Hydridopolycarbosilane Precursors to Silicon Carbide

Abstract

Two different polycarbosilanes that have essentially a “SiH₂CH₂” composition have been examined as potential precursors to stoichiometric silicon carbide. One of these is a high molecular weight, linear polymer with a [SiH₂CH₂]_n repeat unit (PSE). This polymer is the monosilicon analog of polyethylene and was prepared by ring-opening polymerization of tetrachlorodisilacyclobutane, followed by reduction with LiAlH₄. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 0–25 °C. On pyrolysis to 1000 °C, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined in this case by using TGA, mass spec., solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The second polymer is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of Cl₃SiCH₂Cl followed by LiAlH₄ reduction. This polymer thermosets on heating at 200–400 °C (or at 100 °C with a catalyst) and gives near stoichiometric SiC in ca. 80% yield on pyrolysis to 1000 °C. Unlike PSE which is difficult to prepare and only available in small quantities at present, HPCS and its allyl and vinyl derivatives (XHPCS, X= allyl or vinyl) are readily available in high yield from a relatively inexpensive starting material. The application of a 5% AHPCS derivative as a source of SiC matrices for SiC-fiber reinforced composites via a Vacuum Polymer Infiltration and Pyrolysis process is described. The resulting composites exhibit higher flexural strengths than comparably reinforced CVI-SiC matrix composites.