聚铝硅烷与聚碳硅烷共混先驱体制备SiC(Al)纤维

采用聚铝碳硅烷和聚碳硅烷共混制备含铝碳化硅的先驱体,并与直接合成得到的聚铝碳硅烷进行了比较.元素分析表明,共混法能够有效控制聚铝碳硅烷中的铝含量,且共混聚铝碳硅烷先驱体Si—H键含量更高.流变性能研究表明,共混获得的聚铝碳硅烷先驱体黏流活化能从255kJ/mol降至200kJ/mol,先驱体的可纺性提高,所以原纤维的平均直径从19μm降至12μm.预氧化后聚铝碳硅烷原纤维经1800℃一步烧成可得到致密的SiC(Al)纤维;XRD研究表明,纤维中的铝起到抑制碳化硅晶粒长大的作用.

ALUMINUM-CONTAINING SILICON CARBIDE FIBERS PREPARED FROM POLYALUMNINOCARBOSILANE AND POLYCARBOSILANE BLENDS

The preparation of aluminum-containing silicon carbide,SiC(Al),fibers by using polyaluminocarbosilane(PACS) and polycarbosilane(PCS) blends as precursors was investigated.The solubility parameter of PACS was determined to be 18 MPa 1/2 by dissolving PACS in solvents with different solubility parameters.It was very close to the solubility parameter of PCS(18.4 MPa 1/2),indicating the possible compatibility of PACS and PCS.After PACS and PCS both were dissolved in xylene,a translucent yellow blend of PACS and PCS,B-PACS,was obtained by distillation.The composition and structure of B-PACS,PACS and PCS were characterized by chemical analysis,GPC and FTIR.It was found that B-PACS with desirable Al contents could be obtained by adjusting the ratio of PACS(Al content 1.25 wt%) and PCS.The Si—H bond content can be indicated by A_ 2100 cm-1 /A_ 1250 cm-1,where A_ 2100 cm-1,A_ 1250 cm-1 are the absorbance of Si—H bonds at 2100 cm-1 and CH_3 bonds at 1250 cm-1,respectively.The FTIR results showed that the Si—H bond content of B-PACS was higher than that of PACS,while their Al contents were the same.PCS with a higher Si-H content might be responsible for the higher Si—H content of B-PACS.GPC results showed that B-PACS had a high molecular weight fraction as compared with PACS of the same Al content.The apparent activation energy of flow for B-PACS and PACS(Al content 0.6 wt%) was determined by the relationship between temperature and melt viscosity.They were 200 kJ/mol and 255 kJ/mol,respectively.The better spinnability of B-PACS was approved by spinning experiments,and finer B-PACS green fibers were obtained.The tensile strength of Si-Al-C-O fibers from PACS and B-PACS were 2.0 GPa and 2.4 GPa,respectively.The higher strength of Si-Al-C-O fibers from B-PACS might be related with the finer diameter of the green fibers and the high molecular weight fraction in B-PACS.The SiC(Al) fibers derived by curing PACS and B-PACS green fibers at 1800℃ in Ar were compact and smooth,while the SiC fibers derived from PCS had pores and defects.The introduction of Al into SiC fibers may reduce the growth of SiC crystallites in fibers.The average sizes of SiC crystallites in fibers derived from B-PACS,PACS and PCS were 8.5 nm,9.6 nm and 14 nm,respectively.