Construction And Performance Optimization Of Microwave Structural Absorber Based On Porous Biochar

Nowadays,with the rapid development of wireless communication technology,the complex electromagnetic environment poses a severe challenge to human health and the stable operation of equipment.Effective broadband absorption is critical for absorbers due to the broadband detection system and multiband wireless communication technologies.However,the traditional absorption coating and metal structure metamaterials are difficult to achieve broadband absorption.As a practical microwave absorber,besides satisfying the basic broadband absorption performance,the density,cost,and process complexity are considerable factors in radar stealth and electromagnetic protection fields as well.In order to address the critical issue of high density,high cost and complicated technology of broadband absorber,an ultra-broadband structural absorber with light weight,low cost and simple preparation method was proposed in this thesis.Based on the concept of sustainable development of waste utilization,this thesis constructed structural absorber based on banana peel and corncob,and optimized its electromagnetic absorption performance.The porous carbon from biomass can not only overcome the high permittivity of carbon material to optimize the impedance matching,but also reduce the density of absorber.The specific research contents of this thesis are as follows:(1)To improve the pore structure of biochar and form hierarchical porous structure,a low-cost,light-weight,additive-free porous carbon material(BPC)was prepared in one step by using the inherent rich alkali metals in banana peel.The additive-free porous structure formed by the self-activation of banana peel alkali metal can not only enhance the interface polarization,but also optimize the permittivity to improve the impedance matching performance.The absorption performance of BPC can be effectively optimized by researched the influence of experimental conditions.When the thickness is only 1.47 mm,the effective absorption bandwidth is 4 GHz and the electromagnetic absorption performance is-45 d B.(2)In order to further enhance the electromagnetic absorption performance of porous carbon,the porous carbon was treated through composite magnetic materials and reasonable microstructure.Based on corncob waste,the porous carbon composite was successfully synthesized through a simple hydrothermal and high-temperature carbonization process,which is composed of Fe,porous carbon and carbon nanotubes(Fe-NPC/CNTs).Benefiting from the unique hierarchical architecture,the interfacial polarization from diverse interface and defects,the conductive loss from three-dimensional conductive network constructed by CNTs,magnetic loss of Fe nanocrystals,and multiple reflection loss of porous structure are significant improved to enhance microwave absorption performance.The minimum reflection loss of composite can reach-70 d B,and its effective absorption bandwidth can cover 5.0 GHz at a filler loading of as low as 12.5 wt%in paraffin wax.(3)To further broaden the absorption bandwidth of bio-derived porous carbon,the structural absorber was constructed at the macro scale based on the microwave absorption performance of porous carbon at the previous micro-scale.Benefiting from synergistic effect of microscopic and macroscopic scales,the absorber can achieve ultra-wide band absorption of 14 GHz in the microwave band,and it can maintain stable broadband absorption performance under wide incident angles.The lightweight porous biochar provides structural absorber the low-density properties with a surface density of only 0.574 g/cm~2.In order to further optimize the surface density and thickness of the absorber,a multilayer composite structure absorber was designed.The matching layer was designed with porous carbon BPC and the loss absorbing layer was Fe-NPC/CNTs porous carbon composite.Based on this composite multilayer stealth structure,the surface density(0.119 g/cm~2)and thickness(5.5mm)of the absorber were improved effectively,while the 13 GHz broadband absorption performance was guaranteed.Therefore,using biomass waste as carbon precursor to construct ultra-wideband structure absorber with light weight,low cost and simple preparation method can not only realize the effective utilization of biomass waste,but also promote the development of circular economy and social sustainable development.