Preparation Of Hafnium-based Precursors For Ultra High Temperature Ceramics And Ceramic Fibres

Fibre-reinforced ultra-high temperature ceramic matrix composites are the most promising candidates for the thermal protection of hypersonic aircraft under extreme environment.Ultra-high temperature ceramic fibres are the bottleneck for developing ultra-high temperature ceramic matrix composites and further improving the performance of composite materials.Polymer-derived technology is the most promising method for preparing ultra-high temperature ceramic fibres,and the core problem of this technology focuses on synthesizing ultra-high temperature ceramic precursor with excellent molding performance.In this work,precursors with Hf-C bond and Hf-N bond structures in the main chain were constructed via Grignard reaction and amine displacement reaction,respectively.The composition,structures and properties of the precursors were adjusted by the design and optimization of raw materials,synthetic routes and processes.The inorganication processes of the precursors,the composition and high-temperature evolution of pyrolytic ceramics,as well as the ablation resistance of hot-pressed ceramics were studied in detail.Based on these results,the preparation technology of ultra-high temperature ceramic fibres was explored using Hf-N-based precursor with excellent molding properties.Precursor polyhafnocarbosilane?PHCS?with Hf-C bond in the main chain was synthesized by Grignard reaction using Cp₂HfCl₂,BrCH₂CH=CHCH₂Br,?CH₃?₃SiCH₂Cl and Mg as raw materials.FTIR,NMR,XPS,XRD,SEM,TEM,etc.were used to study the composition,inorganicization process,high temperature evolution of pyrolytic products and high temperature resistance of the hot-pressed ceramic.The results show that PHCS with main chain of Hf-C,Si-C and CH=CH groups can be dissolved in tetrahydrofuran,xylene and N,N-dimethylformamide,whose contents of Hf and O are 49.29wt%and 1.19wt%,respectively.Due to Hf-C bond in the main chain and low oxygen content,PHCS could be directly converted into HfC nanocrystallites at 600?,which avoided the adverse effects of large crystal size and loose structure caused by formation of HfO₂.The ceramic yield of PHCS at 1000? is57.49wt%.After annealing at 1600?,the obtained ceramic product is composed of HfC with grain size of 20?70nm,amorphous SiC and free carbon.The weight loss of ceramic product in argon at 1600?2400? is 2.77wt%.The formation of?Hf,Si?C solid solution and SiC grains facilitates the sintering and oxidation resistance of the ceramic.The mass ablation rate and line ablation rate of hot-pressed ceramic under plasma with heat flux density of 6.7MW/m² are 1.73mg/s and 19.00?m/s,respectively.In order to improve the melting properties of PHCS,modified polyhafnocarbosilane?m-PHCS?were synthesized by hydrosilylation reaction of flexible low molecular weight polycarbosilane?LPCS?with PHCS.The composition,structure and inorganication process of m-PHCS,high temperature evolution behavior of pyrolytic products,and ablation resistance of the hot-pressed ceramics were well studied.The results show that by increasing the amount of LPCS,the content of C=C double bond in PHCS main chain decreased gradually.Meanwhile,the solubility of precursors increased from 43g to 70g,and the softening point decreased from 186? to162?.m-PHCS pyrolysed at 600? was directly converted into HfC microcrystalline and amorphous SiC with ceramic yield of 60.39wt%?68.23wt%.After annealed at1600?,the ceramics are mainly composed of HfC,SiC nanocrystals and free carbon.The ceramics showed excellent high temperature performance with weight loss of only0.51wt%at 1600?2200? in argon.The increase of Si content is beneficial for the oxidation resistance of ceramics.The mass ablation rate and line ablation rate of the hot-pressed sintered ceramics under plasma can be as low as to 1.47mg/s and 14.38?m/s.Ployhafnoazocarbanes?PHNC?with Hf-N bond in the main chain were synthesized via amine displacement reaction,with HfCl₄,ethylenediamine and allylamine used as raw materials and TEA/DMF or TEA/AACN used as solvent ligands.The synthesis processes and mechanisms of precursors were studied,and the composition and properties of precursors were characterized in detail.The results show that PHNC contains Hf-N,C-N,N-H and C=C groups,with better flexibility caused by the large bond angle and polarity of Hf-Cl bond.In addition,DMF or AACN can act as electron supply coordination to coordinate with Hf for further increasing the bond angle and polarity of Hf-Cl.Thus,the obtained precursors showed good melt drawability,which can be used for the preparation of fibres.The contents of Hf and O of ployhafnoazocarbane?PHNC-6?coordinated by DMF are 47.71wt%and 3.80wt%,respectively,and the contents of Hf and O of ployhafnoazocarbane?PHNC-11?coordinated by AACN are 51.75wt%and 0.62wt%,respectively.The inorganicization process of PHNC-6,high temperature evolution behavior of pyrolytic products and ablation resistance of the hot-pressed ceramics were studied.The results show that the introduction of oxygen caused by DMF coordination contributed to the formation of HfO₂ crystal at 400?.The precursor showed a ceramic yield of55.38wt%after pyrolyzed at 1000?,with pyrolytic products mainly composed of Hf₂ON₂ nanocrystals,HfO₂ nanocrystals and amorphous carbon.The pyrolytic ceramics annealed at 1400? for 5h or at 1600? for 1h could be completely converted into HfC nanocrystals with some free carbon.The conversion process is based on nitrogen-catalyzed gas-solid reaction between Hf₂ON₂/HfO₂ and CO.The weight loss of the ceramic product is 1.56wt%at 1600?2400? under argon atmosphere.The mass ablation rate and the line ablation rate of the hot-pressed sintered ceramic under plasma are 2.75mg/s and 11.00?m/s,respectively.The inorganicization process of PHNC-11,high temperature evolution behavior of pyrolytic products and ablation resistance of hot-pressed ceramics were studied.The results show that the low oxygen content makes HfC crystallized at 600?.The precursor showed a ceramic yield of 48.44wt%at 1000? with pyrolytic products mainly composed of Hf₂ON₂ and amorphous carbon.The weight loss of the pyrolytic product is 5.44wt%at 1000?2000? under argon atmosphere.The pyrolysis ceramic at1400? is mainly composed of Hf₂ON₂ and free carbon.A very small amount of Hf₂ON₂ was decomposed into HfO and then converted into Hf₂CN by reacting with CO through gas-gas reaction.The pyrolytic ceramics at 2000? are composed of Hf₂ON₂,free carbon and a small amount of Hf₂CN,all of which were completely converted into HfC nanocrystals and graphite at 2400?.The mass ablation rate and line ablation rate of the hot-pressed ceramic under plasma are 2.47mg/s and 20.38?m/s,respectively.PHNC-11 was used as raw material to prepare ultra-high temperature ceramic fibres by melt picking,crosslinking and high temperature treatment.The rheological properties,crosslinking method and mechanism of PHNC-11 were studied,and the composition and structure of ceramic fibres were investigated.PHNC-11 is a stable melt at 113?290?,showing apparent viscosity decreases with temperature increasing.The precursor presents a value of about 55kJ/mol for the activation energy of viscous flow and the optimum temperature of 180?190? for forming fibres.The diameter of PHNC-11 fibres is 6?25?m,with smooth surface and no obvious defects.PHNC-11 fibres can be crosslinked by ultraviolet light or electron beam irradiation combined with borane gas phase.UV light could cross-link C=C,C=N on the surface of the fibres,but showed little effect on the internal structure.Electron beam irradiation can crosslink C=C,N-H,C?N,and CH_x in the fibre surface.After these pre-crosslinking process,the fibres were further crosslinked by reacting borane with groups such as N-H,C=C and C=N.Non-melting PHNC-11 fibres were pyrolyzed at 1600? to transform into ceramic fibres,which mainly composed of HfC,HfB₂ and a small amount of amorphous carbon.The surface of the ceramic fibres is smooth with dense cross section.The chemical compositions of the ceramic fibres prepared by ultraviolet and electron beam irradiation combining with BH₃ crosslinking process were HfC_0.76B_0.53 and HfC_1.39B_0.80N_0.16O_0.15,respectively.