Preparation And Properties Of Mullite Whiskers/Fibers Porous Materials

The running speed of the aircraft is increasing with the development of the aerospace technology.As a result,the aircraft thermal protection system strength should have lower thermal conductivity and higher compressive strength.With the aggravation of energy crisis and the continuous growth of the population,energy conservation is becoming more and more important.And the demand for thermal insulation materials in industrial production is increasing.Mullite fiber porous material has the advantages of low density and low thermal conductivity.And it is widely used in industrial and aerospace fields.In this paper,the mullite whiskers were in situ synthesized on the surface of mullite fiber and high alumina refractory fiber by the method of vacuum impregnation and freeze drying.The microstructure of the frame works could be adjusted by tailoring the solution concentration,heat temperature and heat treatment time.At first,the silicon source,alumina source and fluorine source were introduced into the mullite fibrous brick in turn by the method of vacuum impregnation and freeze drying.After heat treatment,the whiskers were synthesized on the surface of fibers.When impregnated with 0.5 mol/L silica sol,1.5 mol/L Al?NO₃?₃ solution,6.0 mol/L NH₄F solution and heated at 1200^oC for 2 h,the compressive strength of the fibrous brick increased from 0.39 MPa to 1.45 MPa.By adjusting the concentration of impregnation solutions,a series of fiber felt with good resilience under different pressure were prepared.For the preparation of low density mullite fiber/mullite whiskers,the low density mullite fibrous brick?0.2 g/cm³?was prepared by the method of vacuum impregnation and freeze drying.The mass ratio of mullite fiber and SiO₂ was 1:1.The green body was gelled at 15^oC for 1 minute.After heated at 1500^oC for 2 h,the fibrous brick which had 0.24 MPa compressive strength was prepared.When impregnated with0.33 mol/L silica sol,1.0 mol/L Al?NO₃?₃ solution,4.0 mol/L NH₄F solution and heated at 1200^oC for 2 h,the compressive strength of the fibrous brick increased from0.24 MPa to 0.5 MPa.