The evolution of composition and structure of polyaluminocarbosilane (PACS) fiber during curing process in air was investigated by TG-DTA, SEM, GPC, FT-IR, CP/MAS NMR techniques and chemical analysis. Results showed that the PACS fiber reacted with oxygen starting from about 210 ℃ in air and it was an exothermic reaction. The oxygen content in the fiber increased with the elevation of temperature. It was accompanied by a rapid increase of the gel content in the stage of the weight gain of 6～8 wt( and microflaw generated on the surface of the cured PACS fibers. In the initial stage, Si—H bonds in PACS reacted with oxygen to form Si—O—Si bonds and the of the PACS fiber increased greatly, which resulted in a cross-linked structure in the cured PACS fiber. In the medium stage, the remaining Si—H bonds continued to react with oxygen and the reaction between Si—CH₃, Si—H and oxygen took place producing Si—O—C groups. In the last stage, the fiber cross-linked completely and it consisted of SiC₄, SiC₃H, Si—O—Si and Si—O—C groups.

Key words: polyaluminocarbosilane, SiC fiber, curing, evolution of the composition and structure