Pyrolysis of Organometallic Precursors As A Route to Novel Ceramic Materials
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PYROLYSIS OF ORGANOMETALLIC PRECURSORS AS A ROUTE TO NOVEL CERAMIC MATERIALS LEONARD V. INTERRANTE1*, WAYDE R. SCHMIDTI, PAUL S. MARCHETrI 2 AND GARY 2 E. MACIEL IDepartment of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590 2 Department of Chemistry, Colorado State University, Fort Collins, CO 80523. ABSTRACT Homogeneously mixed nanocrystalline composites incorporating SiC, Si 3 N4 , AIN, BN and TiN, and SiC/AIN solid solutions were prepared from mixtures of known precursors to the separate components and specially prepared singlesource precursors. These novel materials are of potential interest as tough, abrasion-resistant coatings, continuous fibers and matrices for high temperature composites and, possibly, superplastic ceramics. Solid state NMR spectroscopy along with other chemical and materials characterization methods have been employed in studies of the precursor-to-ceramic conversion process and the characterization of the final ceramic products. The results of these studies are described and their implications with respect to the relationship between precursor structure, pyrolysis chemistry and the final ceramic composition and microstructure are discussed. INTRODUCTION The pyrolysis of organometallic polymers has been widely applied to the preparation of the simple carbides and nitrides of various main group and transition elements 1 . Among the potential advantages of such precursors arc their amenability, as molecular systems, to such processing methods as solution casting, fiber drawing, etc., allowing the production of coatings, continuous fiber, and other desired final forms for the ceramic product. The use of this methodology in a deliberate fashion to prepare homogeneous mixtures or solid solutions of pseudobinary combinations of ceramic materials has received much less attention 2 . The potential interest in such composite ceramic materials stems in part from observations of improved mechanical properties (such as toughness) and microstructural stability in certain cases. For example, the addition of AIN to SiC was reported to reduce grain size, improve microstructural uniformity, and vary thermal conductivity and expansion in hot-pressed samples 3 . In this case, the formation of a solid solution with the 2H structure over a broad range of AIN/SiC compositions was evidenced by TEM/SAD studies of samples obtained by hot-pressing AIN/SiC mixtures. Recently, such solid solutions were obtained by pyrolysis of organometallic 7 6 5 4 precursors . Similarly, the additions of AIN to BN , BN to Si 3 N4 , and TiN to BN have been found to result in improved toughness, sinterability, and other specific property improvements. Our work on the Si3N 4 /BN system has evidenced a marked decrease in the activation energy for the surface area reduction of precursor-derived amorphous Si 3 N 4 powder in the presence of BN, while at the same time crystallization of the Si 3 N4 at 1600 *C was inhibited 8 . The recent discovery of ductility and even superplasticity for nanocrystalline ceramics and cera
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