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Electrically-assisted flame aerosol synthesis of fumed silica at high production rates
Chemical Engineering and Processing 39 (2000) 219 – 227 www.elsevier.com/locate/cep
Electrically-assisted flame aerosol synthesis of fumed silica at high production rates Hendrik K. Kammler, Sotiris E. Pratsinis * Institut fu¨r Verfahrenstechnik, ETH Zentrum, ML F26, Sonneggstrasse 3, 8092 Zurich, Switzerland Received 19 March 1999; received in revised form 14 June 1999; accepted 14 June 1999
Abstract The control of particle size by external electric fields is investigated during flame synthesis of particles at high production rates (up to 87 g h − 1). Here needle and plate electrodes are used during synthesis of fumed silica from hexamethyldisiloxane (HMDS) in a coflow double diffusion flame at atmospheric pressure. The average primary particle diameter is reduced by a factor of two, when the applied electric field strength between two needle electrodes was increased from 0 to 1.5 kV cm − 1. It was demonstrated that electrode placement is crucial for this process. Furthermore, the powder composition (soot content less than 1 wt%) was controlled by the electric field. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Electric fields; Double diffusion flame; Fumed silica; High production rates
1. Introduction First commercial production of fumed silicas, marketed under the name Aerosil, was realized in the early 1940s [1]. Since then these products have found wide applications in different industrial branches, e.g. as fillers in car tires, toothpaste, thickening and thixotropic agents [2], carriers for catalysts [3] and as base material for optical fibers [4]. Through many process variables can affect the characteristics of flame made particles, external electric fields are intriguing as they can be readily implemented and are quite effective for precise particle size control [5]. The effect of electric fields on oxide particle formation in flames has been studied though in rather low particle production rates. Hardesty and Weinberg [6] injected hexamethyldisiloxane (HMDS) vapor into a premixed methane/air flame and found that the primary particle size of SiO2 powders was reduced by a factor of three with increasing applied electric potential. Katz and Hung [7] used SiCl4 and SiH4 as precursor in a
This article is dedicated to Professor em. Dr.-Ing. E.U. Schlu¨nder on the occasion of his 70th birthday. * Corresponding author. Tel.: +41-1-632-2510; fax: + 41-1-6321141. E-mail address: [email protected] (S.E. Pratsinis)
counterflow diffusion H2/O2 flame and found that the particle diameter increased (as determined by light scattering) with increasing electric potential along the reactant flow. Vemury and Pratsinis [8] generated 1 g h − 1 TiO2 by oxidation/hydrolysis of TiCl4 in a methane/air double diffusion flame in the presence of an electric field created by two needle electrodes across the flame. They showed that the average primary particle diameter and rutile content was reduced by a factor of 2 by increasing the applied potential from 1.2 to 2.0 kV cm − 1. They found also that positioning the needles closer to the burner face (0.5 cm) is more effective than positioning the electrodes further away (\ 1.0 cm). Vemury and Pratsinis [9] demonstrated the controlled synthesis of SiO2-particles in a premixed methane/air flame from SiCl4 while investigating various field configurations, using needle–needle, needle–plate, and plate–plate electrodes at production rates up to 4 g h − 1 SiO2. They found that the electric field created by needles influenced most significantly the product powder characteristics. Furthermore, Vemury et al. [10] investigated the application of electrical fields as a tool for the precision synthesis of nanophase TiO2, SiO2, and SnO2 at production rates up to 3 g h − 1. Increasing the applied field strength resulted in an increase of the specific surface areas up to a factor of 1.8.
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In contrast to this, Spicer et al. [11] showed that increasing the electric field intensity between plate electrodes increased the average primary particle size when making composite silica-soot powders in a premixed flame from SiCl4 and acetylene at production rates up to 15 g h − 1. Morrison et al. [12] used fourier transform infrared (FTIR) spectroscopy to determine the flame temperature, gas composition, and particle size of particle laden flames. Specifically, they studied the effect of an external electric field created by plate electrodes on the flame temperature and found that electrical fields modestly increase the flame temperature. Katzer and Schmidt [13] investigated the influence of an external electric field created by needle electrodes producing TiO2 particles by oxidation of TiCl4 in a H2/O2 diffusion flame. At production rates of less than 1 g h − 1 the mobility equivalent diameter of the product particles was reduced up to a factor of 1.7 using a field strength of 5 kV cm − 1. Briesen et al. [14] found that axial electric fields created by ring electrodes during flame synthesis of SiO2 by HMDS oxidation reduced also the average primary particle size by a factor of 2.8 at production rates of 10 g h − 1. However, at higher production rates (\ 10 g h − 1) the applied electric field had little effect on the product primary particle size and even increased soot formation at the highest production rates, 30 g h − 1, regardless of the field intensity. Up to now, the influence of an electric field on flame synthesis of particles was only investigated at rather small production rates. Here the effect of external electric fields during flame synthesis of silica by combustion of HMDS is investigated at relatively high production rates up to 87 g h − 1. The effect of needle and plate electrodes as well electrode location along the flame axis on the specific surface area, composition and morphology of the product particles are investigated as a function of the field intensity and HMDS concentration. 2. Experimental Fig. 1 shows a schematic of the experimental set-up. Clean, dried argon gas (Wright Brothers, 99.9%) is bubbled through a 1 l glass flask, filled up to 10 cm from the bottom with liquid hexamethyldisiloxane (HMDS, Gelest Inc). An Allihn condenser is placed between the burner and the precursor flask to prevent droplet entrainment resulting in 100% saturation of the Ar-HMDS stream [15]. The condenser, the HMDS vapor manifold and the burner are heated with heating tape 20 K higher than the HMDS flask temperature. Before starting an experiment, an oxygen-rich methane flame was used to heat up the burner and collection system. After each experiment, the HMDS delivering tubes were cleaned by flowing argon gas to prevent possible HMDS oxidation there.
The burner consists of 8 concentric quartz tubes of 1.5 mm wall thickness (Fig. 2). The diameter of the inner (first) tube is 25 mm (ID) while the spacing between the following four tubes is 1.5, 4.0, 1.5 and 2.0 mm, respectively. Oxygen (Wright Brothers, 99.9%) is delivered in the first (center) and the fifth tube, nitrogen (Wright Brothers, 99.9%) is fed in tubes 2, 4, and 7 while the HMDS-Ar is delivered through the third tube (Fig. 2). The flow rates are measured at 25°C by calibrated mass flow controllers (1159 B, MKS Instruments) except for the flow rate of nitrogen through tubes 2 and 4 which is measured by rotameters (Gilmont). For production of 87 g h − 1 of SiO2 the argon stream is set to 5.0 l min − 1 while 6.4 and 3.6 l min − 1 of oxygen flow through tubes 1 and 5, respectively. The flow rates of nitrogen through tubes 2 and 4 were set to 1.5 and 2.0 l min − 1, respectively. These flow rates are chosen so the burner exit velocities of oxygen and the HMDS-Ar streams are about 30 cm s − 1 each resulting in a stable flame. At production rates of 54 g h − 1 the argon gas flow through the HMDS flask was decreased to 3.0 l min − 1, hence, 4.6 mmol min − 1 less HMDS is transferred into the burner. Here, the missing energy amount is replaced by flowing 0.65 l min − 1 methane through tube 3. The combustion energy of methane is − 807 kJ mol − 1 [16] so this CH4 stream contains the equivalent energy to complete combustion of 4.6 mmol HMDS min − 1 using equal Wobbe-values [17] for both fuels (CH4-Ar and HMDS-Ar). Also 1.35 l min − 1 argon gas is added (Fig. 1) to match the total gas flow through the burner for both experiments. The external electric field is created by two stainless steel needle-like electrodes with sharp ends (100 mm [9]) or by two plate electrodes (25 mm high, 38 mm wide, and 4 mm thick), respectively (Fig. 1). The electrodes are positioned at the side of the flame, so the field lines are at cross flow with respect to the flame. One electrode is connected to a high voltage DC power supply (Gamma High Voltage) while the other is connected to the ground. The distance between the two needles is 6.5 cm, while that between the plate electrodes is 10 cm. In a distance of 6.5 cm the flame touched the plate electrodes at potentials of 1.5 kV cm − 1 and therefore the larger distance had to be chosen. The distance between the burner face and the tips of the needle electrodes was 0.2 cm for the measurements varying the electric field strength, while the lower edge of the plate electrodes is positioned at the level of the burner mouth. A schematic of the burner and electrode positioning is shown in Fig. 2. The product particles are collected on a 54 cm long, 8 cm in diameter NOMEX baghouse filter (BWF Inc.) coated with 0.54 g m − 2 polytetrafluoroethylene (Teflon) connected to a vacuum cleaner (Dayton Inc.).
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Fig. 1. Schematic set-up of the diffusion flame aerosol reactor.
The filter is 34 cm above the burner (Fig. 1) and is not enclosed in the typically employed metal sleeve [15] to avoid interaction with the electrical field. The specific surface area, AS, is measured by nitrogen adsorption (Gemini 2360, Micromeritics) at 77.3 K. Before the adsorption, the samples are degassed (Flow Prep 060, Micromeritics) at 150°C for 2 h. Assuming uniformly sized spherical primary particles within the aggregates, the equivalent average primary particle diameter is dp =6/(AS · rp) where rp is the density of SiO2 (2.2 g cm − 3). The HMDS delivery rate is determined by weighing the flask before and after each experiment operated at a constant argon flow rate. The temperature of the precursor flask is determined by a thermometer. Also, the baghouse filter is weighed before and after the experiment. From this, the yield is calculated while saturation
is determined by a mass balance for the precursor flask, using the HMDS equilibrium vapor pressure [18]. All powders are reproduced at least twice and the specific surface area also is measured twice or thrice for each sample. Data points are averages of these results while the error bars show two times the corresponding standard deviation. For microscopic analysis, 3 mg of powder are suspended in 3 cm3 ethanol and dispersed in an ultrasonic bath for 15 min; then one droplet of this suspension is dried on an alumina mount for scanning electron microscopy (SEM). After sputtering the sample with a thin gold layer, SEM-pictures are obtained on an SEM (Hitachi S-4000 Field Emission Gun) operating at 20 kV. The soot content of the product powder is obtained by weighing it before and after oxidizing samples of 0.5
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unit are fixed on lab jacks to facilitate precise location of the electrodes to the burner, which are fixed stationary on the Plexiglas box (Fig. 1) covering the burning zone and the filter unit to minimize disturbance of the flame by air motion.
3. Results and discussion
Fig. 2. Burner geometry, supplied gases in the respective tubes, and location of the needle electrodes.
to 0.7 g each at 800°C in a furnace (Blue M, 4880 Watts) assuming that the weight difference is only a result of soot combustion. The heating time from 25 to 800°C was 30 min, while the residence time at this temperature is set to 10 min following Spicer et al. [11] and preliminary experiments while the cooling time to room temperature is 2 h. The determination of the soot content was made separately two times for each powder. The data from this method were within 6% of those by thermogravimetric analysis (TGA/STA 851, Mettler Toledo). Flame temperatures are measured with a K-type thermocouple (Omega Engineering) and are corrected for radiation loss according to Collis and Williams [19]. Flame temperatures are only measured downstream of the flame in order not to destroy the thermocouple that can act as electrode. The thermocouple is inserted into the inner flame front (1.5 cm off center) where the highest temperatures are located [20], 3.5 and 4.5 cm above the burner face as not to disturb the electric field across the flame. The temperature data are the average of five single measurements. The diffusion flame reactor and the HMDS delivery
Fig. 3 shows a ring-like double diffusion flame during high rate of powder production (87 g h − 1) in the absence of applied electric fields (0 kV cm − 1). This is a stable flame with two combustion fronts as the oxygen diffuses from two sides (Fig. 2) to react with fuel/precursor HMDS. The change of the flame shape can be seen when applying an electric field and increasing its intensity from 0 to 1.0, 1.5 and 2.0 kV cm − 1, regardless of polarity. Here, the field is generated by two needle electrodes where a positive and then a negative potential is applied to the right hand side electrode, while the left hand side electrode is connected to the ground. The ionic wind splits the flame apart along the axis of the two electrode tips and substantially disturbs the uniformity of the flame creating turbulence. Flame temperature measurements at two different heights above the burner are shown in Fig. 4 as a function of applied field strength. The electric field was generated by needle electrodes located 0.2 cm above the burner face. Though in the absence of electric field there is a 200 K difference between the locations, there is little difference in the presence of electric fields. This can be attributed to the mixing created by the ionic wind (Fig. 3). The flame temperature decreases up to 400 K with increasing the field intensity to − 1.75 kV cm − 1 mostly as a result of the turbulence due to ionic wind. This is in qualitative agreement with Vemury and Pratsinis [9] who observed a decrease of the flame temperature of a premixed flame as much as 200 K at 8 cm above the burner when applying − 1.4 kV cm − 1 across needle electrodes. Fig. 5 shows representative scanning electron micrographs in the absence (Fig. 5a) and in the presence (Fig. 5b) of applied electric field ( −1.5 kV cm − 1). It can be seen clearly that the average primary particle diameter
Fig. 3. The effect of the applied electric field by needle electrodes on double diffusion flames producing 87 g h − 1 of silica powder at applied potentials of: − 2.0, −1.5, −1.0, 0 kV cm − 1, + 1.0, + 1.5, and +2.0 kV cm − 1.
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Fig. 4. The flame temperature at two different heights above the burner face as a function of the applied field strength (production rate 87 g h − 1).
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the ionic wind occurs most prominently for needle electrodes and there is only a little difference in the specific surface area of the product powders this explains the slightly stronger influence of these electrodes. Therefore, the major effect when applying an electric field (created by needle or plate electrodes), is attributed to the charged particles. Unipolarly charged aerosol particles have the tendency to expand due to mutual electrostatic repulsion, thereby undergoing a concentration decrease with time [21]. This decrease of particle concentration was also observed with FTIR by Morrison et al. [12]. Hence, the reduced particle concentration leads to subsequently less particle collisions and growth, while as the particles move quicker out of the hot temperature flame zone, also the residence time of the particles in the hot region of the flame decreases and, as a result, the particle diameter decreases.
is reduced by the application of the electric field in agreement with nitrogen adsorption (BET). Also the particle size varies significantly in the presence of the electric field, which was also observed by Vemury and Pratsinis [9] in a premixed flame for low production rates. Some relatively large particles of the size mostly produced in the absence of electric fields and significantly smaller particles are observed when applying the electric field (Fig. 5b). Apparently the ionic wind in the flame broadens the residence time distribution of the particles in the high temperature regions of the flame. Consequently the particle size distribution widens as particles experience unequal residence times and sintering temperatures.
3.1. Effect of applied field and electrode configuration Fig. 6 shows the specific surface area, AS, of the flame generated silica particles as a function of the applied potential for positive and negative field intensities at silica production rates of 87 g h − 1 (diamonds) and 54 g h − 1 (circles) using needle electrodes and 87 g h − 1 (squares) using plate electrodes. For production rates of 87 g h − 1 the specific surface area increased by about a factor of two (Fig. 6) at an applied field strength of 1.5 kV cm − 1 using needle electrodes. The average primary particle diameter was reduced from 157 to 72 nm for positive and to 85 nm for negative coronas. For plate electrodes the specific surface area of the product powder increases by a factor of 1.8 from 17 to 30 m2 g − 1 powders (Fig. 6) when increasing the field strength from 0 to 1.25 kV cm − 1 for a negative potential. The reduction of the average primary particle size is slightly less pronounced than in the case of needle electrodes in agreement to Vemury and Pratsinis [9]. As
Fig. 5. Scanning electron micrographs, magnification 30 000, of particles produced in the double diffusion flame with SiO2 production rates of 87 g h − 1, for no electric field (a) and 1.5 kV cm − 1 (negative potential) (b).
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Fig. 6. Effect of the applied electric potential on the specific surface area AS created by needle electrodes at production rates of 87 g h − 1 (diamonds) and 54 g h − 1 (circles); and by plate electrodes at 87 g h − 1 (squares).
Using plate instead of needle electrodes leads to another mechanism of charge introduction. When needle shaped electrodes are used, particles are charged by corona discharge (‘charge spray’) of unipolar ions [22] while using plate electrodes no new ions are introduced. The plate electrodes merely attract the oppositely charged flame generated ions [10]. However, as the particles are rather small they are getting charged by the movement of the flame ions to the plate electrode and as a result the particle residence time in the decisive region for particle growth in the flame is reduced as discussed before. The present results are also in qualitative agreement to Xiong et al. [23] who showed theoretically that charges can effectively slow down the particle growth process by coagulation.
cm away from the burner face. This is the most decisive region for particle formation and growth as particle charging at the early stages of particle inception slows down particle growth by coagulation. When the electrodes were placed at this point, the specific surface area of the product silica was the highest, 33 m2 g − 1. This is in agreement to Vemury and Pratsinis [8] who found that charging does not affect the particle size when the electrodes were moved away from the particle growth region even though shorter distances between the needles and much smaller production rates were used. In contrast, Katzer and Schmidt [13] found that the average mobility diameter increased merely from 50 to 57 nm when increasing the distance between the needle electrodes and the burner from 2 to 5 cm for TiO2 production rates less than 1 g h − 1 in an oxy-hydrogen diffusion flame. However, with a positive corona they observed that the mobility diameter increased from 42 to 72 nm varying the distance between the electrodes and the burner in the same way. In a distance of 5 cm the average mobility particle diameter of the powders produced without electric field is reached in agreement with Fig. 7 and with Vemury and Pratsinis [8].
3.3. The effect of production rate For both production rates, increasing the electric field strength from 0 to 1.5 kV cm − 1 increases the specific surface area of the product powder by a factor of two. Increasing the production rate at constant total flow rates also resulted in larger primary particle size (Fig. 6) as the initial particle concentration increases
3.2. Effect of electrode location The effect of electrode location, the distance between burner face and needle electrodes, on the specific surface area of the product particles was investigated (Fig. 7) at a production rate of 87 g h − 1 and electric field strength of − 1.5 kV cm − 1. As the distance between the electrodes and burner increases, the influence of the electric field decreases. At a distance of 5 cm the specific surface area was 17 m2 g − 1 which is close to that in the absence of electric fields (16 m2 g − 1). This shows that the location of the electrodes is crucial for observing the effect of electric fields. The position of the needle electrodes that most significantly reduces the particle size or increases the specific surface area is right at the beginning of the most luminous flame zone at 0.4–1.0
Fig. 7. Effect of the distance between the needle electrodes and the burner face on the specific surface area, AS, for SiO2 production rates of 87 g h − 1 at a constant electrical field strength of 1.5 kV cm − 1 (negative potential).
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and therefore the collision frequency of the particles increases. This is in agreement to Pratsinis et al. [24] studying different TiCl4 concentrations in methane-air diffusion flames at production rates up to 10 g h − 1. Limitations to the effect of external electric fields on product particle size were observed by Briesen et al. [14]. At small production rates (10 g h − 1) they observed that the specific surface area of particles produced by oxidation of HMDS was increased by a factor of 2.8 when applying an external electric field with ring electrodes. The distance between the two ring electrodes was 5 cm. At this small production rate they noticed a significant increase or decrease of the flame height when the top electrode was connected to a negative or positive potential, respectively. With increasing production rate, the flame height increased as the HMDS acts as fuel itself. For production rates larger than 10 g h − 1 the flame was larger than the distance between the electrodes resulting in more contact with the top electrode (breakdown) and some particle deposition on the rings [14]. A significant change in the flame height with applied electric field was not observed at high production rates. Here, it is believed that as the flame height changed with increasing production rate, the electric field was not optimally positioned as the distance between flame and electrodes was always fixed. In the current work it is shown that the application of electric fields using needle and plate electrodes is not limited on the production rate as was observed by Briesen et al. [14]. Here increasing the production rate of almost a factor of two and three compared to the work by Briesen et al. [14] showed that the external electric field still has a significant effect (Fig. 6).
3.4. Soot formation At the employed particle production rates, limited soot formation is observed when applying the electric field resulting in gray colored powders. The soot content in the product powders increases up to 1 wt% with increasing electric field strength regardless of the polarity of the corona or the electrode choice (Fig. 8). Slightly larger amounts are observed for high HMDS concentration in agreement to Briesen et al. [14] who also observed small amounts of soot at their highest production rate of 30 g h − 1. Place and Weinberg [25] studying the influence of an external electric field on soot formation in a counterflow diffusion burner, found that increasing the electric potential to 1 kV dropped down the soot formation to 2%. Here, the electric field enhanced the combustion of soot. Spicer et al. [11] observed an increase of soot content in soot – silica powders and a decrease of specific surface area of the same carbon– silica powders with increasing electric field strength.
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Fig. 8. Soot content by weight in the product particles for production rates of 87 g h − 1 (diamonds) and 54 g h − 1 (circles) using needle and for 87 g h − 1 using plate electrodes (squares) as a function of applied electric field strength (positive and negative potential).
The enhanced soot formation [11] and also enhanced soot oxidation [22,26] by the electric field may lead to higher temperatures on the particle surface increasing the sintering of the particles and therefore to reduce the specific surface area. The latter is also in agreement with the formation of smaller soot particles in the presence of electric fields [26] by accelerated combustion [25]. As in the present study the soot content in the powder is too small (only 1/30 of the one by Spicer et al. [11]) this cannot affect significantly silica sintering. The increase of the soot content with increasing potential results from of the decreased particle residence time at high temperatures, when the particles are attracted away from the high-temperature region of the flame [6] by repulsive forces and/or by the ionic wind. It is also expected that carbon is mostly on the surface of the silica particle, as soot particles grow by surface growth [27]. The freshly formed silica particles provide a surface on which soot formation takes place. Oxidizing the soot-silica powders in a furnace at 800°C showed that soot could be removed easily as perfectly white powders were obtained after this treatment. Here, the specific surface area of the oxidized product particles was not changed (within the error bar of measurement). The burner-electrode distance affects also the composition of the product powders. As shown in Fig. 9, the soot content drops from almost 0.8 wt% for a distance of 0.2 cm between the burner face and the needle electrodes, to less than 0.2 wt% at a distance of 5.0 cm. Most likely the ionic wind prevents complete combustion of the HMDS in the presence of electric fields.
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References
Fig. 9. Soot content by weight in the product particles as a function of the distance between the needle electrodes and the burner face for production rates of 87 g h − 1 at a field strength of 1.5 kV cm − 1 (negative potential).
4. Conclusions The role of external electric fields on flame synthesis of silica was investigated at high powder production rates. Fumed silica was made in a double diffusion flame by oxidation of hexamethyldisiloxane. The average primary particle size was reduced by a factor of two using needle electrodes, and by a factor of 1.8 by plate electrodes. The monotonic increase of specific surface area with field intensity shows that electric fields can be used to precisely control the specific surface area of the product particles. Increasing the HMDS concentration leads to larger particles. Proper positioning of the electrodes is necessary for a successful use of electric fields. The optimum electrode location with respect to particle size reduction was at the beginning of the luminous zone of the flame. Soot formation steadily increased (up to 1 wt%) with increasing field intensity. This is explained by the broad particle residence times in the high-temperature regions of the flame as the ionic wind enhances the flame turbulence. As a result, electric fields cannot only facilitate the synthesis of ceramic powders with precisely controlled specific surface area, but can also control powder composition.
Acknowledgements This work was initiated at the University of Cincinnati, USA, and supported in part by the US National Science Foundation and DAAD (CTS-9619392 and INT-9603196) and Swiss National Science Foundation.
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Electrically-assisted flame aerosol synthesis of fumed silica at high production rates Hendrik K. Kammler, Sotiris E. Pratsinis * Institut fu¨r Verfahrenstechnik, ETH Zentrum, ML F26, Sonneggstrasse 3, 8092 Zurich, Switzerland Received 19 March 1999; received in revised form 14 June 1999; accepted 14 June 1999
Abstract The control of particle size by external electric fields is investigated during flame synthesis of particles at high production rates (up to 87 g h − 1). Here needle and plate electrodes are used during synthesis of fumed silica from hexamethyldisiloxane (HMDS) in a coflow double diffusion flame at atmospheric pressure. The average primary particle diameter is reduced by a factor of two, when the applied electric field strength between two needle electrodes was increased from 0 to 1.5 kV cm − 1. It was demonstrated that electrode placement is crucial for this process. Furthermore, the powder composition (soot content less than 1 wt%) was controlled by the electric field. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Electric fields; Double diffusion flame; Fumed silica; High production rates
1. Introduction First commercial production of fumed silicas, marketed under the name Aerosil, was realized in the early 1940s [1]. Since then these products have found wide applications in different industrial branches, e.g. as fillers in car tires, toothpaste, thickening and thixotropic agents [2], carriers for catalysts [3] and as base material for optical fibers [4]. Through many process variables can affect the characteristics of flame made particles, external electric fields are intriguing as they can be readily implemented and are quite effective for precise particle size control [5]. The effect of electric fields on oxide particle formation in flames has been studied though in rather low particle production rates. Hardesty and Weinberg [6] injected hexamethyldisiloxane (HMDS) vapor into a premixed methane/air flame and found that the primary particle size of SiO2 powders was reduced by a factor of three with increasing applied electric potential. Katz and Hung [7] used SiCl4 and SiH4 as precursor in a
This article is dedicated to Professor em. Dr.-Ing. E.U. Schlu¨nder on the occasion of his 70th birthday. * Corresponding author. Tel.: +41-1-632-2510; fax: + 41-1-6321141. E-mail address: [email protected] (S.E. Pratsinis)
counterflow diffusion H2/O2 flame and found that the particle diameter increased (as determined by light scattering) with increasing electric potential along the reactant flow. Vemury and Pratsinis [8] generated 1 g h − 1 TiO2 by oxidation/hydrolysis of TiCl4 in a methane/air double diffusion flame in the presence of an electric field created by two needle electrodes across the flame. They showed that the average primary particle diameter and rutile content was reduced by a factor of 2 by increasing the applied potential from 1.2 to 2.0 kV cm − 1. They found also that positioning the needles closer to the burner face (0.5 cm) is more effective than positioning the electrodes further away (\ 1.0 cm). Vemury and Pratsinis [9] demonstrated the controlled synthesis of SiO2-particles in a premixed methane/air flame from SiCl4 while investigating various field configurations, using needle–needle, needle–plate, and plate–plate electrodes at production rates up to 4 g h − 1 SiO2. They found that the electric field created by needles influenced most significantly the product powder characteristics. Furthermore, Vemury et al. [10] investigated the application of electrical fields as a tool for the precision synthesis of nanophase TiO2, SiO2, and SnO2 at production rates up to 3 g h − 1. Increasing the applied field strength resulted in an increase of the specific surface areas up to a factor of 1.8.
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In contrast to this, Spicer et al. [11] showed that increasing the electric field intensity between plate electrodes increased the average primary particle size when making composite silica-soot powders in a premixed flame from SiCl4 and acetylene at production rates up to 15 g h − 1. Morrison et al. [12] used fourier transform infrared (FTIR) spectroscopy to determine the flame temperature, gas composition, and particle size of particle laden flames. Specifically, they studied the effect of an external electric field created by plate electrodes on the flame temperature and found that electrical fields modestly increase the flame temperature. Katzer and Schmidt [13] investigated the influence of an external electric field created by needle electrodes producing TiO2 particles by oxidation of TiCl4 in a H2/O2 diffusion flame. At production rates of less than 1 g h − 1 the mobility equivalent diameter of the product particles was reduced up to a factor of 1.7 using a field strength of 5 kV cm − 1. Briesen et al. [14] found that axial electric fields created by ring electrodes during flame synthesis of SiO2 by HMDS oxidation reduced also the average primary particle size by a factor of 2.8 at production rates of 10 g h − 1. However, at higher production rates (\ 10 g h − 1) the applied electric field had little effect on the product primary particle size and even increased soot formation at the highest production rates, 30 g h − 1, regardless of the field intensity. Up to now, the influence of an electric field on flame synthesis of particles was only investigated at rather small production rates. Here the effect of external electric fields during flame synthesis of silica by combustion of HMDS is investigated at relatively high production rates up to 87 g h − 1. The effect of needle and plate electrodes as well electrode location along the flame axis on the specific surface area, composition and morphology of the product particles are investigated as a function of the field intensity and HMDS concentration. 2. Experimental Fig. 1 shows a schematic of the experimental set-up. Clean, dried argon gas (Wright Brothers, 99.9%) is bubbled through a 1 l glass flask, filled up to 10 cm from the bottom with liquid hexamethyldisiloxane (HMDS, Gelest Inc). An Allihn condenser is placed between the burner and the precursor flask to prevent droplet entrainment resulting in 100% saturation of the Ar-HMDS stream [15]. The condenser, the HMDS vapor manifold and the burner are heated with heating tape 20 K higher than the HMDS flask temperature. Before starting an experiment, an oxygen-rich methane flame was used to heat up the burner and collection system. After each experiment, the HMDS delivering tubes were cleaned by flowing argon gas to prevent possible HMDS oxidation there.
The burner consists of 8 concentric quartz tubes of 1.5 mm wall thickness (Fig. 2). The diameter of the inner (first) tube is 25 mm (ID) while the spacing between the following four tubes is 1.5, 4.0, 1.5 and 2.0 mm, respectively. Oxygen (Wright Brothers, 99.9%) is delivered in the first (center) and the fifth tube, nitrogen (Wright Brothers, 99.9%) is fed in tubes 2, 4, and 7 while the HMDS-Ar is delivered through the third tube (Fig. 2). The flow rates are measured at 25°C by calibrated mass flow controllers (1159 B, MKS Instruments) except for the flow rate of nitrogen through tubes 2 and 4 which is measured by rotameters (Gilmont). For production of 87 g h − 1 of SiO2 the argon stream is set to 5.0 l min − 1 while 6.4 and 3.6 l min − 1 of oxygen flow through tubes 1 and 5, respectively. The flow rates of nitrogen through tubes 2 and 4 were set to 1.5 and 2.0 l min − 1, respectively. These flow rates are chosen so the burner exit velocities of oxygen and the HMDS-Ar streams are about 30 cm s − 1 each resulting in a stable flame. At production rates of 54 g h − 1 the argon gas flow through the HMDS flask was decreased to 3.0 l min − 1, hence, 4.6 mmol min − 1 less HMDS is transferred into the burner. Here, the missing energy amount is replaced by flowing 0.65 l min − 1 methane through tube 3. The combustion energy of methane is − 807 kJ mol − 1 [16] so this CH4 stream contains the equivalent energy to complete combustion of 4.6 mmol HMDS min − 1 using equal Wobbe-values [17] for both fuels (CH4-Ar and HMDS-Ar). Also 1.35 l min − 1 argon gas is added (Fig. 1) to match the total gas flow through the burner for both experiments. The external electric field is created by two stainless steel needle-like electrodes with sharp ends (100 mm [9]) or by two plate electrodes (25 mm high, 38 mm wide, and 4 mm thick), respectively (Fig. 1). The electrodes are positioned at the side of the flame, so the field lines are at cross flow with respect to the flame. One electrode is connected to a high voltage DC power supply (Gamma High Voltage) while the other is connected to the ground. The distance between the two needles is 6.5 cm, while that between the plate electrodes is 10 cm. In a distance of 6.5 cm the flame touched the plate electrodes at potentials of 1.5 kV cm − 1 and therefore the larger distance had to be chosen. The distance between the burner face and the tips of the needle electrodes was 0.2 cm for the measurements varying the electric field strength, while the lower edge of the plate electrodes is positioned at the level of the burner mouth. A schematic of the burner and electrode positioning is shown in Fig. 2. The product particles are collected on a 54 cm long, 8 cm in diameter NOMEX baghouse filter (BWF Inc.) coated with 0.54 g m − 2 polytetrafluoroethylene (Teflon) connected to a vacuum cleaner (Dayton Inc.).
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Fig. 1. Schematic set-up of the diffusion flame aerosol reactor.
The filter is 34 cm above the burner (Fig. 1) and is not enclosed in the typically employed metal sleeve [15] to avoid interaction with the electrical field. The specific surface area, AS, is measured by nitrogen adsorption (Gemini 2360, Micromeritics) at 77.3 K. Before the adsorption, the samples are degassed (Flow Prep 060, Micromeritics) at 150°C for 2 h. Assuming uniformly sized spherical primary particles within the aggregates, the equivalent average primary particle diameter is dp =6/(AS · rp) where rp is the density of SiO2 (2.2 g cm − 3). The HMDS delivery rate is determined by weighing the flask before and after each experiment operated at a constant argon flow rate. The temperature of the precursor flask is determined by a thermometer. Also, the baghouse filter is weighed before and after the experiment. From this, the yield is calculated while saturation
is determined by a mass balance for the precursor flask, using the HMDS equilibrium vapor pressure [18]. All powders are reproduced at least twice and the specific surface area also is measured twice or thrice for each sample. Data points are averages of these results while the error bars show two times the corresponding standard deviation. For microscopic analysis, 3 mg of powder are suspended in 3 cm3 ethanol and dispersed in an ultrasonic bath for 15 min; then one droplet of this suspension is dried on an alumina mount for scanning electron microscopy (SEM). After sputtering the sample with a thin gold layer, SEM-pictures are obtained on an SEM (Hitachi S-4000 Field Emission Gun) operating at 20 kV. The soot content of the product powder is obtained by weighing it before and after oxidizing samples of 0.5
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unit are fixed on lab jacks to facilitate precise location of the electrodes to the burner, which are fixed stationary on the Plexiglas box (Fig. 1) covering the burning zone and the filter unit to minimize disturbance of the flame by air motion.
3. Results and discussion
Fig. 2. Burner geometry, supplied gases in the respective tubes, and location of the needle electrodes.
to 0.7 g each at 800°C in a furnace (Blue M, 4880 Watts) assuming that the weight difference is only a result of soot combustion. The heating time from 25 to 800°C was 30 min, while the residence time at this temperature is set to 10 min following Spicer et al. [11] and preliminary experiments while the cooling time to room temperature is 2 h. The determination of the soot content was made separately two times for each powder. The data from this method were within 6% of those by thermogravimetric analysis (TGA/STA 851, Mettler Toledo). Flame temperatures are measured with a K-type thermocouple (Omega Engineering) and are corrected for radiation loss according to Collis and Williams [19]. Flame temperatures are only measured downstream of the flame in order not to destroy the thermocouple that can act as electrode. The thermocouple is inserted into the inner flame front (1.5 cm off center) where the highest temperatures are located [20], 3.5 and 4.5 cm above the burner face as not to disturb the electric field across the flame. The temperature data are the average of five single measurements. The diffusion flame reactor and the HMDS delivery
Fig. 3 shows a ring-like double diffusion flame during high rate of powder production (87 g h − 1) in the absence of applied electric fields (0 kV cm − 1). This is a stable flame with two combustion fronts as the oxygen diffuses from two sides (Fig. 2) to react with fuel/precursor HMDS. The change of the flame shape can be seen when applying an electric field and increasing its intensity from 0 to 1.0, 1.5 and 2.0 kV cm − 1, regardless of polarity. Here, the field is generated by two needle electrodes where a positive and then a negative potential is applied to the right hand side electrode, while the left hand side electrode is connected to the ground. The ionic wind splits the flame apart along the axis of the two electrode tips and substantially disturbs the uniformity of the flame creating turbulence. Flame temperature measurements at two different heights above the burner are shown in Fig. 4 as a function of applied field strength. The electric field was generated by needle electrodes located 0.2 cm above the burner face. Though in the absence of electric field there is a 200 K difference between the locations, there is little difference in the presence of electric fields. This can be attributed to the mixing created by the ionic wind (Fig. 3). The flame temperature decreases up to 400 K with increasing the field intensity to − 1.75 kV cm − 1 mostly as a result of the turbulence due to ionic wind. This is in qualitative agreement with Vemury and Pratsinis [9] who observed a decrease of the flame temperature of a premixed flame as much as 200 K at 8 cm above the burner when applying − 1.4 kV cm − 1 across needle electrodes. Fig. 5 shows representative scanning electron micrographs in the absence (Fig. 5a) and in the presence (Fig. 5b) of applied electric field ( −1.5 kV cm − 1). It can be seen clearly that the average primary particle diameter
Fig. 3. The effect of the applied electric field by needle electrodes on double diffusion flames producing 87 g h − 1 of silica powder at applied potentials of: − 2.0, −1.5, −1.0, 0 kV cm − 1, + 1.0, + 1.5, and +2.0 kV cm − 1.
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Fig. 4. The flame temperature at two different heights above the burner face as a function of the applied field strength (production rate 87 g h − 1).
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the ionic wind occurs most prominently for needle electrodes and there is only a little difference in the specific surface area of the product powders this explains the slightly stronger influence of these electrodes. Therefore, the major effect when applying an electric field (created by needle or plate electrodes), is attributed to the charged particles. Unipolarly charged aerosol particles have the tendency to expand due to mutual electrostatic repulsion, thereby undergoing a concentration decrease with time [21]. This decrease of particle concentration was also observed with FTIR by Morrison et al. [12]. Hence, the reduced particle concentration leads to subsequently less particle collisions and growth, while as the particles move quicker out of the hot temperature flame zone, also the residence time of the particles in the hot region of the flame decreases and, as a result, the particle diameter decreases.
is reduced by the application of the electric field in agreement with nitrogen adsorption (BET). Also the particle size varies significantly in the presence of the electric field, which was also observed by Vemury and Pratsinis [9] in a premixed flame for low production rates. Some relatively large particles of the size mostly produced in the absence of electric fields and significantly smaller particles are observed when applying the electric field (Fig. 5b). Apparently the ionic wind in the flame broadens the residence time distribution of the particles in the high temperature regions of the flame. Consequently the particle size distribution widens as particles experience unequal residence times and sintering temperatures.
3.1. Effect of applied field and electrode configuration Fig. 6 shows the specific surface area, AS, of the flame generated silica particles as a function of the applied potential for positive and negative field intensities at silica production rates of 87 g h − 1 (diamonds) and 54 g h − 1 (circles) using needle electrodes and 87 g h − 1 (squares) using plate electrodes. For production rates of 87 g h − 1 the specific surface area increased by about a factor of two (Fig. 6) at an applied field strength of 1.5 kV cm − 1 using needle electrodes. The average primary particle diameter was reduced from 157 to 72 nm for positive and to 85 nm for negative coronas. For plate electrodes the specific surface area of the product powder increases by a factor of 1.8 from 17 to 30 m2 g − 1 powders (Fig. 6) when increasing the field strength from 0 to 1.25 kV cm − 1 for a negative potential. The reduction of the average primary particle size is slightly less pronounced than in the case of needle electrodes in agreement to Vemury and Pratsinis [9]. As
Fig. 5. Scanning electron micrographs, magnification 30 000, of particles produced in the double diffusion flame with SiO2 production rates of 87 g h − 1, for no electric field (a) and 1.5 kV cm − 1 (negative potential) (b).
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Fig. 6. Effect of the applied electric potential on the specific surface area AS created by needle electrodes at production rates of 87 g h − 1 (diamonds) and 54 g h − 1 (circles); and by plate electrodes at 87 g h − 1 (squares).
Using plate instead of needle electrodes leads to another mechanism of charge introduction. When needle shaped electrodes are used, particles are charged by corona discharge (‘charge spray’) of unipolar ions [22] while using plate electrodes no new ions are introduced. The plate electrodes merely attract the oppositely charged flame generated ions [10]. However, as the particles are rather small they are getting charged by the movement of the flame ions to the plate electrode and as a result the particle residence time in the decisive region for particle growth in the flame is reduced as discussed before. The present results are also in qualitative agreement to Xiong et al. [23] who showed theoretically that charges can effectively slow down the particle growth process by coagulation.
cm away from the burner face. This is the most decisive region for particle formation and growth as particle charging at the early stages of particle inception slows down particle growth by coagulation. When the electrodes were placed at this point, the specific surface area of the product silica was the highest, 33 m2 g − 1. This is in agreement to Vemury and Pratsinis [8] who found that charging does not affect the particle size when the electrodes were moved away from the particle growth region even though shorter distances between the needles and much smaller production rates were used. In contrast, Katzer and Schmidt [13] found that the average mobility diameter increased merely from 50 to 57 nm when increasing the distance between the needle electrodes and the burner from 2 to 5 cm for TiO2 production rates less than 1 g h − 1 in an oxy-hydrogen diffusion flame. However, with a positive corona they observed that the mobility diameter increased from 42 to 72 nm varying the distance between the electrodes and the burner in the same way. In a distance of 5 cm the average mobility particle diameter of the powders produced without electric field is reached in agreement with Fig. 7 and with Vemury and Pratsinis [8].
3.3. The effect of production rate For both production rates, increasing the electric field strength from 0 to 1.5 kV cm − 1 increases the specific surface area of the product powder by a factor of two. Increasing the production rate at constant total flow rates also resulted in larger primary particle size (Fig. 6) as the initial particle concentration increases
3.2. Effect of electrode location The effect of electrode location, the distance between burner face and needle electrodes, on the specific surface area of the product particles was investigated (Fig. 7) at a production rate of 87 g h − 1 and electric field strength of − 1.5 kV cm − 1. As the distance between the electrodes and burner increases, the influence of the electric field decreases. At a distance of 5 cm the specific surface area was 17 m2 g − 1 which is close to that in the absence of electric fields (16 m2 g − 1). This shows that the location of the electrodes is crucial for observing the effect of electric fields. The position of the needle electrodes that most significantly reduces the particle size or increases the specific surface area is right at the beginning of the most luminous flame zone at 0.4–1.0
Fig. 7. Effect of the distance between the needle electrodes and the burner face on the specific surface area, AS, for SiO2 production rates of 87 g h − 1 at a constant electrical field strength of 1.5 kV cm − 1 (negative potential).
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and therefore the collision frequency of the particles increases. This is in agreement to Pratsinis et al. [24] studying different TiCl4 concentrations in methane-air diffusion flames at production rates up to 10 g h − 1. Limitations to the effect of external electric fields on product particle size were observed by Briesen et al. [14]. At small production rates (10 g h − 1) they observed that the specific surface area of particles produced by oxidation of HMDS was increased by a factor of 2.8 when applying an external electric field with ring electrodes. The distance between the two ring electrodes was 5 cm. At this small production rate they noticed a significant increase or decrease of the flame height when the top electrode was connected to a negative or positive potential, respectively. With increasing production rate, the flame height increased as the HMDS acts as fuel itself. For production rates larger than 10 g h − 1 the flame was larger than the distance between the electrodes resulting in more contact with the top electrode (breakdown) and some particle deposition on the rings [14]. A significant change in the flame height with applied electric field was not observed at high production rates. Here, it is believed that as the flame height changed with increasing production rate, the electric field was not optimally positioned as the distance between flame and electrodes was always fixed. In the current work it is shown that the application of electric fields using needle and plate electrodes is not limited on the production rate as was observed by Briesen et al. [14]. Here increasing the production rate of almost a factor of two and three compared to the work by Briesen et al. [14] showed that the external electric field still has a significant effect (Fig. 6).
3.4. Soot formation At the employed particle production rates, limited soot formation is observed when applying the electric field resulting in gray colored powders. The soot content in the product powders increases up to 1 wt% with increasing electric field strength regardless of the polarity of the corona or the electrode choice (Fig. 8). Slightly larger amounts are observed for high HMDS concentration in agreement to Briesen et al. [14] who also observed small amounts of soot at their highest production rate of 30 g h − 1. Place and Weinberg [25] studying the influence of an external electric field on soot formation in a counterflow diffusion burner, found that increasing the electric potential to 1 kV dropped down the soot formation to 2%. Here, the electric field enhanced the combustion of soot. Spicer et al. [11] observed an increase of soot content in soot – silica powders and a decrease of specific surface area of the same carbon– silica powders with increasing electric field strength.
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Fig. 8. Soot content by weight in the product particles for production rates of 87 g h − 1 (diamonds) and 54 g h − 1 (circles) using needle and for 87 g h − 1 using plate electrodes (squares) as a function of applied electric field strength (positive and negative potential).
The enhanced soot formation [11] and also enhanced soot oxidation [22,26] by the electric field may lead to higher temperatures on the particle surface increasing the sintering of the particles and therefore to reduce the specific surface area. The latter is also in agreement with the formation of smaller soot particles in the presence of electric fields [26] by accelerated combustion [25]. As in the present study the soot content in the powder is too small (only 1/30 of the one by Spicer et al. [11]) this cannot affect significantly silica sintering. The increase of the soot content with increasing potential results from of the decreased particle residence time at high temperatures, when the particles are attracted away from the high-temperature region of the flame [6] by repulsive forces and/or by the ionic wind. It is also expected that carbon is mostly on the surface of the silica particle, as soot particles grow by surface growth [27]. The freshly formed silica particles provide a surface on which soot formation takes place. Oxidizing the soot-silica powders in a furnace at 800°C showed that soot could be removed easily as perfectly white powders were obtained after this treatment. Here, the specific surface area of the oxidized product particles was not changed (within the error bar of measurement). The burner-electrode distance affects also the composition of the product powders. As shown in Fig. 9, the soot content drops from almost 0.8 wt% for a distance of 0.2 cm between the burner face and the needle electrodes, to less than 0.2 wt% at a distance of 5.0 cm. Most likely the ionic wind prevents complete combustion of the HMDS in the presence of electric fields.
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References
Fig. 9. Soot content by weight in the product particles as a function of the distance between the needle electrodes and the burner face for production rates of 87 g h − 1 at a field strength of 1.5 kV cm − 1 (negative potential).
4. Conclusions The role of external electric fields on flame synthesis of silica was investigated at high powder production rates. Fumed silica was made in a double diffusion flame by oxidation of hexamethyldisiloxane. The average primary particle size was reduced by a factor of two using needle electrodes, and by a factor of 1.8 by plate electrodes. The monotonic increase of specific surface area with field intensity shows that electric fields can be used to precisely control the specific surface area of the product particles. Increasing the HMDS concentration leads to larger particles. Proper positioning of the electrodes is necessary for a successful use of electric fields. The optimum electrode location with respect to particle size reduction was at the beginning of the luminous zone of the flame. Soot formation steadily increased (up to 1 wt%) with increasing field intensity. This is explained by the broad particle residence times in the high-temperature regions of the flame as the ionic wind enhances the flame turbulence. As a result, electric fields cannot only facilitate the synthesis of ceramic powders with precisely controlled specific surface area, but can also control powder composition.
Acknowledgements This work was initiated at the University of Cincinnati, USA, and supported in part by the US National Science Foundation and DAAD (CTS-9619392 and INT-9603196) and Swiss National Science Foundation.
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