Preparation Of Porous(Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δ High-entropy Ceramics By Adding Pore-forming Agent

Hypersonic aircraft has increasingly become the strategic commanding heights in the global aerospace field.The thermal environment brought by long time high Mach flight and aerodynamic heating is extremely harsh,which directly affects the service safety of aircraft.Thus,higher requirements for thermal protection materials of hypersonic aircraft are put forward.In the field of hypersonic thermal protection,porous ceramics stand out because of their excellent properties such as high melting point,low thermal conductivity and high specific strength.High-entropy ceramics is a new material developed in recent years.Among those,high-entropy fluorite ceramics（HEFCs）have both low thermal conductivity and excellent mechanical properties,so it is a good choice for porous ceramic matrix.In order to develop excellent thermal protection materials,(Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δHEFCs was selected as the matrix,and porous materials have been in-situ prepared by adding pore-forming agent in this thesis.Meanwhile,the influences of important process parameters on the composition,structure and properties of porous ceramics have been researched.Taking CeO₂、ZrO₂、TiO₂、SnO₂and CaCO₃ powder as main raw materials,corn flour with different particle size as the pore-forming agent,fluorite structure(Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δporous high-entropy ceramics have been successfully in situ prepared.The main factors of the experiment were studied through orthogonal experimental design,and the results showed that the sintering temperature and the amount of pore-forming agent are the significant factors which affect the phase composition,sintering shrinkage,apparent porosity and thermal conductivity.The particle size of pore-forming agent was not significant factor.The optimal experimental scheme was determined as following:particle size of pore making agent below 0.075mm,dosage of pore making agent 40 wt.%and sintering temperature 1420℃.In view of the significant effects of sintering temperature and the amount of pore-forming agent,single factor experiments have been carried out respectively.The results of sintering temperature study show that single phase fluorite structure sample can be obtained when sintering temperature reaches 1420℃.As the sintering temperature continues to increase,the porosity of the sample decreases gradually in the range of 1400～1480℃.The grain size increases obviously and the thermal conductivity increases when temperature is above 1440℃.The amount of pore-forming agent has little effect on the sample composition and grain size,but has significant effect on the sintering shrinkage.When the amount of pore forming agent is more than 60 wt.%,the sintering shrinkage increases rapidly with the increase of the amount of pore forming agent,the change of porosity is small ranging from 72.2%to 74.3%,and the thermal conductivity varies from 0.035 to 0.040 W·m^-1·K^-1.Considering the properties,the economy and application,the optimal processing parameters are drawn.The particle size of pore forming agent is less than 0.075 mm,the sintering temperature is 1420℃and the amount of pore forming agent is 60 wt.%.The apparent porosity of the prepared material is 72.2%,the bulk density is 1.54 g/cm³,and the thermal conductivity is as low as 0.04 W·m^-1·K^-1.