leaving the neutralizer contains positive and negative particles, as well as neutral ones. The charged fraction of this aerosol was removed by means of a parallelplate condenser (PPC). In this manner, the aerosol exiting the PPC contains only neutral particles of known size—this is actually the test aerosol, which is subsequently made to flow through the test charger, a second ESP, and a CNC (TSI 3025) where particle number concentration is measured. The ESP consisted of a grounded copper tube, 10 mm ID and 10 cm in length, with a coaxial wire, 1 mm in diameter, connected to a DC power source. For each selected particle diameter, preliminary measurements were done with an EM (TSI 3068A) to establish the voltage ranges required for removal of free ions and charged particles in the ESP. The measurement of PCHOFF , the penetration of neutral particles through the charger in nonoperating mode, was carried out from comparison between the particle number concentrations measured when the aerosol was alternately passed through the test charger (OFF) and the bypass route. To this end, the length of the bypass tube AD was made equal to the sum of the lengths of the tubes AB and CD.

4. Results and discussion 4.1. Particle formation during corona discharge Corona discharge can promote the formation of new particles, either from erosion and sputtering of the corona-emitting electrode itself [21,22], or from gaseous contaminants present in the system [23]. For the latter, the following mechanism has been proposed [24]: (a) contaminant molecules are ionized in the corona discharge field; (b) these ions form cores around which water molecules are bonded; (c) the contaminant-water elementary units grow into molecular clusters and finally, by coagulation, into nanometer-sized particles. The presence of water is thought to constitute a decisive factor in the particle generation process. It has been shown that particle generation from organic vapors in air by corona discharge does not occur in the absence of water [25]. We have examined the possible formation of undesired particles in our corona discharge. For this purpose, room air at a flow rate of 2 l min1 was passed through a silica gel dryer, an absolute filter to remove all the particulate matter, and the corona charger. The stream leaving the charger was admitted either into the particle counter (CNC) or the aerosol EM. The charger was operated at increasing voltages. Initially, particles could be detected by both the CNC and the EM at typical corona voltages (2.6–3.6 kV), but their concentration gradually decreased, and vanished after several hours. We tried to measure the size distribution of these particles inserting a DMA between the charger and the particle concentration measuring device (CNC or EM). However, our DMA, operated under extreme conditions (e.g. sheath flow rate as low as 5 lpm), can classify particles of at most 200 nm in diameter. Under these extreme conditions, no particle could be classified. Hence, the diameter of the detected particles must be

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4.2. The nt-product The nt-product (mean ion number concentration
mean aerosol residence time) is a fundamental parameter to describe the operation of an aerosol charger, because it is intimately related to the extent to which the particles can become charged. Firstly, it is obvious that, for a given number concentration of aerosol particles, there must be a minimum number concentration of ions below which the aerosol population cannot attain its maximum charging state. In practice, the chargers are operated under the condition that the ion concentration is always much higher (generally by two or more orders of magnitude) than the aerosol concentration. As for the aerosol mean residence time in the charger, there are two competing processes to consider. On the one hand, the larger the residence time, the larger the mean number of charges on the aerosol. (In bipolar charging, a steady state is reached at which the rate of formation of newly charged particles equals the rate of disappearance of charged particles by recombination with ions of opposite polarity. In contrast, in unipolar charging, since there is no recombination, a truly steady state cannot be attained). But, on the other hand, losses of charged and neutral particles to the charger walls increase with time. Too low a residence time yields too few charged particles, whereas too long residence times improve the extent of charging but at the same time result in large

particle losses. As a consequence, there must be an optimum residence time at which the output of charged particles is a maximum. The mean number concentration of ions within the charger in the absence of aerosol particles was estimated using the expression n ¼ I=eZEA, where I is the current deposited on the grounded conical-shaped wall, e the elementary charge, Z the ion mean mobility, E the electric field, and A the inner surface area of the metallic cone (charger outlet) where the ion current is collected. The positive ion mean mobility was measured in separate experiments running the DMA at an aerosolto-sheath flow rate ratio of 6/40 l min1, and it turned out to be 1.1 cm2 V1 s1, a value which lies within the range of positive air ion mobility measured by a number of former researchers [26–28]. Since the separation distance between the rod electrode and the wall decreases along the charger axis, we used for E the mean value of the applied field (this is actually a very rough estimation because neither space charge nor the high non-uniformity of the field near the electrode tip have been considered). Finally, although one should consider t as the mean aerosol residence time in the charging zone, we have instead calculated it taking the total charger volume (2 cm3) because it is difficult to establish the exact boundaries of the charging zone. Fig. 4 shows the values of the thus estimated nt product as a function of the applied voltage. For voltages larger than about 3.7 kV, the spark-over phenomenon occurred, the current fluctuated in an uncontrollable manner and no measurement could be made (see also the discussion on particle formation in the preceding section). The order of magnitude of nt

4.0

3.5

3.0 nt [107 s/cm3]

larger than 200 nm. This, along with the already mentioned fact that particle concentration decreased steadily with time, led us to think that the cause of these foreign particles was simply dirtiness, either in the charger or in the connecting tubes. After elimination of these unknown particles, the counters (CNC or EM) could not detect particles any longer for charger voltages up to 3.6 kV. However, larger voltages promoted the formation of new particles, again too large to be classified with our DMA and, furthermore, with very high concentrations unable to be measured by the CNC and the EM. These particles probably come from the electrode by erosion or sputtering. In summary, it can be stated that a clean corona charger operated under an appropriate range of voltages, does not produce new additional particles provided the aerosol does not contain impurities or certain chemicals (such as aromatic compounds with water). Therefore, a corona charger can be used with confidence in most of the usual research works with aerosols of known composition, but probably is not a good option to use it for, say, measuring the size distribution of particles present in exhausts having contaminants which may undergo the gas-to-particle conversion process in the corona discharge.

209

2.5

2.0

1.5

1.0 3.0

3.1

3.2

3.3 3.4 voltage [kV]

3.5

3.6

Fig. 4. The nt-product as a function of the charger voltage.

3.7

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(between 1 and 4
107 s cm3) is the same as that reported by Kruis and Fissan [14] for their twin Hewitt charger.

For other particle diameters, the numerical values of the ESP voltages required for the measurement of C 2 and C 3 were different, but the tendency of the current/ voltage curves was similar as that shown in Fig. 5.

4.3. Free ions and charged particles removal in ESP

4.4. Intrinsic charging efficiency

The electrostatic precipitation of the ions and charged particles that leave the corona charger is shown in Fig. 5 for the case of 15.8 nm aerosol particles. As shown, the maximum ion current intensity is about 1.2 pA, which corresponds to a number concentration of 2
105 cm3 (calculated as I=eQ, where I is the current, e is the elementary charge—we assume the ions are singly charged—and Q is the aerosol flow rate through the ESP. The actual ion concentration at the charger outlet is larger than this, because diffusion and space-charge losses in the ESP must be accounted for. Of course, the number concentration of free ions depends on the aerosol particle size and number concentration, but the order of magnitude was the same in all the experiments. Free ions are completely eliminated with an applied voltage of about 5 V. Beyond this, the measured current keeps a constant value up to around 20 V: in the range between 5 and 20 V, all the ions are removed but the charged particles remain unaffected. At ESP voltages above 20 V, charged particles start being eliminated and they become completely removed at around 200 V. For this specific case of 15.8 nm particles, the concentration C 2 was measured while keeping the ESP voltage ðV IR Þ fixed at 10 V; for the measurement of C 3 , the ESP voltage ðV PR Þ was set at 300 V.

The intrinsic charging efficiency, which represents the capability of the charger to produce charged particles, regardless of whether they can subsequently be lost to the walls, is plotted in Fig. 6 as a function of particle diameter and charger voltage. In qualitative agreement with expectations, the intrinsic efficiency increases with particle size and applied voltage. Note that for a sufficiently high value of the voltage (or of the ntproduct), the intrinsic efficiency is practically 100% for particle diameters above 20 nm. It means that the ionizer is capable of charging all of the originally neutral particles in this size range. Unfortunately, parts of these charged particles are deposited to the walls mostly by the electrostatic mechanism and, consequently, the extrinsic charging efficiency, which is the relevant quantity in practice, is generally much lower. 4.5. Diffusion losses Particle diffusion loss, calculated using Eq. (16), is plotted in Fig. 7 as a function of particle size and charger voltage. The uppermost curve, valid for charger voltages below 2.5 kV, is actually the diffusion loss fraction of neutral particles, given by Eq. (17). As explained previously, Eq. (17) is appropriate when no ions are generated in the charger. As soon as corona 1.0

charger voltage [kV] 2.8 3.0 3.2 3.4 3.6 1.0

ion removal 0.5

0.8 intrinsic efficiency, εi [ - ]

current intensity at ESP outlet [pA]

1.5

charged particle removal

0.6

0.4 voltage [kV] 3.0 3.2 3.4 3.6

0.2

0.0 0.1

1

10 ESP voltage [V]

100

1000

Fig. 5. Ion and particle removal in the ESP after the charger. Current intensity refers to the charge carried by the particles and ions which leave the ESP.

0.0

0

5

10 15 20 particle diameter [nm]

25

30

Fig. 6. The intrinsic charging efficiency as a function of particle size for different charger voltages.

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0.5

0.6 voltage [kV]

voltage [kV] < 2.5 3.0 3.2 3.4 3.5

3.0 3.2 3.4 3.6

0.5 electrostatic loss, LE [ - ]

diffusion loss, LD [ - ]

211

0.1

0.4

0.3

0.2

0.1

0.01

0

5

10 15 particle diameter [nm]

20

25

0.0

0

5

10 15 20 particle diameter [nm]

25

30

Fig. 7. Diffusion loss fraction as a function of particle diameter and charger voltage.

Fig. 8. Electrostatic loss, as defined by Eq. (14), as a function of particle diameter and charger voltage.

discharge occurs, ions are formed, some particles become charged, the intrinsic efficiency becomes greater than zero and Eq. (16) should be used instead. As the charger voltage is increased, the fraction of charged particles ð
i Þ increases and diffusion losses decrease. The reason is, as noted earlier, that electrostatic precipitation is by far the predominant loss mechanism for charged particles. Another conclusion that can be drawn from Fig. 7 is that, regardless of the operating state of the charger (ON or OFF), diffusion losses are negligible for particle diameter above 20 nm; the main reason is that the mean aerosol residence time in the charger is quite low (0.06 s at the flow rate of 2 lpm employed in these experiments). In comparison with our former CI [15], diffusion losses are very similar. Therefore, our objective to reduce the particle diffusional loss by modificating the charger geometry has failed.

particles are charged to a lesser extent (
i is lower, see Fig. 6) so that the number concentration of particles which can be electrostatically deposited on the wall is, consequently, smaller. Larger particles are certainly charged to a higher extent but, at the same time, their electrical migration velocity is smaller. This is the qualitative reason for the appearance of a maximum in each curve plotted in Fig. 8. The alternative definition of electrostatic loss, given by Eq. (15), is perhaps more instructive, because it tells us what fraction of the charged particles is lost to the wall by electrostatic precipitation; it is a relative measure which does not depend on how many particles become charged. The corresponding results, as a function of particle size and charger voltage, are shown in Fig. 9. As it could be reasonably expected, the relative electrostatic loss increases with increasing charger voltage and decreasing particle size. We have examined a possible correlation between L0E and the particle electrical migration velocity, ZE, using the estimated value for the electric field as explained before. The results are shown in Fig. 10. For each particle size a linear correlation between L0E and log (ZE) exists, but we have not been able to find a single correlation valid for all particle sizes.

4.6. Electrostatic losses As in the case of diffusion loss, the particle electrostatic loss can be calculated in two manners, by referring it to the total original aerosol number concentration or only to the fraction of particles which get charged. The first case, described by Eq. (14), is illustrated in Fig. 8. The electrostatic loss as defined by Eq. (14) depends on two competing factors. On the one hand, it depends on how many particles can acquire a charge (this increases with particle size), and on the other hand it also depends on the electrical migration velocity of the charged particles (which decreases with particle size). Smaller

4.7. Extrinsic charging efficiency As a result of particle losses, the extrinsic charging efficiency is generally much lower than the intrinsic efficiency. Moreover, since electrostatic loss and intrinsic efficiency both depend on the applied voltage, the

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1.0

voltage [kV] 3.0 3.2 3.4 3.6

0.8

Dp = 31.0 nm 0.8 extrinsic efficiency, εe [ - ]

relative electrostatic loss, L'E [ - ]

1.0

0.6

0.4

0.6 23.8 0.4 18.5 0.2

0.2

13.6 9.6 5.8

0.0

0

5

10 15 20 particle diameter [nm]

25

30

Fig. 9. Electrostatic loss, as defined by Eq. (15), as a function of particle diameter and charger voltage.

0.0 2.6

2.8

3.0 3.2 3.4 charger voltage [kV]

3.6

3.8

Fig. 11. The extrinsic charging efficiency as a function of particle size and charger voltage, determined in two slightly different ways (see text for a full explanation).

1.0 Dp = 3.9 nm

implies that when one uses this type of corona charger operated at a fixed voltage for, say, measuring the particle size distribution of a polydisperse aerosol, the charger performance may be optimum for some size fractions but not for the rest. And, of course, one has to keep this in mind when inverting the mobility distribution to obtain the particle size distribution.

relative electrostatic loss, L'E [ - ]

5.8 0.8

7.8

0.6

11.1

4.8. After-charging effects 0.4 18.5 26.4 0.2

0.0

10

100 1000 electrical migration velocity, ZE [cm/s]

5000

Fig. 10. Electrostatic loss fraction, as defined by Eq. (15), as a function of the particle electrical migration velocity.

extrinsic efficiency also depends on the electric field applied. Indeed, our experimental measurements have shown that the extrinsic efficiency attains a maximum value for a certain field strength and, furthermore, this optimum value of the field depends on particle size, as shown in Fig. 11. In general, the smaller the particle size, the larger the value of the optimum electric field at which the extrinsic efficiency attains its maximum. This

We have examined the possible effect of the presence of free unattached ions downstream of the charger on the measurements. The results are shown in Fig. 11. The open symbols represent the results obtained by measuring the three concentrations C 1 , C 2 and C 3 in the manner already explained in detail in a former section. In particular, C 2 was measured by setting the ESP at the voltage V IR to remove the free ions. In the other set of experiments, charging efficiency was determined as in former investigations, setting the ESP voltage to zero for the measurement of C 2 , that is, letting the free ions to coexist with the aerosol particles all the way downstream of the charger up to the CNC. The corresponding results are shown as solid symbols in Fig. 11. Except for a few points, the results obtained using the two methods are very much the same, and in some cases the coincidence is nearly perfect (these are the case where there appears only one symbol, because the open symbol lies under the solid one). The conclusion to be drawn from this fact is that charging beyond the corona charger is rather insignificant.

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4.9. Comparison with other aerosol chargers

5. Conclusions

The extrinsic charging efficiency for applied voltage of 3.2 kV is plotted in Fig. 12 as a function of particle size. For comparison, our results are shown along with those of previous researchers using other types of chargers. The curve labelled as ‘‘bipolar’’ represents the mean of the efficiency values reported by a number of authors using radioactive chargers [2–6]. As the plot clearly reveals, the efficiency attainable with unipolar chargers is considerably higher than with bipolar ones, mainly because recombination of charged particles with ions of opposite polarity is avoided. For particle diameter above 10 nm, the extrinsic charging efficiency is resonably high (more than 40%). However, the efficiency decreases drastically for particles in the size range of a few nanometers, because electrostatic losses are too high as a consequence of their very large electrical migration velocity. (Note: the results reported in the article of Chen and Pui [13] have been multiplied by 1/4 in order to express them on a particle number concentration basis.) Another issue of interest is that of multiple charging. Because of lack of adequate experimental facilities at the time of this work, it has not been possible to investigate the presence or absence of multiply charged particles. It is possible that a non-negligible fraction of the largest particles examined in this work (around 30 nm) might be, at least, doubly charged. As far as we know, multiple charging has not been addressed neither in former articles on unipolar charging of ultrafine aerosols.

The performance of a corona-type electrical charger for aerosol particles, of simple design and easy fabrication, has been evaluated. The attainable extrinsic charging efficiency for nanometer-sized particles is about one order of magnitude higher than that of commercially available bipolar chargers. In addition, our simple device gives practically the same efficiency as other unipolar chargers of more complex designs. It has been shown that no additional new particles are formed by the corona discharge under certain conditions: clean charger without dirtiness operating in the appropriate voltage range, and aerosol not containing contaminants (such as organics) which might undergo a gas-to-particle conversion process.

extrinsic efficiency, εe [ - ]

1

0.1

0.01

1

10 particle diameter, Dp [nm]

50

Fig. 12. Extrinsic charging efficiency as a function of particle size. Comparison with available results of other unipolar chargers, as well as with the efficiency attainable in bipolar chargers. (K) This work, 3.2 kV; (.) Hernandez-Sierra et al. [15]; (,) Kruis and Fissan [14]; (’) Chen and Pui [13]; (&) Buscher et al. [11]; and (~) bipolar chargers.

Acknowledgment This work was supported by Spain’s Ministerio de Educacio´n y Ciencia under Grant no. PTR1995-0745OP.

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