Porous carbon materials based on MOFs in microwave absorbing

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With the rapid development of science and technology, especially the fast progress in the radar detection technology, lots of efforts have been made to obtain high performance microwave absorption materials. In recent years, carbon-based microwave absorption materials have received much attention because of their high-performance electromagnetic wave absorption performances, special microstructure and low density. However, they do not own an orderly structure, usually, and the impedance mismatching between carbon and air may hinder the incident electromagnetic waves enter into the microwave absorption materials and weaken its absorbing performance. Metal-organic frameworks (MOFs) has become a research hotspot almost in every filed of material chemistry due to its orderly structure, functional designability, etc. But the stability of MOFs is generally lower than the traditional porous materials. Under high temperature conditions, the structure may collapse in some extent. MOFs are often used as precursors to obtain target materials. After pyrolysis, porous carbon (NPC) and composite materials with relatively regular structure, a high specific surface area and a certain pore structure can be obtained, which is convenient for incorporating other materials with different loss mechanism in the NPC. At the same time, the carbonized NPC could be optimized to achieve good chemical stability, light weight, strong electromagnetic attenuation capability, and show excellent microwave absorption performance. In this paper, the properties of nanoporous carbon microwave absorption materials based on MOFs and their relationship between pore size and pore structure are discussed. Appropriate porosity provides a channel for EM waves to enter the microwave absorption materials, which helps to improve the impedance matching between electromagnetic waves and absorbers, thereby increasing the absorbing properties. The sizes and pore structures of the porous carbon microwave absorption materials also have an important effect on the interaction between the electromagnetic waves and the microwave absorption body. Three methods for synthesizing porous carbon of derived from MOFs are described, the advantages and disadvantages of these three methods are also analyzed. One is the one-step pyrolysis of MOFs for synthesis of porous carbons. The other is that the porous carbon obtained by pyrolysis of MOFs is recombined with different materials to carry on secondary calcination. The third method is introducing different nanoparticles into the micropores of MOFs before calcination, and then calcinating them together to prepare porous carbon composite microwave absorption materials. Previous studies show that the smaller the pore size, the more uniform the distribution, and the better the absorbing properties of the composite materials would be obtained. Porous carbon materials with different pore structures provide various propagation paths for electromagnetic waves to reflect and scatter in materials, resulting in different energy losses. The research status of microwave absorption of porous carbon materials based on magnetic metal MOFs, heteronuclear metal MOFs and non-magnetic metal MOFs are reviewed respectively. The microwave absorption mechanism of the porous carbon materials based on different metals MOFs examples or their influences on microwave absorption are also discussed. Porous carbon materials based on magnetic MOFs can improve the microwave absorbing properties by increasing the magnetic losses, and those based on heteronuclear MOFs combine the advantages of two or more materials, impart new chemical and physical properties to the materials and obtain high microwave absorption efficiency. The non-magnetic MOFs are carbonized to obtain non-magnetic metal oxide with low dielectric constant and their porous carbon composite materials, which could achieve excellent absorbing performance by improving impedance matching. Finally, the development trend of this kind of novel excellent absorption materials based on MOFs, with the advantages of strong loss, broad frequency bandwidth, thin thickness and lightweight, in the direction of diversification, compounding, lightweight and industrialization are prospected.