Aerosol synthesis of AlN by nitridation of aluminum vapor and clusters
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Theresa Guiton and Alan W. Weimer Advanced Ceramics Laboratory, The Dow Chemical Co., 1776 Building, Midland, Michigan 48674 (Received 12 July 1994; accepted 14 November 1994)
Aluminum nitride (A1N) powders were produced in an aerosol flow reactor by direct nitridation of aluminum vapor with ammonia and nitrogen in flowing argon. In the presence of excess NH 3 , pure A1N powders were obtained from 1400 to 1873 K. These powders had higher specific surface areas than commercially available A1N powders. The effects of reactor temperature and gas flow rate on A1N purity, yield, and grain and crystallite size were investigated. The BET grain diameter of A1N increased as the process temperature increased, indicating sintering controlled particle formation and growth. Both the crystallite size and the BET grain size of pure A1N powders slightly decreased as ammonia flow rate was increased. Increasing temperature and flow rate increased the A1N yield.
I. INTRODUCTION Polycrystalline aluminum nitride (A1N) ceramics exhibit high thermal conductivity, good electrical resistivity, and a thermal expansion coefficient that matches that of silicon. As a result, A1N can be used as a semiconductor substrate for high power, high density integrated circuits. Furthermore, A1N has excellent chemical resistance, so it can also be used as a crucible for molten salts and superalloys. Aluminum nitride can be manufactured by various methods including carbothermal nitridation,1 direct nitridation of aluminum metal,2 nitridation of aluminum containing organic compounds such as triethylaluminum,3 and nitridation of aluminum halides such as A1C13.4 Both the quantity and type of waste are important considerations when evaluating alternative processes for the manufacture of new materials. Cleaner processes that minimize waste while simultaneously satisfying cost and quality requirements have the best chance to be commercialized. In comparing the different routes for manufacturing A1N, the direct nitridation of Al metal has the significant advantage of no by-products. In addition, since high purity Al metal is readily available at low cost, there is no requirement for waste-producing post-processing acid leaching steps to remove metallic impurities. A third caveat is the fact that energy requirements are minimized by the exothermic nature of the direct nitridation reaction.
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to whom correspondence should be addressed. J. Mater. Res., Vol. 10, No. 3, Mar 1995
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Hotta et al.5 made hollow A1N particles, 4 - 1 2 /xm in diameter, by floating Al particles in N 2 between 1623 and 1823 K. The hollow particles were easily milled with mean sizes of 0.2 yam. When using pure nitrogen and reaction temperatures less than 1673 K, the conversion was less than 10%. A rapid increase in the reaction rate was obtained at temperatures above 1673 K. Kimura et al.6 synthesized A1N by nitridation of suspended aluminum particles (15 /xm in diameter) with N 2 and/or NH3 between 1423 and 1573 K. The per
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