Investigation On Structure Stability Of Porous Mullite Fiber Networks Fabricated By Freeze Casting

With the rapid development of science and technology,especially the continuous improvement of aerospace,the requirements of sealing materials are also increasing.The density,strength and high temperature stability of sealing materials directly affect the performance and safety of products.In recent years,the polycrystalline mullite fibers have drawn more and more attention due to its famous properties such as low density,high strength,good high temperature stability and other advantages.The three dimension framework(network)directly affects the performance of short fiber ceramic.Good three dimension structure is helpful in keeping the original three-dimensional dispersion which leaded to both high porosity and strength.Inspired by the bird nest structure,freeze casting was first used to fabricate unoriented three dimension framework of fibrous porous ceramics.Compared with vacuum filtration,this method was helpful in keeping the original three-dimensional dispersion which leaded to both high porosity and strength.This paper has optimized the recipe of the raw materials used to fabricate fibrous porous ceramics and studied the influence of fiber aspect ratio,fiber content,the type and content of binder and sintering temperature on the bulk density,porosity,mechanical behavior and other properties of porous ceramic.The distribution of fibers in samples were observed and analyzed by X-ray tomography.We compared the differences of mechanical and high temperature mechanical of fabrication of mullite porous ceramics with freeze casting and vacuum filtration.The results indicated that t the optimum fibers and silica sol content were 2 vol% and 5 wt%,the fiber aspect ratio and sintering temperature is 25-50 and 1400 ?C,respectively,samples prepared by freeze casting shows an outstanding structure and properties(the bulk density is 0.33g/cm3,porosity is 88.9 %,the thermal conductivity is 0.051 W/(m·K)and the compressive strength is 2.27 MPa)).What’s more,the silica sol is more suitable than SiO2 power for high-temperature binder when the sintering temperature is 1400 ?C.Compared with vacuum filtration,freeze casting is more suitable for preparing porous ceramic with 3-D structure.It can be seen by X-ray CT that the sample prepared by vacuum filtration had a laminar type internal structure,with the fiber generally oriented parallel to the vacuum-formed surface.During the vacuum forming process,water-based slurry was drawn onto a screen,and colloidal binders adhered to the fiber surfaces which resulted in the fiber plane being perpendicular to the board thickness,thus creating anisotropy with respect to physical properties.Fibers in sample prepared by freeze casting show a uniform distribution and random orientation in 3D space.The structure of samples remains stable for about seven hours and showed lower thermal shrinkage and better stability because of the three dimensional orientations under a high temperature environment with a load of 70% of the maximum compressive strength at 1200 ?C.However,the samples with the same bulk density prepared by vacuum filtration can hardly remain stability under a high temperature environment with a load of 70% of the maximum compressive strength at 1200 ?C.The compressive strength of samples prepared by freeze casting is more than 2.5 times than samples prepared by vacuum filtration.