Z.H. Yang

Recent progress in some composite materials and structures for specific electromagnetic applications

Abstract This review aims to summarise the progress in some materials and structures for electromagnetic applications, such as microwave absorption, electric shielding and antenna designs, which have been developed in recent years. Composites with spherical powders for microwave absorption focus mainly on those based on ferrites (especially hexagonal), carbonyl iron and related alloys and various newly emerged nanosized materials. Composites with long conductive fibres as fillers will be summarised, with speical attentions to prediction, measurment and evaluation of their performances. Metamaterials include structures for microwave absorbing applications, tunable materials or structures with reflection or transmission coefficients that are tunable by external magnetic or electric fields, and specially designed structures for microwave absorbing applications, with thickness much smaller than that of conventional composite materials and performances that can be optimised by the physical properties of substrates, and new metamaterials constructed with ferrite cores wound by metallic wire coils that exhibited unique magnetic properties, with extremely high real and imaginary permeability, which are adjustable or tunable by varying their configurations. Magnetodielectric materials, with matching permeability and permittivity, together with sufficiently low magnetic and dielectric loss tangents, with potential applications in antenna miniaturisation, will be discussed.

Molten-salt-mediated synthesis of SiC nanowires for microwave absorption applications

Silicon carbide (SiC) nanowires were synthesized by a reaction of multiwall carbon nanotubes (MWCNTs) and silicon vapor from molten salt medium at 1250 °C. The phase, morphology, and microstructure of the nanowires were systemically characterized by X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission electron microscopy. The results revealed that the nanowires were of single-crystalline β-SiC phase with the growth direction along [111] and had diameters of 20–80 nm and lengths up to several tens of micrometers. The molten salt introduced facilitated the evaporation of Si (vapor) onto MWCNTs (solid) and the growth of SiC nanowires followed the vapor–solid process. The investigation of microwave absorbability indicated that a minimum reflection loss of −17.4 dB at 11.2 GHz could be achieved with 30 wt% SiC nanowires as the filler in the silicone matrix. The attenuation of microwave could be attributed to the dielectric loss and a possible absorption mechanism was also discussed.

Enhancement photocatalytic activity of the graphite-like C3N4 coated hollow pencil-like ZnO

The pencil-like ZnO hollow tubes with 9-12 μm in length, 350-700 nm in width, 200 nm in wall thickness coating with g-C3N4 have been prepared via a chemical deposition process. As compared with uncoated ZnO or g-C3N4, these g-C3N4/ZnO composites showed the enhanced photocatalytic activity which can be attributed to the heterojunction structure. Furthermore, it is worth pointing out that the weight ratios of g-C3N4 to ZnO (g-C3N4/ZnO) played a significantly influence on the photodegradable properties. With increasing the mass ratio, the photocatalytic activity increased firstly and then decreased after reaching to an optimal photocatalytic performance. It can be inferred that the appreciation of g-C3N4 on the ZnO surface can improve the contact area which resulted in high separation of electrons and holes. However, excessive g-C3N4 may hinder the electrons transferring from the g-C3N4 to ZnO, and thus worse its photocatalytic performance. In our study, the g-C3N4/ZnO sample prepared with 10 wt% of g-C3N4 exhibited the optimal photodegradable efficiency which 94% of Rhodamine B (RhB) has been degraded just in 2 h.

Enhanced microwave magnetic and attenuation properties for Z-type barium ferrite composites with flaky fillers

Microwave attenuation characteristics and magnetic and dielectric properties have been studied for Ba3CoxZn2-xFe24O41 with x = 0.8 and 2.0 ferrite flaky-filler composites. Results show that μ0′ and μmax″ are enhanced by >150 and >70%, respectively, and ɛ′ and ɛ″ are almost the same, as compared to conventional composites. Therefore, the percentage bandwidth WP is expanded from 110% to 150% for reflectivity RL ≤−10 dB for the flaky-filler composites, which achieves 75% of the theoretically maximum bandwidth. For all composites, two matching frequencies are found. The first matching frequency is attributed to the quarter-wavelength resonator and the second matching frequency is mainly determined by the values of both μ′ and ɛ′. At the second matching thickness, the flaky-filler composites also exhibit good attenuation characteristics with low reflectivity of RL <−20 dB and WP= 40% at L band.

Ultrabroad bandwidth of single-layer electromagnetic attenuation composites with flaky fillers

Z-type barium ferrite flakes were prepared for use as fillers in electromagnetic attenuation composites. The composites with the flaky fillers exhibit greatly enhanced complex permeability, thus achieving an ultrabroad attenuation bandwidth. μ0′ and μmax″ are enhanced by above 150%, and the relative bandwidth WR of 6–7 can be achieved. The bandwidth achieves 75% of the theoretically maximum relative bandwidth. The composites with the flaky fillers are powerful and potential candidates as electromagnetic attenuation materials with ultrabroad bandwidth in L, C, and S bands.

Facile Synthesis and Hierarchical Assembly of Flowerlike NiO Structures with Enhanced Dielectric and Microwave Absorption Properties

In this work, two novel flowerlike NiO hierarchical structures, rose-flower (S1) and silk-flower (S2), were synthesized by using a facial hydrothermal method, coupled with subsequent postannealing process. Structures, morphologies, and magnetic and electromagnetic properties of two NiO structures have been systematically investigated. SEM and TEM results suggested that S1 had a hierarchical rose-flower architecture with diameters in the range of 4-7 μm, whereas S2 exhibited a porous silk-flower architecture with diameters of 0.7-1.0 μm. Electromagnetic performances indicated that the NiO hierarchical structures played a crucial role in determining their dielectric behavior and impedance matching characteristic, which further influenced the microwave attenuation property of absorbers based on them. Due to its hierarchical and porous architectures, S2 had higher microwave absorption performances than S1. The maximum RL value for sample S2 can reach -65.1 dB at 13.9 GHz, while an efficient bandwidth of 3 GHz was obtained. In addition, the mechanism of the improved microwave absorption were discussed in detail. It is expected that our NiO hierarchical structures synthesized in this work could be used as a reference to design novel microwave absorption materials.

Study of matching characteristics for Ni0.97−xZnxCo0.03Fe2O4 spinel ferrites

A perfect matching chart is designed to obtain the matching frequency fm and the matching thickness tm of electromagnetic (EM) attenuation material. Based on the chart, the relationship between matching parameters (fm and tm) and high-frequency dynamic parameters [μ′(f), μ″(f), and ϵ′(f)] are studied for Ni0.97−xZnxCo0.03Fe2O4 (NiZnCo) spinel ferrites with x=0.5 and 0.65. At tm, the spinel ferrites exhibit excellent EM attenuation characteristics with low reflectivity and broad bandwidth at very high frequency and ultrahigh frequency bands.

Magnetic relaxation in Zn-Sn-doped barium ferrite nanoparticles for recording

Abstract Magnetic relaxation analysis has been carried out on Zn–Sn-doped barium ferrite BaFe 12−2 x Zn x Sn x O 19 nanoparticles with 0⩽ x ⩽0.5 at room temperature. The choice of these two ions combination was based on the consideration that Zn 2+ substitution could help to increase saturation magnetization M s and Sn 4+ substitution could decrease the temperature dependence of coercivity d H c /d T . Logarithmic behavior on the time dependence of the magnetization for these particles has been observed. The measured viscosity coefficient S was found to vary with the applied field and peaked around the coercive field H c . With increased doping concentration x , thermal relaxation of magnetization increased, mainly due to the decrease of particle size and anisotropy field, as well as their wider distributions. The activation volume V ac and fluctuation field H f were determined from the magnetic viscosity coefficient S together with the irreversible magnetic susceptibility χ irr . V ac was found to be smaller than the particles physical volume, indicating incoherent magnetic reversals in these particles.

Formation mechanism and magnetic properties of hollow Fe3O4 nanospheres synthesized without any surfactant

Hollow structured magnetite spheres were fabricated by a simple solvothermal process, with the assistance of various ammonium salts, where ethylene glycol was used as the solvent and reducing agent. The results of X-ray diffraction, scanning electron microscopy and transmission electron microscopy show that the as-synthesized products are pure single-phase Fe3O4 with good crystalline state, and all the samples are hollow structures except for the S-d sample, which was obtained by the assistance of NH4HCO3. The X-ray photoelectron spectrometry manifests that the Fe3+ ions on the surface of the hollow spheres exist in the form of Fe3O4, and Mossbauer measurements reveal that the hollow spheres are similar to the stoichiometric Fe3O4. The possible formation mechanism of hollow magnetite spheres with various sizes was discussed in detail. Meanwhile magnetic properties were determined by using a vibrating sample magnetometer at room temperature. The magnetic properties investigation showed that the Fe3O4 spheres were ferromagnetic with small hysteresis loops. The values of saturation magnetization are 82.989, 78.049 and 87.417 emu g−1 for the S-a, S-b and S-c samples, which decreases with increasing particle size. This phenomenon may be caused by the content of Fe3+ on A- and B-sites of the spinel ferrite. Based on experimental results, the relationship between their microcomponents also has been studied.

Synthesis and enhanced microwave properties of uniform hollow Fe nanospheres and their core–shell silica nanocomposites

Monodispersed hollow magnetite nanospheres with diameter of about 462 nm and a shell thickness of approximately 90 nm were successfully synthesized through a simple solvothermal process in ethylene glycol (EG) in the presence of urea. The electron and transmission microscopy images showed that the obtained magnetite spheres are composed of small particles and confirmed their hollow interior. Furthermore, by annealing in an H2 atmosphere, the crystalline structure is converted from fcc (Fe3O4) to bcc (Fe) which has been confirmed by X-ray diffraction (XRD) results. The Ms value of reduced samples (hollow Fe nanospheres) increases to 198 emu g−1 which is mostly equal to that of bulk Fe. Relevant hollow Fe/SiO2 core–shell nanospheres were prepared through a modified Stober process. The enhanced microwave attenuation can be achieved for this core–shell nanospheres which is attributed to the improved electromagnetic impedance match and strong and broadband natural resonance at high frequency for the metal nanospheres.