Preparation Of Thermal-stable Mullite Fibers/mullite Whiskers Aerogels And Mechanism Of In-situ Formation Of Mullite Whiskers

On the background of thermal insulation protective materials with low thermal conductivity and to satisfy the demand of high temperature resistance/low density/high strength,this study focused on the mechanical brittleness,thermal instability of traditional SiO₂-Al₂O₃ aerogels.Secondary-pore structure was constructed by in-situ synthesized crystalline mullite whiskers converted from amorphous oxide aerogel particles on mullite fibers skeleton.Two routes,internal addition and external addition of catalyst,were designed to prepare the mullite fibers/mullite whiskers aerogels composite material?MF/MW?.The key parameters during these two routes were systematic studied.And the in-situ generation of MW during vapour-solid reaction process and the mechanism of improving thermal stability had been discussed in detail.In catalyst external addition process,it was found that the length and diameter of in-situ growth mullite whiskers were 10?m²0?m and<1?m,respectively.And the whiskers interlaced with each other to form many secondary-pores with diameters ranging from 1?m to 5?m,partly filled the macropores of MF preform.However,the morphology and properties of MF/MW were not uniform and stable due to the different concentration of catalytic gas during diffusion.In the internal addition process,AlF₃ was uniformly introduced by vacuum immersion and freeze-drying techniques.After?NH₄?₃AlF₆ decomposed into AlF₃ and high temperature vapour-solid reaction,the MF/MW with homogeneous structure and excellent properties was prepared.The secondary-pore structure formed by overlapped fibers and whiskers in MF/MW reduced thermal conductivity(as low as 0.049 W?m^-1?K^-1,which was much lower than that of MF preform 0.070 W?m^–1?K^-1)and exhibited excellent thermal stability after 1400°C heat treatment(0.050 W?m^-1?K^-1)due to the macropores were decreased and gaseous heat transfer being effectively weakened.Moreover,the secondary-pore structure deformed to partly cushion the external stress and improve the mechanical strength of MF/MW.The compressive strength of MF/MW under 10%strain was 0.326 MPa,which was more than 247%higher than that of MF preform?0.094 MPa?.It was shown that constructing secondary-pore structure was an effective way to improve the compressive strength of fibers preform.