Preparation Of Mullite Whisker/fiber Composites And Research On Whisker Growth Mechanism

Porous ceramics have been widely used in heat insulation,sealing,catalysis,separation,filtration and other fields（especially in some special environments,such as high temperature,acid-base,radiation,etc）because of their good chemical stability and high temperature tolerance.Porous ceramics constructed by ceramic fibers have become an important branch of porous ceramics due to their outstanding high porosity,good thermal shock resistance and high temperature structural stability.Ceramic fiber porous materials have good advantages and prospects in the application of high temperature gas filtration.Previous studies have shown that the filtration performance of the porous network can be further improved by the growth of secondary structure on the surface of mullite fibers.However,the growth control mechanism and material strength of mullite whiskers need to be further studied.In this paper,the growth mechanism of mullite whisker in Al₂O₃-SiO₂-Al F₃ system with Al₂O₃ sol and SiO₂ sol as aluminum and silicon sources was studied,and mullite whisker/fiber composites based on mullite short-cut fiber porous ceramics were prepared by extraction filtration and freeze-drying.The effects of molar ratio of Al F₃to SiO₂,the mass ratio of active powder（Al₂O₃-SiO₂ xerogel mixed with Al F₃ powder,hereinafter referred to as active powder）to fiber and sintering temperature on the microstructure and phase composition of the composite were investigated as well as the properties of the composite comparing with the mullite fiber porous ceramic.The study showed that the extraction filtration combined with freeze-drying process can evenly distribute the high-temperature binder on the fiber surface and its lap joints,which is beneficial to obtain the fiber porous material with low density and high strength.Al₂O₃-SiO₂ xerogel and Al F₃ powder mixture produce inter-alcohol condensation water during the heating process,which promotes the decomposition of Al F₃ to form Al OF and hydrogen fluoride（HF）.Samples at 700℃,800℃ and 900℃,thermal insulation for 1 h can get acicular mullite,rod-shaped topaz,flake corundum as main phase of the materials,respectively.In the preparation process of composite materials,whether the molar ratio of Al F₃ to SiO₂ is too low or too high will lead to the co-existence of whiskers and Al₂O₃ in the products;the growth state of whiskers in fiber matrix changed from weak to even to disorderly when the mass ratio of active powder to fiber was changed 0.6 and 1.4;length to diameter ratio of the whiskers increase with the increase of heat treatment temperature（800～1300℃）.When Al F₃/SiO₂=21:10（mol ratio）,active powder/fibers=1:1（mass ratio）,sintering at1300℃ for 2 h,whisker/fiber composites with volume density of 0.43 g/cm³,porosity of 86%,and compressive strength of 2.23 MPa were prepared.Compared with mullite porous ceramics（porosity of 85.4%,compressive strength of 2.1 MPa）of pure fiber systems,Whisker/fiber composites achieve higher strength on the basis of similar porosity.