Preparation And Properties Of Morphology-controlled Mullite Fiber-based Elastic Thermal Insulation Ceramics

High temperature thermal insulation material is an important part of the hypersonic vehicle thermal protection system.In the process of passing through the atmosphere,hypersonic vehicles would encounter a high temperature extreme environment,and an extreme stress caused by the thermal expansion of thermal insulation materials would occur in the surface of the vehicle surface.This thermal stress could cause the cracking of the insulation materials,resulting in the failure of the heat sealing of the thermal insulation system,consequently threatening the safety of the aircraft.In this paper,in order to solve the mentioned thermal sealing problems,a single-stage mullite fiber-based lightweight elastic ceramics with a fixed node and a hierarchical mullit e fiber-based lightweight elastic ceramics with sliding joints were successfully fabricated according to the unique three-dimensional skeleton structure of the natural bird's nest.The microstructure and mechanical properties of the single mullite fiber and the microstructure,phase composition,physical properties,compressive rebound properties and thermal insulation performance of the mullite fiber-based ceramics were discussed across-the-aboard.Firstly,the mechanical properties of single mullite fiber?including the tensile strength and the compressive strength?and the bonding strength of the binder/fiber interface were investigated.With the temperature increasing from 1000^oC to 1500^oC,the average tensile strength of the mullite fiber decreased from 0.954 GPa to 0.191GPa,the Weibull modulus of the tensile strength distribution of the mullite fibers decreased from 5.1 to 1.8,the Young's modulus of the mullite fiber decreased from220 GPa to 170 GPa,and the compressive strength of the mullite fiber decreased from0.547 GPa to 0.096 GPa,which was attributed to the growth of the grains in the mullite fiber and the decrease of the homogeneity in the mullite fiber.On the contrary,as the temperature increased,the bonding strength of the binder/fiber interface increased and the bonding strength of the SiO₂-B₂O₃ binder/fiber interface was higher than that of the SiO₂-Al PO₄/fiber interface at the same temperature.The above results suggested that the sintering temperature and the content of the binder would play important roles in the properties of the fibrous ceramics.Inspired by the bird's nest structure in nature,mullite fibrous ceramics with fixed node were synthesized by the infiltration method using the mullite fiber as the matrix and SiO₂-Al PO₄ as the high-temperature binder.Products with low density?0.549⁰.619 g/cm³?,relatively high compressive strength?0.96².38 MPa?,low Young's modulus?13.1³8.9 MPa?and low thermal conductivity?0.152⁰.176W/?m·K??were successfully fabricated after sintering at different temperatures?1000¹500^oC?and the with the addition of different binder content?15³0 wt%?.Combined with the analyses of the mechanical properties of the single mullite fiber and the bonding strength of the binder/fiber interface,the stress transferring path can be divided into two ways.If the bonding between the fiber/binder was very firm,the stress could effectively transfer into the interior of the products.However,if the bonding between the fiber/binder was weak,the stress would firstly cause the densification of both ends of the sample and then gradually transferred into the interior of the sample,finally leading to the compaction of the whole sample.The compression-rebound tests showed that the fibrous ceramics exhibited excellent elastic property with the compression-resilience ratio higher than 90%.The deformation of the products was provided by the bending of the mullite fiber in the fibrous skeleton,and the high strength and the rebound property were attributed to the fixed nodes in the fibrous skeleton.In order to simplify the preparation process of mullite fibrous ceramics and decrease the sample density,polymethylsilsesquioxane?MK resin?was selected as both the room temperature binder and high temperature binder,and the molding method was used to fabricate the mullite fibrous ceramics.The results showed that compared with the mullite fibrous ceramics with SiO₂-Al PO₄ binder fabricated by filtration method,the mullite fibrous ceramics with MK resin binder fabricated by molding method exhibited lower density,lower thermal conductivity and higher porosity,and was a potential high-temperature elastic thermal insulation material.To improve the stress sensitivity of the products,mullite fibrous ceramics with hierarchical structure were successfully designed and synthesized using sol-gel method by introducing alumina borate whiskers and alumina platelets as the sliding nodes.In the experiment,by adjusting the mole ratios of Al/B and the sintering temperature,the mullite fibrous ceramics with the hierarchical structure(Al₄B₂O₉nanowhiskers/mullite fiber,Al₁₈B₄O₃₃ nanowhiskers/mullite fiber and Al₂O₃ plate Al₁₈B₄O₃₃ nanowhiskers/mullite fiber)were successfully fabricated.The growth process of the formed aluminium borate nanowhiskers followed the VLS mechanism.The fabricated mullite fibrous ceramics with hierarchical structure possessed a low density of 0.452⁰.468 g/cm³,high porosity of 81.1⁸3.3%,high specific surface area of 79.2²30.7 m²/g,low Young's modulus of 20.1²5.2 MPa and low thermal conductivity of 0.132⁰.152 W/?m·K?.Besides,the mullite fibrous ceramics with hierarchical structure also presented excellent elastic property with high compression-resilience ratio of 100%and relatively high strain under low stress,which was derived from the bending of the mullite fiber and the sliding of the intersected whiskers.