Fabrication Of Silicon Carbide Fibers And Fiber Tubes By Pyrolysis Of Polycarbosilane Precusor Fibers By γ-ray、electron Beam Irradiation

Silicon carbide(Si C)ceramic fibers have been used as reinforcement material in advanced ceramic matrix composites,which are applied in the fields of aerospace,nuclear energy,advanced weapon,watercraft,etc.This is because Si C fibers exhibit an excellent mechanical behavior,creep resistance,high tensile strength and stiffness,good thermal resistance at elevated temperature,and good miscibility with ceramic,metal and polymer matrix composites.Currently,continuous Si C fibers are prepared from organosilicon polymer fibers,usually it is polycarbosilane(PCS)fibers.This method includes four process,synthesis of PCS,and then PCS are melt-span,cured,and pyrolysed.Curing is the most important process,there many methods to cure the PCS fibers such as thermal oxidation curing,chemical vapor curing,irradiation curing,etc.In this paper,PCS fibers were irradiated by γ-ray and electron beam(EB)respectively in air atmosphere,and then the cured fibers were converted into Si C ceramic fibers.Chemical structure,gel content,molecular weight distribution and pyrolysis characteristic were studied by FT-IR,GPC,TGA,etc.Results showed that bridge structures of Si-CH2-Si and Si-O-Si were formed in the PCS fibers,which irradiated by γ-ray or EB.Gel content increased with absorbed dose when the absorbed dose higher than the beginning gel dose point.Molecular weight distribution became wider gradually along with the absorbed dose which below the beginning gel point dose.The ceramic yield was also increased with the absorbed dose.The performance and structure of the Si C ceramic fibers were characterized by SEM,XRD,draw-testing,oxygen content testing,etc.Results showed that the surface and cross-section of the Si C fibers were dense and smooth.XRD testing revealed that β-Si C micro crystals were formed in the sintered fibers,and the crystal size was about 9.2nm and 10.5nm.The tensile strength and oxygen content of the Si C fibers cured by γ-ray or EB had the same trend.The tensile strength of the Si C fibers cured by γ-ray were 0.9GPa~ 2.1GPa,and the oxygen content were 7.9wt%~18.7wt%.The Si C fibers whose precursor fibers cured dose below gelling point dose were thermal-crosslink by heat treatment had a higher tensile strength was up to 2.3GPa,and a lower oxygen content was under 10.4wt%.The tensile strength of the Si C fibers cured by EB were 0.3GPa ~1.9GPa,and the oxygen content were 5.9wt%~15.9wt%.The Si C fibers whose precursor fibers cured dose below gelling point dose were thermal-crosslink by heat treatment had a higher tensile strength was up to 2.5GPa,and a lower oxygen content was under 8.8wt%.PCS fibers were irradiated by EB under the air atmosphere,the surface of the PCS fibers were oxidated,and then the oxidated surface became crosslinked by heating up to 350 degree Celsius in He gas.The PCS fiber tubes were formed by dissolving the un-crosslinked core with the solvent of tetrahydrofuran(THF),the PCS fiber tubes converted to Si C ceramic fiber tubes by pyrolysis in an inert gas at high temperature.Result showed that the oxygen was imported into PCS by reacting with Si-H and Si-CH3 to form the structures of Si-OH and C=O,the bridge structures of Si-O-Si and Si-CH2-Si were detected in the crosslinked zone of the PCS fibers.The Si C fiber tubes could be obtained from the precursor fibers which absorbed dose from 2.0 to 3.5MGy.The thickness of the ceramic tube could be controlled by the absorbed dose.