Fabrication And Applications Of 3D Printed Aramid Nanofiber-Based Composite Aerogels

Aerogels were used in fields such as thermal insulation and aerospace due to their low density and high specific surface area,but the practical application of aerogels were still limited by weak mechanical properties and poor processing performance.Aramid had an important position in aerospace and electrical and electronic fields due to their high strength,modulus and heat resistance.Aramid nanofiber（ANF）aerogels were low density,high porosity,which could effectively improve the weak mechanical properties of traditional aerogels.Using ANF aerogels as a matrix,the introduction of nano-functional components such as carbon nanotubes（CNT）and metal organic backbone（MOF）into the system was expected to expand its applications.However,the shape of aerogels prepared by solvent casting method were limited by the mold,and it was difficult to meet the requirements of different application scenarios for the complex shape of the material.Moreover,the agglomeration of nanoparticles would cause the problems of difficult molding of composite aerogels and low addition of functional components.In addition,the bulk aerogels,due to its large size,the solvent exchange process was slow and the solvent and polymer were prone to macroscopic phase separation,resulting in the internal aerogels obtained by drying often showing pore sizes of tens or even hundreds of microns,which could not give full play to the advantages of aerogels.In order to solve these problems,we used 3D printing technology to obtain PPTA hydrogel aerogels with customizable shapes and nanopores.Meanwhile,hybrid aerogels with high CNT and MOF loading were prepared by hybrid and post-growth methods,and the microstructures of the aerogels and their applications in electromagnetic shielding and adsorption were investigated.The specific results of the investigation are as follows:（1）Based on 3D printing technology,CNT/ANF dispersion was used as the printing ink,supplemented with freeze-forming,and the printing parameters were adjusted to ensure the molding accuracy of the wet gel to finally obtain the shape designable CNT@ANF hybrid aerogels.By adjusting the content of CNT,the mechanical strength of the hybrid aerogels decreased with the increase of porosity.A small amount of carbon nanotubes（2.5 vol%）played a role in mechanical enhancement of the material skeleton structure,but when the CNT reached above 5.2 vol%it would destroy the integrity of the matrix material and reduced the mechanical properties of the hybrid aerogels.The CNT content on the electrical conductivity and electromagnetic shielding performance of the hybrid aerogels was also investigated,and the results showed that the electrical conductivity and electromagnetic shielding performance of the hybrid aerogels were enhanced when the volume fraction of CNT increased,and when the CNT content reached 8.2 vol%,the electromagnetic shielding performance of the hybrid aerogels in the range of 8.2-12.4 GHz could reach 22.8 dB,and the specific electromagnetic shielding performance could reach 16239.5 dB·cm²·g^-1.（2）ANF dispersions were used as 3D printing inks to fabricate ANF wet gels with desirable shapes.ZIF-67@ANF hybrid aerogel was obtained by synthesis ZIF-67 by post-growth method using cobalt nitrate hexahydrate（Co（NO₃）₂-6H₂O）as the cobalt source and 2-methylimidazole（2-MIM）as the ligand.It was demonstrated that the loading process of ZIF-67 was the formation of coordination bonds between free Co²⁺and deprotonated N^-in ANF,followed by the ligand stacking of N in imidazole and Co after 2-MIM entered the wet gel to form rhombic orthododecahedral ZIF-67 crystals loaded into the ANF aerogel.The loading of ZIF-67 in the hybrid aerogel was regulated by adjusting different growth templates,solvent types,cobalt ion concentration and soaking time of 2-MIM,and the optimized ZIF-67@ANF hybrid aerogel had high specific surface area（756.6 m²/g）and high loading（63.4%）.The hybrid aerogel had an important role in the field of water purification,with adsorption capacities of up to 243.1 mg/g and 312.5 mg/g for organic dyes（methyl orange）and metal ions（Cu（Ⅱ））.