Preparation And Application Of Watermelon Pulp-based Biochar Magnetic Absorbing Material

The rapid development of modern technology,a new generation of communication and electronic equipment as well as detection and detection equipment emerged,in improving the standard of living of people and the modern combat capabilities of the national army at the same time will produce serious electromagnetic pollution and more military threats.In order to reduce the electromagnetic pollution caused by electromagnetic wave radiation and enhance the combat survival and counterattack ability of the army,it is urgent to develop high-quality electromagnetic wave absorbing materials with wide absorption bandwidth,high reflection loss and thin matching thickness.Biomass,as a resource abundant in nature and green and renewable at the same time,becomes an ideal raw material for the preparation of carbon materials.After pyrolysis,biomass retains its unique pore structure and heteroatoms,which makes it have better dielectric loss capability and is considered as one of the promising materials in the field of electromagnetic wave absorption.Based on the development of biomass carbon absorbing materials,the introduction of magnetic particles and three-dimensional structure is also an effective way to optimize the absorbing performance of the materials.In this thesis,three types of electromagnetic wave absorbing materials（WPC,Co₃Fe₇/WPC and Co₃Fe₇/WPC/cotton biomass carbon）were prepared using watermelon melon flesh and cotton as the biomass carbon sources.The Co₃Fe₇ obtained by conversion of Co Fe₂O₄as the magnetic source,using the preparation processes of thermal reduction and solvothermal methods.The chemical structure,microscopic morphology and wave absorption properties of the materials were characterized by XRD,Raman,SEM,TEM and other characterization methods.The details of the study are as follows.（1）The thin graphene-like multilayer carbon nanosheets were synthesized by a simple hydrothermal method and subsequent carbonization of watermelon flesh.The unique twisted and folded structure not only improves the conductive loss,but also increases the multiple reflection and scattering through extending the propagation path of electromagnetic wave.The minimum reflection loss of the watermelon pulp biochar reaches-50.0 d B at a thickness of 3mm,and the effective absorption bandwidth reaches 3.51 GHz at a thickness of 2 mm.This work provides a new direction and preparation skill to design advanced electromagnetic wave absorbing materials.（2）The pretreated watermelon pulp（PWP）containing hydrochar was prepared by hydrothermal reaction.After mixed with Co Fe₂O₄,Co Fe₂O₄ was transformed into Co₃Fe₇during the annealing process.The Co₃Fe₇/watermelon pulp biomass carbon（WPC）composites were obtained.Under the synergistic effect of multiple scattering and reflection of the wrinkled structure,the polarization caused by the heterogeneous interface between WPC and Co₃Fe₇particles,magnetic resonance of Co₃Fe₇ and excellent impedance matching,the reflection loss（RL）of the absorbent reaches-38.5 d B with the thickness of 2 mm,and the effective absorption bandwidth is 6.0 GHz（12-18 GHz）covering the Ku band.The excellent electromagnetic wave absorbing properties of the Co₃Fe₇/WPC provide a new tactic for the preparation of thin-thickness,lightweight and wide-band-width biomass carbon-doped magnetic particle absorbing materials.（3）Two kinds of biomass（watermelon flesh and cotton）were used as carbon sources,and Co₃Fe₇/WPC/CC composites with vine like three-dimensional structure were successfully synthesized by hydrothermal and high temperature pyrolysis preparation processes.The successful establishment of the three-dimensional structure enhanced various loss mechanisms such as interfacial polarization,conductive loss,multiple reflections and scattering of the composites,which enabled the material to obtain excellent wave absorption properties.The minimum RL value is-46.9 d B when the filling amount is 25 wt.%,and the effective absorption bandwidth reaches 5.7 GHz at a thickness of 2 mm.