Pingan Yang

Flower-like carbonyl iron powder modified by nanoflakes: Preparation and microwave absorption properties

In this paper, flower-like carbonyl iron powder (CIP) is prepared under normal temperature and pressure by a simple method of chemical reduction. This flower-like morphology is conducive to forming discontinuous network, enhancing diffuse scattering of the incident microwave and polarizing more interface charges. Those are all in favor of electromagnetic wave penetration and absorption. The test results show that compared with the unmodified CIP, the electromagnetic wave absorbing property of flower-like CIP is significantly improved in X-band. It is concluded that this research paves a way to enhance the microwave absorption properties of spherical metal particles.

Preparation and characterization of a novel magnetorheological elastomer based on polyurethane/epoxy resin IPNs matrix

This paper proposes the preparation of a novel magnetorheological elastomer (MRE) with improved damping and mechanical properties. This MRE is based on polyurethane (PU)/epoxy resin (EP) graft interpenetrating polymer networks (IPNs). The tensile strengths, thermal stability, magnetorhelogical behavior, and damping properties of the MRE are studied systematically in terms of composition. The Fourier transform infrared (FTIR) spectra verifies the formation of IPN structures, and thermogravimetric analysis (TGA) revealed that the thermal decomposition temperature was raised by the addition of IPN structures. The test results from the materials test machine and the rheometer show that the presence of IPN can significantly improve the tensile strength and damping properties of the MRE. In addition, the mechanism for enhancing tensile strength and damping properties is proposed. The experiment results suggest that the damping performance of the MRE has a significant correlation with the magnetic strength, content of EP, and temperature. As the thermal endurance properties, tensile strength, and loss factor are improved by incorporating EP/PU IPN structure, it is expected that the PU/EP IPN MRE can be used as an intelligent structural damping material.

Study on the characteristics of magneto-sensitive electromagnetic wave-absorbing properties of magnetorheological elastomers

Magnetorheological (MR) materials are a class of materials whose mechanical and electrical properties can be reversible controlled by the magnetic field. In this study, we pioneered research on the effect of a uniform magnetic field with different strengths and directions on the microwave-absorbing properties of magnetorheological elastomers (MREs), in which the ferromagnetic particles are flower-like carbonyl iron powders (CIPs) prepared by an in situ reduction method. The electromagnetic (EM) absorbing properties of the composites have been analyzed by vector network analysis with the coaxial reflection/transmission technique. Under the magnetic field, the columnar or chainlike structures were formed, which allows EM waves to penetrate. Meanwhile, stronger Debye dipolar relaxation and attenuation constant have been obtained when changing the direction of the applied magnetic field. Compared with untreated MREs, not only have the minimum reflection loss (RL) and the effective absorption bandwidth (below −20 dB) greatly increased, the frequencies of the absorbing peaks shift about 15%. This suggests that MREs are a magnetic-field-sensitive electromagnetic wave-absorbing material and have great potential in applications such as in anti-radar camouflage, due to the fact that radar can continuously conduct detection at many electromagnetic frequencies, while the MR materials can adjust the microwave-absorption peak according to the radar frequency.

A dimorphic magnetorheological elastomer incorporated with Fe nano-flakes modified carbonyl iron particles: preparation and characterization

This paper describes a simple and convenient approach for the synthesis of Fe nano-flakes coated spherical carbonyl iron particles (CIP-Nano-Fe). The morphology and composition of CIP-Nano-Fe were characterized using electron scanning microscope and x-ray diffraction analysis. The results indicated that the CI particles were coated with uniform and continuous Fe nanostructures. Partial substitution of CI particles with CIP-Nano-Fe constituted a novel dimorphic magnetorheological elastomer (D-MRE), and the influence of the content of CIP-Nano-Fe on the viscoelastic performance of the magnetorheological elastomers (MREs) were systematically studied. The magnetorheological properties and the damping properties of the D-MRE samples were analyzed to evaluate their dynamic properties. The experimental results indicated that the MR effect, the max loss factor and the magneto-induced loss factor in the sample 3 (CIP-Nano-Fe weight content 6 wt%) were approximately 1.32, 1.45 and 1.56 times that in the sample 1 (non-doped MRE). The approach to synthesize CIP-Nano-Fe reported here can be readily explored for fabricating particles modified by other metal nanostructures, and the resulting D-MREs are expected to be applied in various applications, especially in the field of vibration and noise control, involving vibration isolators, tunable engine mounts, noise insulation devices, and so forth.

Rheological properties of dimorphic magnetorheological gels mixed dendritic carbonyl iron powder

Magnetorheological gels typically are colloids of spherical micrometer magnetic particles dispersed in a high-viscosity polymer matrix. However, some major parameters of this kind of mono-disperse magnetorheological gel, such as the shear storage modulus and yield stress, cannot meet the needs of practical engineering application. In this study, a systematic experiment was investigated on the use of dendritic carbonyl iron particles to enhance the comprehensive performance in conventional (microsphere-based) magnetorheological gels formulated in polyurethane matrix. Two kinds of carbonyl iron particles that have very similar magnetic properties but very different morphologies (one is spherical and the other is dendritic) are employed. The dimorphic magnetorheological gels were prepared by adding dendritic carbonyl iron particles into the conventional spherical carbonyl iron particle–based magnetorheological gel and a series of dimorphic magnetorheological gel samples were prepared. The magnetorheological properties of those samples, both under oscillatory and rotational shear rheometry, were systematically tested. It was found that the dendritic carbonyl iron particle additives can significantly improve the shear storage modulus and response time of the dimorphic magnetorheological gels compared with conventional magnetorheological gels. Meanwhile, when the mass ratio of dendritic particles to carbonyl iron particles is 25% and 50%, the shear stress, static shear yield stress, and dynamic shear yield stress of dimorphic magnetorheological gels can also be greatly enhanced.

The field-dependent conductivity of dimorphic magnetorheological gel incorporated with iron nanowire:

In this article, iron nanowire is synthesized by reducing Fe2+ ion with excessive sodium borohydride in deionized aqueous solution. A kind of dimorphic magnetorheological gel is prepared by partial substitution of carbonyl iron particles with Fe nanowires. Several experimental devices based on the dimorphic magnetorheological gel were fabricated, and the magneto-resistance characteristics under a magnetic field of those devices are systematically tested to research the influence of the Fe nanowire on the conductivity of magnetorheological gel. The experimental results indicated that by adding a certain amount of Fe nanowire, the conductivity of the dimorphic magnetorheological gel can be greatly improved. Moreover, it can be seen that the conductivity of sample 4 (with 6 wt.% Fe nanowire) is increased by about 100 times than sample 1 (without Fe nanowire). The mechanism of Fe nanowire enhances the conductivity of the dimorphic magnetorheological gel, which is investigated by microstructure analysis.

Preparation and high-performance microwave absorption of hierarchical dendrite-like Co superstructures self-assembly of nanoflakes

Dendritic-like Co superstructures based on the self-assembly of nanoflakes that could efficiently suppress the eddy current were successfully synthesized via a facile, rapid, and energy-saving chemical reduction method. Since crystal structure, size, and special geometrical morphology, magnetism have a vital influence on microwave absorption properties, the as-obtained products were characterized by x-ray diffraction, scanning electron microscopy, vibrating sample magnetometry, and vector network analysis. The prepared dendritic Co possesses abundant secondary branches that extend to the 3D space. Their dimensions, spacing, sheet-like blocks, and high-ordering microstructures all contribute to the penetration, scattering, and attenuation of EM waves. The composites present attractive microwave absorption performances in the X band, as well as in the whole S band (2-4 GHz). This work investigates the mechanism of absorption for the as-obtained Co, offers a promising strategy for the fabrication of hierarchical Co microstructure assemblies by multi-leaf flakes and introduces the application of dendritic-like Co as a highly efficient absorber in the S band and X band.