Langmuir 1987, 3, 254-259
254
It is likely that the emphasis on these future studies will be on some of the problems we have identified here, for example, narrowing the experimental gap between large molecules formed in the early chemistry and the smallest particles detected optically and achieving a more detailed understanding of the oxidation process. Additionally there is significant interest in understanding the processes controlling incipient soot particle formation as well as the mechanisms governing surface growth. To date little specific information is available in either of these areas. Furthermore an understanding of particle agglomeration and its dependence on particle properties is of importance in the areas of particle oxidation and transport as well as for optical diagnostic measurements. Each of these topics
represents a complex problem which spans several related disciplines and may require study outside of a diffusion flame or even a combustion environment. However, given the present level of interest and rate of progress, it is likely that a detailed understanding of soot formation in laminar flames will be available before the end of this century.
Acknowledgment. We acknowledge with great appreciation the contributions of our co-workers at NBS in this work, particularly J. J. Horvath, W. G. Mallard, H. G. Semerjian, K. C. Smyth and T. T. Yeh. R.J.S. acknowledges partial support of this project under the Air Force Office of Scientific Research (AFOSR Contract AFOSR-ISSA 86-0006).
Morphology of Flame-Generated Soot As Determined by Thermophoretic Sampling R. A. Dobbins* and C. M. Megaridis Division of Engineering, Brown University, Providence, Rhode Island 02912 Received August 12, 1986. I n Final Form: October 31, 1986 Soot particle morphology in a laminar coannular ethene diffusion flame is studied by means of thermophoretic sampling, using a variety of probes. Soot particles that are small compared to the molecular mean free path are driven to the cold probe surface by the thermophoretic drift velocity that is independent of particle size. The apparatus controlling the motion of the probe is described; probe exposure times within the flame as short as 30 ms are achieved. The sampling surfaces employed are the standard specimen grids and the bulk specimen carrier. Soot particle morphology is displayed in regions of the flame where agglomeration coexists with either particle formation, surface growth, or oxidation. Observations of a series of electron micrographs show that the primary particle size at the region of maximum soot volume fraction (annular region) increases with height in the low and intermediate regions of the flame. The primary size subsequently decreases through oxidation. Our studies further show that, at any height in the lower part of the diffusion flame, the primary particle size and the state of agglomeration peak at the annular region.
Introduction Interest in the formation, transport, and partial or complete oxidation of soot in flames stems from the needs for greater efficiency in the conversion of chemical energy and for improved control of air pollution. Laser light scattering and transmission tests for analyzing the soot field in flames are widely u ~ e d . l - These ~ methods have considerable appeal because they are nonintrusive and allow observation of the soot field without intervening in the chemical and physical processes which are present. Direct sampling of the particles, an intrusive technique, has received more limited attention in the past. A compelling need for the information contained in the morphological character of the soot particle field provides the motivation to overcome the practical difficulties involved in perfecting particle sampling methods. Some of these methods are based on the fact that soot deposition rates on cold targets immersed in flames are dominated by particle thermophoresis, Le., particle drift down the temperature gradient. More recently the role of thermophoretic has provided a quantitative understanding of the drift of particles across a shear layer to the surface that is at a lower temperature than the surrounding gases. Soot particles have often been collected on cold surfaces7J0-’*for subsequent analysis by electron micros+ Presented a t t h e symposium on “Fine Particles: High Temperature Synthesis”, 60th Colloid and Surface Science Symposium, June 15-18. 1986, Atlanta, GA; G. Mulholland, Chairman.
0743-7463/87/2403-0254$01.50/0
copy. In this work we describe a refinement of thermophoretic sampling which seeks to improve the quality of information that this method can provide. All measurements were performed on a laminar axisymmetric ethene diffusion flame at atmospheric pressure.
Thermophoretic Sampling Theory. Thermophoretic deposition is driven by the presence of a temperature gradient in the vicinity of a cold wall inside the flow field of a particle-laden gas. This gradient is readily established by introducing briefly into the hot flame gases a probe surface which is initially at (1) D’Alessio, A.; Di Lorenzo, A.; Borghese, A.; Beretta, F.; Masi, S. Sixteenth Symposium (International) on Combustion;The Combustion Institute, 1977; p 695. (2) D’Alessio, A. In Particulate Carbon Formation During Combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum: New York, 1981; p 207. (3) Blockhorn, H.; Fetting, F.; Meyer, U.; Reck, R.; Wannemager, G. Eighteenth Symposium (International)on combustion; The Combustion Institute, 1981; p 1137. (4)Kent, J. H.; Jander, H.; Wagner, H. Gg. Eighteenth Symposium (International) on Combustion;The Combustion Institute, 1981; p 1117. ( 5 ) Kent, J. H.; Wagner, H. Gg. Combust. Flame 1982, 45, 53. (6) Haynes, B. S.;Wagner, H. Gg. Ber. Bunseges. Phys. Chem. 1980, 84, 499. (7) Jagoda, I. J.; Prado, G.; Lahaye, J. Combust. Flame 1980,37, 261. (8) Goren, S.L. J. Colloid Interface Sci. 1977, 61, 77. (9) Eisner, A. D.; Rosner, D. E. Combust. Flame 1985, 61, 153. (10) Tesner, P. A,; Snegiriova, T. D.; Knorre, V. G. Combust. Flame, 1971, 17, 253. (11)Skolnik, E. G.; McHale, E. T. Combust. Flame 1980, 37, 327. (12) Prado, G.; Jagoda, J.; Neoh, K.; Lahaye, J. Eighteenth Symposium (International)on Combustion; The Combustion Institute, 1981; p 1127.
0 1987 American Chemical Society
Langmuir, Vol. 3, No. 2, 1987 255
Morphology of Flame-Generated Soot
We can then estimate the required exposure time re, here defined to be the time required for a 10% coverage of the probe surface by soot particles, to be given by re =
MOUTH
w Figure 1. Sketch of thermophoretic probe sampling of a coannular diffusion flame. The flame coordinate system and the radius r, of maximum soot volume fraction are also shown.
room temperature. The probe exposure time 7, should be long enough to capture a significant sample but short enough to present a cold surface to the flame-born particles. This cold surface serves a second important purpose which is that it freezes heterogeneous reactions of the particles that are already captured. This chemical freezing action prevents changes in the soot morphology after the particles have impacted upon the cold surface. An estimate of the quench time, i.e., the time for a typical soot particle to traverse from the flame to the probe surface, is given in the Appendix. A schematic diagram of the probe, the diffusion flame, and the burner based coordinate system applicable for our experiments is shown in Figure 1. The theoretical description of the thermophoretic transport of particles is provided by the analysis of the particle drift across a thermal boundary layer formed over a solid surface in a particle-laden, hot gas ~ t r e a m .As~ suming particles of uniform diameter D, in the free molecular regime, this analysis provides the particle flux to the cold wall as
where Jw is the particle number flux, K is the thermophoretic velocity coefficient, u, is the kinematic viscosity of the host gas mixture at the outer edge of the gas thermal boundary layer, f,,, is the particle volume fraction a t the outer edge of the gas thermal boundary layer, D , is the particle diameter, T,, Tgare the wall and carrier gas temperatures, K is the exponent appearing in the dependence of gas mixture thermal conductivity on temperature (it is K = 0.84), and (dT/dy)lwis the temperature gradient in the cross stream direction a t the wall. We approximate
!q dY
=-T g - T w w
at
where 6, is the thermal boundary layer thickness. Then
0.4 ~
irJwDp2
(3)
Using in eq 2 and 3 values corresponding to the characteristics of the soot field at z = 10 mm, Le., K N 0.55 for free molecular v, = 2.81 cm2/s corresponding to air at Tg, f,,,N 1.5 X lo4 cm3/cm3 from ref 13, Tw = 380 K from probe temperature measurements, Tg= 1700 K from ref 14, D, = 12 nm (see Results and Discussion), and 6, = 0.06 cm from heat transfer analysis, we estimate T , to be of the order of 9 ms. In practice somewhat longer exposure times are employed. An additional time scale of importance, the quench time rq, is the time required for a soot particle to traverse from the hot flame gases to the cold surface. This time is derived in the Appendix and is found to be
(4) We take the kinematic viscosity to be ua = 1.26 cm2/s corresponding to air a t the average temperature of Ta = 1040 K. Using the values corresponding to the characteristics of the soot field at z = 10 mm, we estimate from eq 4 rq to be approximately 4 ms. This rapid transport of the particles to the cold probe surface promotes the freeziqg of the chemical reactions during the sampling process. Equations 2 and 4 are based on the assumption that the aggregated particles are small compared with the mean free path of the gas molecules. This condition is fulfilled in most regions of the flame. Experiment. The sampling surfaces employed in this work are the two devices that are used to hold specimens in the electron microscope and modified versions of these materials. These devices are the widely used circular porous grid of 3.05" diameter and the bulk specimen carrier used to examine bulk specimens through scanning electron microscopy (SEM). The more familiar porous grid, Figure 2A, is advantageous for transmission electron microscopy (TEM) work, especially when coated with a film 200 8, thick of elemental carbon or silicon monoxide (SiO) by evaporative deposition. The carbon films are extremely stable under the electron beam, possess a fine background structure giving high-resolution capabilities, and have a composition closely related to that of the soot particles. Carbon films are readily oxidized and therefore can be exposed only for very short time intervals. Films of S i 0 are more durable but tend to produce a lower background contrast. The 200-mesh copper grid, made up of 40-pm bars forming 85-pm square holes, was used in these tests. The image quality produced by soot particles captured on grids is very high and good particle statistics are potentially obtainable from the electron micrographs. A disadvantage of the porous grid is its inability to afford precise spatial correspondence between the flame position and electron micrograph coordinates. Nevertheless position correspondence to about 1.0 mm between flame (13)Santoro, R.J.; Semerjian, H. G.;Dobbins, R. A. Combust. Flame 1983,51, 203. (14)Santoro, R. J.; Yeh, T. T.; Semerjian, H. G . 23rd ASMEIAIChE National Heat Transfer Conference, Denver, CO, 1985.
256 Langmuir, Vol. 3, No. 2, 1987 (A)
E - M Grid
Dobbins and Megaridis
(B) Bulk Specimen Carrier
I
C
D
(C) T y p e I Probe
Modified Probes
Figure 2. Sketch of thermophoretic probe devices discussed in the text: (A) circular grid; (B) bulk specimen carrier; (C) type I probe; (D) type I1 probe. position and the micrograph coordinate is achievable with this device. The bulk specimen carrier, shown in Figure 2B, is a thin strip of metal which can be used to provide high position correspondence by aligning its edge on the flame axis as is illustrated in Figure 1, which is a schematic diagram of the probe inserted into the diffusion flame. The carrier is made of an alloy of copper, zinc, and nickel and has dimensions 11.0 x 3.5 x 0.1 mm. In order to provide a chemically inert surface to capture the particles, the carrier was coated with a gold layer of nominal thickness of 200 A. The distance from the end of the carrier when placed in the microscope provides a reference to link the flame radial position and the micrograph coordinates to within about 0.2 mm. Magnifications up to 50000 with adequate resolution are obtainable in the SEM mode. A disadvantage of the use of the bulk specimen carrier is that it allows only SEM observations which provide poor image quality (low contrast) of the low-density soot particles and does not yield quantitative information on the particle morphology. Modifications of these probes have been made in order to combine the unique advantages that each affords. The first modification consists of replacing a sector of the specimen carrier with a 200-mesh nickel screen (type I probe in Figure 2C). The grid material is coated with the carbon or silicon monoxide film by evaporative deposition after being bonded to the modified carrier. This type of probe has been used mainly for observations of soot present in the lower part of the flame where gas velocities are low and it offers good position correspondence between flame and micrograph coordinates as well as the high image quality afforded by TEM. However, this probe is fragile and more difficult to fabricate and coat uniformly because of the large grid surface. A second modification consists of a carrier through which a slot has been milled (type I1 probe in Figure 2D). Conventional coated grids are bonded to the carrier with epoxy cement to provide a durable and more easily constructed probe. This probe has a larger thermal inertia than the type I probe, is inexpensively fabricated from the
Figure 3. Schematic diagram of the probe control system and related equipment. commercially available carbon-coated circular grids and the bulk specimen carrier, and is highly durable. This version of the thermophoretic probe has been found to be the most advantageous. The soot samples collected on the probes have been examined on a Philips 420 electron microscope operating in either the SEM or TEM mode. This instrument provides magnifications as high as 820000 and possesses a point-to-point resolving power approaching 0.3 nm in the TEM mode. This capability is adequate to resolve the soot particles except in the very lowest portion of the flame where the smallest discernible size is determined by the instrument resolving power. Soot particles of 1nm would be clearly discernible, but particles smaller by a factor of 4 would escape detection. Probe Control System and Burner. The probe control system and the related equipment is shown in Figure 3. The precise spatial positioning of the probe within the flame is achieved by mounting the mechanical components on a rigid aluminum table. The burner (a) with the chimney (b) and the flow restrictor (c) are securely held by a post (d) with a vertical screw drive mechanism. The vertical adjustment of the probe with respect to the burner mouth is measured by a cathetometer, and changes in the height above the burner are measured by means of a dial indicator (e). The vertical position control of the thermophoretic probe is about 0.1 mm. The ideal probe control system would instantaneously insert the thermophoretic probe into the flame for a well-defined,controllable time interval without causing any disturbance to the motion of the flame gases. The actual probe control system has been designed and calibrated to move the probe with the maximum speed to a precisely defined position in the flame. The mechanical actuator for this system is a double-acting pneumatic cylinder (9) of 14-mm i.d. and 25-mm stroke that is driven by an air pressure of 4.5 atm through a pneumatic directional control valve (h). A linear transducer (i) is mounted on the piston shaft and provides a precise record of the probe trajectory on the screen of a storage oscilloscope (k). With this configuration we have been able to achieve transit times rt for the entry of the probe into the flame equal to 12 ms. The exposure time T, of the probe to the flame environment is controllable and can be reduced to as short as 30 ms. The entire probe trajectory is recorded by the linear transducer during each test. Figure 4 shows the probe trajectory and documents the brevity of the transit time as well as the precise definition of the residence time. There is a difficulty in sampling the soot aerosol in the fuel-rich region of the flame. This region is surrounded by an annulus of high soot volume fraction that peaks at
Morphology of Flame-Generated Soot
. .. ',- ..
,.
,
.
Langmuir, Vol. 3, No. 2, 1987 251
~
: ;
+,
c-->c.'~" ,; '
. ,. c.4
Figure 4. Typical oscilloscope record of the probe trajectory. Transit time T~ = 16 ms, exposure time T~ = 56 ms.
r = r,. The value of r, decreases with increasing height, as shown in Figure 1. The probe surface sampling the fuel-rich region can be contaminated by particles from the region of high soot volume fraction during insertion. This suggests that observations of the morphology of particles collected at r < r, must be very carefully examined for the presence of soot originating from the annular region. The soot material samples from r r, is free of such contamination. A series of high-speed cine photographic studies were conducted to determine the influence of the probe motion on the flame. These tests showed that the probe enters the flame very smoothly but a flow induced by the probe support rod distorts the flame severely after the probe motion has ceased. This unsteadiness of the flame position destroys the position correspondence between flame and micrograph which has been a goal of our effort. To prevent the bending of the flame we have added a flow deflector plate, shown schematically in Figure 1, through which the probe enters the flame. The deflector plate remains stationary in space thus deflecting any air current induced by the motion of the probe support. Cine photography of the probe equipped with the flow defledor shows the flame to be perfectly steady throughout the time period during which the probe is in motion. The temperature of the carrier probe during the sampling has been measured by means of temperature-indieating liquids. A t z = 10 mm above the burner mouth, where gas temperature has reached a local maximum" as high as 1900 K, the carrier probe reaches a temperature of 385 10 K when the residence time is 65 ms. In an equal time period the grid material would assume a higher temperature since its thermal inertia is substantially lower. These tests also showed that the probe is subject to significant radiant heating prior to insertion into the flame unless a protective flow of cold air in the probe support annulus is sustained during this period. A timing circuit is used to control the motion of the probe as well as the cooling air flow. This circuit turns off the air flow about 1 s before the extension of the probe. The high-speed photographic studies proved this time to be adequately long for the flame to recover from its influence. The burner is identical in design with that used previo ~ ~ l y for ~ ~the ' ' extensive laser scattering and transmission measurements on the ethene coannular diffusion flame. The burner tube of 11.1-mm i.d. is surrounded by an outer tube of 101.6-mm i.d., and a ceramic honeycomb fills the annular space. An airflow through the ceramic provides the coannular air stream which overventillates the flame. A mixing chamber in the base of the burner is filled with glass beads to eliminate nonuniformities in the air flow field. Throughout the tests reported herein, the volume flow rates of ethene and air were 3.85 cm3/s and 713 cm3/s, respectively. These flow rates produce the "n~nsooting"'~
Figure 5. TEM micrographs of particles deposited on a carbon substrate supported by a copper grid at z = 20 mm. Exposure time T. = 100 ms. flame of 8%" height which displays the bright yellow incandescence characteristic of soot but which releases no soot to the surroundings. Results and Discussion A vertical survey of spherule size has been performed using carbon-coated grids positioned inside the annular region of the flame for exposure times varying from 30 to 120 ms. The longer probe exposure times were used at low regions of the flame, since the soot volume fraction there is considerably lower than that higher in the flame. Examining the soot samples collected this way, we were able to verify the thermophoretic effect on particle deposition inside a flame environment. This effect is clearly shown in Figure 5, depicting soot aggregates collected on a carbon-coated grid from a height of z = 20 mm. The term aggregate is used from now on to depict a discrete, rigid entity composed of extensively coalesced primary particles fused together. The aggregates are highly rigid and stable under the high-energy electron beam (accelerating voltage of 80 KV) during the EM observations. It is obvious from Figure 5 that the particle concentration is higher in the vicinity of the grid bar which is playing the role of the cold target attracting the particles. A series of TEM micrographs reproduced in Figure 6 show typical particles which are present near r = r, for the indicated values of height z. Based on an examination of these and similar micrographs we come to the following conclusions. Large aggregates dominate the particle morphology at the annular region of the flame a t all heights. These aggregates consist of primary particles of rather uniform size a t low and intermediate heights. The primary particle size strongly increases as the particles move toward higher regions of the flame (Figure 6A-D). This observation clearly indicates the strong effect of surface growth on soot morphology at low and intermediate heights of the flame. Low in the flame there exist intense particle inception and intense coagulation. This causes a rapid broadening of the size distribution and a reduction of the number concentration of aggregates immediately after the cessation of the particle inception pulse. A t
258 Langmuir, Vol. 3,No. 2,1987 40.0
Dobbins and Megaridis
,
00
100
200
1
I
300
400
I
600
800
Height above burner Zmm)
Figure 7. Primary particle size as function of height, near r = r.. Particles me captured on mhon film suhstrates, at re varying from 30 to 120 ms. Each size measurement is an average over about 100 primary particles.
(El
IF1
Figure 6. TEM micrographs of soot particles collected from the annular region of the ethene coannular diffusion flame as a function of increasing height (2). For all casea re = 30 ms except (A) where re = 120 ms: (A) z = 10 mm; (B) z = 20 mm; (C) z = 30 mm; (D)z = 40 mm; (E) z = 50 mm; (F) z = 80 mm.
intermediate heights of the flame the number concentration has decreased through coagulation and surface growth becomes the dominant phenomenon. Heterogeneous reactions result in material deposition on the exposed surfaces of the primary particles that make up the aggregates, thus retaining their nearly spherical form while growing. This growth mechanism is compatible with the series of micrographs of Figure 6. The initial effects of soot particle oxidation are apparent from a height of z = 40 mm (Figure 6E) and above. Large aggregates of considerably smaller primary particles are apparent. As the particles move toward the highest regions of the flame, these large clustem dominate showing a smaller primary particle size and a higher degree of agglomeration (Figure 6F). This dramatic decrease of the primary particle sue with increasing height above the burner shows that the particles are oxidized as they move toward the tip of flame. All the above observations were repeated with identical results, hy performing the same measurements with the u8e of SiOcoated grids. There was no ohviow dependence of particle morphology on the substrate material. Measurements were also performed with various exposure times (30-120 ms) of the carbon-coated grids within the flame at a specified height. No change of either the spherule size or the degree of agglomeration was apparent. These observations lead to the conclusion that changes of the degree of agglomeration or the size of the primary particles do not take place on the probe provided that the probe surface coverage by the particles is of small order, 10% or less. Further support for this conclusion is provided by sampling a methane diffusion flame where soot concentrations are dramatically lowerI4 (factor of 10-15 in soot volume fraction). The examination of samples collected from this flame show that single particles and very small aggregates are present, proving that agglomeration on the probe does not occur in that case. Examining a series of electron micrographs at different
heights on a Sh4I Micro-comp data acquisition system, we were able to produce quantitative data on the primary particle size distribution as a function of height z above the burner, for particles collected from the annular region of the flame. With the use of a digitizing tablet the primary particle diameter of about 100 particles a t each height was measured. A basic statistical analysis a t each height, performed by the same system, reduced the mean primary particle diameter D as a function of z. The values of D, vs. z are plotted in Ifigure 7. These values of D, compare favorably with the sizes observed during the SEM measurements on the soot sample collected with the bulk specimen carrier. The maximum size shown in Figure 7 occurs at z = 40 mm which coincides with the axial paition of the maximum local soot volume fraction found by optical tests (See curve labeled “NS annulus” of Figure 7A in ref 13). We consider this level to mark the lower boundary of the oxidation region. A detailed comparison of the particle sizes found here with those of the optical methodI3 is not possible at this time because of the lack of quantitative data on the aggregate morphology. It must he pointed out here that particle deposition occurs on both sides of the film. However, as Figure 6 shows, the low coverage of the film surface by soot aggregates, as well as the narrow depth of field at these microscopic scales, would clearly discem particles deposited on opposite sides of the probe. Observations of soot collected using the type I probe have recently been performed and analyzed. Measurements were made at heights of 10 and 30 mm. A t both heights the primary particle size as well as the degree of agglomeration peak at r = r,. For particles on the oxygen-rich side of the annulear region (r > rc),the size and state of agglomeration decreases drastically with increasing radius. These observations are consistent with our previous SEM measurements using the original bulk specimen carrier as the thermophoretic probe. On the fuel-rich side of the annular region ( r < re),the particles are also of smaller primary size and display a smaller degree of agglomeration. However, some clusters of larger primary particle size were present, probably collected from the annular region as suggested by their morphology. Statistical analysis of the morphological data of soot collected from the annular region yielded the mean primary particle diameter D, at these two heights. These values are shown on Figure 7 and are in very good agreement with the mean sizes found by using the grids. A detailed vertical survey to explore the soot morphology of the coannular ethene
Langmuir 1987, 3, 259-265 diffusion flame is currently in progress.
Conclusions A technique for extracting soot or other particulates from flames has been devised that affords a detailed examination of particle morphology and its regional variation within an annular diffusion flame. The method is based on subjecting a cold probe surface for exposure times as short as 30 ms to precisely defined regions of the flame. The transit times achieved are as low as 12 ms. Several thermophoretic probes exposed to the flames are examined by TEM for best image quality to reveal particle morphology. The morphological features so derived provide not only valuable qualitative information on particle agglomeration, surface growth, and oxidation but also quantitative data on primary particle size as a function of flame coordinates. Observations with a series of probes consistently show an increasing primary particle size with height above the burner in the low and intermediate regions of the flame. The size then decreases through oxidation as the particles move to the tip of the flame. Our studies further show that, a t intermediate heights in the flame, the primary particle size and the degree of agglomeration peak on the radius of maximum soot volume fraction (r = r,). Both TEM and SEM observations with two different probes show that the primary particle size and the state of agglomeration are substantially reduced on the oxygen-rich size (r 2 r,). A similar trend is present for the fuel-rich side (r < r,) but the observations of this region are less reliable. The technique described can be used for the study of a variety of flame-generated particles and affords important supplementary information to the data yielded by light scattering observations. Appendix Some simplifying assumptions can lead to an estimate of the quench time T ~ Le., , the time for a typical soot
259
particle to traverse from the flame to the probe through the thermal boundary layer of thickness 6,. The thermophoretic velocity driving the particles that are in the free molecular regime is given by
where K is the thermophoretic velocity coefficient, v the kinematic viscosity of the host gas mixture, and T the local temperature. We can approximate
where T, is the temperature of the probe, T the temperature of the host gas, T, the average thermak boundary layer temperature, v, the kinematic viscosity of the gas mixture at Ta, and 6, the thermal boundary layer thickness. The quench time is then given by
Acknowledgment. The instructions of Michael Sosnowski in the operation of the Philips 420 EM are acknowledged with deep appreciation. Roger L. Follansbee made major contributions to the design of the probe control system and was responsible for its construction. Professor Harry Kolsky generously provided high-speed photographic equipment and guidance in its use. This research was sponsored by the Center for Fire Research of the National Bureau of Standards under Grant NB83NADA4025. Registry No. Ethene, 74-85-1.
Formation of Ultrafine @-SiliconCarbide Powders in an Argon Thermal Plasma Jet? Peter C. Kong and E. Pfender” Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455 Received August 13, 1986. I n Final Form: November 25, 1986 Ultrafine @-siliconcarbide powders are synthesized in a thermal plasma jet, generated with a swirl-stabilized plasma torch, using methane and silicon monoxide as reactants. High temperatures (>lOOOO K) combined with ultrarapid quench rates (>lo6K/s) lead to a high degree of supersaturation, resulting in homogeneous nucleation of ultrafine Sic particles. The maximum conversion of Si0 to Sic determined by thermogravimetric analyses is 97.3%. Particle size analyses show a bimodal distribution with the majority of the particles falling in a size range from 2 to 40 nm. Larger particles with sizes greater than 80 nm are also observed. Triangular and hexagonal Sic single-crystal platelets are observed throughout this work, and the formation of these particles is discussed in this paper.
Introduction The plasma utilized for these experiments is classified as a thermal or equilibrium plasma which may be produced by means of electric ~ C orS rf discharges. Thermal plasm= provide high temperatures (>15000 K) and a chemically +Presented a t the symposium on “Fine Particles: High Temperature Synthesis”, 60th Colloid and Surface Science Symposium, June 15-18, 1986, Atlanta, GA; G. Mulholland, Chairman.
0743-7463/87/2403-0259$01.50/0
pure environment in contrast to conventional chemical flames. Over the past two decades thermal plasmas have gained wide recognition because of their potential for material processing. Thermal plasmas are used in extractive metallurgy and mineral e~ploitation,l-~ decomposition of in(1)Sayce, I. G. Adv. Extr. Metall. Refin. Proc. Int. Symp. 1971, 241. (2) Rykalin, N. N., Pure A p p l . Chem. 1976, 48,179.
0 1987 American Chemical Society
