Preparation of a liquid boron-modified polycarbosilane and its ceramic conversion to dense SiC ceramics

A boron-modified ethynylhydridopolycarbosilane (B-EHPCS) was successfully prepared via the hydroboration reaction of ethynylhydridopolycarbosilane (EHPCS) with 9-borabicyclo-[3.3.1]nonane (9-BBN). The as-synthesized B-EHPCS with a branched structure was characterized by means of gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). The structural evolution of ceramic conversion of B-EHPCS was investigated by solid-state NMR. The (13)C magic angle spinning (MAS) NMR results indicated that the C=C and C C groups of B-EHPCS take part in the hydrosilation cross-linking at a relatively low temperature (170 degrees C). According to the (29)Si MAS NMR analysis, the CSiH(3) end groups are most reactive hydride functionality involved in the hydrosilation cross-linking. With increasing curing temperature, the C(2)SiH(2) and CSiH(3) units are completely consumed, while C(3)SiH units remain even after curing at 600 degrees C. The TGA results show the 1200 degrees C ceramic yield of B-EHPCS reaches 86%, which is 10% higher than that of the parent EHPCS (76%). At high temperatures, the introduction of <1 wt% boron significantly inhibits silicon carbide (SiC) crystallization. The 1800 degrees C ceramics derived from B-EHPCS are found to be significantly denser than that from EHPCS. Copyright (C) 2010 John Wiley & Sons, Ltd.