Study On Pitch-based Carbon Materials Doped With Si-Zr/Si-Zr-B And C/C-SiC-ZrC Composites

Depending on the excellent thermal performance and high-temperature structural properties,C/C composites become the most potential high temperature structural materials in many advanced fields.Once the temperature is above 450?,the materials react with oxygen in aerobic environment.Continuous oxidation of the materials can result in the degradation of their properties,which restricts their service life and application environment.In order to further improve their oxidation resistance property,the Si-Zr doped coal-tar pitch and the Si-Zr-B doped coal-tar pitch were synthesized by co-pyrolysis of xylene soluble pitch,silicon carbide precursor,zirconium boride precursor and zirconium carbide precursor.The pitch-based carbon materials doped with Si-Zr and Si-Zr-B were prepared through co-carbonization,cold moulding and the following high temperature heat treatment process using the obtained Si-Zr and Si-Zr-B doped coal-tar pitch,respectively.Meanwhile,the C/C-SiC-ZrC composites were prepared through precursor infiltration and pyrolysis(PIP)process using the obtained Si-Zr doped coal-tar pitch as the impregnant.The phase composition,microstructure and the anti-oxidation properties of the samples were characterized by XRD,SEM,EDS,TG-DSC and other testing methods.The influence of the heat treatment temperatures and ratio of ceramic precursors on the anti-oxidation properties of the obtained pitch-based samples was studied and the high temperature ablation resistant mechanism of the C/C-SiC-ZrC composites was also discussed.The main results are listed as follows:1.The Si-Zr doped coal-tar pitch and the Si-Zr-B doped coal-tar pitch were synthesized by co-pyrolysis of a certain amount of xylene soluble pitch and different ratio of silicon carbide precursor,zirconium boride precursor and zirconium carbide precursor.The pitch-based carbon materials doped with Si-Zr and Si-Zr-B,which are evenly distributed by SiC,ZrC and SiC,ZrC,ZrB2,were prepared through co-carbonization,cold moulding and heat treatment process using the obtained Si-Zr and Si-Zr-B doped coal-tar pitch,respectively.2.The mass ratio of the polycarbosilane and zirconium carbide precursor has significant influence on the oxidant resistance of the pitch-based carbon materials doped with Si-Zr.The weight loss of the doped carbon materials is increased with the decrease of the content of silicon carbide precursor in the Si-Zr doped coal-tar pitch.The doped carbon materials obtained from the Si-Zr doped coal-tar pitch,which has a mass ratio of the polycarbosilane and zirconium carbide precursor of 4:1,shows an excellent oxidant resistance.After oxidation in air at 1500? for 4h,its weight loss is 28.3wt%.A dense SiO2 glassy oxide layer formed during oxidation in air can effectively reduce the rate of oxygen diffusion and thus improve the oxidation resistance of the carbon materials derived from the doped coal-tar pitch.3.The mass ratio of the polycarbosilane,zirconium carbide precursor and zirconium boride precursor has significant influence on the oxidant resistance property of the pitch-based carbon materials doped with Si-Zr-B.The weight loss of the doped carbon materials is decreased firstly and then increased with the decrease of the content of silicon carbide precursor in the Si-Zr-B doped coal-tar pitch.The doped carbon materials obtained from the Si-Zr-B doped coal-tar pitch,which has a mass ratio of the polycarbosilane,zirconium carbide precursor and zirconium boride precursor of 1:2:1,shows an excellent oxidant resistance property.After oxidation in air at 1500? for 4h,its weight loss and oxidation depth is 27.5 wt%and 0.7 mm,respectively.The outstanding anti-oxidation property of the sample is mainly attributed to the dense and continuous SiO2-ZrO2-B2O3 glass film generated on the surface of the material during the oxidation process.4.The heat treatment temperatures greatly influenced on phase composition,microstructure and oxidant resistance of the pitch-based carbon materials doped with Si-Zr-B.Neither too high nor too low of the heat treatment temperature is conducive to promote its antioxidant performance.The weight loss of the doped carbon materials is decreased firstly and then increased with the increase of the heat treatment temperatures.The doped carbon materials obtained from the Si-Zr-B doped coal-tar pitch with a mass ratio of the polycarbosilane,zirconium carbide precursor and zirconium boride precursor of 3:2:2 obtained at heat treatment temperature of 1600? shows an excellent oxidant resistance.After oxidation in air at 1500? for 4h,its weight loss and oxidation depth is-14.6 wt%and 0.4 mm,respectively.This may be attributed to that the ceramic precursors are completely pyrolyzed into inorganic ceramics with suitable particle size under the temperature.5.The C/C-SiC-ZrC composites were prepared through high press precursor infiltration and pyrolysis(PIP)process using the obtained Si-Zr doped coal-tar pitch as the impregnant.The holes of obtained C/C-SiC-ZrC composites are full filled and uniformly dispersed by ceramic phases.Furthermore,the ceramic phases exist throughout the thickness direction of the materials.6.The linear ablation rate and the mass ablation rate of the C/C-SiC-ZrC composites are decreased firstly and then increased with the increase of the content of zirconium carbide precursor in the impregnant.The C/C-SiC-ZrC composite with the mass ratio of the polycarbosilane and zirconium carbide precursor of 1:4,shows an excellent oxidant resistance property.After ablation in air for 80s,the linear ablation rate and mass ablation rate is 0.011 mm·s-1 and 0.0009 g·s-1,respectively.Different kinds of protective coatings form on the three different regions of the ablated surface,which can effectively reduce the rate of oxygen diffusion and thus improve the ablation resistance of the C/C composites.Therefore,the materials have excellent ablation resistance.