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

Abstract

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 (CoCl₂·6H₂O) and sodium hydroxide (NaOH) at 110 °C for 24 h using pyromellitic acid (C₁₀H₆O₈) as the complex agent and sodium hypophosphite (NaH₂PO₂·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 CoCl₂·6H₂O, 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.