| With the advent of 5G era,the number and density of modern electronic devices are increasing rapidly,which leads to the problem of electromagnetic interference(EMI)in the process of signal transmission,which will affect the performance of surrounding precision equipment and even threaten human health.Currently,how to prepare high performance electromagnetic shielding materials has become one of the hot topics of research.In recent years,the graphene-based composite film has greatly developed in the field of electromagnetic shielding.However,it is still a huge challenge to prepare graphene-based composite films with excellent mechanical properties,conductivity and electromagnetic shielding properties.Therefore,taking carbon nanofibers and graphene as the research objects,graphene films with different pore sizes were prepared,and their mechanical properties,electrical conductivity and electromagnetic shielding properties were studied.The main research work is as follows:1.Firstly,the electrospun aPAN nanofibers were treated with alkaline solution,and the carboxyl group was formed on the fibers surface,which could interact with graphene(GO)to improve the mechanical properties of the composite films.The GO-aPAN composite films were then prepared by ultrasonic dispersion and vacuum filtration.Finally,the electrospun carbon nanofibers enhanced graphene films(GCFs)were prepared by pre-oxidation and high temperature annealing treatment.Microscopically,carbon nanofibers(CNFs)are intercalated into the graphene sheets,and microgasbag structures are formed during the heat treatment process.The special structure makes GCFs possesses superior tensile strength(10.4 MPa)at 5%strain.After repeated folding over 1000 times,the films still demonstrate excellent structural integrity and flexibility.In addition,the graphene-based composite films with 10 wt%aPAN nanofibers exhibit an extremely low density of about 0.678 g/cm~3 and excellent electrical conductivity of 1.72×10~5 S/m.An outstanding electromagnetic shielding effectiveness(SE)of 55-57 d B was achieved,and the corresponding value of the specific SE/thickness can reach 67601-70059 d B·cm~2/g,which is the highest among reported graphene-based shielding materials.The significant electromagnetic shielding performance is due to the synergistic enhancement effect brought by the excellent conductivity of carbon nanofibers and graphene,the formed effective conductive network and the microgasbags structure.Electromagnetism simulation further clarified that the underlying mechanism should be mainly attributed to the conduction loss and multiple reflections caused by the special structure of GCFs.2.Aerogels were obtained by hydrothermal reactions and freeze drying with GO solution and electrospun aPAN nanofibers.Further,the carbon nanofiber reinforced graphene aerogels(GCA)were obtained by pre-oxidation and high temperature annealing.Finally,the graphene-based aerogels films were obtained by a method of pressing.The final results show that the electrical conductivity of the pure graphene aerogel films(GA)is only4.7 S/cm.After adding carbon nanofibers,the electrical conductivity of the GCA films has been greatly improved,especially the conductivity of GCA-10 can reach 20.5 S/cm.For the electromagnetic shielding properties,change from 73.7 d B(GA)to 78.8 d B(GCA-10),and the overall shielding effectiveness of the aerogel films can reach above 99.999995%.In addition,the aerogel films also exhibit excellent photothermal conversion properties,which is expected to be applied to the fields such as light absorption and crude oil transmission.3.On the above research work,the alternating composite films were prepared by the method of scraping,which taking the GO/aPAN nanofiber as solution and the PVA aerogel film as a substrate.Then graphene based alternating films(GC/C)with macroporou-microporou structures were prepared by high temperature annealing,and the effects of different layers on the properties of the films were investigated.When the total number of alternating layers increases from 1 to 7,the thickness of the film also shows an increasing trend(3-160μm).Although the increase of pores in the alternating layer will reduce the contact between graphene nanosheets,the film(GC/C-7)still shows an excellent conductivity(117 S/cm)and a prominent tensile strength(3.5 MPa).In addition,the films are provided with excellent structural stability and show stable electrical conductivity and mechanical strength even after 250 cycles of bending.In the range of 8-12 GHz,the electromagnetic shielding value is increased from 32 d B to 80 d B,and the shielding effictiveness increases from 99.87%to 99.99999%.In addition,through the analysis of their shielding mechanism,it can be found that absorption is the main mechanism for their excellent electromagnetic shielding effect.Due to the microgasbags of graphene and carbon nanofiber composite film and the porous structure of alternating carbon materials,the multiple reflection loss and absorption of electromagnetic waves are promoted.Therefore,the alternative composite film can not only realize the advantages of light weight and small thickness of graphene film,but also realize high conductivity,high mechanical strength and electromagnetic shielding performance. |
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