| Continuous SiC fibers are critical for high-performance composites. The strategy to prepare SiC fibers is in four steps:(1) synthesis of a precursor polymer,usually polycarbosilane (PCS),consisting of Si and C elements,(2) spinning of the precursor polymer into "green fibers",(3) curing the fibers to make the polymer chains crosslinked and (4) pyrolysis of the cured fibers at above 1300 in inert atmosphere. Although SiC is famous in being durable in rigorous environment,in practice,the temperature limitation of general grade SiC fibers is under 1200 due to high content of oxygen,free carbon and coarseness of crystalline. This work is focusing on preparing SiC fibers with higher temperature resistance.Two new precursors,PACS (polyalumicarbosilane) and hybrid BN-PCS-PMS (borazine modified polycarbosilane-polymethylsilane) were synthesized,from which two kinds of SiC fibers consisting small amount of Al,or SiC(Al),and B-N,or SiC(BN),were prepared respectively. The reaction mechanism,composition,structure and properties of the precursors and the ceramic fibers were studied systematically.The synthesis of PACS was preformed using the reaction of polysilocarbosilane,or PSCS,with Al(AcAc)3 at 500 and ambient pressure. The reaction mechanism was found to be related to Kumada rearrangement,while the connection between Si-H bond in PSCS with Al(AcAc)3 leads into Si-O-Al and Si-Al-Si units in the structure. The PACS was suitable for spinning into green fibers owing to its adjustable soften point around 200 with good Theological properties. The green fibers were cured by slight air-oxidation followed by thermal-crosslinking to minimize the oxygen introduction. SiC(OAl) fibers were thus derived at 1300 and SiC(Al) fibers were finally obtained by sintering at 1800 Another precursor,hybrid BN-PCS-PMS,was synthesized at 100 by hybridization of BN-PCS with BN-PMS by weight ratio of 1:4. BN-PMS was prepared at 70 by the reaction of PMS (polymerized by dehalo-coupling of MeHSiCl2 with sodium) with lwt.% borazine (synthesized by reaction of NaBH4 with (NH4)2SO4);and BN-PCS was obtained at 300 by the reaction of PCS (prepared by thermolysis of PDMS) with lwt.% polyborazine (PBN).With the addition of small amount of PBN,the molecular weight of PCS increased significantly and the ceramic yield of BN-PCS was much higher than that of PCS due to higher crosslinking and some new Si-N bonds. In a similar way,the introduction of smallamount of BN into PMS makes the molecular weight 4 times higher and ceramic yield increased from 30% of PMS to 83% of BN-PMS due to higher crosslinking catalyzed by borazine. Because of the similar molecular structure and the bonds formed with each other,the above two polymers yield the hybrid BN-PCS-PMS as a homogeneous system. BN-PCS-PMS hybrid is infusible but soluble. From the solution,BN-PCS-PMS fibers were prepared by hand-drawing. The fibers were then cured by thermal-crosslinking,thus SiC(BN) fibers were finally obtained by pyrolysis at 1400.The composition of SiC(Al) fibers was described in the formula Si1CL15O0.026A10013,in which there was mainly P-SiC grains in size of 95nm,0.84wt.% aluminum,small amount of a-SiC,SiCxOy phase and free carbon. The content of SiCxOy phase and free carbon was much lower than that of Nicalon fibers. The content of oxygen and silicon on the surface of the fibers were some higher than inside;while Si,C,O and Al are uniformly distributed inside the fibers. The SiC(BN) fibers was mainly composed of P-SiC in addition to small amount of O,B and N,in which the surface is a little rich in oxygen and silicon,while the inside distribution of Si,C,O,B and N are uniform.The average tensile strength of SiC(Al) and SiC(BN) fibers were 1.4GPa and l.SGPa,with the average diameter 12um and 13um,respectively. The initial tensile strength of the both ceramic fibers remained 95% after the treatment at 1400 for Ih in argon. After sintering at 1800 for Ih in argon,SiC(Al) fibers remained initial tensile strength by 71%;while SiC(BN) by 80%. The high temperature stability...
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