Preparation Of Mullite Fibrous Porous Ceramics In-situ Consolidated By Propylene Oxide

High temperature thermal protection system has attracted broad attention since last century.With the development of the hypersonic aircraft,the required performance of the material used in high temperature thermal protection system has become more demanding in many countries.The interior and external three-dimensional structures of the fibrous porous ceramic directly influence the thermal conductivity,weight and mechanical properties of the thermal protection system.To prevent the diffusion of silica particles from the inside to the outside of the sample during the drying process and prepare homogeneous mullite fibrous insulation tiles,a propylene oxide catalyzed sol-gel in-situ consolidation process was introduced into the fabricating procedures.In this paper,mullite fibrous insulation tiles were fabricated by molding method using mullite fibers as the matrix,ethyl orthosilicate as binder and propylene oxide as a catalyst.The necessary conditions of silica sol gelation were studied and the feasibility of in situ consolidation of silica sol was verified.Factors which had influence on the density and the compressive strength of the mullite fibrous insulation tiles were also investigated by changing the content of ethyl orthosilicate or propylene oxide.The results showed that:the density of mullite fiber insulation tiles increased from 0.507g/cm³ to 0.598 g/cm³ with the increase of silica sol concentration from 9 wt%to 18wt%,meanwhile the average strength of the sample increased from 0.73 MPa to 1.72MPa.However,with the molar ratio of PO/SiO₂ increased from 0.43 to 1.72,the density and strength of the sample maximized at 0.551 g/cm³ and 1.53 MPa,respectively,the molar ratio of PO/SiO₂ being 0.86.Further more,by introducing silicon boron mixed sol instead of silica sol as binder and using propylene oxide induced gelling in-situ curing method,mullite fiber insulation tiles with better mechanical properties were obtained by changing the molar ratio of silicon to boron and the sintering temperature.The specimens with different Si-B molar ratio all reached the maximum strength when sintered at 1200?,in which the specimens with Si-B molar ratio of 7:3 and 6:4 reached 4.6 MPa and 6.6 MPa,respectively.