Surface Structure Design And Properties Of Fibers To Reinforce Al₂O₃-SiO₂ Aerogel Composites

The fiber/aerogel materials have low thermal conductivity of the aerogels and high strength of the insulation tiles,which can be used as thermal protection material in extreme environments.However,aerogels have poor thermal stability,high infrared radiation transmittance and weak interfacial adhesion to fibers,which leads to the decrease of thermal insulation and mechanical properties of the materials.In this paper,the effects of SiO₂deposition on thermal stability of the Al₂O₃-SiO₂aerogel（ASA）were investigated.The effects of different coating structures（including ZrSiO₄and TiO₂）on the thermal insulation,thermal stability,infrared shielding and mechanical properties of the composites were investigated by preparing different structural coatings on the surface of fibers.The results can be used for the further development of aerogel composite materials.Firstly,to improve the thermal stability of the ASA,SiO₂was deposited on the surface and neck of the gel particles to reduce the surface diffusion force by particles growth and skeleton coarsening,and it can restrain the growth ofγ-Al₂O₃by forming more Si-O-Al bonds in the system.When the heating temperature exceeds 1000°C,the mullite phase begins to appear.The crystallization activation energy of the Si-ASA（1019.24 KJ/mol）is higher than that of the ASA（975.95 KJ/mol）,indicating that the Si-ASA overcomes higher potential barrier in the transformation process from amorphous to crystalline state.Secondly,to improve the infrared shielding and mechanical properties,the quartz fiber was coated with ZrSiO₄nano-coating with high refractive index to form a high reflective layer without affecting the skeleton structure of the fibers.The infrared transmittance（wavelength 2.5-10μm,0.1%）is significantly reduced and the radiation heat conduction was inhibited.The aerogel composite has excellent thermal insulation and temperature resistance（more than 1200°C）.The high density ZrSiO₄gathers at the fiber lap joint and acts as a high-temperature binder.The compressive strength of the composite increases by 89%when the deformation is 10%.Finally,to improve the infrared shielding and the interfacial adhesion to fibers,the rutile TiO₂with different morphology（including microspheres,nanorods,and nanowires）were directly grown on the surface of ZrO₂,quartz and mullite fibers,respectively.The interfacial adhesion between fibers and aerogels was significantly improved through the high surface roughness and rich surface hydroxyl groups of TiO₂coating.In-situ growth of TiO₂coating can reduce the gaseous conductivity by increasing the interfacial adhesion force between fibers and aerogels,and reduce the radiation heat transfer by shielding infrared radiation.The novel‘fiber/TiO₂/aerogel’composite exhibited excellent thermal insulation（0.071-0.089 W/m·K at 1100°C）,infrared radiation shielding performance(an extinction coefficient greater than 150cm^-1and an infrared reflectivity greater than 85%),fire resistance（more than1200°C）,and compressive strength（0.37 MPa,10%strain）.The larger the thickness or particle size of the coating,the higher the infrared shielding,but the increase of solid content will increase the solid thermal conductivity of the materials.Therefore,the strategy of TiO₂nanorods coating on the surface of fibers can significantly improve the comprehensive properties of the material while maintaining the same density of the aerogel composite.This method solves the problem of uneven dispersion of infrared shielding agent and can realize large-scale preparation and production.This strategies are applicable to multi-type fibers reinforced silica-based aerogel materials used for insulation and protection in extreme environments.