Preparation And Properties Optimization Of Mullite Fiber Based Porous Ceramics

With the rapid development of Aeronautics and Astronautics,and the increasing attention of human to energy conservation and environmental protection,higher requirements have been put forward for materials and structures such as heat insulation,heat preservation and filtration.It is necessary to develop a new type of thermal protection materials with smaller density,better insulation performance and stronger mechanical properties.Therefore,it is of great strategic significance to develop a light,heat insulation and high strength heat insulation material to meet the urgent needs of high temperature applications and to promote the rapid development of China’s aerospace industry.At present,high temperature resistant fiber based porous ceramics are one of the most important candidate materials.In this paper,the three-dimensional framework of the bird’s nest was modeled and the mullite fiber porous ceramics with nodes were designed,and the microstructure,basic physical properties and compression properties of the porous ceramics were systematically studied by the preparation process.In order to prepare the porous fiber based porous ceramics,the formation of the nodes,the construction of the fiber network and the enhancement of the strength of the nodes and fiber networks are the key to the success:1.Mullite fiber porous ceramics with nodes were prepared by vacuum filtration method using mullite fiber as matrix and SiO2 sol as high temperature binder.By changing the drying temperature(30 ~oC~100 ~oC),drying(drying and freeze-drying)and pumping pressure(10 KPa~40 KPa),the mullite fibrous porous ceramics with low density(0.28~0.33 g/cm~3),high porosity(77%~84%),low thermal conductivity(0.068~0.079 W/(m·K))and relatively high compressive strength(0.78 MPa~2.01 MPa)were prepared.The porous microstructure of mullite fiber prepared by freeze-drying and pumping pressure of 20 KPa presents a three-dimensional network lap for fiber,and the bonding agent bond fibers form a fixed node,and the properties of the fibers are optimal.2.In order to further improve the mechanical properties of mullite fiber porous ceramics prepared by vacuum pumping-freeze drying-sintering process,the affinity of fiber and high temperature binder was changed by adding surface active agent in the slurry,and the content of the internal binder was improved;Changing the sintering temperature and promoting the melting of the binder can enhance the strength of the joint and the bonding strength between the binder and the fiber;Changing the freezing temperature and promoting the fast curing of the binder on the surface of the fiber forms a protective film on the surface of the fiber to improve the strength of the fiber.The results show that when the samples contented 0.5 wt%polyacrylamide(PAM),freezed at-196 ~oC and sintered 1300 ~oC,the density,compressive strength and thermal conductivity of samples are 0.37 g/cm~3,2.13 MPa,0.098W/(m K),respectively.3.In order to explore the process of preparing fiber based porous ceramics with complex pore structure,the preparation process of fiber based composites was further expanded,with 1.5 vol%mullite fiber as the matrix and 13.5 vol%silica powder as binder,mullite fiber porous ceramics were prepared by directional freezing casting process.By changing the freezing rate and the ratio of fiber length to diameter,the redistribution process of fiber and binder in the process of directional freezing was studied,and the microstructure,basic physical properties and mechanical properties of mullite fiber porous ceramics were studied.The results show that in mullite fiber porous ceramics prepared by directional freezing casting process,silica binder forms a top-down binder layer,and mullite fibers form a fiber bridge linking adjacent binder layers between layers.