Preparation And Property Study Of Porous Mullite Fiber Matrix Composites With Low Thermal Conductivity And Relatively High Compression Resilience

High temperature sealing problem has received much attention in recent years. Thehigh temperature sealing materials should have the compressibility to absorb thecompressive stress. And then they should rebound to original state when the systemcools to room temperature. And this material can prevent the gas outflow to realizethe gas seal effect. In our project, the air seal can be realized by the dense coating, andthe thermal seal and the required elastic property can be realized by the mullite fiberporous ceramics. In this paper, we mainly discussed the fabrication and properties of apotential high-temperature elastic insulator material. And this design originates fromthe bird’s nest structure in nature.In this paper, we successfully fabricated mullite fiber matrix porous ceramic byTBA-based gel-casting, in which the polycrystalline mullite fiber was used as thematrix and SiO₂was used as the high temperature binder. In this paper, the tert-butylalcohol （TBA） was selected as the solvent for three reasons. Firstly, TBA can be usedas pore-forming agent. The porous SiO₂ceramic prepared between the fibers wouldact as the weak interface to realize the compressive resilience. Secondly, the dryingtime would be reduced because the TBA has a high saturation vapor pressure. Thirdly,there would be small linear shrinkage of the samples due to the low surface tensionforce of the TBA, thus we can control the3-D fiber structure effectively. Theinfluences of the sintering temperature, the solid loading, the ratio of the mullite fiberand SiO₂and the aspect ratio of the mullite fiber on the phase, the microstructure, themechanical properties and the thermal conductivity of samples were analyzed in thisstudy, respectively. The study showed that when setting the solid loading as45wt%,the sample sintering at1600°C has a high porosity of70.92%, a low bulk density of0.79g/cm³, a relatively high compressive strength of13.22MPa and a low thermalconductivity of0.22W/（m·K）. It should be noted that the compression resilienceratios of samples decreased from100%to66%after a strain of2%when the sinteringtemperature increased from1300°C to1500°C. As the solid loading increased from10to45wt%, the bulk density increased from0.404to0.620g/cm³, while theporosity decreased from84.70%to75.12%. Meanwhile, the thermal conductivity andthe compressive strength increased from0.08to0.17W/（m·K）, and from0.58to1.71MPa, respectively. It should be found from the stress-strain curve of the samples with a solid loading of45wt%that there are many distinct and sudden drops with theincrease in displacement. By setting the sintering temperature as1400°C, when theratio between the mullite fiber and SiO₂decreased from3:1to1:1, it can be clearlyseen from the microstructure that there were more melting SiO₂between the fiberjoints. Thus the compressive strength of the samples increased from1.2to2.4MPa,while the porosity decreased from82%to76%. It should be found that when thecontent of the SiO₂increased, the binder between fbers would fracture in a brittle way.The results showed that there was negligible influence of the aspect ratio of the fiberon the linear shrinkage, the weight loss and the thermal conductivity of the samples.As the aspect ratio of the fiber decreased, the structure became denser than that ofsample with a large aspect ratio. And the stress-strain curve showed that the decreaseof the aspect ratio of the fiber would increase the stiffness of the mullite fiber matrixporous ceramic.In this paper, we also explored the preparation process of the resilient sheet forengineering application, including the design of the mould, the forming process of thegreen body, the drying process of the green body, the sintering process of the samples.Now we can successfully prepare the smooth resilient sheet with a thickness rangefrom0.5to2mm.