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

Abstract

Abstract 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°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°C. The TGA results show the 1200°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°C ceramics derived from B‐EHPCS are found to be significantly denser than that from EHPCS. Copyright © 2010 John Wiley & Sons, Ltd.