Study On Electron Beam Irradiation And Thermal Oxidation Curing Of Polycarbosilance Fiber Under Oxygen Atmosphere

Silicon carbide (SiC) fibers which have high tensile strength, high elastic modulus, and good thermal stability are one of the candidate materials for the reinforcement fibers of ceramic matrix composites. In recent years, preparation of ceramic fibers by the pyrolysis of an organic polymer has attracted considerable attention and has become a steadily growing field. The method includes the synthesis of polycarbosilane (PCS) precursor, melt spinning, curing and pyrolysis at high temperature. Pure PCS is difficult to spin, but it can be obviously improved by blending with little polypropylene. This dissertation synchronously studied the crosslinking of PCS and PCS-5P, a blend that contains 5wt% polypropylene (PP).The paper used two different curing processes, one is electron beam irradiation curing under low oxygen partial pressure, the other is thermal oxidation curing in air. Then, cured PCS fibers were annealed at different temperature under inert atmosphere, and pyrolyzed at 1250℃to give SiC fibers. Both two technologies took the variation of oxygen as main spindle. Physical, chemical and structural changes during curing were detected by IR, NMR, TG-DTA and so on. Properties of SiC fibers were characterized by XRD, SEM and EPMA. Following are the main conclusions indicated by this study:The infusibilization of PCS and PCS-5P fibers were achieved using electron beam irradiation under low oxygen atmosphere, but irradiation dose must not be less than 2.0GMy. Cured fibers can be further crosslinked after annealing at 200-500℃under inert atmosphere. Crosslinking degree and oxygen content can be controlled by regulating processing parameters such as oxygen partial pressure, irradiation dose, annealing temperature, and so on. The average tensile strength of SiC prepared from PCS and PCS-5P is 2.40 and 1.81GPa, respectively. The average tensile modulus is 170.1GPa and 168.7GPa, respectively. SEM and EPMA results show that SiC fibers prolyzed at 1250℃have a shell-core structure. The shell is oxygen-rich and the core is oxygen-deficient. Oxygen presents a gradient distribution inside the fiber.Thermal-oxidation curing in air can also obtain cured fibres, but one premise is that curing temperature must be above 170℃. Because the fibers will merge during thermal-oxidation curing process, PCS-5P fibers can not use this method. Compared with irradiation oxidation curing, SiC fiber prepared using thermal-oxidation curing do not have obvious shell-core structure, although their surface are still oxygen rich. Besides, the tensile strength is 2.27GPa and tensile modulus is 145.15GPa, a little lower than fibers prepared with electron beam irradiation, but high-temperature resistance is better than it.When irradiated or heated under oxygen atmosphere, Si-H bonds in the molecular structure of PCS will react with oxygen to give Si-OH group, and Si-CH₃ to give compounds which contain C=O group. During annealing, Si-OH undergo dehydration condensation reactions and compounds that contain C=O group decompose to give small molecular when the temperature is blow 400℃, but above 400℃, Si-H and Si-CH₃ will decompose and polymerize to produce Si-CH₂-Si linkage, and so on.