Construction Of Fe₃O₄ And Fe-loaded Microwave Absorption Composites From Coal Hydrogasification Residues

With the advent of the 5G era,the problem of electromagnetic radiation pollution has become more and more serious,and a series of microwave absorption materials have been developed to solve this problem.In particular,magnetic carbon-based composite materials have been extensively studied with high absorption strength,wide absorption frequency bandwidth,and stable physical and chemical properties.However,the industrial development of carbon-based composites in the field of microwave absorption is limited due to the high cost of carbon-based materials,complex process flow,and difficulty in large-scale production.On the other hand,due to the characteristics of my country’s energy structure with rich coal,poor oil and little gas,coal gasification technology,the core content of clean coal technology,is booming in our country.But at the same time,a new type of solid waste,coal gasification residue is produced.Based on the above,in this paper,a series of carbon-based microwave absorbing composites loaded with magnetic components are prepared via a simple liquid-phase impregnation method and an in-situ carbothermic reduction method using coal hydrogenation residue as a matrix.The results show that the composite materials have good microwave absorption properties,which is not only expected to promote the industrial development of carbon-based microwave absorption materials in the field of microwave absorption,but also provides an effective way for the comprehensive utilization of coal gasification residues.And the specific work is as follows:（1）We used coal hydrogenation residue as the source of carbon,ferric nitrate solution as the source of magnetic particles,and the ferric nitrate salt solution was introduced into the coal hydrogenation residue matrix via simple liquid-phase impregnation,the composite absorption materials Fe₃O_4/SC and Fe/SC with different magnetic components were obtained via calcination at different temperatures in an argon atmosphere.The magnetic particles Fe₃O₄ and Fe were in-situ growth on the surface of the matrix via using the carbon of the matrix itself as the reducing agent for the reaction,and the whole process did not need to introduce other reducing gases,so it was safer and lower cost.The results showed that the prepared composites had a certain amount of graphitized carbon,and the loading of magnetic metal Fe particles could significantly improve the absorption performance.The position of the reflection loss peak and the corresponding reflection loss（RL）could be changed via adjusting the coating thickness.In particular,when the coating thickness was 2.5 mm,the minimum value of the RL of the sample Fe SC800 reached-39.3 d B at 9.0 GHz,corresponding to a frequency bandwidth of about 2.5GHz,and when the coating thickness was 1.5 mm,the bandwidth was up to 3.9 GHz.The excellent microwave absorption performance was mainly attributed to the good impedance matching,conductivity loss,and multiple interface polarization of the Fe SC800.（2）The magnetic contents of the composites were optimized via adjusting the concentration of the ferric nitrate precursor solution.In particular,the sample 30wt%Fe SC500exhibited double absorption peak when the coating thickness was 5.5 mm.The lowest reflection loss(RL_min)at 5.6 GHz was-18.1 d B,the effective absorption bandwidth was 2.0 GHz,and the RL_min at 17.9 GHz was-17.8 d B,the effective absorption bandwidth was 1.3 GHz.When the coating thickness of the sample 15wt%Fe SC800 was only 1.5 mm,the corresponding effective bandwidth could reach 4.5 GHz.The|Δ|values were calculated via the incremental function method indicated that the impedance matching of the composites could be tuned via changing the loading of the magnetic contents,thereby affecting their microwave absorption properties.（3）The carbon-based microwave absorption composites Fe₃O₄/PSC and Fe/PSC with richer porosity were prepared via simple wet chemical method and moderate temperature calcination after high-temperature pore-expansion treatment of coal hydrogenation residue matrix.Since the matrix PSC lost a part of the carbon with high activity after the high temperature treatment,the graphitization degree of the composites was poor,and the microwave absorption performance was not improved.Compared with the treatment without hole expansion.The RL_min of the sample Fe PSC1000 could reach-25.3 d B,and the effective absorption bandwidth was 3.8 GHz.But the composite materials after hole expansion showed a lower bulk density（0.159 g/cm³）,and the coal gasification residue after after high temperature treatment was more lightweight as the carrier of the microwave absorption materials,so the application prospect was more extensive.