Production of aluminum and aluminum coatings by thermal decomposition of aluminum alkyls
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THE
importance of chemical vapor deposition in the electronics industry,~ in the manufacture of r e f r a c tory metal parts,2 and in the production of nickel by the Mond-Langer process, 3 is established. The majority of the important reactions of this nature are described in the book by Powell and coworkers.4 However, one c h e m i c a l vapor deposition reaction of importance has received scant attention in the l i t e r a t u r e : the deposition of aluminum by pyrolysis of aluminum alkyls. Alkyls are typically manufactured by the reaction of an olefin and hydrogen with a l u m minum at elevated pressures: 6R-CH2 + 3H2 + A1= 2(RHCH2)3AI Common alkyls are triethyl aluminum (henceforth TEA), for which R is the group CH2, trinormalp r o p y l aluminum (TnPA, R is CHs-CH) trinormalb u t y l aluminum (TnBA) and triiso-butyl aluminum (TiBA). The lower a l l y l s are spontaneously inflamm a b l e liquids and are used as intermediates and polymerization catalysts by the organic chemical industry. At elevated temperatures and n o r m a l pressures the reaction g i v e n above is r e v e r s e d and aluminum is produced, together with olefin and hydrogen. The reaction can therefore be exploited t o yield aluminum coatings on a heated substrate. Alternatively, the formation of alkyl from impure aluminum, followed by the decomposition of the a l l y l in a separate rea c t o r might be employed in a process for producing bull quantities of aluminum. Ziegler%6 suggested such a s c h e m e for the production of aluminum of high p u r i t y while the scheme has been incorporated in patented processes for the production of aluminum from nonbauxite sources.7,8 Such a process is depicted in Fig. 1, based on one of these patents. The objective of the investigation described in this A. MALAZGIRT, formerly GraduateStudent, Department of Materials Science and Mineral Engineering, University of California, Berkeley, is now withTegal Corporation, Novato, CA. J. W. EVANS is Associate Professor of Metallurgy, Department of Materials Science and Mineral Engineering, University of California, Berkeley, Berkeley, CA 94?20. Manuscript submittedMarch 12, 1979. METALLURGICAL TRANSACTIONSB
paper was t o provide knowledge of the a l l y l decomposition reaction which would be required for the design of a process of the kind shown in Fig. 1 and for equipment in which the reaction is t o be exploited for the deposition of aluminum coatings on substrates. Emphasis was placed on the kinetics of the decomposition of gaseous TnPA, TnBA, and TiBA, but the form of the deposits produced, the content of carbon (the principal impurity), and the compositions of the gaseous reaction products were also determined. PREVIOUS
INVESTIGATIONS
The thermal decomposition of aluminum allyls has been studied by several investigators. %%~24 However, many of these investigations have been concerned with liquid phase decomposition, with the initial step in the decomposition (the formation of a diallyl hydride by elimination of an olefin) or with improvements in the t
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