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

摘要

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℃). 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_2SiH_2 and CsiH_3 units are completely consumed, while C_3SiH units remain even after curing at 600℃. The TGA results show the 1200℃ 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℃ ceramics derived from B-EHPCS are found to be significantly denser than that from EHPCS.