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.     

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.     

High-frequency ultrasonic transducer fabricated with lead-free piezoelectric single crystal

High-frequency (25 MHz) ultrasonic transducers with Na(0.5)Bi(0.5)TiO(3)-BaTiO(3) (NBT-BT) lead-free piezoelectric single crystal as the active elements are fabricated and characterized. The impedance measurement reveals that the poled [001]-oriented NBT-BT single crystal exhibits a high thickness electromechanical coefficient k(t) of 0.52 and a low clamped dielectric constant of 80. The -6-dB bandwidth of the transducer is 46.16% and the insertion loss at the center frequency is -31.89 dB. The good performance of the transducer indicates that the NBT-BT single crystal would be a promising lead-free material for ultrasonic transducer applications.     

Study on microstructure, piezoelectric and dielectric properties of 3-3 porous PZT composites

Porous lead zirconate titanate (PZT) composites are used for low frequency hydrophones due to their high hydrostatic figure of merit, and low sound velocity. Porous PZT composite materials manufactured using varied percentage of polymethyl methacrylate (PMMA) as a pore-forming agent was studied, to understand the effect of PMMA on microstructure and dielectric properties. Properties such as the density, porosity, transverse piezoelectric coefficient, dielectric constant, and dielectric loss factor were studied as a function of PMMA. Porous PZT composites (with 30% PMMA) show lower piezoelectric transverse coefficient (−78 pC/N) and dielectric constant (116) as compared to homogeneous PZT. At a frequency of 1 MHz, the dielectric constant decreases and dielectric loss increases with increase in PMMA. This paper describes the effect of polymethyl methacrylate (PMMA) as a pore-forming agent on dielectric properties and microstructure of porous PZT composites.     

Fabrication of high frequency spherically shaped ceramictransducers

Difficulty in obtaining well focused efficient ultrasound transducers has limited the development of new high frequency applications of B-mode imaging. This paper describes a method for fabricating high frequency (53 MHz) spherically focused lead zirconate titanate (PZT) transducers. A transducer is fabricated by bonding a malleable backing layer onto a thin plate of PZT and then pressing the plate into a spherically shaped well. The backing layer evenly distributes stresses across the material when it is pressed into the well. Local concentrations of stress which lead to fracture are avoided and the material can be deformed without macroscopic cracking. The characteristics of a 2 mm diameter 53 MHz PZT transducer with a 4 mm focal length are described. A lateral beam width of 68 μm and a 12 dB depth of field of 1.5 mm were obtained. The minimum two-way insertion loss of the system was -25 dB and the 6 dB bandwidth of the pulse echo response was 30%. An image of a resolution phantom and an in vivo skin image illustrate the excellent imaging characteristics of the transducer     

Conduction analysis of Li2O doped 0.2[Pb(Mg1/3Nb2/3)]–0.8[PbTiO3–PbZrO3] ceramics fabricated by columbite precursor method

We have fabricated 0.2Pb(Mg_1/3Nb_2/3)O₃–0.8Pb(Zr_0.475Ti_0.525)O₃ [PMN–PZT] ceramics doped with various amounts of Li₂O (0, 0.05, 0.1, 0.2, 0.3 wt.%) using the columbite precursor method. The effects of Li-doping on the conduction behavior of PMN–PZT ceramics are discussed in relation to the low frequency dielectric dispersion and frequency domain measurement. The Li-doped PMN–PZT ceramics sintered at 950 °C showed a sufficient densification with large dielectric constant and low dielectric loss. The incorporation of Li⁺ ion in PMN–PZT ceramics led to an appreciable reduction in electrical conductivity and further enhanced the ferroelectric and piezoelectric properties. The activation energies of PMN–PZT + xLi₂O (x = 0, 0.05, 0.1, 0.2, 0.3 wt.%) ceramics calculated from ac conductivity measurement using the Arrhenius relation were 1.05, 1.25, 1.27, 1.38 and 1.41 eV, respectively. The conduction behavior is examined in the low frequency and high temperature region and the results are discussed in detail through crystal defect mechanism.     

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.     

Fabrication and characterization of annular thickness mode piezoelectric micro ultrasonic transducers

Micromachining techniques, in combination with low temperature ceramic composite sol-gel processing, have been used to fabricate annular array thickness-mode piezoelectric micro ultrasonic transducers (Tm-pMUTs). The processing techniques of low temperature (710 degrees C) composite sol-gel ceramic (sol + ceramic powder) deposition and wet etching were used to deposit and structure 27-microm thick lead zirconate titanate (PZT) films on silicon substrates to produce annular array Tm-pMUTs. Using these techniques, high quality PZT materials with near bulk permittivity have been obtained. The Tm-pMUT devices were shown to resonate at approximately 60 MHz in air and 50 MHz in water. From resonance measurements k(t) values ranging between 0.2 and 0.47 have been calculated and shown to depend on the level of porosity within the film. Lower values of kt were observed for films with higher levels of porosity, which was attributed to the relative decrease in the effective piezoelectric coefficient epsilon(33) with respect to stiffness and permittivity as a function of increasing porosity. This paper presents the successful micro-fabrication of a Tm-pMUT device and discusses the optimization of the poling conditions and effect of PZT microstructure on the coupling coefficient k(t). Pulse echo measurements in water, showing a -6 dB center frequency of 53 MHz and 47% -6 dB bandwidth, using a target 15 mm away from the transducer, have been included to demonstrate successful operation of the device. Full analysis of these results will be conducted in later publications.     

25 MHz ultrasonic transducers with lead- free piezoceramic, 1-3 PZT fiber-epoxy composite, and PVDF polymer active elements

This paper presents the fabrication and characterization of single-element ultrasonic transducers whose active elements are made of lead-free piezoceramic, 1-3 PZT/polymer composite and PVDF film. The lead free piezoelectric KNNLT- LS(K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.10S_0.06b)O₃ powders and ceramics were prepared under controlled humidity and oxygen flow rate during sintering. Due to its moderate longitudinal piezoelectric charge coefficient (175 pC/N) and k_t of 0.50, the KNN-LT-LS composition may be a good candidate for highfrequency transducer applications. PZT fibers with 25 μm diameter formed by the viscose suspension spinning process were incorporated into epoxy to fabricate 1-3 composites with the averaged k_t = 0.64 and d₃₃ = 400 pC/N. Using KNN-LS-LT ceramic, 1-3 PZT fiber composite, and PVDF film, 3 different unfocused single element transducers with center frequencies of 25 MHz were fabricated. The acoustic characterization of the transducers demonstrated that wideband and low insertion loss could be obtained employing KNN-LS-LT ceramic. The ?6 dB bandwidth and insertion loss were 70% and ?21 dB, respectively. In comparison, the insertion loss of the ceramic transducer was much smaller than those made with 1-3 composite and PVDF film. This was attributed to closer electrical impedance match to 50 Ω and higher thickness coupling coefficient of the ceramic transducer.     

Design of efficient, broadband single-element (20-80 MHz) ultrasonic transducers for medical imaging applications

This paper discusses the design, fabrication, and testing of sensitive broadband lithium niobate (LiNbO/sub 3/) single-element ultrasonic transducers in the 20-80 MHz frequency range. Transducers of varying dimensions were built for an f# range of 2.0-3.1. The desired focal depths were achieved by either casting an acoustic lens on the transducer face or press-focusing the piezoelectric into a spherical curvature. For designs that required electrical impedance matching, a low impedance transmission line coaxial cable was used. All transducers were tested in a pulse-echo arrangement, whereby the center frequency, bandwidth, insertion loss, and focal depth were measured. Several transducers were fabricated with center frequencies in the 20-80 MHz range with the measured -6 dB bandwidths and two-way insertion loss values ranging from 57 to 74% and 9.6 to 21.3 dB, respectively. Both transducer focusing techniques proved successful in producing highly sensitive, high-frequency, single-element, ultrasonic-imaging transducers. In vivo and in vitro ultrasonic backscatter microscope (UBM) images of human eyes were obtained with the 50 MHz transducers. The high sensitivity of these devices could possibly allow for an increase in depth of penetration, higher image signal-to-noise ratio (SNR), and improved image contrast at high frequencies when compared to previously reported results.     

Improvement electrical properties of sol–gel derived lead zirconate titanate thick films for ultrasonic transducer application

In this work, a fabrication process of piezoelectric PZT [Pb(Zr_0.52Ti_0.48)O₃] thick films up to 60 μm deposited on silicon and aluminum substrates is reported. Crystalline spherical modified PZT powder about 300 nm in diameter was used as filler. PZT polymeric precursor produced by Chemat Inc. was used as the matrix material. Spinning films were annealed at 700 °C for one hour in the furnace in air. The thickness of the thick films was measured using a scanning electron microscope (SEM). Compared with previous piezoelectric PZT composite films, the modified piezoelectric thick films exhibit better dielectric properties. The dielectric constant is over 780 and dielectric loss is 0.04 at 1 KHz. Using a PiezoCAD model, the high frequency transducer was designed and fabricated. It showed a bandwidth of 75% at 40 MHz.     

Characterization and microstructure of porous lead zirconate titanate ceramics

Porous lead zirconate titanate (PZT) ceramics are widely used because of their low acoustic impedance, high figure of merit and high hydrostatic sensitivity. In the present work, porous PZT ceramics were fabricated by incorporating polyethylene oxide (PEO) as pore-forming agent. Both PZT powder and PEO were mixed with a binder at different ratios and compaction was carried out. The samples were slowly heated to remove the pore-forming agent and binder without cracks, followed by controlled sintering and electrode forming. Samples were poled using corona poling technique. The ferroelectric properties and microstructure of the prepared ceramics were characterized. The correlation of porosity with microstructure and ferroelectric properties were discussed.     

High-frequency transducers based on integrated piezoelectric thick films for medical imaging

A screen-printed PZT thick film with a final thickness of about 40 microm was deposited on a porous PZT substrate to obtain an integrated structure for ultrasonic transducer applications. This process makes it possible to decrease the number of steps in the fabrication of high-frequency, single-element transducers. The porous PZT substrates allow high acoustic impedance and attenuation to be obtained, satisfying transducer backing requirements for medical imaging. The piezoelectric thick films deliver high electromechanical performance, comparable to that of standard bulk ceramics (thickness coupling factor over 45%). Based on these structures, high-frequency transducers with a center frequency of about 25 MHz were produced and characterized. As a result, good sensitivity and axial resolution were obtained in comparison with similar transducers integrating a lead titanate (PT) disk as active material. The two transducers were integrated into a high-frequency imaging system, and comparative skin images are shown.     

80-MHz intravascular ultrasound transducer using PMN-PT free-standing film

[Pb(Mg(1/3)Nb(2/3))O(3)](0.63)[PbTiO(3)](0.37) (PMN-PT) free-standing film of comparable piezoelectric properties to bulk material with thickness of 30 μm has been fabricated using a modified precursor coating approach. At 1 kHz, the dielectric permittivity and loss were 4364 and 0.033, respectively. The remnant polarization and coercive field were 28 μC/cm(2) and 18.43 kV/cm. The electromechanical coupling coefficient k(t) was measured to be 0.55, which was close to that of bulk PMN-PT single-crystal material. Based on this film, high-frequency (82 MHz) miniature ultrasonic transducers were fabricated with 65% bandwidth and 23 dB insertion loss. Axial and lateral resolutions were determined to be as high as 35 and 176 μm. In vitro intravascular imaging on healthy rabbit aorta was performed using the thin film transducers. In comparison with a 35-MHz IVUS transducer, the 80-MHz transducer showed superior resolution and contrast with satisfactory penetration depth. The imaging results suggest that PMN-PT free-standing thin film technology is a feasible and efficient way to fabricate very-high-frequency ultrasonic transducers.     

Dielectric characteristics of Ba(Mg1/3Ta2/3)O3 ceramics sintered at low temperatures

Dielectric characteristics of Ba(Mg_1/3Ta_2/3)O₃ ceramics (BMT ceramics) sintered at low temperatures with 2–3 wt% NaF additives were determined. A dielectric constant of 25 and extremely low dielectric loss (< 0.0001) were measured at 100 kHz and 1 MHz in BMT ceramics sintered under these conditions, and no frequency dependence of the dielectric constant was observed. This suggested that NaF as sintering additive had no harmful influence on the dielectric properties of the ceramics.     

Novel ferroelectric materials for phased array antennas

Various composites of barium strontium titanium oxide (BSTO) combined with other nonelectrically active oxide ceramics have been formulated for application in phased array antennas. In general, the composites have adjustable electronic properties which can be tailored for use in phased array antennas and other phase shifting devices. The dielectric constant and the loss tangents have been reduced to enhance the overall impedance matching and thereby lowering the insertion loss of the device. In addition, the overall tunability, the change in the dielectric constant with applied voltage, is maintained at a sufficiently high level. In order to address a broad frequency range in the microwave region, the composites have been fabricated in bulk ceramic, thick film, and thin film form. This article discusses the processing, material characterization, and electronic properties of the composites in MHz and GHz frequencies     

PZT陶瓷中由氧空位与畴壁相互作用引起的内耗

采用低频倒置扭摆内耗仪对组分为 Pb(Zr0.7Ti0.3)O3 (PZT73)和 Pb(Zr0.3Ti0.7)O3 (PZT37) 的两种陶瓷的内耗 Q-1及振动频率的平方 f 2（正比于材料的剪切模量 G）与温度的关系进行了 测定。在纯三角相的 PZT73陶瓷中发现两个内耗峰。高温内耗峰 PM起源于材料的顺电－铁 电相变 ,低温内耗峰 P1本质上是一个宽化的 Debye峰,可归因于氧空位作用下的畴壁振荡弛豫。 对纯四方相的 PZT37陶瓷,除了 P1和 PM峰外,在 P1与 PM峰之间另有内耗峰 P2,这与 900畴附 近的氧空位团簇的弛豫有关 ,其特征可用描述强关联系统的关联态模型（ Coupling model）描述。     

Tunable capacitors employing BZN/BST thin films for RF applications

Tunable parallel-plate capacitors employing Bi(1.5)Zn(1.0)Nb(1.5)O(7)/Ba(0.5)Sr(0.5)TiO(3) (BZN/BST) thin films for RF applications are reported. The intermediate frequency measurements indicate that the BZN/BST-based varactors demonstrate large tunability of 39% at 40 V and high device quality factor of 300 at 1 MHz. The devices maintain quite low leakage current density even under a high applied bias. The quality factor analysis shows that the device quality factor is highly dependent on conductor loss of electrodes at frequencies above 1 MHz. The phase shifter employing BZN/BST-based varactors exhibits lower insertion loss than does employing semiconductor diodes at a designed frequency of 445 MHz, demonstrating the potential of tunable capacitors employing BZN/BST thin films for RF applications.     

Low field ac study of PZT/PVDF nano composites

Composites of nanocrystalline Pb_0.96Sr_0.04(Zr_0.53,Ti_0.47)O₃ (PZT) and α-phase PVDF have been developed using solution casting technique. Characterization of the composites has been done using XRD, FEGSEM, DSC and impedance analysis. XRD and FEGSEM determined the size range of PZT as 22–40 nm. XRD shows the successful incorporation of PZT into PVDF matrix and also confirms that no new phase is developed. DSC of the nanocomposites showed decrease in crystallinity with increasing PZT content. Broadband impedance analysis has been carried out to study the effect of the addition of PZT on the low field ac electrical properties of PVDF. Room temperature dielectric permittivity measurement of the PZT-PVDF composites at 1 kHz determined using impedance analyzer gives values of permittivity 2–4 times higher as compared to neat PVDF. It is found that dielectric permittivity values at the lower frequency edge are affected by space charges while the higher frequencies show the influence of relaxation effects in the materials. It is suggested that PZT/PVDF composites are the preferred materials for high temperature and high frequency applications. However, for low frequency use at higher temperatures, these composites do not offer any specific advantage. At room temperature, the composites are again the better choice in the 1 mHz–1 MHz frequency range.     

Effects of PNN/PZT ratios on phase structure,electric properties and relaxation behavior of PZN–PNN–PZT ceramics

Pb(Zn_1/3Ni_2/3)_c(Ni_1/3Nb_2/3)_a(Zr_xTi_y)_bO₃ (PZN–PNN–PZT, the ratios of PNN/PZT a/b were 0.88, 1 and 1.136) piezoelectric ceramics were prepared by a traditional solid-state reaction method. The effects of PNN/PZT ratio on phase structure, microstructure and electric properties as well as the relaxation behaviors of PZN–PNN–PZT ceramics were investigated. The XRD patterns showed that all ceramics samples had a pure perovskite phase structure. Meanwhile, it was found that the phase structure undergoes a tetragonal, tetragonal-rhombohedral to rhombohedral transition as ratios of PNN/PZT increased. With the increasing of a/b from 0.88 to 1.136, the dielectric constant and diffusive phase coefficient decreases, it was indicated that relaxation behaviors also decreased. When ceramics with a/b was 1.136, the dielectric relaxation γ reached the minimum and electrical properties were poor. The electric properties of ceramics with a/b was 1.00 have an excellent properties, it was indicated that ceramics reached an optimization at the MPB structure.