Synthesis Of SiBCN And SiBCN-Zr Ceramic Precursors And Their Pyrolysis Behavior

Ultra-high temperature ceramics have been widely used to modify the oxidation and ablation resistance of C/C composites for its excellent high temperature performance.Ultra-high temperature ceramics mainly include single phase and multiphase ceramics.However,single phase ceramics only worked in a relatively narrow range of temperatures for oxidation protection.While ceramics consist of multiply ceramics could make up the shortages of single-phase ceramics,and different ceramics together get an anti-oxidation synergistic effect,which improved the thermomechanical and thermochemical properties of C/C composites.Therefore,multiphase ceramics show broad application prospects in the oxidation and ablation modification of C/C composites.However,the direct modification of C/C composites using ultra-high temperature multi-phase ceramic particles couldn't solve the problem of dispersion uniformity of ceramic phases in matrix carbon and carbon fiber bundles,which limited the full utilization of synergistic antioxidant capacity between different ceramics.Multiphase ceramic modified C/C composites can be also obtained by multiple impregnation and pyrolysis of mixing various single-phase precursor solutions,but the problem of phase separation of precursors and the resulting uniform dispersion of ceramics.So it is particularly necessary to search for a preparation method for multiphase ceramics by which ceramic materials could get well dispersibility.In this dissertation,multiphase ceramic precursor was synthesized by a single source precursor method,by which the target element of multi-phase ceramic was chemically bonded to form a single-source macromolecule,and the structure and properties of the precursor were regulated at molecular or atomic scale,then uniform multi-phase ceramic could be obtained by pyrolysis at high temperature.The structures of precursors,and the microstructures and morphology of ceramics were tested and characterized by various analytical methods.The single-source precursor for multi-phase ceramic is expected to be used for the matrix modification of the C/C composite,achive the well dispersion of each ceramic phase in the matrix carbon and the carbon fiber bundle,which would promote the anti-oxidation and anti-ablation properties of C/C composite.The main work and conclusions are as follows:?1?Two kinds of SiBCN ceramic precursors?named PSNB and PBSZ?with different molecular structures were prepared by using pyridine borane?PdB?and boron trichloride?BTC?asboronsource,methyldichlorosilane?DCS?and methylvinyldichlorosilane?MVDCS?as silicon source,hexamethyldisiloxane as nitrogen source,respectively.Then corresponding SiBCN ceramics were obtained by pyrolysis of precursors.The effects of different process conditions on properties of the precursors and ceramics were investigated.The results indicated that higher synthesis temperature of precursors lead to higher ceramic conversion rate.The ceramic conversion rate was 30.86%,38.66%,41.23%and 42.69%when polymerized at 140^oC,160^oC,180^oC and 200^oC,respectively.Besides,the ceramic was remain amorphous at high temperature because of the addition of boron element,which could inhibit the nucleation and growth of SiC grains in Si-based ceramics.Comparing with PBSZ,PSNB could converted to ceramics at about 600^oC,lower than that of PBSZ?about800^oC?,while the ceramics conversion rate was slightly worse than PBSZ?about 58%?.And the pyrolysis producucts of the two precursors at 1800^oC were mainly composed of SiC,Si₃N₄ and BN.?2?Using above liquid SiBCN ceramic precursors?LPSNB and LPBSZ?as raw materials,zirconocene chloride?Cp₂ZrCl₂?as zirconium source,SiBCN-Zr ceramic precursors?PSNB-Zr and PBSZ-Zr?with different Zr content were prepared.The ceramization process of PSNB-Zr was primitively discussed and the antioxidant performance at 1200-1600^oC of pyrolysis products of PSNB-Zr5 at 1600^oC was tested.It was discovered that the ceramics obtained by pyrolysizing precursors at 1400^oC were still amorphous.When pyrolysis temperature continued to rise,the ceramic nucleus was grown,and the ceramic particle size increased significantly,ceramics such as Zr₂CN and Si₃N₄ were gradually decomposed and recombined to obtain different ceramic compositions.The ceramic conversion rate of precursors increased with the increase of Zr content in PSNB-Zr,and small molecules like H₂,NH₃,CH₄ would released during the ceramization process.The oxidation experiment results indicated that SiBCN-Zr ceramics showed excellent oxidation resistance at 1200-1600^oC,and the weight loss after oxidizing for 2 h at 1600^oC was about 0.35%.