- Email: [email protected]
Design and properties 1–3 multi-element piezoelectric composite with low crosstalk effects
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Design and properties 1–3 multi-element piezoelectric composite with low crosstalk effects ⁎
Bo Geng, Dongyu Xu , Shanling Yi, Guangpeng Gao, Hongchao Xu, Xin Cheng
⁎
Shandong Provincial Key Lab. of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Piezoelectric composite Multi-element Piezoelectric properties Ultrasonic
Piezoelectric composites are gaining increasingly importance in ultrasonic fields due to their superior properties. Here novel 1–3 multi-element piezoelectric composites were developed by using piezoelectric ceramic as functional phase, epoxy resin as matrix phase, and silica gel and polyurethane as decoupling materials. The effects of decoupling materials and composite thickness on dielectric, piezoelectric and electromechanical coupling properties of the composites were investigated. The coupling response among various elements of the composites was discussed by setting up an ultrasonic testing platform. The results show that the multi-element piezoelectric composites have larger piezoelectric voltage factor than piezoelectric ceramic, however, less relative permittivity and piezoelectric strain factor. With decreasing the composite thickness, the thickness resonant peaks of the piezoelectric composite shift toward high frequency direction, and no obvious high-order and coupling resonant peaks appear. The multi-element piezoelectric composites have larger thickness electromechanical coupling coefficient kt and less mechanical quality factor Qm than piezoelectric ceramic. When composite thickness is 5 mm, the epoxy/silica piezoelectric composite has a maximum kt value of 70.41%, and a minimum Qm value of 11.29. The coupling response testing results show that epoxy/silica piezoelectric composite shows less crosstalk effect than epoxy/epoxy and epoxy/polyurethane piezoelectric composites.
1. Introduction Piezoelectric transducers can realize conversion of electrical energy and sound energy, which are widely used in ultrasonic fields due to high electro-acoustic efficiency, large power capacity and structural designability. Piezoelectric composites were usually used as core element of piezoelectric transducers because of the flexible acoustic matching ability, large electromechanical coupling coefficient and piezoelectric voltage factor [1–3]. According to connectivity of twophase composite, piezoelectric composites can usually be divided into ten basic connectivity patterns [4,5]. The 1–3 piezoelectric composite is composed of piezoelectric phase in one dimension and matrix phase in three-dimensions, which is paid comprehensive attention for its high sensitivity, low acoustic impedance and density [4,6,7]. The 1–3 piezoelectric composites were originally developed by Safari et al. [8], and Wang, Auld et al. [9,10] established a theoretical model of 1–3 piezoelectric composites with piezoelectric ceramic (Pb(ZrTi)O3, PZT) column cycle. In the following years, the theoretical and experimental findings further push forward the development of various novel piezoelectric composites [11–17].
⁎
Recently, with the increasingly requirement on online monitoring technology, the common 1–3 piezoelectric composite consisting of piezoelectric ceramic and epoxy resin cannot meet the requirements of piezoelectric transducer on low coupling and high response because of the crosstalk effect between piezoelectric elements. The novel 1–3 piezoelectric composites with low crosstalk effect attract researchers’ interest. Xu et al. [18] developed 1–3 piezoelectric composites with varied piezoelectric phase distribution to reduce the crosstalk effect. Qin et al. [19] developed triphase multli-element piezoelectric composite through adding decoupling material in the matrix to decrease the coupling vibration between each element. Zhou et al. [20] investigated the properties of 1–3 multli-element piezoelectric composites with silicone as the decoupling material. The decoupling material has a high acoustic attenuation coefficient, which can ensure the emission independence of each acoustic beam by effectively blocking the acoustic energy propagation in the composite. Therefore, the crosstalk effect can be effectively reduced by adding decoupling material in the piezoelectric composite. In this research, 1–3 multi-element piezoelectric composites were fabricated by using secondary cutting-casting method, where PZT ceramic and epoxy were used as
Corresponding authors. E-mail addresses: [email protected] (D. Xu), [email protected] (X. Cheng).
http://dx.doi.org/10.1016/j.ceramint.2017.08.047 Received 12 June 2017; Received in revised form 5 August 2017; Accepted 6 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Geng, B., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.08.047
Ceramics International xxx (xxxx) xxx–xxx
B. Geng et al.
Table 1 Properties of PZT piezoelectric ceramic. Properties
kt (%)
d33 (pC N−1)
tanδ (%)
εr
g33 ((mV) m N−1)
Qm
PZT
60
400
2
1500
30
70
(Note: kt-thickness electromechanical coupling coefficient; d33-piezoelectric strain factor; tanδ-dielectric loss; εr-relative permittivity; g33-piezoelectric voltage factor; Qm-mechanical quality factor).
Table 2 Parameters of the decoupling materials.
Fig. 2. Photos of the 1–3 multi-element piezoelectric composites.
Polymer
Acoustic impedance (kg s−1 m−2)
Density (103 kg m−3)
Elastic modulus (MPa)
Sound speed (m s−1)
Epoxy resin
0.32
1.20
1000.0
Polyurethane
1.72
1.04
39.4
Silica gel
1.01
1.12
7.8
2400– 2900 1600– 1650 1320
piezoelectric and matrix phase, and silica gel and polyurethane were used as decoupling materials, respectively. The effects of decoupling materials and composite thickness on dielectric, piezoelectric and electromechanical coupling properties of the composites were investigated. Fig. 3. Ultrasonic testing platform for coupling response test.
2. Experiments 2.2. Performance testing 2.1. Experimental procedure Model ZJ-3A d33 quasi-static meter was used to test piezoelectric strain factor d33 of the composites. Agilent 4294A impedance analyzer was used to measure the resonant frequency fs, anti-resonance frequency fp, capacitance C and dielectric loss tanδ of the composites. The ultrasonic testing platform consisting of Tektronix MDO3024 oscilloscope and AFG3022B function signal generator was built up to test the coupling response among various piezoelectric element, as shown in Fig. 3. The conductive copper tape was used as electrode of each element in the composite, and the pulse signal was applied to 1# element with a frequency of 120 kHz and an amplitude 10 V, and the response signals from 2#, 3# and 4# elements were recorded by the oscilloscope.
The basic properties of PZT ceramic, epoxy resin, silica gel and polyurethane were shown in Tables 1 and 2. The fabrication procedure of the piezoelectric composites was shown in Fig. 1. Firstly, the pre-polarized PZT piezoelectric ceramic block with a dimension of 14.5 mm × 14.5 mm × 13.3 mm was accurately cut along the polarization axis direction, and a foundation of 2 mm was left. The mixture of epoxy resin and curing agent with a mass ratio of 4:1 was poured into the grooves of 0.5 mm in width, and the samples were cured for 24 h around 25 °C. Then the pre-polarized PZT piezoelectric ceramic block was again cut vertical to the first cut, and epoxy resin, polyurethane and silica gel were poured into the grooves. Finally, the ceramic foundation was cut off and the silver paint were coated on surfaces of the composites after polishing. The dimension of PZT piezoelectric ceramic rod in the composites was 2.0 mm × 2.0 mm × 11.3 mm, and the piezoelectric ceramic volume fraction was 51.7 vol%. The fabricated piezoelectric composites were shown in Fig. 2, which are termed as epoxy/epoxy piezoelectric composite (EPC), epoxy/polyurethane piezoelectric composite (PPC) and epoxy/silica piezoelectric composite (SPC).
3. Results and discussion 3.1. Piezoelectric and dielectric properties The relative permittivity εr, piezoelectric voltage factor g33 of the piezoelectric composites were calculated by Eqs. (1) and (2).
Fig. 1. Fabrication schematic of the 1–3 multi-element piezoelectric composites.
2
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Fig. 4. Dielectric properties of the composite with different thicknesses.
Fig. 5. Piezoelectric properties of the composite with different thicknesses.
εr =
g33 =
Ct ε0 A d33 εr ε0
3.2. Electromechanical coupling properties (1) Fig. 6 shows the impedance versus frequency curves of EPC, PPC, SPC and PZT at different thicknesses. It can be seen that there exists series of resonant peaks in the frequency range of 10–500 kHz, which are mainly the thickness resonant peaks. With decreasing the composite thickness, the thickness resonant peaks of the piezoelectric composite and PZT ceramic shift towards high frequency direction. Comparing with PZT ceramic, the multi-element piezoelectric composites have smooth single thickness resonant peak without appearance of obvious high-order resonant peak and coupling resonant peak. This is because that the elastic modulus of the polymer is much lower than that of PZT ceramic, the acoustic wave has strong attenuation in the polymer, and only a small part of the acoustic energy can be transmitted to the polymer, which accordingly reduce the lateral resonant and coupling crosstalk effect between the adjacent piezoelectric ceramic rods. Therefore, the interference of other vibration modes on thickness vibration can be effectively reduced in the 1–3 piezoelectric composites, which is benefit for application of piezoelectric ultrasonic transducer. Based on impedance vs. frequency spectra, the thickness electromechanical coupling coefficient kt and the mechanical quality factor Qm of the composites can be calculated by Eqs. (3) and (4), as shown in Table 3.
(2)
where t and A are thickness and electrode area of the specimen, respectively, ε0 is the vacuum permittivity whose value is 8.85 × 10– 12 F/m, and symbol C was capacitance measured at 1 kHz. The dielectric and piezoelectric properties of multi-element piezoelectric composites as a function of the composite thicknesses were shown in Figs. 4 and 5. It can be observed that εr and d33 values of the multi-element piezoelectric composites are less than those of PZT ceramic, while g33 value is obviously larger than that of the PZT ceramic. It is known that epoxy resin, silica gel and polyurethane have no piezoelectric effects, thus the d33 value of the piezoelectric composites is less than PZT ceramic. Additionally, because the relative permittivity εr of epoxy resin, silica gel and polyurethane is far less than that of PZT ceramic, εr value of the piezoelectric composites is also obviously less than that of PZT ceramic. Based on Eq. (2), it is known that small εr value is benefit to improve the piezoelectric voltage factor g33, therefore, the multi-element piezoelectric composites are suitable for fabricating piezoelectric transducers with high receiving sensitivity. In comparison, SPC has smaller εr value and larger g33 value. In addition, the difference of the dielectric loss tan δ is slight among PZT ceramic and the piezoelectric composites, which fluctuates between 2% and 4%. Nevertheless, the influence of composite thickness on dielectric and piezoelectric properties of the composites is not obvious.
k t2 =
⎛ f − π fs π p ⋅ ⋅ tan ⎜⎜ ⋅ 2 fp fp ⎝2
Qm−1 = 2πfs RC
3
fs ⎞ ⎟ ⎟ ⎠
(3)
f p2 − fs2 f p2
(4)
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Fig. 6. Impedance vs. frequency spectra of (a) PZT; (b) EPC; (c) PPC; (d) SPC.
piezoelectric composites increases at resonance, so Qm value of the piezoelectric composites is significantly less than that of PZT ceramic. It is known that electromechanical coupling coefficient indicates the energy conversion ability of piezoelectric materials, and the mechanical quality factor means the energy exhausting extent of piezoelectric materials at resonance, which is also related to the bandwidth of the piezoelectric transducer. The larger the kt value, the higher energy conversion ability of the piezoelectric composite. The less the Qm value, the broader bandwidth of the piezoelectric transducer. In comparison, when the composite thickness is 5 mm, SPC has a maximum kt value of 70.41%, and a minimum Qm value of 11.29, so SPC is more useful for tailoring piezoelectric ultrasonic transducer.
Table 3 Thickness electromechanical coupling coefficients kt and mechanical quality factor Qm. Composite
Thickness (mm)
fs (kHz)
fp (kHz)
△f (kHz)
Qm
kt (%)
EPC
5. 0 6.7 8.5 11.3 5.0 6.7 8.5 11.3 5.0 6.7 8.5 11.3 5.0 6.7 8.5 11.3
300.25 232.00 204.72 146.05 283.88 219.20 179.18 136.42 279.10 222.40 182.21 148.52 119.30 112.78 110.40 102.05
380.74 292.00 255.84 173.82 376.63 290.40 236.55 169.42 374.86 286.80 230.14 171.08 143.96 138.68 135.20 124.33
80.49 60.00 51.12 27.78 92.75 71.20 57.38 33.00 95.76 64.40 47.93 22.55 24.66 25.91 24.80 22.28
19.79 19.42 18.27 17.10 20.51 12.55 19.83 18.27 16.10 14.36 13.72 11.29 67.81 67.36 66.77 66.53
65.32 64.57 63.84 58.14 69.41 69.29 68.99 63.17 70.40 66.92 64.92 53.49 65.79 62.09 61.62 61.02
PPC
SPC
PZT-5
3.3. Coupling response analysis The coupling response test of multi-element piezoelectric composites with a thickness of 5 mm was carried out by using the ultrasonic testing platform. Fig. 7 shows the time-domain spectra of the ultrasonic wave received by different elements of the multi-element piezoelectric composites. It can be observed that different piezoelectric composites show a similar variation rule of the ultrasonic wave shape. The waveforms and amplitudes of the ultrasonic wave received by elements 2# and 3# of the same piezoelectric composite is very similar, however, obviously different from that received by elements 4#. It can also be easily seen that the ultrasonic attenuation of element 4# is faster than that of elements 2# and 3#, while the wave amplitude is obviously less than that of elements 2# and 3#. As for the same element of different piezoelectric composites, SPC has the most notably ultrasonic attenuation, and EPC has the longest ultrasonic duration time. In addition, the
where R is the minimum impedance value at resonant frequency fs. It can be seen from Table 3 that, kt value of multi-element piezoelectric composites under the same thickness is larger than that of PZT ceramic, and increases with decreasing the composite thickness, while Qm value is less than that of PZT ceramic, and decreases with increasing the composite thickness. The radial inhibition effects induced by polymer matrix contribute to the energy concentration in thickness resonance, that is, the resonance enhancement in thickness increases the thickness electromechanical coupling coefficient kt. In addition, due to introduction of the polymer, the mechanical loss of the 4
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Fig. 7. Time-domain spectra of ultrasonic wave received by different elements of the composites. (Note: Letters a, b and c mean EPC, PPC and SPC. Numbers are the elements of the piezoelectric composite, that is, number 1 is the emission element, and numbers 2, 3, 4 are the receiving elements).
decoupling material of the multi-element piezoelectric composites. The ring count method from acoustic emission technique [21] was employed to evaluate the attenuation and the coupling response between elements of the piezoelectric composites. Here the damping ratio and ringing count were mainly considered, as shown in Eq. (5) and Fig. 8.
ζ=
1 1 + 4π 2 /ln2(A1/A2)
(5)
where ζ is the damping ratio, A1 and A2 are half amplitudes of the largest peak and the following peak. The response amplitudes and attenuation of ultrasonic signal received by different elements of the composites were shown in Table 4. It can be seen that, as for the same piezoelectric composite, the half amplitudes A1 and A2 of element 4# are obviously less than those of elements 2# and 3#, which is due to the fact that the receiving element 4# is far away from the emission element 1#. In comparison,
Fig. 8. Typical acoustic emission wave shape and parameters duration.
peak-peak value of SPC is also less than other composites for the same testing element. This indicates that silica gel has better decoupling capability than epoxy and polyurethane, and is more suitable as 5
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Table 4 Signal response of various elements in different piezoelectric composites. Element
a (1–2)
a (1–3)
a (1–4)
b (1–2)
b (1–3)
b (1–4)
c (1–2)
c (1–3)
c (1–4)
Half amplitude A1/mV Half amplitude A2/mV Damping ratio ζ Ringing count
131 62 0.029 23
139 62 0.026 31
68 27 0.039 17
132 77 0.044 26
143 62 0.041 28
45 30 0.058 25
39 26 0.066 10
39 30 0.054 10
10 10 0.076 8
References
SPC has the least half amplitudes A1 and A2, which indicates that silica gel exhibits better decoupling property than other polymers. The greater the damping ratio is, the stronger the attenuation of ultrasonic wave is. It can be observed that, as for the same piezoelectric composite, the damping ratio of elements 2# and 3# is similar, obviously less than element 4#. The damping ratio of SCP is higher than that of EPC and EPP, and damping ratio of element 4# of SCP is 0.076. In addition, the ringing count can also be used to reflect the ultrasonic attenuation. As for the same piezoelectric composite, the ringing count of elements 2# and 3# is similar, and larger than that of element 4#. In comparison, SPC has a least ring count. Based on above analysis, silica gel has best decoupling effect. This is because that the elastic modulus of silica gel is much smaller than that of polyurethane and epoxy resin. The silica gel with low elastic modulus absorbs the lateral vibration energy between the elements, so the lateral vibration decays rapidly. Therefore, the epoxy/silica piezoelectric composite shows better coupling response than epoxy/epoxy and epoxy/polyurethane piezoelectric composites.
[1] I. Noboru, M. Naohiko, T. Sadayuki, Ultrasonic transducers with functionally graded piezoelectric ceramics, J. Eur. Ceram. Soc. 24 (6) (2004) 1681–1685. [2] A. Muc, M. Barski, P. Kędziora, Piezoelectric transducers, Key Eng. Mater. 542 (2013) 75–80. [3] C.N. Della, D.W. Shu, On the performance of 1-3 piezoelectric composites with a passive and active matrix, Sens. Actuators A-Phys. 140 (2) (2007) 200–206. [4] R.E. Newnham, D.P. Skinner, L.E. Cross, Connectivity and piezoelectric-pyroelectric composite, Mater. Res. Bull. 13 (5) (1978) 525–536. [5] B.Q. Dong, Z.J. Li, Cement-based piezoelectric ceramic smart composites, Compos. Sci. Technol. 65 (9) (2005) 1363–1371. [6] G. Li, L.K. Wang, G.D. Luan, J.D. Zhang, S.X. Li, Study of 1-3-2 type piezoelectric composite transducer array, Ultrasonics 44 (2006) 673–677. [7] Y. Sapsathiarn, T. Senjuntichai, R. Rajapakse, Cylindrical interface cracks in 1-3 piezocomposites, Compos. Part B 43 (5) (2012) 2257–2264. [8] A. Safari, R.E. Newnham, L.E. Cross, W.A. Schulze, Perforated PZT-polymer composites for piezoelectric transducer applications, Ferroelectrics 41 (1) (1982) 197–205. [9] Y. Wang, E. Schniidt, B.A. Auld, Acoustic wave transmission through onedimensional PZT-epoxy composites, in: Proceedings of the IEEE Ultrasonics Symposium, 34(3), 1986, pp. 685–689. [10] B.A. Auld, H.A. Kunkel, Y.A. Shui, Y. Wang, Dynamic behavior of periodic piezoelectric composites, in: Proceedings of the IEEE Ultrasonics Symposium, 1983, pp. 554–558. [11] W.A. Smith, Calculating the hydrophone response of piezoceramic-rod/piezopolyer-matrix composites, in: Proceedings of the IEEE Ultrasonics Symposium, 1990, pp. 757–761. [12] G. Li, L.K. Wang, G.D. Luan, J.D. Zhang, S.X. Li, Study of 1-3-2 type piezoelectric composite transducer array, Ultrasonics 44 (4) (2006) 673–677. [13] T.R. Gururaja, W.A. Schulze, L.E. Cross, R.E. Newnham, B.A. Auld, Y.J. Wang, Piezoelectric composite materials for ultrasonic transducer applications. Part I: resonant rod-polymer composites, IEEE Trans. Sonics Ultrason. 32 (1985) 481–498. [14] K.C. Benjamin, Recent advances in 1-3 piezoelectric polymer composite transducer technology for auv/uuv acoustic imaging applications, J. Electroceram. 1 (2001) 26–33. [15] C.P. Chong, H.L.W. Chan, M.W. Ng, P.C.K. Liu, Effect of hybrid structure (1-3 composite and ceramic) on the performance of sandwich transducers, Mater. Sci. Eng. B 99 (2003) 6–10. [16] D.Y. Xu, P. Du, J.X. Wang, P.K. Hou, S.F. Huang, X. Cheng, Design and properties of gaussian-type 1-3 piezoelectric composites, Compos. Struct. 140 (2016) 213–216. [17] B.Q. Dong, Y.Q. Liu, N.X. Han, H.F. Sun, F. Xing, D.D. Qin, Study on the microstructure of cement-based piezoelectric ceramic composites, Constr. Build. Mater. 72 (2014) 133–138. [18] D.Y. Xu, X. Cheng, H.D. Geng, F. Lu, S.F. Huang, Design, fabrication and properties of 1-3 piezoelectric ceramic composites with varied piezoelectric phase distribution, Ceram. Int. 41 (8) (2015) 9433–9442. [19] L. Qin, L.K. Wang, Y. Lu, Decoupling in multi-elements composite for transducer array application, Curr. Appl. Phys. 11 (3) (2011) 368–370. [20] M.J. Zhou, M. Sun, M.M. Li, S.H. Xie, S.F. Huang, Fabrication and properties of 13-2 multi-element piezoelectric composite, J. Electroceram. 28 (2) (2012) 139–143. [21] D.G. Aggelis, D.V. Soulioti, N. Sapouridis, N.M. Barkoula, A.S. Paipetis, T.E. Matikas, Acoustic emission characterization of the fracture process in fibre reinforced concrete, Constr. Build. Mater. 25 (11) (2011) 4126–4131.
4. Conclusion The 1–3 multi-element piezoelectric composites were prepared via secondary cutting-casting method aiming to reduce the crosstalk effects between adjacent piezoelectric elements by using epoxy as matrix phase, silica gel and polyurethane as decoupling materials. The piezoelectric, dielectric and electromechanical properties of all elements in the piezoelectric composites exhibit a good consistency. The multi-element piezoelectric composites have larger piezoelectric voltage factor than PZT piezoelectric ceramic, while less relative permittivity and piezoelectric strain factor. With decreasing the composite thickness, the thickness resonant peaks of the piezoelectric composite shift toward high frequency direction, and no obvious high-order and coupling resonant peaks appear. When the composite thickness is 5 mm, epoxy/silica piezoelectric composite has a maximum kt value of 70.41%, and a minimum Qm value of 11.29. The coupling response testing results show that epoxy/silica piezoelectric composite has the least half amplitudes and ring count, and the largest damping ratio, therefore, the epoxy/silica piezoelectric composite shows less crosstalk effect than epoxy/epoxy and epoxy/ polyurethane piezoelectric composites. Acknowledgement This work was supported by Natural Science Outstanding Youth Foundation of Shandong Province (Grant no. ZR201702150286), China Postdoctoral Science Foundation funded project (Grant no. 2016M590611), and National Natural Science Foundation of China (Grant no. 51632003).
6
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Design and properties 1–3 multi-element piezoelectric composite with low crosstalk effects ⁎
Bo Geng, Dongyu Xu , Shanling Yi, Guangpeng Gao, Hongchao Xu, Xin Cheng
⁎
Shandong Provincial Key Lab. of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Piezoelectric composite Multi-element Piezoelectric properties Ultrasonic
Piezoelectric composites are gaining increasingly importance in ultrasonic fields due to their superior properties. Here novel 1–3 multi-element piezoelectric composites were developed by using piezoelectric ceramic as functional phase, epoxy resin as matrix phase, and silica gel and polyurethane as decoupling materials. The effects of decoupling materials and composite thickness on dielectric, piezoelectric and electromechanical coupling properties of the composites were investigated. The coupling response among various elements of the composites was discussed by setting up an ultrasonic testing platform. The results show that the multi-element piezoelectric composites have larger piezoelectric voltage factor than piezoelectric ceramic, however, less relative permittivity and piezoelectric strain factor. With decreasing the composite thickness, the thickness resonant peaks of the piezoelectric composite shift toward high frequency direction, and no obvious high-order and coupling resonant peaks appear. The multi-element piezoelectric composites have larger thickness electromechanical coupling coefficient kt and less mechanical quality factor Qm than piezoelectric ceramic. When composite thickness is 5 mm, the epoxy/silica piezoelectric composite has a maximum kt value of 70.41%, and a minimum Qm value of 11.29. The coupling response testing results show that epoxy/silica piezoelectric composite shows less crosstalk effect than epoxy/epoxy and epoxy/polyurethane piezoelectric composites.
1. Introduction Piezoelectric transducers can realize conversion of electrical energy and sound energy, which are widely used in ultrasonic fields due to high electro-acoustic efficiency, large power capacity and structural designability. Piezoelectric composites were usually used as core element of piezoelectric transducers because of the flexible acoustic matching ability, large electromechanical coupling coefficient and piezoelectric voltage factor [1–3]. According to connectivity of twophase composite, piezoelectric composites can usually be divided into ten basic connectivity patterns [4,5]. The 1–3 piezoelectric composite is composed of piezoelectric phase in one dimension and matrix phase in three-dimensions, which is paid comprehensive attention for its high sensitivity, low acoustic impedance and density [4,6,7]. The 1–3 piezoelectric composites were originally developed by Safari et al. [8], and Wang, Auld et al. [9,10] established a theoretical model of 1–3 piezoelectric composites with piezoelectric ceramic (Pb(ZrTi)O3, PZT) column cycle. In the following years, the theoretical and experimental findings further push forward the development of various novel piezoelectric composites [11–17].
⁎
Recently, with the increasingly requirement on online monitoring technology, the common 1–3 piezoelectric composite consisting of piezoelectric ceramic and epoxy resin cannot meet the requirements of piezoelectric transducer on low coupling and high response because of the crosstalk effect between piezoelectric elements. The novel 1–3 piezoelectric composites with low crosstalk effect attract researchers’ interest. Xu et al. [18] developed 1–3 piezoelectric composites with varied piezoelectric phase distribution to reduce the crosstalk effect. Qin et al. [19] developed triphase multli-element piezoelectric composite through adding decoupling material in the matrix to decrease the coupling vibration between each element. Zhou et al. [20] investigated the properties of 1–3 multli-element piezoelectric composites with silicone as the decoupling material. The decoupling material has a high acoustic attenuation coefficient, which can ensure the emission independence of each acoustic beam by effectively blocking the acoustic energy propagation in the composite. Therefore, the crosstalk effect can be effectively reduced by adding decoupling material in the piezoelectric composite. In this research, 1–3 multi-element piezoelectric composites were fabricated by using secondary cutting-casting method, where PZT ceramic and epoxy were used as
Corresponding authors. E-mail addresses: [email protected] (D. Xu), [email protected] (X. Cheng).
http://dx.doi.org/10.1016/j.ceramint.2017.08.047 Received 12 June 2017; Received in revised form 5 August 2017; Accepted 6 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Geng, B., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.08.047
Ceramics International xxx (xxxx) xxx–xxx
B. Geng et al.
Table 1 Properties of PZT piezoelectric ceramic. Properties
kt (%)
d33 (pC N−1)
tanδ (%)
εr
g33 ((mV) m N−1)
Qm
PZT
60
400
2
1500
30
70
(Note: kt-thickness electromechanical coupling coefficient; d33-piezoelectric strain factor; tanδ-dielectric loss; εr-relative permittivity; g33-piezoelectric voltage factor; Qm-mechanical quality factor).
Table 2 Parameters of the decoupling materials.
Fig. 2. Photos of the 1–3 multi-element piezoelectric composites.
Polymer
Acoustic impedance (kg s−1 m−2)
Density (103 kg m−3)
Elastic modulus (MPa)
Sound speed (m s−1)
Epoxy resin
0.32
1.20
1000.0
Polyurethane
1.72
1.04
39.4
Silica gel
1.01
1.12
7.8
2400– 2900 1600– 1650 1320
piezoelectric and matrix phase, and silica gel and polyurethane were used as decoupling materials, respectively. The effects of decoupling materials and composite thickness on dielectric, piezoelectric and electromechanical coupling properties of the composites were investigated. Fig. 3. Ultrasonic testing platform for coupling response test.
2. Experiments 2.2. Performance testing 2.1. Experimental procedure Model ZJ-3A d33 quasi-static meter was used to test piezoelectric strain factor d33 of the composites. Agilent 4294A impedance analyzer was used to measure the resonant frequency fs, anti-resonance frequency fp, capacitance C and dielectric loss tanδ of the composites. The ultrasonic testing platform consisting of Tektronix MDO3024 oscilloscope and AFG3022B function signal generator was built up to test the coupling response among various piezoelectric element, as shown in Fig. 3. The conductive copper tape was used as electrode of each element in the composite, and the pulse signal was applied to 1# element with a frequency of 120 kHz and an amplitude 10 V, and the response signals from 2#, 3# and 4# elements were recorded by the oscilloscope.
The basic properties of PZT ceramic, epoxy resin, silica gel and polyurethane were shown in Tables 1 and 2. The fabrication procedure of the piezoelectric composites was shown in Fig. 1. Firstly, the pre-polarized PZT piezoelectric ceramic block with a dimension of 14.5 mm × 14.5 mm × 13.3 mm was accurately cut along the polarization axis direction, and a foundation of 2 mm was left. The mixture of epoxy resin and curing agent with a mass ratio of 4:1 was poured into the grooves of 0.5 mm in width, and the samples were cured for 24 h around 25 °C. Then the pre-polarized PZT piezoelectric ceramic block was again cut vertical to the first cut, and epoxy resin, polyurethane and silica gel were poured into the grooves. Finally, the ceramic foundation was cut off and the silver paint were coated on surfaces of the composites after polishing. The dimension of PZT piezoelectric ceramic rod in the composites was 2.0 mm × 2.0 mm × 11.3 mm, and the piezoelectric ceramic volume fraction was 51.7 vol%. The fabricated piezoelectric composites were shown in Fig. 2, which are termed as epoxy/epoxy piezoelectric composite (EPC), epoxy/polyurethane piezoelectric composite (PPC) and epoxy/silica piezoelectric composite (SPC).
3. Results and discussion 3.1. Piezoelectric and dielectric properties The relative permittivity εr, piezoelectric voltage factor g33 of the piezoelectric composites were calculated by Eqs. (1) and (2).
Fig. 1. Fabrication schematic of the 1–3 multi-element piezoelectric composites.
2
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Fig. 4. Dielectric properties of the composite with different thicknesses.
Fig. 5. Piezoelectric properties of the composite with different thicknesses.
εr =
g33 =
Ct ε0 A d33 εr ε0
3.2. Electromechanical coupling properties (1) Fig. 6 shows the impedance versus frequency curves of EPC, PPC, SPC and PZT at different thicknesses. It can be seen that there exists series of resonant peaks in the frequency range of 10–500 kHz, which are mainly the thickness resonant peaks. With decreasing the composite thickness, the thickness resonant peaks of the piezoelectric composite and PZT ceramic shift towards high frequency direction. Comparing with PZT ceramic, the multi-element piezoelectric composites have smooth single thickness resonant peak without appearance of obvious high-order resonant peak and coupling resonant peak. This is because that the elastic modulus of the polymer is much lower than that of PZT ceramic, the acoustic wave has strong attenuation in the polymer, and only a small part of the acoustic energy can be transmitted to the polymer, which accordingly reduce the lateral resonant and coupling crosstalk effect between the adjacent piezoelectric ceramic rods. Therefore, the interference of other vibration modes on thickness vibration can be effectively reduced in the 1–3 piezoelectric composites, which is benefit for application of piezoelectric ultrasonic transducer. Based on impedance vs. frequency spectra, the thickness electromechanical coupling coefficient kt and the mechanical quality factor Qm of the composites can be calculated by Eqs. (3) and (4), as shown in Table 3.
(2)
where t and A are thickness and electrode area of the specimen, respectively, ε0 is the vacuum permittivity whose value is 8.85 × 10– 12 F/m, and symbol C was capacitance measured at 1 kHz. The dielectric and piezoelectric properties of multi-element piezoelectric composites as a function of the composite thicknesses were shown in Figs. 4 and 5. It can be observed that εr and d33 values of the multi-element piezoelectric composites are less than those of PZT ceramic, while g33 value is obviously larger than that of the PZT ceramic. It is known that epoxy resin, silica gel and polyurethane have no piezoelectric effects, thus the d33 value of the piezoelectric composites is less than PZT ceramic. Additionally, because the relative permittivity εr of epoxy resin, silica gel and polyurethane is far less than that of PZT ceramic, εr value of the piezoelectric composites is also obviously less than that of PZT ceramic. Based on Eq. (2), it is known that small εr value is benefit to improve the piezoelectric voltage factor g33, therefore, the multi-element piezoelectric composites are suitable for fabricating piezoelectric transducers with high receiving sensitivity. In comparison, SPC has smaller εr value and larger g33 value. In addition, the difference of the dielectric loss tan δ is slight among PZT ceramic and the piezoelectric composites, which fluctuates between 2% and 4%. Nevertheless, the influence of composite thickness on dielectric and piezoelectric properties of the composites is not obvious.
k t2 =
⎛ f − π fs π p ⋅ ⋅ tan ⎜⎜ ⋅ 2 fp fp ⎝2
Qm−1 = 2πfs RC
3
fs ⎞ ⎟ ⎟ ⎠
(3)
f p2 − fs2 f p2
(4)
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Fig. 6. Impedance vs. frequency spectra of (a) PZT; (b) EPC; (c) PPC; (d) SPC.
piezoelectric composites increases at resonance, so Qm value of the piezoelectric composites is significantly less than that of PZT ceramic. It is known that electromechanical coupling coefficient indicates the energy conversion ability of piezoelectric materials, and the mechanical quality factor means the energy exhausting extent of piezoelectric materials at resonance, which is also related to the bandwidth of the piezoelectric transducer. The larger the kt value, the higher energy conversion ability of the piezoelectric composite. The less the Qm value, the broader bandwidth of the piezoelectric transducer. In comparison, when the composite thickness is 5 mm, SPC has a maximum kt value of 70.41%, and a minimum Qm value of 11.29, so SPC is more useful for tailoring piezoelectric ultrasonic transducer.
Table 3 Thickness electromechanical coupling coefficients kt and mechanical quality factor Qm. Composite
Thickness (mm)
fs (kHz)
fp (kHz)
△f (kHz)
Qm
kt (%)
EPC
5. 0 6.7 8.5 11.3 5.0 6.7 8.5 11.3 5.0 6.7 8.5 11.3 5.0 6.7 8.5 11.3
300.25 232.00 204.72 146.05 283.88 219.20 179.18 136.42 279.10 222.40 182.21 148.52 119.30 112.78 110.40 102.05
380.74 292.00 255.84 173.82 376.63 290.40 236.55 169.42 374.86 286.80 230.14 171.08 143.96 138.68 135.20 124.33
80.49 60.00 51.12 27.78 92.75 71.20 57.38 33.00 95.76 64.40 47.93 22.55 24.66 25.91 24.80 22.28
19.79 19.42 18.27 17.10 20.51 12.55 19.83 18.27 16.10 14.36 13.72 11.29 67.81 67.36 66.77 66.53
65.32 64.57 63.84 58.14 69.41 69.29 68.99 63.17 70.40 66.92 64.92 53.49 65.79 62.09 61.62 61.02
PPC
SPC
PZT-5
3.3. Coupling response analysis The coupling response test of multi-element piezoelectric composites with a thickness of 5 mm was carried out by using the ultrasonic testing platform. Fig. 7 shows the time-domain spectra of the ultrasonic wave received by different elements of the multi-element piezoelectric composites. It can be observed that different piezoelectric composites show a similar variation rule of the ultrasonic wave shape. The waveforms and amplitudes of the ultrasonic wave received by elements 2# and 3# of the same piezoelectric composite is very similar, however, obviously different from that received by elements 4#. It can also be easily seen that the ultrasonic attenuation of element 4# is faster than that of elements 2# and 3#, while the wave amplitude is obviously less than that of elements 2# and 3#. As for the same element of different piezoelectric composites, SPC has the most notably ultrasonic attenuation, and EPC has the longest ultrasonic duration time. In addition, the
where R is the minimum impedance value at resonant frequency fs. It can be seen from Table 3 that, kt value of multi-element piezoelectric composites under the same thickness is larger than that of PZT ceramic, and increases with decreasing the composite thickness, while Qm value is less than that of PZT ceramic, and decreases with increasing the composite thickness. The radial inhibition effects induced by polymer matrix contribute to the energy concentration in thickness resonance, that is, the resonance enhancement in thickness increases the thickness electromechanical coupling coefficient kt. In addition, due to introduction of the polymer, the mechanical loss of the 4
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Fig. 7. Time-domain spectra of ultrasonic wave received by different elements of the composites. (Note: Letters a, b and c mean EPC, PPC and SPC. Numbers are the elements of the piezoelectric composite, that is, number 1 is the emission element, and numbers 2, 3, 4 are the receiving elements).
decoupling material of the multi-element piezoelectric composites. The ring count method from acoustic emission technique [21] was employed to evaluate the attenuation and the coupling response between elements of the piezoelectric composites. Here the damping ratio and ringing count were mainly considered, as shown in Eq. (5) and Fig. 8.
ζ=
1 1 + 4π 2 /ln2(A1/A2)
(5)
where ζ is the damping ratio, A1 and A2 are half amplitudes of the largest peak and the following peak. The response amplitudes and attenuation of ultrasonic signal received by different elements of the composites were shown in Table 4. It can be seen that, as for the same piezoelectric composite, the half amplitudes A1 and A2 of element 4# are obviously less than those of elements 2# and 3#, which is due to the fact that the receiving element 4# is far away from the emission element 1#. In comparison,
Fig. 8. Typical acoustic emission wave shape and parameters duration.
peak-peak value of SPC is also less than other composites for the same testing element. This indicates that silica gel has better decoupling capability than epoxy and polyurethane, and is more suitable as 5
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Table 4 Signal response of various elements in different piezoelectric composites. Element
a (1–2)
a (1–3)
a (1–4)
b (1–2)
b (1–3)
b (1–4)
c (1–2)
c (1–3)
c (1–4)
Half amplitude A1/mV Half amplitude A2/mV Damping ratio ζ Ringing count
131 62 0.029 23
139 62 0.026 31
68 27 0.039 17
132 77 0.044 26
143 62 0.041 28
45 30 0.058 25
39 26 0.066 10
39 30 0.054 10
10 10 0.076 8
References
SPC has the least half amplitudes A1 and A2, which indicates that silica gel exhibits better decoupling property than other polymers. The greater the damping ratio is, the stronger the attenuation of ultrasonic wave is. It can be observed that, as for the same piezoelectric composite, the damping ratio of elements 2# and 3# is similar, obviously less than element 4#. The damping ratio of SCP is higher than that of EPC and EPP, and damping ratio of element 4# of SCP is 0.076. In addition, the ringing count can also be used to reflect the ultrasonic attenuation. As for the same piezoelectric composite, the ringing count of elements 2# and 3# is similar, and larger than that of element 4#. In comparison, SPC has a least ring count. Based on above analysis, silica gel has best decoupling effect. This is because that the elastic modulus of silica gel is much smaller than that of polyurethane and epoxy resin. The silica gel with low elastic modulus absorbs the lateral vibration energy between the elements, so the lateral vibration decays rapidly. Therefore, the epoxy/silica piezoelectric composite shows better coupling response than epoxy/epoxy and epoxy/polyurethane piezoelectric composites.
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4. Conclusion The 1–3 multi-element piezoelectric composites were prepared via secondary cutting-casting method aiming to reduce the crosstalk effects between adjacent piezoelectric elements by using epoxy as matrix phase, silica gel and polyurethane as decoupling materials. The piezoelectric, dielectric and electromechanical properties of all elements in the piezoelectric composites exhibit a good consistency. The multi-element piezoelectric composites have larger piezoelectric voltage factor than PZT piezoelectric ceramic, while less relative permittivity and piezoelectric strain factor. With decreasing the composite thickness, the thickness resonant peaks of the piezoelectric composite shift toward high frequency direction, and no obvious high-order and coupling resonant peaks appear. When the composite thickness is 5 mm, epoxy/silica piezoelectric composite has a maximum kt value of 70.41%, and a minimum Qm value of 11.29. The coupling response testing results show that epoxy/silica piezoelectric composite has the least half amplitudes and ring count, and the largest damping ratio, therefore, the epoxy/silica piezoelectric composite shows less crosstalk effect than epoxy/epoxy and epoxy/ polyurethane piezoelectric composites. Acknowledgement This work was supported by Natural Science Outstanding Youth Foundation of Shandong Province (Grant no. ZR201702150286), China Postdoctoral Science Foundation funded project (Grant no. 2016M590611), and National Natural Science Foundation of China (Grant no. 51632003).
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