Electromagnetic Characteristics And Microwave Absorption Property Of Heteroatoms Doped Graphene

Graphene,as one kind of carbon materials,has the advantages of high electrical and thermal conductivity as well as low density;which make it potentially an ultra-light microwave absorption material.It can remedy the high density and large loading content of traditional microwave absorption materials.However,pure graphene is diamagnetic,its electromagnetic matching characteristic need to be improved when it is used as the absorbing material individually.Based on the above issues,non-metal elements doping was proposed to improve the magnetic property and electromagnetic matching characteristic of graphene,and thus modulating its microwave absorption property.Nitrogen doping was adopted to improve the magnetic property,and microwave absorption performance of graphene.The influences of doping methods,reaction conditions and graphene oxide(GO)precursors on doping contents,doping sites and magnetic properties were studied.(i)The experimental results of hydrothermal method demonstrated that pyrrole-type nitrogen contributed the most to the magnetic moments which are consistent with the theoretical results that each pyrrole-type nitrogen contributed the highest magnetic moment(0.94 μB).(ii)The thermal treatment method indicated that the incorporation of doped nitrogen atoms into the graphene sheets were through certain specific oxygen-containing groups,and there was a competitive relationship between doping and reduction process.GO after 900°C heat-treatment can get a saturation magnetization(Ms)of 0.60 emu/g,but at the same heating temperature further doping with nitrogen,the Ms can be increased to 0.67 emu/g.(iii)On the basis of heat-treatment method,we studied three different GO precursors(rippled graphene oxide: h-GO,folded graphene oxide: s-GO and flower-like graphene oxide: g-GO),and explored the influence of precursors on the magnetic property.The results demonstrated that the Ms of the three GO samples after nitrogen doping were 0.31,0.38 and 1.31 emu/g,all of which were more than twice of their GO precursors(0.10,0.16 and 0.52 emu/g).On the basis of nitrogen doped graphene,dual-doped graphene was further designed and investigated.The influences of doping elements(nitrogen,sulfur)and their doping order on morphology,structure,magnetic property and microwave absorption performance were studied.The first doping decisive effect was found,that is,the first doping stage plays a decisive role on the properties of the final dual-doped graphene samples.It was found that at the same thickness,the single-doped graphene generally has single characteristic absorption peak in the reflection loss(RL)curve;nitrogen-doped graphene absorption peak tend to appear in the frequency range of 5~10 GHz,the absorption peak of sulfur-doped graphene appeares in the frequency range of 11~16 GHz.The dual-doped graphene tend to have two absorption peaks corresponding to the above two frequency ranges.Nitrogen first doped graphene(NSrGO)exhibited a stronger absorption peak in 5~10 GHz,and sulfur first doped graphene(SNrGO)had a stronger absorption peak in 11~16 GHz.This work designed series of doped graphene samples used as electromagnetic wave absorbing materials.The relationship between electromagnetic parameters and absorption performance were studied.The results indicated that when the permittivity of the doped graphene powders/paraffin sample is around 10-bi,good absorption can be obtained owing to the balanced impedance match and attenuation ability.Through separating the RL into material loss and λ/4 cancellation loss,it was revealed that the position of RL absorption peaks are determined by the λ/4 principle,while the material loss accounts more than λ/4 cancellation loss in the total reflection loss.Among all the doped graphene samples,the optimized RL in 2~18 GHz is g-NrGO with-21.7 dB at 10 wt.% loading and d=2mm;the strongest RL is-37.1 dB of h-NrGO with 20 wt.% loading at d=5 mm.Multi-absorption bands and strong absorption performance can be obtained by further finely control the loading content,thickness and doping elements of the doped graphene samples.