Faxiang Qin

A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles

Carbon (C) is a crucial material for many branches of modern technology. A growing number of demanding applications in electronics and telecommunications rely on the unique properties of C allotropes. The need for microwave absorbers and radar-absorbing materials is ever growing in military applications (reduction of radar signature of aircraft, ships, tanks, and targets) as well as in civilian applications (reduction of electromagnetic interference among components and circuits, reduction of the back-radiation of microstrip radiators). Whatever the application for which the absorber is intended, weight reduction and optimization of the operating bandwidth are two important issues. A composite absorber that uses carbonaceous particles in combination with a polymer matrix offers a large flexibility for design and properties control, as the composite can be tuned and optimized via changes in both the carbonaceous inclusions (C black, C nanotube, C fiber, graphene) and the embedding matrix (rubber, thermoplastic). This paper offers a perspective on the experimental efforts toward the development of microwave absorbers composed of carbonaceous inclusions in a polymer matrix. The absorption properties of such composites can be tailored through changes in geometry, composition, morphology, and volume fraction of the filler particles. Polymercomposites filled with carbonaceous particles provide a versatile system to probe physical properties at the nanoscale of fundamental interest and of relevance to a wide range of potential applications that span radar absorption, electromagnetic protection from natural phenomena (lightning), shielding for particle accelerators in nuclear physics, nuclear electromagnetic pulse protection, electromagnetic compatibility for electronic devices, high-intensity radiated field protection, anechoic chambers, and human exposure mitigation. Carbonaceous particles are also relevant to future applications that require environmentally benign and mechanically flexible materials.

Stress tunable properties of ferromagnetic microwires and their multifunctional composites

We report the results of a systematic study on stress tunable absorption of glass-coated amorphous Co68.7Fe4Ni1B13Si11Mo2.3 microwires and their composites. The magnetic microwires possess good stress-impedance properties and yield a stress dependence of absorption at gigahertz frequencies. The stress compensates the reverse effect of magnetic field on absorption. There exist strong stress dependences of the effective permittivity and transmission parameters. Composite failure due to the wire damage results in a dramatic change of the sign and magnitude of effective permittivity. The double peak is identified in the stress dependence of field tunability, in contrast to the single peak for the magnetic field tunability. All these results indicate that the present composites are very promising for detecting the ambient stress levels and interrogating the structural integrity.

Excellent magnetocaloric properties of melt-extracted Gd-based amorphous microwires

We report upon the excellent magnetocaloric properties of Gd53Al24Co20Zr3 amorphous microwires. In addition to obtaining the large magnetic entropy change (−ΔSM ∼ 10.3 J/kg K at TC ∼ 95 K), an extremely large value of refrigerant capacity (RC ∼ 733.4 J/kg) has been achieved for a field change of 5 T in an array of forty microwires arranged in parallel. This value of RC is about 79% and 103% larger than those of Gd (∼410 J/kg) and Gd5Si2Ge1.9Fe0.1 (∼360 J/kg) regardless of their magnetic ordering temperatures. The design and fabrication of a magnetic bed made of these parallel-arranged microwires would thus be a very promising approach for active magnetic refrigeration for nitrogen liquefaction. Since these microwires can easily be assembled as laminate structures, they have potential applications as a cooling device for micro electro mechanical systems and nano electro mechanical systems.

Exceptional electromagnetic interference shielding properties of ferromagnetic microwires enabled polymer composites

We present systematic studies of the electromagnetic interference (EMI) shielding and microwave properties of a new class of shielding material, i.e., the ferromagnetic microwires-embedded polymer composites. We show that at 1–2 GHz the shielding effectiveness (SE) of the continuous-wire composite reaches a high value of 18 dB (98.4% attenuation) for a very low filler loading of 0.024% and a thickness of 0.64 mm. The normalized SE of this new composite is about 70 times higher than that of the bucky paper-based composite and is two to four orders of magnitude higher than those of other shielding candidate materials. Complex permeability, permittivity, and impedance experiments reveal that the absorption of electromagnetic radiation is a dominant mechanism for EMI shielding of the studied composites. The advantages of high shielding efficiency, good physical integrity, low fabrication costs, and multifunctionalities make them an attractive candidate material for a variety of technological applications.

Tuneable Metacomposites Based on Functional Fillers

Metamaterials, traditionally in the form of artificial structures with surprising electromagnetic properties, have triggered unprecedented opportunities to achieve those fascinating applications that previously only exist in science-fiction works, for example, Harry Potter’s cloak. Nevertheless, their massive manufacturing costs incurred by their complicated structures restrict the scale-up and mass production. The ultimate properties are primarily (if not solely) determined by the intrinsic structures of metamaterials that make them merely ‘meta-structures’. In response to these issues, it is desirable to have a genuine engineering composite yet with metamaterial characteristics. Thus, ‘metacomposite’ has been proposed to account for a real piece of composite material. This has subsequently become a nascent area where metamaterial properties are attained under wider operating frequencies with certain tunability towards external magnetic fields or mechanical stresses. In this chapter, we start with an overview of metacomposites containing various dielectric and/or magnetic fillers following the fillers’ dimensions from 0D, 1D to 2D. We then critically discussed some progresses in metacomposites containing ferromagnetic microwires together with unparalleled advantages in microwave sensing and cloaking areas. Finally, the chapter is closed with an outlook of strategies for improving existing metacomposites and some future perspectives.

Combined current-modulation annealing induced enhancement of giant magnetoimpedance effect of Co-rich amorphous microwires

We report on a combined current-modulation annealing (CCMA) method, which integrates the optimized pulsed current (PC) and DC annealing techniques, for improving the giant magnetoimpedance (GMI) effect and its field sensitivity of Co-rich amorphous microwires. Relative to an as-prepared Co68.2Fe4.3B15Si12.5 wire, CCMA is shown to remarkably improve the GMI response of the wire. At 10 MHz, the maximum GMI ratio and its field sensitivity of the as-prepared wire were, respectively, increased by 3.5 and 2.28 times when subjected to CCMA. CCMA increased atomic order orientation and circumferential permeability of the wire by the co-action of high-density pulsed magnetic field energy and thermal activation energy at a PC annealing stage, as well as the formation of uniform circular magnetic domains by a stable DC magnetic field at a DC annealing stage. The magnetic moment can overcome eddy-current damping or nail-sticked action in rotational magnetization, giving rise to a double-peak feature and wider working field range (up to ±2 Oe) at relatively higher frequency (f ≥ 1 MHz).

Novel magnetic microwires-embedded composites for structural health monitoring applications

We report the results of a systematic study of the magnetic, mechanical, magnetoimpedance and field tunable properties of glass-coated amorphous Co68.7Fe4Ni1B13Si11Mo2.3 microwires and composites containing these microwires. The magnetic microwires possess good magnetic and mechanical properties. The magnetoimpedance ratio in the gigahertz range varies sensitively with applied fields below the anisotropy field but becomes unchanged for higher applied fields. The good mechanical properties are retained in the magnetic microwires-embedded composites. The strong field dependences of the effective permittivity and transmission parameters in the gigahertz range indicate that the present composites are very promising candidate materials for structural health monitoring and self-sensing applications.

Fe-based ferromagnetic microwires enabled meta-composites

The microwave properties of polymer-based glass fiber reinforced composites containing amorphous Fe77Si10B10C3 microwires in parallel and orthogonal arrays and their dependencies on external magnetic field have been investigated. Double-negative-index characteristics are confirmed for both wire arrays through the observed transmission window in the 1–7 GHz frequency band. The microwave interaction within inter-wire range is responsible for a multi-peak feature observed in the absorption spectra of the parallel wire array composite when the wire spacing is below 7 mm. We introduce the term of “effective diameter” associated with the microwire domain structure to remedy the discrepancy between the computed and experimentally observed plasma frequency.

In situ microwave characterization of microwire composites under mechanical stress

We present results of an experimental characterization of the dielectric properties and microwave absorption of rubber composite samples containing Fe4Co68.7Ni1B13Si11Mo2.3 amorphous microwires which are submitted to a low uniaxial tension. Measurements of the dielectric loss and microwave absorption as a function of strain over the frequency range of 300 MHz-6 GHz reveal that the uniaxial elongation randomly breaks wires at about 2.8% strain and this has for effect to decrease the loss factor for larger strain. Two possible mechanisms are identified to account for our observations, namely, the stress and shape effects. The ability to control this stretch breaking phenomenon will be instrumental to developing stress tunable microwire composites for sensing applications.

Mechanical–electromagnetic coupling of microwire polymer composites at microwave frequencies

We have fabricated a set of microwire composites with varying wire concentrations and studied their effective complex permittivity under the tensile stress at a frequency range of 1–6 GHz. It has been found that with increasing wire concentration the composite presents increasing effective permittivity and strain sensitivity. The Gaussian molecular network model shows a complex strain dependence of sensitivity due to the composite architecture influenced by the wire concentrations. All these results indicate the proposed composite are excellent candidate materials for the microwave sensing and structural interrogation applications.