Hydrolysis of Aluminum -Are All Gels Created Equal?
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HYDROLYSIS OF ALUMINUM -ARE ALL GELS CREATED EQUAL? T. E. Wood, A. R. Siedle, J. R. Hill, R. P. Skarjune and C. J. Goodbrake Ceramic Technology Center, Science Research Laboratory, and Analytical and Properties Research Laboratory, Building 201-4N-01, 3M Center, St. Paul, MN 55144 ABSIRACT The nuclearity and structure of aluminum cations generated by the hydrolysis of aluminum salt solutions depends markedly on the method of preparation. This speciation affects not only the thermochemistry of processes leading to ceramics, but the microstructures of the ceramics themselves. A brief review of aluminum ion hydrolysis is presented along with a study of the thermal evolution of gels derived from aluminum salt solutions via several different hydrolysis methods. The observation of 5 - coordinate aluminum in a bulk transition alumina by MAS-NMR is reported. A new, high defect precursor of rl-alumina dubbed "high-5 alumina" is described and the far reaching implications of this discovery are discussed. INTRODUCTION Hydrolyzed aluminum solutions and gels are used industrially as precursors to ceramic fibers, powders, and binders [1]. Of particular interest to the ceramist is how differences in the aluminum cation speciation affect the thermal evolution of the derived alumina towards the final ceramic product. Unfortunately, aqueous aluminum chemistry is exceedingly complex and poorly understood. A holistic interpretation of the voluminous data that exists is complicated by the fact that aluminum cation speciation depends strongly on the method of preparation. The diversity of aluminum cation speciation, described below, well illustrates the formidable nature of the aluminum hydrolysis problem. Yet to the ceramist this diversity is also indicative of the tremendous potential and flexibility the polynuclear aluminum cation solutions possess as ceramic precursors. In a sol-gel process, the nature of the aluminum cations in a ceramic precursor sol determines the molecular structure of the basic unit of the resulting gel and the packing of such units in the gel. Therefore, some soluble relationship should exist between the cation speciation in the precursor sol and the thermal evolution of the resulting gel. While the differences between molecular and colloidal approaches to sol-gel synthesis have received considerable attention [21, to our knowledge, a detailed study of the effects of cation speciation on the development of alumina phases has not been reported. In order to begin to elucidate a relationship, we have prepared several gel alumina precursors from aluminum salt solutions by different techniques and examined their thermal behavior. Although the exact identity of most of the complex cations in the precursor sols is not known, solution state 2 7A1 NMR can be used to "fingerprint" such sols so that a systematic comparison of thermal evolution can be made. Differences in structure and nuclearity of these cationic aluminum precursors will be shown to persist and propagate through ambient to high temperature processing reg
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