Silicon-based ceramics from polymer precursors

Abstract

A hyperbranched polycarbosilane of the type [R3SiCH2−]x[−SiR2CH2−]y[−SiR(CH2−)1.5]z[−Si(CH2−)2]l (R = H, –CH2CH=CH2, or OR) has been prepared, which was used as a source of inorganic/organic hybrid materials and, through pyrolysis, as a precursor to inorganic solids with unusual microstructures and properties. A partially allyl-substituted derivative “AHPCS”, nominally ['Si(allyl)0.1H0.9CH2']n, has been extensively studied as a precursor to silicon carbide (SiC) and is now used commercially as a SiC matrix source for C- and SiC-fiber-reinforced composites and binder for particulate ceramics. The alkoxy derivatives, ['Si(OR)2CH2'], (R = Me, Et) yield, after hydrolysis and condensation, carbosilane/siloxane gels with unusually high surface areas (700–900 m2/g) and microporosity that is retained in the resultant SiOxCy ceramics formed after pyrolysis to 1000 °C. The fully condensed ['Si(O)CH2'] gel in the latter case was obtained as thin, adherent films on Si surfaces by spin coating and was found to exhibit dielectric constants as low as 2.0 after heating to 400 °C. The SiC precursor, AHPCS, has also been used recently, along with other polymeric precursors, to make two-phase (SiC/C and SiC/BN) amorphous ceramics that exhibit unusual microstructures and thermal/mechanical properties. These microstructures are formed during the mixing and thermosetting of the constituent polymers, which undergo phase separation due to their immiscibility. Certain of the SiC/C composites, which have the C phase uniformly distributed as ca.1-µm droplets in a SiCx matrix, exhibit high oxidation resistance, and microindentation tests on the SiC/BN system suggest unusual toughness.