| The advancement of gigahertz(GHz)band communication technologies has significantly brought convenience for humans while it also leads to hazardous electromagnetic(EM)pollution and interference for electronic facilities and humans.The electromagnetic waves(EMWs)absorption and interference(EMI)shielding materials could absorb or reflect the incident EMWs,obviously weakening the EM pollution and interference.The former can convert the EM energy into heat energy and dissipate it,and the latter can attenuate the incident EMWs through reflection and absorption,weakening the penetration of incident waves and protecting the electronic components and organisms which are exposed to EM radiation.With the development of miniature and portable facilities,it is of great significance to explore lightweight and efficient EMWs absorption and EMI shielding materials.The porous materials have the advantages of low density,large specific surface area,and rich porous structure,which can reduce the density of EM protection materials and enhance the multiple reflection/scattering of incident waves.Nevertheless,the effective and wide-range control of EM parameters in porous materials is difficult,and the adjustment of impedance matching and attenuation capability is limited,hardly meeting the absorption/shielding requirements of "thin,light,wide,and strong".To solve the above problems,in this study,focusing on the MOFs-derived,aerogel-,and hydrogel-based porous materials,their electromagnetic parameters were controlled by adjusting the composition and designing the interior structure.The effects of various composite and content as well as interior structures on absorption/shielding performances and their contributions to property improvement were explored in depth.By effectively controlling the component and interior structure,the highly efficient EMWs absorption and EMI shielding performance were achieved.The main contents are as follows:(1)Porous Fe7S8/C composites derived from Metal-Organic Frameworks(MOFs)were successfully constructed via a simple and high-temperature sulfurizing method.Benefiting from the alterable component property of MOFs,by controlling the introduced dosage of a ferric salt,the proportion of carbon and Fe7S8 in composites could be adjusted.The carbon provided the conductive loss,and the presence of Fe7S8 inhibited the excessive permittivity of carbon,thus optimizing the impedance matching of the porous Fe7S8/C composites.By adjusting the relative content of two components,the influences of Fe7S8 and carbon on the impedance matching characteristics,electromagnetic loss ability,and absorption performance of composites were explored and optimized.The optimal reflection loss(RLmin)value reaches-68.86 dB at 12.08 GHz with a thickness of 1.67 mm,and the effective absorption bandwidth(EAB)is up to 4.56 GHz at 1.45 mm.The synergistic effect of Fe7S8 and carbon facilitate the EMWs absorption performance.(2)In order to maintain the above advantages in absorption properties and materials preparation,and further optimize the performance in medium and low frequency ranges,introducing the hollow structure and heterostructure to optimize the impedance matching and polarization loss capability,the bimetal MOFs derived NiS/ZnS/C hollow composite microspheres with abundant heterostructure were prepared.The synergistic effect between the conductive/dielectric components,heterogeneous structure,and the impedance matching components on the absorbing performance was explored:dielectric constant and loss capacity of the materials were significantly improved by the NiS with semi-metallic characteristic;ZnS inhibited the excessive permittivity and improved the impedance matching of the composites.Moreover,the abundant heterostructures of NiS/ZnS/C could improve the polarization loss capacity of the composites.Under the synergistic effects,the NiS/ZnS/C hollow composite microspheres exhibited excellent absorbing performance.The RLmin reached-51.45 dB and-56.69 dB at the low frequency of 4.72 GHz and high frequency of 11.12 GHz with matching thicknesses of 3.46 mm and 1.67 mm,respectively.Furthermore,the EAB is 3.68 GHz with matching thicknesses of 1.16 mm.By adjusting components and constructing multiple heterogeneous interfaces,the absorption performance of NiS/ZnS/C composites in the middle and low frequency range was optimized,making up for the deficiencies of porous Fe7S8/C composites.(3)Utilizing the conductive MXene nanosheets and cellulose nanofibers(CNFs)as the matrix,the freestanding porous aerogel materials were prepared by the facile and low energyintensive ambient pressure drying(APD)method.The content of MXene could be adjusted in the wide range of 0-70 wt.%and the porous structures could be still maintained,achieving the combined structural and functional performance.The MXene/CNFs had an RLmin of-52.2 dB with a thickness of 1.63 mm and EAB of 4.48 GHz.As the MXene content further increased to 70 wt.%,the shielding performance of MXene/CNFs aerogel with a thickness of 2 mm was more than 60 dB in the wide frequency range of 8.2-40 GHz.This work could prove that the APD MXene/CNFs porous aerogels possessed the advantages of widely controllable component ratio,achieving the EMWs absorption and EMI shielding performance and extending the application scenario.(4)In order to explore the influence of interior pores on the EMI shielding performance of porous materials,MXene/PVA hydrogels with water-enrich pore structure,high strength,and superior flexibility were prepared by the salting-out method.The synergistic effect of highly conductive MXene nanosheets,water-enrich pores,and MXene/PVA conductive cell walls make hydrogels have excellent EMI shielding properties.The shielding performance of MXene/PVA hydrogel containing only 2.67 wt.%MX ene could reach 48.6-94.2 dB with a thickness of 1.5-8.5 mm in the X-band.In addition,during the 8.2-40 GHz broad frequency range,the EMI shielding performance of MXene/PVA hydrogels was more than 50 dB.Particularly,by controlling the water content of the hydrogel,the relationships between the water in the pores and the shielding performance of the hydrogels were explored.This experiment clarified the synergistic effect of the conductive cell walls and the water-rich pores for promoting shielding performance.(5)In order to explore the influence of pore structure on the EM properties and further reduce the costs of conductive hydrogel-based EMI shields,the "discarded" MXene sediments(MS)during the preparation of MXene were used as conductive fillers to construct the conductive MS-based hydrogels with aligned pore structures through the directional freezing and salting-out method.Thanks to the synergic effects of the highly conductive MS/PVA cell walls,aligned porous structure,and water-enrich pores,the EMI shielding performance of MSbased hydrogels could be adjusted in the range of 31-91 dB with the thickness of 1.0-7.5 mm.During the wide band range(8.2-40 GHz),the MS-based hydrogels with a thickness of 2 mm provided an efficient EMI shielding performance of more than 40 dB.More important,by adding a trace of silver nanowires into the hydrogel and adjusting the aligned pores perpendicular to the incident waves,the EMI shielding performance of hydrogel materials can be further improved.The influence and mechanisms of material composition and porous structure on the shielding performance of hydrogel were explored and clarified,which guided the design of hydrogel-based shielding materials. |
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