Synthesis Of HFC,HfB₂ Organic Precursors And Its Application In Preparation Of C/C-HfB₂-SiC Composites

Ultra high temperature ceramics?UHTCs?have been widely used in anti-oxidation modifications of carbon materials especially C/C composites for their excellent high temperature oxidation resistance and thermal shock resistance.Precursor infiltration and pyrolysis?PIP?method is one of the frequently-used methods in the above anti-oxidation modifications,and then organic ceramic precursors are necessary raw materials of PIP method.However,although there are organic precursors of Hf-containing UHTCs for sale at present,but they are very rare and expensive,the synthesis routes and transformation mechanisms of them are still uncertain,and related research reports of them and their applications in preparation of C/C composites by PIP method are also few.Therefore,a series of basic researches have been carried out in this paper based on the above background,the main work content and conclusions are as follows:1.Two kinds of souble organic precursors for ultra-fine hafnium carbide?HfC?were synthesized via series of substitution and polycondensation reactions using hafnium tetrachloride as hafnium source,acetylacetone as complexing agent,anhydrous methanol as solvent,1,4-butanediol or hydroquinone as carbon source,respectively.The pyrolysis behaviors of the HfC precursors were investigated.?1?The HfC precursor synthesized by using 1,4-butanediol as carbon source can transform into HfC and HfO₂ after heat treatment,but the pyrolysis C provided by this carbon source are not enough for HfO₂ completely transform into HfC.?2?The ceramization of the HfC precursor synthesized by using hydroquinone as carbon source starts at about 600 ?,and the information of the HfC ceramics is initiated at about 1300 ?.At a temperature above 1500 ?,the precursor completely transform into HfC ceramics and free carbon.The as-obtained HfC phaes has a single phase crystal structure and the size of the HfC particles ranges from 50 nm to 100 nm.The formation of the HfC ceramics can be attributed to the carbothermal reduction reaction of the HfO₂,which is produced by pyrolysis of the HfC precursor at a relative low temperature.The growth of the HfC particles is retarded by the free carbon formed during pyrolysis of the precursor.2.BN precursor liquid polyborazine?PBN?was synthesized by using BCl₃,NH₄ Cl and so on as raw materials.And then preparation procedures of HfB₂ organic precursor was explored through liquid way by using HfC precursor prepared from hydroquinone as hafnium,PBN or pyridine borane as boron source.The pyrolysis behaviors of the liquid HfB₂ precursors were investigated.?1?Liquid HfB₂ precursor has been prepared by using PBN as boron source,as-synthesized HfC precursor as hafnium,with the B/Hf molar ratio of 6:1.The precursor can transform into HfO₂ after heat treatment at 1000 ?,and the as-formed HfO₂ begins to change into HfB₂ at about 1300 ?.The formation of HfB₂ get finished at 1500 ?.The crystal structure of the as-obtained HfB₂ phaes is unobvious,and HfB₂ particles grow up while the temperature rise over 1500 ?,the size of the HfB₂ particles ranges from 100 nm to 200 nm.?2?Liquid HfB₂ precursor has also been prepared by using pyridine borane as boron source,as-synthesized HfC precursor as hafnium,with the B/Hf molar ratio of 12:1.The precursor can transform into HfO₂ after heat treatment at 1200 ?,and the as-formed HfO₂ begins to change into HfB₂ at about 1400 ?.The formation of HfB₂ get finished at 1600 ?.The as-obtained HfB₂ phaes has a single phase crystal structure,showing a shape of hexagonal columnar and the average size of the HfB₂ particles is less than 100 nm.?3?The formations of the two kinds of HfB₂ ceramics can all be attributed to the carbothermal/boronthermal reduction reaction of the HfO₂,which are produced by pyrolysis of the HfB₂ precursors at a relative low temperature.Both particle sizes are uniformly distributed,indicating that the boron source and hafnium source are all evenly dispersed;but the latter has more obvious crystal structure and smaller particle size,which may be attributed to the higher activity of pyridine borane.3.C/C-HfB₂-SiC composites with densities about 1.8 g/cm³ were prepared through impregnation and pyrolysis?PIP?using C/C composites with densities about 1.35 g/cm³,impregnants consist of xylene?50 wt%?and mixed HfB₂/SiC precursors with mass ratios of 7:1,9:1,11:1,17:1,respectively.Effects of ablation time and HfB₂/SiC precursor mass ratio on the ablation property of the as-prepared C/C-HfB₂-SiC composites were investigated.?1?The anti-ablation properties of C/C-HfB₂-SiC composites reduce firstly and then enhance subsequently with the increase of the mass ratio of HfB₂/SiC precursors,and they are all better than the one which has not been modified,after ablation for 80 s.The C/C-HfB₂-SiC composites prepared with a HfB₂/SiC precursors mass ratio of 7:1 has the best anti-ablation property,the linear ablation rate and mass ablation rate are 5.33�10-2 mm�s^-1 and 1.971�10-3 g�s^-1,respectively.?2?The anti-ablation property of the C/C-HfB₂-SiC composites prepared with a HfB₂/SiC precursors mass ratio of 7:1 is stable after ablation for 40 s,80 s,120 s,respectively.?3?Different anti-oxidation coatings are formed on the three ablation regions of C/C-HfB₂-SiC composites after ablation,which can reduce the diffusion rate of oxygen and therefore the ablation properties of the composites are improved.