Super-Amorphous Alumina Gels
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SUPER-AMORPHOUS ALUMINA GELS
A. C. PIERRE AND D. R. UHLMANN Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Mass. 02139; *On leave from S.N.I.A.S. Company, St. Medard-en-Jalles, France
ABSTRACT The structures of alumina gels made from aluminum sec butoxide have been studied by X-ray diffraction and scanning electron microscopy as a function of pH as well as of the The structures of temperature of peptization and gelling. these gels appear to be very sensitive to the conditions of preparation, and lead to a variety of gelled products and Specific attention is transformation behaviors on firing. directed to preferential orientation and to structures which are X-ray super-amorphous.
INTRODUCTION The potential applicability of sol-gel technology for a wide range of In the case of A1 2 0 3 and A1 2 0 3 -based applications is now well recognized. systems, one of the attractive applications involves their use as the The technology permits the impregnamatrices of ceramic-ceramic composites. tion of continuous tows of fibers, and leads naturally to benefit from In the present processing techniques developed for resin-matrix composites. case, degradation of fiber properties at firing temperatures used to densify This in turn directs the matrix becomes a matter of considerable concern. attention to the densification behavior and to its dependence on the structure and texture of the low-temperature precursors. The present paper reports an investigation of the structure and textural features of some alumina gels prepared from alkoxides.
BACKGROUND Gelation involves the development of chemical bonding between either atoms or molecules (polymeric gels) or solid particles (colloidal gels). It represents a cooperative phenomenon, with the gel point being the percolation threshold where the bonds are in sufficient number to realize a 3-dimensional infinite network. While the organic gels have been the most extensively studied, an With Si0 2 , e.g., extensive literature exists for oxide gels as well. siloxane bonds =Si-O-Si= lead to gels termed polymeric or colloidal [1], In depending on whether they connect particles smaller or larger than 5 nm. the case of alumina, the situation is much less clear, and the name "gel" has very often been given to a variety of different low temperature forms of alumina and its hydroxides. Alumina hydrates with a fibrous texture which are describable qualitatively as gelatinous have been investigated for many decades, and can be In 1870, Cossa [2] described a obtained under a wide range of conditions. The diverse studies type obtained by direct reaction of Al with water. summarized by Gitzen [3] deal with gels obtained from Al salts (sulfates, Hat. Reas. Soc.
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nitrates, chlorides) and mineral bases. At least some such gels have been shown by Turkerich and Hillier [41 to be composed of tiny spheres arranged in fiber-like structures 100-500 A in diameter. Use of these materi
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