Synthesis And Electromagnetic Wave Absorption Properties Of Cobult-based MOF Derived Composites

The metal-organic framework（MOF）is a kind of materials with aspecial structure of periodic and regular arrangement of metal ions or clusters and organic ligands.By simple heat treatment of MOF,the derivatives of metal and carbon with electromagnetic wave-absorbing properties can be obtained.By combining with low density two-dimensional lamellar materials such as graphene and MXene,lighter absorbing materials can be prepared.Herein,three different cobalt-based MOFs were prepared as three systems,and the effects of different hybridizations,heat treatment temperatures,and structures on the properties and mechanism of cobalt-based MOF derived electromagnetic wave absorbing composite were studied:（1）Co-Fe PBA was prepared by wet chemical method,and CoFe@C composite was prepared by annealing at different temperatures.When the annealing temperature was 600℃,the sample filled with 50 wt%CoFe@C had the best absorption performance.The minimum reflection loss and maximum effective absorption bandwidth are-58.08 d B and 4.98 GHz,respectively.which is mainly attributed to the suitable graphitization degree of the carbon layer outside the CoFe alloy at the temperature,simultaneously meeting the excellent impedance matching and good attenuation characteristics.After the Co-Fe PBA was coated with PDA and annealed at different temperatures,CoFe@C@CNCs was obtained.The external carbon nanocages can prevent the large area adhesion of the highly graphitic carbon shell and form matching layer to improve impedance matching,and at the same time introduce more heterogeneous interfaces to improve the ability of attenuating electromagnetic wave of the derived materials.CoFe@C@CNCs derived at 700℃ can realize a RL_minof-51.57 d B and an EAB of 6.32 GHz.CoFe@C/r GO was obtained by annealing of Co-Fe PBA/graphene at different temperatures.The derivative obtained by annealing at 700℃ can achieve a RL_minof-44.23 d B and an EAB of 4.28 GHz when the content was 20 wt%.The reason is that the addition of graphene reduced the density of the derived material,dispersed the Co-Fe PBA,prevented the large area adhesion of the highly conductive carbon layer,improved impedance matching,and introduced a large number of heterogeneous interfaces to enhance the loss capacity.（2）Co-CoPBA was prepared by wet chemical method and Ti₃C₂T_x MXene was prepared by HCL-Li F etching of Ti₃Al C₂MAX phase.The two were combined by quick freezing and freeze drying,and different Co@CP/MXene composites were obtained by heat treatment at different temperatures.With the increase of temperature,the degree of graphitization of carbon layer outside Coincreased,and the electrical conductivity was enhanced.At the same time,the degree of transformation of MXene into Ti O₂,Ti N and C layer increased,forming more heterogeneous interfaces,resulting in the enhancement of loss capacity.However,ultrahigh electrical conductivity will cause the deterioration of impedance matching.700℃ heat-treated derivatives show the best absorptivity under the combined influence of impedance matching and attenuation characteristics,possessing the RL_minof-45.84 d B and the widest EAB of 3.20 GHz when adding 20 wt%,and showing a trend of multi-frequency absorption.（3）Micrometer-sized flower-like CoBDC and two-dimensional lamellar CoBDC nanosheets were prepared by hydrothermal method and wet chemical method,respectively.They were recombined with MXene by quick freezing and subsequent freeze-drying,and heat treated at the same temperature,respectively.l-Co@C_B/MXene derived from two-dimensional lamellar CoBDC and MXene complex had better absorption performance than f-Co@C_B/MXene derived from flower-like CoBDC and MXene complex.When the filling amount is 20 wt%,the RL_minof-41.54 d B and EAB of 2.56GHz can be obtained,and it had the ability of multi-frequency absorption.This is mainly attributed to the uniform distribution of Co@C on MXene sheets,which can cause more heterogeneous interfaces and improve the polarization loss.At the same time,the particle size of Co@C is smaller,the graphitization degree of carbon shell was higher under the catalysis of Co,and the conductive loss is enhanced.In addition,the smaller particle size of magnetic Cocan also enhance the magnetic loss.