Advances in development of Pb-free piezoelectric materials for transducer applications

Pb-based ferroelectrics and piezoelectrics in the form of bulk polycrystalline and textured ceramics, single crystals, and composites, have been used in sensors, actuators, and other electromechanical devices. However, the toxicity of these materials has been a major concern around the globe for the past few decades. The report of high piezoelectric activity in the lead-free BaTiO3 (BT), (Bi0.5Na0.5)TiO3 (BNT), and (K0.5,Na0.5)NbO3 (KNN) and binary and ternary systems with other compounds has given high hopes for alternatives to Pb-based materials. Recent modifications of KNN-based compositions with BaZrO3 in combination with (Bi0.5,K0.5)HfO3 result in excellent electromechanical properties. Therefore, increased research and development in Pb-free materials brings hope for practical applications closer to reality. In this article, the recent developments on BT, BNT, and KNN reproducible soft and hard Pb-free piezoelectric compositions with a range of electromechanical properties for low- and high-power transducer applications will be reviewed. Several examples in the development of lead-free HF ultrasound transducers will be presented.


Introduction
Ferroelectricity is defined as the ability of a material to retain a switchable spontaneous polarization, P s , by an electric field and was discovered in 1921 in Rochelle salt. Ferroelectric crystals also exhibit unique properties such as spontaneous polarization below the Curie temperature, ferroelectric domains, and Curie-Weiss type behavior of the dielectric constant above the Curie temperature. Since then, ferroelectricity has been discovered in several other structurally different categories of materials, the most common of which is the perovskite ABO 3 structure, in which A is a large cation (1 + , 2 + , and 3 + valances) and B is a small cation with valences of 3 + , 4 + , and 5 + .
Piezoelectricity was discovered by the brothers Pierre and Jacques Curie in 1880 during their study of the effects of pressure on the generation of electrical charge by crystals such as quartz, tourmaline, and Rochelle salt. [1][2][3] Piezoelectrics are polar materials in which an electrical charge can be produced by the application of a mechanical stress (direct piezoelectric effect). The proportionality constant relating the generated charge to the applied force is called the piezoelectric coefficient. In the longitudinal direction, the piezoelectric charge coefficient is known as d 33 with units of pCN −1 . The d 33 can be measured directly using a conventional piezoelectric tester at a lower frequency or at a higher frequency using the methods of the IEEE Standard on Piezoelectricity. The effective d 33 (written d 33 *) is sometimes reported and is calculated from the slope of the strain versus electric field curve. Alternatively, if an electric field is applied to a piezoelectric material, it shows a mechanical displacement or strain (converse piezoelectric effect). All ferroelectrics possess piezoelectric properties; however, not all piezoelectrics show ferroelectricity. There are at least two advantages associated with ferroelectrics, including very high piezoelectric properties and the fact that ferroelectric ceramics can be rendered into a pseudo-single crystal piezoelectric by a cost-effective poling process in lieu of crystallographically orienting a non-ferroelectric piezoelectric single crystal. 4,5) Piezoelectrics are anisotropic materials, and their properties vary in different crystallographic directions. The number of independent piezoelectric coefficients in a third rank piezoelectric tensor depends on the symmetry elements of a specific point group. Poled ceramics with ∞∞m symmetry only have three independent piezoelectric coefficients d 33 , d 31 , and d 15 . 4,6) The degree of conversion between electrical and mechanical energy in piezoelectrics is described by the coupling coefficient (k ij ). It is defined as the square root of the ratio of mechanical energy stored in a piezoelectric to the electrical input energy, or vice versa. Depending on the geometry, and the position of electrodes with respect to the poling direction, a piezoelectric resonator has different fundamental resonance modes, such as thickness (k t ), planar (k p ), longitudinally poled length extension (k 33 ) and transversely poled length extension (k 31 ) modes. Another important property is the quality factor, which generally quantifies the "resistance" to damping of an oscillator or resonator. An oscillator with a higher quality factor shows more stability in performance during operation. In the case of mechanical vibrators such as piezoelectric materials, this is specifically represented by the mechanical quality factor (Q m ). 7) For practical high-power applications such as ultrasonic cleaners, atomizers, and high-intensity focused ultrasound (HIFU) for it is desirable to have a higher vibration velocity ν 0 : . , 1 product of Q m and either longitudinal and transverse piezoelectric coefficients or the coupling of various modes of operation. When a device operates in the resonance mode, the product of the mechanical quality factor and the square of the electromechanical coupling coefficient (Q m .k 2 ) is the critical evaluation factor. 4) For over 7 decades, lead-based piezoelectric ceramics, in particular lead zirconate titanate compositions, Pb(Zr x ,Ti 1−x )O 3 (PZT), have been the most dominant piezoelectric materials due to their remarkable properties and relatively low cost of processing. This was mainly due to the existence of a morphotropic phase boundary (MPB) between two ferroelectric phases (tetragonal-rhombohedral), which results in effective poling with the maximum value of electromechanical properties. The functional properties of MPB materials are notably stable over a broad range of temperature, time, and pressure. 7) Doping of PZT with electron donors such as La 3+ or Nb 5+ is known to produce soft PZT with high electromechanical properties and a low critical temperature T c , remnant polarization P r , coercive field E c , and Q m . On the other hand, acceptor doping of PZT with Fe 3+ , also called hard PZT, gives properties that are opposite those of soft PZT. There exists another important type of phase boundary, namely the polymorphic phase boundary (PPB), in which the functional properties are temperature sensitive, which limits the use of these materials in commercial applications. From a broader perspective, PPBs are regions where a temperature-driven structural phase transition occurs, in which coexisting of phases with comparable free energies, such as rhombohedral-tetragonal (R-T) and rhombohedral-orthorhombic-tetragonal (R-O-T) phases, can be obtained around ambient temperature via chemical modification along with enhancement of the piezoelectric properties. 8) Like a MPB, the phase coexistence in a PPB favors the polarization-rotation phenomenon because the energy profile for transition between different orientations of P s flattens in this region.
In this review article, the most important lead-free ferroelectric and piezoelectric systems with the perovskite structure (ABO 3 ), such as BaTiO 3 (BT), BCT-BZT, (Bi 0.5 Na 0.5 )TiO 3 (BNT), and (K 0.5 Na 0.5 )NbO 3 (KNN), will be reviewed for sensor, actuator, and ultrasound low-and high-frequency transducer applications. Several compositions show relatively large piezoelectric coefficients. However, a few of their main drawbacks are generally either low Curie temperatures (T c ) or a decrease in electromechanical properties with temperature. 9) In order to enhance T c and improve the dependence of piezoelectric and ferroelectric properties with temperature, various binary and ternary solid solutions in the vicinity of the MPB have been developed. Textured ceramics oriented in [001] or [110] directions also show enhanced properties compared to polycrystalline piezoelectric materials.  10) There is also evidence of German research on a TiO 2 -based capacitor, patented in 1936. 9) BT has a relatively high k 33 , however the low Curie temperature (T c = 120°C) restricts the working temperature range and its overall usefulness. 9,11) Many investigations have sought to increase T c and improve the piezoelectric and dielectric properties of BT-based systems. The binary system (1-x)BaTiO 3 -x(Bi 0.5 K 0.5 )TiO 3 (BT-BKT) has been reported by Sasaki et al. 12) Introducing bismuth potassium titanate in BT increases T c to ∼175°C, however, this dilutes the piezoelectric properties considerably. 9) In several studies, Wada et al. have investigated the effects of domain size and texturing on piezoelectric properties in BT ceramics. In 2007, the preparation of 〈110〉-oriented BT ceramics via a template grain growth method using 〈110〉oriented BT particles as seed and nanoscale hydrothermal BT spheres as matrix was reported. 13) The textured ceramics showed a relative density of 96%, Lotgering factor, F 110 = 85%, and an excellent d 33 of 788 pCN −1 . The maximum d 33 was limited, however, by the large grain size of ∼75 μm. The improvement in piezoelectric properties was attributed to both orientation and smaller domain size of ∼800 nm. This effect had been previously demonstrated in single crystal BT, 14) and randomly-oriented BT ceramics with domain size 50 nm. 15) Recently, Khanal et al. fabricated 〈110〉-oriented BT ceramics by reactive templated grain growth, RTGG, using the layered titanate H 1.08 Ti 1.73 O 4 ·nH 2 O (HTO) as a template and BaCO 3 as a reactive matrix. 16) Samples sintered under optimized conditions showed a high degree of 〈110〉-orientation (F 110 = 83%), a smaller grain size of ∼19 μm. The effective d 33 * = 445 pm V −1 was lower than desired property, but still ∼34% higher than from randomly oriented BT ceramic. The lower d 33 * was attributed to a low relative density of 93%. Higher sintering temperatures were shown to increase density at the expense of Lotgering factor.
The  18) It has been shown that this composition develops a MPB, which is located at the triple point of cubic-rhombohedral-tetragonal phases with x = 32% and T = 57°C. Optimum dielectric permittivity, ε r = 3060, piezoelectric coefficient, d 33 = 620 pCN −1 , spontaneous polarization P s = 20 μC cm −2 , and remnant polarization P r = 14.8 μC cm −2 were reported for the 0.5BZT-0.5BCT composition, which is comparable with soft PZT. The application of this composition, however, is again restricted by a very low T C of 93°C. 18 is one of the most prominent leadfree piezoelectric materials, with a large remnant polarization (P r ∼ 38 μC cm −2 ) and high T C . 7,23) BNT is a relaxor-type ferroelectric with a rhombohedral structure at RT. It shows a diffuse phase transition to tetragonal (T R-T ∼ 300°C) and then transitions to the cubic phase at T ∼ 540°C. The maximum dielectric temperature (T m ) of BNT is ∼320°C-340°C and the depolarization temperature is around 185°C. [23][24][25] Pure BNT ceramics have not attracted considerable attention for practical applications due to their difficulty in poling. 18) Binary and ternary BNT-based solid solutions, however, have been reported as potential alternatives for lead-based ferroelectrics. Takenaka et al. reported the structure, phase diagram, and ferroelectric/piezoelectric properties of the complex oxide compositions BNT-BT, BNT-BKT, BNT-BKT-BT, and BNT-BKT-BLT. 26) It has been found that these solid solutions form MPB compositions between rhombohedral and tetragonal phases and exhibit improved piezoelectric properties.  27) The BNT-BT phase diagram has an antiferroelectric phase that occurs at 130°C for the MPB composition. 27) The transition to the antiferroelectric phase causes depolarization of the ferroelectric composition. 28) Above the depolarization temperature, T d , there is a coexistence of ferroelectric and antiferroelectric states. 29,30) Co-substitution of small amounts of monovalent cations such as Li 1+ and K 1+ in the A-site of BNT can increase the T d up to 200°C-220°C.
2.2.2. (Bi 0.5 Na 0.5 )TiO 3 -(Bi 0.5 K 0.5 )TiO 3 -BaTiO 3 (BNT-BKT-BT) compositions. Nagata et al. investigated the dielectric and piezoelectric properties of the ternary system, xBNT-yBKT-zBT (x + y + z = 1, y : z = 2 : 1) focusing on the region around the MPB between the tetragonal and rhombohedral phases. 31) It was shown that a maximum d 33 of 191 pCN −1 , was obtained for 85.2 BNT-12 BKT-2.8 BT in the tetragonal phase near the MPB. The T c , k 33 , and relative permittivity of this composition were found to be 301°C, 0.56, and 1141, respectively. Hiruma et al. however, carried out a detailed study on the phase transition temperatures, T d , T R-T , and piezoelectric properties of the BNT-BKT-BT ceramic system. 29) They have reported that the T R-T of BNT ceramics is about 300°C which is the same as that reported value for single crystal. It was also shown that the T d of the rhombohedral phase (x = 0.94) is 188°C and the MPB phase (x = 0.88) has the highest d 33 of 181 pCN −1 with a lower T d of 113°C. This study also revealed the existence of an intermediate phase with ferroelectric and antiferroelectric properties at temperatures higher than T d around the MPB.
Recently, Yesner et al. have studied the effect of A-site non-stoichiometry in 0.88BNT − 0.08BKT-0.04BT, aiming to further develop these Pb-free soft piezoelectric materials by developing Bi-excess and Alkali deficiency compositions. 32) Bismuth excess caused an increase in d 33 and the dielectric constant and a decrease in the coercive field and critical temperature, which is consistent with the effects of donor doping. The alkali deficient compositions did not show any piezoelectric properties after poling at RT as the depolarization temperature was depressed to below RT.   36) This discovery triggered many investigations into the origin of the enhanced properties in this system and the possibility of further improvement. The textured KNN-LT-LS ceramic has a d 33 of 416 pCN −1 that rivals that of the hard PZT4 composition (Fig. 2). 36) Although it was originally proposed that the high piezoelectric activity of KNN-LT-LS was due to the existence of an MPB, several reports have suggested that the improvement in properties was mainly due to the shift of a polymorphic phase transition between ferroelectric tetragonal and orthorhombic phases in the KNN phase diagram. 37) The presence of such a PPB dictates instability in the electrical properties with temperature, making the composition unsuitable for a variety of applications. Therefore, there have been several investigations into the effects of various dopants to shift the PPB to either higher or lower temperatures and hence avoid the unstable region of the phase diagram. For example, Cu-doping in KNN-LT-LS was studied as a function of dopant concentration by Hagh et al. where an increase in grain size and a change in growth behavior were observed. 38) Moreover, addition of Cu was shown to stabilize the orthorhombic phase by increasing T O-T while showing no change in T c . As a result, d 33 and k p were slightly degraded, and dielectric loss was drastically decreased. Another study carried out by Hagh et al. involved Ba-substitution in KNN-LT-LS. Incorporation of Ba 2+ shifted T C downward and increased the RT relative permittivity. This was accompanied by a broadening of the relative permittivity versus temperature peak. 38 43) It was demonstrated that the enhanced unipolar strain was closely related to a reduction in the remnant strain, S rem , due to the dominant presence of a nonpolar phase at zero electric field. Jo et al. then proposed that the giant strain originated from a combined effect of the intrinsically high poling strain, S pol , of BNT-BT based systems, the presence of a nonpolar phase at zero electric field, which destabilizes and randomizes an electrically induced ferroelectric order, and an easy transition between the nonpolar and ferroelectric phases due to their comparable free energies.
Recently, A. Song et al. developed KNN-based composition by designing Dopant Graded Ceramics with a diffused PPB by introducing a layered distribution of key dopants into the monolithic ceramic with stepwise variation of the PPB temperature along the thickness direction. This resulted in a broadening of the temperature range of phase coexistence, thus improving the attractiveness of KNN-based composition  ferroelectric, and piezoelectric properties of these compositions. 51) Both Mn-and Fe-doping results in a considerable enhancement of Q m in both planar and thickness resonance modes. In the 1.5 mol% Mn-doped composition, planar Q m ∼ 970 and tan δ = 0.88% were reported. In the Fedoped BNT-BKT-BLT composition, a planar Q m as high as 900 was achieved. It was shown that acceptor dopants also resulted in decreased coupling coefficients, d 33 , and dielectric constant. 47,48) Taghaddos also studied the effects of the powder particle size on the grain size and Q m of Mn-doped BNT-BKT-BLT and reported an optimum Q m of 1200 with a fine grain size of ∼15 μm. 52) In addition, G. Yesner has extensively studied the effects of non-stoichiometry on the Q and electromechanical properties of the BNT-BKT-BT system, attempting to develop a soft and hard Pb-free BNT-based piezoelectric ceramic. 32 56) To understand the mechanism of improvement in properties and T d , high-energy synchrotron X-ray powder diffraction data were taken to carefully study the crystal structures of the quenched samples. 56) It was concluded that Bi 3+ ions in the quenched ceramics were displaced more from the center position of the A-site in the perovskite structure, increasing rhombohedral distortion. In another study, Takagi et al. analyzed oxygen tracer diffusion and examined the domain structures of the quenched BNT composition by controlling the quenching rate and compared with those of BNT with soft and hard dopants. 57) The report indicated that the diffusion coefficients, D, of 18 O tracer for the conventionally-fired and quenched samples were 2.5 × 10 -11 and 1.8 × 10 -11 cm 2 s −1 , respectively, and no significant differences were observed in the oxygen vacancies of quenched samples in hard and soft compositions as well. It was indicated, however, that the domain size of BNT in the quenched samples was about twice that of the unquenched BNT ceramics [Figs. 4(c) and 4(d)]. This suggests that quenching of these ceramics increases domain size and decreases domain wall density, inducing lattice distortion that results in increased T d , thus affecting electromechanical properties. 57) In addition to the investigations on soft BNT-BKT-BT, Yesner et al. have evaluated the effect of non-stoichiometric compositions 0.88Bi 0.50−x Na 0.50 TiO 3 -0.08Bi 0.50−x K 0.50 TiO 3 -0.04BaTiO 3 aiming to develop hard Pb-free piezoelectric materials. 32) Hard BNT-based piezoelectrics were developed by Bi-deficient or alkali-excess compositions and were found to have increased Q m . The composition with 2% Bi-deficiency had a Q m of 1200 but showed increased leakage current. The electrical conductivity of the Bi-deficient ceramics was lowered by 3-4 orders of magnitude by adding 0.3-0.5 wt% Bi 2 O 3 into the calcined powders, which made poling easier and slightly improved piezoelectric properties, but reduced T d .
Doshida et al. studied the hardening behavior of 0.85(Bi 0.5 Na 0.5 )TiO 3 −0.15BaTiO 3 (BNT-BT) piezoelectric ceramics was investigated by doping with Bi 0.5 Na 0.5 MnO 3 (BNM) powder. 58) BNM acts as a sintering aid, produces domain pinning, and increases tetragonality based on BaTiO 3 for higher stability. Q m of BNT-BT with 0.75 wt% BNM exceeded 1200 with n 0 twice that of hard-PZT is archived. The equivalent stiffness slightly decreased with strain, and the mechanical nonlinearity was much less than that of hard-PZT. BNBT15-BNM (0.75 wt%) has superior high-power properties, and is expected to be a candidate material for use in leadfree piezoelectric ceramics in high-power applications.

Low temprature sintering of Pb-Free piezoelectric materials
Yesner extensively studied the low temperature sintering and compatibility of Bi-based piezoelectric ceramics and copper electrodes by co-firing BNT-BKT-BLT with copper metal. 59) A combination of Bi 2 O 3 , CuO, ZnO, Li 2 CO 3 , and B 2 O 3 were used as additives to reduce the sintering temperature to 900°C (compared to 1150°C for conventional sintering) with minimal effect on electromechanical properties. The oxygen partial pressure was maintained at 6.1 × 10 −8 atm, which is necessary for the coexistence of Cu and Bi 2 O 3 . The BNT-based ceramics were successfully co-fired with both internal and surface Cu electrodes. These ceramics were successfully polarized, and a d 33 of 77 pCN −1 was reported for the sample with the Cu surface electrode. The polarization-electric field hysteresis of BNT-BKT-BT ceramics with the addition of 0.  Fig. 5(a)]. 32,59) L. Gao et al. have successfully co-fired Li-and Tamodified KNN ceramics with inner Cu electrodes in a reducing atmosphere for possible Pb-free multilayer ceramic (MLC) piezoelectric applications. 60) KNN ceramics with a high relative density of 95% were obtained by sintering the samples at 1050°C in a low oxygen partial pressure of 10 −12 atm. The samples were additionally annealed at 850°C and P O2 = 10 −5 atm to decrease oxygen vacancy concentration. TEM studies of these samples show no evidence of inter-diffusion and/or chemical reactions between Cu and either the grain or grain boundaries of the ceramics. The dielectric permittivity, dissipation factor, and d 33 were 800, 0.036, and 220 pm V −1 , respectively [ Fig. 5(b)].
Kawada et al. investigated the co-firing of KNN-based ceramic with Ni metal inner electrodes to develop Pb-free multilayer actuators (MLA). 61) This study used the composition 0.96(K 0.5 Na 0.5 )NbO 3 -0.04 CaZrO 3 with an additional 3 mol% ZrO 2 and 5 mol% Mn as sintering aids to improve the relative density to 98% as well as the resistivity of ceramic. For the multilayer structure, ε r = 640, k p = 0.34, P r = 9.6 μC cm −2 , and E c = 1.5 kV mm −1 were reported [Figs. 5(c) and 5(d)]. The effective * d 33 of a KNN-based piezoelectric MLC structure actuator with Ni inner electrodes was 360 pm V −1 , which is about 50% less than those of PZTbased ceramics but with the same reliability. The piezoelectric properties of KNN-based MLA were considerably improved by decreasing the ceramic layer thickness and increasing the number of layers.

High frequency lead-free ultrasound transducer
HF ultrasound (⩾20 MHz) has played a vital role in various biomedical fields, such as ultrasonic imaging, acoustic tweezers, photoacoustic imaging, and ultrasonic stimulation, due to its lack of radiation, non-invasive properties, and high-resolution. [62][63][64][65][66] Especially in ultrasonic imaging, HF ultrasound with high resolution images will help clinicians The Japan Society of Applied Physics by IOP Publishing Ltd diagnose diseases more accurately. Theoretically, the lateral resolution (R lateral = λ × f ) and axial resolution (R axial = λ/2BW) of the ultrasound imaging can be determined by the wavelength of the ultrasound in water (λ). Therefore, in HF ultrasonic imaging, the HF causes reductions of both wavelength and pulse-duration, which enhance the spatial resolution in the lateral and axial directions, leading to improved imaging quality. This makes HF ultrasound widely applied in intravascular ultrasound (IVUS) imaging, ocular imaging, and skin imaging. 64,[67][68][69][70] Moreover, in acoustic tweezer applications, HF ultrasound can achieve a smaller focal point in the acoustic field to manipulate or trap micro-particles. 71,72) To transmit an HF ultrasound beam, piezoelectric transducers are widely used. Ultrasound transducers can be classified as single-element transducers or array transducers. 73) Ultrasound array transducers with high frame rates and real-time measurement capabilities can be used for both bio-imaging and bio-stimulation, when compared to singleelement transducers. 74) As the core component for transducers, piezoelectric materials determine the performance of transducers via the properties of acoustic velocity, v, longitudinal piezoelectric coefficient, d 33 ), thickness electromechanical coupling coefficient, k t , and dielectric constant, ε r , etc. To achieve HF ultrasound transducers, piezoelectric materials are designed to have an optimum thickness. The thickness of the piezoelectric material, t = v/2 f in designed frequency is usually determined by the acoustic velocity of the piezoelectric material, v, and the central frequency, f. Traditional fabrication methods for HF piezoelectric materials include the dicing-and-filling method, the sol-gel method, laser etching, and dry etching. [75][76][77][78][79] After which, the ultrasound transducer is assembled and utilized for various applications. Currently, Pb-based piezoelectrics are the dominant material for the fabrication of transducers, owing to their excellent piezoelectric properties (d 33 > 500 pCN −1 , k t > 50%). [80][81][82] Nonetheless, the toxic lead oxide from lead-based piezoelectric materials is harmful to human health as it might block the synthesis of hemoglobin or cause brain damage, especially in imaging diagnosis that requires human body contact. [83][84][85] Therefore, developing eco-friendly, lead-free piezoelectric materials as alternatives is critical for the fabrication of new-generation ultrasound transducers in biomedical areas.
Much effort has been made over the past few decades to develop new eco-friendly, lead-free materials for highlysensitive ultrasound transducers with properties comparable to those expected from Pb-based ceramics. [86][87][88] The leadfree ceramic compositions mainly utilized for the fabrication of ultrasound transducers are BTO, BNT, KNN, and BTZ-BCT ceramics and lithium niobate (LNO) single crystal. Furthermore, lead-free organics such as polyvinylidene fluoride (PVDF) have been widely applied for flexible or wearable transducers. 89,90) LiNbO 3 single crystals possess very low clamped permittivity (ε S 33 /ε 0 ∼ 40), high sound velocity (7340 m s −1 ), and a very high Curie temperature (T C ∼ 1150°C). 38) These materials are mostly used in transducers with a large aperture as well as in the fabrication of nondestructive testing (NDT) ultrasound transducers for high temperature applications. 91) To improve the sensitivity of the broadband transducer and the acoustic impedance matching between the piezoelectric The Japan Society of Applied Physics by IOP Publishing Ltd materials and human tissue, a two quarter wavelength matching layer was employed. The expected acoustic impedances of the first matching layer, Z m1 , and second matching layer, Z m2 were calculated as follows: where Z p is the acoustic impedance of the piezoelectric material and Z l is the acoustic impedance of human tissue. Based on these equations, most of the fabricated ultrasound transducers applied 2-3 μm silver epoxy (Z = 7.3 MRayl) and parylene (Z = 2.5 MRayl) as their first and second matching layers, respectively. E-solder, a conductive silver paste, (Z = 5.3 MRayl) was used as a backing layer to absorb reflected acoustic waves. [92][93][94] The following section aims to discuss the development of HF Pb-free ultrasound singleelement and array transducers. Table I summarizes representative developments in lead-free ultrasound transducers with optimized matching layer, backing layer, and geometric shapes. To achieve higher imaging quality, Chen et al. used (K,Na)NbO 3 -KTiNbO 5 -BaZrO 3 -Fe 2 O 3 -MgO (KNN-NTK-FM) Pb-free piezoelectric ceramic with dimensions 0.45 × 0.55 mm 2 to fabricate a 52.6 MHZ HF ultrasound needle-type transducers with high sensitivity and low insertion loss, as shown in Fig. 7(a). 67) Ex vivo ultrasonic biomicroscopy (UBM) images of an excised porcine eyeball and rabbit aorta samples were obtained by the fabricated transducers with a lateral resolution of 423 μm and an axial resolution of 20 μm at −6 dB. The eye anatomical structures of the cornea, conjunctiva, ciliary body, and the aorta sample with vascular wall and fat tissues were clearly visualized. The increased resolution will help improve the accuracy of diagnoses for intravascular and ocular diseases. Based on this study, Jiang et al. developed a 1-3 ceramic-polymer composite of (K,Na)NbO 3 -(Bi,Na)ZrO 3 -BiScO 3 with added Fe 2 O 3 (Fe-KNN)-based ceramic with a width of 55 μm and a kerf of 15 μm to enhance the acoustic and electric properties for HF ultrasound transducers [ Fig. 7(b)]. 69) The acoustic characterization of the 1-3 composite transducer demonstrated a broader −6 dB bandwidth of 83%, a higher k t of 55%, and a very low insertion loss (9.8 dB). A porcine eyeball with detailed structures was again used to show the outstanding imaging ability of the fabricated transducer. Subsequently, a structural engineering strategy was used to enhance the stability, reliability, and piezoelectric response of KNN-based transducers for HF ultrasound applications. 97 102) The ceramic was sandwiched between epoxy-tungsten backing and silver epoxy matching layers to fabricate a single element transducer. A curved Epotek epoxy lens was cast on the matching layer to improve the acoustic performance. A −6 dB bandwidth of 55%, was reported at 23 MHz (Table I). To utilize a composition with higher piezoelectric properties for an ultrasound transducer, Yan et al. applied (BZT-BCT) lead-free ceramic with a high dielectric constant (2800) and high d 33 (600 pCN −1 ) to assemble a needle-typed IVUS transducer with a central frequency of 30 MHz, −6 dB bandwidth of 53%, and an insertion loss of 18.7 dB. 103

LNO-based ultrasound transducer
As indicated earlier, piezoelectric lithium niobate LiNbO 3 (LNO) as a single-crystal is also another representative lead-free material for HF ultrasound applications. With a low dielectric constant, higher and high k t = 0.60, LNO is used in the HF transducers for ultrasonic imaging and acoustic tweezer applications. Especially for acoustic tweezers, the Pb-free LNO transducers showed expected biocompatibility, non-invasive property, and high acoustic intensity in the smaller focal point. Hwang Fig. 8(b). 104) Resolutions of 6.4 μm laterally and 6.2 μm axially were obtained, and the UBM images of zebrafish eyes were clearly achieved, demonstrating a great potential for ultrasound resolution that is comparable to optical resolution. Lately, the mismatch of acoustic impedance between piezoelectric materials and the loading acoustic matching medium has limited the highsensitivity and broad bandwidth of UHF transducers, especially UHF LNO transducers. To overcome this issue, multilayer structures of polymers and metals with different acoustic impedances were proposed and studied to gain a specific matching effect as a new matching layer for LNO transducer. Yang et al. investigated the effect of a triple-layer polymer-metal-polymer matching layer on the resolution of a 100 MHz LNO ultrasound transducer with a broad bandwidth of 60%, as shown in Fig. 8(c). 105  The Japan Society of Applied Physics by IOP Publishing Ltd ultrasonic transducer using ITO electrodes [ Fig. 8(d)]. 106) The fabricated transparent transducer had a center frequency of 36.9 MHz with −6 dB fractional bandwidth of 33.9%. It achieved 90% optical transmission in the visible-to-nearinfrared spectrum and successfully gained photoacoustic images of mouse-ear vasculatures in vivo. However, the low −6 dB bandwidth of the transparent LNO transducer will need to be further improved in the future since there is no matching layer and backing layer assembled on the transducer. For the next-generation HF transparent LNO transducer, the studies of the selected matching layer and the backing layer will be based on the transparent load medium with matched acoustic impedance to enhance the −6 dB bandwidth and pulse-echo performance of the transducer, which will effectively also increase the ultrasound resolution of the transducer. assembled ultrasound array achieved a −6 dB bandwidth of 46.77%, which could be further applied in IVUS for coronary disease diagnosis.

Polymer-based ultrasound transducer
Recently, polymer-based piezoelectric materials have attracted lots of attention in sensor fabrication as lead-based alternatives due to their moderate piezoelectric properties, high flexibility, and low density. PVDF is a piezoelectric polymer with thin thickness, low acoustic impedance, and broadband receiving properties, thus, it is usually applied in HF ultrasound receivers or detectors. [107][108][109] Zou et al. developed a 9 μm thick PVDF thin film to assemble a HF line-focus beam ultrasound transducer [ Fig. 9(c)]. 110) Through surface acoustic waves propagating on a silicon wafer, the transducer with a 30 × 6.35 mm 2 PVDF foil size was able to achieve a wide frequency response from 10 MHz to over 100 MHz, which can be used to determine the elastic property of silicon wafers. Recently, a PVDF copolymerbased transducer with the dimensions of 30 × 30 × 0.85 mm 3 and the central frequency of 48.5 MHz has been developed by the additive manufacturing method on a polyethyleneimine polymer substrate [ Fig. 9(d)]. 108) The fabrication process provided a novel method to build HF PVDF without introducing additional adhesive layers for material bonding, and two-dimensional surface scanning also demonstrated the potential of PVDF for imaging applications. Furthermore, an acoustic image of a coin with detailed patterns also showed the great ability of the HF PVDF transducer in high resolution acoustic imaging.

Summary
There have been major advances in the development of BT, BNT, and KNN based Pb-free piezoelectric materials for sensor, actuator, and HF ultrasonic transducer applications. In the past several years, significant progress has been achieved in the development of hard piezoelectric ceramics with a high mechanical quality factor Q m in BNT-based compositions and soft piezoelectric materials with high electromechanical properties in KNN-based polycrystalline and textured ceramics. It has been demonstrated that the performance of Mndoped BNT-BKT-BLT ceramics with high vibration velocity, v, is significantly better than its Pb-based counterpart for HIFU applications. In the past few years, significant progress has been achieved in the enhancement of piezoelectric coefficient, d 33 of modified polycrystalline KNN ceramic either by phase boundary engineering or by optimizing lattice distortion. The outstanding electromechanical properties of Hf-modified KNN-based textured ceramic have reached to the same value of soft PZT material but with a higher depolarization temperature. In addition, low-temperature sintering of BNT-and KNN-based ceramics co-fired with Cu or Ni electrodes in an oxygen-controlled atmosphere has been successfully demonstrated.
There have been excellent advances in the development of lead-free HF ultrasound transducers, including piezoelectric ceramic, polymer, and ceramic-polymer composite materials for low and HF transducers for medical imaging and biomedical applications. Advances in textured KNN-based materials will play a pivotal role in replacing Pb-based piezoelectric transducers. The main challenges of KNN based materials lie in low-cost, highly efficient manufacturing, and improving the FOM for high power transducer applications.