Preparation And Characterization Of SiO₂ Nanowires/Al₂O₃-SiO₂ Aerogel Composites

Aerogel is a kind of light solid material with three-dimensional network and porous structure.lt has excellent characteristics such as ultra-low density,low thermal conductivity and up to 98%porosity,which makes aerogel have great application prospects in aerospace,architecture,catalysis,environmental protection,medicine and other fields.At present,Al₂O₃-SiO₂ aerogels mainly use ceramic fiber of micron level to improve its mechanical properties.In practical application,it is easy to fall out of powder,or even cause large cracks,which makes the existing aerogel composite material unable to play a better heat insulation performance.Nanowires have a large aspect ratio and a high specific surface area.As the reinforcement of aerogel brittle matrix,nanowires can make aerogel stronger and less prone to crack when subjected to stress or impact.Therefore,the use of nanowires with large aspect ratio as the reinforcement can greatly improve the mechanical properties of aerogel and solve its brittleness and powder loss problems.The SiO₂ nanowire,which is prepared by soft template,is used as the reinforcement of the Al₂O₃-SiO₂ binary composite aerogel.The effects of different composite methods（physical hybrid method and in situ composite method）and the addition of nanowires on the mierostrUcture,mechanical properties and thermal conductivity of the coruposites were investigated.The enhancement mechanism of SiO₂ nanowire on eryuanqi gel was discussed.Mesoporous silica nanowires were prepared from two templates（CTAB,F127）and ethyl orthosilicate.The diameter of Sio2 nanowires is mainly distributed between 100-180 nm,with a length of over 10 um,and the aspect ratio is about 56-100.The specific surface area reached 551.96 m2/g,the average pore diameter was 3.44 nm,the pore volume was 0.12 cm3/g.SiO₂ nanowires/Al₂O₃-SiO₂ composite aerogels were prepared by in situ composite method.The composite aerogels were seriously broken,the content of SiO₂ nanowires was very low,and the mechanical properties were not improved.The composite still maintains a mesoporous structure.When the-volume of the reaction solution was 3mL,the specific surface area of the composite reached735.43 m2/g.SiO₂ nanowires/Al₂O₃-SiO₂ aerogel composites were prepared by dispersing the nanowires in sol by physical mixing method.The results showed that the SiO₂ nanowires and the matrix aerogel were highly compatible.The specific surface area of the composite reached 726.31 m2/g,the average pore diameter was 16.45 nm,and the pore volume was 4.15 cm3/g.When the mass fraction of SiO₂ nano wires was 18 wt%,the compressive strength of the composite was 0.29 MPa,0.18 MPa higher than the pure aerogel,and the shape variable was 37%.The addition of SiO₂ nanowires improved the compressive strength and toughness of the aerogel.After high temperature heat treatment,the shrinkage of sample was 30%when the mass fraction of SiO₂ nanowire was 18 wt%.The linear shrinkage of aerogel is 40%.By 1000℃,nanowires mass fraction of 12wt%of the composite material has the highest compression strength,1.1 MPa,shape variables is 9.6%.When the mass fraction of nanowires was 0.18 wt%,the thermal conductivity was 0.089 W/（m·K）.The addition of nanowires improved the thermal stability of the samples at high temperature.The composites with a nanowire mass fraction of 12 wt%had the best adsorption performance for methylene blue with a adsorption capacity of 7.494 mg/g,and the adsorption capacity and adsorption equilibrium concentration were in line with the Langmuir adsorption model.