Fabrication Of Coal-based Carbon Foam Composites For Microwave Absorption

With the aggravation of electromagnetic (EM) pollution and the development of modern military, high-performance structural microwave absorption (MA) materiel is extremely demanded for civil and military application. Carbon foam (CF) has been considered as a promising candidate for its 3D porous structure and excellent characteristics including superlight, electrical conductivity and chemical stability. Coal liquefaction residue (CLR) is a suitable and cost-effective precursor with heavy carbonaceous materials. The high value-added utilization of CLR can promote efficiency and benefit of coal industry. In the present thesis, several kinds of CF composites are prepared via template method as ultralight, broad-band and adjustable microwave absorber. A proper MA performance are provided by controlling thermal process, the content of metal and adding epoxy resin. Some progresses are briefly summarized as follows:(1) CF composites embedded with carbon-coated nickel (Ni@C) nanocapsule are fabricated from CLR extract asphaltene via simple template method. The core-shell Ni@C capsules disperse uniform in carbon matrix and spherical or quasi-sphere with particle diameters of ca.50 nm. By simply controlling carbonization temperature and the loading of Ni@C capsule, an optimal MA state can be achieved in the 2-18 GHz band. The maximum reflection loss (RL) of Ni10@CF hybrids reaches-16 dB at 14.3 GHz and the effective bandwidth with RL exceeding-10 dB is up to 5 GHz. The MA capability of Ni10@CF/ER are enhanced to-18.8 dB and effective bandwidth surpass 8 GHz.(2) CF composites are synthesized by sacrificing template method from untreated CLR. The as-prepared CF consists of polyhedron cell with a size around 400-600 ?m, which retains the morphology of polyurethane. The particle-like Fe species are 1-20?m in diameter and dispersed homogeneously, which derived from the catalyst of coal liquefaction process. Those species lead to remarkable enhancement in MA properties of CF hybrids. The effective absorption bandwidth of CF750 is up to 5.8 GHz.(3) The MA mechanism for CF composites is discussed based on its microstructure and properties. Firstly, the abundant pore structure functions as microwave darkroom and facilitates the multi-reflection of EM, which increases EM propagation path and then accelerates its attenuation. Secondly, the presence of Ni and Fe species not only improve the magnetic loss, but induce multiple interface, which is beneficial to interfacial polarization and associated relaxation. Thirdly, the synergistic effects and efficient complementarities between electric carbon matrix and magnetic species enhance the impedance matching and MA capability.