Study On Thermal Insulation/Microwave Absorption Of Fiber-reinforced Polyimide Composite Aerogels

With the rapid development of aerospace technology,the demands for new structural materials are rising.Current researches have been focused on developing lightweight multifunctional materials with thermal insulation and microwave absorption.Polyimide（PI）aerogel has become one of the ideal materials for aerospace applications due to its low density,good thermal insulation,thermal stability and excellent mechanical properties.The low density and complex three-dimensional structure of aerogel can effectively extend the heat transfer path and reduce the solid conduction to obtain excellent thermal insulation performance.As a result,the aerogel has excellent thermal insulation performance.At the same time,aerogel with high porosity could allow most of the electromagnetic waves to enter the inside of material and thus become a good candidate for microwave absorbers.However,PI aerogel has large shrinkage during the preparation process,causing the detrimental to the overall performance.Meanwhile,PI is a low-dielectric polymer.The electromagnetic waves are difficult to attenuate after entering the aerogel.Therefore,this research aims to develop PI aerogel with integrated thermal insulation and microwave absorbing functions,The results are expected to provide a new idea and experimental basis for the next generation of high-speed aircraft that need to integrate the thermal insulation and microwave absorbing functions at the same time.In this research,carbon nanofibers（CNF）and ferroferric oxide（Fe₃O₄）were selected as the reinforcement materials to prepare PI composite aerogels.The bridging effect of CNF with high aspect ratio effectively inhibited the volume shrinkage of PI aerogel.Meanwhile,its high electrical conductivity contributed to the improvement of the microwave absorption of the aerogels.The introduction of Fe₃O₄ further enhanced microwave absorption due to the enhancement of the magnetic loss of in the system.The magnetic loss of Fe₃O₄ and the resistance loss caused by CNF play a synergistic effect on enhancing the absorbing performance.To improve the interface interaction between the reinforcements and polyamic acid（PAA）which was the precursor of PI,CNF was acidified to graft carboxyl groups on the surface to obtain a-CNF and Fe₃O₄ was coated with SiO₂,followed by the amination treatment to obtain SiO₂@Fe₃O₄-NH₂.After mixing PAA with the reinforcing materials,freeze-drying and imidization were carried out,a-CNF reinforced PI aerogel,SiO₂@Fe₃O₄-NH₂ reinforced PI aerogel and a-CNF and SiO₂@Fe₃O₄-NH₂ synergistically reinforced PI aerogel were prepared.The comprehensive performances of the PI composite aerogels were studied,and the detailed research content was as follows:（1）a-CNF reinforced PI（PI-C）aerogel:The thermal insulation,microwave absorption and mechanical properties of PI composite aerogels were studied by adjusting the weight fraction of a-CNF（5 wt%,10 wt%,15 wt%）.The thermal conductivity of PI aerogel with 5 wt%a-CNF content was the lowest,only 0.039 W/（m·K）.With the amount of a-CNF increasing,the thermal conductivity increases slightly.This was mainly because the effect of increasing pore size of aerogel was greater than the intrinsic heat transfer effect of CNF.In the aspect of microwave absorption,the CNF induced the resistance loss of the aerogel.The microwave absorption improved with the increase of the a-CNF amount.The minimum reflection loss（RL）value reached-9.65 d B at 8.41GHz for the PI-C aerogel with 15 wt%a-CNF content.In the aspect of mechanical properties,the compressive strength and modulus of PI-C aerogels effectively improved after addition of 15 wt%of a-CNF,which increased by 0.5 times and 1 times compared with Neat PI,respectively.（2）SiO₂@Fe₃O₄-NH₂ reinforced PI（PI-SF）aerogel:the effects of SiO₂@Fe₃O₄-NH₂loading（10 wt%,20 wt%,30 wt%）on the thermal insulation,microwave absorption and mechanical properties of PI-SF aerogel were studied.In the aspect of thermal insulation performance,the thermal conductivity decreased initially and increased with introducing more amounts of SiO₂@Fe₃O₄-NH_2.Although the addition of Fe₃O₄ increased the thermal conductivity,the volume shrinkage reduced.At the same time,the thermal stability of PI-SF aerogel increased due to the addition of SiO₂.In the aspect of absorbing performance,due to the increase in magnetic loss,the absorbing performance of PI-SF aerogel significantly improved.The lowest RL value of PI-SF aerogel with 30 wt%SiO₂@Fe₃O₄-NH₂ reaches-40.32 d B at 14.42 GHz.In the aspect of mechanical properties,the strength effectively improved due to the good interface compatibility of SiO₂@Fe₃O₄-NH₂ nanoparticles and PAA,but the brittleness of structures was dentrimental to the compression resilience of PI-SF aerogels.（3）a-CNF and SiO₂@Fe₃O₄-NH₂ synergistically reinforced PI（PI-C-SF）aerogel:Due to the brittleness of PI-SF aerogel with 30 wt%of SiO₂@Fe₃O₄-NH₂,15 wt%a-CNF and 20 wt%SiO₂@Fe₃O₄-NH₂ were selected as the reinforcing phases.Although the addition of a-CNF and Fe₃O₄ promoted the electron movements and phonon transportations in the framework,the high porosity of the PI-C-SF aerogel led to a slight increase in the overall thermal conductivity which reached 0.046 W/（m·K）compared with the PI-C and PI-SF aerogel.The absorption peak of the PI-C-SF aerogel moved to the low frequency and the lowest RL value reached-34.71 d B at 7.43 GHz,owing to the synergistic effect of Fe₃O₄ and CNF.In the aspect of mechanical properties,the compressive strength of PI-C-SF aerogels significantly improved to 342.44 k Pa due to the bridging effect of CNF and good interfacial compatibility between SiO₂@Fe₃O₄ and the PAA matrix.