Thermal Oxidation Characteristics Of Polycarbosilane Precursor Fibers Under Negative Pressure

The purpose of this paper aims to reduce the oxygen content of silicon carbide?SiC?fibers.The polycarbosilane?PCS?precursor fibers were thermal oxidation treated by the method of negative pressure thermal oxidation.The thermal cross-linking technique was used to treat the thermal oxidation treatment fibers.In the end,the SiC ceramic fibers were prepared by the pyrolysis of PCS precursor fibers which were thermal oxidation treated under the condition of negative pressure.The effect of atmosphere pressure,treatment temperature and holding time on weight gain,Si—H bond reaction degree,gel content,chemical structure and pyrolysis characteristics of the PCS precursor fibers were systematically studied by means of oxidation weight gain,gel content test,FT-IR and TG-DSC.The results showed that with increasing of negative pressure atmosphere pressure and treatment temperature or prolong holding time,the weight gain,gel content,Si—H bond reaction degree and the ceramic yield of the PCS precursor were presented an increasing trend.When the treatment temperature was 195? and the holding time was 8hr,with increasing of atmosphere pressure from-0.06 to 0MPa,the weight gain increased from 5.40% to 12.55%,the gel content increased from 6.93% to 89.22%,the Si—H bond reaction degree increased from34.27% to 72.53%.The absorption peak of Si—H at 2100cm^-1 gradually weakened,the absorption peak of C=O at 1730cm^-1 and Si—OH at3650cm^-1 gradually increased,the Si—C—Si absorption peak at 1020cm^-1appeared enhanced and broadened.The ceramic yield of fibers increased from 84.59% to 86.53%.The thermal oxidation PCS precursor fibers were thermal cross-linking treated,and the thermal cross-linking effect were characterized by IR,gel content and morphology observation.The results showed that when the weight gain was higher than 6.11%,the oxidation fibers could be successfully treated by thermal cross-linking,and the thermal cross-linking PCS fibers could be obtained.The cross-linking precursor fibers' gel content could reach above 87% which weight gain were higher than 6.11%.When the oxidation precursor fibers treated by thermal cross-linking treatment,the peak of Si—H at 2100cm^-1 weakened,and the Si—C—Si absorption peak at 1020 cm^-1 was enhanced and broadened.The thermal cross-linking consumed Si—H,Si—OH and Si—CH₃,and generated Si—C—Si and Si—O—Si structures.The Si—C—Si and Si—O—Si structure made the thermal cross-linking precursor fibers have higher gel content.The microstructure,crystal structure,oxygen content and tensile strength of the SiC fibers which was directly sintered from thermal oxidation fibers?oxidation weight higher than 6.11%?was systematically characterized by SEM,XRD,oxygen analysis and mechanical properties.The results showed that the shape of the fibers was well maintained,thesurface and cross-section were smooth and compact,no obvious holes and defects after high temperature pyrolysis in inert atmosphere.XRD analysis showed that the Si C fibers prepared by pyrolysis were mainly composed of ?-SiC grains group.The oxygen content of SiC fibers which was directly prepared by the pyrolysis of oxidation precursor fibers was below 17wt%,and the lowest was 8.95%.When the temperature was185?,the holding time was 24 hr and the atmosphere pressure in the range of-0.06⁰MPa.The tensile strength and oxygen content of Si C fibers increased with the increasing of atmosphere pressure.When the atmosphere pressure was-0.02 MPa,the tensile strength of SiC fibers prepared by pyrolysis of oxidation fibers was 0.69 GPa,and the oxygen content was 16.41wt%.