Synthesis of hierarchical cobalt dendrites based on nanoflake self-assembly and their microwave absorption properties

Three dimensional (3D) cobalt dendritic architectures composed of nanoflakes have been successfully fabricated by a facile hydrothermal self-assembly route. This method includes the preparation of cobalt dendritic architectures in a solution of cobalt chloride (CoCl2 center dot 6H(2)O) and sodium hydroxide (NaOH) at 110 degrees C for 24 h using pyromellitic acid (C10H6O8) as the complex agent and sodium hypophosphite (NaH2PO2 center dot H2O) as the reducing agent. The crystal structure, morphology, and microwave absorption properties of the as-prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), scanning electron microscopy (SEM), and a network analyzer. The results show that reaction parameters, such as the molar ratio of C10H6O8 to CoCl2 center dot 6H(2)O, and concentration of NaOH, play a crucial effect on the morphology of the products. A plausible mechanism for the formation of hierarchical cobalt dendrites based on the Ostwald ripening process is proposed. The Co dendrite-paraffin composite containing 65 wt% of Co dendrites shows excellent microwave absorption properties. The optimal reflection loss (RL) of -30.2 dB is obtained at 12.5 GHz and RL below -10 dB covers the frequency range of 10.5-14.5 GHz with a thickness of 1.5 mm. The maximum RL values can reach -35.6 dB at 5.6 GHz with a thickness of 3 mm and the effective bandwidth (RL < -10 dB) is up to 11.8 GHz, in the frequency range of 2.7-14.5 GHz by adjusting the thickness of 1.5-5 mm. The excellent microwave absorption properties can be attributed to multiple dielectric relaxation and magnetic resonances, which contribute to proper impendence matching and a microwave attenuation constant. These results show that the 3D cobalt dendritic architecture is a great candidate for microwave absorption applications.