Electromagnetic Dynamic Regulation Theory And Experimental Studies Of Graphene

Tunable electromagnetic devices have become hotspots due to the advantages of high integration,high efficiency,small size and multifunction.Diodes,micro-electromechanical systems(MEMS),metamaterials or field effect transistors(FET)are used to modulate their electromagnetic parameter such as dielectric constant,resonant frequency,bandwidth.However,these methods generally involve high power consumption,excessive electronic components and limited modulation capacity or other issues.It is urgent to explore new materials,new structures and new principles in the field of tunable electromagnetic devices to improve the modulation performance of microwave equipment.Therefore,we have prepared frequency selective surface,antenna and Salisbury screen devices based on the tunable properties of graphene materials by means of combination with ionic liquids.The main research results of this paper are as follows(1)Electrically tunable graphene/ionic liquid/graphene structure is proposed.The graphene sheet resistance can be modulated by applying bias voltage.Then,the dynamic modulation of electromagnetic wave behavior is realized.Firstly,the types of substrate,ionic liquids and preparation methods of contact electrodes are determined by analyzing the electromagnetic properties of graphene capacitor structure and its influencing factors.Further,the theoretical system is established by using the equivalent circuit of graphene capacitor structure.These laid the experimental and theoretical foundation for tunable devices(2)The graphene capacitor structure was introduced to prepare two kinds of tunable frequency selective surface(FSS)devices:band-pass frequency selective surface and band-stop frequency selection surface.Further,a band-stop type frequency selective surface is prepared,and the structure is only composed of a metal unit and a graphene capacitor structure.By applying bias voltage,the transmission coefficient at the resonant frequency can be changed from 82%to 25%,and the modulation depth is 69%.The band-stop FSS only consisted of metal unit and graphene capacitor structure.By applying bias voltage,the transmission coefficient at the resonant frequency rises from 10.11%to 26.59%,and the modulation depth is 61.9%.In the dynamic adjustment process,the transmission coefficient between the frequency range 12.1 GHz and 15.2 GHz are gradually increased.The transmission coefficient of frequencies above 15.2 GHz and below 12.1 GHz decreases gradually.The graphene capacitor structure has a peak-shaping effect on the transmission parameters throughout the regulation process.(3)By combining graphene capacitor with metal patch antenna,we apply voltages to change the conductivity of graphene,then the radiation performance of the antenna can be changed.When the voltage is applied from 0 V to 2 V,the resonate frequency varies from 10.7 GHz to 9.45 GHz,the S11 parameter varies from-6.05 dB to-23.11 dB,and the gain of the antenna at resonance frequency changes from 4.18 dB to-1.56 dB.It shows that the voltage-controlled graphene capacitor structure works as a switch.(4)The graphene capacitor structure is used to replace the impedance layer of traditional Salisbury screen absorbing structure.Then the input impedance of the whole device can be regulated by applying voltages.By this means,the dynamic regulation of wave absorbing performance can be realized.The effects of sheet resistance,dielectric thickness and medium type on electromagnetic wave absorption efficiency and resonant frequency are analyzed.According to these results,a Salisbury screen with tuntable reflection parameters is prepared.The measurement results show that the reflectivity of the resonant frequency is-42.09 dB when the bias voltage is 0.6 V.At this time,the reflection coefficient is the minimum and the system impedance matching is the best.Finally,equivalent circuit model of tunable Salisbury screen is established.We analyzed theoretically the relationship between the reflectivity and graphene sheet resistance,the relationship between the type of the dielectric,thickness of the dielectric material and the resonant frequency respectively.Our research in this paper provides a new idea and method for tunable electromagnetic devices,and has good potential application in the fields of microwave engineering.