Research Of The Controllable Synthesis And Applications On The Hierarchical Mullite Fiber Network

Ceramic fibers are widely used to fabricate the hot gas filter,because of their excellent corrosion resistance,high mechanical strength,and high heat and creep resistance at high temperature.The fibrous network usually has high apparent porosity and low fluid resistance,furthermore the intersected fibers can capture particulate effectively.The fibrous filter can be coated with catalyst,in which the captured soot particles are burnt at the temperature of the exhaust gases.The problem is how to create a filter which is suitable for the coating.In this thesis,a kind of active powder which was made up of alumina gel,silica gel and AlF3 powder was used to create secondary structure on mullite fiber network.Because of the introduction of the secondary structure,the BET surface area of the fibrous network increased by about 50 times and the particle filtration efficiency for the 0.5 ?m particles increased from 12.9% to 94.2%.In the preparation of the modified fibrous network,the emphasis was on the controllable synthesis of the secondary structure.We controlled the heat treatment temperature,the mass ratio of active powder/ mullite fiber,the type of fibers and so on,to study the reaction mechanism of the active powder and to controllably fabricate the hierarchical mullite fiber network.When the mass ratio of active powder/ mullite fiber was 1:1,after being heat treated at 700?,800?,1000? for 1 h,respectively,tassel-mullite/ mullite fiber,rod-topaz/ mullite fiber and mullite-whisker/ mullite fiber were fabricated.When the mass ratio of active powder/ mullite fiber was 0.3,after being heat treated at 1000? for 1 h,branching-mullite/ mullite fiber was fabricated.Using AlF3 powder instead of the active powder,and the mass ratio of AlF3 powder/ mullite fiber was 0.6,after being heat treated at 1200? for 1 h,hexagon-corundum/ mullite fiber was fabricated.Moreover,some conclusions about the phase transformation reaction of the active powder were drawn:?a?The effect of the growth space on the phase transformation essentially affected the composition of the gaseous intermediate.And SiF4 facilitated the anisotropic growth of the mullite whisker;?b?The mullite fiber promoted the formation of mullite phase;?c?The formation process of mullite-whisker/ mullite fiber was breeding on the mullite fiber surface first and then the mullite seeds kept growing up and branching,at last the mullite whiskers coated the mullite fiber.La_0.5Sr_0.5CoO₃ was used as catalyst for the combustion of carbon which was the substitution of the diesel soot.La_0.5Sr_0.5CoO₃ spreaded on the mullite whisker as nanoparticles.Under the atmosphere of NO?1000 ppm?and O₂?10%??He balance?,carbon in tight contact with the La_0.5Sr_0.5CoO₃ containing mullite fiber burned with a maximum at 417?.