Dianyu Geng

Broadband microwave absorption of CoNi@C nanocapsules enhanced by dual dielectric relaxation and multiple magnetic resonances

Dual dielectric relaxation of the permittivity and multiple magnetic resonances of the permeability (including one natural resonance and two exchange resonance modes) are observed in CoNi@C nanocapsules in the same 5–17 GHz frequency range which leads to a better electromagnetic-wave absorption than earlier reported for nanocomposites. A reflection loss (RL) exceeding −25 dB is obtained in a wide frequency range of 5–17 GHz when an appropriate absorber thickness between 2 and 4.8 mm is chosen. For a 2 mm absorber layer, a RL value exceeding −10 dB is achieved in the broad frequency range 12–18 GHz, which covers the whole Ku-band.

Optimal electromagnetic-wave absorption by enhanced dipole polarization in Ni/C nanocapsules

Electromagnetic-wave (EMW) absorption by Ni/C nanocapsules with similar permeability but different permittivity mainly due to differences in the graphite-shell thickness has been investigated. The optimal working frequency could appear at S-band and C-band and considerable strong EMW absorption was achieved. For the optimal Ni/C nanocapsules, a reflection loss exceeding −20 dB was reached from 2.6 to 8.2 GHz with a maximum value of −40 dB at 3 GHz. The improved absorption can be attributed to an optimal electromagnetic match and an enhanced dipole polarization upon increasing of shell thickness.

Synthesis and characteristics of carbon-coated iron and nickel nanocapsules produced by arc discharge in ethanol vapor

Fe(C) and Ni(C) nanocapsules with low carbon content have been produced via an arc discharge process in ethanol vapor. It is clarified by X-ray diffraction that the core of the Fe(C) nanocapsules consists of gamma-Fe, alpha-Fe and Fe3C phase, while that of the Ni(C) nanocapsules contains only nickel. High-resolution transmission electron microscopy imaging confirms that these particles have a broad size distribution and the core/shell structure. Besides mutually independent nanocapsules with segregate graphitic shells, those with sharing shells are also observed in the Fe(C) nanocapsules. The remanence and the coercivity at room temperature of both the nanocapsules are higher than those of the corresponding microcrystallines, while the saturation magnetization is lower

Microwave-absorption properties of Fe(Mn)/ferrite nanocapsules

Electromagnetic ( EM) wave absorption properties of the paraffin-Fe(Mn)/ferrite nanocapsule composite ( with 40 wt% nanocapsules) were investigated in the 2-18 GHz frequency range. The Fe(Mn)/ferrite nanocapsules synthesized by arc-discharging have a core-shell structure with cores 10-30 nm in diameter and shells about 5 nm in thickness. The reflection loss (RL) calculated from the measured permittivity and permeability was found to exceed -20 dB in the 13-15 GHz range for a thickness of about 1.75-1.95 mm. The optimal RL is -28.2 dB at about 14 GHz with 1.84mm thickness. RL values exceeding -10 dB were obtained for the absorber of 1.7mm thickness in the range 12.7-17.1 GHz, which almost covered the whole Ku-band (12.4-18 GHz). The excellent EM wave absorption properties of the Fe(Mn)/ferrite nanocapsules significantly depend on the core-shell microstructure and the match of dielectric and magnetic losses.

Effect of metal grain size on multiple microwave resonances of Fe/TiO2 metal-semiconductor composite

The dielectric resonance and multiple magnetic resonances which correspond to multiple microwave absorptions in the 2–18 GHz range have been studied in the composite Fe/TiO2. The Fe grain size is found to have great impact on the dielectric resonance in this metal-semiconductor composite. The polarization mechanism is attributed to interfacial polarization. The multiple magnetic resonances can be ascribed to the natural resonance and exchange resonances, which can be explained by Aharoni’s exchange resonance theory.

Excellent microwave-absorption performances by matched magnetic–dielectric properties in double-shelled Co/C/polyaniline nanocomposites

Double-shelled Co/C/polyaniline (Co/C/PA) nanocomposites were prepared by combining the arc-discharge process and in situ chemical oxidative polymerization reaction. The effects of PA shells on the magnetic properties of Co/C/PA nanocomposites were studied, and the electromagnetic properties of Co/C/PA–paraffin composites were investigated in the 2–18 GHz frequency range. The reflection loss (RL) exceeding −10 dB is obtained in 9.9–16.4 GHz for an absorber thickness of 2.5 mm, which covers almost half of the X-band (8–12 GHz) and most of the Ku-band (12–18 GHz). Moreover, some strong absorption peaks exceeding −40 dB can be observed in the low-frequency range (3.5–5.5 GHz). These excellent absorbing performances show that the Co/C/PA nanocomposites have great potential for application in a microwave-absorption field for their strong absorption and broad bandwidth.

Fluorescence and microwave-absorption properties of multi-functional ZnO-coated α-Fe solid-solution nanocapsules

Fluorescence (FL) and microwave-absorption properties of multi-functional α-Fe solid-solution nanocapsules have been investigated. High-resolution transmission electron microscopy and x-ray photoelectron spectroscopy analysis show that the nanocapsules have a shell/core structure with α-Fe solid-solution nanoparticles as the core and amorphous ZnO as the shell. The nanocapsules are ferromagnetic at room temperature. There is a tendency to redshift for the peak at 388 nm in the FL spectra as the Zn concentration decreases. The in-depth study of relative permittivity and permeability reveals that the ZnO-coated α-Fe solid-solution nanocapsules exhibit excellent microwave-absorption properties, because of a proper electromagnetic match in the microstructure, strong natural resonances and dipolar polarization mechanisms.

Magnetic and Microwave-absorption Properties of Graphite-coated (Fe, Ni) Nanocapsules

The structure, magnetic and microwave-absorption properties of graphite-coated (Fe, Ni) alloy nanocapsules, synthesized by the arc-discharge method, have been studied. High-resolution transmission electron microscopy shows that the nanocapsules have a core/shell structure with (Fe, Ni) alloy as the core and graphite as the shell. All (Fe, Ni) alloy nanocapsules/paraffin composites show good microwave-absorption properties. The optimal reflection loss (RL) was found for (Fe(70)Ni(30))/C nanocapsules/paraffin composites, being -47.84 dB at 14.6 GHz for an absorber thickness of 1.99 mm, while the RL values exceeding -10 dB were found in the 12.4-17.4 GHz range, which almost covers the K(u) band (12.4-18 GHz). For (Fe(70)Ni(30))/C nanocapsules/paraffin composites, RL values can exceed -10 dB in the 11.4-18 GHz range with an absorber thickness of 1.91 mm, which cover the whole K(u) band.

Amorphous boron nanoparticles and BN encapsulating boron nano-peanuts prepared by arc-decomposing diborane and nitriding

Amorphous boron nanoparticles were prepared by arc-decomposing diborane, which had ideal morphologies in comparison with that of those fabricated by furnace or laser heating diborane. Peanut-shaped boron nitride encapsulating boron nanocapsules were fabricated by nitridation of amorphous boron nanoparticles. Unique core/void/shell structure of the nanocapsules was observed by using a high-resolution transmission electron microscopy. The mechanism of growing the BN nanocapsules by a catalyst free process was distinctly different from the process of arc discharge or laser heating. The broadening of nonpolar intralayer Raman line of hexagonal BN at about 1370 cm−1 was observed, which was attributed to the small crystal size of BN.

Microwave absorption properties of Ni/(C, silicides) nanocapsules

The microwave absorption properties of Ni/(C, silicides) nanocapsules prepared by an arc discharge method have been studied. The composition and the microstructure of the Ni/(C, silicides) nanocapsules were determined by means of X-ray diffraction, X-ray photoelectric spectroscopy, and transmission electron microscope observations. Silicides, in the forms of SiOx and SiC, mainly exist in the shells of the nanocapsules and result in a large amount of defects at the ‘core/shell’ interfaces as well as in the shells. The complex permittivity and microwave absorption properties of the Ni/(C, silicides) nanocapsules are improved by the doped silicides. Compared with those of Ni/C nanocapsules, the positions of maximum absorption peaks of the Ni/(C, silicides) nanocapsules exhibit large red shifts. An electric dipole model is proposed to explain this red shift phenomenon.