High Yield Polycarbosilane Precursors to Stoichiometric SiC. Synthesis, Pyrolysis and Application
- PDF / 868,046 Bytes
- 11 Pages / 414.72 x 648 pts Page_size
- 13 Downloads / 179 Views
LEONARD. V. INTERRANTE,*, C.W. WHITMARSH@, W. SHERWOOD@, H.-J. WU*#, R. LEWIS", AND G. MACIEL** *Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590* @Starfire Systems, Inc, P.O. Box 2628, Glenville, NY 12302 #current address: SRI International, 333 Ravenswood Ave., Meno Park, CA 94025 "**Departmentof Chemistry, Colorado State University, Fort Collins, CO 80523
ABSTRACT The synthesis and properties of two polycarbosilanes that have essentially a "SiH2CH2" composition is described. One of these polymers is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of Cl3SiCH2Cl followed by LiAIH 4 reduction. This synthesis is amenable to large scale production and we are exploring applications of HPCS as a source of SiC coatings and its allyl-derivative, AHPCS, as a matrix source for SiC- and C-fiberreinforced composites. These polymers thermoset on heating at 200-400 oC (or at 100 °C with a catalyst) and give near stoichiometric SiC with low 0 content in ca. 80% yield on pyrolysis to 1000 oC. The second method involves ring-opening polymerization of 1,1,3,3tetrachlorodisilacyclobutane and yields a high molecular weight, linear polymer that can be reduced to [SiH2CH2]n (PSE), the monosilicon analog of polyethylene. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. On pyrolysis to 1000 oC, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined for PSE by using TGA, mass spec., solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The results of these studies of the polymer-toceramic conversion process and our efforts to employ the AHPCS polymer as a source of SiC matrices are described.
BACKGROUND Research on polymeric precursors to silicon carbide began effectively in the mid- 1970's with the reports by Yajima of the successful application of a "polycarbosilane" derived from thermal isomerization of polydimethylsilane as a "processable" precursor to SiC [1]. Since that time, considerable attention has been directed to the synthesis of polymeric precursors to Sibased ceramics and their application to ceramic processing [2,3]. Of particular interest has been the use of such precursors to prepare continuous, weavable, ceramic fiber for use as the reinforcement phase in both ceramic and metal matrix composites [3]. To date, Nicalon fiber has been the only widely available commercial product to emerge from this effort [3]. This fiber is available in several grades that vary in modulus, strength and thermal stability; however, in general, owing to the presence of C and 0 impurities, excessive loss in weight and mechanical strength above ca. 1200 °C limits its usefulness as a high temperature reinforcement phase for composites. In contrast to the fiber applications, published reports relating to the use of Si-based polymers as matrix sources for CMCs have been r
Data Loading...
