Please cite this article in press as: F.X. Qin, et al., Effects of wire properties on the field-tunable behaviour of continuous-microwire composites, Sens. Actuators A: Phys. (2012), doi:10.1016/j.sna.2012.02.011 ARTICLE IN PRESS G Model SNA-7714; No. of Pages 8 Sensors and Actuators A xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical jo u rn al hom epage: www.elsevier.com/locate/sna Effects of wire properties on the field-tunable behaviour of continuous-microwire composites F.X. Qin a,∗ , H.X. Peng a , M.H. Phan b , L.V. Panina c,d , M. Ipatov d , A. Zhukov d a Advanced Composite Centre for Innovation and Science, Department of Aerospace Engineering, University of Bristol, University Walk, Bristol BS8 1TR, UK b Department of Physics, University of South Florida, Tampa, FL 33620, USA c School of computing, Communication and Electronics, University of Plymouth, Drake Circus, Plymouth, Devon PL4 8AA, UK d Dpto. de Fisica de Materiales, Fac. Quimicas, Universidad del Pais Vasco, Bilbao, San Sebastian 20009, Spain a r t i c l e i n f o Article history: Received 1 September 2011 Received in revised form 9 February 2012 Accepted 9 February 2012 Available online xxx Keywords: Field tunable properties Continuous microwire composites Ferromagnetic microwires Plasma frequency Self-sensing application PACS: 78.55.Qr 62.23.Pq 87.15.La a b s t r a c t The microwire composites consisting of continuous Co-rich amorphous glass-coated ferromagnetic microwires embedded in a E-glass prepreg matrix were fabricated, and the influences of wire period- icity (b), composition and radius on the field-tunable properties have been systematically investigated in a broad microwave frequency range of 0.9–18 GHz. It has been found that the field tunability, effec- tive operational frequency and field of the composites are strongly dependent on these factors. With decreasing b from 15 to 7 mm, the field tunability of effective permittivity (n ε ) increases from 0.77% to 16% m/A by more than 20 times. The detected cups and resonances of the transmission and reflection spectra are identified. Their changes with wire periodicity have been shown to be due to a combination of the dielectric and magnetic response arising from the interactions between microwave and microwires and microwires by themselves. The best possible field tunability occurs below the plasma frequency. The effective magnetic field for realisation of the field-tunable properties has been found to be about 500 A/m, which is associated with the anisotropy field. In addition, field tunability is found to be posi- tively correlated with the magnetic softness and GMI properties of the wire fillers, which are determined by the wire composition and geometry. These findings are of practical importance in developing multi- functional microwire composites for a broad range of engineering applications, such as structural health monitoring, NDT and microwave tunable devices. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Introducing additional functionalities into a structural com- posite material for specific purposes of domestic and engineering applications is an area of topical interest [1–4]. Particular atten- tion has been paid to “multifunctional” composites that, beyond their essential structural function, possess other functionalities achieved by constituent materials in an optimised structure. While the electromagnetic functionality has been extensively explored in a variety of complex composites [5–9], the recent progress in devel- oping new composites with superior microwave tunable properties indeed affords them for a broad range of applications from the high-performance frequency selective surfaces and the self-sensing media for the remote non-destructive test of structural materials to tunable filters and phase shifters [10–14]. This type of com- posites consists of conductive scattering elements embedded in a ∗ Corresponding author. Now at: Lab-STICC, Université de Bretagne Occidentale, CS 93837, 6 Avenue Le Gorgeu, 29238 Brest Cedex 3, France. Tel.: +33 2 98 01 80 09; fax: +33 2 98 01 61 31. E-mail address: faxiang.qin@gmail.com (F.X. Qin). dielectric matrix. It has been shown that the composite is irradiated by electromagnetic wave and its dielectric response characterised by the complex effective permittivity depends intimately on the impedance of the fillers [15]. The fillers are therefore required to possess a somewhat unique property ensuring a sensitive response to the electromagnetic excitation such as giant magneto-/stress- impedance effect (GMI/GSI), which is defined as a large variation of the high frequency impedance caused by an applied magnetic field/stress. As a result, the dispersion characteristics of the effec- tive permittivity depend on magnetic field and/or stress. In order to realise the tunable properties, the following condi- tions concerning the constituents of the composites are prioritised [16]: (i) the fillers should possess GMI effect and good soft magnetic property; (ii) the fillers are also preferred to have large permit- tivity and good conductivity to ensure strong dielectric responses of the composite to the electromagnetic wave. Following these criteria, the Co-based ferromagnetic microwires exactly meet the requirements. They have a unique circular magnetic anisotropy due to coupling between the negative magnetostriction and frozen-in stress. Such anisotropy is important to realise a large and sensitive magnetoimpedance (MI) effect for applications in miniature mag- netic sensors [17]. In addition, its fine size guarantees a minimal 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2012.02.011