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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
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
8/18/2019 Lead-free Piezoelectric Materials
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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
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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
E. Taghaddos, M. Hejazi & A. Safari J. Adv. Dielect. 5, 1530002 (2015)
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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.
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