Characterization of high frequency properties of ultrasonic transducer materials using ultrasonic spectroscopy.

摘要

High frequency ultrasonic transducers have become more important in medical ultrasonic imaging in recent years because of the demand for better resolution in ultrasonic imaging. Design of high frequency ultrasonic transducers requires a complete knowledge of material properties at high frequencies, including both for active and passive materials. It has been found that the frequency dispersion of phase velocity and attenuation of transducer materials becomes significant for frequencies above 30 MHz. This dissertation is devoted to the characterization of acoustic and piezoelectric properties of transducer materials, both active and passive at frequencies above 30MHz.; In order to obtain more accurate data, two improved ultrasonic spectroscopy methods have been developed. Error analysis shows that they are more convenient and have at least the same if not better accuracy because one less predetermined parameter is required, which eliminates one more error source.; Using ultrasonic spectroscopy, the passive transducer materials, including matching, backing and lens materials, are characterized in the frequency range of 25–65 MHz for the first time. The alumina/EPO-TEK301 and tungsten/EPO-TEK301 composites are fabricated with different volume fractions of particle loading. Experimental results demonstrate a monotonic increase in acoustic impedance with increasing particle volume fraction for both composites and a sharp fall in phase velocity in tungsten/EPO-TEK301 composites. The results show agreement with the Denavey model and are different from the results obtained in low frequencies, where Reuss model works better. An attenuation peak is found to occur at approximately 9% volume fraction of particles for both composites. This is different from the behavior in low frequencies, where the attenuation of composites generally decreases with increasing volume fraction of particles.; Because the active element in the vast majority of medical ultrasonic transducer is piezoelectric ceramic, one of the main tasks of this thesis is to investigate the physical properties of piezoceramics using ultrasonic spectroscopy. Acoustic wave propagation in piezoceramics and through the interface between the water and ceramics has been analyzed. The dispersions of phase velocity and attenuation of pure mode waves are measured first, then, quasi-longitudinal and quasi-shear waves are investigated. By measuring the phase velocities of ultrasonic waves in different propagation directions, we have obtained the full matrix elastic constants and piezoelectric constants of piezoelectric ceramic PZT-5H by using an improved scheme of data analysis, in which the Levenberg-Marquardt algorithm is used. The new scheme shows good stability and allows us to obtain the full matrix parameter with only two samples.