Structural Design And Properties Of Aramid Nanofiber Aerogels

Aerogels exhibit excellent thermal insulation performance due to their high porosity,high specific surface area and controllable pore size distribution.In particular,organic aerogels have overcome the advantages of brittleness,good flexibility,flexible molecular design and low density of silicon-based aerogels,thus expanding the application field of aerogels,but they also have some problems such as low structural strength and heat resistance to be improved.Aramid nanofibers have excellent mechanical properties,high thermal stability,good chemical stability and other characteristics,which are ideal elements for the construction of high temperature thermal insulation aerogels.In view of the problems that the structure of aramid nanoaerogels in the current research is not strictly a three-dimensional space fiber network structure,with high density and low mechanical strength,this paper has carried out two aspects of work:(1)The mesh-like aramid nanofibers(mANFs)were prepared by high pressure homogenization,and the effects of the preparation process on the structure and morphology of mANFs were studied.With the aid of directional freezing technology,the aggregation structure of mANFs was regulated by ice crystal growth,and a high porosity mANFs aerogel with directional cell cavity was designed.Thanks to its unique multi-stage pore structure,abundant physical entangles and chemical bonding,MANFS aerogel exhibits excellent high elasticity and fatigue resistance.It can withstand 90%high compressive strain and maintain a recovery of 91.9%after 1000cycles under 50%strain.The density of mANFs aerogel can be controlled in the range of 1.47-5.2 mg/cm³.At the density of 3.7 mg/cm³,the thermal conductivity of mANFs aerogel at room temperature and 200?is 24.0 mW m^-1k^-1 and 26.7mW m^-1k^-1,respectively,showing outstanding thermal insulation performance.(2)In order to improve the flame retardancy of aerogels,carboxylated SiO₂nanoparticles were added into deprotonated aramid nanofibers dispersion(ANF)with high reactivity to prepare ANF/SiO₂ organic-inorganic composite aerogels by sol-gel process.The hydrogen bonding between SiO₂ nanoparticles and nanofibers was used to form a stable porous composite aerogel with a three-dimensional fiber network structure mainly distributed in 20-40 nm.Compared with pure aramid nanoaerogels,the composite porous structure has higher compressive strength(300 kPa).The low total heat release of 2.8 kJ/g during combustion is 94.1%lower than that of pure aramid aerogel.Based on the analysis of the flame retardant mechanism,the composite aerogel rapidly forms a dense nano-carbon layer on the surface during combustion,effectively preventing the flame spread.Meanwhile,the composite aerogel has a low thermal conductivity(27.0 mW m^-1K^-1),which can effectively insulate heat transfer.Combined with its good mechanical properties,heat insulation performance and fire retardant performance,it has a good application prospect in the field of high temperature thermal protection.