| Polybenzoxazine(PBz)aerogel is a novel thermal insulation material with excellent mechanical properties,low thermal conductivity,and low density.Therefore,it has high application value in thermal protection.However,for the high-speed aircraft’s thermal protection material,pure PBz aerogel suffers from low thermal stability and insufficient insulation performance.Additionally,its mass residual rate is also low which seriously restricts its practical application.Therefore,in this study,the hybridization of silicon sources with the inorganic phase was utilized to construct PBz aerogels to improve their thermal stability,thermal insulation performance,and mass residual rate.The effects of introducing silicon sources in various ways on the microstructure and properties of PBz/Si hybridized aerogels have been investigated.Based on this,nano silica(NSi)was introduced to form Si complexes hybridized with PBz aerogels,and the prepared PBz/Si/NSi hybrid aerogels could be used as a matrix for ablative thermal protection composites.Mullite fiber-reinforced PBz/Si/NSi aerogel composites were successfully prepared by preferentially selecting a suitable modification scheme for the composites,and the relationship between their structure and properties was investigated.The main results of the study are as follows:(1)The effects of different methods for introducing silicon sources on the structure and properties of PBz hybrid aerogels have been investigated.The introduction of silicon has strengthened the network structure,increased the mass residual rate and compressive stress,and prevented thermal decomposition in high-temperature oxidation environments.Among them,the pore size distribution of polybenzoxazine/Si hybrid aerogels(PSAs)and polybenzoxazine/Si/chitosan hybrid aerogels(PSCAs)was concentrated between 5-35 nm,which helped in achieving lower thermal conductivity.Additionally,both PSAs and PSCAs exhibited excellent flame-retardant and fire-retardant properties.After thermal treatment at 800 °C,the microscopic morphology,crystalline structure,and silica-related chemical bonding of PSCAs remained largely unchanged,indicating good high-temperature thermal stability.(2)The effect of Si complexes formed from nanosilica combined with PBz aerogels on the structural enhancement and thermal stability of the hybrid aerogels has been investigated.PBz exhibited good compatibility with NSi and was able to prepare PBz/Si/NSi hybrid aerogels(PSNSAs)with a high specific surface area and nanoporosity.With low linear shrinkage,low density,and low thermal conductivity,the aerogels not only exhibited excellent compression properties in high to low temperature environments but also maintained a mass residual rate of 61.14% at 800 °C.Additionally,they demonstrated good resistance to flame and fire in tests.After undergoing hydrophobic treatment,the water contact angle of the material increased from 0° to 134°.The synergistic optimization of skeleton reinforcement and high-temperature resistance in the PSNSAs network has proven the effectiveness of the Si composite in enhancing the compressive properties and temperature resistance of the aerogel,thus laying a foundation for the preparation of fiberreinforced aerogel composites.(3)The preparation and performance testing of mullite fiber-reinforced PSNSAs composites has been investigated.The prepared composites exhibit a strong bond between the reinforcement and the aerogel matrix,and the aerogel matrix fully fills the fiber pores.The composites have low thermal conductivity and density,and the mass residual rate is up to 66.32%.High-temperature thermal treatment and tube furnace high-temperature tests have further verified the excellent thermal stability of the material.After exposure to a high-temperature environment at 1100 °C,the material retained its shape and experienced minimal density loss.The improved mass residual rate and thermal stability of the composites demonstrate the effectiveness of mullite fibers for reinforcing the aerogel structure. |
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