Effect of Temperature and Vapor-phase Encapsulation on Particle Growth and Morphology
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Effect of temperature and vapor-phase encapsulation on particle growth and morphology Sheryl H. Ehrman,a) Maria I. Aquino-Class, and Michael R. Zachariah Chemical Science and Technology Laboratory, Building 221, Room B312, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 16 July 1998; accepted 27 October 1998)
The effect of in situ vapor phase salt-encapsulation on particle size and morphology was systematically investigated in a sodium co-flow/furnace reactor. The temperature of the furnace was varied, and the primary particle size and degree of agglomeration of the resulting silicon and germanium particles were determined from transmission electron micrograph images of particles sampled in situ. Particle size increased with increasing temperature, a trend expected from our understanding of particle formation in a high-temperature process in the absence of an encapsulant. Germanium, which coalesces faster than silicon, formed larger particles than silicon at the same temperatures, also in agreement with observations of particle growth in more traditional aerosol processes. At the highest temperatures, unagglomerated particles were formed, while at low temperatures, agglomerated particles were formed, with agglomerate shape following the shape of the salt coating.
I. INTRODUCTION
Combustion synthesis has been used to produce a variety of important aerosol materials such as carbon black, fumed silica, and titania paint pigments.1 High product purity and ease of scale-up are some advantages of flame synthesis. However, the aerosol products are typically agglomerated, an undesirable characteristic if the materials are to be compacted following the aerosol processing step. Another disadvantage is that the oxidizing environment of the flame limits the range of materials which can be produced. Both of these difficulties can be alleviated by the use of a gas-phase encapsulation process. A diffusion reactor based upon sodium metal/metal halide reaction chemistry has been developed for the production of nonoxide materials.2–4 In this system, the general chemistry can be described by the reaction MClx 1 xNa ! xNaCl 1 M , where M is the desired product (metal or nonmetal), and x is an integer. Thermodynamic calculations show that high yields are feasible if the reaction takes place at low temperatures, less than 1700 K.5,6 Because of the difference between the equilibrium vapor pressure of the desired product (metal or nonmetal) and the salt, the product typically condenses before the salt, forming the encapsulated morphology shown in Fig. 1. A variety of materials have been produced using this reactor configuration including metallic titanium, titanium diboride, and metallic iron,3,7 yet particle growth dynamics in this a)
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http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 4, Apr 1999
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reactor have not been systematically investigated.
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