Fe3O4纳米纺锤体复合材料的制备及其高性能微波吸收

Preparation of Fe_3O_4 nanospindle composites and high performance microwave absorption

采用两步化学法合成纺锤体状Fe_3O_4纳米材料,研究了其微波电磁性能.由于较强的磁形状各向异性,Fe_3O_4纳米纺锤体比Fe_3O_4纳米颗粒材料呈现出更高的自然共振频率(4.5 GHz).自然共振频率的蓝移导致了Fe_3O_4纳米纺锤体在2~18 GHz雷达波段内具有更高的磁损耗,从而表现出优异的吸波性能.Fe_3O_4纳米纺锤体复合材料可以达到3.54 GHz吸波带宽,最小反射率为-41 dB,而且在1.5 mm厚度以上均能够实现微波强吸收,具有很好的阻抗匹配特性.通过双层结构设计,Fe_3O_4纳米纺锤体复合材料可以在2.8~17.0 GHz范围内实现宽带微波吸收.

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 a=5.032 ? and c=13.762 ?, and Fe3O4 is a=8.387 ?. The electromagnetic parameters of the samples were tested by a vector network analyzer in the frequency range 2-18 GHz. With the increase of concentration of Fe3O4, both ε′ and ε″ increase at the same frequency. The ε′ 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 4.5 GHz due to strong shape anisotropy, resulting in higher magnetic loss in the range of 2-18 GHz. 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 3.54 GHz and a minimum reflectivity of-41 dB 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 2.8-17.0 GHz by designing double-layer structure at 80 wt% concentration. The broadband microwave absorption is mainly associated with the multiple λ/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(4.5 GHz) than Fe3O4 nanoparticle, resulting in higher magnetic loss in radar band. Fe3O4 nanospindle composites can achieve a maximum bandwidth of 3.54 GHz and a minimum reflectivity of-41 dB with excellent impedance matching. Fe3O4 nanospindle composites can achieve strong microwave absorption in the range of 2.7-17.0 GHz by designing double-layer structure.