Preparation And Properties Of Porous Ceramics With The Framework Of Polycrystalline Mullite Fibers

With the increasing attention to energy conservation and the rapid developmentof aeronautic industry, the performance of heat seal materials also made more andmore requirements. The high-temperature elasticity and insulation properties of heatseal materials directly affect the economic efficiency of enterprises and the runningsafety factor of the spacecraft. In recent years polycrystalline mullite fiber (PMF)products has drawn much attention in a fields of the high-temperature and aerospacebecause of its high stability of high temperature performance.In this experiment, a new porous network structure with the matrix of oxide fiberand the binder of glassy phase at the crossing points of the fibers was designed. Whensubjected to an external force, the materials achieved its compressibilitycharacteristics because the bending of the fibers. After the force was removed, thesample restored to the original state since the fibers was fixed at the crossing points.In other words, the materials had elasticity.In this study, polycrystalline mullite fiber was chosen as matrix due to itshigh-temperature stability, and the silica sols which were used as inorganic binders orsilica-boron sols which were going to change into glassy phase at high temperaturewere chosen as binders. During the experiment, firstly the organic binder andpolycrystalline mullite fiber were mixed together. After infiltration, the mixture wasprepared into a fiber body. Then the inorganic binders (silica sol or a silicon-boron sol)were impregnated into the fiber body. Finally, porous ceramics of polycrystallinemullite fiber with the fixed crossing points was prepared after infiltration. And themechanical, thermal properties and microstructure was investigated. The conclusionswere as follows:1. The porous ceramics with high elasticity based of the polycrystallinemullite fiber can be prepared using vacuum infiltrating technique byadding the binder. The polycrystalline mullite fiber was used as fibrousmaterials; silica sols were used as inorganic binder. With the silica sol of30wt(SiO2)%against the weight of PMF, the compressive strength of thesample can be achieved1.38MPa after sintered at1100℃. Thecompressive rate and rebound resilience of the sample at room temperature under the compressive stress of1MPa were12%and94%,respectively.2. The porous ceramics with high elasticity based of the polycrystallinemullite fiber can be prepared using vacuum infiltrating technique byadding the binder. The polycrystalline mullite fiber was used as fibrousmaterials; silica-boron sols were used as inorganic binder. With thesilica-boron sol of30wt(SiO2-B2O3)%against the weight of PMF, thecompressive strength of the sample can be achieved2.25MPa aftersintered at1200℃. The compressive rate and rebound resilience of thesample at1000℃under the compressive stress of2MPa were19%and85%, respectively. The thermal conductivity at room temperature was0.0728W/m~2K.3. Through the observation of the microstructure of the above two systems,it is found that the connection effect between the silica sols whichconverted into amorphous phase of silica and the fiber were not goodafter sintered at1100℃. Otherwise, the silica-boron sols which convertedinto glassy phase were connected with the fiber firmly after sinter at1200℃.4. The two systems showed non-brittle fracture characteristics. Thecompressive curve was divided into three stages: linear stage, plateaustage and densification stage. The deformation of the linear stage wasmainly due to the bend of the fiber. During the plateau stage, parts of thecells were destroyed. After the densification stage, all the cells contactwith each other. The structure was destroyed completely.