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.

Controllable Fabrication of SiC@C-Fe3O4 Hybrids and Their Excellent Electromagnetic Absorption Properties

In this work, a batch of novel ternary hybrids (SiC@C-Fe3O4), characterized by SiC nanowires core, carbon shell, and adhered Fe3O4 nanoparticles were controllably synthesized via surface carbonization of SiCnw followed by hydrothermal reaction. Carbon, which was derived from SiC with nanometer thickness, possesses an amorphous structure, while Fe3O4 nanoparticles are in a crystalline state. Simultaneously, the inducement of Fe3O4 nanoparticles can provide significant magnetic loss, which is well-tuned by changing the molar content of iron precursors (FeCl3·6H2O and FeCl2·4H2O). SiC@C-Fe3O4 hybrids show great electromagnetic absorption performance owing to the synergy effect of dielectric and magnetic losses. The minimum refection loss can reach to −63.71 dB at 11.20 GHz with a thickness of 3.10 mm, while the broad effective absorption bandwidth (EAB) can reach to 7.48 GHz in range of 10.52–18.00 GHz with a thickness of 2.63 mm. Moreover, the EAB can also cover the whole X band and Ku band. The outstanding performance of the obtained material implys that it is a promising candidate as an electromagnetic absorber.

Fabrication of Electrospun Ni0.5Zn0.5Fe2O4 Nanofibers Using Polyvinyl Pyrrolidone Precursors and Electromagnetic Wave Absorption Performance Improvement

Ni0.5Zn0.5Fe2O4 nanofibers with an average diameter of 133.56 ± 12.73 nm were fabricated by electrospinning and calcination. According to our thermogravimetric—differential thermal analysis and X-ray diffraction results, the calcination temperature was 650 °C. The microstructure, crystal structure, and chemical composition of the nanofibers were observed using field-emission scanning electron, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Commercial particle samples and samples containing 10 wt% and 20 wt% nanofibers were fabricated, and the electromagnetic properties were analyzed with a vector network analyzer and a 7.00 mm coaxial waveguide. Regardless of the nanofiber content, Ni0.5Zn0.5Fe2O4 was dominantly affected by the magnetic loss mechanism. Calculation of the return loss based on the transmission line theory confirmed that the electromagnetic wave return loss was improved up to −59.66 dB at 2.75 GHz as the nanofiber content increased. The absorber of mixed compositions with Ni0.5Zn0.5Fe2O4 nanofibers showed better microwave absorption performance. It will be able to enhance the performance of commercial electromagnetic wave absorbers of various types such as paints and panels.

Enhanced magnetic performance in exchange coupled CoFe2O4 - LaFeO3 nanocomposites

Nanocomposite oxide system of (x)CoFe2O4 - (100-x)LaFeO3 with different weight percent of core-shell structured CoFe2O4 (x = 0,20,40,50,80,100) and LaFeO3 were fabricated, via a two-step sol-gel wet-chemical synthesis technique. The phase formation of the composites was confirmed by X-ray diffraction and the structural parameters of both the phases were attained from the Rietveld refinement results of XRD patterns. The elemental composition and microstructure of the resulting nanocomposites were examined by using energy-dispersive X-ray spectroscopy (EDX) and high-resolution transmission electron microscopy (HRTEM) technique, respectively. The detailed magnetometry studies at 300 K and 5 K reveal that the inter-and intra-phase magnetic interactions affect the saturation magnetization (MS), remanence magnetization (MR) and coercivity (HC) values of this bi-magnetic system. The remarkable feature of “pinched magnetic hysteresis loop” was evidenced in the [(50) CoFe2O4 - (50)LaFeO3] composite, leading to a lesser magnetic loss factor and better magnetic performance of this sample. The report depicts an improved interfacial exchange coupling at 5 K, for the nanocomposites of core-shell morphology and offers an understanding or explanation of improved magnetic performance for the (50)CoFe2O4 - (50)LaFeO3 nanocomposite and opens up an important way to design new multiferroic applications in low magnetic fields.

MIL-88B (Fe) driven Fe/Fe3C encapsulated in high-crystalline carbon for high-efficient microwave absorption and electromagnetic interference shielding

Dual-functional magnetic/dielectric Fe/Fe3C@C composites were fabricated by pyrolysis of MIL-88B (Fe) in Ar atmosphere, which was used for microwave absorption and electromagnetic interference (EMI) shielding. The Fe/Fe3C nanocrystals were completely encapsulated in crystalline carbon, which can improve the oxidation resistance capacity. Owing to the remarkable impedance matching and strong attenuation constants, Fe/Fe3C@C composites show an optimal RL value of -56.4 dB at 14.0 GHz and broad effective absorption bandwidth (RL ≤ -10 dB) of 4.8 GHz, when the filling ratio and absorber thickness are only 20 wt% and 1.9 mm, respectively. Resulted from the high conductivity of crystalline carbon, magnetic loss of Fe/Fe3C and core-shell structure, the Fe/Fe3C@C composites also show remarkable EMI shielding properties at X band, which are enhanced by increasing the filling ratio of Fe/Fe3C@C composites. When the filling ratio was 50 wt%, the EMI shielding efficiency can reach 35 dB. This work suggests that the magnetic/dielectric Fe/Fe3C@C is a good cadidate in microwave absorption and electromagnetic interference shielding.

Electromagnetic Wave Absorption and Mechanical Properties of CNTs@GN@Fe3O4/PU Multilayer Composite Foam

With the development of intelligent communications and stealth technology in the military field, electromagnetic wave pollution cannot be ignored, and absorbing materials have entered people’s field of vision and gradually become a research hotspot. The ideal absorbing material should have the characteristics of “strong, wide, thin, and light”, but a single absorbing material often cannot meet the above conditions. At present, absorbing metal powder combined with two-dimensional carbon nanomaterials (such as carbon nanotubes, graphene, etc.) has became a trend. This article focus on a three-layer composite of Fe3O4, Carbon nanotubes@ Fe3O4, Carbon nanotubes@Graphene nano-platelets@ Fe3O4, which was synthesized by solvothermal method. The results show that the electromagnetic wave absorption performance of the three-layer foam at a thickness of 3.0 mm is more excellent. The minimum of RL can reach −67.0 dB, and the effective bandwidth is above 5.0 GHz. All this is due to the synergy of dielectric and magnetic loss between Fe3O4, CNTs, and GN, the increase of interface polarization and the path of electromagnetic wave reflection and scattering by three-layer foam.

Correlation between Cutting Clearance, Deformation Texture, and Magnetic Loss Prediction in Non-Oriented Electrical Steels

Manufacturing the magnetic cores in electrical machines impacts the magnetic performance of the electrical steel by inducing stresses near the cutting edge. In this paper, energy loss behaviour in non-oriented electrical steels punched with different cutting clearances before and after annealing is investigated. An experimental shear cutting tool was employed to punch the ring-shaped parts from electrical steels in a finished state with four different values of cutting clearance corresponding to 1%, 3%, 5%, and 7% of the sheet thickness. The effect of cutting clearance on the magnetic losses is derived and analysed by the statistical theory of losses and associated loss separation concept including the analysis of movable magnetic objects. In this framework, this paper assesses the combined effect of cutting clearance, frequency, and heat treatment on the hysteresis loops and iron losses in non-oriented FeSi electrical steels. Measurements have been performed from quasi-static to 400 Hz at peak induction Bp = 1.0 T. Both states before and after heat treatment have been considered. The excess loss is observed as the most sensitive loss component to cutting clearance and its magneto–structural correlation is quantified.

Structural, Magnetic, and Optical Properties of Mn2+ Doping in ZnO Thin Films

MnxZn1−xO thin films (x = 0%, 1%, 3%, and 5%) were grown on corning glass substrates using sol–gel technique. Single-phase hexagonal wurtzite structure was confirmed using X-ray diffraction. Raman analysis revealed the presence of Mn content with an additional vibrational mode at 570 cm−1. The surface morphology of the samples was observed by scanning electron microscopy which suggested that the grain size increases with an increase in Mn concentration. The optical bandgap increases with increasing Mn concentration due to a significant blueshift in UV–visible absorption spectra. The alteration of the bandgap was verified by the I–V measurements on ZnO and Mn-ZnO films. The various functional groups in the thin films were recorded using FTIR analysis. Magnetic measurements showed that MnxZn1−xO films are ferromagnetic, as Mn induces a fully polarised state. The effect of Mn2+ ions doping on MnxZn1−xO thin films was investigated by extracting various parameters such as lattice parameters, energy bandgap, resistivity, and magnetisation. The observed coercivity is about one-fifth of the earlier published work data which indicates the structure is soft in nature, having less dielectric/magnetic loss, and hence can be used as ultra-fast switching in spintronic devices.