Synthesis And Microwave Absorbing Properties Of Polymorphic Structured FeNi/C Composites

In recent years,electrical pollution has gradually increased,causing a great burden on people’s health and the ecological environment.Microwave absorbing materials have attracted the attention of many researchers because of their excellent performance in electromagnetic pollution protection.Studies have shown that single-component microwave absorbing materials（e.g.magnetic metals,carbon materials）have certain wave-absorbing properties,but still suffer from high density,imbalance in impedance matching and mismatch in dielectric parameters.Therefore,at this stage,most of the composite absorbing materials are prepared by combining carbon materials and magnetic metal materials,using carbon materials to reduce the density of metal materials,regulate impedance matching and enhance the reflection loss characteristics.In this paper,FeNi/C composite absorbing materials with various morphological structures were prepared by co-precipitation method and precursor transformation method using FeNi alloy as magnetic material and polyvinylpyrrolidone（PVP）as carbon source,and the absorbing characteristics of absorbing materials under different morphologies were studied to achieve the regulation of impedance matching,as follows.:（1）The magnetic hollow FeNi alloy/porous carbon composite absorbing material was prepared by co-precipitation method.The porous carbon endows the material with great specific surface area and pore volume,which is conducive to the constant reflection and scattering of electromagnetic waves on the surface and inside of the material and helps to regulate the impedance matching.At the same time FeNi alloy is compounded with carbon phase,which makes the mutual balance between complex dielectric constant and complex permeability,and the mutual synergy between dielectric loss and magnetic loss,achieving better reflection loss characteristics.On top of again,the material shows better microwave absorption properties and exhibits unique multi-frequency microwave absorption performance,and the material shows certain wave absorption performance at three frequencies of 2～4 GHz,8～10 GHz and 16～18 GHz.The minimum reflection loss value(RL_min)value of sample HS FNC11 at 14.48 GHz at 1.5 mm thickness was-49.90 d B and the effective absorption bandwidth（EAB）was 7.23 GHz when FeNi:C=1:1.（2）A porous honeycomb microwaveabsorbing material with FeNi alloy nanoparticles uniformly embedded in a carbon matrix was further designed.The porous honeycomb structure allows more electromagnetic waves to enter the material and further optimizes the impedance matching.At the same time,multiple loss mechanisms are used to effectively attenuate electromagnetic waves and improve the reflection loss capability.The results show that a certain degree of graphitization facilitates electron migration,while oxygen-containing polar functional groups can also act as polar sites to generate dipole polarization and increase the dielectric loss capability.Combined with the reflection loss diagram,it was found that the triple RL peak of the material was enhanced,but the absorption performance at low and high frequencies was slightly deficient.At a thickness of 2.5 mm,the RL_minof sample S3 is-53.67d B（11.28 GHz）and the combined absorption effective bandwidth reaches 5.27 GHz.（3）Rod-shaped oxalate FeNi alloy precursors were prepared by precursor transformation,and then PVP was cross-linked and sintered with them to form a three-dimensional network structure.With increasing carbon content,the magnetic saturation strength of the material decreased from 133.18 emu/g to 63.48 emu/g,while the anisotropy value increased,favouring the enhancement of the magnetic loss properties.Mechanistic analysis shows that the composite material has also progressed in terms of multi-frequency absorption characteristics with optimal impedance matching due to the combined effect of loss mechanisms such as interfacial polarisation,eddy current loss and exchange resonance.The RL_minof sample FNC12 was-52.99 d B at 3.6 GHz and the combined effective absorption bandwidth of sample FNC21 reached 5.43 GHz.The impedance of the FeNi/C composite material is optimised and the impedance matching value at the frequencies corresponding to the RL peaks is gradually approaching 1,while the RL capability is increasing.It shows a feasible idea for the research of"thin,light,wide and strong"new microwave absorbing materials.