Low Temperature Routes to Cordierite-Like Ceramics using Chemical Processing
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LOW TEMPERATURE ROUTES TO CORDIERITE-LIKE CERAMICS USING CHEMICAL PROCESSING ZHI-FAN ZHANG, MARTIN L. HOPPE, tJEFFREY A. RAHN, SANG-MAN KOO and RICHARD M. LAINE.* Departments of Materials Science and Engineering, and Chemistry, University of Michigan, Ann Arbor, MI 48109-2136. tDept. of Chemistry, Eastern Washington State University, Cheney, WA. ABSTRACT The reaction of Si0 2 with basic magnesium compounds, in the presence of excess ethylene glycol, provides access to the hexacoordinate silicate complex, MgSi(OCH 2 CH 20) 3. Alkali metal hydroxides provide access to pentacoordinate silicates of the type M2Si 2 (OCH 2 CH2 0) 5, where M = Li, Na, K, and Cs. Alumina also reacts with alkali metal hydroxides and ethylene glycol to give compounds of the type MAI(OCH 2CH 2 0) 2 . HCl neutralization of the alkali metal silicates and aluminates provides the neutral metal glycolates of Si and Al. Stoichiometrically correct mixtures of these complexes form homogeneous, cordierite precursor solutions that are potentially useful for forming films, fibers and membranes of cordierite. These polymer-like materials can be heated directly in air to generate first gt-cordierite and then ct-cordierite. Hydrolysis and condensation reactions, which occur in sol-gel processing, are not required. The phase transformations and chemical changes that occur during pyrolytic transformation of the preceramic materials to their target ceramic products were characterized. Methods developed during the course of these studies led to the development of heating schedules wherein well crystallized g.-cordierite could be formed at 850 0 C. INTRODUCTION Chemical processing of ceramic materials can be divided into two focus areas: sol-gel and organometallic/inorganic polymer processing. The use of organometallic precursors as a route to ceramic materials is typically reserved for processing non-oxide ceramics where hydrolytic processing is normally detrimental to the quality of the final ceramic product. Sol-gel processing is often the route of choice for processing oxide ceramics. One of the failings of sol-gel processing has been in processing multimetallic materials. Problems arise in sol-gel processing of multimetallic oxide ceramics because of the great differences in the hydrolysis rates of the various metal alkoxides[l,2]. Although numerous efforts have been made to stabilize some of the more reactive alkoxides towards hydrolysis, most notably the work of Sanchez and coworkers[2]; it is still difficult to accommodate the wide range of metal-alkoxide hydrolysis rates, found for various metals, using a single general procedure. Consequently, we have begun to explore a complementary approach to solgel processing of multimetallic metal oxides wherein processable inorganic polymers are used *in place of sol-gel methods. We describe here preliminary efforts to develop inorganic polymer precursors to cordierite based on inexpensive, metal oxide derived starting materials. In addition, recent efforts[3-7] to prepare cordierite via sol-gel processing permi
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