Reactor Irradiated Polycarbosilane Pyrolysis Synthesis Of Silicon Carbide Ceramic Fiber

Silicon carbide (SIC) ceramic fibers have been an attractive candidate for reinforcementmaterial in advanced composites, which can be are applied in the fields of aerospace, aviation,nuclear energy, advanced weapon, etc. This is attributed to the excellent performances of SiCfibers, for example, high tensile strength and stiffness, outstanding oxidation resistance atelevated temperature, and good miscibility with ceramics, metals and resins.SiC ceramic fibers have been mainly fabricated by pyrolysis of organosilicon polymerfibers called ceramic precursor fibers such as polycabosilane (PCS). In this process, it isnecessary to cure the polymer fibers before pyrolysis to make them infusible, and curing isusually achieved by way of thermal oxidation, electron beam orγ-rays irradiation.In order to explore a new path to synthesize SiC ceramic fibers, polycarbosilane (PCS)ceramic precursor fibers were irradiated in a nuclear reactor under N₂ atmosphere, thenpyrolyzed under inert atmosphere, and SiC fibers were obtained. Furthermore,polydimethylsilane samples were irradiated by electron beam (EB) from an accelerator tostudy their radiation effect and research into the practicality of synthesizing PCS ceramicprecursor in this way.The effect of irradiation on PCS fibers' gel content and chemical structure were analyzedby extracting and FT-IR, respectively. Results showed that gel content of the fibers increasedwith neutron flux, and kept at about 95％finally; Results of FT-IR manifested that bridgestructure of Si-CH₂-Si was formed in the irradiated products by the rupture of Si-H bonds andsucceeding linkage of active sites.Thermal decomposition characterizations of PCS fibers were studied by TGA, FT-IR,PGC/MS, etc. On this basis, the thermal decomposition mechanism was presumed. TGAcurves indicated that compared with the un-irradiated PCS fibers, the ceramic yield at 1000℃of irradiated ones increased from 67.7％to 93％～95％, and the temperature at which the PCSfibers begin to decompose increased from 330℃to 460℃. The structural analysis of PCSfibers' pyrolysis products suggested that at higher temperature, the organic polymers weretransited to inorganic ceramics more thoroughly. When pyrolysed at 1000℃for 2 hours, PCSfibers were converted to SiC ceramic fibers entirely. FT-IR and GPC/MS illustrated that in theprocess of pyrolysis, Si-H bonds, C-H bonds and Si-CH₃ groups were partly broken down,and the structure of Si-C-Si was formed, at the same time a mass of CH₄ and H₂ gas and asmall amount of silicon hydride were released.The structure and properties of the obtained SiC ceramic fibers were characterized by SEM, XRD, etc. SEM photos showed that the surface and cross-section of SiC fibers weresmooth and dense. XRD curve revealed thatβ-SiC micro crystals were formed in the fibers,and the mean grain size was about 7.5nm. The oxygen content in these SiC fibers was 1.69wt％～3.77wt％. Tensile strength of the SiC fibers was up to 2.7GPa, which exhibited theirmechanical properties were excellent. After the SiC fibers were heat treated at 1400℃in airfor an hour or at 1600℃under Ar gas atmosphere for 30 minutes, their extemal appearancewas still undamaged and compact, which demonstrated that heat resistance of the SiC fiberswas eximious.In addition, polydimethylsilane samples were irradiated by electron beam (EB) in vacuumat room temperature to research their radiation effect and analyse the practicality ofsynthesizing PCS ceramic precursor in the way of radiation chemistry. The results of GC/MSrevealed that a small amount of H2 and CH4 was released, and their radiation yields (G values)were merely in the magnitude of 10^-3/100eV. FT-IR, Raman spectra and XRD curves showedthat the chemical and crystal structure of polydimethylsilane did not change after irradiatedwith high dose of several MGy. These results indicated that PCS ceramic precursor couldn'tbe synthesized in this way, on the other side, it suggested that the radiation tolerance ofpolydimethylsilane was prominent, which might be caused by the delocalizedσelectron ofthe highly ordered Si-Si skeletons.PCS precursor fibers were cured by irradiation in a nuclear reactor, which was a novelmethod at home and abroad. This method was superior to electron beam orγ-rays irradiationin point of ceramic yield. SiC fibers synthesized in this way exhibited excellent properties,which could be applied in the development of advanced composite materials. Moreover, thenew discovery that polydimethylsilane was endowed with outstanding radiation tolerance wassignificant to the preparation of newstyle radiation resistant materials.