| The 5G frequency bands approved by China's Ministry of Industry and Information Technology include 3.4-3.6GHz,4.8-5.0GHz,24.75-27.5GHz and 37-42.5GHz.Nowadays,there is little research about microwave absorbing materials applied in frequency band above 24 GHz,so it is pretty significative to study the high frequency absorbing materials that can be applied in 24.75-27.5GHz and 37-42.5GHz,the high frequency bands of 5G.Graphene(RGO)prepared by chemical oxidation-reduction reaction has more residual groups and structural defects,which can not only reduce the dielectric constant of graphene,but also trigger a variety of polarization effects to increase the loss of electromagnetic waves.Barium ferrite(BaM)has high magneto crystalline anisotropy field and natural resonant frequency up to 42.5GHz,which are beneficial for its absorption property in the millimeter waveband.Nitrile rubber(NBR)is a kind of flexible engineering material that exhibits great resistance in both high and low temperature,and it also has good toughness and strong machinability.In this dissertation,flexible wave-absorbing composites were prepared with graphene,barium ferrite and NBR,where NBR was the substrates.The absorption properties of composites in Ka-band(26.5-40GHz)were analyzed in detail,and its availability in 5G high frequency was also discussed.Firstly,the wave absorbing properties in Ka-band of RGO/NBR composites with different reduction time were studied.The results illustrated that all RGO/NBR composites showed excellent absorbing properties and 3h-RGO/NBR was the best among them for achieving the maximum reflection loss(RL)at most thicknesses.3h-RGO/NBR can achieve the maximum absorption loss of-20 dB at the thickness of 2.0-2.4mm.when the thickness is 2.1mm,the maximum absorption loss can be obtained at 35.4GHz,and the effective absorption bandwidth is 5GHz.The absorption properties of RGO/NBR composite material reduced by 5h and 7h are similar,where the maximum absorption loss of-15 d B can be obtained in all test thicknesses.In addition,when the thickness is 2.1mm and 2.5mm,the RGO/NBR composites with different reduction time can obtain effective absorption of-10 dB at 26.5-27 GHz and 37-40 GHz respectively,which indicated the possibilities used for the absorption materials of 5G high-frequency electromagnetic wave.Compared with RGO/NBR composites with different contents of graphene,RGO/NBR composites could achieve the best absorption loss of-18.2dB and the effective absorption bandwidth of 2.6GHz when the graphene content was 12%.Secondly,this dissertation studied the effect of barium ferrite content and thichkness on the absorption properties in Ka-band of BaM/NBR composites.The results demonstrated that the RL of BaM/NBR composites at different thicknesses were mainly centralized in the high frequency region above 37 GHz.When the thickness was 5mm,BN41 could obtain the maximum reflection loss of-36.2dB at 39.7GHz,the effective absorption spectrum below-10 dB was 38.8-40 GHz,which could cover part of the 5G high frequency band.Therefore,it could be used as a 5G absorbing material of high frequency band.Finally,we replaced part of the barium ferrite with equivalent RGO,prepared RGO/BaM/NBR composites with different mass ratio of absorbing agents,and explored the wave absorbing properties in ka-band of composites.The results showed that the addition of graphene effectively improved the compatibility and absorbing performance of RGO/BaM/NBR composites,and the effective absorption bands below-10 dB of composites with different RGO mass and thicknesses could cover part of 5G high frequency bands.In addition,this paper also compared the reflection loss of the composites resulted from different measuring methods and calculation ways.The results showed that different equipments and calculation methods would make little difference on the absorbing performance for the case of the device itself and the structural parameters of the test materials. |
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