Powder synthesis by thermally induced vapor phase reactions is described. The powder generated by this technique consists of spherical, nonagglomerated particles of high purity. The particles are uniform in size, in the 0.1–0.2 µm size range. Most of the particles are crystalline spheres. A small fraction of the spheres are amorphous. Chain agglomerates account for less than 1% of the spherules

Bowen H. K.

Gobet J.

Jin Jwang Wu

McPhersen R.

Moulsen A. J.

Otto J. Gregory

Prochazka S.

Richard C. Flagan

Ulrich G. D.

English

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Submicron silicon powder production in an aerosol reactor

Wu, Jin Jwang

Flagan, Richard C.

Gregory, Otto J.

Caltech Authors - Main

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Powder synthesis by thermally induced vapor phase reactions is described. The powder generated by this technique consists of spherical, nonagglomerated particles of high purity. The particles are uniform in size, in the 0.1–0.2 μm size range. Most of the particles are crystalline spheres. A small fraction of the spheres are amorphous. Chain agglomerates account for less than 1% of the spherules

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University of Rhode IslandDigitalCommons@URIChemical Engineering Faculty Publications Chemical Engineering1986Submicron silicon powder production in an aerosolreactorJin Jwang WuRichard C. FlaganSee next page for additional authorsFollow this and additional works at: https://digitalcommons.uri.edu/che_facpubsTerms of UseAll rights reserved under copyright.This Article is brought to you for free and open access by the Chemical Engineering at DigitalCommons@URI. It has been accepted for inclusion inChemical Engineering Faculty Publications by an authorized administrator of DigitalCommons@URI. For more information, please contactdigitalcommons@etal.uri.edu.Citation/Publisher AttributionWu, J. J., Flagan, R. C., & Gregory, O. J. (1986). Submicron silicon powder production in an aerosol reactor. Applied Physics Letters,49(2), 82-84. doi: 10.1063/1.97358Available at: http://dx.doi.org/10.1063/1.97358AuthorsJin Jwang Wu, Richard C. Flagan, and Otto J. GregoryThis article is available at DigitalCommons@URI: https://digitalcommons.uri.edu/che_facpubs/34Submicron silicon powder production in an aerosol reactorJin Jwang Wu, Richard C. Flagan, and Otto J. GregoryCitation: Appl. Phys. Lett. 49, 82 (1986); doi: 10.1063/1.97358View online: https://doi.org/10.1063/1.97358View Table of Contents: http://aip.scitation.org/toc/apl/49/2Published by the American Institute of PhysicsArticles you may be interested inNanoparticle production in arc generated fireballs of granular silicon powderAIP Advances 2, 012126 (2012); 10.1063/1.3681283Preparation and analysis of the negative resistance characteristic in an amorphous silicon and silicon–carbidesingle-barrier deviceApplied Physics Letters 65, 2815 (1998); 10.1063/1.112575Submicron silicon powder production in an aerosol reactor Jin Jwang Wu and Richard C. Flagan Division 0/ Engineering and Applied Science, California Institute o/Technology, Pasadena, California 91125 Otto J. Gregory Department o/Chemical Engineering. University 0/ Rhode Island, Kingston. Rhode Island 02881 (Received 24 February 1986; accepted for publication 19 May 1986) Powder synthesis by thermally induced vapor phase reactions is described. The powder generated by this technique consists of spherical, nonagglomerated particles of high purity. The particles are uniform in size, in the O.I--~).2Jim size range. Most ofthe particles are crystalline spheres. A small fraction of the spheres are amorphous. Chain agglomerates account for less than 1 % of the spherules. To produce ceramics with high strength at high tem-peratures, starting powders with carefully controlled prop-erties are required. The ideal powder for such applications would consist of uniformly sized, spherical, nonagglomerat-ed submicron particles with carefully controlled composi-tions. I These properties make it possible to produce very uniform packing of the starting materials, a critical step in ceramics processing. A variety of methods, including solu-tion synthesis, 2.~ laser induced reactions,4.5 and thermally induced vapor phase reactions,6--1O are presently used to gen-erate these starting powders. The production of uniformly sized, nonagglomerated solid particles of spherical shape by gas phase chemical reac-tions is possible only under special circumstances. The initial step in particle formation is homogeneous nucleation which generally leads to very high concentrations of very small particles. These particles can grow by Brownian coagulation or by vapor deposition, either chemical or physical. Brow-nian coagulation of solid particles leads to the formation of low density floes that may be densified by sintering or vapor deposition. Even if the particles coalesce completely, coagu-lation leads to a relatively broad particle size distribution, a limiting case of which is the so-called "self-preserving" par-ticle size distribution which maintains its shape as the parti-cles grow. I I On the other hand, the growth of particles by vapor deposition narrows the particle size distribution as the parti-cles grow. Vapor deposition can dominate over coagulation only when the particle concentration is low since coagula-tion is a second order process. It is, however, very difficult to prevent the formation oflarge numbers of particles by homo-geneous nucleation of refractory species, even though there may be many particles present in the system. To generate particles with the desired characteristics, it is therefore necessary to (i) produce particles much smaller than the desired powder size by homogeneous nucleation, with the number concentration being kept low enough to prevent appreciable coagulation, and (ii) grow those parti-cles by deposition of the products of the gas phase chemical reactions that are carried out at such a rate that additional particle formation from the gas is suppressed. As the parti-cles grow by vapor deposition, they depress the vapor pres-sure of the condensible reaction products and reduce their tendency to nucleate. The influence of growing particles on the nucleation rate has been the subject of numerous theo-retical investigations. 12-16 With this information and knowledge of the reaction kinetics of the system of interest, a reactor can be designed to grow particles with the desired characteristics. Multi-stage reactors have previously been developed in which silicon particles as large as 10 Jim mass median diameter were grown directly from the products of the thermal pyrolysis of silane on 0.1-1.5 Jim diameter seed particles by vapor depo-sition, 17.18 In the primary particle growth stage of the reac-tor, the silane pyrolysis was gradually accelerated from a very low value as the particles grew and became more effec-tive at scavenging condensible vapors. This was accom-plished by ramping the temperature along the length of the flow reactor. In this letter, we describe a method for the synthesis of spherical submicron particles of controlled size by thermally induced chemical reactions. This process is applied to the synthesis of silicon powders in the 0.15-0.25 Jim size range. The properties of these powders are examined. The synthesis of uniform particles of 0.1 Jim diameter requires the use of small numbers of much smaller particles as seeds to be grown by vapor deposition. This is accom-plished in a single stage reactor in which the reaction rate is initially very low, thereby limiting the size and number con-centration generated by nucleation. By ramping the tem-perature along the length of a flow reactor using the five-zone furnace illustrated in Fig. 1, the rate of reaction IS SILANE and NITROGEN MIXTURE STATIC MIXERS INSULATION AEROSOL FLOW ZONE I ZONE 2 ZONE 3 ZONE 4 lONE 5 FIG. I. Schematic of the five-zone aerosol reactor in which silicon particles were generated by silane pyrolysis and the transpired wall system for pro-duct dilution and cooling. 82 Appl. Phys. Lett. 49 (2). 14 July 1986 0003-6951/86/280082-03$01.00 @) 1986 American Institute of Physics 82 1750~-~-,----.-----,----,-----,----, 1500 1250 1000 500 a a Mra..,urtd Wall Trmpl"ratllrt by Chrurlll'i-Alumr! Tht'rmo{ouples 0------ .......... ----.-.. --_ ~ .. ~ M ~ ~ ~ ~, ~ , '00 200 '00 .00 .... /// 0 , 500 pt.~lllun (mmJ / I / // I I fa 500 I I I , I I ~ = 0 , lao ~ '00 900 FIG. 2. Measured temperature profiles on the reactor wall. The heating zones and regions of insulation are indicated. accelerated and the seeds are grown by vapor deposition. The reactor consists of a 12-mm-i.d. quartz tube that is 850 mm long. The first three heating zones are 50 mm long, and are separated by 10 mm oflow density insulation. The fourth and fifth heating zones are 350 and 150 mm long, respective-ly, and again are separated by insulation. Electronic grade silane (Union Carbide, 99.99%) and high-purity nitrogen (further purified and dried by passing over hot copper) were thoroughly mixed with a series of static mixers (Luwa, Inc.) and introduced into the upper end of the reactor tube. The temperature of the first furnace zone was maintained at 770 K, to assure a low initial reaction and nucleation rate. This low initial temperature was fol-lowed by slow heating, as shown in Fig. 2, to allow time for the nuclei to begin to grow. The temperature at the end of the fourth heating zone was increased to 1100 K, a sufficiently high temperature that complete decomposition was assured. In the remaining length of the furnace the temperature was increased to 1523 K to densify the silicon particles. The product aerosol was collected on Teflon mem-brance filters (Millipore). To prevent thermophoretic depo-sition of the small particles in the hot reactant flow on the cool walls of the sampling system, the aerosol was first dilut-ed in the porous tube arrangement shown in Fig. 1. By blow-ing cool (room temperature) nitrogen through the wall of the diluter, the particles are transported away from the vicin-ity of the wall and high-temperature gradients that would otherwise lead to substantial losses of the product particles. 12 I " 10 -0 ,.., 8 E !l. 6 "Cl Z 0' -0 ~ 4 "Cl 21-0 10-2 dp(fLm) FIG. 3. Number distributions of the product aerosol. The histogram is the raw data obtained with EAA. Also shown is an estimate of the actual parti-cle size distribution obtained by applying a modified Twomey algorithm" to correct for cross sensitivities in the EAA instrument response function. 83 Appl. Phys. Lett., Vol. 49, No.2, 14 July 1986 0.1 Jl FIG. 4. TEM photographs of the crystalline silicon particles that accounted for about 99% of the particles produced by rate controlled thermal decom-position of silane. The filter holders were sealed following collection, and taken to a nitrogen glove box where the silicon was transferred to bottles for storage and shipping. The size distribution and number concentration of the silicon aerosol at the reactor outlet were measured using a TSI model 3030 electrical aerosol size analyzer, a Royco model 226 laser optical particle counter, and an Environ-ment One condensation nuclei counter. The total number concentration of the product aerosol was 2 X 10 14/m~. The size distribution is shown in Fig. 3. The particles were highly uniform in size at about 0.15 flm diameter. The silicon powders were brown in color, indicative of high-purity silicon. l'l Electron micrographs showed that the particles were dense, spherical, and uniformly sized. The vast majority of the spherules exhibited the morphologies shown in Fig. 4. The structure of these particles consisted of diamond cubic grains with dimensions of 0.05--{).1O flm x IO-~ L.'r---.........------,----.--__ --,--__ ~--~ 1.35 L' o Br<\gg Peak Droadening -- Uil'!t Squan F.lting o --------------~--~-------------------~ o 0 ,.,.,'------!---+-----!------!'-----f.-------! IIo<lH' ",~dor 1\.(..\-1) FIG. 5. Bragg peak broadening t..K as a function of the magnitude of the wave vector K. Wu, Flagan, and Gregory 83 tal (bj showing extensive twinning and stacking faults. The twin lamellae extend across the entire grain diameter, suggesting that the transformed region sweeps across the entire particle as a planar front upon the amorphous to crystalline phase transformation. A similar microstructure has been observed during the Zr02 tetragonal to monoclinic transformation in the system Zr02-zicron,20 which proceeds by a martensitic phase trans-formation. Frequently associated with these transforma-tions are strain effects due to the large volume changes in-curred. These strain effects were not evident in the transformed silicon particles, and there appeared to be little or no distortion of the particles after transformation. The twinned regions appear to be randomly oriented with respect to neighboring particles, as would be expected for particles that underwent independent processing in the aerosol phase. The average crystallite size was determined from x-ray peak broadening.22 The Bragg peak broadening M as a function of the magnitude of the wave vector K was calculated after performing Rachlinger's and Stoke's correc-tions. 22 The result, as in Fig. 5, shows that the dominant contribution to the line broadening is from small crystal size. The average crystallite size determined was 50 nm, in close agreement with that estimated from dark field transmission electron microscopy measurements. Based on these mea-surements, the silicon particles produced by thermal decom-position of silane were 3-4 times as large as the crystallites. This is consistent with measurements made by Cannon et al .. ~ on silicon powders produced by laser induced pyrolysis of silane. The factor controlling the size of these crystallites produced during the amorphous/crystalline transformation is not certain even though there is some evidence that it is similar to a martensitic phase transformation. The particle size to crystallite size ratio appears to be independent of his-tory. A very small fraction of powders, Fig. 6(a), arefeature-less, no fine structure. Electron diffraction patterns indicate that these particles are truly amorphous. The amorphous particles could also be distinguished from the crystalline powders by the absence of twins and stacking faults in their microstructures. These essentially featureless powders were 84 Appl. Phys. Lett., Vol. 49, No 2,14 July 1986 FIG. 6. (a) TEM photograph showing amor-phous silicon particle produced in the aerosol reactor. (b) TEM photograph of a chain ag-glomerate produced in the aerosol reactor. The small spherical subunits of such particles ac-counted for less than I % of the spherules pro-duced. unaffected by tilting whereas contrast changes were ob-served in the twinned regions upon tilting. Chain agglomer-ates of microcrystalline particles, shown in Fig. 6(b), ac-counted for less than 1 % of the spherules. Some neck formation was observed in the agglomerates of these spher-ules. Infrared absorption spectroscopy was used to explore the possible contamination of the surface with silicon oxides or silicon nitride. No detectable absorption by these species was found. The detection limits for these measurements were estimated to correspond to a layer averaging 7 A thick on the surface of the particles. This work was supported in part by the Flat Plate Solar Array Project, Jet Propulsion Laboratory, California Insti-tute of Technology sponsored by the U. S. Department of Energy through an agreement with NASA. 'H. K. Bowen, Mater. Sci. Eng. 44, I (1980). 'E. A. Barringer and H. K. Bowen, Commun. Am. Ceram. Soc., Decem-ber, C-199 (1982). 'J. Gobet and E. Matijevic, J. Colloid Interface Sci. 100, 555 (1984). ·W. R. Cannon, S. C. Danforth, J. H. Flint, J. S. Haggerty, and R. A. Marra, J. Am. Ceram. Soc. 65, 324 ( 1982). 'w. R. Cannon, S. C. Danforth, J. S. Haggerty, and R. A. Marra, J. Am. Ceram. Soc. 65, 330 (1982). 'Yo Suyama and A. Kato, J. Am. Ceram. Soc. 59, 146 (1976). 7G. D. Ulrich, Combust. Sci. Techno!. 4, 47 (1971). KG. D. Ulrich, B. A. M. Ines, and N. S. Subranmanian, Combust. Sci. Tech-no!. 14, 243 (1976). qG. D. Ulrich and N. S. Subranmanian, Combust. Sci. Techno!. 17, 119 ( 1977). lOS. Prochazka and C. Greskovich, Am. Ceram. Soc. Bull. 57, 579 ( 1978). "F. S. Lai, S. K. Friedlander, J. Pich, and G. M. Hidy, J. Colloid Interface Sci. 39,395 (1972). "E. M. Zaiser and V. K. LaMer, J. Colloid Sci. 3, 571 (1948). "H. Reiss and V. K. LaMer, J. Chern. Phys. 18, I (1950). "A. Pesthy, R. C. F1agan, and J. H. Seinfeld, J. Colloid Interface Sci. 82, 465 (1981). "M. K. Alam and R. C. Flagan, J. Colloid Interface Sci. 97, 232 ( 1984). 10J. E. Stern, J. J. Wu, R. C. Flagan, and J. H. Seinfeld, J. Colloid Interface Sci. (in press). 17M. K. Alam and R. C. Flagan, Aerosol Sci. Techno!. (in press). "J. J. Wu and R. C. Flagan (unpublished). 19 A. J. Moulsen, J. Mater. Sci. 14, 1071 (1979). 20R. McPhersen, B. V. Shafer, and A. M. Wang, J. Am. Ceram. Soc. 65, C-57 (1985). 21G. R. Markowski, Aerosol Sci. Techno!. (in press). "B. E. Warren, X-Ray Diffraction (Addison-Wesley, 1969). Wu, Flagan, and Gregory 84 

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Submicron silicon powder production in an aerosol reactor

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