Xuecang Geng

5I-1 Microfabrication of Piezoelectric Composite Ultrasound Transducers (PC-MUT)

In this paper a piezoelectric composite based micromachined ultrasound transducer (PC-MUT) fabrication technology is presented. PMN-PT single crystal posts with side length of 14 mum and height of > 60 mum were fabricated using a deep dry etching method. High frequency (20-50 MHz) PMN-PT single crystal/epoxy 1-3 composites were prepared and the electromechanical coupling coefficient of the composites was ~0.72. Prototype 40 MHz ultrasound transducers showed promising sensitivity and bandwidth

Very high frequency (beyond 100 MHz) PZT kerfless linear arrays

This paper presents the design, fabrication, and measurements of very high frequency kerfless linear arrays prepared from PZT film and PZT bulk material. A 12-mum PZT thick film fabricated from PZT-5H powder/solution composite and a piece of 15-mum PZT-5H sheet were used to fabricate 32-element kerfless high-frequency linear arrays with photolithography. The PZT thick film was prepared by spin-coating of PZT sol-gel composite solution. The thin PZT-5H sheet sample was prepared by lapping a PZT-5H ceramic with a precision lapping machine. The measured results of the 2 arrays were compared. The PZT film array had a center frequency of 120 MHz, a bandwidth of 60% with a parylene matching layer, and an insertion loss of 41 dB. The PZT ceramic sheet array was found to have a center frequency of 128 MHz with a poorer bandwidth (40% with a parylene matching layer) but a better sensitivity (28 dB insertion loss).

Single Crystal Piezoelectric Composite Transducers for Ultrasound NDE Applications

Single crystal piezoelectric composite transducers including 75 MHz PC-MUT (piezoelectric composite micromachined ultrasound transducers), diced 10 MHz and 15 MHz 1-3 composite transducers were successfully demonstrated with broad bandwidth and high sensitivity. In this paper, the design, fabrication and characterization of composite transducers are reported. C-scan experiments for SiC ceramic samples were performed using these composite transducers as well as some commercial NDE transducers. The results suggest that significant improvements in resolution and penetration depth can be achieved in C-scan NDE imaging using single crystal composite broadband transducers.

Micromachined PMN-PT single crystal composite transducers -- 15–75 MHz PC-MUT

In this paper a piezoelectric composite based micromachined ultrasound transducer (PC-MUT) technology is presented for fabrication of PMN-PT single crystal piezoelectric composite transducers. Micromachined PMN-PT single crystal 1-3 composites with resonance frequencies of 15 MHz-75 MHz were fabricated and characterized with coupling coefficient of 0.67-0.79. Transducers with a frequency of 75 MHz were prototyped and characterized.

Single Crystal Piezoelectric Composites for Advanced NDT Ultrasound

In this paper, the design, fabrication and characterization of PMN-PT single crystal/epoxy composites are reported for NDT ultrasound transducers. Specifically, 1-3 PMN-PT/epoxy composites with center frequencies of 5 MHz - 40 MHz were designed and fabricated using either the dice-and-fill method or a photolithography based micromachining process. The measured electromechanical coefficients for composites with frequency of 5 MHz - 15 MHz were about 0.78-0.83, and the coupling coefficients for composites with frequencies of 25 MHz- 40 MHz were about 0.71-0.72. The dielectric loss remains low (< 0.05). These properties hold promise for advanced NDT ultrasound applications.

Broad band single crystal transducer for contrast agent harmonic imaging

A broadband single element transducer was constructed from single crystal PMN-PT and tested to determine the feasibility of using this material for simultaneous sub-harmonic and second harmonic imaging. The transducer consisted of a 1-3 PMN-PT/epoxy composite with two quarter wave matching layers and a backing. The transducer face was spherically curved to yield a 53 mm focal point. The transducers impulse response had a center frequency of 5.2 MHz and -6 dB bandwidth over 130%. The transmit response for tone burst excitation indicated a shift in center frequency to 6.5 MHz and a 6 dB bandwidth of 100%. Receive sensitivity was high at the lower end of the band and decreased with increasing frequency; this behavior was probably due to electrical impedance mismatch. Contrast agent testing using Sonazoid/spl reg/ showed that the second harmonic could be easily detected. The sub-harmonic was not detected due to the center frequency shift under tone burst excitation. Simultaneous sub, and second harmonic testing could probably be accomplished with a lower center frequency and better electrical impedance matching.

30 MHz medical imaging arrays incorporating 2-2 composites

Methods for fabricating and modeling high frequency 2-2 composites and arrays are presented. The composites are suitable for arrays and small aperture single element devices. Coupling coefficients above 0.65 and lateral mode frequencies near 60 MHz have been achieved. Two prototype 4 element 30 MHz linear arrays were designed and built using this composite. Backing and matching layers were fabricated and characterized while coaxial cable was used to electrically tune each element and broaden bandwidth. The measured properties of passive and active components were used to analyze the design in a time-domain finite element analysis program. Agreement between experiment and theory was excellent.

Design, fabrication, and characterization of a single-aperture 1.5-MHz/3-MHz dual-frequency HIFU transducer

High-intensity focused ultrasound (HIFU) treatment efficiency is critical in maximizing the hyperthermia and reducing the surgery time. In this paper, a single-aperture, 1.5 MHz/3 MHz dual-frequency HIFU transducer was designed, fabricated, and characterized for tissue ablation enhancement. Double PZT-2 layers were configured in serial and dual-frequency ultrasound waves can be concurrently generated by exciting one of the PZT-2 layers. Impulse responses from the prototype showed that the wave amplitudes at 1.5 and 3 MHz were about the same, and both are more than 12 dB larger than those of higher orders of harmonics. Tissue ablation tests demonstrated that higher temperature rise can be achieved with dual-frequency ultrasound than with single-frequency ablation at the same acoustic power.

Electroacoustic response of 1-3 piezocomposite transducers for high power applications

The electroacoustic performance of 1-3 piezoelectric composite transducers with low loss polymer filler was studied and compared to monolithic Pb(Zr,Ti)O(3) (PZT) piezoelectric transducers. The 1-3 composite transducers exhibited significantly high electromechanical coupling factor (k(t) ∼ 0.64) when compared to monolithic counterparts (k(t) ∼ 0.5), leading to the improved bandwidth and loop sensitivity, being on the order of 67% and -24.0 dB versus 44% and -24.8 dB, respectively. In addition, the acoustic output power and transmit efficiency (∼50%) were found to be comparable to the monolithic PZT transducers, demonstrating potential for broad bandwidth, high power ultrasonic transducer applications.

Design, Fabrication, and Characterization of a Bifrequency Colinear Array

Ultrasound imaging with high resolution and large penetration depth has been increasingly adopted in medical diagnosis, surgery guidance, and treatment assessment. Conventional ultrasound works at a particular frequency, with a - 6-dB fractional bandwidth of ~ 70% , limiting the imaging resolution or depth of field. In this paper, a bifrequency colinear array with resonant frequencies of 8 and 20 MHz was investigated to meet the requirements of resolution and penetration depth for a broad range of ultrasound imaging applications. Specifically, a 32-element bifrequency colinear array was designed and fabricated, followed by element characterization and real-time sectorial scan (S-scan) phantom imaging using a Verasonics system. The bifrequency colinear array was tested in four different modes by switching between low and high frequencies on transmit and receive. The four modes included the following: 1) transmit low, receive low; 2) transmit low, receive high; 3) transmit high, receive low; and 4) transmit high, receive high. After testing, the axial and lateral resolutions of all modes were calculated and compared. The results of this study suggest that bifrequency colinear arrays are potential aids for wideband fundamental imaging and harmonic/subharmonic imaging.