Synthesis And Properties Of Mullite Fiber Reinforced SiO₂ Porous Ceramics With High Strength And Low Thermal Conductivity

The thermal insulation materials with high temperature-performance,high strength and low thermal conductivity have broad application prospects in aerospace,high temperature industry and other fields.In this study,we successfully fabricated a novel mullite fiber reinforced silica matrix composite（abbreviated as MS composite）by a facile vacuum impregnation,gel aging,ambient pressure drying and sintering process,in which the mullite continuous fiber preform was used as the reinforcement and silica was used as matrix.The effect of fiber volume content,sintering process and densification process on the mechanical and thermal insulation properties of the MS composites were investigated.The main research work is as follows:In order to obtain high-strength MS composites with low thermal conductivity,the effect of fiber content on the performance of MS composites was studied.As the fiber content increases,the density of the material increases,the strength gradually increases,and the thermal conductivity was slightly increased.The composite fabricated with fiber volume content of 10.8%has excellent properties with low thermal conductivity of 0.132 W/m·K（at 800℃）,high compressive strength of 18.39 MPa.The influence of the sintering process on the properties of the MS material was studied.The final sintering system was determined as follows:the sintering temperature was 1000℃,the heating rate was 3℃/min,and the holding time was0.5h.The composite has excellent properties with low thermal conductivity of 0.146 W/m·K（at800℃）,high compressive strength of 25.2 MPa,and the material has excellent temperature performance at 1200°C.In order to further improve the strength of MS composites,the material densification process was studied,with two times of vacuum impregnation and high temperature sintering as one cycle.When the number of densification cycles increased from 1 cycle to 4 cycles,the density of MS composites increased from 0.81 g/cm³ increased to 1.215 g/cm³,compressive strength increased from 25.2 MPa to 74.38 MPa,bending strength increased from 5.07 MPa to21.78 MPa,thermal conductivity at room temperature increased from 0.1123 W/m·K to 0.2096W/m·K,the linear shrinkage of the composite material increased from 0.85%to 4.21%after heat treatment at 1200℃.The thermal insulation performance optimization of MS composite was studied.The pore structure of MS composite was improved by impregnating aerogel.The strength of the MSA composite has been improved after impregnate Al₂O₃ aerogel,but the thermal insulation performance has not been improved.The novel high-porosity MS composite was used as the reinforcing framework.The MSS composite with strength and thermal insulation performance has significantly improved by impregnating SiO₂ aerogel.The compressive strength at strain 10%is 0.50 MPa,and the thermal conductivity at room temperature is 0.045 W/m·K,but the high temperature performance of the material is reduced.The MSZ composite material obtained by doping Zr O₂into the matrix has further improved thermal insulation performance and high temperature performance.The thermal conductivity at room temperature is 0.08250 W/m·K.After 1400℃heat treatment,the linear shrinkage is still less than 1%.The RS composite material prepared by using rock wool fiber as the reinforcement has significantly improved thermal insulation performance and strength.The thermal conductivity at room temperature is0.1033 W/m·K and the compressive strength is 62.56 MPa,but the temperature performance of the RS composite is reduced.