The Preparation And Properties Of Mullite Porous Hierarchical Fibrous Material

As a kind of high temperature structural materials,mullite fibers framework materials(MF)constructed by the mullite fibers have the advantages of low density,high porosity,low thermal conductivity,good resistance to thermal shock and can be widely used in aerospace and industrial fields.However,the low mechanical property of MF limited their applications in the further.Compared with MF,mullite fibers/whiskers hierarchical framework materials(MFW)can improve compressive strength in a certain extent,which is beneficial to be used in a wider range.The three dimensional network structure-adjustable MFW were prepared by threestep method.The main structure of MFW was constituted by MF consisting of mullite fibers.Mullite grains were formed on the fibers firstly,and then the secondary structures of mullite whiskers were formed from the grains by two-step heat treatments.The growth mechanism of mullite whisker prepared by three-step method and the change of microstructure and properties of different MFW were emphatically studied.Based on fractal theory,the influencing factors of effective thermal conductivity of MFW were discussed.The objective of this paper is to prepare MFW with low density,low thermal conductivity,high porosity and high compression strength at lower temperatures.During the preparing process,low-density MF were prepared by pressurized drainage and freeze casting,and the influences of heat treatment temperature on the surface morphology of fiber and binder were analyzed.The results show that mullite grains were inoculated on the fiber surface of MF after heat treatment at 1200 ℃.These grains were used as mullite grains to reduce the temperature of secondary structure formation.Silica sols,alumina sols and Al F3 were introduced into MF through immersion hydrothermal and freeze casting.The influences of process parameters,i.e.hydrothermal temperature,heat treatment temperature,on microstructure of active powder and secondary structure were analyzed.The hydrothermal reaction causes the rearrangement of the structure in the active powder,which is beneficial to the uniform distribution of the active powder on the fiber surface and provides some energy for the uniform growth of the secondary structure.The mullite seeds and the active powder of uniform distribution contribute to the growth of mullite whiskers on the fiber surface at 800 ℃.The length of whisker and the density of MFW were adjusted by changing the MF density and the amount of raw materials.The microcosmic morphology and pore structure test results show that a new pore size distribution appeared by introducing the secondary structure in MF.Some new pores with a diameter of 0.1～10 μm were found in MFW.But in MF,there were the pores with a diameter of more than 10 μm.Under the influence of MF density and active slurry concentration,the density of MFW gradually increased and maintained between 0.06 g/cm3 and 0.016 g/cm3.The compressive strength was between 0.016 MPa and 0.074 MPa.The thermal conductivity was between 0.0378 W/(m·K)and 0.0514 W/(m·K).All of these property parameters prove that MFW can be used as high temperature thermal insulation materials.Based on fractal theory,the fractal models of MF and MFW were built.Their porosity and thermal conductivity were simulated by using the fractal resistance network of series-parallel combination.The simulation results showed that the porosity,pore size and their spatial distribution possess on a great influence on the effective thermal conductivity of MFW,and the thermal conductivity of the fluid phase determined the minimum MFW can achieve at high porosity.