Fabrication And Properties Of The Mullite Nanofibrous Porous Ceramics

Mullite fibrous porous ceramics are one of the most commonly used high temperature（>1300°C）insulation materials in the fields of aerospace and energy industries owing to their low density,high porosity,excellent thermal shock resistance and low thermal conductivity.However,to keep up with the demand of the thermal protection,it is a challenge to improve the sample strength dramatically without sacrificing the low sample density.In order to solve this problem,we designed a nanofibrous porous ceramics with the three-dimensional porous skeleton formed by mullite nanofibers,based on the idea of the fabrication of the micron-fiber porous ceramics.And we investigated the effects of different preparation techniques and parameters on the properties of the mullite nanofibrous porous material.The key to the preparation of lightweight and high strength thermal insulation materials is to obtain mullite fibrous porous ceramics with uniform three-dimensional skeleton structure.In this paper,mullite nanofibrous porous ceramics with different agarose contents were prepared via agarose gel-casting using the electrospun ceramic nanofibers.The density of the materials could be effectively controlled by changing the content of agarose.The sample with 1.0wt%agarose content in the slurry has a minimum density of about 0.348 g/cm³ and a strength of 1.76 MPa.However,due to the excessive shrinkage（46.35%）,the density of the sample is too high,and the thermal conductivity is high(0.127 Wm^-1K^-1).To obtain low thermal conductivity samples,the mullite nanofiber porous ceramics were prepared by tert-butyl acrylamide gel-casting.The samples increased from 0.114 MPa to 0.158 MPa with the solid content of nanofibers increasing from 1.5wt%to 2.5wt%.In addition,the effect of fiber length on porous ceramics with nanofibers was investigated.Comparing with the samples using the short fibers,the samples using the long fibers as the raw materials show an ultralow density（0.062 g/cm³）and low thermal conductivity(0.0597 Wm^-1K^-1).The mullite nanofibrous ceramics with the density of 0.202 g/cm³ show a much higher compressive strength（0.837 MPa）than that of the mullite micron-fiber ceramics（0.515 MPa）with a little higher density（0.266 g/cm³）.The high-strength mullite nanofibrous porous ceramics is a promising high-temperature insulation material in various thermal protection systems.