1. Build PbTiO₃ transducers with center frequency of 25-100 MHz using parylene as matching and conductive epoxy as backing. Focusing is realized either by acoustic lens or by spherical shaping.
2. Investigate the performance trade-off between the bandwidth and sensitivity, especially for frequencies above 50 MHz.

PbTiO₃ after HIP (heat isotropic pressure) is a novel piezoelectric material for the construction of high frequency transducers. It possesses high thickness coupling coefficient (>0.53). It's high density allows the thickness to be polished to less than 20 mm to meet the requirement of high frequency applications. The major drawback in using PbTiO₃ is its narrow bandwidth caused by acoustic impedance mismatch to human tissue and electrical impedance mismatch to 50W electronics. The bandwidth will be extended by acoustic matching and innovative electronic tuning techniques. For frequencies below 50 MHz, the focus is realized by using an acoustic lens. Focusing at frequencies above 50 MHz, however, is realized by spherical shaping to ensure the best acoustic energy transmission to human tissue.

At present, PbTiO₃ transducers with 75 MHz center frequency have been built with 38 % bandwidth.

1. Build LiNbO₃ transducers for center frequencies of 25-75MHz using a conductive matching layer and backing,and an acoustic lens.
2. Investigate the performance trade-offs in the fabrication of high frequency transducers using an acoustic lens or a spherically focused LiNbO₃ substrate.
3. Design and build transducers for very high frequency operation (75-200MHz).

LiNbO₃ is a novel piezoelectric material for the construction of transducers at very high frequency (25-200MHz).Its high longitudinal velocity (7340m/s in the 36° rotated Y-cut)aids in the fabrication process, as the thickness of the piezoelectric element operating in half wave length mode is large compared to most other piezoceramics and polymers operating at the same resonance frequency. This material also has a high thicknessmode coupling coefficient (kt₃₃=0.499) which allows for highly sensitive devices to be built. The only major drawback to using LiNbO₃ is its poor coupling to water or human tissue. This poor coupling is due to the high acoustic impedance (34Mrayls) of LiNbO₃ when compared to water or tissue (~1.54Mrayls). It is possible though to minimizethis impedance mismatch by casting quarter wave length matching layers on the substrate with impedance values within the range of 2-20Mrayls.

The goal of this project is to perfect a fabrication procedure for the building of broadband, low loss focused transducers with center frequencies ranging from 25MHz to 200MHz. With that, different matching and backing materials will be investigated for their performance in the 25-200MHz range. The use of an acoustic lens for beam focusing as opposed to press focusing the LiNbO₃ will be compared at various frequencies between 25MHz and 200MHz.

At present, two 25MHz transducers have been built with conductive epoxy backing and matching layers, and acoustic lenses also made of epoxy. The results of preliminary testing are excellent. Testing in a pulse-echo arrangement we were able to achieve -6dBb and widths of 67% and 70% and insertion loss values of -17dB and -19dB.

1. Fabricate focused PVDF transducers in l/2 and l/4 wavelength modes.
2. Design and build transducers from 25-100MHz with very wide bandwidth (50-120% or more).
3. Test electrical tuning methods such as transmission line transformers and coaxial cable tuning with high frequency PVDF devices.

With the advent of new fabrication techniques and materials blending, polyvinylidene fluroride (PVDF) and its copolymers such as P(VDF-TrFE) have gained piezoelectric properties more similar to that of piezoceramics without greatly sacrificing acoustical matching or mechanical flexibility. The thrust of this work is designing high frequency (25MHz) focused PVDF transducers using these new materials.

A number of fabrication methods have been employed in designing the transducers. The backing materials used are varied from conductive microballoons to several different epoxies in order to achieve wavelength modes between l/2 and l/4. Microballoons are used to simulate an air backing while retaining the ability to focus the polymer. Multiple layer 9mm PVDF transducers in l/2 mode were made, resulting in lower electrical impedance, higher center frequency and larger bandwidth than single layer 25m PVDF transducers made in the lab. Coaxial cable lengths and transmission line transformers are also being investigated to electrically match the transducers to a 50W system.

Experimental data closely matches that of one-dimensional KLM modeling for 25mm single and 9mm double layers, though there is disparity in the case of single 9mm layer transducers. Work is currently being done on modeling coaxial cable lines to better tune the transducer to the testing system.

Transducers have been made with a center frequency range of 25-50MHz with a -6dBb and width of 40-120%. Wider bandwidth values were for the transducers in l/4 mode. Insertion loss values are approximately -59dB.

Future work involves testing polymer materials from other vendors and comparing piezoelectric properties. Tuning issues will be further studied, and single and double layer 9m PVDF and P(VDF-TrFE) in l/2 wavelength mode will be investigated for use in transducers from 50-150MHz.

1. Characterize electromechanical properties of fiber composite for high frequency transducer application.
2. Build fiber composite transducers with center frequencies of 25-70MHz using a parylene matching layer and microballoons mixed with epoxy as backing material.
3. Develop very high frequency (>70MHz) fiber composite transducers by using smaller diameter (<10mm) PZT fiber.

Fine-scale fiber piezo-composites with 1-3 connection are one of the most promising materials for high frequency (25-70MHz) ultrasonic transducer applications. They possess high coupling coefficients (>0.6), adjustable acoustic and electric impedance, and good mechanical flexibility. The random distribution of PZT fiber in the composite avoids the stopband which occurs in regular 1-3 composite. The major drawback is the difficulty in manufacturing fine-scale PZT fiber using current techniques.

The near future goal of this project is to design and built transducers with a center frequency of 25-70MHz by using different fiber volume fraction composites with fibers of diameter larger than 17mm. In the transducer fabrication, vapor deposited parylene is selected as a matching layer and silvercoated microballoons mixed with epoxy as backing material. The focusing is realized by spherical shaping. The far future goal is to design and built very high frequency (>70MHz) by using smaller diameter (<10mm) PZT fiber which is currently under development in cooperation with the Materials Research Laboratory.

At present, fiber composite transducers up to 60MHz have been built. The pulse/echo test results showed a bandwidth of 72% and insertion loss of -38dB at center frequency.