The Research On The Dielectric Properties And Microwave Absorbing Performance Of Wood-based Porous Carbon And Its Composites

In recent years,biomass have become a sort of important raw material for the high-performance microwave absorbing materials due to their properties of recyclability,vast sources and environmental friendliness.As one of the biomass with abundant reserves,natural network,porous structure,and carbon-rich characteristics,wood provide a realistic possibility for the preparation of highperformance microwave absorbing materials.But up to now,there are few studies on the preparation of microwave absorbing materials based on wood.In this paper,the cubic wood-based porous carbon absorbing materials were obtained by the oxygenfree high temperature carbonization of cubic masson pine wood.Meanwhile,the wood-based porous carbon powder-paraffin composites were obtained by combined process of oxygen-free high temperature carbonization,hot melt casting and cold pressing of masson pine wood and paraffin.Moreover,The dielectric properties and microwave absorbing properties of wood-based porous carbon and wood-based porous carbon powder-paraffin composites were systematically studied.Additionally,the carbonization and preparation parameters of wood-based porous carbon and its composites were optimized,and the microwave absorbing mechanism of these two materials was revealed.The main results are as follows:1)The effect of carbonization temperature on the microstructures and chemical properties of wood-based porous carbon materials was investigated.The results show that cubic wood-based porous carbon materials not only retain the anisotropic anatomical characteristics of wood,but also generate micron-scale axial arrayed carbon tube structure,and increasing carbonization temperature will further the shrinkage of this structure.Moreover,increasing carbonization temperature can promote the growth of graphite microcrystals in the wood-based porous carbon,and meanwhile,it can increase the C/O ratio and reduce the types and contents of polar functional groups of wood-based porous carbon.Furthermore,increasing carbonization temperature can not only enable the cubic wood-based porous carbon materials transform from dielectric to semiconductor,but also further the anisotropy of the conductivity of cubic wood-based porous carbon materials.2)The dielectric properties and microwave absorbing properties of cubic woodbased porous carbon materials treated at different carbonization temperatures were systematically studied by coaxial reflection method.The results show that the effective microwave absorbing bandwidth of 8.47 GHz covering 11.53-20.00 GHz is obtained by only 3 mm thickness in radial section(tangential direction)of cubic wood-based porous carbon materials carbonized at 650?.Moreover,the peak reflection loss of cross section(longitudinal direction)with only 3 mm thickness of cubic wood-based porous carbon materials carbonized at 670? can reach-47.50 dB,which means that the absorptivity of electromagnetic wave is over 99.99%.The dielectric properties and absorbing properties of cubic wood-based porous carbon materials show obvious anisotropy,and increasing temperature will further the anisotropy of these properties of cubic wood-based porous carbon materials.3)The microwave absorbing mechanism of cubic wood-based porous carbon materials was also investigated.The results indicate that the main absorbing mechanism of cubic wood-based porous carbon materials is the polarization response of dipole,and the cubic wood-based porous carbon material carbonized at 650?possesses the best microwave absorbing performance due to the strongest polarization.In addition,the anisotropy of dielectric parameters and absorbing performance of cubic wood-based porous carbon materials is attributed to its axial array carbon tube structure,which makes the radial section(tangential direction)obtain the best dielectric matching at different electromagnetic frequencies,and thus shows relatively better microwave absorbing performance.4)The effects of carbonization temperature,carbon powder content and material thickness on dielectric properties and microwave absorption performance of woodbased porous carbon powder-paraffin composites were systematically studied by coaxial short circuit line method.The results show that this material with the effective absorption bandwidth of 6.80 GHz,covering 11.20-18.00 GHz,originates from the composite with 40%of mass fraction of carbon powder carbonized at 670?at only 2.5 mm of thickness.Meanwhile,the peak reflection loss of this composites is-26.41 dB,which means that the electromagnetic wave absorption rate exceeds 99%.5)The absorbing mechanism of wood-based porous carbon powder-paraffin composites was also discussed.The results show the main absorbing mechanism of these composites is the polarization response of dipole,and the microwave absorbing performance of the composite with 40%of mass fraction of carbon powder carbonized at 670? is the best because of the strongest polarization.In addition,there is a 1/4 rule between material thickness and absorbing properties of these composites.