Preparation And Absorption Performance Of Carbon Nanofiber-Based Composite Aerogels

Over the past few years,driven by the rapid spread of wireless communications and electronic devices,electromagnetic pollution has increased exponentially,which is harmful to human health and complex electronic devices.Common electromagnetic wave absorbing materials can effectively collect and dissipate electromagnetic energy through magnetic loss or electrical loss,which is an easy way to eliminate electromagnetic pollution.The conductivity of carbon nanofiber aerogel is moderate,and the impedance matching performance with air is better,it can be combined with magnetic materials to improve the absorption ability.Most of the research at this stage grinds aerogel into powder as a wave absorbing agent and fills it into the matrix material to test and analyze the wave absorbing performance.In this process,the overall microstructure of the aerogel will be destroyed,and the research on the absorbing properties of intact bulk aerogel materials is very limited.In this paper,through the adjustment of the preparation process of aerogel,the composite magnetic material realizes the carbon nanofiber composite aerogel with dielectric/magnetic properties,and under the premise of not destroying the overall structure of the aerogel,the absorption performance of the aerogel block to electromagnetic waves is tested.First,the in-situ method is used to prepare the magnetic material into the carbon nanofiber,and then the aerogel is prepared,By analyzing and studying the structure and absorbing properties of the composite aerogel prepared by the in-situ method,the preparation process was further optimized,and the idea of a one-pot process was adopted to introduce magnetic materials into the aerogel structure.The main findings are as follows:Carbon nanofiber composite aerogel with dielectric/magnetic properties prepared by in-situ process.Through electrospinning and high-temperature carbonization,magnetic materials were successfully introduced into the interior of carbon nanofibers.Using water-based polyurethane（WPU）and soluble starch（ST）as binders,when the ratio of WPU and ST is 1:1,the composite aerogel has good mechanical properties,with a compressive strength of 4.67 k Pa and a specific strength of 0.197 kPa cm³·mg^-1.The influence of magnetic carbon nanofibers at different carbonization temperatures on the microwave absorption properties of carbon nanofiber composite aerogels was studied.The composite aerogel samples have no effective absorption bands in the frequency range of 2-18 GHz and matching thickness of 2.4-4.0 mm.This is because both WPU and ST used as binders are insulating materials,which destroy the overall dielectric/magnetic properties of the aerogels,resulting in the aerogel having no effective wave-absorbing performance.Therefore,the binder is removed by high-temperature pyrolysis.The effect of different heat treatment temperatures on the microwave-absorbing properties of composite aerogel was studied,The h-Fe CNFAs-400 composite aerogel heat-treated at 400℃ exhibited good microwave absorption properties,it has good absorbing performance within the matching thickness of 2.4-4.0 mm,when the thickness is 3.0 mm,the optimal reflection loss value(RL_min)is-40.27 dB,effective absorption bandwidth（EAB）up to 5.28 GHz,the EAB frequency range is 9.36-14.64 GHz;At a thickness of 2.6 mm,the EAB broadens to 5.68 GHz.This is mainly because at the pyrolysis temperature of 400℃,the shrinkage of the aerogel is relatively uniform,most of the binder is removed,and the dielectric and magnetic properties of the composite aerogel are improved.After heat treatment,it was found that the aerogels showed different degrees of shrinkage,showing an irregular shape with a small top and a large bottom,the reason is that the dispersibility of the magnetic carbon nanofibers in the solution is too poor,and sedimentation occurs during freeze-drying.By optimizing and improving the in-situ process,a one-pot process was adopted to prepare carbon nanofiber composite aerogels with dielectric/magnetic properties.The problem of poor dispersion of carbon nanofibers was solved by acidification modification with concentrated nitric acid.Using ST as the binder,glutaraldehyde（GA）as the crosslinking agent,and ferric nitrate as the precursor of the magnetic material,after different pyrolysis temperatures,the aerogel has a complete structure and uniform shrinkage.When the pyrolysis temperature is 600℃ and 800℃,the composite aerogel has good magnetic properties.The effects of different pyrolysis temperatures and different ferric nitrate contents on the microwave-absorbing properties of the composite aerogel were studied.When the pyrolysis temperature was 600℃ and the ferric nitrate content was 0.056 mmol,the composite aerogel had excellent microwave-absorbing performance.When the thickness is 3.6 mm,the RL_min is-41.7 dB,and the EAB is 3.36 GHz,which can achieve strong absorption of electromagnetic waves.When the thickness is 2.4 mm,the EAB is widened to 5.84 GHz,which has the performance of a wide effective absorption band.When the pyrolysis temperature is 600℃ and the amount of ferric nitrate is 0.028 mmol,the aerogel has a thickness of 2.4 mm,RL_min is-19.22 dB,EAB is further expanded to 6.88 GHz,and the frequency coverage of EAB is 11.12-18 GHz,it can achieve full absorption of electromagnetic waves in the Ku band（12-18 GHz）.This is because the introduction of magnetic materials can well regulate the impedance matching characteristics of the composite aerogel,and also improve the overall dielectric and magnetic properties of the aerogel,and significantly increase the absorption loss of the composite aerogel to electromagnetic waves.In this paper,through the optimization and improvement of the preparation process,carbon nanofiber composite aerogels with dielectric/magnetic properties were prepared.The composite aerogels have good electromagnetic wave absorption properties.It meets the current requirements of"thin,light,wide,strong"for new wave-absorbing materials.