K. K. Shung

Development of a high frequency (35 MHz) linear ultrasonic array using 2-2 composite elements [biomedical applications]

This paper discusses the development of a 35 MHz composite ultrasonic array. This array was designed primarily for ocular imaging applications, and features 2-2 composite elements mechanically diced out of a fine grain high density Navy VI ceramic. Array elements were spaced at a 50 /spl mu/m pitch, interconnected via a custom flexible circuit and matched to the 50 /spl Omega/ system electronics via a 78 /spl Omega/ transmission line coaxial cable. Elevation, or off axis, focusing was achieved using a cylindrical epoxy lens. A 64-element array was fabricated and tested, yielding promising results. An average center frequency of 34 MHz was achieved with an average -6 dB bandwidth of 57.8 % and average -20 dB pulse length of 102 ns. The maximum combined electrical and acoustical crosstalk between adjacent or next adjacent elements was less than -24 dB. The 35 MHz array developed is capable of resolving structures in the human body that are smaller than 0.1 mm.

Multiple matching scheme for broadband 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystal phased-array transducer

Lead magnesium niobate–lead titanate single crystal 0.72Pb(Mg1/3Nb2/3)O3−0.28PbTiO3 (abbreviated as PMN-PT) was used to fabricate high performance ultrasonic phased-array transducer as it exhibited excellent piezoelectric properties. In this paper, we focus on the design and fabrication of a low-loss and wide-band transducer for medical imaging applications. A KLM model based simulation software PiezoCAD was used for acoustic design of the transducer including the front-face matching and backing. The calculated results show that the −6 dB transducer bandwidth can be improved significantly by using double λ/8 matching layers and hard backing. A 4.0 MHz PMN-PT transducer array (with 16 elements) was fabricated and tested in a pulse-echo arrangement. A −6 dB bandwidth of 110% and two-way insertion loss of −46.5 dB were achieved.

Local two-way magnetoelectric couplings in multiferroic composites via scanning probe microscopy

Local two-way magnetoelectric (ME) couplings of a multiferroic composite have been characterized at nanoscale using novel scanning probe microscopy techniques we developed. A bilayer multiferroic composite consisting of lead zirconate titanate (PZT) and TbDyFe (TDF) has been fabricated, and the evolution of ferroelectric domains in PZT induced by an external magnetic field is observed by piezoresponse force microscopy, while the evolution of magnetic domains in TDF induced by an external electric field is observed by magnetic force microscopy, confirming the two-way ME couplings in the multiferroic composite. The technique will be useful in characterizing nanoscale ME couplings in a wide range of multiferroic composites.

A kerfless 30 MHz linear ultrasonic array

In this study a 30 MHz kerfless linear array design was developed and was compared to a 30 MHz piezo-composite array of identical dimensions. A 2-D finite element model (PZFlex) was used to optimize and compare the performance of both arrays. The modeled results suggested that the kerfless array with two matching layers would have a two-way single element sensitivity 7 dB greater than that of the composite array with one acoustic matching layer. However analysis of one-way directivity patterns for single array elements showed a significant reduction in acceptance angle for the kerfless design due to the expected increase in electromechanical crosstalk. The modeled -6 dB acceptance angles were 28o and 50o for the kerfless array element and composite array element, respectively. Based upon these encouraging results kerfless and composite 30 MHz arrays were fabricated and tested. On average the sensitivity of the kerfless array echo response was only 0.8 dB higher than that of the composite array. The average echo center frequency and bandwidth was slightly higher than that of the composite array, whereas the maximum measured crosstalk for the kerfless array was 14 dB larger than that of the composite array. As expected, this crosstalk increase resulted in a reduction of element acceptance angle. The measured -6dB acceptance angle was 22o for the kerfless array and 35o for the composite array. Based upon these results it is expected that kerfless array technology will provide a viable alternate for high frequency linear array designs but should be considered only when an adequate composite is not available.

Piezoelectric PZT thick films on LaNiO(3) buffered stainless steel foils for flexible device applications.

In this paper, we report on 4.5µm piezoelectric Pb(Zr(0.52)Ti(0.48))O(3) (PZT) thick films deposited on flexible stainless steel (SS) foils with LaNiO(3) (LNO) buffer layers using a ceramic powder/sol-gel solution modified composite method. The polycrystalline thick films show a hysteresis loop at an applied electric field of 900 kV cm(-1) with remanent polarization and coercive electric field values of 27µC cm(-2) and 85 kV cm(-1), respectively. At 1 kHz, the dielectric constant is 653 and the dielectric loss is 0.052. The leakage current density of the film is lower than 1.55 × 10(-5) Acm(-2) over the range of 0 to ±150V. The conduction current shows ohmic behaviour at a low electric field and space-charge-limited current characteristics at a high electric field.

Fabrication of sol-gel modified piezoelectric thick films for high frequency ultrasonic applications

In this work, a fabrication process of piezoelectric PZT thick films up to 60 /spl mu/m deposited on silicon and aluminum substrates is reported. Crystalline spherical modified PZT powder about 0.3 /spl mu/m in diameter was used as filler. PZT polymeric precursor produced by Chemat Incorporated was used as the matrix material. Spinning films were annealed at 700/spl deg/C in the furnace for half an hour 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 very good 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.

Comparison of different piezoelectric materials for GHz acoustic microscopy transducers

Today acoustic microscopy transducers are based on zinc oxide, ZnO, films as piezoelectric material. Especially in the GHz range the insertion loss of existing ZnO transducers has to be improved for better imaging. The motivation for this work was to substitute the ZnO by the piezoelectric material lead zirconate titanate, PZT, with better electro mechanical properties. Novel technologies of sol-gel and metal organic chemical vapor deposition method (MOCVD) were used to realize high frequency PZT thin film transducers with 200 ¿m aperture diameter. The transducers were characterized by electrical and acoustical measurement. Their performance for acoustic microscopy was evaluated.

High frequency piezoelectric micromachined ultrasonic transducers for imaging applications

Methods for fabricating high frequency ultrasound transducer and array based on piezoelectric films and MEMS technology are presented in this paper. Piezoelectric PZT films up to 30 μm-thick deposited on silicon substrate have been prepared by a modified sol-gel process. The raw materials included lead acetate trihydrate and zirconium n-propoxide and titanate isoproxide. The sol-gel PZT solutions were prepared using above materials and 2-methoxyethanol as the solvent. Spin-coated films were annealed at 750 oC by a rapid thermal annealing (RTA) process. Thicker PZT films were fabricated by repeating this process and using a modified PZT composite solution. The high frequency single element transducers actuated by the PZT films were fabricated and pulse-echo measurement results show the transducers had a broad bandwidth and high central frequencies. The beam profile of one 103 MHz transducer was measured using a 8 μm diameter wire and a lateral resolution of 33 μm was observed. A micromachined process to fabricate high frequency linear array will be also presented.

PMN-PT single crystal thick films on silicon substrate for high-frequency micromachined ultrasonic transducers

In this work, a novel high-frequency ultrasonic transducer structure is realized by using PMNPT-on-silicon technology and silicon micromachining. To prepare the single crystalline PMNPT-on-silicon wafers, a hybrid processing method involving wafer bonding, mechanical lapping and wet chemical thinning is successfully developed. The active element is fixed within the stainless steel needle housing. The measured center frequency and -6 dB bandwidth of the transducer are 35 MHz and 34%, respectively. Owing to the excellent electromechanical property of PMNPT film, the transducer shows good energy conversion performance with a very low insertion loss down to 8.3 dB at the center frequency.

PIN-PMN-PT single crystal high frequency ultrasound transducers for medical applications

High frequency ultrasonic transducers were fabricated using lead indium niobate-lead magnesium niobate-lead titanate (0.24PIN-0.44PMN-0.32PT) as the active piezoelectric material. The measured center frequency and bandwidth of the device were 35 MHz and 48 % respectively. The electrical impedance magnitude was 71 Omega and phase was - 38deg. Insertion loss was measured to be 15 dB. A phantom which consists of 20-mum tungsten wires was imaged to assess spatial resolution of the transducer.