X-band Absorption Research Articles

Light weight RGO/Fe3O4 nanocomposite for efficient electromagnetic absorption coating in X-band

Reduced graphene oxide (RGO)/magnetite (Fe3O4) nanocomposite has been synthesized by an in-situ facile hydrothermal method. The XRD pattern reveals the development of nanocomposite in which both phases are coexistent. Raman Spectroscopy shows the main characteristics peaks of D and G bands at 1349 cm−1 and 1595 cm−1 for graphitic structures. The intensity ratio (ID/IG) is also calculated, which indicate the degree of defects in the material. This ratio (ID/IG), increases from 0.84 for GO to 0.91 for RGO/Fe3O4 nanocomposite and promotes the defects which are beneficial for electromagnetic (EM) absorption. The SEM image depicts that, Fe3O4 spherical nanoparticles are dispersed over the surface of graphene sheets and provide a thermal conducting path for heat dissipation between different layers of graphene. The EM absorption properties have been analyzed at 2–18 GHz of RGO and RGO/Fe3O4. The addition of proper content of Fe3O4 magnetic nanoparticles in RGO sheets improved the Reflection Loss (RL) from − 13.5 dB to − 20 dB at a frequency of 9.5 GHz. Moreover, due to magnetic loss and interfacial polarization, the effective bandwidth increases from 2.5 GHz to 3.8 GHz at a coating thickness of 1.5 mm. Hence this light weight nanocomposite is an excellent material for strong EM absorption in X-band.

Metasurface absorber based on water meta “molecule” for X-band microwave absorption

Two different sizes of water “atoms” embedded in a flexible silicone matrix are combined together to form water “molecules” and used as the resonating elements of the metasurface to achieve wideband microwave absorption. The sizes of the two “atoms” are chosen such that their resonating frequencies couple to give a whole X-band absorption with a bandwidth of –10 dB. The design and simulation of the unit cell are carried out to achieve the desired results with an overall absorber thickness of ∼3.0 mm (i.e., only λ/12 of the lower cut off frequency). A wide absorption bandwidth of 4.2 GHz with an absorption efficiency of 90% is observed for the fabricated metasurface absorber in an X-band waveguide environment. The evaporation of water is prevented by embedding the water “molecules” in a silicone substrate, and its flexibility helps in retaining the designed shape. The arrayed structure of the water “molecules” provides the required effective permittivity and permeability of the metasurface by generating Mie resonance. Numerical investigation shows the polarization insensitive absorbance of the symmetrical unit cell structure and wide-angle absorption performance of the absorber for both transverse magnetic and electric modes.

A universal permittivity-attenuation evaluation diagram for accelerating design of dielectric-based microwave absorption materials: A case of graphene-based composites

Since Materials Genome Initiative has been proposed for demonstrating promising approaches in achieving high-performance materials, here we employ such concept to present a Materials Genome Initiative inspired universal permittivity-attenuation evaluation diagram (PAED) for accelerating design of highly efficient dielectric-based microwave absorption composites. Initially, parameter sweeping has been applied to screening the best microwave absorption parameters based on the absorption-permittivity mode, aiming to creating a novel PAED. For well demonstrating the validity of such diagram, experiments are applied to scalable fabricate various graphene-based composites with a simple approach, achieving strong microwave absorption (−57 dB with full-band qualified absorption in X-band) and considerably high radar cross scattering reduction (24.25 dBm2 at 90°) with the guide of the proposed diagram. Noticeably, implication of the verification results from experiments, previous literature and simulation indicates validity of such novel PAED, and thus it highlights a new general roadmap for rationally designing high-performance microwave absorption.

Comment on “Dual resonating C-band with enhanced bandwidth and broad X-band metamaterial absorber” in Appl. Phys. A (2016) 122:166

In a recent paper, Agarwa et al. (Appl Phys A 122:166, 2016) proposed a structure of metamaterial unit cell, which could realize dual-band absorption in C-band, and by altering its design parameters, broadband absorption in X-band could also be easily achieved, and its peak absorptivity is over 99 %. However, we find that the peak absorptivity is 40 % in C-band and 32 % in X-band, since the ostensible good return loss performance is caused by the polarization rotation rather than the absorption.

Effects of carbon nanotube dispersion methods on the radar absorbing properties of MWCNT/epoxy nanocomposites

Radar absorbing materials (RAMs) for practical applications are expected not only to have strong microwave absorption and a wide absorption bandwidth, but also to be lightweight, to have a fine thickness and acceptable structural performance, as well as being cost-effective. Although the dispersion of carbon-nanofillers in polymer matrices is a key factor determining the microwave absorbing properties of the composites, there have few studies on these effects. To our knowledge, to date, the realization of pristine multi-walled carbon nanotube (MWCNT)/polymer composites as RAMs in industrial production has been restricted, due to high CNT contents or large composite thicknesses. Thus, in this work, two MWCNT dispersion processing methods, a solution process with surfactant-aid and a ball-milling dispersion, were investigated to fabricate pristine MWCNT/epoxy nanocomposites. The effects of the different dispersion processes, CNT loading, and composite thickness on CNT dispersion in the matrix, were observed by TEM, and the electrical conductivity and X-band absorbing performance of the composites were assessed. The use of an ionic surfactant to aid the dispersion of CNTs in solution resulted in the best RAMs, with a good compromise among effective X-band absorption, small composite thickness, and very low CNT content. The ball-milling method also resulted in materials with a low CNT content and microwave absorbing performance acceptable for industrial applications. Moreover, it offers a very simple and efficient route suitable for low-cost, mass production of RAMs. The results showed that by facile approaches of dispersing pristine commercial MWCNTs in an epoxy resin matrix, composites of only 2–3 mm thickness and as little as 0.25–0.5 wt% CNT loading could be obtained, with a relatively wide X-band operating bandwidth and maximum absorptions exceeding 18–25 dB.

Preparation and Dielectric Properties of Nanostructured ZnOWhiskers

By a novel controlled combustion synthesis method, a large number ofnanostructured ZnO whiskers with different morphologies, such astetra-needles, long-leg tetra-needles and multi-needles, are preparedwithout any additive in open air at high temperature. The morphologiesand crystalline structures of the as-prepared ZnO nanostructuredwhiskers are investigated by SEM and XRD. The possible growth mechanismon the nanostructured ZnO whiskers is proposed. The experimental resultsindicate that the dielectric constants and losses of the nanostructuredZnO whiskers are very low, demonstrating that the nanostructured ZnOwhiskers are low-loss materials for microwave absorption in X-band.However, obvious microwave absorption in nanostructured ZnO whiskers isobserved. The quasi-microantenna model may be attributed to themicrowave absorption of the ZnO whiskers.

The Effects of Irradiation of 100 keV Electrons on Alkali Halide Crystals; The Point Defect Coagulates in NaCl and KCl Irradiated at Room Temperature

Thin films of KCl and NaCl single crystals prepared by chemical polishing were examined by an electron microscope. During the observation, the thin films were damaged by electron beam used for the image formation. As a result of electron irradiation at room temperature, plate shaped defects parallel to one of the {100} planes and elongated in one of the directions were formed in both kinds of crystals. The maximum dimension of them was about 1 µ in long axis 1000∼3000 A in short axis and ∼100 A in thickness. It is supposed that these plates correspond to the small alkali metal colloids which were considered to be the origin of X-band absorption in the optical measurement, because the behaviour of the plates during heat treatment between room temperature and 300°C showed good agreement with that of the colloids.