Controllable Preparation of Fe3O4@RF and Its Evolution to Yolk–Shell-Structured Fe@C Composite Microspheres with High Microwave Absorbing Performance

Fe3O4@RF microspheres with different phenolic (RF) layer thicknesses are prepared by adjusting the polymerization time. With the prepared Fe3O4@RF as the precursor, Fe@C composite microspheres with rattle-like morphology are obtained through one-step controlled carbonization. This method simplifies the preparation of rattle-shaped microspheres from sandwich microspheres. Fe@C microspheres exhibit excellent microwave absorbing properties. The morphology and composition of the product are investigated depending on the effects of carbonization temperature, time and thickness of the RF layer. When the carbonization temperature is 700 °C, the carbonization time is 12 h and the polymer shell thickness is 62 nm, the inner hollow Fe3O4 is completely reduced to Fe. The absorption properties of the materials are compared before and after the reduction of Fe3O4. Both Fe@C-12 and Fe3O4@C-700 show excellent absorbing properties. When the filler content is 50%, the maximum reflection loss (RLmax) of the rattle-shaped Fe@C microspheres is −50.15 dB, and the corresponding matching thickness is 3.5 mm. At a thickness of 1.7 mm, the RLmax of Fe3O4@C-700 is −44.42 dB, which is slightly worse than that of Fe@C-12. Both dielectric loss and magnetic loss play a vital role in electromagnetic wave absorption. This work prepares rattle-shaped absorbing materials in a simple way, which has significance for guiding the construction of rattle-shaped materials.

Microwave absorption properties of rare earth alloy Nd2Co17@C/Na2SiO3 composite at elevated temperature (293–723 K) in X band

A variety of new challenges are being faced in the development of high temperature microwave absorbing materials in the X band. Recently, some of the 2:17 phase rare-earth soft magnetic alloys with high permeability and curie temperature have potential to be a novel X band high temperature microwave absorbing material. In this paper, a high temperature microwave absorbing material (Nd2Co17@C/Na2SiO3) is prepared with Nd2Co17 as a raw material. After carbon cladding and Na2SiO3 treatment, the composite can work stably at 723 K. The calculated absorption properties display that the reflection loss (RL) intensity of the composite with a thickness of 1.5 mm is below -6 dB in the whole X band. Moreover, the thermogravimetric (TG) analysis results and static magnetic properties before and after sintering indicate that the material has excellent resistance to oxidation. Rare earth alloy materials provide a new possibility for the research of innovative high temperature absorbing materials.

Comparative Study on Microwave Absorbing Heating Characteristics and Microwave Deicing Performance of Airport Pavement Modified Concrete

In this paper, the airport pavement concrete has been taken as the main research object, three kinds of absorbing materials, namely silicon carbide (SiC), iron oxide (Fe3O4) and graphite, have been respectively mixed into the concrete, and an open microwave testing system has been established. Based on this system, the basic mechanical properties, microwave heating characteristics, microwave deicing effect and its influencing factors of modified concrete are systematically studied. In addition, a comparative analysis of the influence mechanism of different absorbing materials on the strength and absorbing performance of pavement concrete is carried out. The results showed that the addition of SiC, Fe3O4, and graphite could effectively enhance the microwave effect of pavement concrete, and the more the addition, the more obvious the improvement. Furthermore, under the same mixing amount, the degree of improvement of microwave deicing performance of each absorbing material from large to small is graphite, Fe3O4, SiC. However, the addition of graphite will form several weak links in concrete, thereby reducing its overall mechanical properties. SiC can slightly improve the mechanical properties of pavement concrete, but it has no significant effect on the microwave absorption properties. With the addition of Fe3O4, the strength of concrete changes little, and the effect of microwave absorbing heating and microwave deicing is remarkable. In general, the comprehensive performance of microwave deicing of Fe3O4 modified concrete is optimal. This study has high scientific and practical significance, and can be widely applied to deicing projects on airports and high-grade highways.

In situ regulation of microstructure and microwave-absorbing properties of FeSiAl through HNO3 oxidation

Wrapping insulation of coatings is effective for enhancing the microwave-absorbing properties (MAPs) of ferromagnetic absorbents (FMAs). However, the process is still limited by the low bonding strength with the matrix. Herein, an in situ regulation strategy based on the preparation of thin thickness and strong adhesion insulating layers through HNO3 oxidation was developed to address the limitations. The oxidation process of FeSiAl (FSA) powders was carried out by HNO3 following three main steps. First, the original oxide layer first reacted with HNO3 to form Fe3+ and Al3+. Second, the oxide layer composed of Al2O3 and Fe3O4 was preferentially formed due to the negative change in Gibbs free energy. Finally, the oxide and pigment-deposition layers were subjected to competitive growth and dissolution accompanied by the dissolution of Fe and Al atoms. Oxidation time up to 10 min resulted in the formation of a bilayer structure with a thickness of ∼50 nm on the FSA surface, as well as an outer layer crammed of Al(OH)3 and Fe(OH)3, and an inner layer containing mixed Fe2O3, Fe3O4, Al2O3, and SiO2. The MAPs of as-treated FSA achieved minimum reflection loss (RL) of −25.90 dB at 13.36 GHz, as well as absorption bandwidth of 5.61 GHz (RL < −10 dB) at 10.13–15.74 GHz and thickness of 2.5 mm. In sum, the developed route looks promising for the preparation of high-performance FMAs.