| In order to improve the processing technology and accessional value of natural microcrystalline graphite as well as meet the demand of thin,light,broad and strong microwave-absorbing materials,the preparation of graphene from natural microcrystalline graphite was studied in this paper.The effect of reduction process on the structure and composition of graphene was described.Furthermore,in this work the electromagnetic properties of the prepared graphene were studied,and the mechanism of the formation and electromagnetic wave absorption of the prepared graphene was also analyzed.The main contents of this study are listed as below:1.The preparation of graphene by liquid phase oxidation reduction with natural microcrystalline graphite as raw material was studied.The results show that The prepared graphene has an area of about 2 ?m2.The number of carbon atoms is less than that of the layer,which is mostly 3-4 layer with a thickness of about 1.1 nm.The molar ratio of carbon to oxygen atom is about 6.5:1,and the conductivity is up to 358.423 S·m-1.2.The effects of chemical reduction method and solvothermal method on the structure and composition of graphene were investigated.The results show that the graphene oxide can be reduced effectively by solvothermal reduction method and chemical reduction method.The prepared graphene by solvothermal reduction method has smaller layer thickness,less carbon atom number,and there are more internal defects in this method.In the process of solvothermal reduction,the reaction temperature has a great influence on the morphology and structure of graphene and ethanol concentration mainly affects the composition of graphene.3.Based on the analyses of the microstructure,crystal structure and elemental composition of the prepared graphene,the process of the transition from natural microcrystalline graphite to graphene oxide and then to reduced graphene oxide was studied.The process includes three stages:oxidation,stripping and reduction,and the effect of reduction on the quality control of the prepared graphene were significant.4.The effects of ethanol concentration and reaction temperature on the structure and composition of the prepared graphene were investigated.The results show that the reaction temperature has a great influence on the morphology and structure of graphene and ethanol concentration mainly affects the composition of graphene.With the increase of reaction temperature,the particle size of graphene becomes smaller,the edge of graphene become more wrinkled and graphene become easier to agglomerate.With the increase of ethanol concentration,the oxygen content of graphene decreases and the molar ratio of carbon atoms and oxygen atoms increases.When the ethanol concentration was 50%,the molar ratio of carbon atoms and oxygen atoms is 7.4:1.5.The effects of ethanol concentration and reaction temperature on the electromagnetic properties of the prepared graphene were studied.The results show that graphene/paraffin composites with a thickness of 4 mm have better absorbing properties when the ethanol concentration is up to 40%,and the mass fraction is 5%under the reaction temperature of 180? and the ethanol concentration of 40%.Meanwhile,the effective bandwidth of less than-5 dB and-10 dB are 7.2 GHz and 1.76 GHz,respectively.There are double absorption peaks in this graphene/paraffin composites,and the reflection loss can reach-11.17 dB.6.The electromagnetic wave absorption mechanism of natural microcrystalline graphene composite material was analyzed.The electromagnetic wave dissipation may be derived from the dual Debye relaxation process,which is caused by the graphene layer structure of the multiple reflection of electromagnetic wave and the dipole scattering of internal defects and impurities formation.This study will provide the scientific bases for the further improvement of the comprehensive utilization and the level of deep processing technology of natural microcrystalline graphite mineral resources. |
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