Preparation And Microwave Absorbing Properties Of Graphene/Cement-based Materials

Electromagnetic(EM)wave absorbing materials are the key materials to prevent EM wave radiation pollution.At the present stage,cement is the most common basic material used in the buildings.The research and development of the new high-performance cement-based composite absorbing materials is of great significance to solve the indoor EM wave pollution.Recent years,the researchers at home and abroad have made a lot of explorations in improving the microwave absorption performance and broadening the application frequency band of cement-based composites,but there are still some unresolved issues in the materials,such as the high density,narrow absorbing bandwidth and low absorption performance.The permittivity and permeability of the absorbent are the key parameters that affect the EM wave absorption performance in the cement-based absorbing materials.In particular,the pore structure and the distribution of absorbent of the cement-based composites will directly affect the EM wave absorption performance.It is of great theoretical significance and practical value to improve the absorbing performance of cement-based composites by studying the distribution characteristics of absorbers and explaining the EM loss mechanism of absorbers in porous materials.ln this paper,cement-based composites are prepared by using cement as the matrix material,adding graphene absorbent and hollow glass microbeads respectively,and combining with foaming to make pore.Characterization methods,such as TEM,SEM,EM and mechanical properties tests are used to study the microstructure,EM absorption properties and related mechanisms of the materials.The composites are prepared using ordinary Portland cement and sulphoaluminate cement as the matrix and the graphene as the absorbent.The absorbing properties of the composites in the frequency range of 2-1 8GHzare studied.The results show that the EM wave absorption performance of the composites made of ordinary Portland cement and sulphoaluminate cement is similar,and the added graphene can interact with the electric field component of the incident EM wave to generate conduction loss and thus EM wave loss.When the content of graphene gradually increases,a local conductive network will be formed in the cement matrix,and a current will be formed in the conductive network,thus the EM energy can be converted into heat energy.At the same time,the dipole on the surface of graphene will resonate with EM wave,converting EM energy into heat energy and attenuating it.Therefore,the increase of the amount of graphene can significantly improve the EM wave absorption performance of the cement-based composites.When the graphene content reaches 0.06wt.%and the thickness of the composite is 20mm,the maximum reflection loss(RL)is-36dB at 4.7GHz,and the absorption bandwidth(RL<-5dB)is up to 7.9GHz.The continuous increase of graphene cannot improve the absorbing properties of the composites indefinitely.Compared with pure cement plate,the reasonable amount of graphene is about 0.06wt.%,and its matching thickness is 20～30mm.In order to improve the incident capacity of the EM wave,hollow glass microbeads are added into the cement matrix as a wave transparant material to prepare glass microbeads cement-based composites.With the use of graphene nanosheet absorbent,the absorbing performance of the composites is further improved.In the case that only hollow glass microspheresare added.EM waves enter the cement and are absorbed by the magnetic substance Fe2O3 and hydrate in the cement under the joint action of impedance matching improvement.The EM wave absorption performance is improved with the increase of the number of hollow glass microbeads.When the dosage is 20vol.%and 40vol.%,the absorption intensity of-5dB can be achieved in the frequency range of 2-18GHz,and the performance of the dosage of 40vol.%is better than that of 20vol.%.However,when the glass beads content increases to 60vol.%,the absorbing performance begins to decline.When the glass microbeads content are 40vol.%～60vol.%,the best matching thickness is 20mm.When the glass beads content is small,the addition of graphene has a significant effect on the absorbing property of the composite.When the graphene content is 0.1kg/m3 and the glass microbeads content is 10vol.%and 20vol.%,the absorption bandwidth(RL<-5dB)is 13GHz.When the graphene content is 0.3kg/m3 and the glass microbeads content is 40vol.%and 60vol.%,the absorption bandwidth(RL<-5dB)is 14GHz.The best matching thickness of the glass microbeads cement-based composite is 20mm.In order to further improve the absorbing properties of cement-based composites,a preparation method of the foam-loaded graphene is proposed to study the effects of graphene,porosity and material thickness on the impedance matching performance and wave-absorbing property of the new cement-based porous materials,and analyze the EM wave loss mechanism.The results show that the absorption performance of foamed cement-based composite is improved continuously with the increase of porosity.When the porosity is 52.1%,the absorption performance of the composites is the best,reaching-10dB within the frequency range of 8-18GHz.When the porosity increases to 57.8%,the absorbing property of the composites begin to decrease.The thickness of the material has a significant impact on its EM wave absorption performance.In the range of 10-40mm,the thicker the foamed cement-based porous composite is,the better the wave absorption performance will be.When the thickness of the composite is 40mm,the absorption bandwidth(RL<-7dB)is within the frequency range of 2～18GHz.that is,the absorption rate in the full frequency band is greater than 80%.Compared with the traditional absorbing materials,the introduction of pores makes the input wave impedance of absorbing materials match better with the free space wave impedance,and the absorption performance is not affected by the increase of thickness.In terms of the economy and absorbing property,20mm is the best thickness to match foamed cement-basedporous composite.The analysis shows that the introduction of graphene foam can adjust the matching performance of foamed cement-based composite porous materials with free space impedance,and graphene can improve the EM wave dissipation ability of composite,thus effectively improving the wave-absorbing performance in the frequency range of 2～18GHz.When the porosity is greater than 46%,the effective absorption bandwidth(RL<-10dB)is over 13GHz.The mechanical properties r of the graphene cement-based composite demonstrate that the mechanical properties decrease obviously with the increase of porosity.The addition of the graphene improved its compressive strength to a certain extent,increased by 2.6%.The compressive strength of graphene cement-based composites can reach 5.3MPa after curing 28 days.Due to the building materials used for decoration material on the surface,the strength can completely satisfy the practical requirements.