The role of transport phenomena in the plasma synthesis of fine particles: The production of silicon powder
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I. I N T R O D U C T I O N
IN recent
years, thermal plasmas have found a variety of uses in the materials processing field, Ell61 but the synthesis of monodispersed powders appears to be one of the more promising applications of plasma systems. In such operations, as sketched in Figure 1, one or more precursor streams are injected into a thermal plasma jet where thermal decomposition occurs. Typical examples of such systems include: (1) silicon carbide synthesis using methane and silane as reactants; (2) single-phase high temperature tungsten carbide from tungsten oxide and methane as reactants; (3) silicon powder utilizing silane Sill4, halides SIC14 or SiF4, and hydrogen; (4) heating and quenching of metastable superconducting compounds with a plasma jet; (5) reaction of powder boron with a propane-butane mixture in a hydrogen/argon plasma jet. The requirements for satisfactory operation of these systems are quite stringent in that the precursor must be mixed with hot plasma gas to facilitate the reaction; but at the same time, the dwell time of the products must be carefully controlled in order that nucleation and growth may proceed in a manner to provide a monodispersed solid product. In a sense, these two requirements are orthogonal, because effective mixing is essential for the first step, while plug flow behavior would be ideal for ensuring uniform particle growth. In this regard, these systems have to be quite different from those usually employed in combustion (or in flash roasting operations) where recirculation is thought to be highly desirable. While considerable effort is currently being devoted
to plasma synthesis, most of this has been directed at equipment design and product characterization with less attention being paid to the transport phenomena within these systems. In recent years, major advances have been made regarding our understanding of heat and fluid flow phenomena in thermal plasma, t~7-221 and the purpose of the present paper is to draw on these recent findings in order to evolve rational design criteria for thermal plasma reactors used in the synthesis of monodispersed solid particles. This subject matter is thought to be appropriate to the King Memorial Symposium in view of Professor King's strong interest in plasma phenomena; indeed, both authors had the privilege of discussing plasma problems with Professor King at MIT.
II. FORMULATION Let us consider a system such as that sketched in Figure 2. It is seen that this consists of a nontransferred thermal plasma jet which is made to discharge into a reaction chamber. This plasma jet is made of an inert gas, argon, whose role is to provide the heat source. The gaseous reactant, silane in the present case, is introduced into a system through an annular orifice which is coaxial with the inlet nozzle of the thermal plasma jet. As the silane is heated, through its contact with the plasma jet, it will decompose according to the following reaction: [23,24,25] SiH4(g)--+ Si(s) + 2Hz(g)
AH = - 3 0 K j / m o l
[1]
the rate of which is give
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