Abstract: Polyaluminocarbosilane (PACS) precursors are key raw materials for the preparation of heat-resistant silicon carbide (SiC) fibers. To overcome the technical limitations of the traditional “two-step” high-temperature pyrolysis route for PACS precursor synthesis, including complex processing and difficult regulation of composition and structure, a “one-step” atmospheric-pressure pyrolysis route was proposed in this study. Using polydimethylsilane (PDMS) and aluminum acetylacetonate (Al(AcAc)₃) as starting materials, the synthesis of the PACS precursor, the regulation of its composition and structure, the ceramization transformation process, as well as the composition and structure of the SiC ceramic products were systematically investigated. The results showed that by controlling the mass fraction of Al(AcAc)₃ in the feedstock (0-4%), the Al content and number-average molecular weight of the precursor could be effectively tailored within the ranges of 0.07%-0.47% and 1909-4302, respectively. When the Al(AcAc)₃ loading was 2%, the obtained PACS-2 precursor exhibited a softening point of 192.5 ℃ and a number-average molecular weight of 4302. The reaction mechanism of the “one-step” pyrolytic synthesis can be described as follows: at 450 ℃, PDMS undergoes the Kumada rearrangement, in which Si-Si bonds cleave and rearrange into Si-C-Si bonds with the concurrent generation of abundant Si-H bonds; these Si-H species further react with Al(AcAc)₃ to yield the PACS precursor containing Si-O-Al structures. The high-temperature inorganic transformation of the PACS precursor proceeds through three stages: (1) room temperature to 300 ℃, dominated by the volatilization of adsorbed water and residual solvent; (2) 300~700 ℃, dominated by the cleavage of side-chain alkyl groups and the scission of the main chain; (3) 700~1000 ℃, dominated by the elimination of residual oxygen and hydrogen. Pyrolyzed at 1000 ℃, the PACS-2 precursor afforded a ceramic yield of 67.09%. The PACS-2 precursor also exhibited good melt-spinning performance, from which continuous green fibers with an average diameter of (11.42±0.42) μm could be stably produced. After pyrolysis at 1800 ℃, the ceramic product derived from the PACS-2 precursor consisted mainly of the β-SiC crystalline phase, accompanied by minor amounts of α-SiC, graphite and an amorphous SiC_xO_y phase, with a β-SiC grain size of 38.8 nm at this temperature. The high ceramic yield and good melt-spinning performance of the PACS precursor endow it with promising application prospects in the field of continuous SiC fiber fabrication. Furthermore, the “one-step” synthesis strategy proposed in this study provides new technical insights for the design and synthesis of low-cost SiC precursors and other ceramic precursors.

Key words: polyaluminocarbosilane, composition and structure regulation, precursor conversion method, SiC ceramics, SiC fibers