Preparation And Properties Of Novel Ultra-high Temperature Thermal Insulation Materials

Hypersonic vehicles are inevitably subject to harsh environments in the service process,such as ultra-high temperature,high aerodynamic loading and high-speed airflow scouring.In addition,the scramjet engines of hypersonic vehicles are exposed to extreme environment conditions with ultra-high temperature water vapor and high aerodynamic pressure.Under such service environments,the present thermal insulation materials can not meet the requirements of thermal protection systems for future hypersonic vehicles.Therefore,searching for novel ultra-high temperature insulation materials with low density and high strength is of great significance for the development of hypersonic vehicles.Based on the above background,the preparation and properties of novel ultra-high temperature thermal insulation materials were investigated in this dissertation.The main contributions are as follows:1.In order to increase the porosity of porous ultra-high temperature ceramics(UHTCs),a novel method combining in-situ reaction and partial sintering is developed for preparing porous UHTCs.A new reaction for the synthesis of YB₂C₂was designed,and the porous YB₂C₂ceramics with high porosity(57.17%?75.26%)and high compressive strength(17.47?98.57 MPa)were successfully prepared through this in-situ reaction/partial sintering method using Y₂O₃,BN and graphite as raw materials for the first time.The relationship between the density of the green body and the sintered density,porosity,radial shrinkage and compressive strength was established.The results indicated that the microstructures and mechanical properties of porous YB₂C₂ceramics could be controlled by the density of the green body.2.In order to meet the needs of ultra-high temperature thermal insulation applications,porous transition metal carbides were prepared by the in-situ reaction/partial sintering method for the first time.The thermodynamics of carbothermal reduction reactions,as well as the gas-releasing and volume shrinkage process during carbothermal reduction reactions were investigated.The corresponding process parameters were designed.The carbothermal reduction processes were controlled by adjusting the heating rates of the reactions.The porous Zr C,Hf C,Nb C,Ta C and Ti C ceramics with excellent shape stability were successfully prepared by the in-situ reaction/partial sintering method.The porous transition metal carbides ceramics possess high porosity(68.74%?80.24%),small grain size(200?800 nm)and low thermal conductivity(0.63?1.12 W·m^-1·K^-1).The results indicated that the thermal conductivity of porous UHTCs could be effectively reduced by increasing the porosity and decreasing the grain size.3.In order to further reduce the thermal conductivity of porous UHTCs,porous high entropy(HE)UHTCs were prepared by the in-situ reaction/partial sintering method for the first time.The gas-releasing and volume shrinkage process during reactions were investigated.The corresponding process parameters were designed.The porous HE(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C and(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)B₂caramics with excellent shape stability were successfully prepared by the in-situ reaction/partial sintering method.The thermal conductivities of porous HE UHTCs are lower than that of single components.The RT thermal conductivities of porous HE(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C and(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)B₂are 0.39 W·m^-1·K^-1and0.51 W·m^-1·K^-1,respectively.In addition,porous HE UHTCs exhibit excellent high-temperature stability.The combination of unique properties makes porous HE(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C and(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)B₂promising as ultra-high temperature thermal insulation materials.4.For transpiration cooling applications,porous HE(Y_0.2Yb_0.2Sm_0.2Nd_0.2-Eu_0.2)B₆ceramics were synthesized through boron reduction reaction and boron carbide reduction reaction for the first time,respectively.The composition of HE rare earth metal hexaborides,as well as the boron reduction reaction and boron carbide reduction reaction for the synthesis of HE rare earth hexaboride were designed.The gas-releasing and volume shrinkage process,as well as phase evolution during boron reduction reaction and boron carbide reduction reaction were investigated.And the corresponding reaction paths were determined.Porous HE(Y_0.2Yb_0.2Sm_0.2Nd_0.2-Eu_0.2)B₆ceramics with high porosity(?70.92%),high compressive strength(?31.59MPa),low thermal conductivity(?1.81 W·m^-1·K^-1)and high permeability(?2.73×10^-11m²)are promising candidates for transpiration cooling applications.