With the rapid development of technology and the widespread use of electronic communication equipment, microwave interference and radiation has become a problem that cannot be ignored. The electromagnetic pollution caused may adversely affect human health and the operation of precision electronic equipment. In order to solve these problems, the design and preparation of microwave absorption materials of various frequency bandwidths have always been the focus of research. The ideal absorbing material should have excellent comprehensive characteristics, such as ultra-wide absorption bandwidth, strong absorption, light weight and thin thickness.
So far, there are many ways to adjust the impedance matching of microwave absorbing materials and improve the reflection loss (RL). Among them, optimizing the composition and structure of materials has become a hot spot in recent years. Introducing magnetic nanoparticles into dielectric loss materials is an effective method. This composite material has the following advantages:
(1) Magnetic nanoparticles can become polarization centers at the material interface. Dipolar polarization and interface polarization can improve the absorbing performance.
(2) Magnetic nanoparticles have the advantages of nanometer scale and large surface area, leading to the scattering of incident microwaves.
(3) The complementarity of dielectric loss and magnetic loss is conducive to achieving proper impedance matching and improving the absorbing performance. The spatial confinement effect is conducive to the dissipation of microwaves. At present, a wave absorbing material with a rattle, multi-shell or tubular structure has been developed. The incident microwave will be effectively absorbed because it can be reflected multiple times in the cavity.
Recently, researchers such as Zhang Baoliang (corresponding author) of Northwestern Polytechnical University designed and prepared a new type of microwave absorbing material (TCF@Fe3O4@NCLs) through a single-step pyrolysis process of precursors. Based on the dual synergistic effect of dielectric/magnetic loss, the introduction of multi-layer heterostructure and conductive network, TCF@Fe3O4@NCLs material exhibits excellent wave absorption performance, with a reflection loss (RL) value of -43.6 dB, and an effective absorption bandwidth ( EBA) is 4.6 GHz (8.2-12.8 GHz) and the filling amount is 10 wt%, which has great potential in the application of absorbing materials. Related work was published in the internationally renowned journal "Journal of Colloid and Interface Science" with the title of "Fabrication of magnetic tubular fiber with multi-layer heterostructure and its microwave absorbing properties".
Paper link: https://doi.org/10.1016/j.jcis.2020.05.058
TCF@Fe3O4@NCLs are prepared using confined domain self-condensation, solvothermal method and dopamine (DA) polymerization technology. The obtained material is a tubular carbon nanofiber (TCF) structure composed of embedded Fe3O4 nanoparticles, dispersed Fe3O4 nanoparticles and a nitrogen-doped carbon layer (NCL). Tubular carbon nanofibers provide the main dielectric loss. Fe3O4 nanoparticles can significantly improve the low-frequency microwave absorption capacity and provide appropriate magnetic loss. NCL improves electrical conductivity and promotes the generation of multiple polarization effects, which leads to increased dielectric loss. The author further clarified the electromagnetic wave absorption mechanism.
TCF@Fe3O4@NCLs material synthesis strategy includes four steps, it has a Fe3O4 and nitrogen doped carbon layer from the inside to the outside. The advantages of this material are reflected in the following aspects. The multi-layer heterostructure is conducive to optimize impedance matching. A layered hybrid structure with cavities can improve multiple reflections and interface polarization effects. Under the protection of nitrogen-doped carbon layer, the stability of Fe3O4 nanoparticles is improved. Nitrogen doping causes surface defects and promotes dipole formation and interface polarization. The above inherent characteristics are beneficial to enhance the microwave absorption performance of the absorber.
The microscopic morphology of the product was observed with an electron microscope, and tubular fibers with inorganic nanoparticles inside and outside were observed. In contrast, it is obvious that FeCl3 aggregates more Fe3O4 nanoparticles after carbonization in the position occupied by PF@Fe3O4. The average particle size of Fe3O4 loaded on the surface increased from 7 nm to 15 nm.
The maximum RL values for samples with 7.5% wt, 10% wt, 12.5% wt and 15% wt were -21.4 dB (8.5 GHz, 4 mm), -43.6 dB (9.9 GHz, 3.3 mm), -31.7 dB (17.9 GHz, 1.9mm) and -15.1 dB (17.6 GHz, 1.7mm). The maximum value of RL increases first as the filling amount increases, and then decreases. When the filling amount is 10 wt%, the absorption performance is the best. The maximum value of RL moves toward the high-frequency direction, and the matching thickness decreases as the load increases. This phenomenon shows that the effective absorption bandwidth can be adjusted by changing the amount of filler added.
The wave absorbing mechanism is: the multi-layer heterostructure optimizes the impedance matching of the absorber. Multiple interfaces introduce more polarization centers, triggering multiple polarization effects and further improving dielectric loss. The three-dimensional conductive network formed by the overlapping of one-dimensional materials can increase the conduction loss. The introduction of magnetic Fe3O4 nanoparticles provides additional magnetic losses. Magnetic loss mechanisms include natural resonance and eddy current loss. The hollow tubular structure, porous structure and Fe3O4 nanoparticles cause multiple scattering and reflection, which is beneficial to the absorption and dissipation of microwaves.
In summary, a new microwave absorption material with a multi-layer heterostructure was prepared by a simple four-step method. It is systematically analyzed and characterized. The contribution of each component to microwave absorption performance was studied by comparison. In addition to the performance of a single functional component, a reasonable microstructure and synergy between the components are two important factors for improving microwave absorption. The presence of the porous secondary structure is conducive to multiple reflections and scattering of incident microwaves. The multi-polarization effect introduced by the interface of the multi-layer heterostructure can increase the dielectric loss of the absorber. A three-dimensional conductive network formed by stacked one-dimensional materials increases the conduction loss, and the effective absorption bandwidth can be adjusted by changing the filling amount in the range of 2–18 GHz.