Study On Preparation Of Composite Aerogel And Microwave Absorption And Thermal Insulation Based On CNTs

With the application of microwave-absorbing materials in military fields where require radar stealth technology,such as aerospace,cruise missiles and supersonic vehicles,this extremely complex and severe application environment has put forward extremely high requirements for microwave-absorbing materials due to the obvious warming effect of strong aerodynamic friction on the surface of weapons and equipment,leading to the emergence of novel microwave-absorbing materials that muster both microwave absorption and thermal insulation.Despite some new materials with both microwave absorption and thermal insulation have been reported,it is still a great challenge to effectively integrate these functions by simple and efficient methods.In addition,the current research lacks in-depth investigation on the structure-activity relationship among structure and function for novel multifunctional microwave absorption materials.Herein,this topic furnishes a simple fabrication method for new microwave-absorbing material with both microwave-absorbing and heat-insulating properties,and investigates the relationship between structure and microwave-absorbing and heat-insulating properties in depth to provide a new strategy for the structural design and performance study of multifunctional microwave-absorbing materials.Firstly,Co Fe₂O₄/CNTs/WPU(CCW)aerogel was successfully prepared by fully mixing cobalt ferrite(Co Fe₂O₄),carbon nanotubes(CNTs)and waterborne polyurethane(WPU)and using a one-step directional freeze-drying method.The effects of different electromagnetic component ratios on the aerogel properties were investigated by the coaxial method.The results show that the samples possess different microwave absorption properties with different electromagnetic component ratios,where the CCW aerogel exhibits the best microwave absorption effect when the mass ratio of CNTs to Co Fe₂O₄ is 3:2,and its minimum reflection loss value(RL_min)is-45.8 d B,while the CCW aerogel with minimum addition of CNTs has the best thermal insulation performance with the lowest thermal conductivity of 36.9 m W m^-1 K^-1.Besides,the systematic study on the mechanism of microwave absorption and thermal insulation of CCW aerogel was also carried out.This work provides a novel way for the simple preparation of new microwave absorbing materials with both microwave absorption and thermal insulation properties.Subsequently,Co Fe₂O₄ nanoparticles were synthesized on the surface of CNTs using Co²⁺and Fe³⁺by a solvent-thermal assisted co-precipitation method,and thus CNTs@Co Fe₂O₄ composite was successfully prepared.The anisotropic,multifunctional CNTs@Co Fe₂O₄/PI(CCP)aerogel was successfully prepared by mixing the obtained CNTs@Co Fe₂O₄ composite with polyamide acid(PAA),a precursor of polyimide(PI),through directional freeze-drying and thermal imidization.Using the coaxial method,the effects of different electromagnetic component ratios and anisotropic structures on the aerogel properties were analyzed with emphasis.The results exhibit that when the mass ratio of Co Fe₂O₄ prepared by Co²⁺and Fe³⁺ions to CNTs is 2:3,the corresponding CCP-3 aerogel achieves the optimal microwave absorption effect in the radial direction(lamellar structure direction)with an RL_min of-54.4 d B and a matching thickness of 2.6mm.While the CCP-3 aerogel has the maximum effective absorbing broadband in the axial direction(honeycomb structure direction)of 7.15 GHz,corresponding the thickness of 3 mm.In addition,the CCP aerogel also displays obvious anisotropy in terms of thermal insulation performance.The radial thermal insulation performance of all samples are better than axial,and the lowest thermal conductivity emerges in the radial direction of CCP-3 aerogel,which is 29.6 m W m^-1 K^-1.Therefore,when CCP-3 aerogel was placed on the 120?heating table for 40 min,the surface temperature of the CCP-3 aerogel in the radial direction is only 32.0?.The aerogel integrates various functions including hydrophobicity,microwave absorption,thermal insulation and excellent flame retardancy,endowing it promising for applications in various complex and harsh environments.More importantly,the different and mechanism between microwave absorption and thermal insulation properties induced by the anisotropic structure are further explored,and this work provides new ideas for the elaborately structural design and performance study of multifunctional microwave-absorbing materials.