| Carbon materials have great advantages as light absorbing materials due to their light weight,cheap raw materials,stable chemical properties,and adjustable dielectric loss.Among them,3D porous pure carbon and 1D magnetoelectric composite absorbing materials tend to get more excellent absorbing performance,especially in terms of effective absorption bandwidth.However,there are some problems that need to be resolved urgently.(i)There are still few reports about 3D porous carbon absorbing materials,and most of them are obtained by direct carbonization of biomass,and it is impossible to fine-tune the network porous structure to obtain the best performance;ii)Although the 1D magnetoelectric composite materials have good performance,the existence of magnetic substances also destroys the lightweight and stability to a certain extent,which is not conducive to popularization and application;(iii)During the researches of 3D porous carbon,the dependence of microstructure such as structure closure,pore size and network structure on microwave absorption performance is still poorly studied and unclear.In this paper,based on the metal backplane model,the distribution of electromagnetic parameters of non-magnetic materials with high absorbing properties was studied.In theory,materials with low dielectric constant and high dielectric loss tend to obtain-large effective absorption bandwidth.Based on this,series of carbon-based absorbing materials with long-distance network structure/porous structure such as large-sized flake-like carbon,large-sized hollow cubic carbon,3D continuous porous carbon,and B4C/SiC hybrid nanowires are constructed.Their microwave absorption properties and their corresponding electromagnetic loss mechanisms have been studied.At the same time,the effects of microstructure such as structure closure,pore size and network structure on absorbing performance have been preliminarily revealed.The main contents are as follows:(1)Large-size flake carbon materials were prepared by constructing a two-dimensional confined growth space through dissolution and recrystallization with NaCl and glucose as templates and carbon sources,respectively.Compared with the bulk carbon obtained by direct pyrolysis,these carbons exhibited irregular flake-like structure with rougher surface,larger specific surface area(>291 m2g-1)and mesoporous volume(>0.14 cm3g-1),and the size was up to tens of microns.At 850?,high-temperature NaCl molten salt not only maximized the yield of flaky carbon,but also introduced a richer mesoporous structure,and enhanced the graphitization degree of carbon materials.Compared with bulk carbon,flake carbon materials showed significantly enhanced microwave absorption performance:at 850?,the optimal reflection loss(RL)reached-37.5 dB,and the corresponding effective absorption bandwidth(EAB)was 3.4 GHz;At 950?,the optimal reflection loss reached-41.4 dB,and the corresponding EAB covered the entire X-band.The excellent absorbing performance of the flake carbon material could be attributed to the large specific surface area which enhances the polarization loss induced by the heterointerfaces,and the introduced mesoporous structure which improved the impedance matching,being quite beneficial to absorb more electromagnetic waves energy.(2)Large-sized hollow cubic carbon materials were prepared by combining the NaCl template method and in-situ polymerization method.The carbon materials exhibited a hollow cubic structure with an average size of about 3.6 ?m and a thickness of about 30 nm,which possessed a very large closed cavity volume.N2 adsorption-desorption experiments confirmed that hollow cubic carbons have a high specific surface area(366 m2g-1)and a rich mesoporous structure in the carbon shell.The carbon material exhibited excellent microwave absorption performance when the filling ratio was only 1.5 wt.%:at the coating thickness(d)of 2.9 mm,the EAB reached 8.0 GHz,and the corresponding optimal RL also reached-28 dB,and within 2.6-3.3 mm,their EAB were all above 6 GHz,covering the entire X and Ku bands.The analysis results of electromagnetic parameters showed that the material attenuated electromagnetic waves through polarization loss and conduction loss,and the latter played a dominate role.The large-sized cavity structure also promoted multi-level reflection of electromagnetic waves to extend its transmission path,thereby enhancing electromagnetic attenuation.Further structural damage experiments found that the closed large-sized hollow carbon materials had better absorption performance than the broken structure,because the large-sized hollow closed structure not only enhanced the electromagnetic loss ability,but also improved impedance matching.The advantages of simple preparation process,controllability and environmental protection,and light weight of the materials are beneficial to the practical promotion of the carbon material.(3)A series of 3D continuous porous carbon materials(3D-Void@C)with different pore sizes(206?617 nm)were prepared by combining SiO2 template method and cold pressing sintering method.These carbon materials are filled with continuous pores of uniform size and present a long-range and large-scale porous network structure.When studying the effect of different filling ratios on the absorption performance,it was found that 8 wt.%and 11 wt.%3D-Void@C-617(aperture:617 nm)composite samples both showed outstanding broadband absorption characteristics,and their corresponding maximum EAB values reached 8.2 GHz and 8 GHz.Through a comparative study with dispersed hollow carbon spheres(Void@C-617)of the same pore size,it was found that the 3D-Void@C-617 samples had excellent absorbing performance because of its special porous network structure,which significantly improved impedance matching.When studying samples with different pore sizes,it was found that when the pore diameter of 3D-Void@C was larger,the impedance matching could be improved without significantly degrading the attenuation ability by increasing its size,thereby enhancing microwave absorption performance.Combining the absorbing performance research of Void@C-617 sample under different filling ratios,it was found that large-sized hollow structures were the main influencing factor of broadband absorption characteristics of 3D-Void@C samples rather than the network connection between hollow carbon spheres.(4)B4C/SiC hybrid nanowires were prepared by the improved VLS method with the carbon fiber obtained from carbonization of absorbent cotton as the carbon source.The synthesized B4C nanowires with abundant stacking faults exhibit excellent microwave absorption properties.Heterointerface engineering of the B4C nanowires was realized by introducing SiC nanoparticles into the nanowires through a multi-step VLS process.The SiC embedded B4C hybrid nanowires with unique worm-like structure achieved a broadband effective absorption up to 4.7 GHz(13.3?18 GHz)at 2.5 mm and a broadband absorption up to 4.9 GHz(7.4?12.3 GHz)at 3.7 mm,which covered the whole X-band.Furthermore,the optimized reflection loss value got to-50.81 dB at the thickness of 3.3 mm,almost triple higher than the B4C nanowires.The significantly improved microwave absorption ability primarily resulted from the enhanced dual dielectric relaxation,that is,electric dipole polarization and interfacial polarization.The low reflection loss values and wide absorption bandwidth of the SiC/B4C hybrid nanowires,together with its high temperature stability and light weight make it a good candidate as highly efficient electromagnetic wave absorbers under harsh conditions. |
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