Stretchable Chemically Crosslinked And Nitrogen Doped Graphene Films With High Electrical And Electromagnetic Shielding Properties

In the field of aerospace,aircraft such as airplanes and satellites are equipped with important electronic components or devices,which are susceptible to interference from strong electromagnetic(EM)fields.Besides the electromagnetic protection function,some parts of the aircraft should also meet the mechanical requirements of large deformation and stretchability during the service.Excellent electrical conductivity is vital to fabricate highly EM shielding materials.However,present flexible electronic devices are hard to withstand large deformations such as stretching or bending while maintaining their excel ent electrical conductivity stably.The reduced graphene oxide films(r GO)are light and flexible,with high electrical conductivity and electromagnetic interference(EMI)shielding properties,which can be fabricated on the large scale and at the low cost.However,due to structural defects,the residual oxygen atoms and the contact resistance between the graphene sheets,the electrical property of the rGO films is much inferior to the theoretical value.Besides,graphene films are rigid,which are hard to be stretched with large deformations.The purpose of this paper is to improve the electrical and EMI shielding properties of rGO films.Therefore,the effects of cross-linking,size of graphene flakes,graphitization temperature and nitrogen doping on the properties of the r GO films are investigated.Based on a two-layer stretchable structure,the stretchable graphene/PDMS composites with high shielding effectiveness(SE)are fabricated.In this paper,the influence of cross-linking on the electrical conductivity and EMI shielding properties of carboxylated graphene films is studied.The results reveal that the modified reagents such as ethylenediamine,butanediamine and p-phenylenediamine can improve the electrical and EMI shielding properties of graphene films.The main reasons for the improvement of the shielding performance of the cross-linked graphene films include nitrogen doping effect,chemical reduction of carboxylated graphene sheets,and cross-linking structure.In order to further improve EMI shielding performance of r GO films,GO is graphitized at high temperature.This paper investigates the influence of graphitization temperature and size of graphene flakes on the electrical conductivity,thermal conductivity and EMI shielding performance of graphene films.When graphitized at 2000?,rGO film possesses SE of 23.7 d B.Both the increase of graphitization temperature and size of GO flakes would improve electrical conductivity,thermal conductivity and EMI shielding performance of the rGO films.To study the effect of porous structure on the EMI shielding performance of the r GO films,the model of multi-layer graphene laminates is established.The results indicate that increase of the space between graphene laminates is beneficial to enhance SE of the rGO films.Combining graphitization at high temperature and nitrogen doping,electrical conductivity and EMI shielding performance of r GO films are further improved.Experimental results demonstrate that nitrogen doped graphene film has three kinds of nitrogen doping forms: pyrrole nitrogen,pyridine nitrogen and graphite nitrogen.Compared with the rGO film,graphene films modified with ethylenediamine show an increase of 28.5% in the electrical conductivity.EMI shielding performance of nitrogen doped graphene films is mainly based on absorption loss.N itrogen doping and improvement of electrical conductivity are beneficial to enhance the absorption loss of the rGO films.The first principle method is applied to study the influence of cross-linking and nitrogen doping on the electron transport properties of graphene nanoribbon(GNR)devices.The results show that cross-linking and nitrogen doping enhance the transport properties of the GNR device.When the bias voltage is 2.0 V,the currents of the ethylenediamine,butanediamine and p-phenylenediamine crosslinked GN R devices are increased to 259 nA,248 nA and 5340 nA,respectively,which are much larger than the current of the GNR device(0.74 nA).When the bias voltage is 1.5 V,graphite nitrogen doping increases the transport current of the GNR-44 device by 3 times.The increase of graphite nitrogen atoms would further improve the transport properties of the GNR device.Based on the nitrogen doped graphene film and the wavy PDMS substrate,the stretchable and highly conductive composite film with excellent EMI shielding performance is obtained.For the wavy PDMS substrate,the suspended part of the graphene film can be regarded as a thin shell structure with two ends fixed.The simulation results show that the maximum strain occurs at the constraint of two ends,that is,the junction of the bonded part and suspended part of the graphene film.By pre-stretching the PDMS substrate biaxially,the stretchable conductive composite film with 50.5% strain is obtained.When the tensile strain is 50.5 % and number of tensile-relaxation cycle is 500,the resistances of the graphene film are 1.41 ? and 1.38 ?,respectively,with the changing rate of only 5.2% and 2.9%;the SEs of the composite film are 55.4 dB and 56.1 dB,which are almost the same as the nitrogen doped graphene film.