Preparation And Microwave Absorbing Properties Of Carbon-based Composites Derived From Ion Self-assembly

With the rapid development of wireless communication technology,high-frequency electronic equipment inevitably brought serious electromagnetic radiation and pollution.Therefore,the development of multi-functional and high-efficiency electromagnetic wave（EMW）absorber is urgent.Polyoxometalates（POMs）are a large class of excellent metal-oxygen anion cluster compounds.Among them,Keggin-type POMs have become excellent building blocks in the field of supramolecular hybrid self-assembly because of their suitable size,unique structure and excellent physicochemical properties.In the article,Keggin-type POMs were used to form organic-inorganic hybrid precursors by electrostatic self-assembly with different organic ligands and magnetic materials,followed by carbonization to obtain a series of novelty carbon-based compound absorbing materials with excellent performance.（1）Molybdenum dioxide/iron cobalt alloy/nitrogen phosphorus doped carbon（Mo O₂/FeCo/NPC）nanocomposites were prepared by combination of supramolecular self-assembly and high-temperature carbonization.The synergistic effect between the dispersion of dielectric Mo O₂ system and the magnetic coupling network formed by FeCo nanoparticles effectively improved the impedance matching.In addition,local charge accumulation caused by Schottky barrier between two interfaces also contributed to enhance the interface polarization.Internal oxygen vacancy defects and unique electric double layer structure on the surface of ionic crystal Mo O₂,as well as the existence of capacitor-like structure between carbon nanosheets further optimized absorption performance of Mo O₂/FeCo/NPC.Its minimum reflection loss（R_L）reached-42.3 d B at 10.4 GHz and 2.1 mm thickness.In the range of 1.5-3 mm,the effective absorption bandwidth（EAB）of 8.4 GHz could be achieved（R_L≤-10 d B）.（2）By virtue of supramolecular self-assembly in water-in-oil system and carbonization strategy,hollow flower-like molybdenum dioxide/cobalt-nickel alloy/nitrogen phosphorus co-doped carbon（Mo O₂/Co Ni/NPC）microspheres were constructed.The components Mo O₂ and NPC combined to form nanosheets and assembled into flower-like morphology,while Co Ni nanoparticles were anchored in pores on the surface of flower-like spheres.By adjusting the ratio of phosphomolybdic acid to dopamine and the solvent polarity of the oil phase,the morphology,structure and electromagnetic parameters of composites could be controlled.When the mass ratio of the phosphomolybdic acid and dopamine hydrochloride was 3:4,and the oil phase solvent was glycol,microspheres presented outstanding EMW absorption performance.Its minimum R_L was-54 d B,and the EAB was 4.72 GHz at 1.97 mm.The maximum EAB was 6.48 GHz（11.52-18GHz）with a thickness of only 1.7 mm.（3）The precursor PW₁₂/PDA/Co Fe₂O₄ was synthesized by self-assembly of phosphotungstic acid,organic ligand dopamine hydrochloride and magnetic cobalt ferrite in Tris buffer solution.After that,the precursors were carbonized at different temperature（700℃,800℃and 900℃）to prepare polycrystalline and multiphase tungsten-based composites.The multiple magnetic components derived from Co Fe₂O₄ optimized impedance matching and provided magnetic loss.In addition,the heat treatment process created more heterogeneous interfaces and defects within the sample,which contributed to the dielectric loss.The results manifested that the W/WO₂/FeCo/NC nanocomposites obtained by carbonization at 700℃exhibited the most outstanding absorption performance.Specifically,under 35 wt%filler loading,the minimum R_Land EAB of W/WO₂/FeCo/NC reached-54.4 d B at 2.6 mm and 7.04 GHz at 2.2 mm respectively.