Design Of Metal-Organic Frameworks Derived Microwave Absorbing Materials With Optimized Electromagnetic Properties

The development of lightweight and highly efficient microwave absorbing materials is vital for electromagnetic stealth and electromagnetic pollution control.Metal-organic frameworks(MOFs)is a kind of novel crystal with ultra-high surface area and extremely ordered porous structure,which can be designed to synthesize various kinds of excellent microwave absorbing materials with the aid of advanced materials technology.Nevertheless,related research about materials synthesis and electromagnetic properties control is still in infancy.In the beginning,the effects of thermal annealing,types of metal elements and organic linkers on the electromagnetic properties of monometallic MOF-derived carbon composites are systematically investigated.It is found that the increase of annealing temperature would enhance the electromagnetic attenuation ability,while worsen the impedance matching behavior.The type of metal element largely decides the components of MOF-derived materials.From the viewpoint of electromagnetic attenuation ability,magnetic metals would possess stronger attenuation ability than nonmagnetic metals and metal oxides.The type of organic linkers would take effect on the reduction of metal elements and the release of nonmagnetic elements.Generally,micromolecule organic linkers would lead to low carbon content in MOF-derived materials,thus researchers should carefully choose organic linkers to obtain desired composition.However,electromagnetic attenuation ability of monometallic MOF-derived microwave absorbing materials is highly restricted by impedance matching behavior,which can hardly be improved by optimizing heat treatment conditions and materials synthesis processes.Then,we have developed bimetallic MOF-derived microwave absorbing materials to strengthen the electromagnetic attenuation ability.The effects of type,content of metal elements,heat treatment conditions and post-treatment process are also investigated.Optimal electromagnetic parameters can be achieved by simply changing the molar ratio of Zn/Co of BMZIF.The existence of extra metal compounds benefit the growth of carbon nanotubes.Meanwhile,interfacial polarization would be enhanced due to the existence of multicomponents,including Co,Co₃Zn C,Zn O and C.As for derivatives of Fe^III-MOF-5,the chemical state of Fe would be changing with the increasing annealing temperature,which would be in favor of enhanced magnetic loss and mitigation of contradictions between electromagnetic attenuation ability and impedance matching behavior.Hollow graphite spheres embedded in porous amorphous carbon matrix can be obtained by acid washing of derivatives of Fe₂Ni MIL-88 nanorods.The existence of one-dimensional and two-dimensional structure would decrease the percolation threshold of lightweight carbon materials.Although the filling ratio of bimetallic MOF-derived microwave absorbing materials has been decreased a lot(lower than 10 wt%),the effective absorption frequency bandwidth is quite narrow,caused by the lack of strong magnetic loss.Therefore,MOFs composites have been employed to fabricate novel microwave absorbing materials with enhanced magnetic loss.The effects of type,content of magnetic components,interfacial structure and natural resonance frequency are discussed.Magnetic Co nanoparticles can be uniformly inserted into Cu/C composites derived from Cu₃(btc)₂ by introducing Co²⁺solution into pores of Cu₃(btc)₂.It is proved that higher content of magnetic Co nanoparticles is propitious to the enhancement of both dielectric and magnetic loss,which further improves the low-frequency microwave absorption behavior.Magnetic Co/C composites with unique interface structures can be synthesized from Co₃[HCOO]₆?DMF/glucose mixtures.It should be noticed that strong magnetic loss provide by Co and controllable dielectric loss offered by different carbon coatings would result in the optimized impedance matching behavior at low frequency and ameliorated reflection loss behavior.A series of Fe-based composites can be gained by annealing Fe₃O₄@PB composites under different heating rate.Natural resonance frequency would be shifted to high frequency,which contributes to the reinforcement and rearrangement of electromagnetic attenuation mechanisms.Therefore,the working frequency of ferrites could be extended.This thesis aims at the development and optimization of materials fabrication techniques to ameliorate and investigate the electromagnetic properties of MOF-derived materials,which would establish excellent foundations for the research about MOF-derived lightweight highly efficient microwave absorbing materials and detailed electromagnetic attenuation mechanisms.