Fabrication And Properties Of Mullite Fibrous Aerogels Based On Electrospun Fibers

Aerogels are well known for their ultralow density,ultralow thermal conductivity and ultrahigh porosity,playing a key role in thermal insulation and preservation systems of various aircrafts,missiles and furnaces.However,the 3D nanoporous microstructure of the traditional nanopartical-based aerogel would collapse at high temperatures（higher than 1200^oC）due to the ultrahigh activity of nanoparticals.Therefore,the fabrication of high-temperature-resistant aerogel materials that can be used for a long time above 1300^oC in air atmosphere has remained an extremely challenging.This paper jumps out of the thinking inertia of the research and development of nanoparticle-based aerogel system,and based on the design idea of nanofibrous aerogel widely used in the field of functional materials,this paper designed a mullite-based nanofibrous aerogel with the three-dimensional porous skeleton formed by high-temperature-resistant mullite-based nanofibers.Moreover,the preparation process of mullite-based nanofibers and the molding process of mullite-based nanofibrous aerogel were studied in detail.Obtaining high temperature resistant mullite-based nanofibers is the basis of this experiment.In order to solve the problem of low crystallization temperature of manophasic mullite precursor,a diphasic mullite precursor prepared by using aluminum tri-sec-butoxide as the aluminum source and polyhydromethylsiloxane as the silicon source was proposed,and the mullite-based nanofibers with different molar ratios of alumina to silica（3:2,3:1）were successfully prepared by electrospinning.The experiment results indicate that the diameter of mullite-based nanofiber prepared by the diphasic mullite precursor was about 450nm after sintering at 1400^oC,and there was no obvious crystallization on the fiber surface.In addition,pure alumina nanofibers were also fabricated in this study,due to the monophasic precursor prepared by aluminum source,the obtained alumina fibers partially melted after sintering at 1400^oC.On the basis of preparation of high-temperature-resistant mullite-based nanofibers,a kind of high-temperature-resistant mullite-based nanofibrous aerogel was successfully prepared via the gel-casting and freeze drying methods using the electrospun ceramic nanofibers with different alumina/silica molar ratios（3:2,3:1 and 3:0）as the matrix.All mullite-based nanofibrous aerogels show a similar multilevel pore structure.The major pores was caused by the sublimation of the ice crystal,while the minor pores were formed by the overlaps of nanofibers.This unique multilevel pore structure make the mullite-based nanofibrous aerogels still exhibit an low density（34.64⁴8.89 mg/cm³）,low thermal conductivity(0.03274⁰.04317 Wm^-1K^-1)at room temperature and low thermal conductivity(0.07341 Wm^-1K^-1)at high temperature（1000^oC）after sintering at 1400^oC.Therefore,the service temperature of aerogel materials is increased from 1200^oC to 1400^oC.These mullite-based nanofibrous aerogels prepared in this experiment have wide applications in the thermal protection system of spacecraft,high temperature furnace and so on.