ARTICLE IN PRESS Sensors and Actuators A xxx (2012) xxx xxx

Transcription

1 G Model ARTICLE IN PRESS 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: 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: Qr Pq 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 periodicity (b), composition and radius on the field-tunable properties have been systematically investigated in a broad microwave frequency range of GHz. It has been found that the field tunability, effective 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 positively 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 multifunctional microwire composites for a broad range of engineering applications, such as structural health monitoring, NDT and microwave tunable devices Elsevier B.V. All rights reserved. 1. Introduction Introducing additional functionalities into a structural composite material for specific purposes of domestic and engineering applications is an area of topical interest [1 4]. Particular attention 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 developing 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 composites consists of conductive scattering elements embedded in a Corresponding author. Now at: Lab-STICC, Université de Bretagne Occidentale, CS 93837, 6 Avenue Le Gorgeu, Brest Cedex 3, France. Tel.: ; fax: 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-/stressimpedance 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 effective permittivity depend on magnetic field and/or stress. In order to realise the tunable properties, the following conditions 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 permittivity 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 magnetic sensors [17]. In addition, its fine size guarantees a minimal /$ see front matter 2012 Elsevier B.V. All rights reserved. doi: /j.sna

2 ARTICLE IN PRESS G Model 2 F.X. Qin et al. / Sensors and Actuators A xxx (2012) xxx xxx Fig. 1. Schematic graph of the composite containing continuous microwires. disturbance on the resultant composite thereby making them suitable as multifunctional fibres for self-sensing composite media. With above considerations, a kind of composites consisting of Co-based ferromagnetic microwires embedded in a nonconductive polymer matrix is proposed in our work. Specifically, these microwires were embedded into a aerospace-grade epoxy matrix in a parallel and continuous fashion (see Fig. 1). This configuration not only structurally meets the stereotype of reinforcing fibre polymer composites, but also functionally provides a robust geometrical parameter, i.e., wire periodicity, as a tuning factor in contrary to the random composites with complicated mesostructure that is hard to control. We systematically studied the effects of wire periodicity, composition and radius on the field-tunable microwave properties of the continuous-wire composites. The purpose is to provide the readers a comprehensive picture of the mesostructure-property relationship for this kind of microwire composite and shed some light on how to formulate their field tunable properties via manipulating the essential elements of the composite architecture for relevant sensing and other engineering applications. 2. Experimental details Three soft magnetic glass-coated (Co,Fe,Ni) (B,Si,Mo) microwires (see Table 1) were fabricated by a modified Taylor Ulitovskiy method [18,19]. They possess vanishing magnetostriction coefficient ( 10 7 ) and low magnetic anisotropy field. A set of microwire composites were prepared using these Co-rich wires and E-glass 913 prepregs, as summarised in Table 1. The continuous-wire composites were manufactured in the following steps [20]: (1) Fifty centimetre wire-pieces were laid out at zero degree along the glass-fibre direction between the two layers of prepregs with in-plane size of 50 cm 50 cm. The wire periodicity (b) was controlled at an attempted value in the perpendicular direction as shown in Fig. 1. (2) Other two layers were laid up on the top and bottom of the wire-embedded layers in the same direction, giving a layup of four prepreg layers containing short wires. (3) After bagging the composite on an aluminium plate with air sucked out to the required vacuum of kpa, the material was cured in an autoclave. The curing process: the temperature was raised at a rate of 2 C/min to 127 C and kept for 80 min before cooling down naturally to room temperature. At a rate of 69 kpa/min the pressure was increased to 30 psi and kept at this level for 30 s and then 690 kpa for 600 min before decreasing at a rate of the 20.7 kpa/min. The S-parameters were measured in the frequency range of GHz in the presence of external field ranging up to 3 ka/m applied through a plane coil with turns perpendicular to the electrical field in the incident wave. The effective permittivity spectra were deduced from S-parameters using Reflection/Transmission Epsilon Fast Model. More details of the experimental setup can be found in Ref. [11]. The field tunability n in the presence of an external magnetic field H is defined as: n = X(H) X(0), (1) H where X(H) and X(0) can be specified as effective permittivity, transmission or reflection parameter at a given magnetic field H and zero magnetic field, respectively. 3. Results and discussions 3.1. Influence of wire periodicity Effective dielectric properties The continuous-wire composites has been theoretically demonstrated to have a characteristic plasma dispersion behaviour of the effective permittivity ε, expressed as [7,11,15]: ( ) 2 fp ε = ε m, (2) f where ε m is the permittivity of the matrix. f p is the plasma frequency given by [11,15]: fp 2 = c 2 2b 2 ln(b/a), (3) where b is the spacing between the wires, termed here as wire periodicity and a is the wire radius. Since the plasma frequency is dependent on the wire impedance [21], the effective permittivity can then be modulated by the external magnetic field or stress through the impedance. According to Eq. (3), the characteristic plasma frequency of a composite containing continuous ferromagnetic wires varies with wire radius and periodicity. Therefore, one can expect that the tunable properties vary with the magnetic properties of different wires. In other words, the investigation into tunable behaviour of continuous-wire composites in terms of influencing factors can be carried out in three major aspects, namely, the influence of wire geometry, wire composition determined magnetic properties and the wire periodicity on the microwave tunable properties of the composites concerned. Herein the present study is centred on these effects, which dominate the macroscopic properties of such kind of heterogeneous composites [9]. Fig. 2 displays the frequency dependence of the real part of effective permittivity (ε ) taken at different magnetic fields for the composites with different b. The calculation using Eq. (3) gives two different values of the plasma frequency: 5.1 GHz and 6.6 GHz, respectively. The strong frequency dependence of ε is observed below the corresponding plasma frequencies. To further clarify this intriguing feature, the magnetic field dependence of ε taken at f = 1 GHz is plotted in the inset of Fig. 2. It can be seen that with increasing b from 7 up to 15 mm, the magnetic field dependence of the effective permittivity is strongly varied. This dependence can be understood as this that the wire periodicity determines the intensity of polarisation at the metal/dielectric interface, thus influencing the dielectric relaxation patterns [15]. The field tunability of the effective permittivity (n ε ) reduces from 16% m/a for the b = 7 sample to 0.77% m/a for the b = 15 sample. This clearly indicates that small wire periodicity is preferable to field-tunable property. However, a decrease of wire periodicity will increase the plasma frequency and hence the skin effect. As too strong a skin effect would be detrimental to the field tunability, the wire diameter may also need to be decreased to compensate the reduction of skin depth. Thereby, there exists an optimum value of wire periodicity matching the diameter for the desirable microwave tunable

3 G Model ARTICLE IN PRESS F.X. Qin et al. / Sensors and Actuators A xxx (2012) xxx xxx 3 Table 1 List of the wire composition, wire core radius (a), wire periodicity (b) and plasma frequency (f p) for the studied composite samples in the present work. Composite label Wire composition a ( m) b (mm) f p (GHz) 1 Co 68.7Fe 4Ni 1B 13Si 11Mo Co 68.7Fe 4Ni 1B 13Si 11Mo Co 68.7Fe 4Ni 1B 13Si 11Mo Co 68.7Fe 4Ni 1B 13Si 11Mo Co 67.05Fe 3.85Ni 1.44B 11.53Si 14.47Mo Co 67.05Fe 3.85Ni 1.44B 11.53Si 14.47Mo Co 67.05Fe 3.85Ni 1.44B 11.53Si 14.47Mo properties. This is consistent with the theoretical prediction that for b = 0.5 mm, the best tunability will be achieved for a wire radius of 5 10 m [22] Transmission properties Fig. 3 shows the transmission spectra (S21) of the composites with different b. Overall, at zero magnetic field (ZMF), all the samples show two maximum and one minimum cups at frequencies of 8 GHz, 15 GHz and 12 GHz, respectively. These cups become more pronounced for composites with larger b. Noticeably, the amplitude of S21 significantly reduces as the magnetic field (H) is applied. For H > 500 A/m, an additional dip occurs at 3 GHz and this dip shifts to a lower frequency as b is increased. For the samples investigated, the amplitude of S21 reduces strongly with H < 500 A/m and levels off for H > 500 A/m. This implies that there exists an upper limit of the field tunability of the composites, which can be related to the effective anisotropy field of a multi-wire system. It should also be noted that the field tunability of the transmission parameter is diminished as the frequency is increased. In the vicinity of the plasma frequency, the transmission parameter becomes independent of the external magnetic field. This indicates that the field tunable properties are limited not only by the magnetic field but also by the frequency, both of which should be carefully considered in practical applications. To further elucidate this, we show in Fig. 4(a) and its inset the magnetic field dependence of field tunability (n S21 ) and S21 at 4 GHz, a frequency near the minimum of S21 in the presence of field of 1kA/m and below the plasma frequency. The field tunability varies with magnetic field and decreases with increasing b until H exceeds 360 A/m. This can be understood as increasing Fig. 2. Frequency plots of the real (ε ) and imaginary part (ε ) of effective permittivity for composite #2 and #3 with different wire periodicity (a) b = 7 mm; (b) b = 9 mm. The inset shows the field dependence of ε for composite #2, #3 and #4 with different wire periodicity b = 7, 9 and 15 mm. Fig. 3. Transmission spectra (S21) for composite #1, #2 and #3 with varying wire periodicity. (a) b = 3 mm; (b) b = 7 mm; (c) b = 9 mm.

4 ARTICLE IN PRESS G Model 4 F.X. Qin et al. / Sensors and Actuators A xxx (2012) xxx xxx Fig. 4. Field tunability of transmission parameter S21 as a function of external field for composite #1, #2 and #3 with varying wire periodicity b = 3, 7 and 9 mm at 4 GHz (a) and plasma frequencies (b). The ordinate profiles are divided by the factor Inset graphs are the corresponding field dependence of S21. amount of wires and an elevated interaction between the wires caused by decreasing b encouraged the response to applied magnetic field [15,23]. The maximum tunability is shifted to a higher field for composites with larger b, which clearly indicates the significant impact of the wire periodicity on the field tunable properties of the composites. This can be explained by the aforementioned mechanism that the increase of b dwarfs the wire interaction and consequently its field response. Fig. 4(b) illustrates the magnetic field dependence of the field tunability at the plasma frequency, which is theoretically expected to excite the maximum response to electric field. However, the best field tunability occurs below the plasma frequency, which can be attributed to the different dynamic dielectric response and magnetic response. The application of an external magnetic field reduces the transmission till c.a. 500 A/m where the maximum absorption can be inferred Reflection properties Fig. 5 shows the frequency dependence of reflection parameter (S11) taken at different magnetic fields for the composites with b = 3 mm, 7 mm and 9 mm. It can be seen that the shape of the curves varies remarkably as the wire periodicity increases from 3 to 9 mm. For the b = 3 mm sample, the spectra can be divided into two frequency zones at 7.6 GHz. Below 7.6 GHz, the reflectivity decreases as the magnetic field is applied due to the absorption effect. However, an opposite trend is observed for f > 7.6 GHz. For the b = 7 and 9 mm samples, one more zone is found for f > 16.3 GHz and f > 14.4 GHz, respectively. The frequency at which the signal of S11 changes with magnetic field is considered as a characteristic frequency. It is worth noting that as b increases from 3 mm to 9 mm, the characteristic frequency decreases from 7.6 GHz to Fig. 5. Reflection spectra (S11) for composites #1, #2 and #3 with varying wire periodicity. (a) b = 3 mm; (b) b = 7 mm; (c) b = 9 mm. 3.8 GHz (between the zone 1 and 2). One can also see that the characteristic frequency (between the zone 2 and 3) decreases from 16.3 GHz for the b = 7 mm sample to 14.4 GHz for the b = 9 mm sample. For the b = 3 mm sample, although the characteristic frequency due to limited measurement frequency range is formidable to be identified, it predicts to be higher than those for the b = 7 mm and 9 mm samples. This finding points to an important consequence that the characteristic frequency shifts to a lower value for composites with larger wire periodicity in the reflection spectra. The marked feature displayed in the reflection spectra of S22 (Fig. 6) is the resonance at 15.9 MHz and 14.2 MHz, which originate from the influence of magnetic field on the wave propagation through the composite media. Both graphs share a common phenomenon that the resonance decreases with the magnetic field and the resonance frequency blueshifts. For the composite with b = 7 mm, the resonance frequency shifts from 15.9 GHz for ZMF to 16.5 GHz with an applied field of 1 ka/m; the resonance value changes from 42.3 to 38.9 db. For the b = 9 mm composite, the resonance frequency shifts from 14.2 for ZMF to 14.6 for H = 1 ka/m; the resonance intensity varies from 52.7 to 40.9 db. A larger

5 G Model ARTICLE IN PRESS F.X. Qin et al. / Sensors and Actuators A xxx (2012) xxx xxx 5 variation in the resonance value of S22 and a smaller shift of the resonance frequency with the applied field occur as the wire periodicity is increased. A plausible underlying reason is that the larger wire periodicity diminishes the influence of magnetic field on the wave propagation in the interfaces of microwires and composite matrix, consequently presents less influence on the resonance frequency. But the value of the resonance is dependent on both induced polarisation and magnetic field dependence of wave interaction, which are more sensitive to the applied field with larger periodicity. Similar to the transmission spectra, in order to guarantee an effective response to the magnetic field, there exists an upper limit at 500 A/m and the plasma frequency for the applied magnetic field and operational frequency, respectively Influence of wire composition Fig. 6. Reflection spectra (S22) for composites #1, #2 and #3 with varying wire periodicity (a) b = 7 mm; (b) b = 9 mm. This part of work aims to establish the relationship between wire composition and tunable properties so that the suitable composition can be identified in the early stage to ensure the final composite product meet the requirements such as operational frequency and field sensitivity as specified by targeted applications. Fig. 7 shows a comparison between the composites containing wires of different composition for b = 7 mm (samples #2 and #5) and b = 9 mm (samples #3 and #6), respectively. While both wirecomposites present rather good dispersion properties, composite #2 and #3 show a more sensitive response of effective permittivity to the magnetic field in the lower frequency range than #5 and #6, respectively. The scattering spectra for #5 composite with periodicity of 7 mm are presented in Fig. 8. In the spectra of S11, as compared with sample #2 (Fig. 5(b)), composite #5 exhibits similar characteristic frequencies but apparently differing field tunability. Quantitatively, at f = 0.9 GHz, the variation of S21 from ZMF to 1 ka/m is 6.1 db for #2 against 4.8 db for #5. In the spectra of S21, at the first dip induced by the application of field of 1 ka/m, the tunability of S21 is 2.9 db m/a for #2 (Fig. 3(b)) versus 2 db m/a. The comparison of spectra S22 reveals that both the shifts of resonance and resonance frequency are smaller for Fig. 7. Frequency plots of real part of effective permittivity for composites #2 (a) and #5 (b), #3 (c) and #6 (d) with the external field as a parameter. The inset shows M H curves of Co 68.7Fe 4Ni 1B 13Si 11Mo 2.3 (wire 1) and Co 67.05Fe 3.85Ni1.44B 11.53Si 14.47Mo 1.66 (wire 2) measured in the field along the wire axis.

6 ARTICLE IN PRESS G Model 6 F.X. Qin et al. / Sensors and Actuators A xxx (2012) xxx xxx Fig. 9. Frequency dependence of ε with external field as a parameter for composites #6 (a) and #7 (b) containing wires of different radius a. Fig. 8. Frequency dependencies of magnitude of S-parameters for composite #5 (containing amorphous wires Co 67.05Fe 3.85Ni 1.44B 11.53Si 14.47Mo 1.66 spaced at 7 mm): (a) S11, (b) S21 and (c) S22. #5. All these differences can be attributed to that of soft magnetic properties as shown in the magnetisation curves of the two wires (the inset of Fig. 7). This can be reasoned as follows. A better magnetic softness indicates a larger dynamic magnetic permeability at concerned frequency range, giving rise to a larger relaxation parameter according to cos 2 [22], where is the static magnetisation angle. As it has been proved that field tunable effects are positively dependent on the relaxation parameter, the excellent magnetic softness would be preferable to realise strong field tunable effects. In considering the soft magnetic features of microwires are sensitive to the composition [24,25], one should try to achieve a precise control of composition in the fabrication process and appropriate tailoring in the post-fabrication treatments before proceeding to the manufacture of microwire composites for field-tunable functionality Influence of wire radius Fig. 9 shows that composites containing wires with large radius (#6) presents a higher field tunability than the other with a smaller radius (#7), which can be explained by the wire geometry dependence of GMI effect. It has been well established that, at given glass-coat thickness, the GMI effect is enhanced with increasing wire radius [26]. Accordingly, the dielectric response of the composite containing wires of larger radius is stronger than the otherwise. This can also be understood from the perspective of skin effect: the field tunable effect tends to be smaller with stronger skin effect, therefore, the thinner wires will present weaker field effects. This is fully consistent with the latest results reported by Panina et al. [22]. A further comparison between the transmission spectra (cf. Fig. 10) reveals that the radius of the wire has a profound impact on the intensity of S21 but much less effect on the tunability. The variation of radius of several microns has a negligible influence on the plasma frequency of 5 GHz, and that the frequencies at which S21 reaches the minimum are about 3 GHz for both of them. It follows that the plasma frequency plays a dominant role in formulating the patterns of transmission spectra. At 3.0 GHz, for #6 the value of S21 changes from 4.5 to 6.4 db under the applied field of 200 A/m. While a smaller change of S21 occurs to the #7 from 3.5 to 4.7 db with the applied field of 300 A/m. At the plasma frequency of 5 GHz, S21 decreases by 8 db m/a for #6 in comparison with 4.3 db m/a for #7. The fact that #6 presents a better tunability than #7 can also be attributed to the difference of wire radius. The tunable property illustrated in reflection spectra is in favour of #7. Concerning the whole frequency range, #6 displays minor response to the applied field as compared with #7. Particularly, the maximum tunability of 5 db m/a occurs at 0.9 GHz, which is below the plasma frequency of 5 GHz for #6 whilst the value is 6.1 db m/a for #7. The second largest tunability of 1.9 db m/a occurs at 9.5 GHz, which is above the plasma frequency for #6, and the value is 2.5 db m/a for #7. These three characteristic frequencies are in fairly good agreement for #6 and #7, which evidences the crucial role of plasma frequency in harnessing the reflection spectra. Combined with discussions on the transmission spectra in the previous section, it is concluded that the patterns of scattering spectra are correlated to the plasma frequency. In addition, at 9.5 GHz, the relaxation type of S11 for #6 is in sharp contrast to a resonant S11 for #7; this can also be explained as the variation of wire radius leading to different microwave properties through giant magnetoimpedance. Herein, it is further confirmed that the wire geometry plays an imperative role in influencing the patterns of scattering spectra. Because the plasma frequency is determined by the wire radius and periodicity, which can be categorised as the microstructure of the composite, the ultimate conclusion is that the microstructure of composites determines the scattering spectra of the macroscopic scale. The phase shift of S11 in the presence of external field (cf. Fig. 11) suggests a promising sensing application of the composite. For #6, the phase going through ± is completely suppressed when it is

7 G Model ARTICLE IN PRESS F.X. Qin et al. / Sensors and Actuators A xxx (2012) xxx xxx 7 Fig. 10. Frequency dependence of scattering spectra with external field as a parameter for composites #6 (a and c) and #7 (b and d) containing wires of different radius a. 4. Conclusions Fig. 11. Frequency dependence of S11 phase with external field as a parameter for composites #6 (a) and #7 (b) containing wires of different radius a. under a small field of 25 A/m. For #7, the concerned phase shifts to a lower value from 3.7 in the absence of field to 3.0 in the presence of field of 70 A/m. These considerable changes suggest the ferromagnetic microwires enable their composites with the self-monitoring capacity-any stress change occurred to the wire through the composite can be detected via the microwave tunable spectra. The composites composed of continuous Co-rich ferromagnetic microwires embedded in a aerospace-grade prepreg matrix in the parallel manner have been fabricated. The influences of wire parameters including wire radius, composition and periodicity on the field-tunable properties have been thoroughly discussed with detailed presentation of feature differences in all comparisons. The results obtained reveal that the field-tunable properties of the composites in the microwave frequency range depend intimately on these wire properties and their configuration via the skin effect, GMI effect and the relaxation parameter. It is viable to modulate or optimise the microwave dielectric properties and transmission/reflection patterns of the composites through tuning these wire parameters, which can also be utilised to extend the limitation of the effective operation frequency range. When it comes to the design and manufacture of such kind of microwire composite for tunable functionality, careful choice of wires proves to be critical to the ultimate performance of the resultant composites. While a plethora of studies have been performed on the fabrication and tailoring of the microwires [18,24], it remains a challenge to realise an industrial fabrication of the microwire polymer composites to capitalise on the cost-effectiveness and intriguing properties of the microwires. Fundamentally, on one hand, we should continue to look into the microstructure property relationship of the microwires, especially those [27] fabricated by other techniques other than Taylor Ulitovskiy method, and the mesostructure property of their composites with precise control of the composite preparation in the lab scale within the framework of heterogeneous materials theory [1]. On the other hand, with the successful fabrication of submicron wires [28] and nanowires [29], their intriguing magnetic properties distinguished from microwires aside, they tend to take the form of molecular fillers and trigger a more complicated interactions between wires and polymer chains, which certainly deserve detailed investigations. More perspective applications can also be sought after with the involvement of other type of fillers such as the present-day wondrous materials, nanotubes or graphenes, chemically or physically mixed with the wires in the polymer to generate a unique class of hybrid composites. All these possibilities enrich the content of the multifunctionalities derived from the microwires that one

8 ARTICLE IN PRESS G Model 8 F.X. Qin et al. / Sensors and Actuators A xxx (2012) xxx xxx can imagine and consequently open up new arena of research subjects and innovative technologies such as the application of smart composite materials in aeronautical electronics. Acknowledgements FXQ was supported through the Overseas Research Students Awards Scheme and University of Bristol Postgraduate Student Scholarship. HXP would like to acknowledge the financial support from the Engineering and Physical Science Research Council (EPSRC) UK under the Grant No. EP/FO3850X. The authors also acknowledge valuable discussions with Dr. D. Makhnovskiy and his help in designing the free space measurement installation. FXQ would also like to acknowledge the current financial support from the Conseil général du Finistère, France. References [1] S. Torquato, Random Heterogeneous Materials Springer, New York, [2] S. Torquato, S. Hyun, A. Donev, Multifunctional composites: optimizing microstructures for simultaneous transport of heat and electricity, Phys. Rev. Lett. 89 (2002) [3] J.P. Thomas, M.A. Qidwai, Mechanical design and performance of composite multifunctional materials, Acta Mater. 52 (2004) [4] R.F. Gibson, A review of recent research on mechanics of multifunctional composite materials and structures, Compos. Struct. 92 (2010) [5] A.N. Lagarkov, S.M. Matytsin, K.N. Rozanov, A.K. Sarychev, Dielectric properties of fiber-filled composites, J. Appl. Phys. 84 (1998) [6] A.N. Lagarkov, A.K. Sarychev, Electromagnetic properties of composites containing elongated conducting inclusions, Phys. Rev. B 53 (1996) [7] J.B. Pendry, A.J. Holden, W.J. Stewart, I. Youngs, Extremely low frequency plasmons in metallic mesostructures, Phys. Rev. Lett. 76 (1996) [8] S.M. Matitsine, K.M. Hock, L. Liu, Y.B. Gan, A.N. Lagarkov, K.N. Rozanov, Shift of resonance frequency of long conducting fibers embedded in a composite, J. Appl. Phys. 94 (2003) [9] C. Brosseau, P. Queffelec, P. Talbot, Microwave characterization of filled polymers, J. Appl. Phys. 89 (2001) [10] D.P. Makhnovskiy, L.V. Panina, Field dependent permittivity of composite materials containing ferromagnetic wires, J. Appl. Phys. 93 (2003) [11] D.P. Makhnovskiy, L.V. Panina, C. Garcia, A.P. Zhukov, J. Gonzalez, Experimental demonstration of tunable scattering spectra at microwave frequencies in composite media containing CoFeCrSiB glass-coated amorphous ferromagnetic wires and comparison with theory, Phys. Rev. B 74 (2006) [12] L.V. Panina, D.P. Makhnovskiy, K. Mohri, Magnetoimpedance in amorphous wires and multifunctional applications: from sensors to tunable artificial microwave materials, J. Magn. Magn. Mater (2004) [13] F.X. Qin, N. Pankratov, H.X. Peng, M.H. Phan, L.V. Panina, M. Ipatov, V. Zhukova, A. Zhukov, J. Gonzalez, Novel magnetic microwires-embedded composites for structural health monitoring applications, J. Appl. Phys. 107 (2010) 09A A. [14] F.X. Qin, H.X. Peng, V.V. Popov, L.V. Panina, M. Ipatov, V. Zhukova, A. Zhukov, J. Gonzalez, Stress tunable properties of ferromagnetic microwires and their multifunctional composites, J. Appl. Phys. 108 (2011) 07A310. [15] D.P. Makhnovskiy, L.V. Panina, Field and stress tunable microwave composite materials based on ferromagnetic wires, in: V.N. Murray (Ed.), Progress in Ferromagnetism Research, Nova Science Publishers Inc., Hauppauge, NY, [16] D. Makhnovskiy, L. Panina, Tuneable Microwave Composites Based on Ferromagnetic Wires, 2005, [17] L.V. Panina, Asymmetrical giant magneto-impedance (AGMI) in amorphous wires, J. Magn. Magn. Mater. 249 (2002) [18] V.S. Larin, A.V. Torcunov, A. Zhukov, J. Gonzalez, M. Vazquez, L. Panina, Preparation and properties of glass-coated microwires, J. Magn. Magn. Mater. 249 (2002) [19] G.F. Taylor, Process and apparatus for making filaments, US Patent (1931). [20] H.X. Peng, F.X. Qin, M.H. Phan, J. Tang, L.V. Panina, M. Ipatov, V. Zhukova, A. Zhukov, J. Gonzalez, Co-based magnetic microwire and field-tunable multifunctional macro-composites, J. Non-Cryst. Solids 355 (2009) [21] A.K. Sarychev, V.M. Shalaev, Electromagnetic field fluctuations and optical nonlinearities in metal-dielectric composites, Phys. Rep. 335 (2000) [22] L.V. Panina, M. Ipatov, V. Zhukova, A. Zhukov, J. Gonzalez, Magnetic field effects in artificial dielectrics with arrays of magnetic wires at microwaves, J. Appl. Phys. 109 (2011) [23] M.H. Phan, H.X. Peng, S.C. Yu, M.R. Wisnom, Large enhancement of GMI effect in polymer composites containing Co-based ferromagnetic microwires, J. Magn. Magn. Mater. 316 (2007) e253 e256. [24] A. Zhukov, V. Zhukova, Magnetic Properties and Applications of Ferromagnetic Mircowires with Amorphous and Nanocrystalline Structure, Nova Science Publishers, Inc., New York, [25] M. Vázquez, Advanced magnetic microwires, in: H. Kronmüller, S. Parkin (Eds.), Handbook of Magnetism and Advanced Magnetic Materials, John Wiley & Sons, 2007, p [26] F.X. Qin, H.X. Peng, M.H. Phan, Influence of varying metal-to-glass ratio on GMI effect in amorphous glass-coated microwires, Solid State Commun. 150 (2010) [27] M. Han, D. Liang, L. Deng, Fabrication and electromagnetic wave absorption properties of amorphous Fe 79Si 16B 5 microwires, Appl. Phys. Lett. 99 (2011) [28] H. Chiriac, S. Corodeanu, M. Lostun, G. Ababei, T.-A. Óvári, Magnetic behavior of rapidly quenched submicron amorphous wires, J. Appl. Phys. 107 (2010) 09A301. [29] H. Chiriac, S. Corodeanu, M. Lostun, G. Stoian, G. Ababei, T.A. Ovari, Rapidly solidified amorphous nanowires, J. Appl. Phys. 109 (2011) Biographies Dr. Faxiang Qin is now a postdoctoral research fellow in Lab-STICC at Université de Bretagne Occidentale, Brest, France. He received his first degree from the Beijing University of Chemical Technology in 2003, MSc from the South China University of Technology in 2007, and PhD in Aerospace Engineering from University of Bristol, UK in His research interest lies in soft matter, multifunctional composites and applied physics. Dr. Hua-Xin Peng is now a Reader in Aerospace Materials in the Advanced Composite Center for Innovation and Science at the University of Bristol. He obtained his BSc (July 1990) in Physical Metallurgy from Zhejiang University followed by his MSc (March 1993) and PhD (July 1996) in Composite Materials from Harbin Institute of Technology (HIT), Harbin, PR China. His research interest lies in developing composite materials with novel microstructures and functionality. Dr. Manh-Huong Phan is now a research assistant professor in the Dept. of Physics at University of South Florida. He received his first degree in Physics at Hanoi National University, Vietnam in 2000, MSc in Physics at Chungbuk National University, South Korea in 2003, and PhD in Engineering Physics at University of Bristol, England in His research interests span a wide range of experimental and theoretical topics in magnetism and magnetic materials such as giant magnetoimpedance (GMI) materials and giant magnetocaloric (GMC) materials. Prof. Larissa Panina is now a professor in School of Computing and Mathematics at the University of Plymouth. She received her MSc in physics from Moscow State University in 1981 and PhD in physical and mathematical science from Moscow State University in Her research interest lies in nanostructered plasmonic metamaterials, magnetoimpedance (MI) effect, sub-nano Tesla magnetic sensors and magnetic microwires and thin films. Mihail Ipatov is a doctoral fellow of the Dept. of Material Physics, Basque Country University, Spain. He graduated as an engineer in the electronics in Kishinev in The topic of his PhD is technological studies and applications of glass coated magnetic microwires, being documented in 10 scientific publications. Dr. Arcady Zhukov graduated in 1980 from the Physics-Chemistry Department of the Moscow Steel and Alloys Institute. In 1988 received PhD degree from the Institute of solid state physics of the Russian Acad. Sci. Presently he is a research Ikerbasque professor at the Department of the Materials Physics of the University of Basque Country in San Sebastian. His current fields of interest are novel magnetic materials, amorphous ferromagnetic materials, in particular microwires, giant magneto-impedance, giant magnetoresistance, magnetoelastic sensors and nanocrystalline materials.