Entrapment of organosilicon molecules in nonhydrolytic alumina gels and thermal behavior of the resulting composite
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This work describes the entrapment of tetrakis(trimethylsilyl)silane (TK) and tetrakis(chlorodimethylsilyl)silane (TKCl) in nonhydrolytic alumina gels, and the materials’ thermal behavior. During gelation and drying TK and TKCl are physically entrapped in the gel up to a limit of Si/Al ⳱ 0.33. Above this limit, sublimation and decomposition of TK and TKCl occur during heating. A larger fraction of TKCl decomposition products is retained due to their higher reactivity. Below and above Si/Al ⳱ 0.33, the gel converts to mullite + ␣–Al2O3or mullite + amorphous silica, respectively. Conversion to hexagonal mullite indicates atomic scale homogeneity of Si and Al during firing.
I. INTRODUCTION
Sol-gel techniques are often employed to obtain materials with a large surface area and controlled porosity. The porosity is governed by the chemistry during gelation and by the drying process. Following the initial drying stage the pore evolution is influenced by the subsequent heat treatment conditions (temperature profile, atmosphere, etc.). The pore structure in the gel is of key importance to the entrapment of unique molecules within the gel.1 The main goal of this work is to develop a method for entrapment of polysilicon molecules within an alumina gel matrix, investigation of their entrapment limit, and study of the thermal behavior of the obtained bicomponent material. The matrix used in this work is a nonhydrolytic gel.2–5 Besides the entrapment limit of the polysilicon molecules, we are interested in the effect of their thermal decomposition on physical properties such as the surface area and phase structure of the final material. The organosilicon molecules used in this work are tetrakis(trimethylsilyl)silane (TK) and tetrakis(chlorodimethylsilyl)silane (TKCl). In hydrolytic medium the organosilicon molecules either undergo hydrolysis and decomposition, as in the case of TKCl, or are only slightly soluble as in the case of TK. For example in gels prepared from TEOS only 1 mol% TK was dissolved,6 giving rise to a ratio of Si[from TK]:Si[from TEOS] ⳱ 0.05:1. After heat treatment part of the molecules sublime and the ratio of the Si from TK that stays entrapped is Si[from TK]:Si[from TEOS] ⳱ 0.03:1. The nonhydrolytic gel2–5 system was selected as the matrix material for the entrapment in this work, since the solubility and stability of the entrapped species is expected to be higher J. Mater. Res., Vol. 16, No. 5, May 2001
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in a nonaqueous medium as compared to the hydrolytic analog. For example during gelation with AlCl 3 / diisopropylether system in dichloromethane medium, a ratio of Si:Al ⳱ 2.5:1 can be attained during gelation and as high as Si:Al ⳱ 1.25:1 remain in the gel after heating to 1000 °C. The nonhydrolytic method has been used by Corriu et al.7–10 and by Hay et al.11 in the preparation of pre-mullite and other multicomponent gels. The uniqueness of our work compared to the work of Corriu et al. and Hay et al. is that the entrapped polysilicon molecules a
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