Strong flexural resonant magnetoelectric effect in Terfenol-D/epoxy-Pb „ Zr, Ti … O
3bilayer
J. G. Wan,a兲 Z. Y. Li, Y. Wang, M. Zeng, G. H. Wang, and J.-M. Liu
Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China and International Center for Materials Physics, Chinese Academy of Sciences, Shenyang, China
共Received 26 January 2005; accepted 9 April 2005; published online 12 May 2005兲
We report the bending resonant magnetoelectric effect observed in the bilayer of magnetostrictive Tb0.30Dy0.7Fe2共Terfenol-D兲/epoxy and piezoelectric Pb共Zr0.52Ti0.48兲O3共PZT兲. It has been found that the bilayer has both giant magnetoelectric voltage coefficient and high coupling efficiency when it operates at the first bending resonance mode. The maximum bending resonant magnetoelectric voltage coefficient of 14.6 V / cm Oe at quite low frequency of 12.5 kHz was observed in the bilayer of small sizes of 17.4⫻4.0⫻1.44 mm3. This bending-resonance-type magnetoelectric effect shows promising application in transducers for magnetoelectric energy conversion. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1935040兴
Magnetoelectric 共ME兲 effect has been receiving great and continuous interest due to its potential applications in sensors, memory device, and transducers for magnetoelectric energy conversion.1–3A large ME effect could be realized in a layered composite of the piezoelectric phase and the mag- netostrictive phase by the product property.4That is, a mag- netic field applied to the layered ME composite results in a mechanical strain in the magnetostrictive layer that is subse- quently transferred to the piezoelectric layer by the mechani- cal stress-mediated interface coupling and lead to an electric polarization. In the past several years, various layered ME composites, such as Tb1−xDyxFe2−y共Terfenol -D兲/ Pb共Zr, Ti兲O3共PZT兲, ferrite / PZT, and Terfenol-D/PVDF laminates, were widely investigated.5–11 The reported ME voltage coefficient is generally below⬃5.0 V / cm Oe.
Recently, people found that the frequency affected sig- nificantly the ME coupling in the laminates. When the lami- nate operates in the resonance mode, its ME effect could be enhanced largely, generally yielding a ME voltage output of nearly two orders of magnitude over that of the nonresonant ME laminates.12–16To date, many experiments and calcula- tions have been performed to optimize the resonant ME out- put for the laminates. The developed ME resonance modes include the longitudinal mode 共for the rectangle laminates兲 共Ref. 13兲and radial mode 共for disk laminates兲.12It has been proved that the resonant ME laminates could be used poten- tially for the high-voltage miniature transformer applications.14However, a problem for the current ME reso- nance modes is that the operating frequencies are generally high, which could bring about significant eddy current loss for the magnetostrictive phase, especially for the large mag- netostrictive earth rare alloys such as Terfenol-D, resulting in an inefficient magnetoelectric energy conversion. An alterna- tive solution is to reduce the operating frequency. Unfortu- nately it will make the size of the laminate largely increase, which is disadvantageous to the actual application.
In this letter, we propose a bending-resonance-type ME bilayer. We note that the flexural deformation will take place if an external magnetic field is applied to the
magnetostrictive/piezoelectric bilayer due to the nonsym- metrical stress distribution in the magnetostrictive and piezo- electric layers. When the bilayer operates in the bending vi- bration mode, its resonance frequency will be much lower than that of the other modes; meanwhile its size could be kept quite small and ME coupling is strong. In this Letter, we report our experiments on the flexural resonant ME bilayer of magnetostrictive Terfenol-D 共Tb0.30Dy0.7Fe2兲/ Epoxy and piezoelectric PZT关Pb共Zr0.52Ti0.48兲O3兴, which yields a giant ME output with a quite low operating frequency and a high coupling efficiency.
Our ME bilayer was prepared by stacking and bonding the rectangle Terfenol-D / Epoxy composite 共TEC兲 and PZT-5 pieces with epoxy binder. The PZT layer was polar- ized in the thickness direction and the TEC layer was mag- netized along the longitudinal direction, as shown in Fig. 1.
The volume content of Terfenol-D in the TEC is 0.8. The preparation process of the TEC can be found elsewhere.17 The typical piezomagnetic coefficient d31,t of TEC and the piezoelectric coefficient d31,p of PZT were measured to be 5.9 nm/ A at 1 kOe and 155 pm/ V, respectively.
Both the TEC and PZT layers have the
same sizes of 17.4 mm共length, l兲⫻4.5 mm共width, w兲
⫻0.72 mm共thickness, d / 2兲.
We firstly analyzed the vibrational modes of the bilayer.
Before bonding, the first longitudinal magnetomechanical resonance共MMR兲for the TEC piece and electromechanical resonance共EMR兲 for the PZT piece occur at 50.4 and 86.5 kHz, respectively. After bonding, both the MMR and EMR frequencies of the bilayer tend to be consistent, and the first
a兲Author to whom correspondence should be addressed; electronic mail:
FIG. 1. Configuration and operation principle of the bending vibrational magnetoelectric TEC/PZT bilayer. The arrows P and M show the polarized direction of PZT and the magnetized direction of TEC, respectively.
APPLIED PHYSICS LETTERS 86, 202504共2005兲
0003-6951/2005/86共20兲/202504/3/$22.50 86, 202504-1 © 2005 American Institute of Physics Downloaded 20 Mar 2010 to 219.219.118.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
longitudinal resonance of the bilayer shifts to a frequency between 50.4 and 86.5 kHz due to the acoustic velocity dif- ference between the TEC and PZT. The average acoustic velocity V¯ of the bilayer can be derived from the classical plate theory combined with a simple composite principle
V¯ = 1/
冑
s11, 共1兲where¯ is the average density of the bilayer that is deter- mined by¯ = vtt+vpp, and s11is the equivalent elastic com- pliance of the bilayer that is followed by the parallel com- posite law and given by s11= s11t s11p /共vps11t +vts11p兲, where t
orp are the density of the TEC and PZT, s11t or s11p are the elastic compliance of the TEC and PZT, vt or vp are the volume content of the TEC and PZT, respectively. The lon- gitudinal resonance frequency is thus evaluated by
fnL= n
2lV¯ . 共2兲
Meanwhile, the flexural vibrational modes appear in the bilayer due to the flexural deformation caused by the non- symmetrical stress distribution in the TEC and PZT layers.
Assuming that the two sides of the bilayer are free and ap- plying the one-dimensional approximation, we describe the bending resonance frequencies of the bilayer as follows:
fnB= d 2
冑
12l2V¯n2, 共3兲
where n⬇n + 1 / 2. Using the typical bilayer parameters listed in Table I, we predicted the first, second, and third bending resonance frequencies of the bilayer to be 11.7, 32.5, and 63.7 kHz, and 69.0 kHz for the first longitudinal reso- nance frequency. The further vibration modal analyses for the bilayer were subsequently performed by using a laser doppler vibrometer 共Polytec, Germany兲. Figure 2 presents the measured frequency response of the surface displacement of one free side of the bilayer induced by an ac magnetic field of 5 Oe at magnetic bias of 0.7 kOe. One easily iden-
tifies that the resonance peak at 69.2 kHz is attributed to the first longitudinal resonance, while the resonance peaks at 12.1, 33.2, 60.1 kHz are ascribed to the first, second, and third bending resonance modes, respectively. The experimen- tal and predicted results are well in agreement with each other. Modal analyses for the bilayer indicates that the first bending resonance frequency was much lower, only about one sixth of the first longitudinal resonance frequency. If such low frequency is required for the first longitudinal reso- nance, the length of the bilayer must be extended to be as long as⬃100 mm.
We further investigate the magnetoelectric coupling in the bilayer. Figure 3共a兲 presents the frequency response of magnetic flux density for the TEC and electric impedance for the PZT in the bilayer. The effective electromechanical cou- pling coefficient kpfor the PZT and magnetomechanical cou- pling coefficient kmfor the TEC can be evaluated from the resonance frequency fr and antiresonance frequency fa, in the expression
km or p=关1 −共fr/fa兲2兴1/2. 共4兲 By taking the characteristic frequencies from Fig. 3共a兲, the kp for the first bending resonance is calculated to be ⬃0.33, slightly higher than that for the first longitudinal resonance, which is⬃0.26. While the kmfor the first bending resonance is ⬃0.51, much higher than that for the first longitudinal resonance, which is only ⬃0.21. If the energy loss in the
TABLE I. Bilayer parameters for the TEC and PZT.
torp共kg/ m3兲 s11t or s11p共m2/ N兲 vtorvp
TEC 7.64⫻103 42.5⫻10−12 0.50
PZT-5 7.70⫻103 15.4⫻10−12 0.50
FIG. 2. Frequency response of the surface displacement of one free side of the bilayer induced by an ac magnetic field of 5 Oe at HBias= 0.7 kOe.
FIG. 3.共a兲Frequency response of magnetic flux density for TEC piece and electric impedance for the PZT piece in the bilayer.共b兲Frequency depen- dence of␣Efor the bilayer. An optimal HBias= 0.7 kOe was set for maximum ME output.
202504-2 Wanet al. Appl. Phys. Lett. 86, 202504共2005兲
Downloaded 20 Mar 2010 to 219.219.118.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
interface coupling is neglected, we evaluate the effective magnetoelectric coupling coefficient by keff= kmkp. Accord- ingly, the evaluated keff for the first bending resonance is
⬃0.17, being about three times of that for the first longitu- dinal resonance, which is only⬃0.05.
The ME voltage coefficient␣E= dE / dH, which is deter- mined by the induced electric field E under a small ac mag- netic field H = 5 Oe, was then tested for the bilayer. The ac magnetic field H was superimposed onto a dc magnetic bias HBiasand both were parallel to the longitudinal direction of the bilayer. The induced electric field E was measured using a lock-in amplifier共SRS Inc, SR830兲. Figure 3共b兲 plots the profile of ␣E vs frequency for the bilayer at an optimized HBias= 0.7 kOe at which the ME effect is maximal. The maximum values of␣E= 14.6 V / cm Oe at 12.5 kHz for the first bending resonance and␣E= 19.9 V / cm Oe at 71.6 kHz for the first longitudinal resonance were observed. The ME output in the second and third bending resonance modes were not observed due to their weak vibration. According to the resonant ME frequency frand 3-dB frequency bandwidth
⌬f, we obtained the effective mechanical quality factor Qm
= fr/⌬f to be⬃48 for the first bending resonance and⬃51 for the first longitudinal resonance.
It is well known that the energy conversion efficiency for a transducer is relative to the effective mechanical quality factor Qmand the effective coupling coefficient keff.18 High keffand Qmvalues will lead to a large value. With respect to the ME coupling of the bilayer operated in the first bend- ing resonance mode, the effective magnetoelectric coupling coefficient keffis much higher, while its Qmvalue is compa- rable to that of the first longitudinal resonance, though the␣E
value induced by the first bending resonance is somewhat lower than that of the first longitudinal resonance. Therefore, we infer that the magnetoelastoelectric energy conversion operated in the first bending resonance mode is more effi- cient in the bilayer. In addition, the much smaller geometric size for the bending resonance mode is also advantageous to miniaturize the devices. These advantages show that this magnetostrictive/piezoelectric bilayer operated at the bend- ing resonance mode is more suitable to the transducer appli- cation for magnetoelectric energy conversion.
In summary, a giant ME voltage output operated at quite low frequency has been found in the first bending resonant TEC/PZT bilayer with very small size. It was also shown that the first bending resonant ME coupling efficiency was higher than the others. The current experimental results indi- cate that this bending-resonance-type ME effect has promis- ing applications in the transducer devices for magnetoelectric energy conversion.
This work was supported by the Provincial Nature Sci- ence Foundation of Jiangsu in China 共Grant No.
BK2003412兲, the National Nature Science Foundation of China共Grants No. 50332020, 10474039, 10021001兲, the Na- tional Key Projects for Basic Research of China共Grant No.
2002CB613303兲, and LSSMS of Nanjing University.
1T. H. O’Dell, Electron. Power 11, 266共1965兲.
2L. P. M. Bracke and R. G. van Vliet, Int. J. Electron. 51, 255共1981兲.
3S. X. Dong, J. F. Li, and D. Viehland, Appl. Phys. Lett. 83, 2265共2003兲.
4J. van den Boomgaard and R. A. J. Born, J. Mater. Sci. 13, 1538共1978兲.
5G. Srinivasan, E. T. Rasmussen, J. Gallegos, R. Srinivasan, Yu. I. Bokhan, and V. M. Laletin, Phys. Rev. B 64, 214408共2001兲.
6G. Srinivasan, E. T. Rasmussen, B. J. Levin, and R. Hayer, Phys. Rev. B 65, 134402共2002兲.
7J. Ryu, A. V. Carazo, K. Uchino, and H. E. Kim, Jpn. J. Appl. Phys., Part 1 40, 4948共2001兲.
8J. Ryu, S. Priya, A. V. Carazo, and K. Uchino, J. Am. Ceram. Soc. 84, 2905共2001兲.
9K. Mori and M. Wuttig, Appl. Phys. Lett. 81, 100共2002兲.
10C. W. Nan, L. Liu, N. Cai, J. Zhai, Y. Ye, and Y. H. Lin, Appl. Phys. Lett.
81, 3831共2002兲.
11C. W. Nan, G. Liu, and Y. H. Lin, Appl. Phys. Lett. 83, 4366共2003兲.
12M. I. Bichurin, D. A. Filippov, V. M. Petrov, V. M. Laletsin, N. Paddub- naya, and G. Srinivasan, Phys. Rev. B 68, 132408共2003兲.
13S. X. Dong, J. R. Cheng, J. F. Li, and D. Viehland, Appl. Phys. Lett. 83, 4812共2003兲.
14S. X. Dong, J. F. Li, D. Viehland, J. Cheng, and L. E. Cross, Appl. Phys.
Lett. 85, 3534共2004兲.
15U. Laletsin, N. Padubnaya, G. Srinivasan, and C. P. DeVreugd, Appl.
Phys. A 78, 33共2004兲.
16S. X. Dong, J. F. Li, and D. Viehland, Appl. Phys. Lett. 85, 2307共2004兲.
17J. G. Wan, J.-M. Liu, H. L. W. Chan, C. L. Choy, G. H. Wang, and C. W.
Nan, J. Appl. Phys. 93, 9916共2003兲.
18F. X. Zhang and K. Shun, Piezoelectricity共National Defence Industry Press, China, 1984兲.
202504-3 Wanet al. Appl. Phys. Lett. 86, 202504共2005兲
Downloaded 20 Mar 2010 to 219.219.118.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
