The size-dependent electromagnetic properties of MOF-derived Co/C composites have been optimized to significantly extend the effective microwave absorption bandwidth.

Broadband microwave absorption at low thickness still remains a challenge for most microwave absorbing materials derived from metal–organic frameworks. Herein, a three-step route, including the oxidation of Prussian blue microcubes, polymerization of phenolic resin on Fe₂O₃ microcubes and controllable carbothermal reduction of Fe₂O₃@resin, has been developed to achieve simultaneous enhancement of complex permittivity and permeability, in order to promote reflection loss performance at low thickness. It has been proved that different carbothermal reduction environments generated by different resin contents would have a direct effect on the composition and microstructures of the reduced products. In detail, irregular particles of reduced products tend to grow up and connect with each other with increasing resin content. With low resin content, the reduced products (CR-0.2 and CR-0.4) consist of Fe₃O₄, FeO and Fe. With high resin content, the reduced products (CR-0.6 and CR-0.8) are composed of Fe₃O₄, Fe, Fe₃C and partially graphitic carbon. A higher reduction degree endows CR-0.6 and CR-0.8 with stronger attenuation ability, including conduction loss provided by metallic Fe and graphitic carbon frameworks, enhanced polarization loss generated among multiple interfaces and magnetic loss produced by Fe and Fe₃O₄. Owing to the good balance between electromagnetic attenuation and impedance matching, a broad effective absorption bandwidth of 5.44 GHz can be reached at only 1.5 mm for CR-0.6. Our findings may pave a new way for realizing broadband microwave absorption at low thickness and offer novel insights for designing the composition and structure of MOF-derived materials.

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.

One-dimensional chain-like Fe ₃ O ₄ /C composites possess good microwave absorbing performance at an ultralow thickness of 1.2 mm, owing to high and appropriate permittivity and permeability.