- Email: [email protected]
Metglas laminate
Sensors and Actuators A 153 (2009) 64–68
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Ultra-sensitive detection of magnetic field and its direction using bilayer PVDF/Metglas laminate X.W. Dong a , B. Wang a , K.F. Wang a , J.G. Wan a , J.–M. Liu a,b,c,∗ a b c
Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China School of Physics, South China Normal University, Guangzhou 510006, China International Center for Materials Physics, Chinese Academy of Sciences, Shenyang, China
a r t i c l e
i n f o
Article history: Received 19 January 2009 Received in revised form 19 March 2009 Accepted 23 April 2009 Available online 3 May 2009
a b s t r a c t Bilayer PVDF/Metglas laminates with remarkably anisotropic ME coupling effect have been fabricated. By a careful evaluation of these anisotropic behaviors both experimentally and theoretically, we demonstrate that an ultra-sensitive detection of weak dc magnetic field and its spatial orientation is practically applicable using the bilayer PVDF/Metglas laminate. This magnetic field probe has quite high sensitivity. The reason for such a high magnitude and orientation-dependent magnetoelectric coupling is explained.
PACS: 75.80.+q 85.80.Jm 77.74 Dy
© 2009 Elsevier B.V. All rights reserved.
Keywords: Magnetoelectric coupling Laminate Magnetic field detection
1. Introduction Magnetoelectric (ME) effect characterized by an induced electric polarization under an applied magnetic field and vice versa, can be seen as a bridge between the electric polarization and magnetization [1]. Since the ME effect as a strong function of external dc magnetic field, Hdc , was observed, it has drawn special attention due to its potential and important applications in detection of weak dc magnetic field [1–6]. It is well known that a strong ME effect at room temperature can be easily obtained from a multiferroic composite which is generally made by combining piezoelectric and magnetostrictive components [1–14]. Up to now, a number of multiphase ceramic composites have been synthesized and characterized in order to demonstrate the detection of weak Hdc , noting that the sensitive detection of external ac magnetic field has been demonstrated by Viehland and co-workers [15]. Among these ME composites reported so far, lead zirconate titanate (PZT) ceramics have been widely used as the piezoelectric component, owing to its high piezoelectric constant and large
∗ Corresponding author at: Laboratory of Solid State Microstructures, Nanjing University, Hankou Road No. 22, Nanjing 210093, China. Tel.: +86 25 83596595; fax: +86 25 83595535. E-mail address: [email protected] (J.–M. Liu). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.04.033
electromechanical coupling. At the same time, the giant magnetostrictive alloy Tb1−x Dyx Fe2 (Terfenol-D) has often been used as the magnetostrictive component. The ME effect of these composites is not very anisotropic in response to Hdc due to the fact that the piezoelectric and piezomagnetic coefficients in these composite sensors are not very orientation-dependent in prior to poling. These magnetic sensors are capable of detecting the magnitude component of Hdc given its fixed direction, while a determination of both the magnitude and direction of Hdc seems challenging. In addition, the detection sensitivity using these composite sensors still has space to improve. Furthermore, they are mechanically inflexible and thus inaccessible to some specific applications. It should be reminded that so far only a few reports focused on the anisotropic ME coefficient in response to ac or dc magnetic field. Rashed and Shashank reported the angular dependence of ME coefficient for co-fired BaTiO3 -(Ni0.8 Zn0.2 )Fe2 O4 bilayer composite upon ac magnetic field [16]. More recently, the electric-field-induced magnetization (EIM) anisotropy in laminate composite structure consisting of Terfenol-D component and 0.68[Pb(Mg1/3 Nb2/3 )O3 ]-0.32PbTiO3 component was investigated [17], and an angular dependence of the EIM variation at the electromechanical resonance frequency ∼185 kHz was measured. However, up to date no report on the angular dependence of magnetic-field-induced electric polarization (MIEP) in response to a dc magnetic field Hdc , imposed onto an ac magnetic field Hac has
X.W. Dong et al. / Sensors and Actuators A 153 (2009) 64–68
not yet been available. Such an effect has potential application in detection of weak Hdc rather than Hac . The motivation of this work is to develop a ME composite structure exhibiting anisotropic MIEP. Recently, polynivylidence-fluoride (PVDF) piezopolymer was also incorporated into ME laminate [18–22], because it owns many advantages such as soft elastic constants, high resistivity, and high piezoelectric voltage coefficient, etc. A giant ME effect in a three-layer PVDF/Metglas laminate, which is mechanically flexible, was reported by Viehland and co-workers [22]. As well known, the piezoelectricity of PVDF is generally obtained by submitting the PVDF film to a sufficient high electric field after a mechanical stretching [23]. The most attractive feature is that the anisotropic piezoelectric performance can be obtained by mechanically stretching the PVDF film in one or two perpendicular directions prior to the polarization process. A uni-axial stretching will induce in-plane unidirectional piezoelectric coefficient, while a bi-axial stretching of the film will induce in-plane orientation-dependent piezoelectric property (strictly speaking, biaxial piezoelectric coefficient). As a consequence, an in-plane and highly anisotropic piezoelectric coefficient for PVDF film can be obtained simply by a uni-axial stretching. Similarly, if the magnetostrictive component shows in-plane anisotropic piezomagnetic property too, the resulting ME composite structure would exhibit anisotropic coupling which can be modulated with high degrees of freedom through different combinations of the in-plane piezoelectric and piezomagnetic properties of the two components. Along this line, a ME composite laminate structure for detecting not only the magnitude but also the direction of Hdc (i.e. detecting Hdc as a vector Hdc ) can be designed and fabricated. By a similar strategy, a detection of external ac magnetic field, Hac , is also accessible, which will not be addressed here since it is even simpler than detecting Hdc . In this work, we investigate the ME effect of a bilayer composite structure consisting of PVDF film with in-plane anisotropic piezoelectricity as the piezoelectric component and flexible/thin iron-based Metglas as the magnetostrictive component, and explore its high angular dependence with respect to Hdc and ultra-sensitivity of detecting weak Hdc . The Metglas has the in-plane anisotropic piezomagnetism, high piezomagnetic coefficient and huge magnetic permeability (r > 40,000). The bilayer PVDF/Metglas laminates were prepared, with the piezoelectric coefficients (d31 =25 pC/N) of the PVDF layer aligned along magnetization (M) direction of the Metglas layer, along which the piezomagnetic coefficient is extremely high with respect to Terfenol-D under low magnetic field.
2. Experimental details The PVDF thin films used as piezoelectric components in this work were commercially purchased (Shenzhen Mingschin Highpolymer Technology Co. Ltd., China), with 28 m in thickness. It was transversely poled and fitted with electrode on the top and bottom surfaces. The piezoelectric coefficients are d31 = 25 pC/N, d32 = 2.0 pC/N and d33 = 25 pC/N, indicating large in-plane anisotropy, as shown schematically in Fig. 1(a). The magnetostrictive Metglas-SA1-MP4010XGDC layers were bought from Metglas Inc., USA, with 25 m in thickness and the magnetization was aligned along the longitudinal (length) direction. Both the PVDF layer and Metglas layer were glued together using epoxy binder, with a dimension of 20 mm in length and 10 mm in width. In Fig. 1(b) is given a photograph of the PVDF/Metglas laminate. As for the ME measurement, the detailed procedure can be found in [6]. The ac magnetic field Hac was produced from a sync signal using a Helmholtz coil. This sync signal was generated by a signal generator and amplified by a function amplifier. The dc magnetic
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Fig. 1. (a) An illustration of the laminate structure with three piezoelectric coefficients for the PVDF layer. (b) Photograph of a PVDF/Metglas laminate.
field Hdc was supplied by an electromagnet. The induced voltage and phase in PVDF film were measured using a lock-in amplifier (SRS Inc., SR830). The reference signal for the phase measurement came from the sync signal of the signal generator which produced Hac . 3. Results and discussion We look at the bilayer PVDF/Metglas laminate, which was prepared by aligning the d31 of PVDF layer along the length direction. This is coincident with the direction of M for the Metglas layer. Thus, the d32 of PVDF layer is aligned along the width direction, as seen in Fig. 1. 3.1. Anisotropic ME coupling We first demonstrate the anisotropic ME coupling of this laminate structure. During the measurement, the ac magnetic field was set to Hac = 3.0 Oe with frequency f = 1.0 kHz and such a choice was made without any specific consideration. The low-level Hdc in the range of 0–200 Oe in magnitude was supplied by a high-precision electromagnet. Fig. 2(a) shows the Hdc dependence of ME voltage coefficient (˛ME ) when Hac and Hdc are both applied along the directions of length (L), width (W), and thickness (T) of the laminate, respectively, regardless of the sign of Hdc along this direction. It is clearly seen that the ME coefficient along the length direction, ˛L , is remarkably larger than the other ones (˛W and ˛T ) over the whole Hdc range. As Hdc is low, ˛L increases approximately lin-
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SH33 is the elastic compliance under a constant magnetic field, q33 is the longitudinal piezomagnetic constant, T3m and S3m are the stress and strain in the longitudinal direction of Metglas layer. For the case that the PVDF layer is poled along the thickness direction, the piezoelectric constitutive equations are: S1p = SE11 T1p + d31 E3 , D3 = d31 T1p + εT 33 E3
(2)
where D3 and E3 are the electric displacement and electric field along the thickness direction, S1p and T1p are the strain and stress of PVDF layer imposed by Metglas layer, SE11 and εT33 are the elastic compliances under constant electric field E and dielectric permittivity under constant stress T, and d31 is the piezoelectric constant. Assuming that the layers in the laminate act only in one coupled mode, from Newton’s second law, the piezoelectric and piezomagnetic constitutive equations can be coupled into the equation of motion:
∂T1p ∂2 u(z) ∂T3m + (1 − n) =n , ∂z ∂z ∂t 2
(3)
where = (1 − n)p + nM is the average mass density of the laminate, p and M are the density of PVDF and Metglas, n is the geometric factor of Metglas layer and u(z) is displacement defined along the z-axis (thickness direction). Given the model proposed above, the ME equivalent-circuit under free boundary conditions can be described by substituting Eqs. (1) and (2) into Eq. (3). In the open-circuit conditions, the ME coefficient at low frequency can be written as: ˛E =
Fig. 2. (a) Hdc dependence of anisotropic ME coefficients ˛L , ˛W , and ˛T of the laminate. Hac and Hdc are both applied along the length (L), width (W), and thickness (T) of the laminate. (b) Measured ˛L − A and phase angle as a function of Hdc for laminate A. (c) Hdc dependence of anisotropic ME coefficients ˛ L , ˛W and ˛T of the laminate.
early with increasing Hdc , due to the increment of piezomagnetic coefficient q = d/dHdc and because ˛ME is usually proportional to q. The ˛L − Hdc dependence shows a peak at Hdc ∼ 52 Oe and then decreases slowly in the high-Hdc range because the magnetostriction coefficient for the Metglas layer reaches the saturated value [24]. It is noted that ˛L = 390 mV/cm Oe, ˛W = 3.0 mV/cm Oe, and ˛T = 2.0 mV/cm Oe, at Hdc ∼ 52 Oe, the peak position for ˛L . The anisotropic factors defined by KW = ˛L /˛W and KT = ˛L /˛T reach up to 130 and 195, respectively. The measured data demonstrate the significant ME coupling anisotropy for the laminate. To understand this sharp anisotropy behavior, one can consult to the anisotropic piezoelectricity of PVDF layer, the in-plane anisotropic piezomagnetism and demagnetization factor of Metglas layer. For this type of laminate composite structures, if the demagnetization effect along the longitudinal direction of laminate is ignored, one can propose a model by using the equivalent-circuit approach which is based on magnetostrictive and piezoelectric constitutive equations, noting that the magnetostrictive and piezoelectric layers are coupled through elastic interaction excited by the magnetic field [25]. When a magnetic field is applied in parallel to the length direction of the laminate, a longitudinal strain in Metglas layer is excited. The piezomagnetic constitutive equations for this longitudinal mode are: S3m = SH33 T3m + q33 H3 , B3 = q33 T3m + T 33 H3
(1)
where B3 and H3 are the magnetization and magnetic field along the length direction, T33 is the permeability under constant stress,
nq33 g31p 2 ) + (1 − n)S nSE11 (1 − k31p H33
,
(4)
where g31p is the piezoelectric voltage coefficient. Clearly, the ME coefficient is proportional to piezomagnetic coefficient q33 and g31p (or d31 ). Therefore, one has ˛E ∼ q33 ·g31p = q33 ·d31 /εT33 . Correspondingly, the three ME coefficients, given the magnetic field applied along the three different directions, can be written as: ˛L ∼ q33 ·d31 /εT33 , ˛W ∼ q32 ·d32 /εT33 , ˛T ∼ q31 ·d33 /εT33 , respectively. The model proposed above allows ˛L ˛W and ˛L ˛T , i.e. significant ME coupling anisotropy of the laminate. The reason is quite clear since for the Metglas layer q32 and q31 are both much smaller than q33 , and for the PVDF layer both d31 and d33 are far larger than d32 , although no reliable data on q31 , q32 , and q33 are available. Nevertheless, what is unclear here is the very small difference between measured ˛W and ˛T , given d32 d33 and q31 ∼ q32 . In fact, as mentioned earlier, the anisotropic behavior of the ME coupling is attributed to the anisotropic piezoelectricity of the PVDF layer and the in-plane anisotropic piezomagnetism of the Metglas layer, the demagnetization effect of the Metglas layer should be taken into account too. The ME coefficient is inversely proportional to the demagnetization factor. For Metglas layer, the demagnetization factors are NL = 4.0 × 10−4 , Nw = 1.40 × 10−3 and NT = 0.9982, referring respectively to the length (L), width (W) and thickness (T) directions. It is noted that Nw NT , indicating that the effective field along the thickness (T) direction is much weaker than that along the width (W) direction. The balance (compromise) between the two types of effects makes ˛T ∼ ˛W , although d32 d33 . Further evidence for the ME coupling anisotropy arises from experiment on the laminate which was prepared with d32 direction of PVDF layer along the length of the laminate while d31 along the width of the laminate. The driving ac magnetic field Hac ∼ 3 Oe at f = 1.0 kHz and external dc magnetic field Hdc in the range of 0 ∼ 23 Oe were applied on the laminate just in the same case with the laminate addressed above. Fig. 2(c) illustrates the Hdc dependence of ME coupling coefficients ˛L , ˛W and ˛T . At Hdc = 13 Oe, the values of ˛L , ˛W and ˛T were observed to be 1.5 mV/cm Oe, −54 mV/cm Oe and 3 mV/cm Oe, respectively. How-
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Fig. 4. Measured ME voltage as a function of Hdc . Both Hdc and Hac are applied along the length direction of the laminate.
Fig. 3. (a) Illustration for measuring the dependence of ˛ME , (b) dependence of ˛ME at Hdc = 5 Oe, Hac = 3 Oe at f = 1.0 kHz.
ever, the anisotropic factors KL = |˛W /˛L | and KT = |˛W /˛T | were evaluated to be 36 and 18, respectively.
can fit the measured data by ˛E () = P1 sin (2+P2 ) + P3 , resulting in P1 = 64.49, P2 = 1.36, P3 = 71.49, indicating the effective ratio K of the maximal to the minimal reaches up to K ∼ 19.4. It is indicated that the ME voltage of our PVDF/Metglas laminate structure can show a high angular dependence with respect to Hdc even if Hac operates at a relatively low and non-resonant frequency of 1.0 kHz. Unfortunately, no data at high frequency are available to us in order to make a comparison with earlier work [17]. Regarding this sensitivity of ˛ME to angle , it should be mentioned that for the isotropic PVDF/Metglas laminate with similar dimensions, ˛ME depends on too. However, the measured ˛ME at = n ± /2 is far from zero, making the direction detection much less sensitive. The measured K is only ∼2.0, much lower than K ∼ 19.4 for the present anisotropic laminate, indicating that the latter exhibits much better performance than the former, in terms of detection of the direction of Hdc .
3.2. Anisotropic response to dc magnetic field 3.3. Detection of weak dc magnetic field This fantastic ME coupling anisotropy can then be used for detection of the direction of Hdc . Given the spatial axis along which Hdc is aligned, we can utilize the phase angle of ˛L (with respect to ac signal Hac ) as a function of Hdc to identify the sign of Hdc . The data are presented in Fig. 2(b). We observe a sharp (step-like) phase shift from 0◦ to 180◦ corresponding to the sign reversal of Hdc from a positive value to a negative one. Therefore, phase angle together with the ME coupling anisotropy makes accessible to detect Hdc as a vector, Hdc . We further explore a detection of the direction of Hdc using this laminate structure (the piezoelectric coefficients d31 of the PVDF layer aligned along magnetization M direction of the Metglas layer). The ac field Hac , generated by a cylinder Helmholtz coil surrounding the laminate, always aligns along the length direction. The laminate together with the coil was rotated with angle defined as the planar angle between the direction of Hdc and Hac . At the beginning, Hdc is set to be parallel to Hac at = 0, as shown in Fig. 3(a). The measured ˛ME at Hac = 3 Oe and f = 1.0 kHz under a Hdc = 5 Oe (fixed both in magnitude and direction), as a function of , is presented in Fig. 3(b). We observe remarkable dependence of ˛ME on , demonstrating the high sensitivity of the ME output to the direction of Hdc . Second, the periodic oscillation of ˛ME against with a periodicity of allows for a determination of Hdc over the whole space range. The maximal ˛ME is reached at = n (n = 0, ±1, ±2, . . .), i.e. Hdc is parallel or antiparallel to the length direction of the laminate. At = n ± /2, ˛ME reaches its minimal which is close to zero. The above results can be understood by analyzing the anisotropic behavior of piezomagnetic coefficient q in the Metglas layer. Also, the magnetostrictive coefficient (33 ) of the Metglas layer depends on the angle of applied dc magnetic field. From equations ˛L ∼ q33 d31 /εT33 and q33 = d33 /dHdc , one learns that ˛L is a function of Hdc . Ignoring the influence from both ˛W and ˛T , we
We demonstrate the ultra-sensitivity of this anisotropic laminate in detecting the magnitude of Hdc . Our measurement was performed with Hdc in a broad range, as shown in Fig. 4. While the weak dc field can be reliably generated in a coil with a dc voltage (rectangle wave) as low as ∼0.01 V, noting that a high-reliable resistor of 600 M is series-wound to the coil. The data show that the ME voltage is almost linearly proportional to Hdc in the log–log coordinates. In fact, similar approach was already utilized by a timedomain capture mode [15]. The high sensitivity of the present structure is due to the following reasons: First, the PVDF layer has a large piezoelectric voltage coefficient (with a small dielectric constant), indicating a high output voltage in response to a small variation of strain. Second, the piezomagnetic coefficient of Metglas is also high. Third, one also needs to take into account of the demagnetization factor. The demagnetization factor of the Metglas layer, e.g. along the longitudinal direction, is relatively small. Generally, the ME coefficient is proportional to the piezomagnetic coefficient and piezoelectric voltage coefficient, and inversely proportional to the demagnetization factor. So, it is understandable that the present structure has high sensitivity. 3.4. Discussion For the detection of Hdc using a single laminate structure, a spatial rotation of the laminate is necessary, which may bring difficulty in practice. A more effective detection of Hdc can be achieved by a combined laminate structure consisting of three pieces of laminate A, B and C, which are configured vertically from one and another, with their length directions aligned respectively along x-, y-, and z-axis, respectively. Given an Hdc aligned arbitrarily, the ME voltage
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X.W. Dong et al. / Sensors and Actuators A 153 (2009) 64–68
and phase angle from the three pieces of laminates can be measured as (˛x , x ), (˛y , y ), and (˛z , z ), respectively. By observing Fig. 2(b), it is known that angles x , y , and z can only be ∼0◦ or ∼180◦ . Using the relationship between ˛ME and Hdc , the three components of H can be evaluated as (Hx , x ), (Hy , y ), (Hz , z ). Consequently, H = (Hx cos x , Hy cos y , Hz cos z ) can be easily obtained with high sensitivity. What should be addressed is that this simple detection mode cannot be realized using the isotropic PVDF/Metglas laminate since the ME coefficient ˛i at i = n ± /2 (i = x, y, z) is far from zero in that case. 4. Conclusion In summary, the bilayer PVDF/Metglas laminates with anisotropic ME effect have been fabricated. The remarkably anisotropic ME effect can be achieved by combining the anisotropic piezoelectric property of PVDF layer with anisotropic piezomagnetic property of Metglas layer. By a careful evaluation of these anisotropic behaviors, we demonstrate an ultra-sensitive detection of weak dc magnetic field and its spatial orientation using the bilayer PVDF/Metglas laminate. We propose a simple detector for weak magnetic field in three-dimensional space. Acknowledgements This work was supported by the National Key Projects for Basic Research of China (2009CB623303), the National Natural Science Foundation of China (50832002), and the Jiangsu Natural Science Foundation of China (BK2008024). References [1] M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) R123. [2] M. Avellaneda, G. Harshe, J. Intell. Mater. Syst. Struct. 5 (1994) 501.
[3] J. Ryu, S. Priya, K. Uchino, H.E. Kim, J. Electrocream. 8 (2002) 107. [4] S.X. Dong, J.Y. Zhai, J.F. Li, D. Viehland, Appl. Phys. Lett. 88 (2006) 082907. [5] J.G. Wan, J.–M. Liu, G.H. Wang, C.W. Nan, Appl. Phys. Lett. 88 (2006) 182502. [6] X.W. Dong, Y.J. Wu, J.G. Wan, T. Wei, Z.H. Zhang, S. Chen, H. Yu, J.–M. Liu, J. Phys. D: Appl. Phys. 41 (2008) 035003. [7] S.X. Dong, J.F. Li, D. Viehland, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 50 (2003) 1253. [8] I. Levin, J. Li, J. Slutsker, A.L. Roytburd, Adv. Mater. 18 (2006) 2044. [9] H. Zheng, F. Straub, Q. Zhan, P. Yang, W. Hsieh, F. Zavaliche, Y. Chu, U. Dahmen, R. Ramesh, Adv. Mater. 18 (2006) 2747. [10] R. Ramesh, N.A. Spaldin, Nature Mater. 6 (2007) 21. [11] Y.H. Chu, L.W. Martinl, M.B. Holcomb, M. Gajek, S.-J. Han, Q. He, N. Balke, C.-H. ´ Yang, D. Lee, W. Hu, Q. Zhan, P.-L. Yang, A. Fraile-Rodrlguez, A. Scholl, S.X. Wang, R. Ramesh, Nature Mater. 7 (2008) 478. [12] J. Ma, Z. Shi, C.W. Nan, Adv. Mater. 19 (2008) 2571. [13] J.G. Wan, Z.Y. Li, Y. Wang, M. Zeng, G.H. Wang, J.–M. Liu, Appl. Phys. Lett. 86 (2005) 202504. [14] D.V. Chashin, Y.K. Fetisov, K.E. Kamentsev, G. Srinivasan, Appl. Phys. Lett. 92 (2008) 102511. [15] J.Y. Zhai, Z.P. Xing, S.X. Dong, J.F. Li, D. Viehland, Appl. Phys. Lett. 88 (2006) 062510; J.Y. Zhai, Z.P. Xing, S.X. Dong, J.F. Li, D. Viehland, Appl. Phys. Lett. 89 (2006) 083507; S.X. Dong, J.Y. Zhai, F.M. Bai, J.F. Li, D. Viehland, Appl. Phys. Lett. 87 (2005) 062502. [16] A.I. Rashed, P. Shashank, Appl. Phys. Lett. 89 (2006) 152911. [17] J.P. Zhou, Y.Y. Guo, Z.Z. Xi, P. Liu, S.Y. Lin, G. Liu, H.W. Zhang, Appl. Phys. Lett. 93 (2008) 152501. [18] C.W. Nan, L. Liu, N. Cai, J. Zhai, Y. Ye, Y.H. Lin, Appl. Phys. Lett. 81 (2002) 3831. [19] K. Mori, M. Wuttig, Appl. Phys. Lett. 81 (2002) 100. [20] N. Cai, C.W. Nan, J.Y. Zhai, Y.H. Lin, Appl. Phys. Lett. 84 (2004) 3516. [21] Y.H. Lin, N. Cai, J.Y. Zhai, G. Liu, C.W. Nan, Phys. Rev. B 72 (2005) 012405. [22] J.Y. Zhai, S.X. Dong, Z.P. Xing, J.F. Li, D. Viehland, Appl. Phys. Lett. 89 (2006) 083507. [23] A. Preumont, Vibration Control of Active Structures: An Introduction, Kluwer Academic Publishers, Dordrecht, 2003, p. 388. [24] J.G. Wan, X.W. Wang, Y.J. Wu, M. Zeng, Y. Wang, H. Jiang, W.Q. Zhou, G.H. Wang, J.–M. Liu, Appl. Phys. Lett. 86 (2005) 122501. [25] C.W. Nan, M.I. Bichurin, S.X. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Ultra-sensitive detection of magnetic field and its direction using bilayer PVDF/Metglas laminate X.W. Dong a , B. Wang a , K.F. Wang a , J.G. Wan a , J.–M. Liu a,b,c,∗ a b c
Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China School of Physics, South China Normal University, Guangzhou 510006, China International Center for Materials Physics, Chinese Academy of Sciences, Shenyang, China
a r t i c l e
i n f o
Article history: Received 19 January 2009 Received in revised form 19 March 2009 Accepted 23 April 2009 Available online 3 May 2009
a b s t r a c t Bilayer PVDF/Metglas laminates with remarkably anisotropic ME coupling effect have been fabricated. By a careful evaluation of these anisotropic behaviors both experimentally and theoretically, we demonstrate that an ultra-sensitive detection of weak dc magnetic field and its spatial orientation is practically applicable using the bilayer PVDF/Metglas laminate. This magnetic field probe has quite high sensitivity. The reason for such a high magnitude and orientation-dependent magnetoelectric coupling is explained.
PACS: 75.80.+q 85.80.Jm 77.74 Dy
© 2009 Elsevier B.V. All rights reserved.
Keywords: Magnetoelectric coupling Laminate Magnetic field detection
1. Introduction Magnetoelectric (ME) effect characterized by an induced electric polarization under an applied magnetic field and vice versa, can be seen as a bridge between the electric polarization and magnetization [1]. Since the ME effect as a strong function of external dc magnetic field, Hdc , was observed, it has drawn special attention due to its potential and important applications in detection of weak dc magnetic field [1–6]. It is well known that a strong ME effect at room temperature can be easily obtained from a multiferroic composite which is generally made by combining piezoelectric and magnetostrictive components [1–14]. Up to now, a number of multiphase ceramic composites have been synthesized and characterized in order to demonstrate the detection of weak Hdc , noting that the sensitive detection of external ac magnetic field has been demonstrated by Viehland and co-workers [15]. Among these ME composites reported so far, lead zirconate titanate (PZT) ceramics have been widely used as the piezoelectric component, owing to its high piezoelectric constant and large
∗ Corresponding author at: Laboratory of Solid State Microstructures, Nanjing University, Hankou Road No. 22, Nanjing 210093, China. Tel.: +86 25 83596595; fax: +86 25 83595535. E-mail address: [email protected] (J.–M. Liu). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.04.033
electromechanical coupling. At the same time, the giant magnetostrictive alloy Tb1−x Dyx Fe2 (Terfenol-D) has often been used as the magnetostrictive component. The ME effect of these composites is not very anisotropic in response to Hdc due to the fact that the piezoelectric and piezomagnetic coefficients in these composite sensors are not very orientation-dependent in prior to poling. These magnetic sensors are capable of detecting the magnitude component of Hdc given its fixed direction, while a determination of both the magnitude and direction of Hdc seems challenging. In addition, the detection sensitivity using these composite sensors still has space to improve. Furthermore, they are mechanically inflexible and thus inaccessible to some specific applications. It should be reminded that so far only a few reports focused on the anisotropic ME coefficient in response to ac or dc magnetic field. Rashed and Shashank reported the angular dependence of ME coefficient for co-fired BaTiO3 -(Ni0.8 Zn0.2 )Fe2 O4 bilayer composite upon ac magnetic field [16]. More recently, the electric-field-induced magnetization (EIM) anisotropy in laminate composite structure consisting of Terfenol-D component and 0.68[Pb(Mg1/3 Nb2/3 )O3 ]-0.32PbTiO3 component was investigated [17], and an angular dependence of the EIM variation at the electromechanical resonance frequency ∼185 kHz was measured. However, up to date no report on the angular dependence of magnetic-field-induced electric polarization (MIEP) in response to a dc magnetic field Hdc , imposed onto an ac magnetic field Hac has
X.W. Dong et al. / Sensors and Actuators A 153 (2009) 64–68
not yet been available. Such an effect has potential application in detection of weak Hdc rather than Hac . The motivation of this work is to develop a ME composite structure exhibiting anisotropic MIEP. Recently, polynivylidence-fluoride (PVDF) piezopolymer was also incorporated into ME laminate [18–22], because it owns many advantages such as soft elastic constants, high resistivity, and high piezoelectric voltage coefficient, etc. A giant ME effect in a three-layer PVDF/Metglas laminate, which is mechanically flexible, was reported by Viehland and co-workers [22]. As well known, the piezoelectricity of PVDF is generally obtained by submitting the PVDF film to a sufficient high electric field after a mechanical stretching [23]. The most attractive feature is that the anisotropic piezoelectric performance can be obtained by mechanically stretching the PVDF film in one or two perpendicular directions prior to the polarization process. A uni-axial stretching will induce in-plane unidirectional piezoelectric coefficient, while a bi-axial stretching of the film will induce in-plane orientation-dependent piezoelectric property (strictly speaking, biaxial piezoelectric coefficient). As a consequence, an in-plane and highly anisotropic piezoelectric coefficient for PVDF film can be obtained simply by a uni-axial stretching. Similarly, if the magnetostrictive component shows in-plane anisotropic piezomagnetic property too, the resulting ME composite structure would exhibit anisotropic coupling which can be modulated with high degrees of freedom through different combinations of the in-plane piezoelectric and piezomagnetic properties of the two components. Along this line, a ME composite laminate structure for detecting not only the magnitude but also the direction of Hdc (i.e. detecting Hdc as a vector Hdc ) can be designed and fabricated. By a similar strategy, a detection of external ac magnetic field, Hac , is also accessible, which will not be addressed here since it is even simpler than detecting Hdc . In this work, we investigate the ME effect of a bilayer composite structure consisting of PVDF film with in-plane anisotropic piezoelectricity as the piezoelectric component and flexible/thin iron-based Metglas as the magnetostrictive component, and explore its high angular dependence with respect to Hdc and ultra-sensitivity of detecting weak Hdc . The Metglas has the in-plane anisotropic piezomagnetism, high piezomagnetic coefficient and huge magnetic permeability (r > 40,000). The bilayer PVDF/Metglas laminates were prepared, with the piezoelectric coefficients (d31 =25 pC/N) of the PVDF layer aligned along magnetization (M) direction of the Metglas layer, along which the piezomagnetic coefficient is extremely high with respect to Terfenol-D under low magnetic field.
2. Experimental details The PVDF thin films used as piezoelectric components in this work were commercially purchased (Shenzhen Mingschin Highpolymer Technology Co. Ltd., China), with 28 m in thickness. It was transversely poled and fitted with electrode on the top and bottom surfaces. The piezoelectric coefficients are d31 = 25 pC/N, d32 = 2.0 pC/N and d33 = 25 pC/N, indicating large in-plane anisotropy, as shown schematically in Fig. 1(a). The magnetostrictive Metglas-SA1-MP4010XGDC layers were bought from Metglas Inc., USA, with 25 m in thickness and the magnetization was aligned along the longitudinal (length) direction. Both the PVDF layer and Metglas layer were glued together using epoxy binder, with a dimension of 20 mm in length and 10 mm in width. In Fig. 1(b) is given a photograph of the PVDF/Metglas laminate. As for the ME measurement, the detailed procedure can be found in [6]. The ac magnetic field Hac was produced from a sync signal using a Helmholtz coil. This sync signal was generated by a signal generator and amplified by a function amplifier. The dc magnetic
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Fig. 1. (a) An illustration of the laminate structure with three piezoelectric coefficients for the PVDF layer. (b) Photograph of a PVDF/Metglas laminate.
field Hdc was supplied by an electromagnet. The induced voltage and phase in PVDF film were measured using a lock-in amplifier (SRS Inc., SR830). The reference signal for the phase measurement came from the sync signal of the signal generator which produced Hac . 3. Results and discussion We look at the bilayer PVDF/Metglas laminate, which was prepared by aligning the d31 of PVDF layer along the length direction. This is coincident with the direction of M for the Metglas layer. Thus, the d32 of PVDF layer is aligned along the width direction, as seen in Fig. 1. 3.1. Anisotropic ME coupling We first demonstrate the anisotropic ME coupling of this laminate structure. During the measurement, the ac magnetic field was set to Hac = 3.0 Oe with frequency f = 1.0 kHz and such a choice was made without any specific consideration. The low-level Hdc in the range of 0–200 Oe in magnitude was supplied by a high-precision electromagnet. Fig. 2(a) shows the Hdc dependence of ME voltage coefficient (˛ME ) when Hac and Hdc are both applied along the directions of length (L), width (W), and thickness (T) of the laminate, respectively, regardless of the sign of Hdc along this direction. It is clearly seen that the ME coefficient along the length direction, ˛L , is remarkably larger than the other ones (˛W and ˛T ) over the whole Hdc range. As Hdc is low, ˛L increases approximately lin-
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SH33 is the elastic compliance under a constant magnetic field, q33 is the longitudinal piezomagnetic constant, T3m and S3m are the stress and strain in the longitudinal direction of Metglas layer. For the case that the PVDF layer is poled along the thickness direction, the piezoelectric constitutive equations are: S1p = SE11 T1p + d31 E3 , D3 = d31 T1p + εT 33 E3
(2)
where D3 and E3 are the electric displacement and electric field along the thickness direction, S1p and T1p are the strain and stress of PVDF layer imposed by Metglas layer, SE11 and εT33 are the elastic compliances under constant electric field E and dielectric permittivity under constant stress T, and d31 is the piezoelectric constant. Assuming that the layers in the laminate act only in one coupled mode, from Newton’s second law, the piezoelectric and piezomagnetic constitutive equations can be coupled into the equation of motion:
∂T1p ∂2 u(z) ∂T3m + (1 − n) =n , ∂z ∂z ∂t 2
(3)
where = (1 − n)p + nM is the average mass density of the laminate, p and M are the density of PVDF and Metglas, n is the geometric factor of Metglas layer and u(z) is displacement defined along the z-axis (thickness direction). Given the model proposed above, the ME equivalent-circuit under free boundary conditions can be described by substituting Eqs. (1) and (2) into Eq. (3). In the open-circuit conditions, the ME coefficient at low frequency can be written as: ˛E =
Fig. 2. (a) Hdc dependence of anisotropic ME coefficients ˛L , ˛W , and ˛T of the laminate. Hac and Hdc are both applied along the length (L), width (W), and thickness (T) of the laminate. (b) Measured ˛L − A and phase angle as a function of Hdc for laminate A. (c) Hdc dependence of anisotropic ME coefficients ˛ L , ˛W and ˛T of the laminate.
early with increasing Hdc , due to the increment of piezomagnetic coefficient q = d/dHdc and because ˛ME is usually proportional to q. The ˛L − Hdc dependence shows a peak at Hdc ∼ 52 Oe and then decreases slowly in the high-Hdc range because the magnetostriction coefficient for the Metglas layer reaches the saturated value [24]. It is noted that ˛L = 390 mV/cm Oe, ˛W = 3.0 mV/cm Oe, and ˛T = 2.0 mV/cm Oe, at Hdc ∼ 52 Oe, the peak position for ˛L . The anisotropic factors defined by KW = ˛L /˛W and KT = ˛L /˛T reach up to 130 and 195, respectively. The measured data demonstrate the significant ME coupling anisotropy for the laminate. To understand this sharp anisotropy behavior, one can consult to the anisotropic piezoelectricity of PVDF layer, the in-plane anisotropic piezomagnetism and demagnetization factor of Metglas layer. For this type of laminate composite structures, if the demagnetization effect along the longitudinal direction of laminate is ignored, one can propose a model by using the equivalent-circuit approach which is based on magnetostrictive and piezoelectric constitutive equations, noting that the magnetostrictive and piezoelectric layers are coupled through elastic interaction excited by the magnetic field [25]. When a magnetic field is applied in parallel to the length direction of the laminate, a longitudinal strain in Metglas layer is excited. The piezomagnetic constitutive equations for this longitudinal mode are: S3m = SH33 T3m + q33 H3 , B3 = q33 T3m + T 33 H3
(1)
where B3 and H3 are the magnetization and magnetic field along the length direction, T33 is the permeability under constant stress,
nq33 g31p 2 ) + (1 − n)S nSE11 (1 − k31p H33
,
(4)
where g31p is the piezoelectric voltage coefficient. Clearly, the ME coefficient is proportional to piezomagnetic coefficient q33 and g31p (or d31 ). Therefore, one has ˛E ∼ q33 ·g31p = q33 ·d31 /εT33 . Correspondingly, the three ME coefficients, given the magnetic field applied along the three different directions, can be written as: ˛L ∼ q33 ·d31 /εT33 , ˛W ∼ q32 ·d32 /εT33 , ˛T ∼ q31 ·d33 /εT33 , respectively. The model proposed above allows ˛L ˛W and ˛L ˛T , i.e. significant ME coupling anisotropy of the laminate. The reason is quite clear since for the Metglas layer q32 and q31 are both much smaller than q33 , and for the PVDF layer both d31 and d33 are far larger than d32 , although no reliable data on q31 , q32 , and q33 are available. Nevertheless, what is unclear here is the very small difference between measured ˛W and ˛T , given d32 d33 and q31 ∼ q32 . In fact, as mentioned earlier, the anisotropic behavior of the ME coupling is attributed to the anisotropic piezoelectricity of the PVDF layer and the in-plane anisotropic piezomagnetism of the Metglas layer, the demagnetization effect of the Metglas layer should be taken into account too. The ME coefficient is inversely proportional to the demagnetization factor. For Metglas layer, the demagnetization factors are NL = 4.0 × 10−4 , Nw = 1.40 × 10−3 and NT = 0.9982, referring respectively to the length (L), width (W) and thickness (T) directions. It is noted that Nw NT , indicating that the effective field along the thickness (T) direction is much weaker than that along the width (W) direction. The balance (compromise) between the two types of effects makes ˛T ∼ ˛W , although d32 d33 . Further evidence for the ME coupling anisotropy arises from experiment on the laminate which was prepared with d32 direction of PVDF layer along the length of the laminate while d31 along the width of the laminate. The driving ac magnetic field Hac ∼ 3 Oe at f = 1.0 kHz and external dc magnetic field Hdc in the range of 0 ∼ 23 Oe were applied on the laminate just in the same case with the laminate addressed above. Fig. 2(c) illustrates the Hdc dependence of ME coupling coefficients ˛L , ˛W and ˛T . At Hdc = 13 Oe, the values of ˛L , ˛W and ˛T were observed to be 1.5 mV/cm Oe, −54 mV/cm Oe and 3 mV/cm Oe, respectively. How-
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Fig. 4. Measured ME voltage as a function of Hdc . Both Hdc and Hac are applied along the length direction of the laminate.
Fig. 3. (a) Illustration for measuring the dependence of ˛ME , (b) dependence of ˛ME at Hdc = 5 Oe, Hac = 3 Oe at f = 1.0 kHz.
ever, the anisotropic factors KL = |˛W /˛L | and KT = |˛W /˛T | were evaluated to be 36 and 18, respectively.
can fit the measured data by ˛E () = P1 sin (2+P2 ) + P3 , resulting in P1 = 64.49, P2 = 1.36, P3 = 71.49, indicating the effective ratio K of the maximal to the minimal reaches up to K ∼ 19.4. It is indicated that the ME voltage of our PVDF/Metglas laminate structure can show a high angular dependence with respect to Hdc even if Hac operates at a relatively low and non-resonant frequency of 1.0 kHz. Unfortunately, no data at high frequency are available to us in order to make a comparison with earlier work [17]. Regarding this sensitivity of ˛ME to angle , it should be mentioned that for the isotropic PVDF/Metglas laminate with similar dimensions, ˛ME depends on too. However, the measured ˛ME at = n ± /2 is far from zero, making the direction detection much less sensitive. The measured K is only ∼2.0, much lower than K ∼ 19.4 for the present anisotropic laminate, indicating that the latter exhibits much better performance than the former, in terms of detection of the direction of Hdc .
3.2. Anisotropic response to dc magnetic field 3.3. Detection of weak dc magnetic field This fantastic ME coupling anisotropy can then be used for detection of the direction of Hdc . Given the spatial axis along which Hdc is aligned, we can utilize the phase angle of ˛L (with respect to ac signal Hac ) as a function of Hdc to identify the sign of Hdc . The data are presented in Fig. 2(b). We observe a sharp (step-like) phase shift from 0◦ to 180◦ corresponding to the sign reversal of Hdc from a positive value to a negative one. Therefore, phase angle together with the ME coupling anisotropy makes accessible to detect Hdc as a vector, Hdc . We further explore a detection of the direction of Hdc using this laminate structure (the piezoelectric coefficients d31 of the PVDF layer aligned along magnetization M direction of the Metglas layer). The ac field Hac , generated by a cylinder Helmholtz coil surrounding the laminate, always aligns along the length direction. The laminate together with the coil was rotated with angle defined as the planar angle between the direction of Hdc and Hac . At the beginning, Hdc is set to be parallel to Hac at = 0, as shown in Fig. 3(a). The measured ˛ME at Hac = 3 Oe and f = 1.0 kHz under a Hdc = 5 Oe (fixed both in magnitude and direction), as a function of , is presented in Fig. 3(b). We observe remarkable dependence of ˛ME on , demonstrating the high sensitivity of the ME output to the direction of Hdc . Second, the periodic oscillation of ˛ME against with a periodicity of allows for a determination of Hdc over the whole space range. The maximal ˛ME is reached at = n (n = 0, ±1, ±2, . . .), i.e. Hdc is parallel or antiparallel to the length direction of the laminate. At = n ± /2, ˛ME reaches its minimal which is close to zero. The above results can be understood by analyzing the anisotropic behavior of piezomagnetic coefficient q in the Metglas layer. Also, the magnetostrictive coefficient (33 ) of the Metglas layer depends on the angle of applied dc magnetic field. From equations ˛L ∼ q33 d31 /εT33 and q33 = d33 /dHdc , one learns that ˛L is a function of Hdc . Ignoring the influence from both ˛W and ˛T , we
We demonstrate the ultra-sensitivity of this anisotropic laminate in detecting the magnitude of Hdc . Our measurement was performed with Hdc in a broad range, as shown in Fig. 4. While the weak dc field can be reliably generated in a coil with a dc voltage (rectangle wave) as low as ∼0.01 V, noting that a high-reliable resistor of 600 M is series-wound to the coil. The data show that the ME voltage is almost linearly proportional to Hdc in the log–log coordinates. In fact, similar approach was already utilized by a timedomain capture mode [15]. The high sensitivity of the present structure is due to the following reasons: First, the PVDF layer has a large piezoelectric voltage coefficient (with a small dielectric constant), indicating a high output voltage in response to a small variation of strain. Second, the piezomagnetic coefficient of Metglas is also high. Third, one also needs to take into account of the demagnetization factor. The demagnetization factor of the Metglas layer, e.g. along the longitudinal direction, is relatively small. Generally, the ME coefficient is proportional to the piezomagnetic coefficient and piezoelectric voltage coefficient, and inversely proportional to the demagnetization factor. So, it is understandable that the present structure has high sensitivity. 3.4. Discussion For the detection of Hdc using a single laminate structure, a spatial rotation of the laminate is necessary, which may bring difficulty in practice. A more effective detection of Hdc can be achieved by a combined laminate structure consisting of three pieces of laminate A, B and C, which are configured vertically from one and another, with their length directions aligned respectively along x-, y-, and z-axis, respectively. Given an Hdc aligned arbitrarily, the ME voltage
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and phase angle from the three pieces of laminates can be measured as (˛x , x ), (˛y , y ), and (˛z , z ), respectively. By observing Fig. 2(b), it is known that angles x , y , and z can only be ∼0◦ or ∼180◦ . Using the relationship between ˛ME and Hdc , the three components of H can be evaluated as (Hx , x ), (Hy , y ), (Hz , z ). Consequently, H = (Hx cos x , Hy cos y , Hz cos z ) can be easily obtained with high sensitivity. What should be addressed is that this simple detection mode cannot be realized using the isotropic PVDF/Metglas laminate since the ME coefficient ˛i at i = n ± /2 (i = x, y, z) is far from zero in that case. 4. Conclusion In summary, the bilayer PVDF/Metglas laminates with anisotropic ME effect have been fabricated. The remarkably anisotropic ME effect can be achieved by combining the anisotropic piezoelectric property of PVDF layer with anisotropic piezomagnetic property of Metglas layer. By a careful evaluation of these anisotropic behaviors, we demonstrate an ultra-sensitive detection of weak dc magnetic field and its spatial orientation using the bilayer PVDF/Metglas laminate. We propose a simple detector for weak magnetic field in three-dimensional space. Acknowledgements This work was supported by the National Key Projects for Basic Research of China (2009CB623303), the National Natural Science Foundation of China (50832002), and the Jiangsu Natural Science Foundation of China (BK2008024). References [1] M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) R123. [2] M. Avellaneda, G. Harshe, J. Intell. Mater. Syst. Struct. 5 (1994) 501.
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