Ferromagnetic microwire metacomposites provide multifunctionality

Composite materials are attractive because they can combine the superior properties and functionalities of each of their constituents. Much research currently focuses on developing multifunctional composites, which unite good mechanical (structural) attributes along with functional properties such as thermal, electrical, or magnetic characteristics. A particularly interesting example of such a composite consists of ferromagnetic microwires embedded in a polymer matrix. These composites can exploit phenomena occurring in the wires (such as the giant magnetoimpedance effect) and can also offer emergent properties that arise from the mesoscale arrangement of the microwires (so-called metacomposites). Possible applications include remote monitoring of structural integrity, electromagnetic shielding, and cloaking at microwave wavelengths.

Stress monitoring in a ferromagnetic composite
Figure 1. Stress monitoring in a composite material containing ferromagnetic microwires by means of a microwave contrast imaging technique. The microwire composite’s electromagnetic response (microwave absorption in this case) changes with the local stress state. (Courtesy of Dmitriy Makhnoskiy, University of Plymouth, UK.[6])

Materials today can be conveniently divided into two categories: structural  and functional. Structural materials, for which the most important qualities are their mechanical properties, have undergone development for centuries, due to the lasting need for all kinds of structures, such as buildings, bridges, and vehicles. The pursuit of new materials or new techniques for tailoring materials to obtain better mechanical properties remains one of the major goals of materials research. More recently, the unprecedented development of functional materials (e.g., semiconductors), as well as the devices exploiting them, has to some extent ‘stolen the thunder’ of structural materials. Inert stone could meet the basic needs of ancient humans as it could be built into a robust shelter, but it cannot remotely meet the needs of modern mankind because it cannot take the place of transistors to build a mobile phone or a computer.

It might seem we have nothing to worry about since we have both stone and transistors. But very often, we need both good mechanical properties and functional properties. Composite materials can answer this demand for materials that combine multiple properties. A composite, simply put, is composed of two or more kinds of materials or phases, which are referred to as constituents. A composite is attractive in that it can exhibit properties (structural and functional) of each of its constituents. The development of composites is thus driven by the desire for combinations of superior properties and functionalities that a monolithic material cannot offer.

The aeronautical industry, for example, is making greater use of lightweight carbon-fiber composites to replace metals, but it must also address the issue of invisible damage occurring inside the composite, for which only viable structural health monitoring can ensure the safety of passengers. Other issues include lightning strike protection, de-icing, and electromagnetic shielding, for which the composite parts must have good electrical or thermal conductivity.

Therefore, the current trend in research and development is to bring in additional functionalities (e.g., thermal, electrical, or magnetic functions) to a structural composite, giving rise to what are known as multifunctional composites. The ideal approach is to develop multifunctional composites with indiscriminate properties (i.e., with a good all-round combination of properties rather than a single exceptional quality) that can meet all these structural and functional demands in a most economical way. Of particular interest are multifunctional metacomposites, which can offer ‘new emerging’ properties that may not exist in a single constituent. Metacomposites are similar to metamaterials. A traditional metamaterial is constructed with a precise regular arrangement of mesoscale structures, such as conducting rods and rings, which can result in extraordinary electromagnetic properties at wavelengths longer than this scale. Metacomposites, by contrast, can have a somewhat disordered structure, so they are more amenable to industrial-scale fabrication techniques instead of necessarily requiring delicate nano- or microfabrication.

A particularly interesting approach to realizing multifunctional metacomposites is to incorporate ferromagnetic microwires into a polymer matrix. The polymer matrix largely supplies the structural properties we seek, and the microwires provide the functional (electromagnetic) properties. The science and philosophy behind this specific line of work applies more generally to the development of functional composites using a single kind of fine-sized functional filler, as well as the broader application-driven efforts to develop advanced multifunctional metacomposites, which is exciting and inspiring for the future.

The most prominent work on ferromagnetic microwire composites was contributed by Larissa Panina of the University of Plymouth, UK. As the co-discoverer of the giant magnetoimpedance (GMI) effect in microwires,[1] she was the first to propose developing GMI microwire-based tunable composite media.[2] The GMI effect refers to the change of a material’s electrical impedance when it is submitted to an external magnetic field. The basic idea for exploiting this effect in a composite is to arrange cobalt iron (CoFe)-based microwires in a certain geometry in order to tune the wire media’s response to an incident electromagnetic wave. Depending on the setup, this response can be modulated by a stimulus such as a magnetic field, mechanical stress, or temperature. The dependence of the GMI effect on the external magnetic field helps define the material’s electromagnetic response and thus makes the composite potentially useful for reconfigurable microwave devices. After establishing the theoretical framework, Panina’s group later proposed a metamaterial based on microwires that could display negative permittivity.

From Panina’s work, we should understand that the microwires are the magic key to some specific electromagnetic functionalities. Regarding the GMI microwires themselves, the leading experts are Arkady Zhukov at the University of the Basque Country (UPV/EHU) and Manuel Vázquez at the Institute of Materials Science of Madrid (ICMM), both in Spain. These metallic microwires are often coated in glass, and the GMI effect in them is dependent on a number of factors such as geometry, the metal-to-total-diameter ratio, measuring parameters (e.g., temperature and frequency), and tailoring techniques such as field annealing or stress annealing (i.e., annealing of the wires in the presence of an external magnetic field or under mechanical stress).[3][4] The knowledge gained in Zhukov and Vázquez’s work on tailoring the magnetic structure of microwires and optimizing their GMI performance is essential for understanding how to design and fabricate a microwire composite and how their intrinsic properties relate to the composite performance.

Our group in the Advanced Composites Centre for Innovation and Science (ACCIS) at the University of Bristol, UK, is performing exploratory work on microwire composite materials. We have shown that by embedding a number of microwires in a polymer matrix, the soft magnetic properties and GMI effect can be significantly enhanced as compared to a single embedded wire.[5] The work also provides a basic route to making structural microwire composites that are of engineering interest. In the past few years, our group has started working on the multiple functionalities of microwire composites.[6]

One significant aspect of multifunction is to capitalize on the magnetic field dependency of tunable microwave properties for structural health monitoring applications: see Figure 1.[7] We need fine elements in the composite to serve as sensor arrays that interrogate the material’s structural integrity. This sensor-embedding technique is not new. Researchers have been trying to embed all kinds of sensors (e.g., fiber optics) into composite structures, but this approach typically degrades the composite’s mechanical performance a great deal because of the diameter mismatch between structural reinforcing fibers and the embedded sensors. In the case of microwire composites, the microwires’ fine diameter (e.g., 20μm) proves to have no detrimental effect, and the microwires can potentially fulfill the task of in-flight monitoring.[8]

Microwire composites can also provide electromagnetic shielding. A number of research groups (e.g., Pilar Marín of Complutense University of Madrid, and coworkers[9]) have reported that the wires, being both electrically and magnetically conductive, are excellent candidate materials for shielding or absorption. In either low or high concentrations, the microwire composites can show useful levels (more than 10 dB) of absorption or shielding effectiveness (SE). Notably, as compared to other shielding fillers, the microwires enable a relatively thin composite with low filler concentration to have a very large SE.[10] Furthermore, absorption characteristics (such as the absorption frequency) which we need to adjust for different applications can be easily formulated by changing the wire geometry and patterns. Such microwire composites could be used for wind turbines, which need simultaneously good impact resistance and electromagnetic shielding to ensure that the rotating turbines do not create interference with Doppler radar systems.

The most exciting property of ferromagnetic microwire composites, which we have only recently tapped into, is that of metacomposite behavior. Some initial contributions in this field have been made by Lie Liu of the National University of Singapore,[11] Serghei Baranov of the Institute of Applied Physics in Chisinau, Moldova,[12] Konstantin Rozanov of the Institute for Theoretical and Applied Electromagnetics in Moscow, Russia,[13] and Jorge Carbonell of the Polytechnic University of Valencia in Spain,[14] and their coworkers. As mentioned earlier, metamaterials’ unique properties originate from the precise mesostructure of their constituents. The complicated structure of a metamaterial can manipulate electromagnetic waves to achieve feats such as Harry Potter-like invisible cloaking, but metamaterials are generally not suitable for mass production. Wire metacomposites, with a simpler structure that is susceptible to control and production at engineering scale, can realize a transmission window (i.e., a range of wavelengths) in which the material’s magnetic and dielectric responses are both negative. Such a metacomposite might be used to make a cloaked craft that would be invisible in the microwave range.

It is worth mentioning that another promising direction is the use of nanofillers such as carbon nanotubes to realize multiscale multifunctional composites. The nanotubes can enhance the material’s mechanical and conductive properties, can function as piezoelectric sensors to detect damage, and they could also show metamaterial characteristics.[15][16] However, these nanofillers are hard to manipulate, they are not magnetic, and they are expensive.

Admittedly, multifunctional microwire composites are still in their infancy and are not without limitations. For instance, the orientation of the wires has to be restricted to obtain the best response to an electromagnetic wave. Making a quasi-isotropic composite is no easy task because the wires are not supposed to become a burden to the lightweight structure, and handling large quantities of such fine wires with robust control remains a technical challenge. The next step in this research will be more detailed and systematic work toward the ultimate goal of realizing mass production of microwire prepregs (i.e., microwires pre-impregnated with polymer resin, analogous to commercialized carbon fiber or glass fiber prepregs). Backed by the physics and fundamental understanding exhibited in our recent work, we are convinced that multifunctional microwire composites can have a significant impact, both scientifically and technically, and will find a wide range of engineering applications in the near future.[6]


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