ABSTRACT
Magnetic materials with ingeniously designed structures may be utilized as highly efficient microwave absorbing materials (MAMs) working at ultralow matching thicknesses. However, it remains a challenge to decrease the matching thickness by synergistically tailoring the composition and structure of magnetic MAMs. In this work, a series of magnetic MAMs have been synthesized by sequentially annealing Fe-bdc nanorods in air and hydrogen. The results show that with the increase in hydrogen reduction temperature, the Fe2O3 nanorods would be gradually converted into Fe2O3/Fe3O4/C, Fe3O4/C, Fe3O4/FeO/Fe/C and Fe/C composites. In the meantime, obvious particle shrinkage would first occur and significant crystal growth would then happen, leading to the disappearance of pores, the decrease in axial length and the increase in particle size. In addition, a higher reduction temperature always leads to higher complex permittivity (εr) and permeability (µr), which should be related to the higher content of Fe3O4 and Fe. Nevertheless, one-dimensional structures and surface oxidation may cause abnormal εr and µr. As for HR-350, the one-dimensional structure results in strong conduction loss, the Fe3O4/C interfaces contribute to polarization loss, and semiconductive Fe3O4 and amorphous carbon favor superior impedance matching. Consequently, an RL peak value of -43.77 dB can be obtained with an effective absorption bandwidth of 3.52 GHz, when the thickness is only 1.2 mm. This work may provide novel insights into the design of MAMs with broadband absorption at ultralow matching thicknesses and provide a good reference for the synthesis of MOF-derived magnetic materials.
ABSTRACT
Ultra-thin microwave absorbers have been urgently demanded for electromagnetic applications in recent years. Herein, porous carbon with a "flower cluster" microstructure was synthesized from biomass waste (mango seeds) by a facile activation and carbonization method. The novel structure reduced the density and also improved the impedance matching, dipole polarization, and provided many carbon matrix-air interfaces for interfacial polarization, resulting in superior microwave absorption performance. At an ultra-thin thickness of 1.5 mm, extraordinary microwave absorption was achieved, with a reflection loss (RL) of -42 dB. The effective absorption bandwidth reached 4.2 GHz. The RL can be further improved to -68.4 dB by adjusting the amount of activator to manipulate the structure of porous carbon. In addition, from the simulated radar scattering results, the maximum reduction in the radar cross-section (RCS) reached 30.4 dBm2, which can greatly reduce the probability of equipment being detected by radar. This work provides a low-cost and high-performance microwave absorber for electromagnetic stealth technologies.
Subject(s)
Carbon , Radar , Biomass , Microwaves , PorosityABSTRACT
Extending effective absorption bandwidth at a low filling ratio is still a challenge for metal-organic framework-derived microwave absorbing materials. Herein, varied complex structures based on CNTs have been built on Co/C particles derived from ZIF-67 via melamine-involved annealing routes. It was found that cobalt nanoparticles derived from ZIF-67 act as catalysts for the growth of CNTs, effectively promoting and controlling the content of melamine. Due to the effective control of the CNT-containing complex structure, excellent microwave absorption performance was achieved at a rather low filling ratio of 20 wt%, which can be attributed to improved attenuation ability and ameliorated impedance matching. Results show that highly graphitic CNTs benefit the formation of the electron transport network and enhancement of conduction loss. Unique one-dimensional complex structure and abundant Co/C interfaces strengthen the polarization loss. When the dielectric loss was optimized at different frequencies, appropriate impedance matching was also gained to realize a broad effective absorption bandwidth of 5.6 and 4.4 GHz in Ku and X bands, respectively. This work may provide novel insights into the synthesis and design of CNT-containing metal-organic framework-derived materials with lightweight features and wide frequency response.
