Oxygen evolution in the organic-inorganic transformation of polycarbosilane fibers via temperature-programmed analysis

During the process of preparing SiC fibers by oxidation curing method, the chemical bonding form of oxygen and its content undergo significant changes, and oxygen plays a crucial role in the process of organic-inorganic-polycrystalline structure transformation. Currently, there are certain difficulties in specifically studying the evolution of oxygen chemical structure and the variation regularity of its content during the process of organic-inorganic transformation of precursor fibers. In this work, a novel research strategy, temperature-programmed oxygen-nitrogen analyzer, is proposed to study the evolution of oxygen chemical structure and its content during the organic-inorganic transformation of polycarbosilane (PCS) precursor fibers. Three oxygen partial peaks of different oxygen chemical structures are separated by this method, oxygen is identified as Si-OH, low-oxygen silicon groups (SiOC3 + SiO2C2), and high-oxygen silicon groups (SiO3C + SiO4). Fourier transform infrared (FTIR) and solid-state Si-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) validated these assignments, showing Si-OH depletion above 700 degrees C and progressive conversion of low- to high-oxygen groups with temperature. In addition, after 600 degrees C, the number of high-oxygen silicon groups in the system began to increase significantly. The oxygen content introduced in the low-temperature preoxidation stage significantly affects the evolution of oxygen structure and the change of oxygen content after 700 degrees C. Higher oxygen content (>6%) promotes the formation of more high-oxygen silicon groups in the high-temperature decomposition stage. The method demonstrated reliability for oxygen content below 20%, offering insights into structural evolution critical for optimizing SiC fiber properties.