Catalyticity Properties Of ZrB₂ Based Ultra High Temperature Ceramics

The development of hypersonic flight technology requires good properties for the thermal protection materials. Therefore, the excellent overall performances of ZrB₂-based ultra-high temperature ceramics (UHTCs) attract more attention, and can be qualified for hypersonic vehicle nose, wing leading edge and other key parts of the extreme aerodynamic heating environment, which meeting the aircraft structural integrity in severe requirements. It is a class of promising non-ablative ultra-high temperature structural materials. When a hypersonic vehicle flies in the relatively dense atmosphere, a strong bow shock takes place ahead of the vehicle because of the intense air compression, then the gas inside the shock layer is dissociated and ionizated, leading to the formation of a large number of atoms and ions. It has been found that, in the chemical non-equilibrium flow field, the dissociation enthalpy of the release of chemical recombination at the wall activated by the catalytic properties of the TPS has significant effects on the aerodynamic heating of the vehicle wall and aerodynamic heating environment. catalytic properties of the material can effectively improve the harsh service environment and reduce the stringent requirements of TPS. Therefore, the research of the ultra-high temperature ceramic material with low catalytic properties is an important development trend of non-ablative thermal protection materials.The surface catalytic behavior and thermal response of ZrB₂-based UHTCs in real service environment is performed by the self-designed equipment. The wall temperature response method and photochemical method, which based on the heat balance principle and spectral diagnostic techniques respectively, are developed to meet the qualitative and quantitative characterization of catalytic properties of UHTCs.The components of the material, termperature, and pressure are investigated to study their influences for the catalytic behavior of UHTCs. Experimental results show that, among the different components of the UHTCs materials (ZrB₂-20vol.%SiC-5vol.%W, ZrB₂-20vol.%SiC-5vol.%Cr, ZrB₂-20vol.%SiC, ZrB₂-30vol.%SiC), the catalytic efficiency of ZrB₂-SiC-W is the lowest, making a lower catalytic activity and a better thermal response, while the chemical reaction of ZrB₂-SiC-Cr is intensive with the highest catalytic efficiency. In the meantime, the pre-oxidation samples have a lower the highest surface temperature and less fluctuations of surface temperature compared with the original samples. That is because glass phase of SiO2 prepared by pre-oxidation treation has lower catalytic properities, and has changed the surface condition of material, making material shows lower catalytic activity. In addition, this paper also considered the impact of variation of temperature and gas pressure on material catalytic activity. The results indicate that the catalytic properties of UHTCs are increasing with temperatures increasing from 500°C to 1000°C. The relationship of catalytic behavior and temperature follows Arrhenius law. While the catalytic properties doesn't vary significantly for the pressure 50¹00Pa, and the catalytic efficiency under 50Pa is slightly higher than under 70Pa and 100Pa.