The SiC_xO_y phase in SiC fibers will decompose at high temperature, which results in the loose structure, declining mechanical performance, and lowing the high temperature resistance of the fibers. Introduction of some elements can play the role of sintering assistant at high temperature. Nearly stoichiometric polycrystalline SiC fibers can improve the oxidation resistance of fibers at high temperature. Al and Y are commonly used as sintering additives for SiC ceramics. Al and Y can form yttrium aluminum garnet (YAG) liquid phase at high temperature. The YAG liquid phase is conducive to reduce the sintering temperature and improve the densification of SiC ceramics. If the SiC fibers can form YAG phase at high temperature, the performance of SiC fibers can be improved. In this work, the both Y and Al are introduced for the fabrication of high performance SiC fibers.

A certain amount of polydimethylsilane (PDMS) was placed in a flask, raised to 470 ℃ and hold for 42 h in N₂, then cooled to room temperature, filtered to obtain yellowish viscous polysilacarbosilane (PSCS). 0.4 g yttrium acetylacetone (Y(AcAc)₃) and 0.6 g aluminum acetylacetone (Al(AcAc)₃) were uniformly mixed with 20 g PSCS in a flask, polymerized in N₂, and then dissolved, filtered, and distilled under vacuum to obtain polyyttrio-aluminocarbosilane (PYACS) precursor.

A certain amount of PYACS precursor was added to a spinning cylinder, heated to 300 ℃ in N₂, for defoaming, then reduced to 240 ℃. Using a pressurized N₂ (flow rate of 0.1 L/min), the melt was extruded through a single hole spinneret and the fiber was stretched and wound on a spool by a drafting device at a winding speed of 350 r/min. Curing of the fibers was carried out by exposing fibers in iodine with mass ratio of 3∶5, covered with glass and placed in oven. The system was heated up to 80 ℃ for 3 h, then cooling to room temperature.

PYACS fibers were pyrolysied in N₂ and converted into ceramic fibers at different temperature schdule, such as 20 ℃ to 400 ℃ with a heating rate of 150 ℃/h, 400 ℃ to 800 ℃ with a heating rate of 100 ℃/h, 800 ℃ to 1 000 ℃ with heating rate of 150 ℃/h. Si-C-O-(Y/Al) fibers are fired at 1 000 ℃ for 1 h. The Si-C-O-(Y/Al) fibers were heated to 1 400, 1 600 and 1 900 ℃ in argon according to certain procedures.

Due to the higher molecular weight of polycarbosilane (LPCS), it tends to crosslink during polymerization, which is notably affecting the spinning properties and fiber characteristics. To solve this problem, PYACS fibers were synthesized by addition of Y(AcAc)₃ and Al(AcAc)₃ into liquid PSCS, providing uniform molecular weight with excellent spinnability. The Y and Al elements are effectively introduced into the PYACS precursor as the form of Si—O—Y and Si—O—Al. PYACS fibers were obtained successfully by melt-spinning and iodine vapor curing. The cross-linking of fibers can be achieved with the iodine vapor curing, where iodine plays the critical role of accelerating the cleavage of Si—H, Si—Si, and C—H bonds, and recombination to form cross-linked networks of Si—O—Si. The amorphous Si—C—O—(Y/Al) fibers were obtained by pyrolysising at 1 000 ℃, and the ceramic yield was 73.96%. The effects of different heat treatment temperatures (1 400, 1 600, 1 900 ℃) on the morphology and structure of the Si—C—O—(Y/Al) fibers were investigated. After heat treatment at 1 400 ℃, the fibers remained amorphous, with smooth surface and dense cross section. After heat treatment at 1 600 ℃, the fibers has obvious β-SiC diffraction peak. And the fibers surface and the cross sections are loose porous structure, the diameter becomes smaller. The degradation reaction occurs in the fibers may be the main reason for the formation of porous degraded fibers, SiC grains are formed in Si—C—O—(Y/Al) fibers, and the release of CO gas. After heat treatment at 1 900 ℃, the crystalline degree of SiC fibers doped with Al and Y is higher, the grain size increases, and the fibers diameter decreases further. Al and Y in Si—C—O—(Y/Al) fibers exist as Al—O—Y at 1 900 ℃. The doping of Al and Y plays a role of sintering additives at high temperature.

The main conclusions of this paper are summarized as following. The Y and Al elements are effectively introduced into the PYACS precursor as the form of Si—O—Y and Si—O—Al. The crosslinking of fibers can be achieved with the iodine vapor curing. The amorphous Si—C—O—(Y/Al) fibers were obtained by pyrolysising at 1 000 ℃, and the ceramic yield was 73.96%. The effects of different heat treatment temperatures (1 400, 1 600, 1 900 ℃) on the morphology and structure of the fibers were investigated. After heat treatment at 1 400 ℃, the fibers remained amorphous, with smooth surface and dense cross section. After heat treatment at 1 600 ℃, the fibers has obvious β-SiC diffraction peak. The surface and cross section of the fibers are loose and porous structures. After heat treatment at 1 900 ℃, the crystalline degree of SiC fibers is higher, the grain size of SiC is increased, the diameter of the fibers is further reduced, and the structure is gradually densified. Al and Y in Si—C—O—(Y/Al) fibers exist as Al—O—Y at 1 900 ℃. This study demonstrates that the presence of Al and Y can reduce the formation of SiC crystal particles and nano-pores on the fiber surface at high temperature. The doping of Al and Y plays a role in the sintering densification of SiC fibers. This work has practical significance for the fabrication of SiC fibers containing heterogeneous element.