| An electromagnetic wave absorber is a material that can soak up electromagnetic waves and is used in many electronic communications,radar,and stealth technologies.Efficient electromagnetic wave absorbers can help reduce electromagnetic pollution.Ferrite is a popular microwave absorber due to its exceptional oxidation resistance,high magnetic permeability,non-toxicity and non-pollution,low cost,and ease of access.However,due to their narrow absorption band width,poor impedance matching,and thermal instability,unary ferrite materials have limited utility as electromagnetic wave absorbers.Electromagnetic-type composites are regarded as an excellent class of electromagnetic wave absorbing materials because the mixture of electrically lossy absorbing materials and magnetically lossy absorbing materials can produce good impedance matching and synergistic effects.Carbon-based materials,in particular,aid in the attenuation of electromagnetic radiation due to their high conductivity and low density.Carbon materials can be added to ferrite to increase dielectric loss,reduce weight,better impedance matching,and,eventually,broaden the absorption bandwidth.In this paper,a hollow open-ended Co Fe2O4 nanosphere particle was designed,and after discussing the wave absorption performance of a single Co Fe2O4 sphere particle,excellent electromagnetic wave absorption was obtained by compounding multi-walled carbon nanotubes with nitrogen-doped reduced graphene oxide on this basis,which improved the disadvantages of poor impedance matching,large thickness,and high density of a single Co Fe2O4 nanosphere particle.The details of the work and its conclusions are as follows:（1）The solvent heat technique was used to create spherical Co Fe2O4 particles with hollow holes and rough surfaces.The electromagnetic wave absorption performance of paraffin was investigated when various amounts of hollow open-ended Co Fe2O4 spherical particles were added.The influence of the filler-loading ratio on the microwave absorption performance of wax-based absorbers is thoroughly investigated.The results show that there is no absorption loss lower than-10 d B within the tested thickness of 1–5 mm at a low fill amount,and the absorption intensity of electromagnetic waves is weak.When the fill percentage is 60%,the matching thickness is extended to 10–50 mm,and the reflection loss is obtained at a matching thickness of 30mm,yielding a reflection loss of–28.6 d B.The loss to electromagnetic waves at this point is mainly attributed to the natural loss and eddy current loss generated by Co Fe2O4.However,this is not the characteristic that an excellent electromagnetic wave absorber should have.The main reason is the impedance mismatch caused by the single loss of the ferrite,which produces poor electromagnetic wave absorption performance.（2）The electromagnetic wave absorption performance of hollow,open-ended Co Fe2O4spherical particles was improved by combining them with multi-walled carbon nanotubes（MWCNTs）with electrical losses to create a composite electromagnetic wave absorber with multiple loss mechanisms.When multi-walled carbon nanotubes（MWCNTs）are introduced,dipole polarization and conduction losses occur,and when wrapped around Co Fe2O4 particles,interfacial losses rise where the two materials come into contact.The impact of filler ratio on absorber microwave absorption performance was thoroughly investigated.At 25%filling,the maximum electromagnetic wave absorption performance was achieved with a minimum reflection loss(RLmin)of-36.52 d B,a thickness of 1.9 mm,and an effective absorption bandwidth（EAB）of 3 GHz.A high fill ratio,on the other hand,does not provide excellent absorption performance and results in an excessive dielectric constant,resulting in an impedance mismatch.（3）CoFe2O4/nitrogen-doped reduced graphene oxide（NRGO）composite electromagnetic wave absorbers were prepared by a hydrothermal method.Among them,the N atom was successfully doped into reduced graphite oxide（RGO）using ethylenediamine as the nitrogen source and reducing agent,which destroyed the sp2 hybridization of the C atom and produced structural defects and disordered sites and improved the electrical conductivity,accelerated electron transport,increased the polarization loss in alternating electromagnetic fields,and improved the conduction loss.Meanwhile,a large amount of interfacial polarization was formed by NRGO adsorption on Co Fe2O4 under the effect of electrostatic adsorption.The absorption performance was evaluated under different filler loadings.When the loading is 50%,the minimum reflection loss(RLmin)is-58.13d B at a thickness of 3.3 mm and the maximum effective bandwidth is 6 GHz（12GHz-18 GHz）,covering the whole Ku-band at a thickness of1.9 mm.The NRGO/Co Fe2O4 composite developed in this study shows great potential as a high-performance electromagnetic wave absorber. |
