Preparation of Homogeneous Aluminosr, Icate Gels by sol/gel Methods
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PREPARATION OF HOMOGENEOUS ALUMINOSR,ICATE GELS BY SOL/GEL METHODS M. E. REESE, A. V. McCORMICK* and J. SANCHEZ University of Minnesota, 421 Washington Avenue, Minneapolis, MN 55455.
ABSTRACT The factors affecting the production of homogeneous aluminosilicate gels from alkoxide precursors were investigated. Aluminosilicate gels were made in acidic conditions with a prepolymerization of the silicate precursor followed by the addition of the alumina precursor. Confirming earlier studies, we found that the gel homogeneity is increased when the rate of gelation decreases. The reduction of the water content and/or the aluminum precursor, and more surprisingly the increase of the acid concentration contributed to the gel time increase and thus the gel homogeneity improvement.
INTRODUCTION Sol/gel processing offers many advantages over more traditional processing techniques to produce polymeric aluminosilicate gels. The chief advantage of the molecular-scale homogeneity possible through sol/gel processing is that one can in principle produce aluminosilicates with any arbitrarily specified composition, so one can tailor physicochemical properties which are extremely sensitive to the composition on the molecular scale. However, not all sol/gel techniques work; on the contrary, it is quite difficult to produce homogeneous aluminosilicate polymers. It is the purpose of this paper to study the factors affecting the gel homogeneity.
Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
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HOMOGENEITY OF ALUMINOSILICATE GELS
The degree of homogeneity ideally attained in a gel is the molecular homogeneity. For an aluminosilicate gel, this would correspond to a high concentration of AI-O-Si bonds produced by copolymerization of the silicon alkoxide and aluminum alkoxide precursors. However, the homopolymerization of aluminates is much faster than both the silica homopolymerization and the alumina-silica copolymerization, yielding a separation of alumina and silica-rich phases. Typically, this results in the formation of colloids or even precipitates, instead of gels. Even if a gel is produced, it is cloudy and opaque. The use of double alkoxides (Pouxviel, Yasumori), e.g. (BuO) 2 -Al-O-Si-(OEt) 3 , could in principle improve the homogeneity, but Pouxviel et al. have observed that even for these precursors, only certain regions of sol/gel solution composition yield homogeneous gels, and these compositions correspond to extremely slow gel times. For our present purpose, only the macroscopic homogeneity will concern us. It does not however necessarily imply homogeneity at the molecular level. Macroscopically homogeneous gels appear transparent to the eye, and exempt of precipitates or any visible phase separation.
EXPERIMENTAL The ceramic precursors were silicon tetraethoxide or TEOS ( Si[EtO] 4 ) and aluminum sec-butoxide ( [sec-C 4 H 9 0] 3 A1 ). This aluminum precursor proved to be much superior to aluminum isopropoxide to obtain homogeneous gels. In addition, aluminum isopropoxide does not readily dissolve
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