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Sensors and Actuators A 182 (2012) 95– 100

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

jo u rn al hom epage: www.elsev ier .com/ locate /sna

ead-free piezoelectric single crystal based 1–3 composites for ultrasonicransducer applications

an Zhoua, Kwok Ho Lama, Yan Chena, Qinhui Zhangb, Yat Ching Chiua, Haosu Luob,iyan Daia,∗, Helen Lai Wa Chana

Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, ChinaInformation Materials and Devices Research Center, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 201800, China

r t i c l e i n f o

rticle history:eceived 4 January 2012eceived in revised form 15 May 2012ccepted 16 May 2012

a b s t r a c t

In this work, lead-free 1–3 composites based on piezoelectric 0.947Na0.5Bi0.5TiO3–0.053BaTiO3

(NBT–0.053BT) single crystal and epoxy are fabricated for ultrasonic transducer applications by a modi-fied dice-and-fill method. Excellent properties for ultrasonic transducer applications have been achieved,such as high electromechanical coupling coefficient (kt = 73%), lower acoustic impedance (Z = 16 MRayl)

vailable online 24 May 2012

eywords:ead-freeBT–BT single crystal–3 composite

and moderate dielectric constant. Based on this lead-free piezoelectric single crystal composite, single-element ultrasonic transducer and linear array have been fabricated and characterized. Both types oftransducers exhibit similar performance with broad bandwidth of exceeding 100%. The promising resultsshow that these lead-free composites have the potential to be used for high-performance ultrasonictransducers.

ltrasonic transducer

. Introduction

Lead-based piezoelectric ceramics with a perovskite struc-ure such as lead zirconate titanate (PZT) are most widely usedn the fabrication of ultrasonic transducers for medical diagno-is and industrial non-destructive evaluation (NDE) applicationsue to their relatively high electromechanical coupling factornd piezoelectric constant [1–5]. However, the use of the lead-ased ceramics would cause serious environmental problems.igh lead content (more than 60% lead in PZT by weight) cre-tes hazards in the fabrication process (lead is released into thetmosphere), which may cause lead poisoning. Therefore, it isecessary to develop more environmental friendly piezoelectricaterials for replacing the lead-based ones [6–11]. The solid solu-

ions of (1−x)Na0.5Bi0.5TiO3–xBaTiO3 (NBT–xBT or NBT–BT) areecognized as a leading candidate among lead-free materials. Theiezoelectric constant d33 of NBT–BT ceramics and single crystalsas been reported to be as high as 205 pC/N [12] and 450 pC/N [13],espectively. As the NBT–BT material system has relatively highiezoelectric constants and Curie temperatures [13–15], various

pplications have been proposed [16–19]. The reported NBT–BTingle crystal-based ultrasonic transducer with a bandwidth of 46%

∗ Corresponding author.E-mail address: apdaijy@inet.polyu.edu.hk (J. Dai).

924-4247/$ – see front matter © 2012 Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.sna.2012.05.030

© 2012 Published by Elsevier B.V.

has shown the potential applications of lead-free materials in ultra-sonic transducer [20].

For enhancing the resolution in medical ultrasonic imaging[21–23] and the performance in NDE applications for detectinghighly attenuative materials [24], the PZT/epoxy composites with1–3 connectivity were generally utilized to further improve theelectrical and acoustic properties. In this work, the NBT–BT sin-gle crystal is selected as an active phase of the 1–3 composite.With a modified dice-and-fill technique, the 1–3 composites havebeen fabricated successfully using the very fragile NBT–BT singlecrystal. Based on this lead-free piezoelectric composite, ultrasonictransducers of single element and linear array types were designed,fabricated and characterized.

2. Materials preparation

Large-size and high-quality single crystals of NBT–xBT weregrown by a top-seeded solution growth (TSSG) technique [25] atthe Shanghai Institute of Ceramics. The composition of the NBT–xBTsingle crystals measured by an inductive coupled plasma atomicemission spectrometry (ICP-AES) is x = 0.053 BT content, whichslightly deviated from the rhombohedral–tetragonal morphotropicphase boundary (MPB) near x = 0.06–0.07 [26]. The as-grown sin-

gle crystals are oriented along the [0 0 1] pseudo cubic directionas examined by an X-ray diffractometer with an accuracy of 15′.The [0 0 1] oriented wafers are sliced into plates with a thick-ness of 0.7 mm for fabricating 1–3 composites. In order to avoid

96 D. Zhou et al. / Sensors and Actuators A 182 (2012) 95– 100

for fa

mreNictJkivN1w(kTuerdpd4npts(eaNaiww2

s(ip

tively, as shown in Fig. 2,

c = 2Nt = 2tfa (2)

Fig. 1. Schematic procedures of the modified dice-and-fill method used

ode coupling, the aspect ratio (height to width) of the NBT–BTods embedded in the composites should be at least 2.5. How-ver, the NBT–BT single crystal is very fragile and hard to prepareBT–BT rods even for an aspect ratio of 1.0. Therefore, a mod-

fied dice-and-fill method has been adopted to prepare the 1–3omposites with higher aspect ratio, as shown in Fig. 1. A 60-�m-hick nickel/diamond blade with a DAD 321 dicing saw (DISCO,apan) was used to dice the NBT–BT single crystal. The resultanterf width was found to be 70 �m due to blade vibration. Dur-ng the dicing process, the dicing speed was reduced to a lowalue of 0.46 mm/s to avoid breakage of the crystal. At first, theBT–BT wafer was diced from two perpendicular directions with

mm pitch and 0.5 mm depth to form periodic rods in the waferith 500 �m in height and 930 �m in width. Since the aspect ratio

500/930 = 0.538) was still low, the rods can stand after dicing. Theerfs were then filled by low-viscosity epoxy (Epo-Tek 301, Epoxyechnology, USA), and the composite plate was placed under vac-um to remove the trapped bubbles before solidification. After thepoxy was cured, the NBT–BT rods were fixed tightly to the sur-ounding epoxy. Then, the rods in the epoxy matrix were furthericed into four equal areas by dicing the composite plate in the tworevious perpendicular directions with the same dicing pitch andepth. With the protection of epoxy, the rods of 500 �m height and30 �m width (aspect ratio is 500/430 = 1.163) can be achieved. Theewly formed kerfs were epoxy-filled and cured again. Finally, thelate was further diced into 0.5 mm pitch and 0.5 mm depth to formhe configuration of the composites. The final width of the NBT–BTingle crystal rods is 180 �m and the aspect ratio approaches 2.8500/180 = 2.778). After filling and curing the epoxy in the kerfs,xcess single crystal and epoxy were lapped away from the topnd bottom sides of the composite plate. The volume fraction ofBT–BT rods in the composite was calculated to be 52%. Fig. 2 showsn optical image of the NBT–BT single crystal/epoxy 1–3 compos-te. Chromium gold (Cr/Au) electrodes with a thickness of ∼500 nm

ere sputtered onto both sides of the composite. The compositeas finally poled under an electric field of 4 kV/mm at 60 ◦C for

0 min.Electrical and acoustic properties of the as-prepared NBT–BT

ingle crystal/epoxy 1–3 composite with the dimension of 12.3 mmlength) × 8.0 mm (width) × 0.478 mm (thickness) was character-zed. For comparison, a plate of NBT–BT single crystal cut from therevious wafer with the dimensions of 7.0 mm × 7.0 mm × 0.7 mm

bricating 1–3 composites based on very fragile piezoelectric materials.

was characterized as well. The NBT–BT plate was sputtered withCr/Au electrodes and poled under a lower electric field of 3 kV/mmat 100 ◦C for 20 min. The piezoelectric charge d33 coefficient of thecomposite was directly measured by a Berlincourt d33 meter at55 Hz and the density � was determined from the Archimedes prin-ciples. An impedance analyzer (Agilent 4294A, Santa Clara, CA) wasemployed to measure the frequency dependence of the electricalparameters (electrical impedance, capacitance, conductance, etc.).The electromechanical coupling kt coefficient, acoustic velocity c,acoustic impedance Z and mechanical quality factor Qm were calcu-lated from the measured spectra shown in Figs. 3 and 4, accordingto the IEEE standards on piezoelectricity [27], as the following for-mulae:

kt =√

�

2frfa

tan(

�

2fa − fr

fa

)(1)

where fr and fa are resonant and anti-resonant frequencies, respec-

Fig. 2. Optical image of the NBT–BT single crystal/epoxy 1–3 composite plate.

D. Zhou et al. / Sensors and Actuators A 182 (2012) 95– 100 97

Fa

ws

Z

Q

wos

ε

wa

gtpfiiPlkvb

3

t

ig. 3. The impedance and phase angle spectra for (a) NBT–BT single crystal platend (b) NBT–BT/epoxy 1–3 composite plate.

here Nt is the frequency constant and t is the thickness of theample,

= �c (3)

m = frf+1/2 − f−1/2

(4)

here f+1/2 and f−1/2 are the upper and lower frequencies with halff the conductance obtained at fr from the conductance spectrahown in Fig. 4,

T33 = C0t

Sε0(5)

here C0 is the capacitance measured at 1 kHz, S is the sample areand ε0 is the dielectric constant in vacuum.

The measured and derived properties of the NBT–0.053BT sin-le crystal and its 1–3 composite are listed in Table 1. It is apparenthat the fabricated lead-free NBT–BT single crystal 1–3 compositeossesses comparable performance, such as the piezoelectric coef-cient, dielectric constant, mechanical quality factor and acoustic

mpedance, with the commercial lead-based PZT ceramics [28,29],MN–PT single crystal [30], and their 1–3 composites [30,31]. Theead-free based 1–3 composite is found to exhibit reasonably hight value of 0.73 among the materials as shown in Table 1. The kt

alue is a key parameter for fabricating high sensitivity and broadandwidth transducers.

. Transducer design and fabrication

An as-prepared NBT–BT/epoxy 1–3 composite plate with ahickness of 0.395 mm was cut into a dimension of 4 mm × 4 mm

Fig. 4. The conductance and susceptance spectra for (a) NBT–BT single crystal plateand (b) NBT–BT/epoxy 1–3 composite plate.

for a single element transducer. Meanwhile, the other 1–3 com-posite plate of 0.478 mm thick was cut into a dimension of12.3 mm × 8.0 mm for fabricating a linear transducer array with 8elements. The thickness vibration mode was used in both types ofthe transducers. The resonant frequencies of those two 1–3 com-posite plates were 3.7 MHz and 3.1 MHz, respectively. A front-facematching layer was employed to enhance the bandwidth and sen-sitivity of the transducers. The designated acoustic impedance ofthe single matching layer (Z1 = 3.3 MRayl) was calculated as follows[32]:

Z1 = Z1/30 Z2/3

L (6)

where Z0 (16 MRayl) is the impedance of the NBT–BT/epoxy 1–3composite and ZL (∼1.5 MRayl) is the impedance of the loadmedium (body tissue or water). In this work, the matching layerwas made by mixing a low-viscosity epoxy (Epo-Tek 301) with analumina powder of ∼5 �m in diameter. The measured impedance ofthe fabricated matching layer is 3.9 MRayl which is close to the des-ignated value. In order to eliminate the back reflections and reducethe ring-down time of the transducers, backing material with highacoustic attenuation (∼15 dB/mm) was prepared by mixing thepolyether-modified epoxy resin (LER-0350, Liyi, Shanghai, China)with tungsten powder and micro-bubbles. Fig. 5 shows schematicdiagrams of the single-element and linear array transducers.

The Krimholz, Leedom, and Mattaei (KLM) equivalent circuit-based software package PiezoCAD (Sonic Concepts, Woodinville,

WA) was used for designing the NBT–BT/epoxy 1–3 compositetransducers with the fabricated matching and backing materi-als. The simulated pulse-echo waveform and frequency spectrumof the transducer are shown in Fig. 6. The optimal pulse-echo

98 D. Zhou et al. / Sensors and Actuators A 182 (2012) 95– 100

Table 1Properties of the lead-free NBT–0.053BT single crystal plate and its 1–3 composite plate compared with other common lead-based and lead-free materials and their 1–3composite plates.

Material d33 (pC/N) εT33 (ε0) Tc (◦C) kt Qm c (m/s) Z (MRayl)

NBT–0.053BT single crystal 430 1000 290 0.63 60 4800 29NBT–0.053BT/epoxy 1–3 composite (� ∼ 0.52) 360 600 – 0.73 8 4100 16NBT–0.050BT ceramics [8] 129 600 – 0.45 220 – –PZT-5H ceramic [28,29] 600 3100 350 0.51 65 3900 34PMN–PT single crystal [30] 1980 5214 150 0.60 41 4666 37PZT/epoxy 1–3 composite (� ∼ 0.50) [31] 300–400 400–1000 – 0.58–0.63 – – 8–20PMN–PT single crystal/epoxy 1–3 composite (� ∼ 0.48) [30] – 2000 – 0.8 14 3600 16

Fa

wtwbqbtwif

atvwc

FN

backing layer. The array kerf was 0.07 mm, which was filled usingthe highly acoustic attenuated LER-0350 epoxy so as to furthereliminate the cross-talk effect.

ig. 5. Construction of the NBT–BT/epoxy 1–3 composite single-element and linearrray transducers.

aveform with a bandwidth of 101% can be obtained when thehickness of matching layer was set to be ∼0.2�, where � is theavelength of the acoustic wave in the matching material emitted

y the active element(s) of the transducer at the resonant fre-uency. In this work, the thickness of the matching layer was set toe 0.120 mm for the single-element transducer and 0.145 mm forhe linear array transducer. The thickness of the backing materialas set to be 3.5 mm, which should be thick enough for eliminat-

ng backward transmitted ultrasound. Thus, the transducers wereabricated according to the simulation results.

The fabricated matching and backing layers were lapped downnd polished to the designed thickness, and then mounted on the

op and bottom surfaces of the 1–3 composites by using low-iscosity epoxy (Epo-Tek 301), respectively. A margin of 0.3 mmas left on both surfaces of the 1–3 composites for subsequent

onnection of the core wire(s) and ground wire(s) of the coaxial

ig. 6. Modeled pulse-echo waveform and frequency spectrum of theBT–BT/epoxy 1–3 composite single element ultrasonic transducer.

cable(s) with conductive adhesive (E-Solder® 3022, Von Roll Isola,USA). In this work, a single coaxial cable was used for the sin-gle element transducer, while eight coaxial cables were used forthe eight-channel linear array transducer. The eight coaxial cableswere arranged with equal distance of 1.4 mm between each otherand then connected individually to the composite plate. Individualarray elements with electrodes and a coaxial cable were diced apartusing a dicing saw. To eliminate the cross-talk between neighbor-ing elements, the dicing depth was set as deep as 0.3 mm into the

Fig. 7. (a) Pulse-echo waveform and (b) frequency spectrum of the fabricatedNBT–BT/epoxy 1–3 composite single element ultrasonic transducer.

D. Zhou et al. / Sensors and Actua

Fig. 8. (a) Pulse-echo waveform and (b) frequency spectrum of a single arrayet

4

pTlp5udo5ftbsHavoc

I

[

[

[

[

[piezoelectrics in the alkaline–bismuth–titanate perovskite family, Applied

lement of the fabricated NBT–BT/epoxy 1–3 composite linear array ultrasonicransducer.

. Transducer performance

To evaluate the performance of the transducers, a conventionalulse-echo response measurement method was performed [33].he transducer was mounted in a water tank in front of a thick stain-ess steel target with a distance at the near field/far field transitionoint. By connecting to an ultrasonic pulser-receiver (Panametrics900PR, Olympus, Japan), active element(s) was excited individ-ally by a 1 �J electrical impulse with 1 kHz repetition and 50 �amping. The echo responses were captured by the receiving circuitf the pulser-receiver and displayed on an oscilloscope (Infinium4810A, HP/Agilent, USA). The built-in Fast Fourier Transform (FFT)eature of the oscilloscope was used to obtain the frequency spec-rum of the pulse-echo response. The center frequency (fc) andandwidth (BW) of the transducer were determined from the mea-ured FFT spectrum. By connecting to a function generator (8116A,P/Agilent, USA), the active element(s) was excited individually by

tone burst of a 20-cycle sine wave with a voltage of Vi at fc. Theoltage Vo of the received echo responses was measured by thescilloscope with 1 M� coupling. The two-way insertion loss (IL)an be calculated from the following equation:

L = 20 log(

Vo

Vi

)(7)

[

tors A 182 (2012) 95– 100 99

Figs. 7 and 8 show the pulse echo response and FFT spectrum ofthe fabricated NBT–BT/epoxy 1–3 composite-based single elementtransducer and linear array transducer, respectively. It can be seenthat they have similar pulse-echo waveform and frequency char-acteristics. As shown in the pulse-echo waveforms, the ring downseems to be damped effectively. The fabricated lead-free 1–3 com-posite transducers possess much broader bandwidth (>100%) thanthe commercial PZT (∼60%) and PZT based 1–3 composite trans-ducers (∼74%) [24]. The experimental results are consistent withthe simulation. The two-way IL calculated by Eq. (7) is −21 dB and−30 dB for the lead-free 1–3 composite based single element andlinear array transducer, respectively.

5. Conclusion

The lead-free NBT–BT single crystal/epoxy 1–3 compositeexhibits ultrahigh thickness electromechanical coupling coefficient(kt = 73%), relatively high piezoelectric constant (d33 = 360 pC/N),and relatively low acoustic impedance (Z = 16 MRayl). Both the sin-gle element and array ultrasonic transducers fabricated using thelead-free composite have shown to exhibit good performance. Abroad bandwidth of exceeding 100% has been obtained which ismuch higher than that of the advanced commercial lead-basedultrasonic transducers fabricated using PZT/epoxy 1–3 composites.The results indicate that the lead-free NBT–BT single crystal com-posite is a promising material to be applied in high-performanceultrasonic transducers with broad bandwidth and high sensitivity.

Acknowledgments

This work was supported by the Hong Kong Innovative Technol-ogy Council (Project No. K-ZP1H) and the Centre of Smart Materialsof the Hong Kong Polytechnic University.

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iographies

an Zhou received his B.S. degree from Fudan University, Shanghai, China, in004, and Ph.D. degree from Shanghai Institute of Ceramics, Chinese Academyf Sciences (SICCAS), Shanghai, China, in 2009. From 2009 to 2011, he was with

epartment of Applied Physics at the Hong Kong Polytechnic University, Hongong, China, as a Research Associate working on piezoelectric materials and ultra-onic devices. Dr. Zhou is presently a senior transducer engineer responsibleor R&D of medical ultrasonic transducers in Edan Instruments, Inc., Shenzhen,hina.

tors A 182 (2012) 95– 100

Kwok-Ho Lam received the M.Phil. and Ph.D. degrees from the Hong Kong Poly-technic University in 2002 and 2006, respectively. During his graduate studies, heworked on the processing and characterization of piezoelectric ceramics, polymers,and composites and their applications in sensors and actuators. From 2007 to 2009,he worked as a Postdoctoral Fellow on the applications of smart materials in civilengineering structures at the Hong Kong Polytechnic University. Dr. Lam currentlyworks as a Research Associate for the NIH Resource Center on Medical UltrasonicTransducer Technology at the University of Southern California. His fields of researchinterest are materials science, sensor and actuator applications of smart materials,and biomedical applications of ultrasonic transducers.

Yan Chen was born in Hebei Province, China. She obtained B.E. and M.Phil.degrees in 2006 and 2009, respectively, both from the Jingdezhen Ceramic Insti-tute, Jingdezhen, China. Now she is a Ph.D. candidate in the Dept. of Applied Physics,Hong Kong Polytechnic University, Hong Kong, China. Her research interests arepiezoelectric materials and ultrasound transducers.

Qinhui Zhang was born in Linyi, China in 1983. He received his B.S. degree in Mate-rial Chemistry in 2005 and M.E. degree in Applied Chemistry in 2008 from HarbinUniversity of Science and Technology, Harbin, China, and Ph.D. degree in 2011 fromShanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), Shanghai,China. His research interests are growth and properties of ferroelectric relaxor singlecrystals.

Chiu Yat Ching received her B.S. degree from the Hong Kong Polytechnic Univer-sity in 2009 with a major in Engineering Physics. From 2009 to 2011, she wasa Research Assistant in Applied Physics Department, responsible for assisting inultrasonic transducers fabrication and characterization processes.

Haosu Luo received his B.S. degree from Nanjing Normal University, Nanjing, China,1982, and his Ph.D. degree from the Shanghai Institute of Ceramics, Chinese Academyof Sciences (SICCAS), Shanghai, China, 1992. He became a Professor at SICCAS in1997. His research interests include the growth, characterization, and practicalapplications of ferroelectric single crystals. Current researches involve the growthof high-Tc relaxor-based single crystals, lead-free piezoelectric single crystals, anddevice applications of these single crystals in ultrasonic transducers, high-strainactuators, infrared detectors, ultrasonic motors, and various sensors. He is involvedin more than 200 publications.

Jiyan Dai received his B.Sc. degree in Physics from Fudan University in 1988, hisM.S. degree in Electrical Engineering from Tsinghua University in 1991, and his Ph.D.degree in Materials Physics from the Chinese Academy of Sciences in 1994. He hasworked at Northwestern University as a Research Associate for three years, and afterone year working in the Institute of Materials Research and Engineering Singapore,he joined Chartered Semiconductor Manufacturing Ltd. in Singapore in failure anal-ysis. His research interest is materials science and medical ultrasound transducers.He joined the Department of Applied Physics at the Polytechnic University in 2001 asLecturer and currently is Associate Professor. He has been working on a few projectsin fabricating endoscopic and high-frequency ultrasound transducers and imagingsystems.

Helen Lai Wa Chan received the B.Sc. and M. Phil. degrees in Physics from the Chi-nese University of Hong Kong in 1970 and 1974, respectively, and the Ph.D. degreein Physics from Macquarie University, Australia, in 1987. She worked as a ResearchScientist in the National Measurement Laboratory of the CSIRO Division of AppliedPhysics, Sydney, Australia, from 1987 to 1991, where she was responsible for setting

up the Australian standards for medical ultrasound transducer calibration. She thenworked as a Senior Acoustic Engineer at GEC-Marconi Pty., Australia, for a year onhydrophone arrays for underwater acoustics before she returned to Hong Kong in1992. Prof. Chan is currently the Chair Professor and Head of Applied Physics at TheHong Kong Polytechnic University. She is a senior member of IEEE.