Synthesis And Pyrolysis Behavior Of Ta₄HfC₅ And SiC-Ta₄HfC₅ Liquid Phase Precursors

Ta₄HfC₅ ceramics have the highest melting point?4215K?among the known compounds,and have excellent resistance to oxidation and ablation.It is one of the ideal material systems for extreme thermal environment applications.At present,the preparation methods of Ta₄HfC₅ ceramics mainly include sintering method,direct chemical method,solid phase reaction method and sol-gel method.Ta₄HfC₅ ceramics prepared by sintering,direct reaction or solid state reaction have low purity and poor mechanical properties,which is difficult to meet the use requirements.The sol-gel method has the advantages of low reaction temperature and uniform ceramic particles,but also has the disadvantages of high raw material cost,long process cycle and difficult process control.The liquid precursor pyrolysis method overcomes the shortcomings of the sol-gel method,and the ceramic particle size is small and the composition is uniform.However,there are few reports on the preparation of Ta₄HfC₅ ceramics via liquid precursor pyrolysis method.Based on the above background,Ta₄HfC₅ ceramic prepared via liquid precursor pyrolysis method is proposed in this paper.On this basis,the high temperature properties of Ta₄HfC₅ ceramic is improved by doping Si C,the main work content and conclusions of this paper are as follows:1.The precursors containing Ta and Hf were synthesized via liquid phase precursor method.The effects of carbon source?1.4-butanediol,hydroquinone,and phenolic resin?and the molar ratio of Ta and Hf on the composition and structure of precursor cracking products were investigated.The pyrolysis behavior of the Ta₄HfC₅ precursor is investigated by FT-IR,XRD and TG-DSC,and the solid solution mechanism of Ta C and HfC was investigated.?1?The precursor synthesized by using 1,4-butanediol as carbon source can transform into HfC,HfO₂ and Ta C at 1800°C,but pyrolysis C provided by1,4-butanediol is not enough for HfO₂ completely transform into HfC.Although the precursor synthesized by using hydroquinone as a carbon source can provide sufficient pyrolytic carbon for carbothermal reduction,the small molecular weight of hydroquinone can not provide sufficient dispersion for Ta atoms and Hf atoms.so Ta C and HfC are the final products of precursors synthesized by using hydroquinone as carbon source at 1800°C.The precursor synthesized by using phenolic resin as carbon source can successfully transform into Ta₄HfC₅ ceramics at 1800°C.?2?The molar ratio of Ta and Hf has an important effect on the composition of precursor cleavage products.When the molar ratio of Ta:Hf is 4:1,the product of the precursor at 1800°C is Ta₄HfC₅ ceramic;when the molar ratio of Ta:Hf is 1:1,the product of the precursor at 1800°C is Ta HfC₂ ceramic;when the molar ratio of Ta:Hf is1:4,the product of the precursor at 1800°C is HfC and Ta_xHf_1-xC ceramic.It shows that as the molar ratio of HfCl₄ increases,the synthesis of solid solution ceramics becomes more difficult.?3?The pyrolysis of Ta₄HfC₅ ceramic precursor was studied in this paper.The precursor decomposes completely at about 600°C,after crystal transformation,carbothermal reduction reaction,and solid solution reaction,Ta₄HfC₅ ceramics were formed at 1800°C.The as-obtained Ta₄HfC₅ phaes has a single phase crystal structure and the size of the Ta₄HfC₅ particles ranges from 50nm to 100nm.The Ta₄HfC₅ crystal is wrapped by an amorphous carbon layer with a thickness of 2?3nm.And the growth of the Ta₄HfC₅ particles is retarded by the free carbon layer.The formation of Ta₄HfC₅ceramic phase is produced by the solid solution reaction of Ta C and HfC.Accelerating the reaction rate of Ta C and HfC and reducing the reaction energy barrier of Ta C and HfC are the keys to achieve low-temperature synthesis of Ta₄HfC₅ ceramics.?4?The solution process of TaC and HfC was further tracked by the pyrolysis of the TaHfC₂ ceramic precursor.It is found that the essence of the solid solution reaction of TaC and HfC is the interdiffusion between Ta atom and Hf atom,so that Ta C and HfC gradually form a single-phase solid solution.The diffusion rate of Hf atom is faster than the Ta atom.The Ta HfC₂ ceramic structure is similar to the Ta₄HfC₅ ceramic,the particle size of the Ta HfC₂ ceramic is 200?300nm,and the crystal is surrounded by a layer of amorphous carbon with a thickness of 2?3nm.2.Two kinds of SiC-Ta₄HfC₅ ceramic precursors were synthesized via sol-gel method and solution method using TEOS as silicon source.The effect of doping amount of TEOS on the composition and structure of pyrolysis precursor products was investigated.The pyrolysis of SiC-Ta₄HfC₅ ceramic precursor was studied by means of FT-IR,XRD,TG-DSC.Based on the above research work,the main work content and conclusions are as follows:?1?The SiC-Ta₄HfC₅ ceramic precursor is successfully synthesized via the sol-gel method and the solution method using TEOS as the silicon source.The formation of HfSiO₄ during the pyrolysis of SiC-Ta₄HfC₅ precursor can promote the solid solution reaction of HfC and TaC,and reduce the formation temperature of Ta₄HfC_5.The chemical bond force of the HfSiO₄ crystal is smaller than the HfC crystal,and energy required for the Hf atom to detach from the HfSiO₄ crystal lattice is less than the energy required to detach from the HfC crystal lattice.?2?The molar ratios of Si,Ta and Hf have an important effect on the composition of the pyrolysis products of precursor synthesized via sol-gel method.When the molar ratio of Si:Ta:Hf is 4:4:1,the pyrolysis products of precursor is Ta₄HfC₅ and a small amount of SiC;When the molar ratio of Si:Ta:Hf is 6:4:1,the pyrolysis products of precursor is Ta_xHf_1-xC,HfC and SiC;When the molar ratio of Si:Ta:Hf is 8:4:1,the the pyrolysis products of precursor is Ta_xHf_1-xC,HfO₂ and SiO₂.?3?The SiC-Ta₄HfC₅ precursor synthesized via the sol-gel method transform into Ta₄HfC₅ and SiC at 1600°C,and the particle size of the ceramic powder is 300?400nm.The SiC-Ta₄HfC₅ precursor synthesized via the solution method under the same conditions transform into Ta₄HfC₅ and SiC at 1600°C,and the particle size of the ceramic powder is in the range of 50?100nm.