Preparation And Microwave Absorption Properties Of MOF Derived Carbon-based Composites

With the rapid development of the 5G era and the changes in the modern battlefield,microwave absorbing materials are required to be lightweight,strong absorption,broadband and thin thickness.Carbon material has become one of the potential microwaves absorbing materials owing to the advantages of low density,high electrical conductivity,and large specific surface area.Metal-organic framework compounds（MOF）are also a class of highly tunable porous crystalline materials,and its derivatives have the advantages of structural diversity,tunable chemical properties,and high porosity,which are beneficial to improve microwave absorption ability of the materials.Therefore,a class of high-performance microwave absorbing materials with low density,adjustable structure and electromagnetic balance can be obtained by combining carbon materials with MOF derivatives.The thesis mainly focuses on MXene/MOF-derived carbon-based composites.The effects of the microstructure and electromagnetic parameters of the composites on the microwave absorption properties were investigated.The specific research content is as follows:（1）The Ti₃C₂T_x/Co-MOF composites were successfully prepared by thermosolvent method with in-situ growth of Co-MOF on the surface of Ti₃C₂T_x MXene.TiO₂/Co/C composites were obtained by annealing the Ti₃C₂T_x/Co-MOF precursors.The effects of Co-MOF particle size and Ti₃C₂T_x addition on the microstructure and wave absorption properties of the composites were investigated.The results showed that the wrapping of Co-MOF to Ti₃C₂T_x changed from tight to lose as the particle size of Co-MOF increased.The TiO₂/Co/C composites achieved the optimal microwave absorption performance when the content of Ti₃C₂T_x was 50 mg,and the particle size of Co-MOF was 2.0μm.The minimum reflection loss(RL_min)was-50.45 d B at a thickness of 3.0mm.The effective absorption bandwidth（EAB）reached 6.1 GHz at a thickness of only 1.8 mm.the TiO₂/Co/C composites exhibited strong microwave absorption capability over the entire band range from C-band to Ku-band.The carbon material and magnetic Co nanoparticles in the composites effectively improve the dielectric and magnetic loss capability of the composites.The multiple interfaces in the composites also promote the dipole polarization and interfacial polarization processes.（2）The ZIF-67/MXene composites were prepared by in-situ chemical reaction,and the mixture of ZIF-67/MXene precursors and melamine was pyrolyzed at high temperature to obtain the TiO₂/Co/C（CNTs）composites with satisfactory electromagnetic wave absorption properties.The effects of the melamine content on the microstructure and electromagnetic parameters of the composites were investigated.The factors affecting the microwave absorption performance and the wave absorption mechanism of the TiO₂/Co/C（CNTs）composites were further explored.The results showed that the in situ-grown carbon nanotubes linked the ZIF-67/Mxene derivatives to form a heterogeneous network structure.Magnetic Co nanoparticles were dispersed at the end of CNTs.The RL_min value of the TiO₂/Co/C（CNTs）composites reaches-52.43 d B with the EAB value of 6.18 GHz when the thickness was 1.5 mm.The EAB value reached 5.32 GHz with RL_min value of-41.35 d B at the thickness of 1.7 mm.The three-dimensional conducting network structure in the composites can also effectively modulate the electromagnetic parameters and optimize impedance matching.（3）Heterogeneous metal CoNi-MOF and the MXene/CoNi-MOF composites were prepared by hydrothermal method.The microwave absorption properties and mechanism of the carbon-based absorbing materials derived from CoNi-MOF were systematically investigated.The effect of the molar ratio of cobalt and nickel ions and the amount of MXene in CoNi-MOF on the microstructures and microwave absorption properties of the composites were discussed.The results showed that the morphologies of CoNi-MOF changed from nano-flowers to nano-sheets with the increase of the Co/Ni molar ratio.The CoNi-MOF in MXene/CoNi-MOF composites also showed the same transition with the increase of MXene addition.When the addition of MXene in the M/CoNi/C composites was 50 mg and the molar ratio of Co/Ni was 1:1,the composites exhibited excellent microwave absorption performance with the EAB value of 5.2 GHz and the RL_min value of-61.34 d B at a thickness of 1.9 mm only.The absorbing efficiency of the composites at 1.5～2.5 mm thickness can reach above 99%and the EAB can achieve above 3.0 GHz.The optimized M/CoNi/C composite has abundant heterogeneous interfaces and defects.Moreover,the introduction of magnetic metal alloys can effectively adjust the electromagnetic parameters of the materials,enhance the dielectric loss and magnetic loss,improve the impedance matching and enhance the microwave absorption capability.（4）The MOF-derived T/CoFe/C（CNTs）composites were prepared by introducing melamine as an external carbon source into the MXene/CoFe-MOF composites.The effects of the mass ratio of the heterogeneous metal elements Co and Fe,and annealing temperature on the microwave absorption properties of the composites were investigated.The results showed that the CoFe-MOF with different morphologies were obtained by adjusting the mass ratio of Co/Fe.When the MXene addition was 50 mg,the melamine addition was 3.0 g,the mass ratio of Co/Fe was 3:1,and the annealing temperature was 800℃,the composites showed excellent microwave absorption performance in the thinner thickness and high frequency region,and the RL_min value of the T/CoFe/C（CNTs）composites was-45.77 d B only at 3.0 mm thickness.The EAB achieved 4.16GHz at a thickness of 1.6 mm.The CNTs with magnetic particles at the end form a conductive and magnetically coupled network structure,which increases the path of electromagnetic wave transmission and electron transfer.At the same time,a large number of defects and interfaces are introduced to enhance the dielectric loss.The three-dimensional network structure optimizes the impedance matching,which achieves strong absorption in the microwave band.