P.H. Zhou

Characterization and microwave resonance in nanocrystalline FeCoNi flake composite

To explore the resonance mechanism of nanocrystalline FeCoNi flake composites, characterization of the flakes was investigated. The structural and magnetic properties of flakes manifestly tailored the features of microwave permeability spectrum of flake composites, relating to the physical effects of nanograins. The exchange mode with a few modifications was used to explain the origin of composite microwave performance, and the computed results were close to the experiment. It is believed that the exchange energy, dominating in the microwave resonance of spherical nanoparticles, plays an equally important role in nanocrystalline flakes.To explore the resonance mechanism of nanocrystalline FeCoNi flake composites, characterization of the flakes was investigated. The structural and magnetic properties of flakes manifestly tailored the features of microwave permeability spectrum of flake composites, relating to the physical effects of nanograins. The exchange mode with a few modifications was used to explain the origin of composite microwave performance, and the computed results were close to the experiment. It is believed that the exchange energy, dominating in the microwave resonance of spherical nanoparticles, plays an equally important role in nanocrystalline flakes.

Electromagnetic and absorption properties of urchinlike Ni composites at microwave frequencies

In this paper, nearly monodispersed urchinlike Ni powders were synthesized by a simple hydrogen-thermal reduction method. Electromagnetic and absorption characteristics were then investigated at 0.5–18 GHz. The permeability spectra present four resonance peaks over the whole frequency range. The resonance absorption property was discussed by fitting the permeability spectrum using the well-known Landau-Lifshitz-Gilbert equation and Maxwell-Garnett mixing rule. Correspondingly, the magnetic loss of the first band observed is attributed to the natural resonance, while the other three bands are considered to originate from non-uniform exchange resonance in the permeability spectra. The maximum reflection loss can reach −43 dB at about 10 GHz with 2 mm in absorber thickness.

The hierarchical architecture effect on the microwave absorption properties of cobalt composites

The effect of hierarchical architecture on the electromagnetic properties of microwave absorber candidates is important for absorption design. This work prepares dendritic cobalt powders by a simple hydrothermal reduction method. The study of microwave electromagnetic properties reveals that dielectric polarization relaxation dominates over the power loss owing to the superstructure with a large volume of branches. Compared with the conventional spherical filler, a composite consisting of dendritic filler shows an improved electromagnetic absorption performance in low frequencies due to the enhancement of both the impedance match to free air and the loss induced by interfacial polarization and Ohmic resistance.

Optimization of the Carbon Coating of Honeycomb Cores for Broadband Microwave Absorption

A simple and fast coating method of honeycomb cores for microwave absorption has been described. The honeycomb cores with two different thicknesses (5 and 20 mm) coated with thermoplastic resin filled with carbon powder as lossy filler in 5, 10, 15, and 20 wt% have been tested for microwave absorption in 2 - 18-GHz frequency range. The 5-mm-thick honeycomb has shown absorption bandwidth of 14 GHz for maximum absorption of -6 dB (75%) with 15 wt%. filler content. However, the percentage of the filler was decreased to 10 wt% in 20-mm-thick honeycomb absorber for maximum absorption over a wide frequency range. The honeycomb sample with 10 wt% filler has bandwidth of 18 GHz for -7 dB (80%) reflection loss. The reflection loss measurements of coated honeycomb cores have also shown that use of E-glass fiber/epoxy composite can enhance the performance of the honeycomb absorber. The combination of a microwave absorbing nanocomposite and the coated honeycombs has been resulted in inferior absorption properties in 2 - 18-GHz frequency range.

Correlation of electrical conductivity, dielectric properties, microwave absorption, and matrix properties of composites filled with graphene nanoplatelets and carbon nanotubes

The DC electrical conductivity, percolation threshold, and dielectric properties of Graphene Nanoplatelets (GNPs) filled epoxy composites are studied and correlated with microwave absorption. The properties of GNPs filled composites are also compared with multiwalled carbon nanotubes (MWCNTs) composites, and GNPs are observed to have superior conductivity than MWCNTs. In all batches, the nanofillers have 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 wt. %. All composites irrespective of the type of nanofiller and viscosity of the matrix have shown electrical percolation threshold at 3.0 wt. %. The dielectric properties, i.e., complex permittivity, tan loss, and AC conductivity, are studied in 100 Hz–5.5 MHz. The DC and AC electrical conductivities (at and below the percolation) measured in 100 Hz–5.5 MHz are correlated to the GNPs and MWCNTs epoxy composites in the microwave frequency range (11–17 GHz). The maximum return loss of −12 dB and −6 dB was determined for MWCNTs and GNPs, respectively. The effects of na...

Spatial orientation and position distribution effect on the effective permeability of composites consisting of aligned flakes

In this work, the effective permeability of two-phase composite materials is studied based on a three-dimensional model. In this model, the magnetic inclusions embedded in a dielectric matrix are set with aligned orientation and calculated by finite-difference time-domain method in both lossless and lossy cases. First, the sensitivity of the effective permeability of composites to the spatial orientation and position distribution of inclusions is analyzed. Second, since it is difficult to give an accurate effective permeability with classic effective medium theories, we have modified the well-known Maxwell–Garnett mixing rule by introducing the equivalent demagnetization factors and two fitting parameters α and β, indicating the extent of position distribution chaos. An excellent agreement is obtained between the theoretical and simulation results in the quasistatic limit. Finally, in the case of inclusions oriented at several directions in the matrix, an “averaging method” is proposed to calculate the ef...

Bounds on the dynamic magnetic properties of multiresonant nanostructure composites in exchange resonance model

Existing bounds expression established on the sum of the gyromagnetic losses is limited by static magnetic interactions. In this paper, we deduce a bounds relation from the Bloch–Bloembergen theorem to depict both the uniform resonance and exchange resonance. It is shown to agree with experiments, of particular relevance to the competition between static magnetic force and exchange interaction. An analytic explanation on favorable multiresonance phenomenon in nanocrystalline iron flakes is achieved.

Thickness-Dependent Magnetic Properties of Patterned FeCoBSi Amorphous Thin Films on Silicon Substrate

With the rapid development of high frequency and microwave technology, soft magnetic thin film with high saturation magnetization and high permeability, has been intensively studied in the several decades. In recent years, many research effort have been placed on the controlling of resonance frequency (fr)[1-3]. One of the useful ways to tuning the fr is by patterning the magnetic thin film into periodic strip structure[4-6]. But in previous studies, there have paid no attention to how the thickness influences the magnetic properties of strip patterned film in details. In addition, we believed that the fringe morphology of strips, usually be ignored, should be careful analysis.

Microwave Properties of Fe-based Nanocrystalline Alloy Particles

Microwave properties of Fe-based nanocrystalline alloy particles mainly depend on the crystal structure and particle morphology. It’s difficult to build up their relationship directly, so magnetic properties are introduced as intermediaries. In this paper, nanocrystalline Fe73.5Cu1Nb3Si13.5B7alloy powders were prepared and tested by the structural, magnetic and microwave measurement. It’s found that besides particle size, the structural properties such as grain size or lattice constant have important influence on the sample’s static magnetic properties, and such influence effectively extends to its complex permeability. Moreover, the exchange coupling and surface-effect were discussed in this issue.

Resonance Frequency of Ferromagnetic Thin Film Controlled by Rectangle Antidot Arrays

We have investigated both the static and dynamic magnetic properties of rectangle antidot arrays in a ferromagnetic thin film. The thin films are Fe66Co17B16Si1 amorphous alloy with different thicknesses (40, 80, 160, and 320 nm) on a Si substrate. Vibrating sample magnetometry and microstrip transmission line measurements are conducted to associate the microwave magnetic analysis of the antidot arrays with hysteresis studies. The patterned films result in the change of ferromagnetic resonance from single-band to multiband, and the destruction of in-plane uniaxial anisotropy. Hence, the mechanisms of multiresonance are proposed to be related to the special domain-wall motion and spin-wave modes what is caused by the demagnetizing field in per-unit area. To study the magnetic torques in per-unit area, The Object Oriented MicroMagnetic Framework (OOMMF) is used to simulate the dynamic magnetization process.