lead-free piezoelectric materials

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Lead-free piezoelectric materials and ultrasonic transducers for medical imaging

Elaheh Taghaddos, Mehdi Hejazi and Ahmad Safari*

Glenn Howatt Electroceramics Laboratories

Department of Materials Science and Engineering

Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, USA

*[email protected]

Received 2 March 2015; Revised 4 May 2015; Accepted 7 May 2015; Published 19 June 2015

Piezoelectric materials have been vastly used in ultrasonic transducers for medical imaging. In this paper, firstly, the most promising

lead-free compositions with perovskite structure for medical imaging applications have been reviewed. The electromechanical

properties of various lead-free ceramics, composites, and single crystals based on barium titanate, bismuth sodium titanate,

potassium sodium niobate, and lithium niobate are presented. Then, fundamental principles and design considerations of ultrasonic

transducers are briefly described. Finally, recent developments in lead-free ultrasonic probes are discussed and their acoustic

performance is compared to lead-based transducers. Focused transducers with different beam focusing methods such as lens

focusing and mechanical shaping are explained. Additionally, acoustic characteristics of lead-free probes including the pulse-echo

results as well as their imaging capabilities for various applications such as phantom imaging, in vitro intravascular ultrasound

imaging of swine aorta, and in vivo or ex vivo imaging of human eyes and skin are reviewed.

Keywords: Lead-free; piezoelectric; ultrasonic transducers; medical imaging.

1. Introduction

Piezoelectric materials generate polarization under applica-

tion of a mechanical force. The generated polarization is

proportional to the applied force via a proportionality coef-

ficient which is called the piezoelectric constant (d ij ). Pie-

zoelectric materials also exhibit a reverse piezoelectric effect

which is an electric field-induced displacement. The me-

chanical strain generated in these materials is also propor-tional to the applied electric field through the piezoelectric

constant.1–5

Ferroelectric materials are a subcategory of piezoelectrics

in which the direction of polarization can be switched by

application of an external electric field. Due to the internal

friction required to nucleate and switch ferroelectric domains

in different crystallographic orientation, ferroelectrics have a

characteristic hysteresis loop with a coercive field (the elec-

tric field required to switch the domains) and a remnant po-

larization. The remnant polarization in ferroelectrics is

permanent and does not disappear upon removing the electric

field. This is opposite to nonferroelectric piezoelectrics such

as ZnO, AlN, and quartz as well as anti-ferroelectrics whichdo not possess a remnant polarization after the electric field is

removed.1,2,4–8

Due to their special characteristics, piezoelectric materials

have found hundreds of civil, military, and energy-related

applications. Automotive, computer, medical, and electronic

industries are the main customers of piezoelectric materials.

Disposable patient monitors, heart monitors, catheters, and

ultrasonic transducers for imaging and noninvasive therapy

are some of the medical applications of piezoelectrics.1,2,4–8

Materials used in aforementioned applications are mostly

based on lead-containing ferroelectric compositions such as

lead zirconate titanate PbZr 0:5Ti0:5O3 (PZT), lead magnesium

niobate–lead titanate Pb(Mg1=3Nb2=3)O3–PbTiO3 (PMN–

PT), and lead magnesium niobate–lead indium niobate–

lead titanate Pb(In1=2Nb1=2)O3–Pb(Mg1=3Nb2=3)O3–PbTiO3

(PIN–

PMN–

PT).2,9–12 Lead is a volatile element with lowvapor pressure which can enter the atmosphere during high

temperature processing or recycling of piezoelectric cera-

mics. It can be directly (through inhalation) or indirectly

(contaminated food, rain, etc.) absor bed into the human body

and cause numerous side effects.10–14 Some of the common

lead-poisoning symptoms with various degrees of severity are

mentioned in Table 1.

Therefore, in order to protect the environment and public

health, it is essential to explore new lead-free piezoelectrics

which can be used as alternatives for their lead-based coun-

terparts. During last two decades, noticeable amount of re-

search has been devoted to study lead-free piezoelectrics.Although lead-free compositions in general show inferior

electromechanical properties compared to their Pb-based

counterparts, for some applications promising lead-free

ceramics have been introduced. There has been a remarkable

progress in development of lead-free piezoelectric ceramics

with improved electromechanical properties in the last de-

cade. Soft lead-free piezoelectrics with high piezoelectric

This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 3.0

(CC-BY) License. Further distribution of this work is permitted, provided the original work is properly cited.

JOURNAL OF ADVANCED DIELECTRICS

Vol. 5, No. 2 (2015) 1530002 (15 pages)

© The Authors

DOI: 10.1142/S2010135X15300029

1530002-1

Review

http://dx.doi.org/10.1142/S2010135X15300029http://dx.doi.org/10.1142/S2010135X15300029


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coefficient and low Curie temperature have been introduced

for actuator and ultrasonic transducer applications. On the

other hand, hard lead-free piezoelectrics with a high elec-

tromechanical quality factor have been developed for high

power devices where minimal heat dissipation and power

consumption is needed.5,9–11,15–17

Ferroelectric materials with perovskite structure (ABO3)

such as BaTiO3 (BT), (Bi0:5Na0:5)TiO3 (BNT), KNbO3 (KN),

and (K 0:5Na0:5)NbO3 (KNN) are the most significant lead-free piezoelectric materials. These compositions show rela-

tively large piezoelectric and dielectric properties and can be

utilized as active elements of ultrasonic transducers. In the

following sections, viable lead-free compositions are briefly

reviewed. Then, the acoustic performance of lead-free ultra-

sonic transducers are discussed and compared to lead-based

probes used for medical imaging.

1.1. BaTiO 3-based piezoelectrics

The first discovered ferroelectric oxide with perovskite

structure is barium titanate (BaTiO3).2 BT has relatively high

electromechanical properties, high dielectric constant, andlow Curie temperature (T C 120

C). BT-based ceramics

have been mainly used for capacitor applications. Their low

Curie temperature restricts the working temperature range in

which these materials can be used.11 Numerous investigations

have been devoted to increase the Curie temperature and

enhance the electromechanical properties of BT-based

ceramics. The binary system of ð1 x ÞBaTiO3– x (Bi0:5K 0:5)TiO3 was studied by Sasaki et al.

19 Introducing bismuth

potassium titanate in BT system increased the Curie tem-

perature T C, however, it diluted the piezoelectric proper-

ties.11,13 Substitution of Ba and Ti by small amount of Sr and

Zr resulted in distortion of the tetragonal unit cell.20

Ba1 x Sr x TiO3 and BaZr x Ti1 x O3 ceramics exhibited high

dielectric tunability and dielectric constant. A high clamped

permittivity " S33="0 of about 1350 and d 33 of 300 pC/N werereported for (Ba0:95Sr 0:05)(Zr 0:05Ti0:95)O3 composition.

21,22

The binary system of Ba(Ti0:8Zr 0:2)O3–(Ba0:7Ca0:3)TiO3(abbreviated as BZT– x BCT) was investigated by Liu and

Ren.23 The highest electromechanical properties were

achieved at BZT–50BCT composition around the morpho-

tropic phase boundary (MPB). An outstanding piezoelectric

coefficient d 33 of 560–620 pC/N was attained for this

composition which was noticeably higher than that of other

lead-free piezoelectrics. This ceramic also showed a re-

markable clamped dielectric constant "S33="0 of about 2820. However, the low Curie temperature of BZT–50BCT

ceramic (T C 93C) restr icts the application of this lead-free

piezoelectric ceramic.23,24

1.2. BNT-based ceramics

BNT-based piezoelectrics with a high remnant polarization

38 C cm2 and moderate depolarization temperature( 200C) are one of the promising lead-free materials.25,26

Pure BNT ceramics, however, suffer from high conductivity

and a large coercive ( 73kV cm1) field which makes thepoling process difficult. In order to enhance the electro-

mechanical properties and decrease the coercive field, bi-

nary or ternary solid solutions in the vicinity of MPB have

been developed. BT, Bi0:5K 0:5TiO3 (BKT), Bi0:5Li0:5TiO3(BLT) are the most widely used materials which have been

added to BNT ceramics to improve the electrical proper-ties.27,28 A-site substituted BNT-based ceramics exhibit a

lower coercive field, lower sintering temperature and higher

electrical resistivity compared to pure BNT. It has been

found that solid solutions of BNT with BKT, BLT, and

BT form an MPB between rhombohedral and tetragonal

phases.11,29–33 The MPB compositions exhibit the highest

electromechanical properties which are suitable for soft

piezoelectric applications. On the other hand, the rhombo-

hedral structure shows higher mechanical qualit y factor

which is desired for high power applications.26,34,35 Table 2

provides properties of several BNT-based ceramics with

different compositions.

1.3. K 0.5 N 0.5 NbO3-based ceramics

Another family of lead-free piezoelectrics with perovskite

structure is based on ANbO3 where A is an alkali metal. The

ferroelectricity in potassium niobate KNbO3 (KN) was dis-

covered by Matthias.13,37 Sodium potassium niobate with the

general formula of K 1 x N x NbO3 (KNN hereafter) is a solid

solution of ferroelectric KN and antiferroelectric NaNbO3compounds which shows promising electromechanical

properties. KNN ceramics possess high Curie temperature

(T C ¼ 420C) and noticeable ferroelectric properties (Pr ¼

33 C/cm2).38 The phase transitions and variations of di-

electric properties of KNN versus temperature are reminiscent

Table 1. Symptoms and signs of lead poisoning.13,18

Mild Moderate Severe

Lethargy Anemia Convulsions

Anorexia Headache Coma

Abdominal discomfort Abdominal cramps Encephalopathy

Arthralgia Gingival lead linePeripheral neuropathy

Renal failure

Table 2. Properties of BNT-based piezoelectric ceramics.

Ceramic composition "T33="0 T d (C)

d 33(pC N1) k 33(%) Ref.

0.88BNT–BKT–BT 1000 113 181 56 34

0.94BNT–BKT–BT 490 185 92 48 34

BNT–0.06BT 730 150 125 55 30

0.88BNT–BKT–BT 440 220 84 47 31, 34, 36

0.76BNT–BKT–BLT 1160 170 174 61 31, 34, 36

E. Taghaddos, M. Hejazi & A. Safari J. Adv. Dielect. 5, 1530002 (2015)

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of BT, yet every transition occurs at a higher temperature.39,40

The highest piezoelectric properties in KNN system was

achieved in K 0:5Na0:5NbO3 composition. The volatility of

alkaline elements and hygroscopic nature of potassium nio-

bate are the obstacles associated with development of KNN.

Sodium potassium niobate ceramics have been chemically

modified to obtain enhanced piezoelectric response andbetter processing repeatability. The effects of incorporation

of LiTaO3 (LT) and LiSbO3 (LS) on the structure, phase

transition, and electrical properties of the KNN ceramics

have been extensively investigated by several research

groups.41–51 Ta addition enhanced the piezoelectric properties

due to the shifting of the transition temperatures.50 A di-

electric constant of 1255, d 33 of 230 pC/N, and k p of 50% was

reported for f(K 0:5Na0:5)0:07Li0:03g(Nb0:8Ta0:2)O3 ceramicsreported by Saito et al.51 The simultaneous addition of Li and

Sb via LiSbO3 decreased the tetragonal–orthorhombic phase

transition temperature T TO while not significantly affecting

the Curie temperature. Shifting T TO

down to room temper-

ature considerably improved the electromechanical proper-

ties. A dielectric constant of 1380 and piezoelectric

coefficient d 33 of 265 pC/N were reported for KNN–LS

ceramics.46,52–54 The combination of KNN–LT and KNN–

LS systems resulted in advent of KNN–LT–LS ceramics

with remarkable dielectric and piezoelectric properties as

presented in Table 3.55

1.4. LiNbO3 single crystals

LiNbO3 single crystals possess very low clamped permittivity

("S33="0 40), high sound velocity (7340 m/s), and very highCurie temperature (T C 1150

C).59 These materials are

mostly used in fabrication of single element high frequency

transducer where a large aperture is required.63

LiNbO3 hasalso been used in fabrication of non-destructive testing

(NDT) ultrasound transducers for high temperature applica-

tions because of their high Curie temperature nature.64,65

1.5. Piezoelectric composites

Besides piezoelectric ceramics and single crystals, piezo-

electric/epoxy composites have also been used as the active

layer in ultrasonic transducers particularly in high frequency

probes for medical imaging. These composites offer several

advantages in comparison to monolithic piezoelectric cera-

mics or polymers. High coupling coefficient, low acoustic

impedance, better acoustic matching to the human body,adjustable dielectric constant, and mechanical flexibility are

some of the benefits of piezoelectric composites.62 Com-

posite materials show higher g33 coefficient (g33 ¼ d 33="T 33)

than ceramics which results in a better sensitivity in the re-

ceiver mode.

Dice-and-fill method is a t raditional technique for fabri-

cation of 1–3 composites.60,62 Injection molding, lost mold,

tape lamination, relic processing, laser ultrasonic cutting, jet

machining, 3D printing and reticulation are some of the other

techniques used for fabr ication of piezoelectric composite

with various structures.64–66 Figure 1 illustrates a modified

dice-and-fill method to prepare 1–3 piezoelectric/epoxy

composites. Table 4 provides properties of several lead-free

composites used in fabrication of ultrasonic transducers.

1.6. Ultrasonic transducers

Ultrasonic transducers are composed of three main compo-

nents: a piezoelectric material, a backing material and one or

Table 3. Properties of KNN-based piezoelectric ceramics.40

Ceramic

composition "T 33="0

T C(C)

T t O(C)

d 33(pC N1) k p(%) Ref.

KNN–BKT 1260 376 75 251 56

KNN–LT 540–1256 323 70 200–230 36–51 41, 51

KNN–

LS 1380 368 35 265 50 46, 52KNN–LT–LS 665–1865 265–290 60 315 48.4 57, 58Ba-doped

KNN–LT–LS

1173 266 70 210 34.8 57

CuO-doped

KNN–LT–LS

1230 264 40 260 48 59

Fig. 1. Schematic illustration of the modified dice-and-fill method used for fabrication of 1–3 composites.60


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multiple matching layers. Piezoelectric materials are the core

component of ultrasonic transducers for generating the ul-

trasound beam as well as receiving the echo signal. The

acoustic performance of transducers is prominently influ-

enced by electromechanical and dielectric properties of the

piezoelectric layer. For medical imaging applications,

broadband transducers are required. The bandwidth (BW(%))

of the transducer is defined as57:

BW ¼ f H f L

f c 100; ð1Þ

where f c is center frequency, and f L and f H are low and high

frequencies at 6 dB of the frequency response spectrum.Broadband transducers can be operated at multiple

transmit/receive frequencies. Transmit at lower frequencies

followed by receiving the echo signal at higher frequencies

enhances the sensitivity and resolution during a medical

imaging event.69,70 Soft piezoelectric ceramics and single

crystals with high coupling coefficient and low mechanical

quality factor offer more efficiency in conversion of electr ical-

acoustic signals which results in a broader bandwidth.69,70

The penetration depth of ultrasound beam in human bodyis a function of frequency and acoustic output power. Ultra-

sound beams with high frequencies provide a better image

resolution but they are highly attenuated by human tissue and

as a result, the penetration depth is reduced. Usually piezo-

electrics with a high dielectric constant suggest higher

acoustic output pressure. This is particularly important for

matrix arrays with small element size which demand high

permittivity materials for a better electrical impedance

matching. On the other hand, lower dielectric permittivity is

required in high frequency single element transducers to

improve the resolution and sensitivity of the transducer. The

frequency of the transducer is determined based on the

dimensions and characteristics of the tissue and the age of the

patient to be imaged.12,69,70

In order to decrease the reflection of acoustic wave at

the transducer –body interface, one or two matching layers are

employed on the front side of the transducer. To maximize

transmission of acoustic energy from transducer to the

medium (human body) and broaden the bandwidth, the

thickness of matching layers needs to be equal to =4(where is the wavelength of acoustic wave in the matchinglayer at the center frequency). The acoustic impedance of

the matching layer ( Z M ) can be calculated either by Eq. (2) or

Eq. (3)36,71–73:

Z M ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z water Z Ceramic

p ; ð2Þ

Z M ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z 2water Z ceramic

3

q : ð3Þ

For a double matching layer design, the appropriateacoustic impedance for the first and second layer can be

calculated through following equations36,71–73:

Z M 1 ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffi Z 3water Z

4Ceramic

7

q ; ð4Þ

Z M 2 ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffi Z 6water Z

1Ceramic

7

q : ð5Þ

The acoustic impedance of piezoelectric ceramics and

human body is about 25–35 MRayls and 1.5 MRayls,

respectively. Materials such as silver-epoxy, Epotek epoxy,

graphite, and parylene with acoustic impedance of 3–7

MRayls are used as a matching layer.71

In ultrasonic transducers, a piezoelectric layer is bondedon a backing material with relatively high acoustic attenua-

tion to reduce the ringdown caused by the echo from backside

of the transducer. Mixture of epoxy with metallic/oxide par-

ticles (such as tungsten) and micro-bubbles are widely used

for the backing layers.71,74,75 The thickness of the backing

layer is chosen to provide an attenuation around 20–30 dB.

The natural focal point ( N ) for flat transducers occurs at

the transition f rom near to far field region which can be

calculated by36

N ¼ a 2

¼ a2

f c

C ; ð6Þ

where a is the radius of the transducer, C is the sound ve-locity, and is the wavelength corresponding to the trans-ducer center frequency ( f c).

The lateral ( Rlat ) and axial ( Rax) resolutions of the trans-

ducer can be calculated from Eqs. (7) and (8), respectively36:

Rlat ¼ F

2a; ð7Þ

Rax ¼ C

2 f ; ð8Þ

where the F is the focal length, 2a is the diameter of trans-

ducer, C is the sound velocity and f is the frequency width

( f high

f low

).

Lens-focusing is the most common method to focus the

ultrasound beam and improve the resolution. Usually convex

lenses made of elastomers such as room temperature vulca-

nization (RTV) and Sylgard silicone with a sound velocity

less than the speed of sound in water ( 1480 m/s) are bondedon top of outer matching layer. The focal length (F ) of the

lens-focused transducers can be calculated by the following

equation76:

F R 1 C 2

C 1

1

; ð9Þ

Table 4. Properties of lead-free composites used in fabrication of ultrasonic

transducers.

Material d 33 (pC/N) "T 33="0 k t Z (MRayl) Ref.

KNN–LT composite 140 300 0.65 6.6 61

BNT–BT composite 360 600 0.73 16 60

BNT–

BT fiber composite

72 588 0.71 —

62


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where R is the radius of curvature of the lens, C 1 is the sound

velocity in the lens, and C 2 is the sound velocity in water.

Figure 2 shows a typical structure of a single element

ultrasonic transducer with backing layer, one matching layer,

and a focusing lens. In this particular case, a concave lens

design was used because the sound velocity in the lens ma-

terial (Epotek epoxy) was higher than that of water.36

Insertion loss (IL) is one of other important characteristics

of transducers which is defined as the ratio of the output

power (Po) to input power (Pi).77 Low IL is desired for a

higher output pressure and less heat generation in the trans-

ducer.36

IL ¼ 10 log po

pi

¼ 20 log

V o

V i

; ð10Þ

2. Lead-Free Ultrasonic Transducers for Medical

Imaging

2.1. BT-based transducers

As mentioned in Sec. 1, an outstanding piezoelectric constant

(d 33 600 pC/N) and high dielectric constant ("S33="0

2800) have been reported for lead-free BZT–50BCTceramics.23,24 Yan et al. used this composition to fabricate a

30 MHz needle type transducer for intravascular imaging

application. The pulse-echo waveform and frequency spec-

trum of the probe exhibited a 6 dB bandwidth of 53%(Fig. 3) with an insertion loss of 18.7 dB. The performanceof this lead-free transducer for biomedical applications was

evaluated by in vitro intravascular ultrasound (IVUS) imaging

of coronary artery. As illustrated in Fig. 4, adequate resolu-

tion and contrast to diff erentiate the vessel wall and fibrous

plaque were achieved.24

Lee et al. prepared a single element 40 MHz transducer

with (Ba0:95Sr 0:05)(Zr 0:05Ti0:95)O3 composition (BSZT).22

The BSZT ceramic has a piezoelectric constant of d 33 ¼300 pC/N and a thickness coupling coefficient of k t ¼ 0:45.Figures 5(a) and 5(b) show the pulse echo response and

frequency spectrum of the transducer. The bandwidth of

76.4% with an IL of 26 dB was achieved in this lead-freedevice. The axial and lateral resolutions were 22mmand 96 m, respectively. Broad bandwidth and high sensi-tivity were achieved due t o high electromechanical coupling

coefficient of the ceramic.22

2.2. Bi0.5 Na0.5TiO 3-based transducers

Hejazi et al. designed and fabricated a high frequency

BNT-based transducer.36 A piezoelectric ceramic with com-

position of 0.88Bi0:5Na0:5TiO3–0.08Bi0:5K 0:5TiO3–0.04Bi0:5-

Li0:5TiO3 (BNKLT88) was chosen as the active element of

Fig. 2. Schemat ic structure of a single-element ultrasonic transducer

(not to scale).36

Fig. 3. The pulse-echo response and frequency spectrum of the

BZT–50BCT transducer.24

Fig. 4. An in vitro IVUS image acquired by the BZT–50BCT

transducer.24


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the transducer. This composition exhibited a thickness cou-

pling coefficient (k t ) of 0.45 and clamped dielectric constant

of 350. Low dielectric constant is desired in single element

high frequency transducers for a better electrical matching

and improved lateral resolution (due to the larger aperture). In

addition, BNKLT88 ceramics possess a higher depolarization

temperature compared to other BNT-based compositions.36

The ceramic with thickness of 109 m was sandwiched be-tween epoxy-tungsten backing and silver epoxy matching

layers to fabricate a single element transducer. The thickness

of backing layer was 1.3 mm to provide 30 dB round tripacoustic attenuation around 20 MHz. The matching layer

thickness was determined based on the quarter wavelength

design rule. A curved Epotek epoxy lens was casted on the

matching layer to improve the acoustic performance.36

Figure 6 shows the pulse-echo waveform and frequency

spectrum of the focused transducer. The center frequency and

6 dB bandwidth were measured to be about 23 MHz and55%, respectively.

Figure 7 illustrates an ultrasound image of a phantom

made of copper wires (30m in diameter) acquired by aBNT-based transducer. The results indicated that this

transducer with a lateral resolution of 260m could beconsidered as a candidate for replacement of lead-based

ultrasonic transducers.36

Chen et al. developed a high frequency transducer based on

BNT-BT single cryst als grown by top-seeded solution growth

(TSSG) technique.78 The clamped dielectric constant and

thickness coupling coefficient of the crystal were 80 and 0.52,

respectively. The crystal was polished down to 87m toachieve a center frequency of about 25 MHz. A thin layer of

(a)

(b)

Fig. 5. (a) Pulse-echo waveform and (b) frequency spectrum of the

BSZT transducer.22

-1

-0.5

0

0.5

1

1.5

13.0 13.5 14.0 14.5 15.0

Time (µs)

N o r m a l i z

e d A m p l i t u d e

EXP

KLM

(a)

EXP

KLM

-30

-25

-20

-15

-10

-5

0

5

5 10 15 20 25 30 35 40

Frequency (MHz)

M a g n i t u d e ( d

B )

(b)

Fig. 6. Simulated (KLM model) and measured (a) pulse echo

responses and (b) frequency domains of a BNT-based lens-focused

transducer.36

Fig. 7. The image of a wire phantom (30m diameter) formed by ahigh frequency BNT-based transducer.36


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parylene (20 m) was deposited on crystal as the matching

layer. Figure 8 shows the pulse-echo waveform and frequencyspectrum of the unfocused BNT-based transducer with a

6 dB bandwidth and IL of 46% and31.9 dB, respectively.78

BNT–epoxy composites have also been investigated for

ultrasonic imaging applications. Zhou et al. used a modified

dice-and-fill method to fabricate 1–3 BNT-based compo-

sites.60 BNT–BT single crystal with a composition close to

the rhombohedral–tetragonal MPB was grown by TSSG

technique. The width and height of diced elements in the

composite (52% crystal volume fraction) were 180 and

500 m, respectively. In order to minimize the cross-talkbetween adjacent elements, an aspect ratio of higher than 2.5

was considered in the composite design.60 Table 5 provides

the properties of BNT–

BT single crystals and 1–

3 compo-sites. The composite showed lower acoustic impedance and

higher coupling coefficient than single crystal which would

improve the bandwidth and sensitivity of the ultrasonic

transducer.60

The BNT-based 1–3 composites reported by Zhou et al.

were used in fabrication of single element and linear arr ay

transducers with center frequencies of about 3–4 MHz.60 A

mixture of Epotek 301 epoxy and alumina powder with

acoustic impedance of 3.9 MRayl was used as the matching

layer. An attenuative backing materials (15 dB/mm) was

prepared by mixing epoxy, tungsten powder, and micro-

bubbles. To reduce the cross-talk between neighboring ele-

ments in the array, the dicing depth was extended into the

backing layer and the array kerf (70 m width) was filledwith an attenuative epoxy. The 6 dB bandwidth of bothsingle element and linear array transducers exceeded 100%.60

In order to image fine tissues for applications such as

ophthalmology and dermatology, high fr equency transducers

with enhanced resolution are required.62 Traditional dice-

and-fill method has limitation for fabrication of composites

with very fine elements and small pitches. Fiber –epoxy

composites can be regarded as an alternative method for

preparation of high frequency transducers.62 Using sol–gel

method, Wang et al. prepared BNT–BT fibers with diameter

of about 150 m.62 1–3 composites were processed by

aligning fibers in a plastic tube followed by epoxy filling(Fig. 9).

The ceramic volume fraction and thickness of the com-

posite were 30% and 143 m, respectively. The thicknesscoupling coefficient k t of the composites was 0.71 which

was considerably greater than that of monolithic ceramic

(k t 0:45). A 14 MHz focused transducer was fabricated byforming a convex shape on the composite attached to sili-

cone rubber as the backing material. The 6 dB bandwidthand IL of t he transducer were measured to be 80% and

34.8 dB.62

2.3. KNN-based transducers

KNN-based transducers for medical imaging application

were developed by Jadidian et al.55 The acoustic perfor-

mance of a 25MHz single element transducer with

(K 0:44Na0:52Li0:04)(Nb0:84Ta0:10Sb0:06)O3 (abbreviated to

KNN–LT–LS) active element was compared to a PZT

fiber 1–3 composite transducer. The properties of KNN-

based ceramics and 1–3 PZT composite are given in

Table 6. The KNN–LT–LS transducer exhibited a 6 dBbandwidth of 70% and IL of 21 dB. The electrical im-pedance of KNN-based piezoceramic was close to 50,

Fig. 9. Scanning Electr on Microscopy image of a BNT–BT fiber/

epoxy 1–3 composite.62

Table 5. Properties of monolithic BNT–BT single crystal and BNT–BT/

epoxy 1–3 composite.60

Material d 33 (pC/N) "T 33="0 k t Qm V (m/s) Z (MRayl)

BNT–BT single

crystal

430 1000 0.63 60 4800 29

BNT–BT/epoxy

1–3 composite

360 600 0.73 8 4100 16

Fig. 8. Pulse-echo waveform and frequency spectrum of the BNT–

BT single crystal transducer.78


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which resulted in smaller IL compared to the lead-based

transducer. Pulse-echo responses and frequency spectra of

KNN–LT–LS and PZT-fiber composite transducers are il-

lustrated in Fig. 10.55

Hagh et al.57 prepared Ba2þ-doped KNN–LT–LS cera-

mics for fabrication of a low frequency transducer with a

center frequency of 5.5 MHz. The transducer with a single

layer matching design ( Z m ¼ 4:3 MRayl) showed a band-width of 50%. It was demonstrated that the acoustic

performance of Ba2þ-doped KNN–LT–LS transducer was

comparable to a PZT–5H transducer.57

The (K 0:5Na0:5)0:97Li0:03(Nb0:9Ta0:1)O3 (abbreviated to

KNN–LT) composition with clamped dielectric constant of

"S33="0 ¼ 890, piezoelectric coefficient of d 33 ¼ 245 pC/N,electromechanical coupling factor of k t ¼ 0:42, and Curietemperature of T C 320

C was also used to fabricate a high

frequency transducer by Wu et al.79 The design parameters

used for an unfocused 40 MHz transducer are summarized in

Table 7. The 6 dB bandwidth (Fig. 11) and two-way in-sertion loss were 45% and 18 dB, respectively. The lens-focused transducer demonstrated axial and lateral resolutions

of 32 m and 102 m, respectively.79

Z a, Z m1, Z m2 and Z b are acoustic impedance (in MRayl); t a,

t m1, t m2 and t b are thickness (in m, except t b).Mechanical dimpling technique has been employed to

fabricate focused transducers with wide bandwidth for appli-

cations such as IVUS. Dimpled ceramics have a continuous

change in thickness which produces multi-resonance fre-

quencies and as a result, boarder bandwidth could be

Table 6. Properties of KNN–LT–LS ceramic and 1–3 PZT fiber composite.55

Material d 33 (pC/N) "T 33="0 "

S33="0 tan k t

KNN–LT–LS 175 644 506 0.022 0.39

1–3 PZT fiber composite 400 541 296 0.013 0.64

(a) (b)

(c) (d)

Fig. 10. The time and frequency domain spectra of (a)-(b) the 1–3 PZT fiber/polymer composite and (c)-(d) KNN–LS–LT transducer.55


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achieved.80–82 A lead-free focused probe with MnO-doped

0.97(K 0:5Na0:5ÞNbO3–0.03(B0:5K 0:5ÞTiO3 ceramic (KNN–BKT) was fabricated and characterized by Chen et al.82

Table 8 compares the performance of dimpled KNN–BKT,

dimpled PMN–PT and plane PMN–PT transducers. Assessing

the ultrasound images of the swine aorta revealed that the

artery wall and fatty tissues were discernable as depicted in

Fig. 12. However, the KNN–BKT probe showed lower sen-

sitivity and higher IL compared to its lead-based counterpart.82

Shen et al. developed a broadband transducer using a 1–3

KNN–LT composite.61 First, the (Na0:535K 0:485)0:95Li0:05(Nb0:8Ta0:2)O3 ceramics were prepared by spark plasma

sintering technique. Then, KNLNT/epoxy composite with

50 m element width and pitch size of 100 m was fabricatedby modified dice-fill method. In order to achieve broader

bandwidth and higher sensitivity two matching layers

were used. The center frequency, 6 dB bandwidth, andtwo way IL of the transducer were 29 MHz, 90% and 25 dB,

respectively.61

Bantignies et al. reported acoustic performance of a

30 MHz linear array tr ansducer based on potassium niobate

(KN) 1–3 composites.83 The linear array contained 128 ele-

ments with 100 m pitch size. It was validated that 30 MHzlead-free transducer with 59% bandwidth was suitable for

skin imaging due to its high sensitivity and large depth of

(a)

(b)

Fig. 12. In vitro imaging of swine aorta acquired by (a) a dimpled

PMN–PT and (b) a dimpled KNN-based probes.82

Table 7. Acoustic impedance and thickness of materials used in KNN-LT

high frequency transducer.

Piezoceramic

First matching

silver epoxy

Second matching

silver epoxy Backing material

Z a t a Z m1 t m1 Z m2 t m2 Z b t b

31 75 7.3 8.4 2.5 13.0 5.9 > 3 mm

Table 8. Summary of results obtained by dimpled and plane transducers.82

Material

F c(MHz)

6 dBBW%

IL

(dB)

Axial

resolution

(m)

Lateral

resolution

(m)

Dimpled KNN–BKT

ceramic

40 72 28.8 44 125

Dimpled PMN–0.28PT

single crystal

34 75 22.9 58 131

Plane PMN–0.28PT

ceramic

29 30 21.8 —- —-

Fig. 11. The pulse-echo characteristics of a 40 MHz KNN–LT

transducer.79


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employed to enhance the lateral resolution and performance

of the transducers. Lens thickness nonuniformity and pro-

cessing flaws resulted in degradation of sensitivity of the

lens-focused transducer. On the other hand, the press-focused

transducer produced higher sensitivity and lower incretion

loss. Table 12 summarizes the acoustic characteristics of

LiNbO3-based transducers. Figure 15 depicts in vivo and ex

vivo ultrasound biomicroscopy (UBM) images of human eye.

The images exhibit outstanding signal-t o-noise ratio, pene-

tration depth, and appropriate contrast.87

Inversion layer design could be applied to fabricate

broadband transducers.88–90 The sign of the piezoelectric

constant and generated strain in the ferroelectric inversion

layer are opposite to those of the regular layer.88–91

Nakamura et al. have shown that the transducer performance

depends on the location of inversion layer.89 The inversion

layer ratio is another key parameter in designing these devi-

ces which can be controlled by annealing temperature, heat-

ing time, or via mechanical bonding. It has been

demonstrated that by increasing the thickness of the inversion

layer to half thickness, most of the energy was transferred tothe second harmonic.88,89 Zhou utilized LiNbO3 single

crystals to develop half-thickness inversion layer high-

frequency ultrasonic transducer. The results showed that the

center frequency of the transducer was increased to twice as

large as the original frequency. The center frequency, 6 dBbandwidth, and two way IL were 60 MHz, 80%, and28 dB,respectively. The pulse echo characteristics of this transducer

are depicted in Fig. 16.88

The concept of dual frequency tr ansducers have

been demonstrated by several researchers.92–95 In regular

transducers there is always a trade-off between the ultrasound

beam penetration depth and image resolution. On the other

hand, in dual frequency transducers, the low frequency

transmit element provides a deep penetration while the high

frequency receive element creates an image with enhanced

resolution. Kim et al. fabricated annular dual element arrays

for high frequency ophthalmic imaging.92 The outer ring

element (12 mm in diameter) was designed to transmit at

20 MHz while the inner circular element (5 mm in diameter)

received the second harmonic signal at 40 MHz. The thick-

ness of LiNbO3 single crystals for transmit and receive

elements were 150 and 77 m, respectively. Press-focusingtechnique was used to create a radius of curvature of 30 mm

to place the focal point at the retina. A double matching layer

design (Insulcast silver epoxy and parylene) was used to

further improve the bandwidth and resolution of the trans-

ducers. The backing material was a centrifuged E-solder

silver epoxy. Figure 17 compares the images of the posterior

segment of an excised pig eye acquired by single element

(fundamental imaging) and dual element (harmonic imaging)

transducers. It was clearly observed that the harmonic image

Table 11. Acoustic properties of several lead-free and lead-based high

frequency transducers.86

Material F c (MHz) BW (%) IL (dB)

Pulse

length (ns)

LiNbO3 crystal 44.5 74 21.3 56

1–

3 PZT Fiber composite 53.6 47 34.4 67PVDF 48.1 118 45.6 44PbTiO3 ceramic 45.1 47 23.7 74

Table 12. Acoustic characteristics of LiNbO3-based high frequency trans-

ducers.87

F c (MHz)

Focusing

technique

Aperture

size (mm)

6 dBBW (%)

IL

(dB)

Pulse

length (ns)

22 Lens 10 72 19.5 145

23 Spherical shaping 12 60 13.4 140

45 Lens 3.0 74 21.3 56

50 Spherical shaping 6.0 60 9.6 54

73 Lens 1.8 57 20.3 40

78 Spherical shaping 3.0 73 13.5 36

(a)

(b)

Fig. 15. (a) An UBM image of the anterior portion of an excised

human eye acquired by 40 MHz LiNbO3 transducer. (b) A wide-

angle view of a normal anterior human eye segment taken in vivo by

a 50 MHz transducer.87


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produced by dual element probe had better resolution than the

image formed by a single element transducer.92

3. Conclusions

In this paper, the current state of lead-free ultrasonic trans-

ducers for medical imaging applications was reviewed.

Electromechanical properties of lead-free ceramics, single

crystals, and composites were summarized and promising

candidates with enhanced properties were identified. The

characteristics of the most encouraging lead-free transducers

along with the properties of their piezoelectric layer are

provided in Table 13.

Lead-free piezoelectrics have been successfully used in

fabrication of single element transducers, linear arrays, dual

frequency annular arrays, and inversion-layer transducers.

LiNbO3 single crystals are suitable for fabrication of single

element transducers with a large aperture. Depending on their

dielectric and electromechanical properties, BNT and KNN-

based ceramics and single crystals could be utilized in single

element or array transducers. Transducers with wide range of

frequencies (3 to 80 MHz) for variety of imaging applications

have been developed. In last decade, a remarkable progress in

enhancement of electromechanical properties of lead-free

ceramics was made and as a result, ultrasonic transducers

with a performance level comparable to PZT-based probes

Table 13. Properties of lead-free and lead-based piezoelectrics along with

characteristics of their ultrasonic transducers.

Material " S33="0

d 33(pC/N) k t

F c(MHz)

6 dBBW (%)

IL

(dB) Ref.

BSZT 1346 300 0.45 42 76 26 22

BZT–

BCT 2817 597 0.41 30 53 19 22, 24LiNbO3 crystal 39 35 0.49 45 54 21 87

KNN–LT–LS 506 175 0.39 26 72 21 55

Baþ2 doped

KNN–LT–LS

(1173) 210 0.37 5.5 50.5 — 96

KNN–LT 890 245 0.42 40 45 18 79

BNT–BT crystal 80 210 0.52 25 46 32 78

BNT–BKT–BLT 353 84 0.45 22 61 28.6 36

KNN–BKT 730 189 0.50 40 72 28.8 82

PMN–0.33PT

(Pb-based)

797 1430 0.58 44 45 15 97

PbTiO3(Pb-based) 200 50 0.49 45 47 24 86

(a) (b)

Fig. 16. (a) Pulse-echo waveform and (b) frequency spectrum for LiNbO3 half-thickness inversion layer transducer.88

(a) (b)

Fig. 17. Images of the posterior segment of an excised pig eye (a) fundamental imaging using the single element transducer and (b) harmonic

imaging using the dual element transducer.92


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have been introduced. The bandwidth of lead-free transducers

is in the range of 45–76% with IL of 18–29% at different

frequencies.

Acknowledgments

The authors would like to appreciate the financial support of

Glenn Howatt foundation for continuous support of research

on lead-free piezoelectrics.

References

1D. Damjanovic, Hysteresis in piezoelectric and ferroelectric

materials, The Science of Hysteresis, ed. G. B. D. Mayergoyz,

Chapter 4 (Academic Press, Oxford, 2006), pp. 337–465.2

G. H. Haertling, Ferroelectric ceramics: History and technology,

J. Am. Ceram. Soc. 82(4), 797 (1999).3

K. M. Ok, E. O. Chi and P. S. Halasyamani, Bulk characterization

methods for non-centrosymmetric materials: Second-harmonicgeneration, piezoelectricity, pyroelectricity, and ferroelectricity,

Chem. Soc. Rev. 35(8), 710 (2006).4

D. Damjanovic, Ferroelectric, dielectric and piezoelectric properties

of ferroelectric thin films and ceramics, Rep. Prog. Phys. 61(9),

1267 (1998).5

J. R€odel, W. Jo, K. T. P. Seifert, E.-M. Anton, T. Granzow and

D. Damjanovic, Perspective on the development of lead-free

piezoceramics, J. Am. Ceram. Soc. 92(6), 1153 (2009).6

N. Setter, Electroceramics: Looking ahead, J. Eur. Ceram. Soc. 21

(10–11), 1279 (2001).7

K. K. Shung, J. M. Cannata and Q. F. Zhou, Piezoelectric mate-

rials for high frequency medical imaging applications: A review,

J. Electroceramics 19(1), 141 (2007).8

S. Tadigadapa and K. Mateti, Piezoelectric MEMS sensors: State-of-the-art and perspectives, Meas. Sci. Technol. 20(9), 092001

(2009).9

A. Safari and M. Abazari, Lead-free piezoelectric ceramics and

thin films, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57

(10), 2165 (2010).10

S. O. Leontsev and R. E. Eitel, Progress in engineering high strain

lead-free piezoelectric ceramics, Sci. Technol. Adv. Mater. 11(4),

044302 (2010).11

T. Takenaka and H. Nagata, Current status and prospects of lead-

free piezoelectric ceramics, J. Eur. Ceram. Soc. 25(12), 2693

(2005).12

Y. Hosono and Y. Yamashita, Piezoelectric ceramics and single

crystals for ultrasonic medical transducers, J. Electroceramics 17

(2–

4), 577 (2006).13

P. K. Panda, Review: Environmental friendly lead-free piezo-

electric materials, J. Mater. Sci. 44(19), 5049 (2009).14

J. N. Gordon, A. Taylor and P. N. Bennett, Lead poisoning: Case

studies, Br. J. Clin. Pharmacol. 53(5), 451 (2002).15

M. D. Maeder, D. Damjanovic and N. Setter, Lead free piezo-

electric materials, J. Electroceramics 13(1–3), 385 (2004).16

E. Ringgaard and T. Wurlitzer, Lead-free piezoceramics based on

alkali niobates, J. Eur. Ceram. Soc. 25(12), 2701 (2005).17

T. Takenaka, H. Nagata, Y. Hiruma, Y. Yoshii and K. Matumoto,

Lead-free piezoelectric ceramics based on perovskite structures,

J. Electroceram. 19, 9035 (2007).

18J. N. Gordon, A. Taylor and P. N. Bennett, Lead poisoning: Case

studies, Br. J. Clin. Pharmacol. 53(5), 451 (2002).19

A. Sasaki, T. Chiba, Y. Mamiya and E. Otsuki, Dielectric and

piezoelectric properties of (Bi0.5Na0.5)TiO3?(Bi0.5K0.5)TiO3

systems, Jpn. J. Appl. Phys. 38(9S), 5564 (1999).20

S. Sen and R. N. P. Choudhary, Impedance studies of Sr modified

BaZr0.05Ti0.95O3 ceramics, Mater. Chem. Phys. 87(2–

3), 256(2004).

21Y. Xu and Y. He, Ferroelectric-dielectric solid solution and com-

posites for tunable microwave application, Ferroelectrics —

Material Aspects, ed. M. Lallart (InTech, 2011), pp. 211–249.22

S. T. F. Lee, K. H. Lam, X. M. Zhang and H. L. W. Chan, High-

frequency ultrasonic transducer based on lead-free BSZT piezo-

ceramics, Ultrasonics 51(7), 811 (2011).23

W. Liu and X. Ren, Large piezoelectric effect in Pb-free ceramics,

Phys. Rev. Lett. 103(25), 257602 (2009).24

X. Yan, K. H. Lam, X. Li, R. Chen, W. Ren, X. Ren, Q. Zhou and

K. K. Shung, Correspondence: Lead-free intravascular ultrasound

transducer using BZT-50BCT ceramics, IEEE Trans. Ultrason.

Ferroelectr. Freq. Control 60(6), 1272 (2013).25

G. O. Jones and P. A. Thomas, Investigation of the structure andphase transitions in the novel A-site substituted distorted perov-

skite compound Na0:5 Bi0:5 TiO3, Acta Crystallogr. B 58(2), 168

(2002).26

Y. Hiruma, K. Yoshii, H. Nagata and T. Takenaka, Phase transition

temperature and electrical properties of (Bi 1/2 Na 1/2) TiO3–

(Bi 1/2 A 1/2) TiO3 (A ¼ Li and K) lead-free ferroelectric cera-mics, J. Appl. Phys. 103(8), 084121 (2008).

27Y. Hiruma, T. Watanabe, H. Nagata and T. Takenaka, Piezo-

electric properties of (Bi1/2Na1/2)TiO3-based solid solution

for lead-free high-power applications, Jpn. J. Appl. Phys. 47(9S),

7659 (2008).28

M. Zhu, L. Liu, Y. Hou, H. Wang and H. Yan, Microstructure and

electrical properties of MnO-doped (Na0.5Bi0.5)0.92Ba0.08TiO3

lead-free piezoceramics, J. Am. Ceram. Soc. 90(1), 120 (2007).29

S. O. Leontsev and R. E. Eitel, Progress in engineering high strain

lead-free piezoelectric ceramics, Sci. Technol. Adv. Mater. 11(4),

044302 (2010).30

T. Takenaka, K. Maruyama and K. Sakata, (Bi1/2Na1/2)TiO3-

BaTiO3 system for lead-free piezoelectric ceramics, Jpn. J. Appl.

Phys. 30(9S), 2236 (1991).31

Y. Hiruma, H. Nagata and T. Takenaka, Phase-transition tem-

peratures and piezoelectric properties of (Bi1/2Na1/2)TiO3- (Bi1/

2Li1/2)TiO3-(Bi1/2K1/2)TiO3 lead-free ferroelectric ceramics,

IEEE Trans. Ultrason. Ferroelectr. Freq. Control 54(12), 2493

(2007).32

Y.-J. Dai, X.-W. Zhang and K.-P. Chen, An approach to improve

the piezoelectric property of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)

TiO3–

BaTiO3 lead-free ceramics, Int. J. Appl. Ceram. Technol. 8(2),423 (2011).

33Z. Luo, J. Glaum, T. Granzow, W. Jo, R. Dittmer, M. Hoffman and

J. R€odel, Bipolar and unipolar fatigue of ferroelectric BNT-based

lead-free piezoceramics, J. Am. Ceram. Soc. 94(2), 529 (2011).34

T. Takenaka, H. Nagata and Y. Hiruma, Phase transition tem-

peratures and piezoelectric properties of (Bi1/2Na1/2)TiO3-and

(Bi1/2K1/2)TiO3-based bismuth perovskite lead-free ferroelectric

ceramics, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 56(8),

1595 (2009).35

Y. Hiruma, H. Nagata and T. Takenaka, Depolarization tempera-

ture and piezoelectric properties of (Bi1/2Na1/2)TiO3–(Bi1/2Li1/2)


1530002-13


14/15

TiO3–(Bi1/2K1/2)TiO3 lead-free piezoelectric ceramics, Ceram.

Int. 35(1), 117 (2009).36

M. M. Hejazi, B. Jadidian and A. Safari, Fabrication and evalu-

ation of a single-element Bi0.5Na0.5TiO3-based ultrasonic

transducer, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 59(8),

1840 (2012).37

F. Jona and G. Shirane, Ferroelectric Crystals, Una Rep edn.(Dover Publications, New York, 1993).

38S. Priya and S. Nahm (eds.), Lead-Free Piezoelectrics (Springer,

New York, 2011).39

B. Jaffe, Piezoelectric Ceramics (Elsevier, 2012).40

S. Priya and S. Nahm (eds.), Lead-Free Piezoelectrics (Springer,

New York, 2011).41

Y. Guo, K. Kakimoto and H. Ohsato, (Na0.5K0.5)NbO3–LiTaO3

lead-free piezoelectric ceramics, Mater. Lett. 59(2–3), 241 (2005).42

Y. Chang, Z. Yang, Y. Hou, Z. Liu and Z. Wang, Effects of Li

content on the phase structure and electrical properties of lead-free

(K0.46−x/2Na0.54−x/2Lix)(Nb0.76Ta0.20Sb0.04)O3 ceramics,

Appl. Phys. Lett. 90(23), 232905 (2007).43

F. Rubio-Marcos, P. Ochoa and J. F. Fernandez, Sintering and

properties of lead-free (K,Na,Li)(Nb,Ta,Sb)O3 ceramics, J. Eur.Ceram. Soc. 27(13–15), 4125 (2007).

44Y. Wang, D. Damjanovic, N. Klein, E. Hollenstein and N. Setter,

Compositional inhomogeneity in Li- and Ta-modified (K, Na)

NbO3 ceramics, J. Am. Ceram. Soc. 90(11), 3485 (2007).45

Y. Wang, D. Damjanovic, N. Klein and N. Setter, High-temperature

instability of Li- and Ta-modified (K,Na)NbO3 piezoceramics,

J. Am. Ceram. Soc. 91(6), 1962 (2008).46

S. Zhang, R. Xia, T. R. Shrout, G. Zang and J. Wang, Piezoelectric

properties in perovskite 0.948(K0.5Na0.5)NbO3–0.052LiSbO3

lead-free ceramics, J. Appl. Phys. 100(10), 104108 (2006).47

E. Hollenstein, M. Davis, D. Damjanovic and N. Setter, Piezo-

electric properties of Li- and Ta-modified (K0.5Na0.5)NbO3

ceramics, Appl. Phys. Lett. 87(18), 182905 (2005).48

D. Lin, K. W. Kwok and H. L. W. Chan, Microstructure, phasetransition, and electrical properties of (K0.5Na0.5)1-xLix(Nb1-

yTay)O3 lead-free piezoelectric ceramics, J. Appl. Phys. 102(3),

034102 (2007).49

M.-S. Kim, S.-J. Jeong and J.-S. Song, Microstructures and pie-

zoelectric properties in the Li2O-Excess 0.95(Na0.5K0.5)NbO3–

0.05LiTaO3 ceramics, J. Am. Ceram. Soc. 90(10), 3338 (2007).50

Y. Dai, X. Zhang and G. Zhou, Phase transitional behavior in

K0.5Na0.5NbO3–LiTaO3 ceramics, Appl. Phys. Lett. 90(26),

262903 (2007).51

Y. Saito and H. Takao, High performance lead-free piezoelectric

Ceramics in the (K,Na)NbO3-LiTaO3 solid solution system,

Ferroelectrics 338(1), 17 (2006).52

S. Zhang, R. Xia, T. R. Shrout, G. Zang and J. Wang, Charac-

terization of lead free (K0.5Na0.5)NbO3–

LiSbO3 piezoceramic,Solid State Commun. 141(12), 675 (2007).

53J. Wu, Y. Wang, D. Xiao, J. Zhu, P. Yu, L. Wu and W. Wu,

Piezoelectric properties of LiSbO3-Modified (K0.48Na0.52)

NbO3 lead-free ceramics, Jpn. J. Appl. Phys. 46(11R), 7375

(2007).54

J. Wu, D. Xiao, Y. Wang, J. Zhu, P. Yu and Y. Jiang, Composi-

tional dependence of phase structure and electrical properties in

(K0.42Na0.58)NbO3-LiSbO3 lead-free ceramics, J. Appl. Phys.

102(11), 114113 (2007).55

B. Jadidian, N. M. Hagh, A. A. Winder and A. Safari, 25 MHz

ultrasonic transducers with lead- free piezoceramic, 1–3 PZT

fiber-epoxy composite, and PVDF polymer active elements, IEEE

Trans. Ultrason. Ferroelectr. Freq. Control 56(2), 368 (2009).56

H. Du, W. Zhou, F. Luo, D. Zhu, S. Qu, Y. Li and Z. Pei, Poly-

morphic phase transition dependence of piezoelectric properties in

(K0.5Na0.5)NbO3–(Bi0.5K0.5)TiO3 lead-free ceramics, J. Phys.

Appl. Phys. 41(11), 115413 (2008).57

N. M. Hagh, B. Jadidian, E. Ashbahian and A. Safari, Lead-freepiezoelectric ceramic transducer in the donor-doped K1/2Na1/

2NbO3 solid solution system, IEEE Trans. Ultrason. Ferroelectr.

Freq. Control 55(1), 214 (2008).58

N. M. Hagh, B. Jadidian and A. Safari, Property-processing re-

lationship in lead-free (K, Na, Li) NbO3-solid solution system,

J. Electroceramics 18(3–4), 339 (2007).59

N. Marandian Hagh, K. Kerman, B. Jadidian and A. Safari, Di-

electric and piezoelectric properties of Cu2þ-doped alkali Nio-bates, J. Eur. Ceram. Soc. 29(11), 2325 (2009).

60D. Zhou, K. H. Lam, Y. Chen, Q. Zhang, Y. C. Chiu, H. Luo,

J. Dai and H. L. W. Chan, Lead-free piezoelectric single crystal

based 1–3 composites for ultrasonic transducer applications, Sens.

Actuators Phys. 182, 95 (2012).61

Z.-Y. Shen, J.-F. Li, R. Chen, Q. Zhou and K. K. Shung, Micro-scale 1–3-type (Na,K)NbO3-based Pb-free piezocomposites for

high-frequency ultrasonic transducer applications, J. Am. Ceram.

Soc. Am. Ceram. Soc. 94(5), 1346 (2011).62

D. Y. Wang, K. Li and H. L. W. Chan, High frequency 1–3

composite transducer fabricated using sol–gel derived lead-free

BNBT fibers, Sens. Actuators Phys. 114(1), 1 (2004).63

Q. Zhou, K. H. Lam, H. Zheng, W. Qiu and K. K. Shung, Pie-

zoelectric single crystal ultrasonic transducers for biomedical

applications, Prog. Mater. Sci. 66, 87 (2014).64

A. Baba, C. T. Searfass and B. R. Tittmann, High temperature

ultrasonic transducer up to 1000 C using lithium niobate single

crystal, Appl. Phys. Lett. 97(23), 232901 (2010).65

X. Jiang, K. Kim, S. Zhang, J. Johnson and G. Salazar, High-

temperature piezoelectric sensing, Sensors 14(1), 144 (2013).66

A. Safari, M. Allahverdi and E. K. Akdogan, Solid freeform

fabrication of piezoelectric sensors and actuators, J. Mater. Sci. 41

(1), 177 (2006).67

E. Koray Akdogan, M. Allahverdi and A. Safari, Piezoelectric

composites for sensor and actuator applications, IEEE Trans.

Ultrason. Ferroelectr. Freq. Control 52(5), 746 (2005).68

A. Bandyopadhyay, R. K. Panda, V. F. Janas, M. K. Agarwala,

R. van Weeren, S. C. Danforth and A. Safari, Processing of pie-

zocomposites by fused deposition technique, in Proc. Tenth IEEE

Int. Symp. Applications of Ferroelectrics, 1996. ISAF '96 , Vol. 2

(1996), pp. 999–1002.69

J. Chen and R. Panda, Review: Commercialization of piezoelectric

single crystals for medical imaging applications, 2005 IEEE

Ultrasonics Symp., Vol. 1 (2005), pp. 235–

240.70

J. Chen, R. Panda and B. Savord, Realizing dramatic improve-

ments in the efficiency, sensitivity and bandwidth of ultrasound

transducers, Philips.71

J. D. Bronzino, The Biomedical Engineering Handbook. 1

(Springer Science & Business Media, 2000).72

G. M. Lous, I. A. Cornejo, T. F. McNulty, A. Safari and

S. C. Danforth, Fabrication of piezoelectric ceramic/polymer

composite transducers using fused deposition of ceramics, J. Am.

Ceram. Soc. 83(1), 124 (2000).73

J. H. Goll, The design of broad-band fluid-loaded ultrasonic

transducers, IEEE Trans. Sonics Ultrason SU-26(6), 385 (1979).


1530002-14


15/15

74H. Wang, T. A. Ritter, W. Cao and K. K. Shung, High

frequency properties of passive materials for ultrasonic trans-

ducers, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 48(1),

78–84 (2001).75

K.-B. Kim, D. K. Hsu, B. Ahn, Y.-G. Kim and D. J. Barnard,

Fabrication and comparison of PMN-PT single crystal, PZT and

PZT-based 1–

3 composite ultrasonic transducers for NDE appli-cations, Ultrasonics 50(8), 790 (2010).

76K. K. Shung, M. B. Smith and B. M. W. Tsui, Principles of

Medical Imaging (Academic Press, San Diego, 1992).77

D.-Y. Wang and H. L.-W. Chan, A dual frequency ultrasonic

transducer based on BNBT-6/epoxy 1–3 composite, Mater. Sci.

Eng. B 99(1–3), 147 (2003).78

Y. Chen, X. P. Jiang, H. S. Luo, J. Y. Dai and H. L. W. Chan,

High-frequency ultrasonic transducer fabricated with lead-free

piezoelectric single crystal, IEEE Trans. Ultrason. Ferroelectr.

Freq. Control 57(11), 2601 (2010).79

D. W. Wu, R. M. Chen, Q. F. Zhou, K. K. Shung, D. M. Lin

and H. L. W. Chan, Lead-free KNLNT piezoelectric ceramics for

high-frequency ultrasonic transducer application, Ultrasonics 49

(3), 395 (2009).80K. H. Lam, Y. Chen, K. F. Cheung and J. Y. Dai, PMN-PT single

crystal focusing transducer fabricated using a mechanical dim-

pling technique, Ultrasonics 52(1), 20 (2012).81

Y. Chen, K. H. Lam, D. Zhou, W. F. Cheng, J. Y. Dai, H. S. Luo

and H. L. W. Chan, High frequency PMN-PT single crystal fo-

cusing transducer fabricated by a mechanical dimpling technique,

Ultrasonics 53(2), 345 (2013).82

Y. Chen, W. B. Qiu, K. H. Lam, B. Q. Liu, X. P. Jiang,

H. R. Zheng, H. S. Luo, H. L. W. Chan and J. Y. Dai, Focused

intravascular ultrasonic probe using dimpled transducer elements,

Ultrasonics 56, 227 (2015).83

C. Bantignies, E. Filoux, P. Mauchamp, R. Dufait, M. Pham Thi,

R. Rouffaud, J. M. Gregoire and F. Levassort, Lead-free high-

frequency linear-array transducer (30 MHz) for in vivo skin im-aging, in 2013 IEEE Int. Ultrasonics Symp. (IUS) (2013), pp. 785–

788.84

F. Levassort, E. Filoux, M. Lethiecq, R. Lou-Moler, E. Ringgaard

and A. Nowicki, P3Q-3 curved piezoelectric thick films for high

resolution medical imaging, in IEEE Ultrasonics Symposium,

2006 (2006), 2361–2364.85

F. Levassort, J. Gregoire, M. Lethiecq, K. Astafiev, L. Nielsen,

R. Lou-Moeller and W. W. Wolny, High frequency single element

transducer based on pad-printed lead-free piezoelectric thick films,

in 2011 IEEE Int. Ultrasonics Symp. (IUS) (2011), pp. 848–851.86

K. A. Snook, J.-Z. Zhao, C. H. F. Alves, J. M. Cannata,

W.-H. Chen, R. J. Meyer, T. A. Ritter and K. K. Shung, Design,

fabrication and evaluation of high frequency, single-element

transducers incorporating different materials, IEEE Trans. Ultrason.


J. M. Cannata, T. A. Ritter, W.-H. Chen, R. H. Silverman and

K. K. Shung, Design of efficient, broadband single-element

(20–80 MHz) ultrasonic transducers for medical imaging appli-

cations, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 50(11),

1548 (2003).88Q. Zhou, J. M. Cannata, H. Guo, C. Huang, V. Z. Marmarelis

and K. K. Shung, Half-thickness inversion layer high-frequency

ultrasonic transducers using LiNbO/sub 3/single crystal, IEEE

Trans. Ultrason. Ferroelectr. Freq. Control 52(1), 127 (2005).89

K. Nakamura, K. Fukazawa, K. Yamada and S. Saito, Broadband

ultrasonic transducers using a LiNbO/sub 3/plate with a ferro-

electric inversion layer, IEEE Trans. Ultrason. Ferroelectr. Freq.

Control 50(11), 1558 (2003).90

T. Funasaka, M. Furuhata, Y. Hashimoto and K. Nakamura, Pie-

zoelectric generator using a LiNbO3 plate with an inverted do-

main, 1998 IEEE Ultrasonics Symp., 1998. Proc., Vol. 1 (1998),

pp. 959–962.91

K. Nakamura and H. Shimizu, Local domain inversion in ferro-

electric crystals and its application to piezoelectric devices, inUltrasonics Symp., 1989. Proc., IEEE 1989, Vol. 1 (1989), pp.

309–318.92

H. H. Kim, J. M. Cannata, R. Liu, J. H. Chang, R. H. Silverman

and K. K. Shung, 20 MHz/40 MHz dual element transducers

for high frequency harmonic imaging, IEEE Trans. Ultrason.


E. J. W. Merks, A. Bouakaz, N. Bom, C. T. Lancee, A. F. W. van

der Steen and N. de Jong, Design of a multilayer transducer for

acoustic bladder volume assessment, IEEE Trans. Ultrason. Fer-

roelectr. Freq. Control 53(10), 1730 (2006).94

H. J. Vos, M. E. Frijlink, E. Droog, D. E. Goertz, G. Blacquière,

A. Gisolf, N. de Jong and A. F. W. van der Steen, Transducer for

harmonic intravascular ultrasound imaging, IEEE Trans. Ultrason.


K. Nakamura, K. Fukazawa, K. Yamada and S. Saito, An ultra-

sonic transducer for second harmonic imaging using a LiNbO3

plate with a local ferroelectric inversion layer, IEEE Trans.

Ultrason. Ferroelectr. Freq. Control 53(3), 651 (2006).96

N. M. Hagh, B. Jadidian, E. Ashbahian and A. Safari, Lead-free

piezoelectric ceramic transducer in the donor-doped K1/2Na1/

2NbO3 solid solution system, IEEE Trans. Ultrason. Ferroelectr.

Freq. Control 55(1), 214 (2008).97

Q. Zhou, X. Xu, E. J. Gottlieb, L. Sun, J. M. Cannata, H. Ameri,

M. S. Humayun, P. Han and K. K. Shung, PMN-PT single crystal,

high-frequency ultrasonic needle transducers for pulsed-wave

Doppler application, IEEE Trans. Ultrason. Ferroelectr. Freq.

Control 54(3), 668 (2007).


lead-free piezoelectric materials

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