Preparation of Fe<sub>3</sub>O<sub>4</sub> nanospindle composites and high performance microwave absorption

Abstract

<p indent="0mm">With the development of electronic communication technology, microwave absorption and shielding are becoming increasingly important in information security, electronic warfare and human body protection. Especially with the rapid development of device integration and multi-band radar technology, the demand for ultra-thin broadband absorbing materials is highly urgent. Magnetic material is easier to achieve impedance matching with air due to their high permeability, which is beneficial to increase the absorbing bandwidth and decrease the thickness of materials. Therefore, it is the first choice for low-frequency microwave absorbing materials. But the microwave permeability is drastically lowered above the resonance frequency, resulting in deterioration of absorbing properties at high frequency, due to Snoek's limit. Therefore, research on magnetic absorbing materials have focused on increasing the permeability and resonance frequency. The Snoek's limit of magnetic materials can be broken by increasing the magnetic anisotropy of materials, including magnetocrystalline anisotropy, shape anisotropy, and surface/interface anisotropy, to increase the permeability and magnetic loss in the radar band. A one-dimensional nanomaterial with strong magnetic anisotropy is proved to be able to significantly increase the natural resonance frequency, thereby enhancing the absorbing properties of the magnetic materials in the high-frequency radar band. In this paper, Fe₃O₄ nanospindle was prepared by a two-step chemical reaction method. First of all, Fe₂O₃ nanospindle was synthesized by hydrothermal method using FeCl₃·6H₂O and NaH₂PO₄·2H₂O as raw materials. Then, Fe₃O₄ nanospindle was obtained by activated carbon reduction in Fe₂O₃ nanospindle. The spindle structure of α-Fe₂O₃ is clearly observed in scanning electron microscope (SEM) image, and Fe₃O₄ holds the same nanostructure and size in transmission electron microscope (TEM) images as α-Fe₂O₃. The X-ray diffraction (XRD) pattern shows that the lattice constants of α-Fe₂O₃ are <italic>a</italic><sc>=5.032 Å</sc> and <italic>c</italic><sc>=13.762 Å,</sc> and Fe₃O₄ is <italic>a</italic><sc>=8.387 Å.</sc> The electromagnetic parameters of the samples were tested by a vector network analyzer in the frequency range <sc>2−18 GHz.</sc> With the increase of concentration of Fe₃O₄, both <italic>ε</italic>′ and <italic>ε</italic>″ increase at the same frequency. The <italic>ε</italic>′ for each sample slightly decreases with increasing frequency owing to the behavior of Debye relaxation. Compared with Fe₃O₄ nanoparticle, the natural resonance frequency of Fe₃O₄ nanospindle shifts to the higher frequency <sc>4.5 GHz</sc> due to strong shape anisotropy, resulting in higher magnetic loss in the range of <sc>2−18 GHz.</sc> The peaks of RL move to lower frequency with the increase in thickness. At the same concentration, Fe₃O₄ nanospindle has better absorbing properties than Fe₃O₄ nanoparticle. The Fe₃O₄ nanospindle composites have a maximum bandwidth of <sc>3.54 GHz</sc> and a minimum reflectivity of <sc>-41 dB</sc> at 70 wt% concentration. Furthermore, the material can achieve good impedance matching at various thicknesses, which is critical for broadband design of microwave absorption. The Fe₃O₄ nanospindle composites achieve strong microwave absorption of 90% covering a wide frequency range of <sc>2.8−17.0 GHz</sc> by designing double-layer structure at 80 wt% concentration. The broadband microwave absorption is mainly associated with the multiple <italic>λ</italic>/4 resonances. In conclusion, Fe₃O₄ nanospindle was prepared by two-step chemical reaction method. The complex permittivity and permeability, and microwave absorption for Fe₃O₄ nanospindle were investigated. Due to the strong magnetic anisotropy, Fe₃O₄ nanospindle exhibits a higher natural resonance frequency <sc>(4.5 GHz)</sc> than Fe₃O₄ nanoparticle, resulting in higher magnetic loss in radar band. Fe₃O₄ nanospindle composites can achieve a maximum bandwidth of <sc>3.54 GHz</sc> and a minimum reflectivity of <sc>-41 dB</sc> with excellent impedance matching. Fe₃O₄ nanospindle composites can achieve strong microwave absorption in the range of <sc>2.7−17.0 GHz</sc> by designing double-layer structure.