A 3.7 MHz phased array probe using 0.91Pb(Zn(1/3)Nb(2/3))O(3)-0.09PbTiO(3 )

A novel 128-channel phased array probe for echocardiography with a center frequency of 3.7 MHz using 0.91Pb(Zn(1/3)Nb(2/3))O(3)-0.09PbTiO(3 ) (PZN-9%PT) single crystal has been fabricated to realize greater sensitivity and broader bandwidth properties. The echo amplitude of the PZN-9%PT single-crystal probe is about 5 dB higher than that of the conventional lead airconate titanate (PZT) ceramic probe, and the fractional bandwidth is about 25 percentage points broader. The quality of B mode images obtained by the PZN-9%PT probe satisfies the performance of the two types of conventional PZT ceramic probes that have center frequencies of 2.5 and 3.75 MHz. At the reference frequency of 3 MHz, the Doppler sensitivity of the PZN-9%PT probe is about 5 dB higher than that of the 3.75 MHz PZT probe; the blood flow of a pulmonary vein in a hard-to-image patient is much more clearly imaged than in the case of using the PZT probe. These superior images are attributable to the use of sufficiently large PZN-9%PT single crystals obtained by the self-flux method.     

A 20 MHz single-element ultrasonic probe using 0.91Pb(Zn(1/3 )Nb(2/3))O(3)-0.09PbTiO(3) single crystal

A 20 MHz single-element ultrasonic probe using 0.91Pb(Zn(1/3 )Nb(2/3))O(3)-0.09PbTiO(3) (PZN-PT 91/9) single crystal has been fabricated. The single crystal of PZN-PT 91/9 orientated to the (001) plane has longitudinal coupling factor of k(33)>90%, which is much larger than the k(33)=70 to 80% of conventional Pb(Zr(1-x),Ti(x))O(3) (PZT) based ceramics. A single crystal of PZN-PT 91/9 without inclusion or crack has been grown with dimensions of about 25x15x5 mm by the self-flux method. Because mechanical strength in the fabrication of disk transducers orientated to the (001) plane was sufficiently strong, under the same conditions as are applied to conventional PZT ceramics, a piston single-element probe with a diameter of 2.0 mm and a frequency of 20 MHz was successfully fabricated. The bandwidth of the PZN-PT 91/9 probe was 13-26 MHz, which was 4 MHz broader than that of the conventional PZT probe.     

Finite element comparison of single crystal vs. multi-layer composite arrays for medical ultrasound

Finite element (PZFlex; Weidlinger Assoc., New York, NY and Los Altos, CA) simulations predict that for a 2-MHz phased array element with a single matching layer, the three-layer hybrid structure increases the pulse echo signal-to-noise ratio (SNR) by 16 dB over that from a single layer PZT element and -6 dB pulse echo fractional bandwidth from 58% for the PZT element to 75% for the hybrid element. Analogous finite element method (FEM) simulations of single crystal material [lead zinc niobate (PZN)-8% lead titanate (PT)] showed increased SNR by only 3.1 dB, but a -6 dB bandwidth of 108%     

Single crystal PZN/PT-polymer composites for ultrasound transducer applications

Single crystal relaxor ferroelectrics of PZN-8%PT were investigated for potential application in ultrasound transducers. The full set of electromechanical properties was determined using combined resonance and laser interferometry techniques. Ultra-high length extensional coupling (k(33)) of 0.94 was observed, a 25% increase over Navy Type VI PZT ceramics. The thickness extensional coupling (k(t)) of 0.48 was comparable to PZT compositions, and the compliance S(33)(E) was a factor of six greater. To maximize height extensional coupling (k'(33)), while minimizing length extensional coupling k(31) in array elements, it was necessary to align the elements along the 100 crystallographic direction in the x-y plane. Mode coupling plots and test samples for array elements determined that width-to-height ratios of less than 0.5 were desired, similar to the requirement for polycrystalline PZT ceramics. Modeling of 1-3 composites and experimental results demonstrated that thickness coupling greater than 0.80 could be achieved with a 40% to 70% volume fraction of PZN-PT. Although this is a substantial increase over PZT 1-3 composites, with a thickness coupling coefficient of 0.66, it represents a smaller fraction of the length extensional coupling k(33). This reduction may be a consequence of the increased compliance of PZN-PT, which results in significant clamping by the polymer matrix. Ultrasonic transducers fabricated using PZN-8%PT 1-3 composites achieved experimental bandwidths as high as 141%. The pulse-echo responses displayed good agreement with modeled results using the Redwood equivalent circuit.     

Multi-layered PZT/polymer composites to increase signal-to-noise ratio and resolution for medical ultrasound transducers part II: thick film technology

For pt.I, see ibid., vol.46, no.4, p.961-71 (1999). Increasing transducer bandwidth and signal-to-noise ratio (SNR) is fundamental to improving the quality of medical ultrasound images. In previous work, we have proposed the use of multi-layer 1-3 PZT/polymer composites to increase both, but have encountered significant fabrication challenges. Thus, we have developed a multi-layer composite hybrid array that will not require post alignment. Starting from a 2-MHz, three-layer PZT-5H, thick film transducer designed for 1.5-D arrays, cuts are made only through the top layer and back-filled with epoxy, forming a composite layer on top of two ceramic layers. Finite element (PZFlex) simulations show that for a 2-MHz phased-array element with a single matching layer, the three-layer hybrid structure increases the pulse echo SNR by 11 dB versus a single layer PZT element and improves -6 dB pulse echo fractional bandwidth by a factor of 1.4. Composite hybrid arrays fabricated in our laboratory showed an improvement in SNR of 6 to 11 dB over a PZT control and an increase in -6 dB bandwidth by a factor of 1.1. Images from a phased-array scanner confirmed these improvements     

Piezoelectric ceramics with high dielectric constants for ultrasonic medical transducers

Complex system ceramics Pb(Sc(1/2)Nb(1/2))O3-Pb(Mg(1/3)Nb(2/3))O3-Pb(Ni(1/2)Nb(1/2))O3-(Pb0.965,Sr0.035) (Zr,Ti)O3 (PSN-PMN-PNN-PSZT abbreviated PSMNZT) have been synthesized by the conventional technique, and dielectric and piezoelectric properties of the ceramics have been investigated for ultrasonic medical transducers. High capacitances of the transducers are desired in order to match the electrical impedance between the transducers and the coaxial cable in array probes. Although piezoelectric ceramics that have high dielectric constants (epsilon33t/epsilon0 > 5000, k'33 < 70%) are produced in many foundries, the dielectric constants are insufficient. However, we have reported that low molecular mass B-site ions in the lead-perovskite structures are important in realizing better dielectric and piezoelectric properties. We focused on the complex system ceramics PSMNZT that consists of light B-site elements. The maximum dielectric constant, epsilon33T/epsilon0 = 7, 200, was confirmed in the ceramics, where k'33 = 69%, d33 = 940 pC/N, and T(c) = 135 degrees C were obtained. Moreover, pulse-echo characteristics were simulated using the Mason model. The PSMNZT ceramic probe showed echo amplitude about 5.5 dB higher than that of the conventional PZT ceramic probe (PZT-5H type). In this paper, the electrical properties of the PSMNZT ceramics and the simulation results for pulse-echo characteristics of the phased-array probes are introduced.     

Multi-layered PZT/polymer composites to increase signal-to-noise ratio and resolution for medical ultrasound transducers

Increasing transducer bandwidth and signal-to-noise ratio (SNR) is fundamental to improving the quality of medical ultrasound images. In previous work, the authors have proposed the use of multi-layer 1-3 PZT/epoxy composites to increase both but have encountered significant fabrication challenges. These difficulties include making the bond thickness between the layers extremely small relative to the ultrasound wavelength and aligning the posts of the composite to increase the coupling coefficient. The authors have routinely achieved a bond thickness of less than 5 mum but aligning the posts is more complicated. Finite element (PZFlex; Weidlinger, Assoc., New York, NY and Los Altos, CA) simulations show that the pulse-echo SNR and bandwidth degrade significantly with misalignment of the posts. Alignment of greater than 90% of the post pitch (i.e., tolerance of 10 to 20 mum) is required to obtain significant increases in SNR and bandwidth relative to conventional transducer arrays. This will be a difficult tolerance for large-scale production. Thus, the authors have developed a multi-layer composite hybrid array that will not require post alignment. This structure consists of a layer of 5 MHz 1-3 composite material on top of conventional 5 MHz PZT, which will provide greater SNR relative to conventional composites and increased bandwidth over multi-layer PZT. PZFlex simulations show that for a 2 MHz linear array element, the 2 layer hybrid structure increases the pulse-echo SNR by 7.5 dB over that from a single layer PZT element. Even without a matching layer, an increase in the -6 dB pulse-echo fractional bandwidth from 22% for the PZT element to 35% for the hybrid element was also predicted. Experimentally, in a 32 element array, the authors achieved an increase of 5.2 dB in SNR and an increased -6 dB bandwidth from 23 to 30%. In vitro and in vivo images showed corresponding improvements.     

Modeling of elastic nonlinearities in ferroelectric materials including nonlinear losses: application to nonlinear resonance mode of relaxors single crystals

(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) and (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) single crystals are considered to behave like soft Pb(Zr,Ti)O3 (PZT) ceramics because of their small mechanical quality factor Qm and poor stability under external disturbances (Qm > 500-1000 for hard PZT ceramic, and Qm < 100 for soft PZT and PMN-PT and PZN-PT single crystals). At weak signal excitation of the first resonance mode, the displacement at the end of a lateral bar is proportional to the Q31d31 figure of merit that is very close to that found for hard PZT. Indeed the very large piezoelectric coefficient compensates the low Qm. But increasing alternating current (AC) field results in the appearance of strong non-linearities through a shift of the resonance frequency and jumps phenomenon observed on increasing and decreasing frequency sweep. It is shown in this paper that these nonlinearities are due to the nonlinear elastic compliance that can be modeled by a third order development of the constitutive piezoelectric equations. Experiments on PMN-PT and PZN-PT single crystals are used for comparison with the model to show the viability of the approach. Both the frequency shift and jumps phenomenon are simulated with a very good agreement with experimental results. The importance is also shown of losses associated with the third order term responsible for the large decrease of the mechanical quality factor for high strain levels. Thus, the nonlinear losses are related to the hysteresis of domain wall motion when subjected to large displacements.     

Nanostructured ceramics of 0.92PbZn(1/3)Nb(2/3)O(3)-0.08PbTiO(3) processed by SPS of nanocrystalline powders obtained by mechanosynthesis

This work reports the successful use of a combination of non-conventional methods of synthesis (mechanosynthesis) and sintering (spark plasma sintering, SPS) for the preparation of nanostructured 0.92PbZn(1/3)Nb(2/3)O(3)-0.08PbTiO(3) (PZN-PT) ceramics. With this approach we achieve not only the stabilization of the PZN-PT perovskite phase in ceramics when sintering is carried out at temperatures between 823 and 873?K, but also good control of the grain growth, necessary to produce nanostructured materials with grain sizes of 15-20?nm. This reduction of the size results in relaxor-type electric behaviour.     

Micro-machined high-frequency (80 MHz) PZT thick film linear arrays

This paper presents the development of a micromachined high-frequency linear array using PZT piezoelectric thick films. The linear array has 32 elements with an element width of 24 μm and an element length of 4 mm. Array elements were fabricated by deep reactive ion etching of PZT thick films, which were prepared from spin-coating of PZT sol-gel composite. Detailed fabrication processes, especially PZT thick film etching conditions and a novel transferring-and-etching method, are presented and discussed. Array designs were evaluated by simulation. Experimental measurements show that the array had a center frequency of 80 MHz and a fractional bandwidth (-6 dB) of 60%. An insertion loss of -41 dB and adjacent element crosstalk of -21 dB were found at the center frequency.     

Lead zirconate titanate/poly(vinylidene fluoride-trifluoroethylene) 1-3 composites for ultrasonic transducer applications

A new procedure for preparing lead zirconate titanate (PZT)/poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) 1-3 composites with both phases piezoelectrically active is described. Sintered PZT rods are inserted into a prepoled copolymer matrix, and the composite is repoled under a lower electric field. Using this new procedure, the dipoles in the two phases are aligned in either the same or opposite directions. Composite disks, of 12.7-cm diameter and 0.33- to 0.60-mm thicknesses, have been fabricated with PZT rods of 0.8 or 1 mm diameter distributed in a square pattern with 3 mm center-to-center separation. The ceramic volume contents of the composite disks are 3.6 and 5.6%, respectively. The resonance characteristics of the composite disks consist of the resonance modes of the two constituent phases, but they are dominated by the coupled longitudinal thickness mode (H-mode) of the PZT rods. The coupled radial mode (L-mode) resonance of the PZT rods is significant only for thin disks. The observed resonance frequencies of the H- and L-modes agree well with the values calculated from the coupling theory. The thickness mode resonance of the copolymer matrix (T-mode) is present but hardly observable in thick disks. The composite disks have been fabricated into transducers with air-backing and with no front face matching layer, and their performance characteristics have been evaluated in water. The transmitting and receiving voltage responses of a PZT/P(VDF-TrFE) composite transducer are better than those of a PZT/epoxy composite transducer. The transmitting and receiving voltage responses are improved when the PZT rods and copolymer matrix are poled in opposite directions, especially when the resonance frequencies of the H- and T-modes are approximately equal. When the phases are poled in the same direction and the resonance peaks associated with the Hand T-modes just overlap, the bandwidth is improved. Using 0.33-mm thick composite disks, a transducer can be produced with three operating frequencies by poling the constituent phases in the same direction, or with two operating frequencies at equal efficiency by poling the constituent phases in opposite directions. The PZT/P(VDF-TrFE) 1-3 composite transducer, especially the one with multiple operating frequencies, should be very promising in the applications of medical ultrasonic imaging.     

Electric-field and stress effects on depoling and overpoling phenomena in [001]-poled transverse mode Pb(Zn1/3Nb2/3)O3-(6-7)%PbTiO3 single crystal of [110] length cut

Solid-solution Pb(Zn(1/3)Nb(2/3))O(3)-PbTiO(3) (PZN-PT) single crystals, touted as next-generation piezoelectric materials, have been studied extensively in the past decade. This work addresses the advantages and limitations of transducers made of transverse mode PZN-(6-7)%PT single crystals of [110](L) X [001](T)(P) cut. This cut exhibits superior electromechanical properties, with k(31) ≈ 0.85 and d(31) ≈-1450 pC/N, and an extremely high d(31)/S(E)(11) value of >35 C/m(2). It also has relatively high overpoling, i.e., rhombohedralto- tetragonal phase transformation, field of ≈2 kV/mm. This overpoling field further decreases with increase in axial compressive stress. Despite these good attributes, this crystal cut has a low depoling field of ≤ 0.3 kV/mm, a result of low coercive fields of [001]-poled relaxor-based single crystals, which decreases further with increasing axial compressive stress, limiting its bipolar drive capability. The axial compressive stress required to cause overpoling via rhombohedral-to-tetragonal phase transformation of relevant domain variants in the crystal is found to be >90 MPa. In contrast, this crystal cut depolarizes at comparatively low axial tensile stress of ≈15 MPa, the magnitude of which is not significantly affected by the moderate forward field applied.     

Large electric-field-induced strain in ferroelectric crystals by point-defect-mediated reversible domain switching

Ferroelectric crystals are characterized by their asymmetric or polar structures. In an electric field, ions undergo asymmetric displacement and result in a small change in crystal dimension, which is proportional to the applied field. Such electric-field-induced strain (or piezoelectricity) has found extensive applications in actuators and sensors. However, the effect is generally very small and thus limits its usefulness. Here I show that with a different mechanism, an aged BaTiO(3) single crystal can generate a large recoverable nonlinear strain of 0.75% at a low field of 200 V mm(-1). At the same field this value is about 40 times higher than piezoelectric Pb(Zr, Ti)O(3) (PZT) ceramics and more than 10 times higher than the high-strain Pb(Zn(1/3)Nb(2/3))O(3)-PbTiO(3) (PZN-PT) single crystals. This large electro-strain stems from an unusual reversible domain switching (most importantly the switching of non-180 degrees domains) in which the restoring force is provided by a general symmetry-conforming property of point defects. This mechanism provides a general method to achieve large electro-strain effect in a wide range of ferroelectric systems and the effect may lead to novel applications in ultra-large stroke and nonlinear actuators.     

Characterization of flux-grown PZN-PT single crystals for high-performance piezo devices

Relaxor ferroelectric Pb(Zn(1/3)Nb(2/3))O(3-x)PbTiO(3) (PZN-PT) and Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3)(PMN-PT) single crystals are the potential candidates for future high-performance piezoelectric devices due to their exceptionally high dielectric and piezoelectric properties. Characterization on flux-grown PZN-PT single crystals of different orientations revealed that PZN-(6-7)%PT single crystals show good homogeneity in dielectric and electromechanical properties and composition. When poled in [001] direction, these crystals exhibit high longitudinal-mode properties with dielectric constant (K(T)) approximately equal to 7000, piezoelectric coefficients (d(33)) approximately equal to 2800 pC/N, and electromechanical coupling factors (k(33)) > or = 0.92. For [011]-cut crystals, optimally poled PZN-7%PT single crystal exhibits very high transverse-mode dielectric and piezoelectric properties with K(T) > or = 5000, d(32) approximately equal to -3800 pC/N and k(32) > or = 0.90. [011]- poled PZN 6%PT has d(32) approximately equal to -3000 pC/N and comparable k(32) and K(T) values. In comparison with melt-grown PMNPT single crystals, flux-grown PZN-PT single crystals show good compositional homogeneity, superior and consistent dielectric and electromechanical properties, and higher depolarization temperatures (TDP).     

Extruded PZT/polymer composites for electromechanical transducer applications

PZT/polymer composites having a 1–3 parallel connectivity were fabricated by impregnating a sintered, extruded honeycomb configuration of PZT with various polymers. The resultant composites were found to have densities less than 2900 kg/m³, a dielectric constant of ～500 and a piezoelectric d₃₃ of ～300×10^?12 C/N. The 1–3 connectivity increases the piezoelectric voltage coefficient (g₃₃) from 22×10^?3 Vm/N (solid PZT) to ～70×10^?3 Vm/N.The composites have thickness mode electromechanical coupling coefficients (k_t) which are ～25% greater than that of homogeneous PZT, and are readily adaptable for broad bandwidth operation. This combination of electromechanical properties makes these composites ideal for low voltage displacement and pulse echo applications.     

0-3型(PZT,KTN)/P(VDF-TrFE)三相铁电复合材料的电学性能

以偏氟乙烯-三氟乙烯（P（VDF-TrFE））共聚物为基体,锆钛酸铅（PZT）铁电颗粒为功能相,钽铌酸钾（KTN）颗粒为增强相,制备了0-3型（PZT,KTN）/P（VDF-TrFE）三相铁电复合材料。利用SEM及EDAX技术,分析了复合材料的显微结构及PZT和KTN相的分布。测试了具有不同KTN 体积分数的复合材料的电性能。实验结果表明:PZT和KTN相的颗粒分布均匀,存在少量的团聚体;随KTN体积分数的增加,三相复合材料的极化漏电流I、介电常数ε_r和介电损耗tanδ增加,压电系数d₃₃降低,而热释电系数p₃先增加后降低,但其d₃₃和p₃均高于具有相同PZT体积分数的PZT/P（VDF-TrFE）两相复合材料。      

Complete set of material properties of [011](c) poled 0.24Pb(In(1/2)Nb(1/2))O(3)-0.46Pb(Mg(1/3)Nb(2/3))O(3)-0.30PbTiO(3) single crystal

A complete set of elastic, piezoelectric, and dielectric constants of [011](c) poled multidomain 0.24Pb(In(1/2)Nb(1/2))O(3)-0.46Pb(Mg(1/3)Nb(2/3))O(3)-0.30PbTiO(3) ternary single crystal has been determined using resonance and ultrasonic methods and the temperature dependence of the dielectric permittivity has been measured at 3 different frequencies. The experimental results revealed that this [011](c) poled ternary single crystal has very large transverse piezoelectric coefficient d(32) = -1693 pC/N, transverse dielectric constant ε(11)/ε(0) ~ 7400 and a high electromechanical coupling factor k(32) ~ 90%. In addition, its coercive field is 2 times of that of the corresponding binary 0.7Pb(Mg(1/3)Nb(2/3))O(3)-0.30PbTiO(3) single system with much better temperature stability. Therefore, the crystal is an excellent candidate for transverse mode electromechanical devices.     

Optical properties of (1−<Emphasis Type="Italic">x</Emphasis>)PbZn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>−<Emphasis Type="Italic">x</Emphasis>PbTiO<Subscript>3</Subscript> single crystals

The temperature dependence of the optical transmission and small-angle light scattering with and without applied constant electric field was studied in relaxor single crystals of 0.91PbZn_1/3Nb_2/3O₃-0.09PbTiO₃ (PZN-PT 91/9) and 0.93PbZn_1/3Nb_2/3O₃-0.07PbTiO₃ (PZN-PT 93/7) solid solutions in the region of two phase transitions: (i) from cubic paraelectric to tetragonal ferroelectric phase at T=T _c and (ii) from tetragonal ferroelectric to rhombohedral ferroelectric phase at T=T _rt. In the absence of external electric field, only the phase transition at T _c proceeds in both PZN-PT 91/9 and PZN-PT 93/7 crystals according to a percolation mechanism and is accompanied by the appearance of a sharp maximum in the small-angle light scattering intensity curve. In PZN-PT 93/7 crystals, the application of a relatively weak electric field induces an additional percolation type phase transition at T _rt.     

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128 x 128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.     

Harmonic 3-D echocardiography with a fast-rotating ultrasound transducer

Although the advantages of three-dimensional (3-D) echocardiography have been acknowledged, its application for routine diagnosis is still very limited. This is mainly due to the relatively long acquisition time. Only recently has this problem been addressed with the introduction of new real-time 3-D echo systems. This paper describes the design, characteristics, and capabilities of an alternative concept for rapid 3-D echocardiographic recordings. The presented fast-rotating ultrasound (FRU)-transducer is based on a 64-element phased array that rotates with a maximum speed of 8 Hz (480 rpm). The large bandwidth of the FRU-transducer makes it highly suitable for tissue and contrast harmonic imaging. The transducer presents itself as a conventional phased-array transducer; therefore, it is easily implemented on existing 2-D echo systems, without additional interfacing. The capabilities of the FRU-transducer are illustrated with in-vitro volume measurements, harmonic imaging in combination with a contrast agent, and a preliminary clinical study.