Synthesis and Ceramic Conversion Reactions of 9-BBN-Modified Allylhydridopolycarbosilane:  A New Single-Source Precursor to Boron-Modified Silicon Carbide

The design, synthesis, and properties of a new type of poly(borocarbosilane) polymer, 9-borabicyclo[3.3.1]nonane-modified allylhydridopolycarbosilane (AHPCS−9-BBN), that has proven to be an excellent processible single-source precursor to dense silicon carbide ceramics is reported. The polymers were synthesized in high yields by the reaction of allylhydridopolycarbosilane (AHPCS) with 9-borabicyclo[3.3.1]nonane (9-BBN). The spectroscopic data for the AHPCS−9-BBN polymers, along with results of the hydroboration reaction of 9-BBN with the model compound allyldimethylsilane, are consistent with predominantly anti-Markovnikov hydroboration of the polymer allyl groups. The degree of polymer hydroboration was controlled by varying the reactant ratios with the compositions of the modified polymers ranging from (SiH2CH2)0.95(Si(allyl)2CH2)0.044(Si(allyl)(CH2CH2CH2-9-BBN)CH2)0.006 to (SiH2CH2)0.95(Si(allyl)2CH2)0.012(Si(allyl)(CH2CH2CH2-9-BBN)CH2)0.038. The AHPCS−9-BBN polymers are soluble and stable in hydrocarbons and aromatics. Solution properties analysis indicated molecular weight growth and intrinsic viscosity decrease with increasing boron incorporation. Thermal gravimetric analyses showed that the AHPCS−9-BBN polymers have thermal stabilities similar to that of the parent AHPCS polymer and thus maintain the processibility needed for their use as precursors to matrix materials. Bulk pyrolyses at 1600 °C of the AHPCS−9-BBN polymers produced SiCB ceramic chars with boron contents ranging from 0.1 to 0.8% depending upon the degree of polymer modification. XRD studies showed that the 1800 and 2000 °C AHPCS−9-BBN ceramic chars had larger grain sizes than ceramics derived from the parent AHPCS polymer. Most significantly, in agreement with previous studies that have shown that the presence of small amounts of boron (<1%) can enhance SiC sintering, it was found that with increasing temperature the AHPCS−9-BBN-derived ceramics exhibited dramatic increases in their densities compared to those derived from the parent AHPCS.