Preparation Of Three-dimensional Carbon Matrix Composites And Study On Their Wave-absorbing Properties

Nowadays,electromagnetic wave has been proverbially utilized in communications,military,aerospace,electrical appliances,etc.However,the ensuing problems of electromagnetic wave pollution and radiation are more and more serious and have become the new pollution source after water,air and noise.Therefore,it is quite critical to develop efficient electromagnetic wave absorbers.Due to their low density,high thermal stability,oxidation resistance and tunable dielectric constant,carbon-based materials have drawn lots of attention in electromagnetic wave absorption.In this work,based on the theory of structure determinning performance,three-dimensional mesoporous carbon hollow microspheres(PCHMs)are proposed as efficient electromagnetic wave absorber.Subsequently,sulfur doped and in-situ growth of magnetic nanoparticles on PCHMs are carried out,it is expected to promote electromagnetic waves absorption through enhanced interfacial polarization,dipole polarization as well as the coordination of dielectric and magnetic properties.The main research contents and results are as follow:(1)Three-dimensional carbon spheres with mesoporous shell and hollow cavity were obtained by combining the hard template method and pyrolytic etching process.The influence of calcination temperature on the structure of PCHMs(mesopores,shells,defects,etc.)was studied,and the difference dielectric response deriving from calcination temperatures was analyzed.The results show that the particle size of PCHMs gradually increases with the temperature rising,and the size of inner cavity and mesoporous shell also increases.Different temperatures have no significant effect on the formation of the mesoporous shell,but higher temperatures may destroy the mesoporous structure.It can be found that,except for PCHMs-750,the dielectric properties of other samples show regular changes with temperature,the obvious difference can be attributed to the strong interference of polarization relaxation on the materials.In addition,the complex permeability of PCHMs responds to dielectric conversion due to electromagnetic coupling effects.Based on the enhanced dielectric loss,the minimum reflection loss(RL_min)of PCHMs-650 at 3.6 mm is-39.4 d B.The widest effective absorption bandwidth(EAB)can be extended to 5.28 GHz.(2)A facile hydrothermal method was used to dope PCHMs with heteroatoms using sulfur sources,and the effects of sulfur doping with different contents on the microstructure and dielectric properties of the samples were studied.The results show that the successful doping of sulfur element provides more defects and residual functional groups for the materials,which plays a positive role in the improvement of dielectric properties.The increase in the conductivity of the doped materials will improve the imaginary part of the complex permittivity,thereby enhancing the dielectric loss.Moreover,the interface polarization after the introduction of sulfur atoms is also a major factor in improvement of performance.Obviously,an appropriate amount of doping will bring good dielectric properties through inherent conduction loss and polarization relaxation.Finally,S-doped PCHMs-10 exhibits the best electromagnetic wave absorption behavior with a RL_min value of-51.83 d B at 15.05 GHz,and EAB reaches 6.08 GHz with a thickness of only 1.82 mm.(3)In-situ growth of magnetic inorganic nanoparticles on PCHMs is an effective means to improve electromagnetic wave absorption.The structure and magnetic properties of samples with different loadings of magnetic particles are studied,and the synergy of good dielectric and magnetic properties brought about by the appropriate loading is discussed.The results show that as the load increases,the mesopores in the sample may be partially covered,and the saturation magnetization will gradually increase.In addition,the Co-CoFe₂O₄@PCHMs composite shows a RL_min of-65.31 d B and a maximum EAB of 8.48 GHz under the dominance of excellent impedance matching due to the combination between the components.