Preparation And Properties Of Hierarchically Porous Microwave Absorption Materials

The combination of structure and components is an important method to achieve excellent microwave absorption(MA)performance.In comparison with single-component materials,the superimposed effects between different components can effectively improve impedance matching.Among various of microstructures,hierarchically porous structure with high specific surface area and multi-sized pore units can simultaneously enhance the polarization loss capacity and thereby is conducive to improving MA performance.However,there remains some challenges in the construction method of reported hierarchically porous structure,such as environmentally hazardous fabrication process,complex preparation technology,and high cost.In this thesis,from the perspective of microstructure design,several hierarchically porous composites with the features of strong absorption efficiency,wide absorption bandwidth,thin thickness,and lightweight have been prepared by cheap raw materials and simple methods.Moreover,the MA mechanism has been investigated thoroughly.The main content is divided into the following two parts:(1)A series of hierarchically porous C/Fe3C composites have been successfully prepared through drying the gels of gelatin/metal nitrates and following carbonization treatment.During the drying process,the micro-scale pores are formed due to the self-foaming of the gels.At a lower carbonization temperature,metal nitrates are converted into metal oxides rapidly.With the extension of carbonization process,the nano-scale pores are produced by the etching reaction between the gelatin-derived carbon arid metal oxides.The as-prepared composite demonstrates the outstanding MA performance with the maximum reflection loss(RLmax)of as high as-57.1 dB and a broad efifective absorption bandwidth(EAB)of 8.16 GHz.Detailed investigation manifests that such an excellent MA performance results from the synergistic effect between dual-loss components and hierarchically porous structure.In order to verify this mechanism,the hierarchically porous C/Co and C/Ni composites are prepared via carbonizing the dried gels of gelatin/cobalt nitrate and gelatin/nickel nitrate,respectively.Similarly,the C/Co and C/Ni composites acquire the broad EABs of 6.32 and 8.80 GHz,respectively.Therefore,it is believed that this method provides a reference for the preparation of MA absorbent with strong absorption,broad bandwidth,thin thickness,and lightweight.(2)The lightweight hierarchically porous C/Co foams have been successfully prepared by a sequential process of stirring,freeze-drying,and carbonization.A large amount of foam is produced in the stirring process,which is solidified with the aid of liquid nitrogen.In the freeze-drying process,the ice template is removed by sublimation and the macro-scale pores are retained on the foam skeleton.During the carbonization step,nano-scale pores are formed through the etching reaction between gelatin-derived carbon and metal oxides produced by the decomposition of cobalt nitrate.Therefore,the hierarchically porous C./Co foam is successfully obtained.By adjusting the carbonization temperature and filler loading in the paraffin,the obtained C/Co-700 foam exhibits the RLmax of-56.5 dB and a broad EAB of 6.00 GHz at a low filler loading of 15 wt%.Detailed absorption mechanism reveals that the combination of dielectric loss and magnetic loss components in foam is a promising strategy to fabricate MA absorbents ellexcent performance.