Modification of polycarbosilane by polyborazine as a precursor for oxygen-free SiC fibers

Abstract

Infusible but soluble polycarbosilane (PCS) was obtained by heating Yajima PCS at 300 °C for 1 h with 1– wt.% of polyborazine (PBN). The simple addition of PBN to PCS and subsequent heat treatment produced multiple effects on thermal conversion. It improved the ceramic yield of the polymers from 64% to 78%, and increased the molecular weight, indicating that PBN promotes the dehydrocoupling of Si–H in PCS, as determined by ¹H-NMR and mass spectrometry. It promoted the conversion of SiHC₃ to SiC₄ at over 300 °C, and thus accelerated ceramization at the intermediate phase, and it significantly suppressed crystalline growth at high temperatures. The modified PCS with 5 wt.% PBN was dry-spun into green fibers using a hand-drawing method. The fibers produced were pyrolysed without an additional curing step, and retained dimensional stability at low oxygen contents and a smooth surface with nanocrystallinity up to 1700 °C. These results suggest that this modified PCS might be useful for the economic production of oxygen-free SiC fibers.