Surface Architecture of Ni-Based Metal Organic Framework Hollow Spheres for Adjustable Microwave Absorption

Ni-based metal organic framework (MOF) hollow spheres were successfully synthesized by a hydrothermal method. After carbonization in nitrogen at 600 °C, a surface layer made of different Ni architectures is formed. The shape and size of the surface architectures (e.g., needles or pillars) can be adjusted effectively by controlling the duration of the hydrothermal reaction, leading to variable anisotropy and electromagnetic properties. The investigation of the microwave attenuation performance reveals that the paraffin wax composite (50 wt %) of Ni-MOF with 8 and 10 h reaction time could exhibit multiple resonance behaviors in permeability curves due to a different anisotropy of the magnetic Ni architectures on the surface compared to that in the shell of the hollow Ni-MOF spheres. Such multiresonance behavior could also benefit the bandwidth of the Ni-MOF hollow sphere absorber. It is found that the 8 and 10 h samples could reach a broad effective absorption bandwidth (EAB, reflection loss < −10 dB) of about 6.8 and 6.2 GHz with a thickness of around 1.8 and 4.6 mm, respectively. It is believed that this study not only offers a promising type of lightweight and broadband Ni-MOFs absorbers but also demonstrates a feasible approach for broadening the bandwidth of absorbers through making surface structures with different anisotropies.