Preparation And Research Of Electromagnetic Properties Of Zeolitic Imidazolate Framework ZIF-67-Derived Carbon-Based Composites

With the continuous development of science and technology,more and more communication methods use electromagnetic waves as a medium,but the ubiquitous electromagnetic waves also bring serious impact on the environment and precision equipment.Zeolitic imidazolate frameworks（ZIFs）are a branch of metal organic frameworks（MOFs）with excellent chemical and thermal stability and are widely used in gas adsorption,gas storage,drug release and catalytic reactions.ZIF-67 is one of the zeolitic imidazolate frameworks bridged by 2-methylimidazole anion and cobalt metal cation.To improve the microwave absorption performance of ZIF-67,Co/C composites were prepared by carbonization,using the carbonized organic backbone to form carbon chains while reducing the cobalt ions to highly dispersed metallic cobalt.This Co/C magnetic carbon matrix composite can further improve the microwave absorption performance and is suitable for practical applications.In this paper,the derived Co/C composites and their composite properties with other substances were investigated using ZIF-67 as a precursor,which is studied in this paper as follows:（1）The dodecahedral structured ZIF-67 precursors were synthesized by hydrothermal method and calcined under inert gas protection to obtain Co/C composites.The Co/C composites were obtained at three temperatures by varying the calcination temperatures（600℃,700℃and 800℃,respectively）.It was found that the conversion of amorphous carbon to graphitized carbon was favored with the increase of calcination temperature,and the dodecahedral structure of ZIF-67 also increased the collapse degree with the increase of calcination temperature.By analyzing the electromagnetic parameters of Co/C composites as well as impedance matching,the increase of calcination temperature is favorable to increase the dielectric and magnetic losses,but not to impedance matching.The sample Co/C-2（700°C）has the best electromagnetic wave absorption performance by reflection loss calculation.When the filling ratio is 45 wt%,the RL_min value is-28.24 d B at 8.4 GHz,and the effective absorption bandwidth is 4.6GHz with a thickness of 3.0 mm.（2）The hydrothermally synthesized ZIF-67 precursor was homogeneously dispersed into methanol solution,and ZIF-67 was etched by different concentrations of cerium nitrate hexahydrate（Ce（NO₃）₃·6H₂O）to form rhombic dodecahedral Co/C composites with porous hollow structure after calcination under N₂ protection.The effects of different Ce（NO₃）₃·6H₂O contents on the etching degree,specific surface area,impedance matching and electromagnetic wave absorption properties of the Ce-Co/C composites were investigated.It was concluded that when the mass ratio of cerium nitrate hexahydrate to ZIF-67 was 1:2（sample Ce-Co/C-2）exhibited the best impedance matching and the strongest electromagnetic wave absorption performance.For the Ce-Co/C-2 composite with a filling ratio of 45 wt%,the RL_min reaches-64.67 d B at 11.12GHz with a thickness of 2.21 mm.the increased specific surface area of the porous hollow structure of Ce-Co/C composite facilitates the impedance matching enhancement and improves the microwave absorption performance.（3）Hydrothermal synthesis of ZIF-67 was followed by calcination under inert gas protection to form rhombic dodecahedral Co/C composites with porous structures.A layer of rod-shaped polyaniline was coated on the surface of the Co/C composites by in situ polymerization.The effect of the mass ratio of polyaniline to Co/C on the electromagnetic properties of the composites was investigated.The sample Co/C@PANI-2（when 0.5 g Co/C powder was added）had the highest electromagnetic wave absorption performance.When the filling ratio was 50 wt%,RL_min reached-67.67 d B at 10.88 GHz with an effective absorption bandwidth of 5.02 GHz and a thickness of 1.99 mm.rod polyaniline effectively improved the dielectric properties and impedance matching of the composite,thus enhancing the electromagnetic wave absorption performance.