| To improve the high temperature resistance and high temperature oxidation resistance of SiC ceramics and fibers, some hetero-elements, such as Nb, Mo, or Re, were introduced into SiC ceramics and fibers as the sintering aids and grain growth inhibitors.In this paper, three novel preceramic polymers, polyniobiumcarbosilane (PNCS), polymolybdencarbosilane (PMoCS), and polyrheniumcarbosilane (PRCS) were synthesized with liquid polysilane (LPS) and metal pentachloride (MCl5, M= Nb, Mo, or Re) as raw materials. Nb, Mo, and Re-containing SiC fibers were fabricated from the polymers, respectively. The synthesis conditions, composition, molecular structure, reaction mechanism of the precursors, and the conversion process from polymer to ceramis were investigated. The processing, composition, microstructure, high temperature resistance, and high temperature oxidation resistance of the polymer-derived fibers were also studied.The results showed that, PNCS with the soften point 160 ~ 220℃was synthesized by the reaction between LPS and 5～10wt% NbCl5 at reaction temperature 440～460℃and the pyrolysis temperature 500℃for 8hs with a yield about 55%. The chemical formula of PNCS was SiC2.211H7.09O0.035Nb0.0037 when the dosage of NbCl5 was 10wt%. PMoCS with the soften point 150~210℃was synthesized by the reaction of LPS with 4～8wt% MoCl5 at the reaction temperature 440～450℃and pyrolysis temperature 500℃for 10hs with the yield about 51%. The chemical formula of PMoCS was SiC2.073HxO0.034Moy when the dosage of MoCl5 was 8wt%. PRCS with the soften point 200~260℃was synthesized by the reaction of LPS with 1～4wt% ReCl5 at the reaction temperature 440℃and the pyrolysis temperature of 500℃for 6hs with the yield about 60%. The chemical formula of PRCS was SiC2.312HxO0.029Rey when the dosage of ReCl5 was 2wt%. PMCSs were composed of SiC4 and SC3H groups. The hetero-elements in PMCS existed mainly in the manners of Si-M bond and a little in the manners of Si-O-M bond. The hetero-elements, therefore, acted as crosslinking points.The pyrolysis process of PMCSs were similar to that of PCS. The ceramic yields of PMCSs at 1200℃in N2 were more than 75%, which were much higher than that of PCS. Theβ-SiC grain sizes of the pyrolytic products from PMCSs were smaller than that from PCS.When PMoCS was pyrolzed at 1200℃in N2,β-MoSi2 was found according to the XRD results. After pyrolyzed at 1600℃,β-MoSi2 was converted completely toα-MoSi2.The x-ray diffraction peaks of NbC were observed in PNCS-derived ceramics pyrolzed at 1600℃in N2. Most of PMCSs showed good spinnability. Amorphous Si-M-C-O fibers with the average diameter about 11μm and the average tensile strength about 2GPa were prepared from PMCSs. The high temperature resistance and high temperature oxidation resistance were much better than those of Nicalon? fibers. The tensile strength retention of Si-Nb-C-O, Si-Re-C-O, and Si-Mo-C-O fibers, heated to 1600℃in N2 for 30 minutes, were 48%, 44%, and 40%, respectively. Nicalon? fibers, however, lost its shape integrity at this time. Heated up to 1800℃in N2 for 30 minutes, Si-Re-C-O and Si-Nb-C-O fibers still showed the tensile strength retention of 31% and 22%, respectively. Si-M-C-O fibers were mainly composed ofβ-SiC. The maximal grain size ofβ-SiC in Si-M-C-O fibers was about 10.5nm, which was much smaller than that in Nicalon? fibers.Since the SiO2 layer formed on the surface of the Si-M-C-O fibers in air at high temperature, Si-M-C-O fibers showed good high temperature oxidation resistance. The tensile strength retention of Si-Nb-C-O, Si-Re-C-O, and Si-Mo-C-O fibers, heated to 1250℃in air for 6hs, were 50%, 41%, and 34%, respectively. Nicalon? fibers, however, lost completely its strength after treated at the same conditions.Due to introducing Nb, Mo and Re into the SiC fibers, the high temperature resistance and high temperature oxidation resistance of the Si-M-C-O fibers were much better than those of Nicalon? fibers.
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