Thermal energy compensation strategy to achieve rambutan-like CoFe@C-CNTs composites with controllable cross-linked networks for broadband microwave absorption

Combining carbon nanotubes (CNTs) with dielectric/magnetic materials is an attractive strategy to obtain high-performance microwave absorbing materials. However, it remains great challenges to overcome the agglomeration of CNTs and balance magnetic components to achieve impedance matching and broadband absorption. Herein, a thermal energy compensation strategy is designed to construct rambutan-like CoFe@C-CNTs composites with excellent microwave absorption performances via a catalyst chemical vapor deposition method and the subsequent reduction process. This strategy fully utilizes the thermal energy released during the C2H2 carbonization process, allowing the cracking of C2H2 and the growth of CNTs to continue at a temperature of lower than 430 °C. By adjusting the C2H2 introduction time in the cooling process of preheated cobalt iron oxide (CoFeO) nanoparticles, the number and length of CNTs on the carbon shells covering the surface of CoFeO cores can be controlled without forming metal carbides. The abundant interfaces, defects and grain boundaries in CoFe@C-CNTs composites contribute to enhancing the polarization loss via various polarizations and relaxations. Meanwhile, the appropriate 3D conductive network constructed by properly controlled CNTs provides remarkable additional conductivity loss. Furthermore, the effective retention of magnetic components greatly improves the impedance matching of the composites and offers considerable magnetic loss. Therefore, the optimized CoFe@C-CNTs-3 composite shows an effective absorption bandwidth of 9.00 GHz with a thickness of 2.1 mm. The proposed thermal energy compensation strategy provides a novel insight into developing magnetic metal-CNTs composites with potential practical applications.