One-dimensional MOF-derived magnetic composites for efficient microwave absorption at ultralow thickness through controllable hydrogen reduction

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 (epsilon(r)) and permeability (mu(r)), which should be related to the higher content of Fe3O4 and Fe. Nevertheless, one-dimensional structures and surface oxidation may cause abnormal epsilon(r) and mu(r). 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.