Jian-Tang Jiang

Resonance-antiresonance electromagnetic behavior in a disordered dielectric composite

In the present study, a kind of SiCw/wax composite was prepared and its electromagnetic properties were studied experimentally. It is found that Mie resonance can occur in this composite in spite of its disordered structure. The Mie resonance is believed to lead to the resonant-antiresonant electromagnetic behavior accompanied by a negative e″ in the SiCw/wax composite in the 2–18GHz band range. The resulted magnetic behavior in the current composite is believed to be weakened by the random orientation of the whiskers.

Synthesis and microwave electromagnetic properties of CoFe alloy nanoflakes prepared with hydrogen-thermal reduction method

CoFe alloy nanoflakes (NFs) with diameter and thickness on nanoscale were prepared by hydrogen-thermal reduction in CoFe2O4 flakes at 400 °C for 60 min. The effective complex permittivity and permeability of CoFe alloy NFs/paraffin composites were measured and compared with that of CoFe alloy nanoparticles (NPs)/paraffin composites. Due to the two-dimensional shape character, the real part of permittivity and permeability of CoFe alloy NFs was rather higher than that of CoFe alloy NPs. Electromagnetic wave absorbing (EMA) performance of both CoFe alloy NFs and NPs was evaluated by using transmission line theory. The effective EMA band position of the coating with CoFe alloy NFs as fillers was found to locate in the range of 2–4 GHz, while the effective EMA band position of the coating containing CoFe alloy NPs as fillers was located in the range 8–18 GHz. A maximum reflection loss (RLmax) of −57.8 dB was achieved in a coating containing CoFe alloy NFs as fillers, which is much higher than the −16.6 dB of ...

Synthesis of hexagonal Fe microflakes with excellent microwave absorption performance

Fe microflakes with uniform size and well-defined shape were successfully synthesized by hydrogen-thermal reduction of α-Fe2O3 precursor microflakes. The hexagonal Fe microflakes have edge lengths of about 5 μm and thicknesses of about 500–1000 nm. The complex permittivity and permeability of Fe microflakes–paraffin composite were measured using a vector network analyzer in the 2–18 GHz frequency range. Remarkable dielectric relaxation, natural resonance and exchange resonance were observed in the complex permittivity and permeability spectrum. Electromagnetic wave absorption (EMA) performance of samples was evaluated by using transmission line theory. An excellent microwave absorption performance was obtained for a coating containing Fe microflakes as filler, in which the maximum reflection loss is −15.3 dB and effective EMA band (RL < −10 dB) covers the whole frequency range of 12.2–16.6 GHz.

Synthesis of Fe–ferrite composite nanotubes with excellent microwave absorption performance

Fe–ferrite composite nanotubes were successfully prepared by thermal hydrogen reduction of α-FeOOH nanowires. The nanotubes have diameters of about 100 nm and lengths of tens of micrometres. The formation mechanism of Fe–ferrite composite nanotubes is discussed, and the non-equilibrium diffusion between hydrogen and oxygen was found to be responsible for the formation of the hollow interior structure. Because of the high shape anisotropy of the 1-D shape, the coercivity of composite nanotubes was higher than that of reported granular Fe–ferrite composite nanoparticles. Since the eddy current is effectively suppressed by the thin wall characteristic of nanotubes, the composite nanotubes exhibit higher permeability than that of the reported ferromagnetic metal nanowires. Due to the better impedance matching and higher dissipation efficiency, a superior microwave absorption performance was obtained in Fe–ferrite composite nanotubes, in which the maximum reflection loss is −18 dB and the effective absorption band (<−10 dB) covers the entire frequency band of 12.5–17.5 GHz.

Co7Fe3 and Co7Fe3@SiO2 Nanospheres with Tunable Diameters for High-Performance Electromagnetic Wave Absorption

Ferromagnetic metal/alloy nanoparticles have attracted extensive interest for electromagnetic wave-absorbing applications. However, ferromagnetic nanoparticles are prone to oxidization and producing eddy currents, leading to the deterioration of electromagnetic properties. In this work, a simple and scalable liquid-phase reduction method was employed to synthesize uniform Co7Fe3 nanospheres with diameters ranging from 350 to 650 nm for high-performance microwave absorption application. Co7Fe3@SiO2 core-shell nanospheres with SiO2 shell thicknesses of 30 nm were then fabricated via a modified Stöber method. When tested as microwave absorbers, bare Co7Fe3 nanospheres with a diameter of 350 nm have a maximum reflection loss (RL) of 78.4 dB and an effective absorption with RL > 10 dB from 10 to 16.7 GHz at a small thickness of 1.59 mm. Co7Fe3@SiO2 nanospheres showed a significantly enhanced microwave absorption capability for an effective absorption bandwidth and a shift toward a lower frequency, which is ascribed to the protection of the SiO2 shell from direct contact among Co7Fe3 nanospheres, as well as improved crystallinity and decreased defects upon annealing. This work illustrates a simple and effective method to fabricate Co7Fe3 and Co7Fe3@SiO2 nanospheres as promising microwave absorbers, and the design concept can also be extended to other ferromagnetic alloy particles.

Permeability calculation in composite media with low filler concentration: A new method of effective media theory application

A new method was established to calculate the intrinsic or effective permeability in composite media with low filler concentrations. The calculation instability of Bruggeman effective media theory was avoided through a media reconstruction, and thus the prediction accuracy and calculation consistence were greatly improved. The established method has been tested in Fe/SiO2 based composite media, and its validity been preliminarily proved. This study then proposed a new way to extend the applicability scope of the Bruggeman effective media theory.

Thickness-controllable coating of SiO2 on Co microspheres with tunable electromagnetic properties and enhanced oxidation resistance

A modified Stober method was utilized for coating SiO2 on Co microspheres with tunable thickness as a filler for electromagnetic absorbing coatings with enhanced oxidation resistance. Co microspheres with diameters of 1.5–3.5 μm were prepared using an aqueous-reduction process, and Co@SiO2 core–shell microspheres with different shell thicknesses were subsequently fabricated by a modified Stober method using tetraethyl orthosilicate (TEOS) as a Si source. The phase, morphology, and structure of composite microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TGA). Both e and μ of Co@SiO2 microspheres increase with the increasing filling ratio. No strong eddy current effect induced by local agglomeration was observed as the presence of a SiO2 shell protects the Co particles from agglomeration and the filling ratio is up to 45 vol%. Due to the presence of the SiO2 shell, the core–shell Co@SiO2 composite microspheres exhibit better antioxidation capability than that of pure Co microspheres. The oxidation temperature of Co@SiO2 is up to 720 °C, much higher than that of Co microspheres (380 °C). The effects of SiO2 shell thicknesses and annealing treatment on microstructure evolution and on EM parameters of Co@SiO2 composites were also investigated.

Rational Construction of Uniform CoNi-Based Core-Shell Microspheres with Tunable Electromagnetic Wave Absorption Properties

Core-shell particles with integration of ferromagnetic core and dielectric shell are attracting extensive attention for promising microwave absorption applications. In this work, CoNi microspheres with conical bulges were synthesized by a simple and scalable liquid-phase reduction method. Subsequent coating of dielectric materials was conducted to acquire core-shell structured CoNi@TiO2 composite particles, in which the thickness of TiO2 is about 40 nm. The coating of TiO2 enables the absorption band of CoNi to effectively shift from Ku to S band, and endows CoNi@TiO2 microspheres with outstanding electromagnetic wave absorption performance along with a maximum reflection loss of 76.6 dB at 3.3 GHz, much better than that of bare CoNi microspheres (54.4 dB at 17.8 GHz). The enhanced EMA performance is attributed to the unique core-shell structures, which can induce dipole polarization and interfacial polarization, and tune the dielectric properties to achieve good impedance matching. Impressively, TiO2 coating endows the composites with better microwave absorption capability than CoNi@SiO2 microspheres. Compared with SiO2, TiO2 dielectric shells could protect CoNi microspheres from merger and agglomeration during annealed. These results indicate that CoNi@TiO2 core-shell microspheres can serve as high-performance absorbers for electromagnetic wave absorbing application.

Synthesis of Zn(II)-Doped Magnetite Leaf-Like Nanorings for Efficient Electromagnetic Wave Absorption

We report the thermal annealing-induced formation of ring-like structure of Zn(II)-doped magnetite from iron alkoxide leaf-like nanoplate precusor. The phase, structure and morphology of magnetite nanorings were comprehensively characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscope, scanning electron microscope, and transmission electron microscope. The obtained Zn(II)-doped magnetite nanorings are of 13–20 nm in edge width, 70–110 nm in short axis length and 100–150 nm in long axis length. The growth mechanism was possibly due to a combined effect of decomposition of the organic component and diffusion growth. Zn(II)-doped magnetite nanorings delivered saturation magnetization of 66.4 emu/g and coercivity of 33 Oe at room temperature. In addition, the coatings containing Zn(II)-doped magnetite nanorings as fillers exhibit excellent microwave absorption properties with a maximum reflection loss of −40.4 dB and wide effective absorbing band obtained in coating with thin thickness of 1.50 mm.