Study On The Synthesis Of ZnO@ZIF And Microwave Absorption Properties Of Its Pyrolyzed Derivatives

Electromagnetic wave absorbing materials have broad application scenarios in electromagnetic pollution prevention and control,anti-radar stealth technology and new wearable electronic devices.Traditional absorbing materials often have problems such as high density,thick matching thickness,low absorption intensity,and narrow effective absorption bandwidth.Metal-organic framework-derived carbon-based absorbing materials are expected to be a new generation of high-performance"light,thin,wide and strong"absorbing materials due to their low density,good weather resistance,low filling content and diverse material structures.Zeolitic imidazolate frameworks（ZIFs）are a series of MOFs formed by the coordination of organic imidazolates and transition metal ions(Zn²⁺,Co²⁺),which have the advantages of low preparation cost and simple preparation.ZIF could form porous graphitized carbon derivatives dispersed with magnetic nanoparticles after pyrolysis.ZIF can form porous graphitized carbon derivatives dispersed with magnetic nanoparticles after pyrolysis,which can synergistically optimize impedance characteristics and attenuation characteristics,and is an effective method to prepare high-efficiency wave absorbing materials.In this paper,ZIF and its derivatives are the main research objects,and ZnO@ZIF with core-shell structure is prepared by ZnO self-sacrificial template method,aiming to develop carbon-based composites with low filling content,low density and excellent wave absorbing properties.The effects of pyrolysis temperature,doping amount of cobalt ions and shell thickness on the morphology,electromagnetic parameters and absorbing properties of derivatives were systematically studied.The work is as follows:（1）The effects of mixed solvent volume ratio（N,N-dimethylformamide and water）and reaction time on the formation of T-ZnO@Zn/Co-ZIF core-shell structure were studied using tetrananorod ZnO（T-ZnO）as a self-sacrificing template.The study showed that with the increase of water content and reaction time in the mixed solvent,the content of Zn/Co-ZIF nanoparticles also gradually increased.When the mixed solvent V_DMF:V_H2O is 3:1,the optimal core-shell structure T-ZnO@Zn/Co-ZIF is formed.At the same time,when the Zn/Co-ZIF layer was grown for 24 h and pyrolyzed at 800°C,the T-ZnO could be completely reduced to Zn⁰ and volatilized to form the tetrapod-shaped hollow derivative PDH31-24-800 with best microwave absorbing performance,its RL_min is-16.40 d B and EAB_max is 4.48 GHz at 10 wt%filler loading.（2）Using spherical zinc oxide（S-ZnO）as a self-sacrificing template,the doping concentration of cobalt ions has little effect on the morphology of the precursor S-ZnO@Zn/Co-ZIF,the precursors show a mixture of rhombic dodecahedron and spherical morphologies.The composition and morphology of ZnO@Zn/Co-ZIF with different Co²⁺content is different after pyrolysis.For the sample of SE1-2（the mass ratio of ZnO and Co（NO₃）₂·6H₂O is 8:5）,the reduced cobalt atoms were uniformly dispersed in the carbon matrix during the pyrolysis of ZnO@Zn/Co-ZIF at 700°C,800°C and 900°C.For the sample of SE2-2（the mass ratio of ZnO and Co（NO₃）₂·6H₂O is 8:10）,with the pyrolysis temperature increasing,the zinc,cobalt and carbon atoms in ZnO@Zn/Co-ZIF first agglomerated to form a nano-sized bimetallic carbide Co₃Zn C,and then the zinc atoms in the Co₃Zn C crystal gradually volatilized at high temperature and promoted the cobalt atom to form a single atom and disperse in the carbon matrix.For the sample of SE3-2（the mass ratio of ZnO and Co（NO₃）₂·6H₂O is 8:15）,there are more Co₃Zn C nanocrystals when pyrolyzed at700°C compared with PSE2-2-700.When the pyrolysis temperature was higher,the residual zinc atoms in the Co₃Zn C crystals volatilized and Cobalt atoms tended to agglomerate to form cobalt nanoparticles.Among the nine derivatives obtained by pyrolysis of SE1-2,SE2-2 and SE3-2 at different temperatures,the EAB_max of PSE1-2-800 reached 7.36 GHz,and the RL_min of PSE3-2-800 reached-51.24 d B.（3）S-ZnO@ZIF-8 with a core-shell structure was prepared using S-ZnO as a template without the addition of Co（NO₃）₂ 6H₂O,and the thickness of the ZIF-8 layer increases with prolonging the growth time of the shell layer.When the pyrolysis temperature is 800°C,a hollow structure non-magnetic pyrolysis derivative is formed,and the doping of zinc and nitrogen atoms in the carbon layer effectively improves the dielectric loss performance of the material.When the sample thickness is 2.50 mm,the derivative of PZZ2-800 has a EAB_max of 6.50 GHz（10.80-17.30 GHz）at a filling amount of 30 wt%.By adjusting the pyrolysis temperature,the Co²⁺doped in the core-shell structure ZnO@ZIF precursor can form single-atom doping or magnetic nanoparticles dispersion after pyrolysis,and ZnO undergoes a carbothermal reaction to form a hollow structure carbon-based derivative.The self-sacrificial template method to construct the hollow structure improves the impedance matching characteristics and electromagnetic wave loss ability of the material.This research has reference value for improving the absorbing properties of ZIF derivatives and provide a basis for further research on high-performance carbon-based absorbing composites.