Improving SiC grain growth and reducing carbon content in pyrolyzed polycarbosilane using nanosized Si particles

• Published in: Ceramics international 2024-07, Vol.50 (13), p.22771-22782
• Main Authors: Mei, Yu, Zhang, Li, Mo, Gaoming, Huang, Qing
• Format: Article
• Language: English
• Subjects: Carbothermic reaction; Ceramic yield; Polycarbonsilane; Silicon
• ISSN: 0272-8842; 1873-3956
• DOI: 10.1016/j.ceramint.2024.03.379

Polycarbosilane (PCS) serves as a crucial precursor for silicon carbide (SiC) fibers and ceramics. However, both of the precursor and the resulting SiC ceramics are carbon-rich, which is often viewed as detrimental to their relevant high-temperature properties. In this work, PCS is modified using nanosized Si particles to improve SiC grain growth and reduce carbon content in the resulting SiC ceramics, and the structural evolution during the polymer–ceramic conversion process has been meticulously investigated. The findings reveal that the incorporation of nano-Si into PCS raises its actual ceramic yield and accelerates the organic-inorganic conversion process. This is attributed to the formation of a more crosslinked structure at 200 °C–350 °C, resulting from the reaction between the Si–OH groups in nanosized Si and the Si–H and Si–CH3 groups in PCS. The carbothermic reaction between nano-Si and the free carbon starts at approximately 1200 °C at the periphery of nanosized Si particles, and is accomplished at around 1400 °C. This leads to the formation of large-sized reactive β-SiC. Simultaneously, PCS-derived β-SiC with smaller sizes is also formed at 1300 °C–1400 °C. At 1600 °C–1800 °C, the growth of the reactive and the derived β-SiC in nano-Si-modified PCS is both accelerated due to a substantial decrease in the free carbon content. Consequently, the grain size of β-SiC is much larger than that of unmodified PCS. Additionally, the residual amorphous free carbon transforms into nanocrystalline carbon at around 1400 °C and evolves into graphite carbon at 1600 °C–1800 °C, which exhibits a more ordered structure with larger size compared to that in unmodified PCS.