Preparation Of Zn,Co Based MOFs Derived Carbon Composites And Their Electromagnetic Wave Absorption Properties

Due to the advantages of light weight,low density and high conductivity,carbon materials are widely used in the field of electromagnetic wave absorption.ZIFs,especially ZIF-8(Zn)and ZIF-67(Co),have the benefits of simple preparation method,large specific surface area,high porosity,and good structural stability,which are widely used to fabricate porous carbon-based materials.ZIFs derived carbon-based materials not only inherit the advantages of large specific surface area and high porosity of MOFs precursors,but also overcome the disadvantages of complex experimental process and risk during the preparation of traditional carbon materials.Therefore,we prepare ZIFs derived carbon materials through high temperature carbonization route by using Zn and Co based ZIFs as the precursors.The carbonization process is studied in detail,and the microwave absorption performance is investigated.The main results are as follows:1.Insight to the enhanced microwave absorption of porous N-doped carbon driven by ZIF-8.Utilizing the low boiling point of Zn,porous N-doped carbon is fabricated by carbonization of ZIF-8(Zn)at different temperature,and the microwave absorption performance was investigated.By increasing the carbonization temperature,the contents of Zn and N elements are decreased;the graphitization degree is improved;however,the specific surface area and porosity are increased first and then decreased.When the carbonization temperature is 1000℃,the porous N-doped carbon behaves enhanced microwave absorption.With an ultrathin thickness of 1.29 mm,the ideal RL reaches-50.57 d B at 17.0 GHz and the broad effective absorption bandwidth is 4.17 GHz.The mechanism of boosted microwave absorption is ascribed to the competition of graphitization and porosity as well as N dopants,resulting in high dielectric loss capacity and good impedance matching.The porous structure can prolong the pathways and raise the contact opportunity between microwaves and porous carbon,causing multiple scattering,interface polarization and improved impedance matching.Besides,the N dopants can induce electron polarization and defect polarization.These results suggest that the microwave absorption of carbon can be tuned by regulating the graphitization and porosity.2.Dynamically observing the formation of ZIF-67 driven Co/N-doped carbon nanocomposites by in situ transmission electron microscope and their application as highefficient microwave absorbent.ZIFs derived magnetic carbon-based nanocomposites have drawn widespread attentions due to the well distributed nanocrystals in carbon matrix.Herein,taking ZIF-67 as an example,the pyrolysis process is investigated by in-situ TEM assisted with ex-situ characterizations.The results indicate that the nanocrystals grow bigger by increasing pyrolysis temperature.At 600-800℃,the carbon texture transfers from amorphous to crystalline.At 700-1100℃,tiny Co nanocrystals escape from the carbon skeleton,and some of them evaporate.The residual Co nanocrystals catalyze the formation of carbon nanotubes.Due to the synergistic effect between Co and carbon as well as the porous structure,the nanocomposites show high-efficient microwave absorption performance.When the mass fraction is 30 wt%,the nanocomposites obtained at 600 or700℃ display remarkable microwave absorption with optimal RL smaller than-70 d B and effective absorption band larger than 4.9 GHz.These findings are helpful to understand the formation of MOFs derived carbon-based composites and expand their practical applications,especially for microwave absorption.3.Preparation and microwave absorption properties of hollow nitrogen doped porous Co/C@C composites derived from ZIF-67@ZIF-8.Hollow structure can not only reduce the density of the material,but also increase the propagation path of electromagnetic waves in material,providing channels for multiple scattering.Herein,based on previous studies,ZIF-67@ZIF-8 precursors were synthesized by a two-step precipitation method,in which ZIF-67 is grown on the surface of ZIF-8.Hollow nitrogen-doped Co/C@C composites were prepared by carbonization of the ZIF-67@ZIF-8 precursors at different temperatures.When the carbonization temperature is 800℃,the ZIFs derived materials show hollow structure.When the mass fraction and thickness are 20 wt% and 2.01 mm,respectively,the minimum RL value can reach-75.59 d B at 13.97 GHz,and the corresponding effective absorption bandwidth is 4.77 GHz.Compared with the ZIF-67 derived nitrogen-doped Co/C composite,the hollow nitrogen-doped Co/C@C composite not only greatly reduces the density of the material,but also achieves good microwave absorption performance at low filling ratio.