The Research On Microwave Absorbing Properties Of Graphene Composites

Due to the high demands in military field and the rapid development of electronical and communication industry where electromagnetic?EM?wave has been utilized extensively,leading to severe EM interference problems,the fabrication of light weight,small thickness,strong absorption and high effective bandwidth EM wave absorbing material has been an urgency.Graphene,possessing excellent physical properties,has been viewed as revolutionary material and received lots of attention from researchers once it had been found.Owing to high specific surface area and excellent thermal conductivity,graphene can be used as promising EM wave absorbing material.However,imbalanced impedance of graphene can't meet the practical requirements.Thus,in order to improve the impedance match and EM wave absorbing properties,the hybrids of graphene and magnetic particles have been developed which combined the dielectric loss and magnetic loss.In this thesis,graphene oxide has been prepared by Hummers method,followed by conversion into reduced graphene oxide?RGO?using hydrazine hydrate as reduced agent.When the mass ratio of RGO in paraffin was 30%,the effective bandwidth of RGO-paraffin composites reaches 4.7 GHz?13.3-18.0 GHz?and the maximum reflection loss?RL?of-18.3 dB at 15.9 GHz has been achieved with the thickness of 1.5mm.The improved EM wave absorbing property originates from the dipolar polarization relaxation and conductivity loss of RGO.Matrimony vine-like ?-Fe₂O₃/RGO has been prepared by a facile method,followed by transformation into Fe₃O₄/RGO through annealing at 500 ? with the particle size and morphology significantly unchanged.By changing the mass ratio of iron oxide,the hybrids display optimal electromagnetic wave absorption performance.The maximum reflection loss value of ?-Fe₂O₃/RGO is-46.6 dB at 5.6 GHz with a thickness of 4.0 mm and the highest effective absorption bandwidth reaches 4.9 GHz at the thickness of 1.8 mm.Interestingly,after annealing treatment,the hybrids in which ?-Fe₂O₃ were converted into Fe₃O₄ exhibit an effective absorption bandwidth of 4.6 GHz and the maximum reflection loss value reaches-42.8 dB at 13.3 GHz with the thickness of only 1.8 mm,indicating Fe₃O₄/RGO can be used as an efficient electromagnetic wave absorbing material in high temperature and moisture environment,where Fe₃O₄ will be oxidized into ?-Fe₂O₃.Results show the enhancement of electromagnetic wave absorption performance originates from synergistic effects of dielectric and magnetic loss,including dipolar polarization relaxation,interfacial polarization relaxation,conductivity loss,natural resonance,exchange resonance,etc.Crystalline FeCo/RGO high performance EM wave absorber has been prepared by combining co-precipitation and sintering methods and the EM wave absorbing properties of FeCo/RGO with different mass ratio of FeCo have been researched.An effective bandwidth of 5.0 GHz?12.3-17.3 GHz?has been achieved at the thickness of only 1.6 mm,while the maximum RL reaches-41.7 dB?17.1GHz?at the thickness of 1.4 mm.Results show that the excellent EM wave absorbing properties benefit from the synergistic effects of dielectric and magnetic loss.Furthermore,impedance matching ratio and attenuation constant were calculated to disclose the absorbing mechanisms.At last,the relationship between absorber thickness and peak frequency was explained according to quarter-wavelength matching model.