Microwave Tunable Composites with Melt- Extracted Magnetic Microwires

1 General Introduction A key to the realization of effective microwave tunable devices, among others, is to develop adaptive materials with superior adjustability of electromagnetic parameters. Since there are two fundamental electromagnetic parameters, namely, permittivity and permeability, the tunable property can then be readily approached by manipulating (i) permittivity directly via the electric field, such as in ferroelectrics and conducting polymer [1, 2], or (ii) permeability via magnetic field, such as in ferromagnetic materials [3], or (iii) permittivity (permeability) indirectly by magnetic field (electric field) such as in multiferroics [4]. A unique class of composite materials is considered here to realize such tunable properties: ferromagnetic microwires with excellent soft magnetic properties and giant magnetoimpedance effect [5,6] were introduced into a polymer matrix material so that an indirect control of permittivity can be tuned by the magnetic field. The principle is that the magnetic field can induce significant variation of the impedance via the skin effect at high frequency. The current distribution along the wire then changes accordingly and induces the dipolar polarization [7]. There have been a few relevant studies on this subject in the open literature. Panina et al. [8] and Acher et al. [9] used the microwire arrays to realize the left-hand materials and tunable properties in the presence of magnetic field. Starostenko et al. [10] gave an in-depth discussion of the magnetic bias effect on the permittivity for random wire composites and proved the feasibility of using the wire composites to control attenuation. Di et al. [11], Marin et al. [12] and Zhang et al. [13] have demonstrated the perspective of microwire composites as microwave absorbers. In addition, our previous studies have shown that this kind of composite prove to be multifunctional for a wide range of potential engineering applications such as sensing and structural health monitoring (see e.g. [14-16]). The aim of this paper is to utilize instrumentation [17] dedicated to precise in-situ measurements of electromagnetic parameters to characterize the microwire composites over a broad frequency range and the field effect on the microwave behaviors of the wire composites for potential structural health monitoring (SHM) application.