Investigation On Design,Preparation And Performance Of MOFs-based Electromagnetic Functional Materials

In recent years,with the rapid development and extensive application of communication technology and electronic equipment,electromagnetic pollution has been made increasingly prevalent.In this context,the materials that could protect against electromagnetic wave have attracted much attention for scientific research.As the requirements placed on working environment and functionality are made increasingly higher for equipment,both electromagnetic shielding materials and microwave absorbing materials are expected to demonstrate high capacity of electromagnetic wave absorption.In the meantime,the development of new electromagnetic materials is also targeted at achieving such characteristics as lightweight,flexibility,corrosion resistantance and so on.Characterized by unique porous structure and large specific surface area,metal-organic frameworks（MOFs）and their derivatives have a large number of active sites and intrinsic defects on their surface.These characteristics allow the space required for the design of interface structure and the performance optimization of electromagnetic materials.In this paper,the micro-composition regulation,micro-and macro-structure design of MOFs and their derivative materials are carried out to construct three-dimensional structural materials with both dielectric and magnetic losses.An investigation is conducted into the mechanisms followed by the impact of multi-scale interfaces,lattice defects,porous structures and three-dimensional networks on the electromagnetic properties of materials and the attenuation of electromagnetic wave.Not only could the prepared electromagnetic materials produce high shielding effectiveness,it can also achieve the efficient absorption of incident electromagnetic waves.The main research contents and results are presented as follows:（1）A highly absorbing C-MIL-88B/GNP film with high electromagnetic shielding（EMI）performance was designed by constructing a laminate structure of MOFs-derived Fe₃O₄with GNP.By combining the absorption characteristics of MIL-88B-derived magnetic materials and the shielding characteristics of GNP,this layered structure is conducive to the construction of a macroscopic sandwich interface,thus enhancing the dielectric loss capability of the material and achieving high absorption characteristics and excellent shielding performance for the material.When the C-MIL-88B/GNP film consists of 5 layers,the electromagnetic shielding performance can reach up to 28d B,while the electromagnetic wave absorption coefficient could reach as high as 86%.（2）Three-dimensional（3D）honeycomb lightweight aerogels were prepared using a directional freeze casting method,with MOFs as the structural units based on the synergistic effect of multi-level structural elements.An investigation was carried out into the mechanism followed by the impact of multi-level pore structure with multiphase components on the shielding and absorption of electromagnetic wave.Not only could the multi-stage pore structure of MOFs honeycomb aerogel structure reduce the density of materials,it can also contribute to constructing a large number of macroscopic interfaces,thus providing more paths for the scattering of electromagnetic waves.Due to the magnetic particles contained in the MOFs-derived materials,the intrinsic defects,and the losses caused by the conducting network of the aerogel,the aerogel exhibited absorption-dominated electromagnetic shielding properties.The aerogel carbonized at 900°C achieved an electromagnetic shielding effectiveness of 35.1 d B at X-band with a density as low as 1.74 mg/cm³.In the meantime,the corresponding specific shielding effectiveness reached up to 20,172.4 d B cm³/g.（3）The self-assembly technique was applied to grow MOFs in situ on one-dimensional fibers and they were prepared by means of carbonization to obtain string-like CNF@Co/C composite aerogels.An investigation was conducted into the mechanisms followed by the impact of multi-scale heterogeneous interfaces,three-dimensional continuous networks,and lattice defects on electromagnetic shielding.Due to the beaded microstructure and the synergistic effect between different components,the impedance matching of CNF@Co/C aerogel was improved and the electromagnetic properties of the composite aerogel were enhanced.More specifically,it produced an electromagnetic shielding performance of 56.7 d B at the density of 0.023 g/cm3,and an electromagnetic wave absorption coefficient of 79%.Meanwhile,the composite aerogel demonstrated high mechanical strength,which contributes a fresh idea to the research on novel MOFs-based flexible electromagnetic functional materials.（4）A technique combining freeze casting and carbonization was applied to grow one-dimensional carbon nanotubes in MOFs-based aerogels in situ,based on which a three-dimensional network structure with multi-component and multi-level pores was obtained,which is effective in solving metal phase agglomeration and increasing the heterogeneous interface.An investigation was carried out into not only the effect of three-dimensional porous and continuous structures on the absorption of electromagnetic wave,but also the mechanism behind the impact of lattice defects,multiple interfaces and conducting networks on the attenuation of electromagnetic wave.The prepared aerogel maintained consistency in electromagnetic wave absorption properties when the temperature ranged from 700to 900°C.More specially,CNS/Co@NCNTs-900 exhibited the most satisfactory performance in the absorption of electromagnetic wave,the maximum reflection loss value reached-27.5d B at 4.6GHz,and the comprehensive effective absorption bandwidth reached 14.3GHz（1-5.5mm）.