Preparation And Microwave Absorbing Properties Of Composites Based On Graphene

Microwave absorbing material has gotten people's attention because of the emergence of radar and stealth aircraft.While,with the development of electromagnetic equipment,electromagnetic(EM)hazard is an increasing problem.And people start to pay more attention to electromagnetic pollution which can not only reduce the accuracy of equipment,but even injure health.Based on this circumstances,microwave absorbing material is increasingly expanded from military field to civil field.At the same time,extensive efforts have been made to develop EM wave absorption materials by countries.But unfortunately,the existing microwave absorbing material has high density and poor microwave absorbing property.As a new type of two-dimensional(2D)material,graphene draws more attention of the society since it has been found.With a series of prominent property,such as extraordinary electrical and thermal conductivity,excellent strength and transmittance as well as the large surface area and the very low density,graphene is a potentially promising candidate for microwave absorbing material that can meet the requirements of microwave absorbing material.But it is harmful to its EM absorption in terms of impedance match mechanism if graphene is microwave absorber only.Therefore,one of the most effective ways to solve the problem is to couple graphene with other types of absorbers.In this thesis,graphene oxide(GO)is prepared by the improved Hummer method and reduced graphene oxide(rGO)/Fe2O3 composites are fabricated by the hydrothermal method.Scanning electron microscope(SEM)and TEM reveal that some Fe2O3 nanoparticles,with porosity,anchored uniformly on the graphene sheets.Some Fe2O3 nanoparticles are attached on the surface of the graphene sheets,and there is significant portion of the Fe2O3 nanoparticles encapsulated inside the graphene sheets,which can prevent the aggregation of particles efficiently.In addition,we prepare large-area rGO/PVA EM wave absorbing film using a simple solution processing method.We obtaine the homogeneous GO/PVA solution and then L-ascorbic acid is added into mixture solution stirred at 50 ? for 24 h.The film we made can be cut into various shapes and different sizes by a scissor and it can be also bended at any angle,which is good for the practical application.At last,the microwave absorbing property of rGO/Fe2O3 and rGO/PVA are investigated in detail by vector network analyzer(VNA)and at the same time,we further studied the microwave absorption mechanism.EM parameters and EM wave absorption performance of the as-prepared materials are measured and calculated in detail.The results indicate that components of the composites is major factor that affects the EM wave absorption.Pure rGO,Fe2O3 and porous Fe2O3 have poor EM wave absorption property.But porous Fe2O3 has better EM wave absorption performance than Fe2O3.In contrast,rGO/Fe2O3 shows excellent EM wave absorption property.The minimum RL is-38 dB when the porous Fe2O3 weight ratio is 70%with thickness is 2 mm at 14.78 GHz,and The bandwidths of RL values below-10 dB(90%of EM wave absorption)exceeds 5.8 GHz.Besides,The minimum RL of rGO/PVA is-36.4 dB when the mass percentage of GO is 0.9 wt%and the thickness is 5 mm at 4.5 GHz.In addition,when the thickness of film is 2 mm,the bandwidths of RL values below-10 dB can exceed 7.7 GHz.And pure PVA has very weak EM absorption property at every sample thickness.Furthermore,the thickness of composites is another factor that affected the EM wave absorption.The experimental results show that with the increase of the absorber thickness,the minimum RL corresponding to the maximum absorption gradually decreases,and the corresponding frequency shifts towards lower frequencies.So we can change the thickness of as-prepared composites to adjust the location of RLmax to realize multi-band frequency absorption.In a word,the as-prepared composites achieved excellent EM wave absorption performance.Through the analysis to their microwave absorption mechanism,these two materials can get excellent absorbing property because dielectric loss and a good impedance matching.For rGO/PVA film,PVA possesses a wide range of wave-transparent property change.The substance of wave-transparent is providing channels for transmission of EM wave.