Preparation And Properties Of B?Si And B-Si Doped Pitch Based Carbon Fibers

Isotropic pitch-based carbon fibers are widely applied in thermal protection systems,brake systems,cement reinforcements,water purification and other fields because of its simple preparation process,abundant source and low preparation cost.However,the carbon fibers will be seriously oxidized once the temperature exceed 350°C in an aerobic environment.Continuous oxidation will lead to rapidly decline of their mechanical properties,and restrict their service life and application environment.In order to further improve their oxidation resistance property,the Si doped petroleum pitch,B doped petroleum pitch and B-Si doped petroleum pitch were synthesized by co-pyrolysis of petroleum pitch and polycarbosilane(PCS),pyridine borane(PYB),Polyborosilazane(PSNB),respectively.The Si,B and B-Si doped pitch-based carbon fibers with were prepared by using the obtained Si,B and B-Si doped petroleum pitch through melt spinning,pre-oxidation and high temperature heat treatment processes.The phase composition,microstructure and the anti-oxidation properties of samples were characterized by FT-IR,XRD,SEM,EDS,XPS,TG-DSC and elemental analysis.The effects of the heat treatment temperatures and ratio of dopant on the the mechanical and anti-oxidation properties of the prepared pitch-based carbon fibers were investigated.The high temperature ablation resistant mechanism of the three kinds of pitch-based carbon fibers was also discussed.The main results are listed as follows:1.The PCS,PYB and PSNB were mixed with petroleum pitch at a certain mass ratio,then dissolved in xylene solvent.Si,B and B-Si doped petroleum pitch were synthesized by low temperature co-pyrolysis.The pitch-based carbon fibers doped with Si,B and B-Si were prepared through melt spinning,pre-oxidation and high temperature heat treatment process using the obtained Si,B and B-Si doped petroleum pitch.The pitch-based carbon fibers doped with Si,B and B-Si showed regular round shape,uniform thickness distribution without paralled yarn and bridging phenomenon.The elements of C,Si,B,N and O were homogeneously distributed on cross-section and surface of the prepared fibers.2.The mechanical properties of the pitch-based carbon fibers were decreased,on the contrary,the antioxidant properties were significantly improved with the doping of PCS.After oxidized for 2h at 600°C in air.The oxidation weight loss rate of the pitch-based carbon fibers treated at 1400°C was 17.6wt%,while the oxidation weight loss rates of PSCF5 and PSCF10 were 13wt% and 14.5wt%,respectively.The Si O2 generated in oxidation process played a certain role in oxidation protection which can effectively inhibit the diffusion of oxygen into the fibers.It can effectively improve the oxidation resistance of pitch-based carbon fibers with doping of PCS.3.The B doped pitch-based carbon fibers were treated at 900°C after oxidized for 2h at 550°C in air.The oxidation weight loss rates of CF,PBCF3,PBCF4,PBCF5 and PBCF7 were 77.8wt%,74.4wt%,66.7wt%,66.5wt% and 61.8wt%,respectively.With the increase of the doping amount of pyridine borane,the oxidation resistance of carbon fibers in the air environment of 550°C increased gradually.4.The B-Si doped pitch-based carbon fibers were treated at 1400°C showed good antioxidant properties in the air environment of 600°C.After 4h of oxidation,the weight loss rate of CF were 33wt%,and the weight loss rate of PBSCF7 and PBSCF10 is 25.3wt% and 25.5wt%,respectively.It can effectively improve the antioxidant properties of the pitch-based carbon fibers with doping of PSNB.