CHEMICAL VAPOR DEPOSITION OF SILICON DIOXIDE, ALUMINUM OXIDE, AND MULLITE FROM MIXTURES OF ALUMINUM TRICHLORIDE, CHLOROSILANE, CARBON DIOXIDE, AND HYDROGEN

摘要

The objective of this study is the comprehensive investigation of the kinetics of the codeposition of silica (SiO_2), alumina (Al_2O_3), mullite (3Al_2O_3·2SiO_2), and other aluminosilicates from mixtures of silicon tetrachloride, aluminum trichloride, carbon dioxide, and hydrogen in order to prepare coatings for the protection of SiC-based ceramics from oxidation in high-temperature applications. In an attempt to elucidate some aspects of the codeposition process, the deposition of silica and alumina from silicon tetrachloride and aluminum trichloride, respectively, in carbon dioxide and hydrogen is also studied. Experiments are conducted in a tubular, hot-wall chemical vapor deposition reactor, coupled to an electronic microbalance. The effects of process parameters on deposition rate and film composition are examined. The results of the deposition experiments show that manipulation of the temperature of the reaction and the residence time of the mixture in the reactor offers a way to control the composition of the codeposited films in SiO_2 and Al_2O_3, obtaining deposits with significant alumina and aluminosilicate (e.g., mullite) content. In order to account for the complex chemistry of the formation of the oxide films, detailed homogeneous and heterogeneous mechanisms are developed for the deposition of silica, the deposition of alumina, and the codeposition process. The kinetic mechanisms encompass several reaction sequences for the generation of deposition precursors and the formation of solid phases. The mechanism of the codeposition process and the results of the kinetic studies are employed to formulate routes for the preparation of mullite and functionally graded mullite/alumina coatings from SiCl_4-AlCl_3-CO_2-H_2 mixtures.