Preparation And Properties Of Mullite Fibrous Porous Ceramics Based Thermal Inslution System

To meet the urgent need of hypersonic vehicles for the reusable ceramic fibrous insulation tiles,the preparation,characterization and performance evaluation of mullite fibrous porous ceramics,high emission coatings and high-temperature-resistant adhesives were symmetrical investigated.In order to solve the problems of the low strength of mullite fibrous porous ceramics,the poor matching between high emission coating and matrix,and the high brittleness of high temperature adhesives,the mullite fibrous porous ceramic anti-insulation component with excellent performance was successfully prepared by structural regulation of mullite micron(nano)fibrous porous ceramics,high emission coating composition control and whisker toughening of high temperature adhesive.The main research results are as follows:As the most important thermal insulation component in thermal protection system,how to improve the strength of mullite fibrous porous ceramics while ensuring low density and low thermal conductivity is always a difficult problem in the field of thermal protection.In this study,silicon powder with both fluidity and temperature resistance was used as the high temperature binder,which effectively solved the shortcomings of poor wettability of SiO₂ and poor temperature resistance of SiO₂-B₂O₃.The experimental results showed that,when the mass ratio of fiber to silicon was 1:0.25,the mullite fibrous porous ceramics prepared by gel-casting process exhibited low density(0.43 g/cm³),low thermal conductivity(0.091 W/mK),high porosity(85.25%)and high compressive strength(1.21 MPa).In order to further improve the strength of mullite fibrous porous ceramics,the idea of using mullite nanofibers instead of mullite micron fibers to prepare new mullite nanofibrous porous ceramics with high strength was proposed from the perspective of improving the intrinsic characteristics of the internal structural unit(single mullite fiber).During the preparation process of mullite nanofibers,in order to overcome the poor temperature resistance of mullite nanofibers,the design idea of biphasic mullite spinning precursor based on polymethylsiloxane was first put forward,and the biphasic electrospun mullite precursor was successfully obtained.The physical properties of the mullite nanofibrous porous ceramics were successfully controlled by adjusting the amount of glycerol and agar during the freeze casting process.The results show that when the densities of two kinds of samples were both0.30 g/cm³,the strength of mullite nanofibrous porous ceramics(1.61 MPa)was twice that of traditional mullite micron fibrous porous ceramics(0.73 MPa).MoSi₂-borosilicate glass high emissivity coating was prepared on the mullite fibrous porous ceramics by a two-step method to increase surface radiation and reduce surface temperature.The composition of the coating binder was carefully regulated in order to improve the structural compatibility between the glass coating and the porous substrate,and thereby the oxidation of the emittance agents was restrained at a wide temperature range.A dense MoSi₂-borosilicate glass coating with excellent thermal shock resistance,high interface bonding strength and emissivity of 0.89 was prepared.As the high brittleness of traditional aluminium phosphate-based high temperature resistant adhesives,it is difficult to ensure the safety of bonding parts under conditions of the multi-stress impacting and thermal cycling.Therefore,a novel strengthening method by mullite whiskers in situ growing was proposed in this chapter to toughen and strengthen aluminium phosphate-based high temperature resistant adhesives.The results showed that both strength and toughness of whisker-growth adhesive got greatly improved after calcination at 1300°C,which were increased by 75%and 156%,respectively,and the corresponding bonding strength was enhanced to 23.81 MPa.